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Adsorption of perfluorooctanoic acid and perfluorooctanesulfonic acid to iron oxide surfaces as studied by flow-through ATR-FTIR spectroscopy


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The kinetics and mechanisms of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) adsorption to nanoparticulate hematite (α-Fe2O3) from aqueous solutions were examined using in situ, flow-through attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Results indicate that both PFOA and PFOS molecules are retained at the hydrophilic hematite surface and the adsorption shows strong pH dependence. However, ATR-FTIR data reveal that PFOA and PFOS are bound to the iron oxide by different mechanisms. Specifically, in addition to electrostatic interactions, PFOA forms inner-sphere Fecarboxylate complexes by ligand exchange, whereas the PFOS sulfonate group forms outer-sphere complexes and possibly hydrogen-bonds at the mineral surface. Both solution pH and surface loading affect adsorption kinetics. Faster adsorption was observed at low pH and high initial PFC concentrations. Sorption kinetics for both compounds can be described by a pseudo-second-order rate law at low pH (pH 3.0 and 4.5) and a pseudo-first-order rate law at high pH (pH 6.0). Sorption isotherm data for PFOA derived from spectroscopic results exhibit features characteristic of ionic surfactant adsorption to hydrophilic charged solid surfaces.
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Adsorption of perfluorooctanoic acid and
perfluorooctanesulfonic acid to iron oxide surfaces as
studied by flow-through ATR-FTIR spectroscopy
Xiaodong Gao
and Jon Chorover
Department of Soil, Water and Environmental Science, University of Arizona, Tucson,
AZ 85721, USA.
Present address: Department of Earth Science, Rice University, Houston, TX 77251, USA.
Corresponding author. Email:
Environmental context. Perfluoroalkyl compounds are organic contaminants that exhibit strong resistance to
chemical- and microbial-degradation. As partitioning between solid and aqueous phases is expected to control
the transport of perfluoroalkyl compounds, we studied the molecular mechanisms of their adsorption–
desorption at a representative Fe oxide surface using in situ molecular spectroscopy. The results provide
valuable information on the types of bonds formed, and enable a better understanding of the transport and fate of
these organic contaminants in natural environments.
Abstract. The kinetics and mechanisms of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)
adsorption to nanoparticulate hematite (a-Fe
) from aqueous solutions were examined using in situ, flow-through
attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Results indicate that both PFOA and
PFOS molecules are retained at the hydrophilic hematite surface and the adsorption shows strong pH dependence.
However, ATR-FTIR data reveal that PFOA and PFOS are bound to the iron oxide by different mechanisms. Specifically,
in addition to electrostatic interactions, PFOA forms inner-sphere Fe–carboxylate complexes by ligand exchange, whereas
the PFOS sulfonate group forms outer-sphere complexes and possibly hydrogen-bonds at the mineral surface. Both
solution pH and surface loading affect adsorption kinetics. Faster adsorption was observed at low pH and high initial PFC
concentrations. Sorption kinetics for both compounds can be described by a pseudo-second-order rate law at low pH (pH
3.0 and 4.5) and a pseudo-first-order rate law at high pH (pH 6.0). Sorption isotherm data for PFOA derived from
spectroscopic results exhibit features characteristic of ionic surfactant adsorption to hydrophilic charged solid surfaces.
Received 29 September 2011, accepted 15 February 2012, published online 30 April 2012
Perfluoroalkyl compounds (PFCs) such as perfluorooctanoic
acid (PFOA) and perfluorooctanesulfonic acid (PFOS) (Fig. 1)
are surfactants that have been used as fire extinguishing agents
in aqueous film forming foam (AFFF) for decades.
products usually comprise 1–5 % w/w PFCs, including PFOS,
PFOA and their short-chain homologues.
The unique chemi-
cal and thermal stabilities of PFCs are the principal reasons for
their use in AFFFs.
The physicochemical properties of PFCs
differ significantly from their hydrocarbon analogues. Substi-
tution of H by electronegative F in the aliphatic chain structure
gives rise to very high C–F bond covalency. Relative to their
hydrocarbon analogues, fluorination increases the rigidity of the
carbon chains, compound thermal stability and Brønsted acidity
of the acid group (Table 1).
Historically, effluents from AFFF fire-fighting activities
were not pre-treated before discharge to waste-water treatment
systems or to the environment, and military activities are an
important contributor. For example, because of the use of large
quantities of flammable liquids, the USA military consumes
,75 % of total nationwide AFFF production each year, resulting
in numerous PFC contaminated sites within the Department of
Defence complex that require cleanup.
It is estimated that a
total of 50–100 t of PFOA and 3–30 t of PFOS have been
released to the environment from historical use of AFFF
products in the USA.
PFCs can cause developmental toxicity,
immunotoxicity, hepatotoxicity and hormonal effects even at
very low concentrations.
Thus, it is essential to establish a
mechanistic understanding of the geochemical processes that
govern the transport and fate of PFCs in soil and water systems,
and that can be used in remediation technologies for effective
contaminant removal.
The global occurrence and distribution of PFCs has only
recently become a focus of study. Despite the fact that
PFCs have been widely identified in surface water,
public drinking water systems
sea water,
little is known about their transport and fate in
the environment. Because of their remarkable resistance
to chemical- and microbial-degradation in geomedia,
sorption–desorption dynamics are expected to be a key control
on environmental fate. Upon release to soils and sediments, the
reactive transport of PFCs in terrestrial systems is governed
largely by surface interactions with soil and sediment particles.
A recent soil column study reported that PFOS in street runoff
Environ. Chem. 2012,9, 148–157
Journal compilation ÓCSIRO 2012
Research Paper
was not removed during the infiltration process and in fact the
concentration of PFOS was observed to increase along the flow
path, which was attributed to its formation from precursor
This study and several other recent reports
suggest that PFCs can be relatively mobile in the subsurface and
may cause groundwater contamination. PFCs are strong acids
that dissociate to anionic form in most natural environments.
Thus, anionic PFCs are expected to adsorb to positively charged
surfaces (e.g. Al and Fe oxides) by electrostatic or covalent
bonding interactions. In addition, because of its unique compo-
sition, the fluorocarbon tail exhibits both hydrophobic and
lipophobic properties, and fluorocarbon chain length controls
the extent to which the compound undergoes intermolecular
aggregation and adsorbs to hydrophobic surfaces such as natural
organic matter (NOM).
The energetics of surfactant adsorption result from the
additive effects of multiple bonding modes, including the Gibbs
energies of molecular and surface hydration, and intermolecular
associations. In general, molecular mechanisms of adsorption
can include covalent (inner-sphere) or electrostatic (outer-
sphere) complexation at surface sites, hydrophobic interaction
(including intermolecular associations such as surface admicelle
and hemimicelle formation), ion or water bridging and hydrogen
Prior macroscopic studies indicate that adsorption
of PFCs is strongly influenced by aqueous geochemistry and
sorbent properties. Adsorption of PFCs to several natural sedi-
ments, clays, synthetic iron oxide (goethite, no organic carbon)
and activated carbon shows positive correlation with organic
matter content
and compound fluorocarbon chain
indicating that PFC hydrophobicity contributes to
sorption energetics. Adsorption of PFCs to sediments also
increases with decreasing pH and charge fraction of bivalent
cation (Ca
suggesting that electrostatic interaction
of the polar anionic PFC head with charged solids and bridging
cations plays an important role in adsorption. Sorption mechan-
isms, however, cannot be unambiguously determined from these
macroscopic experiments. Molecular-scale adsorption mechan-
isms of PFCs at solid surfaces remain largely unexplored.
In addition, PFCs can potentially form surface-induced hemi-
micelles at solid–water interfaces well below the critical micelle
concentration (CMC) (0.1–1.0 % of the CMC).
As a result,
the adsorption mechanisms of PFCs are likely to vary signifi-
cantly across the environmentally relevant concentration range.
Studies are clearly needed to develop a comprehensive under-
standing of molecular-scale sorption processes as affected by
geochemical conditions as they control the transport and fate of
PFCs in natural soil and water systems.
Attenuated total reflection Fourier-transform infrared (ATR-
FTIR) spectroscopy is an established in situ technique for
interrogating molecular sorption mechanisms, redox processes
and reaction kinetics in aqueous systems.
Recently, ATR-
FTIR has been increasingly applied to the determination of
adsorption kinetics and detailed molecular-level information of
the coordination and structure of various adsorbates at the solid–
water interfaces, including inorganic anions (e.g. sulfate, phos-
phate and arsenate),
low-molecular-weight (LMW) car-
boxylic acids,
extracellular polymeric substance (EPS) and
intact microbial cells
and hydrocarbon ionic surfac-
To our knowledge, however, there are no prior
ATR-FTIR studies of adsorption of PFCs from aqueous solution
onto environmental solid surfaces. The primary objective of this
study, therefore, was to examine the binding mechanisms and
kinetics of PFOA and PFOS, the two most common components
of PFCs in AFFF products, onto the surface of a representative
oxide (hematite, a-Fe
) across a gradient in pH that is
characteristic of natural pore waters present in soils and sedi-
ments. Hematite was chosen as the model surface because it is
one of the most abundant Fe (oxyhydr)oxides in highly weath-
ered soils or sediments, and it often occurs naturally in nano-
particulate, high specific surface area forms, which can serve as
a high affinity sorbent for PFCs.
ATR-FTIR spectroscopy
was employed in conjunction with a flow cell technique to
elucidate in situ dynamic changes in spectroscopic data of
PFOA and PFOS as affected by solution chemistry and binding
to solid surfaces.
Experimental methods
Chemicals and materials
PFOA (96 %) and PFOS as the potassium salt (.98 %) were
purchased from Sigma–Aldrich Co. (St Louis, MO) and used as
received. Synthesis of nanoparticulate a-Fe
was performed
following the methods of Schwertmann and Cornell.
detailed synthetic procedure and characterisation have been
given previously.
The synthetic hematite has a particle size of
,10–20 nm in diameter, a Brunauer–Emmett–Teller (BET) N
specific surface area of 69.3 0.3 m
, and a measured iso-
electric point (IEP) of ,7.7–7.8 in 1 mM NaNO
All solutions were prepared using Barnstead
Nanopure (BNP) water (Thermal Scientific, Waltham, MA;
18.2 MOcm) in a background electrolyte solution containing
10 mM NaCl (except for the ionic strength study) with pH
adjustment by addition of NaOH or HCl.
FTIR spectroscopy measurements
ATR-FTIR spectra were obtained with a Magna-IR 560 Nicolet
spectrometer (Madison, WI) equipped with a purge gas
generator and a deuterated triglycine sulfate (DTGS) detector.
Fig. 1. Chemical structure of (a) perfluorooctanoic acid (PFOA,
COOH) and (b) perfluorooctanesulfonic acid (PFOS, C
Table 1. Physico-chemical properties of the perfluoroalkyl com-
pounds (PFCs) perfluorooctanoic acid (PFOA) and perfluorooctanesul-
fonic acid (PFOS)
CMC, critical micelle concentration
PFCs Molecular
Water solubility
(mg L
CMC (mM) log K
3400 2.6 8.7–9.0 4.3
570 ,1 2.0 5.3
PFOA and PFOS adsorption to hematite
A458trapezoidal germanium internal reflection element (IRE)
(56 10 3mm
) in a flow-through cell (Pike Technologies,
Madison, WI) was employed. The Ge IRE in the flow cell was
coated with ,1.65 mg nanoparticulate hematite as described by
Gao et al.
and placed on a horizontal ATR sample stage
inside the IR spectrometer. The thickness of the hematite coat-
ing was calculated to be ,0.6 mm. A new coating was prepared
for each experiment, and spectra of dry films were collected to
determine the consistency of the coating. The cell was con-
nected to a reaction vessel containing 1 L of background elec-
trolyte solution, with and without PFCs, continuously stirred
with a magnetic bar. A peristaltic pump was used to deliver the
background electrolyte or PFC solutions from the reaction
vessel through the flow cell at a constant flow rate of 0.5 mL
. The effluent from the flow cell was collected to waste.
All spectra were acquired at 4.0 cm
resolution with 400
scans over the spectroscopic range of 4000–800 cm
using the
autogain function and aperture set at 100 at room temperature.
For each experiment, background electrolyte solution (10 mM
NaCl) was first pumped through the cell, allowing the hematite
coating to equilibrate with the background solution. Successive
background spectra of 10 mM NaCl at the hematite-coated Ge
IRE were collected during the equilibrium time under continu-
ous flow conditions. The final background spectrum was col-
lected when no further changes in the spectra were observed and
this background was used for the remainder of the experiment.
PFOA or PFOS in 10 mM NaCl electrolyte solution was
then injected into the cell to initiate the adsorption experi-
ment. Spectra were collected as a function of pH (3.0–6.0)
and compound concentration (50–1000 mM for PFOA and
50–250 mM for PFOS). Experiments were also conducted at
pH 9.0, which is higher than the point of zero net proton charge
) of hematite (pH 7.7–7.8), but spectroscopic intensities
were observed to decrease with increasing pH and became
undetectable by FTIR at pH 9.0. PFOS adsorption was examined
over a lower concentration range due to its lower water solu-
bility and CMC (Table 1). The pH of the solution in the reaction
vessel was monitored throughout the experiments, and adjusted
as necessary by addition of 10 mM NaOH or HCl. Adsorption
kinetics of PFOA or PFOS on hematite were monitored by
collecting spectra at 15 min intervals untiladsorption equilibrium
was attained as indicated by no further changes between succes-
sive spectra. Following the adsorption experiment, PFC-free
background electrolyte solution was introduced again to the
ATR cell to assess desorption kinetics. Spectra were collected
every 15 min until no change was observed in subsequent
spectroscopic data over a time scale of ,120 min and this was
defined as the apparent desorption equilibrium. The effect of
ionic strength on PFOA adsorption was also examined in a
separate experiment in 1, 10 and 100 mM NaCl background
electrolyte using the same method described above.
All spectra (adsorption and desorption) were obtained by
subtracting the final background spectrum of hematite in the
corresponding NaCl background electrolyte. Data collection
and spectroscopic processing, including background subtraction
and baseline correction, were performed using the OMNIC
program (Thermo Nicolet, Co., Madison, WI). The GRAMS/
AI software (Thermo Electron Corp., Madison, WI) was used
for peak deconvolution and to determine the position and area of
the IR bands. A linear baseline was applied to the raw spectra.
The baseline corrected spectra were then fitted with Lorentzian
peaks. No constraints were placed on any of the fitting para-
meters (e.g. peak position, width or intensity).
Results and discussion
FTIR spectra of dissolved and hematite-adsorbed PFOA
The ATR-FTIR spectrum of aqueous PFOA on the Ge IRE in the
absence of nanohematite coatings represents that of the dis-
solved species. Adsorption to the Ge IRE is expected to be
negligible because of electrostatic repulsion between the nega-
tively charged Ge IRE and the anionic surfactant. The spectrum
is dominated by strong asymmetric and symmetric stretching
bands of the CF
and CF
groups of the hydrophobic tail and the
deprotonated carboxylate head groups (COO
) (Fig. 2).
Detailed peak position and mode assignments are given in
Table 2.
As PFOA, a monocarboxylic acid with rel-
atively high acid strength (pK
¼1.31–2.8 depending on
experimental conditions),
is nearly fully deprotonated
throughout the pH range probed by this study ($pH 3.0), the
carboxylate stretching bands in aqueous form are not expected
to exhibit systematic changes with solution pH.
In the presence of the hematite coating, absorbance intensi-
ties in the PFOA spectrum are much greater (,10-fold higher)
than those for the aqueous form (Fig. 2). The positive effect of
hematite on PFOA adsorption can be attributed to favourable
electrostatic interactions between the positive-charged hematite
surface (pH
and the anionic surfactant, in
addition to an increase in total surface area of the solid–water
interface probed by the IR beam when nanoparticulate hematite
is present.
The spectrum of hematite-adsorbed PFOA is very
similar to its aqueous form, exhibiting major bands correspond-
ing to the stretching bands of COO
and CF
Despite overall similarities between the two spectra (Fig. 2),
close examination reveals that significant changes in the car-
boxylate stretching region results from the presence of hematite,
indicating that the negatively charged COO
groups play a role
in interfacial reaction. For example, compared with the aqueous
PFOA spectrum, the peak position of n
) in the presence
of hematite is shifted to a higher wavenumber, from 1667 to
1677 cm
, whereas the n
) is shifted to a slightly lower
wavenumber (from 1414 to 1408 cm
), resulting in an increase
1800 1600 1400 1200 1000
Wavenumber (cm1)
1677 1408
1363 &
1102 & 1016
@ Ge IRE
@ Hematite
Fig. 2. Attenuated total reflection Fourier-transform infrared spectra of
500-mM perfluorooctanoic acid adsorbed on Ge internal reflection element
(IRE) and hematite surface in 10 mM NaCl solution at pH 6.0. The
absorbance intensity of the spectrum at Ge IRE was increased 10 times for
X. Gao and J. Chorover
in separation (D~
n) between these two stretching bands. Similar
shifts in the n
and n
peak positions have been previously
observed for LMW carboxylic acid (L-lactate) adsorbed on
hematite nanoparticles.
These shifts were attributed to the
formation of inner-sphere complexes between lactate carboxyl-
ate groups and hematite surface Fe atoms. In addition, the
intensity ratio of n
to n
for COO
stretching increased upon
PFOA adsorption to hematite (Fig. 2). Prior studies indicate that
increased values of the n
) intensity ratio
upon adsorption are a diagnostic feature of inner-sphere com-
plexation with mineral surface metal centres.
Thus, the
results suggest that the carboxylate head groups of PFOA form
inner-sphere (i.e. covalent) bonds at the hematite surface. The
asymmetric stretching bands of CF
groups were also shifted
slightly to higher wavenumbers, apparently because of local
changes in the PFOA molecular environment encountered at the
solid–water interface upon adsorption (Fig. 2).
Although ATR-FTIR data clearly indicate that PFOA is
adsorbed to hematite by inner-sphere complexes, the additional
contribution of outer-sphere complexes cannot be excluded
under the experimental conditions, as the hematite surface and
surfactant molecules are oppositely charged. Variation in ionic
strength is often used as an indirect test of the contribution of
inner- and outer-sphere complex modes of metal or anion
adsorption to charged surfaces.
If inner-sphere complexation
is the sole sorption mechanism, adsorption should not be
significantly affected by a change in the ionic strength of
‘indifferent’ (non-surface complexing, e.g. NaCl) background
electrolyte, whereas if outer-sphere complexation is a predomi-
nant sorption mechanism, the extent of adsorption should
decrease with increasing ionic strength. In the present study,
PFOA adsorption to hematite was measured as a function of
ionic strength (NaCl as background electrolyte) at three differ-
ent pH values (3.0, 4.5 and 6.0). The adsorption, as represented
by the absorbance of the n
) band centred at 1208 cm
decreases with increasing ionic strength at all pH values,
indicating that, in addition to ligand exchange reactions,
outer-sphere complexation also plays a role in PFOA adsorption
(Fig. 3). A negative effect of ionic strength on adsorption was
also observed in a batch study of PFOS adsorption on goe-
In the pH range of the present study (3.0–6.0), the
electrostatic attractive force between the positively charged
hematite surface and negatively charged PFOA molecules was
evidently diminished by charge screening or adsorptive compe-
tition with the background ions (i.e. Na
and Cl
). Hence,
although the ATR-FTIR data clearly show a ligand exchange
reaction for PFOA at the hematite surface, decreased adsorption
with increasing ionic strength suggests that a portion of PFOA
may be adsorbed by electrostatic interaction. The fact that a
distinct second n
) peak characteristic of outer-sphere
complexation was not observed at a lower wavenumber suggests
that both complexed and non-complexed carboxylate vibrat-
ional modes may be overlapping to give rise to the broad
1677 cm
FTIR spectra of dissolved and hematite-adsorbed PFOS
The structure of PFOS is similar to that of PFOA, but with
sulfonate replacing carboxylate in the polar head group, along
with the addition of a single CF
group (Fig. 1). Consequently,
PFOS exhibits stronger acidity (pK
,1) and a lower CMC
(2.0 mM) than PFOA.
Spectra of PFOS at the Ge IRE (dis-
solved) and hematite surface (adsorbed) are shown in Fig. 4. The
two spectra are very similar except for the intensity difference.
Similar to PFOA, PFOS exhibits a much higher affinity for the
hematite surface as a result of electrostatic attraction and
increased interfacial area produced by the hematite film. The
peak positions of major sulfonate and CF
stretching bands
occur with consistent relative intensities and peak locations in
pH 3.0
pH 4.5
pH 6.0
Ionic stren
th (mM)
Fig. 3. Effect of ionic strength on perfluorooctanoic acid adsorption on
hematite at pH 3.0, 4.5 and 6.0 shown by the absorbance of the n
) band
centred at 1208 cm
Table 2. Positions (cm
) and mode assignments of major IR bands in attenuated total reflection Fourier-
transform infrared spectra of aqueous perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)
(pH 6.0) on the Ge internal reflection element
asymmetric stretch; n
symmetric stretch; n
axial stretch
Wavenumber (cm
) Mode assignment Wavenumber (cm
) Mode assignment
1667 n
) 1372 n
1414 n
) 1329 n
1363 n
) 1283 n(CF
1319 n
) 1267 n(CF
1238 n
) 1243 n
1204 n
) 1215 n
1149 n
) 1152 n
1102 n(C–C) 1066 n
1014 n(C–C) 1037 n(C–C)
PFOA and PFOS adsorption to hematite
the spectroscopic region 1300–1100 cm
. Precise assignment
of individual IR bands was obtained by peak fitting and
deconvolution. As shown in Fig. 5a, the set of bands determined
by peak deconvolution is consistent with those previously
reported in the literature.
The absorption bands at 1283,
1267, 1243 and 1202 cm
correspond to asymmetric and sym-
metric CF
stretching bands that are also present in the spectrum
of PFOA. The additional band at ,1215 cm
can be assigned to
Sulfonate groups are very sensitive to changes
in local coordination geometry. For example, a symmetry change
of sulphate and sulfonate groups may result in splits or shifts of
the asymmetric vibrations (n
The sulfonate group in the
PFOS structure has C
symmetry, which is expected to exhibit
doublet peaks corresponding to a doubly degenerate band at a
higher wavenumber (,1240 cm
) and a non-degenerate band at
a lower wavenumber (,1210 cm
Thus, we attribute
the strong band centred at 1243 cm
to a combination of
). This assignment is also supported by
the fact that the band became the strongest band in the spectrum
whereas the other n
) band (i.e. 1204 cm
) is the strongest
band in the spectrum of PFOA in the absence of interference
from sulfonate stretching. Band assignments are summarised
in Table 2.
Peak fitting results of the hematite-adsorbed PFOS spectrum
are very similar to those obtained for the aqueous phase species
(Fig. 5b). The n
) manifests as a doublet corresponding
to the C
symmetry, indicating that the symmetry of the
sulfonate group was not changed upon adsorption, and therefore,
no direct chemical bond formed between the sulfonate groups
and hematite surface. Nevertheless, we observed slight spectro-
scopic changes in the region of asymmetric sulfonate stretching.
Compared with the spectrum of aqueous PFOS, one of the
) bands shifted from 1215 to 1220 cm
, and the
intensity of this band was diminished relative to other bands in
the spectrum (Fig. 5). Similar changes were observed in a
previous study of sodium dodecyl sulfonate (SDS, a hydrocar-
bon anionic surfactant) adsorption to hematite.
The two
) bands correspond to a doubly degenerate E vibration
and a second non-degenerate A vibration with the directions of
the transition dipole moment perpendicular to each other.
The A vibration at 1220 cm
is more sensitive to direct contact
with charged solid surfaces, whereas the E vibration mode at
1243 cm
is sensitive to lateral surfactant–surfactant interac-
Thus, the change in the A vibration mode of
) upon adsorption to hematite indicates a change in
the local environment of the sulfonate head, possibly attribut-
able to hydrogen-bond formation between the sulfonate group
and hematite surface hydroxy groups.
Effect of solution pH on PFOA and PFOS adsorption
Several recent ATR-FTIR spectroscopic studies of adsorption of
LMW carboxylic acids on various hydroxylated mineral sur-
faces (e.g. a-Fe
and TiO
) indicate that carboxylate
groups can form either inner-sphere or outer-sphere complexes
at the oxide–water interface, and that solution pH plays an
important role in the process.
The effect of pH on PFOA
adsorption was, therefore, examined in this study. Adsorption of
1400 1300 1200 1100 1000 900
Wavenumber (cm1)
@ Hematite
@ Ge IRE (intensity 5)
Fig. 4. Attenuated total reflection Fourier-transform infrared spectra of
250-mM perfluorooctanesulfonic acid adsorbed on Ge internal reflection
element (IRE) and hematite surface in 10 mM NaCl solution at pH 5.0. The
absorbance intensity of the spectrum at Ge IRE was increased five times.
1400 1300 1200 1100 1000
Wavenumber (cm1)
Wavenumber (cm1)
νas(CF2) νas(R–SO3
νas(CF2) νas(R–SO3
(a) @ Ge IRE
1066 1037
1400 1300 1200 1100 1000
(b) @ Hematite
1066 1037
1370 1329
Fig. 5. Peak deconvolution of the perfluorooctanesulfonic acid spectra
(250 mM) on (a) Ge internal reflection element (IRE) and (b) hematite in
10 mM NaCl solutions at pH 5.0. The solid line is the experimental data. The
red dash line represents the non-linear least square fit. The blue lines are the
deconvoluted peaks.
X. Gao and J. Chorover
PFOA to hematite, as measured by FTIR absorbance intensities,
exhibits strong pH dependence (Fig. 6). Adsorption increases
substantially with decreasing pH from 6.0 to 3.0 (no detectable
IR signals at pH $7.0 under the flow conditions), consistent
with prior macroscopic adsorption studies.
The stronger
adsorption at low pH can be partially attributed to favourable
electrostatic forces. At pH ,pH
of hematite (7.7–7.8),
adsorption is certainly affected by electrostatic attractions
between the negatively charged carboxylate and positively
charged hematite surface functional groups. Adsorption is
negligible at pH .pH
of hematite where, in the presence of
an indifferent background electrolyte, PFOA and the hematite
surface are both net negatively charged. Despite similarities
among the spectra across the pH range, small pH-dependent
changes in carboxylate stretching vibrations were observed. The
intensity ratio of n
) increases with
decreasing pH (Fig. 6, inset), suggesting that inner-sphere
metal–carboxylate complexes are favoured at low pH, similar to
what has been previously reported for hematite adsorption of
LMW carboxylic acids
and carboxylate groups of microbial
surface biomacromolecules.
Therefore, in addition to elec-
trostatic attraction, ligand exchange of carboxylate groups at
mineral surface hydroxy groups also contributes to enhanced
adsorption at low pH.
Similar to PFOA, the adsorption of PFOS to hematite also
shows strong pH dependence (Fig. 7). Adsorbate surface excess
increases with decreasing pH, consistent with prior batch sorp-
tion studies.
Other than increased intensity, no detect-
able changes were observed in the spectroscopic shape across
the pH range, indicating that electrostatic attraction is domi-
nantly responsible for the strong pH-dependence of adsorption.
Adsorption kinetics of PFOA and PFOS
Integrated absorbances of IR bands have been used previously to
assess surface excess of surfactants at solid surfaces.
penetration depth (d
) of the IR evanescent wave (0.16–1.6 mm
for the spectroscopic range of 4000–400 cm
with the 458Ge
IRE used in this study) may be greater than the layer of inter-
facial sorption and extend into the bulk solution. Thus, in
addition to adsorption at the solid–water interface, the measured
IR absorbance may also contain contributions from isotropic
bulk solution.
However, as discussed above, as the
absorbance intensities of the aqueous PFOA spectrum are ,1/10
of those for the hematite-adsorbed form, we assume that the
contributions from the isotropic bulk media are negligible in this
experiment. Therefore, the kinetics of PFOA adsorption to
hematite were assessed by directly monitoring the intensity
change of its fluorocarbon IR band – specifically the n
band centred at 1208 cm
that does not shift significantly with
solution chemistry and surface loading during the flow through
experiment. Adsorption was relatively rapid at all pH values,
reaching equilibrium within a few hours, but the rate and extent
of the adsorption increased significantly with decreasing pH
(Fig. 8). At pH 6.0, adsorption increased progressively with
reaction time and achieved equilibrium after 6 h. Adsorption at
1800 1600 1400 1200 1000
Wavenumber (cm1)
pH 3.0
pH 4.5
pH 6.0
1800 1700 1500 1400
Wavenumber (cm1)
pH 3.0
pH 4.5
pH 6.0
Fig. 6. Attenuated total reflection Fourier-transform infrared spectra of
500-mM perfluorooctanoic acid adsorbed to hematite surface as a function
of pH.
1400 1300 1200 1100 1000 900
Wavenumber (cm1)
250 µM PFOS @ hematite
pH 3.0
pH 4.5
pH 5.0
pH 6.0
Fig. 7. Attenuated total reflection Fourier-transform infrared spectra of
250-mM perfluorooctanesulfonic acid (PFOS) adsorbed to hematite surface
as a function of pH.
0 100 200 300 400
pH 3.0
pH 4.5
pH 6.0
Absorbance of νas(CF2)
Time (min)
10 Fe
Zeta potential (mv)
Fig. 8. Adsorption kinetics of 500-mM perfluorooctanoic acid on hematite
as represented by the IR absorbance of n
) band at 1208 cm
. Fitting
results of the pseudo-first-order (pH 6.0) and pseudo-second-order models
(pH 4.5 and 3.0) are plotted as dash lines. The inset presents the zeta potential
of hematite (0.1 g L
) as a function of pH in 1 mM NaNO
PFOA and PFOS adsorption to hematite
pH 3.0 was much more rapid, reaching an adsorption maximum
within 30 min. The difference can be attributed to the pH-
dependent surface charge of hematite (Fig. 8, inset). At pH 3.0,
hematite surface hydroxy groups that can undergo protonation
are in a fully protonated state, giving rise to maximum positive
surface charge and, therefore, exhibit high affinity for anionic
surfactant molecules. The kinetics of ligand exchange are also
expected to be increased under these conditions, where inner-
sphere complexation of the PFOA carboxy group is promoted by
the dissociation of the water molecule from the Fe
metal centre
Fe OHþ
Relative to its neutral (Fe–OH) or deprotonated forms
), protonation of the hematite surficial hydroxy groups
reduces the activation energy for ligand exchange and, thereby
increases their substitutional lability.
Thus, a large concen-
tration of protonated sites at low pH evidently increases the
reaction rate of Eqn 1. With increasing pH, the hematite surface
becomes progressively deprotonated, resulting in a decrease in
adsorption kinetics and equilibrium extent. The slower second
step adsorption kinetics at high pH is consistent with a sorption
barrier generated by structural re-arrangement and surfactant
self-assembly of adsorbed PFOA at the solid–water inter-
In addition, intra- and interparticle diffusion into the
porous hematite film may also contribute to the slow kinetics.
This observation contrasts with prior studies pertaining to
adsorption of hydrocarbon analogues of PFOA and PFOS.
Adsorption of hydrocarbon ionic surfactants to charged particles
is generally rapid, reaching sorption equilibrium within minutes
irrespective of solution pH.
One possible explanation for
this discrepancy is the size and hydrophobicity differences
between the two types of compounds. Substitution of H by F in
the aliphatic chain structure increases the molecular size and
hydrophobicity of PFCs relative to hydrocarbon analogues. This
may limit their diffusion in the hydrophilic sorbent film and
thereby contribute to the slow sorption kinetics.
Adsorption kinetic data were fit to pseudo-first-order (Eqn 2)
and second-order (Eqn 3) models to facilitate interpretation of
the rate controlling sorption processes.
logðqeqtÞ¼log qek1
2:303 tð2Þ
where q
is the adsorbed mass of PFCs at time t(represented by
the absorbance of the n
) band); q
is the equilibrium
adsorbed mass, and k
and k
are the first and second-order
adsorption rate coefficients. As shown in Fig. 8 and Table 3,
adsorption at pH 3.0 is well described by a pseudo-second-order
reaction equation with the correlation coefficient R
and the calculated sorbate concentration at equilibrium (q
close to the measured value (q
). Adsorption at pH 4.5 is also
fairly well fit to pseudo-second-order kinetics (R
However, the q
is much larger than q
in this case, sug-
gesting that adsorption equilibrium may not have been reached
during the experimental time frame. As shown in Table 3,
adsorption kinetics at pH 6.0 are better described by a pseudo-
first-order model (Table 3). However, this model still clearly
over-predicts adsorption at early times (,250 min) (Fig. 8). The
kinetics of PFOS adsorption to hematite exhibits similar trends
(Fig. 9). Increasing pH diminished both the PFOS uptake rate
and extent.
The concentration of surfactant in the bulk solution signifi-
cantly affects PFOA adsorption kinetics. As shown in Fig. 10,
increasing the initial PFOA concentration from 50 to 1000 mM
not only increased the extent of equilibrium uptake, but the
adsorption rate coefficient as well. Time to adsorption equilib-
rium decreased with increasing concentration. In the case
of adsorption to hematite, this observation can be explained
by the specific adsorption mechanisms. The adsorption of
aqueous organic acid anions to hydrous metal oxides often
involves a two-step process with rapid formation of an outer
sphere complex followed by slower ligand exchange reaction
with the elimination of H
O molecules.
Increasing the
Table 3. Fitting parameters of the pseudo-first-order and pseudo-second-order models for perfluorooctanoic acid (PFOA) adsorption on hematite
from 500 lM PFOA aqueous solution at different pH
, experimental equilibrium sorption uptake as represented by IR absorbance (peak area) of n
); q
, calculated equilibrium sorption uptake
Solution pH q
Pseudo-first-order Pseudo-second-order
3.0 29.337 29.851 0.0105 0.9998
4.5 7.150 – – – 8.489 0.0022 0.984
6.0 5.190 6.994 0.0092 0.932 7.530 0.0007 0.774
0 60 120 180 240 300
250 μM PFOS (pH 3.0)
250 μM PFOS (pH 5.0)
Absorbance of νas(CF2)
Time (min)
Fig. 9. Adsorption–desorption kinetics of perfluorooctanesulfonic acid
(PFOS) on hematite as represented by the IR absorbance of the n
pH 3.0 and 5.0
X. Gao and J. Chorover
concentration of protonated reactant surface hydroxy groups
) evidently increases the reaction rate of ligand
exchange, resulting in an accelerated second step for high PFOA
aqueous concentration. It is noteworthy, however, that Qu
et al.
reported similar variation in adsorption kinetics of
PFOA onto powdered activated carbon in response to changing
initial PFOA aqueous concentration, which presumably did not
involve an inner-sphere metal complexation mechanism.
A small decrease in adsorption was observed at early reaction
times for a concentration of 1000 mM PFOA (Fig. 10). This
decrease is likely attributable to a surfactant-induced hematite
dispersion effect whereby PFOA at high concentration formed
complete bilayers on the hematite surface, reversing the charge
on the Fe oxide particles, which resulted in partial displacement
of the hematite film as a result of shear forces associated with
fluid flow. Desorption is relatively rapid at all concentrations.
When PFOA-free electrolyte solution (i.e. 10 mM NaCl) was
introduced to the flow through cell, the adsorbate mass
decreased quickly, and only a small sorbate mass remained after
60 min (Fig. 10).
Adsorption isotherm
The results from the PFOA adsorption experiment at pH 3.0 are
depicted in the form of an adsorption isotherm in Fig. 11 using
the equilibrium absorbance of the n
) band at 1208 cm
a measure of surface excess. The spectra are very similar across
the concentration range from 50–1000 mM, indicating that the
dominant adsorption mechanisms are largely independent of
surface loading. The log–log scale isotherm exhibits charac-
teristic features of ionic surfactant adsorption to charged mineral
surfaces, as reported previously in the literature.
The iso-
therm contains several distinct regions : a low surface excess
region (,100 mM), where the adsorption is dominated by
covalent chemical bonding and electrostatic forces; an inter-
mediate region with very steep slope (from 100 to ,250 mM)
where lateral surfactant–surfactant hydrophobic interactions are
postulated to predominate resulting in the formation of hemi-
micelle-like monolayer clusters on the hematite surface; and a
plateau region at concentrations $500 mM where the surface is
occupied by surfactant bilayers.
The adsorption mechanisms, kinetics and isotherms of PFOA
and PFOS from aqueous solution to the hematite surface were
examined using in situ flow through ATR-FTIR spectroscopy.
To our knowledge, this is the first spectroscopic study of PFC
adsorption to environmental surfaces. Our results suggest that
adsorption of both PFCs to the hydrophilic hematite surface
exhibits strong pH-dependence. The data also reveal different
sorption mechanisms controlling the adsorption of PFOA and
PFOS to hematite. In addition to electrostatic interactions,
PFOA forms inner-sphere complexes at Fe metal centres, and
such direct covalent bond formation may also contribute sorp-
tion energetics at low pH, whereas PFOS forms only outer-
sphere complexes at the mineral surface. The kinetics of the
adsorption of PFCs to hematite was generally rapid, reaching
equilibrium within a few hours. Both solution pH and surface
loading had profound effects on the adsorption kinetics. Faster
adsorption was observed at low pH and high surface loading.
Research support was provided by the Binational Agricultural Research and
Development (BARD) fund, Grant # IS-3822-06, the Water Research
Foundation (Award #4269) and the University of Arizona Water Sustain-
ability Program. The comments and views detailed herein may not be nec-
essarily reflecting the views of the Water Research Foundation, its officers,
directors, affiliates or agents. Analyses in the ALEC were supported by NSF
grant CBET-0722579.
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PFOA and PFOS adsorption to hematite
... Currently, relatively little is known about the importance of inorganic components in WWRs to PFAS partitioning. WWRs, especially those generated by plants recycling high aluminum (Al) and iron (Fe) drinking water treatment residuals, often contain significant quantities of non-crystalline aluminum (Al) and iron (Fe) substances, respectively (Gravesen and Judy, 2020), which have been linked to PFAS partitioning behavior in soils (Gao and Chorover, 2012;Du et al., 2014;Oliver et al., 2019). Here, we investigate the relative importance of WWR inorganic chemical composition (i.e., total Fe and Al content, amorphous Fe and Al content) and organic chemical characteristics (i.e., OM content, dissolvable organic carbon (DOC), and protein content) to PFAS partitioning from WWRs into water. ...
... Of these PFAS, all except for PFOS are PFCAs, suggesting that PFCAs were more likely to partition to non and poorly crystalline Al surfaces. Previous mechanistic spectroscopic sorption studies with an Fe-oxide (hematite (α-Fe 2 O 3 ) showed the formation of an Fe-PFCA inner-sphere complex via ligand exchange while a perfluoroalkyl sulfonate (PFSA) formed an outersphere complex via columbic forces (Gao and Chorover, 2012;Du et al., 2014). This is consistent with the results of by-class regression (Table 3) where PFCAs are significantly associated with both Fe ox and Al ox content of WWRs. ...
Per- and polyfluoroalkyl substances (PFAS) are a class of highly persistent contaminants that have been linked to human health effects at low exposure concentrations. Public concerns exist that land-application of biosolids may result in the release of PFAS into terrestrial and aquatic ecosystems. The relative importance of inorganic constituents such as Fe and Al, which are known to impact PFAS retention/release behavior in soils, on PFAS release from wastewater residuals (WWRs, i.e., biosolids and sewage sludges) is not well understood. Here, we examine native concentrations and WWR-water partition coefficients of a range of PFAS in the context of WWRs characteristics including oxalate-extractable Fe and Al, organic matter (OM), dissolved organic carbon, and total protein content. Total PFAS concentrations, which included perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, fluorotelomer sulfonates and some sulfonamides, ranged from ∼480 to 3500 μg PFAS kg-1 dry weight. PFAS WWR-water partition coefficients ranged from ∼10 to 20,000 L kg-1, consistent with the literature. PFAS partitioning was significantly correlated to oxalate extractable Al and Fe as well as bulk OM and protein content. These results have important implications for wastewater treatment facilities that recycle Al- and Fe-based drinking water treatment residuals in terms of both PFAS retention and loading.
... The ligand exchange is also possibly occurring during adsorption of PFOA onto iron oxide (≡Fe-OH). By dissociating the water molecule from the Fe 3+ surface metal center, the protonated surface hydroxyl groups could be replaced by PFAS anions (Gao and Chorover, 2012). The ligand exchange reaction can be expressed as follows (Eqs. ...
Per- and polyfluoroalkyl substances (PFAS) have drawn great attention due to their wide distribution in water bodies and toxicity to human beings. Adsorption is considered as an efficient treatment technique for meeting the increasingly stringent environmental and health standards for PFAS. This paper systematically reviewed the current approaches of PFAS adsorption using different adsorbents from drinking water as well as synthetic and real wastewater. Adsorbents with large mesopores and high specific surface area adsorb PFAS faster, their adsorption capacities are high, and the adsorption process are usually more effective under low pH conditions. PFAS adsorption mechanisms mainly include electrostatic attraction, hydrophobic interaction, anion exchange, and ligand exchange. Various adsorbents show promising performances but challenges such as requirements of organic solvents in regeneration, low adsorption selectivity, and complicated adsorbent preparations should be addressed before large scale implementation. Moreover, the aid of decision-making tools including response surface methodology (RSM), techno-economic assessment (TEA), life cycle assessment (LCA), and multi criteria decision analysis (MCDA) were discussed for engineering applications. The use of these tools is highly recommended prior to scale-up to determine if the specific adsorption process is economically feasible and sustainable. This critical review presented insights into the most fundamental aspects of PFAS adsorption that would be helpful to the development of effective adsorbents for the removal of PFAS in future studies and provide opportunities for large-scale engineering applications.
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In a demonstration of modern analytical chemistry at its best, the International Interdisciplinary Conference of Chemical Analysis, APCE-CECE-ITP-IUPAC 2022 was held after two years of COVID-19-related delays in Siem Reap, Cambodia. The quadruple meeting included: 18th Asia Pacific International Symposium on Microscale Separations and Analyses, 17th International Interdisciplinary Meeting on Bioanalysis, 28th International Symposium on Electro- and Liquid Phase-Separation Techniques, and IUPAC Special Symposia by Division of Chemistry and the Environment. While, under normal circumstances, these conferences would take place in different countries, we have decided to bring together analytical chemists from all over the world for a conference covering all aspects of modern analytical chemistry. Our goal remained the same: “bring together scientists from different disciplines who may not meet at other meetings”. With plenary and invited lectures delivered by distinguished scientists, this in-person meeting allowed to broaden our knowledge, meet new friends, and start new collaborations. The organizers want to thank all speakers, sponsors, and participants for their support. Please, check the conference web for more information about the history, programs, photos, and videos.
Polytetrafluoroethylene (PTFE) composites with suitable fillers can exhibit ultra-low wear via transfer and running films formation and the prevention of matrix removal. However, they are often generated simultaneously, thereby making it difficult to distinguish their individual effects. In the present study, dual-pin-on-disk (DPOD) friction pairs are used to tailor the combination of different functions, so that the individual effects of the running and transfer films are identified. The results show that the running film formed on the worn polymer surface is indispensable for achieving ultra-low wear. The high-quality transfer film formed on the counterpart steel surface only slightly improves the wear performance individually, whereas ultra-low wear rates are achieved via the synergistic effect of the running and the transfer films.
Proposed treatment steps to target the removal of short-chain PFAS from contaminated aqueous solutions.
The global occurrence of per- and polyfluoroalkyl substances (PFAS) in aquatic systems has raised concerns about their adverse effects on ecosystems and human health. Adsorption is a promising technique for the remediation of PFAS, yet effective adsorbents with rapid uptake kinetics and high adsorption capacity are still in high demand, and molecular-level understanding of the interfacial adsorption mechanisms is lacking. In this study, we developed a superior layered rare-earth hydroxide (LRH) adsorbent, ultrathin Y2(OH)4.86Cl1.44·1·07H2O (namely YOHCl) nanosheets, to achieve the effective removal of perfluorooctanoic acid (PFOA). YOHCl nanosheets exhibited ultra-high adsorption capacity toward PFOA (up to 957.1 mg/g), which is 1.9 times and 9.3 times higher than the state-of-the-art layered double hydroxides (MgAl-LDH) and benchmark granular activated carbon (GAC) under the same conditions, respectively. Furthermore, YOHCl nanosheets pose stable performance on the removal of PFOA under various water matrices with robust reusability. We also developed YOHCl-based continuous-flow column, demonstrating its promise in simultaneously removing multiple PFAS with wide range of chain lengths at environmentally relevant concentrations. With the molecular-level investigations, we have revealed that multi-mechanism, including ion exchange, electrostatic attraction and bidentate/bridging coordination, contributed to the strong PFOA-YOHCl affinity, leading to the ultra-high adsorption capacity of PFOA. We have provided emerging LRHs-based adsorbents for the effective remediation of PFAS with molecular-level insights on the interfacial mechanisms.
Perfluorosulfonic acids (PFSAs) are a recalcitrant subclass of per- and polyfluoroalkyl substances (PFASs) linked to numerous negative health effects in humans. Scalable technologies that effectively destroy PFSAs will greatly reduce the future health and ecological impact of these "forever chemicals". Herein, we show that several PFSAs undergo facile mechanochemical destruction (MCD) in the presence of quartz sand (SiO2). This process operates in the absence of solvent, at ambient temperature and pressure, generating a benign solid byproduct. Quantitative analysis of milled samples revealed high destruction efficiencies of 99.95% to 100% for five different PFSAs subjected to MCD conditions in the presence of SiO2 only. Extensive nontargeted analysis showed that, during degradation, other PFASs form and are ultimately destroyed upon extended mechanochemical treatment. Direct polarization (DP) and cross-polarization (CP) solid-state nuclear magnetic resonance (SSNMR) spectroscopy showed abundant silicon-fluorine (Si-F) bond formation post-MCD, indicating that fluorine was secured in a stable reservoir. Collectively, these results identified the degradation profile for an environmentally sound and effective PFSA degradation process that is amenable to scale-up.
IntroductionPreparationOther Methods
The fate of nutrients, pollutants and other solutes in natural waters is coupled to their distribution between solid, aqueous and gas phases. The processes of phase distribution are many, including penetration and absorption into one of the phases, or accumulation at the interface between them. The term sorption is defined here as the full range of processes whereby matter is partitioned between the gas, aqueous and solid phases. In geochemical systems, this includes adsorption of matter at the surfaces of solid particles (minerals and organic matter) or at the air—water interface, and absorption into the solids during surface precipitation or solid phase diffusion. The complexity of natural geomedia (Fig. 4.1) implies that both broad classes of “sorption” reaction may occur simultaneously. As discussed in this chapter, recent research into the kinetics and mechanisms of sorption for inorganic and organic species indicates that both processes are indeed important. The relative predominance of a given reaction and sorbate—sorbent structure is a function of time scale, system loading and geochemical conditions.
We investigated the occurrence of perfluorocarboxylates (PFCAs) in Tokyo Bay during dry and wet weather and evaluated the contributions of wastewater effluent, untreated wastewater, and surface runoff by using two sewage markers, caffeine and crotamiton. Sigma 8PFCAs ranged from 11 to 185 ng L(-1). Perfluorononanoate (PFNA) was the major species, followed by perfluorooctanoate (PFOA) and perfluoroheptanoate (PFHpA). Principal component analysis followed by multiple linear regression revealed that the PFCAs were derived mainly from wastewater effluent during dry weather, and jointly from wastewater effluent (59%) and combined sewer overflow (41%) during wet weather. We used caffeine-to-crotamiton ratios to evaluate the contributions of untreated wastewater and wastewater effluent. Estimated concentrations of wastewater-derived PFCAs were much lower than observed concentrations during wet weather, indicating the contribution of surface runoff to contamination. During a combined sewer overflow, surface runoff had a significant effect on contamination in the bay.
In order to study the nature of the theoretically interesting and potentially useful long chain alkanoic and alkenoic acids having terminal perfluoroalkyl segments, synthetic routes to Rf (alkylene)COOH have been devised. Free radical chain reaction of iodoperfluoroalkanes and terminal unsaturated acids gave 90-100% yields of RfCH2CHI(CH2)n,COOH which were reduced to alkanoic acids or hydrolyzed to alkenoic acids having a terminal Rf "tail." Three series of the so-called segmented acids were synthesized, in which Rf is normal, iso- or sec-perfluoroalkyl and the alkylene segment is two to sixteen carbons long. These alkanoic acids are similar to nonfluorinated compounds in reactivity but have unique surface-active and wettability properties. The enhancing effect of CF3(CF2)6 - on the ionization of the acids is transmitted through alkylene segments as long as five carbons.
Adsorption of poly(styrenesulfonate) (PSS) and cetyltrimethylammonium bromide (CTAB) on silica from mixed solutions of the polyelectrolyte and surfactant has been studied. The surface excess of the polyelectrolyte was determined by ultraviolet attenuated total reflectance (UV-ATR) monitoring of the 224 nm styrenesulfonate absorbance band. Complementary information was derived from infrared attenuated total reflectance (IR-ATR) on the same system. Deconvolution of the 2920 cm-1 asymmetric and 2850 cm-1 symmetric methylene bands common to both surfactant and polyelectrolyte gave the adsorbed amount of surfactant, as well as a measure of adsorbed polyelectrolyte. The surface excesses of both polymer and surfactant were determined in situ for different pH values and solution histories. The adsorbed amount of both species was highly dependent on pH and on the order of addition of the polyelectrolyte and surfactant. These results are interpreted in terms of competing interactions between the polyelectrolyte and the surface and between the surfactant and the surface and in terms of cooperative surfactant/polyelectrolyte solution interactions.
Monolayers formed by the chemisorption of CF3(CF2)7(CH2)2SH (FT) at epitaxially grown Au(lll) films were characterized using atomic force microscopy (AFM), electrochemistry, and infrared reflection spectroscopy (IRS). The AFM was used to probe the atomic scale arrangement of the monolayer. The electrochemical and IRS studies provided insight into the surface coverage and spatial orientation of the monolayer. The AFM images exhibit ordered domains of hexagonal periodicity with average nearest- and next-nearest-neighbor separation distances of 0.58+/-0.02 and 1.01+/-0.02 nm, respectively. These spacings agree well with a (2 x 2) adlayer at Au(lll), a two-dimensional arrangement predicted on the basis of considerations of a space-filling model. The sizes of the ordered domains typically range from a few adsorbates up to 30 nm2 and occasionally larger. The results of the surface coverage ((5.7+/-0.7) X 10(10) mol/cm2) and orientational analysis (20-degrees average tilt of the perfluorocarbon chains from the surface normal) support the presence of a (2 X 2) adlayer. Differences in the ability to image the FT monolayer and those formed from alkanethiols (CH3(CH2)nSH) are discussed, along with the advantages of correlating the findings from microscopic and macroscopic characterization techniques.
A new fluorocarbon-chain-containing molecule, CF[sub 3](CF[sub 2])[sub 7]C(O)N(H)CH[sub 2]CH[sub 2]SH, for self-assembly was synthesized using a simple and general technique. Self-assembled monolayers of this molecule were studied using ellipsometry, contact angle measurements, grazing-incidence infrared spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure measurements. The films were found to be uniform and reproducible, presenting a fluorocarbon surface ([theta][sub adv](H[sub 2]O) = 114[degree]) with predominantly CF[sub 3] groups exposed. The C=O and N-H dipoles are found to be oriented parallel to the gold surface, with the fluorocarbon axis oriented perpendicular to the surface. 50 refs., 8 figs., 5 tabs.