Solubilization of Selected Free Fatty Acids in Palm Oil by
Biodegradable Ethoxylated Surfactants
TAU-YEE LIM,†JING-LIANG LI,†AND BING-HUNG CHEN*,†,‡
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent
Ridge Crescent, Singapore 119260, and Department of Chemical Engineering, National Cheng Kung
University, 1 University Road, Tainan 70101, Taiwan
The solubilization of three major components, viz., palmitic, oleic, and linoleic acids, in palm oil by
ethoxylated surfactants was investigated. The results were analyzed in terms of the molecular
properties of surfactants and free fatty acids (FFAs). It was found that the solubilities of these FFAs
in various micellar solutions depend not only on their octanol-water partition coefficients (Kow), but
also on their physicochemical properties. The study on the solubilization kinetics was conducted by
choosing palmitic acid as a model solubilizate and Tergitol 15-S-7 as the model surfactant. A first-
order film diffusion model, which accounts for the direct uptake of organic molecules at a solid surface
into surfactant micelles, was adopted to analyze the effect of surfactant on dissolution of palmitic
acid. It was observed that the presence of surfactant reduced the mass-transfer coefficient. Instead,
the overall mass-transfer rate was enhanced because of the much higher driving force from the
increased solubilization capacity.
KEYWORDS: Solubilization; dissolution; surfactant; free fatty acid; palm oil
Palm oil is one of the world’s most important vegetable oils
(1). It is used mainly for edible purposes and has become an
important raw material for many applications during the past
few decades. Increasing interest in the chemistry and biotech-
nology of palm oil has emerged in recent years, which is mainly
attributed to the fact that oleochemicals are derived from
renewable sources. The major fatty acids in palm oil are
palmitic, oleic, and linoleic acids (2, 3) (Table 1). In palm oil,
each of the saturated palmitic acid and the monounsaturated
oleic acid accounts for ca. 40 wt % of the overall free fatty
acids (FFAs), depending on harvest conditions and locations
of cultivation (3).
Palm oil mills produce a large amount of solid and palm oil
mill effluent (POME) wastes. The quantity of POME varies with
the amount of water used. Typically, the total weight of liquid
effluent is estimated at around 0.6 ton per ton of fresh fruit
bunches (4). The POME wastes are the fiber-free oil components
obtained from the clarification zone of oil mills, where the
temperature of the POME discharge ranges from 50 to 70 °C.
They consist of various suspended components, including a high
content of FFAs between 670 and 1800 mg/L, cell walls,
organelles, a spectrum of carbohydrates ranging from hemicel-
lulose to simple sugars, a wide range of nitrogenous compounds
from proteins to amino acids, and an assembly of minor organic
and mineral constituents (4). Fortunately, they are nontoxic and
When POME wastes are discharged into a river, they increase
the biochemical oxygen demand of water and undoubtedly
reduce dissolved oxygen concentration. In addition, the low
volatility and aqueous solubility of the free fatty acids in the
POME wastes make them persistent. Moreover, a significant
amount of FFAs would precipitate from the effluent, in which
the temperature of the effluent is cooled to ca. 30 °C in nature.
Biological transformation is believed to be a major removal
process of FFAs from soil and aquifer systems. Nonetheless,
the hydrophobic nature of these contaminants results in their
* To whom correspondence should be addressed at the National Cheng
Kung University. Phone: +886-6-275-7575, ext 62695. Fax: +886-6-234-
4496. E-mail: email@example.com.
†National University of Singapore.
‡National Cheng Kung University.
Table 1. Fatty Acid Components of Palm Oila
common namesystematic name
partitioning onto the soil matrix, limiting their bioavailability.
It is also likely that settlement of suspended solids would
produce objectionable banks of septic sludge in slow reaches
of the rivers. Moreover, from an environmental point of view,
it is better to eliminate as much FFAs as possible in POME
before they are discharged or precipitated to the environment.
Consequently, an efficient and economic scheme to enhance
the degradation of these FFAs is critical to successfully achieve
It is generally believed that sorbed hydrophobic pollutants
are not directly available to the microbial population. A well-
designed bioremediation process should consider methods to
mobilize these contaminants from the soil surface and make
them available to the microbial population. One common way
to achieve the higher bioavailability and biodegradation rate is
to increase their aqueous solubilities by adding proper surfactants
that do not compete with the solubilizates as a food source to
the microbes (5-7).
Surfactants are effective in increasing aqueous solubility of
a hydrophobic compound. If the concentration of a surfactant
exceeds a certain threshold, called the critical micelle concentra-
tion (CMC), at a temperature higher than its Krafft temperature,
surfactant monomers in aqueous solution will aggregate to form
micelles of colloidal size. One of the important properties of a
surfactant is that the hydrophobic cores of micelles could offer
good affinity for hydrophobic solutes to stay in the micelles
and, hence, increase the aqueous solubilities of these practically
insoluble solubilizates. Such a process is called solubilization,
typically defined as the spontaneous dissolving of an insoluble
substance by reversible interaction with the micelles of a
surfactant in a solvent to form a thermodynamically stable
isotropic solution with reduced thermodynamic activity of the
solubilized material (16). Under proper conditions, surfactants
could solubilize considerable amounts of palm oils and tri-
glycerides in oil-containing micelles, called swollen micelles
or microemulsions (8-10).
In this study, solubilization of major FFAs in the palm oil
by some ethoxylated surfactants is investigated in an attempt
to enhance their bioavailability by mobilizing them from
precipitates. These oils are mainly the palmitic, oleic, and
linoleic acids, listed in Table 2. Five ethoxylated surfactants,
shown in Table 3, having slightly different molecular structures
and hydrophile-lipophile balance (HLB) values were chosen
in an attempt to differentiate the contribution of their molecular
structures to the solubilization processes. Indeed, two of them
have been successfully demonstrated in the solubilization and
extraction of polycyclic aromatic hydrocarbons (11, 12).
MATERIALS AND METHODS
Reagents. Reagent grade palmitic acid, oleic acid, and linoleic acid
were purchased from Tokyo Kasei (TCI), Sigma, and Aldrich. At
ambient temperature, palmitic acid is a solid, while oleic and linoleic
acids are in the liquid state. Selected physical and chemical properties
of FFAs are given in Table 2.
Tergitol 15-S-5 (T-S-5) and Tergitol 15-S-7 (T-S-7), obtained from
Dow, are nonionic secondary ethoxylated alcohols. A mixed nonionic
surfactant, T-S-Mix, was made by mixing T-S-7 with T-S-5 in a weight
ratio of 3:1, respectively, which gives T-S-Mix an average ethylene
oxide (EO) number of 6.7. Another nonionic surfactant, Neodol 25-7
(N-7), a primary ethoxylated alcohol, was obtained from Shell
Chemicals. Two anionic ethoxylated surfactants, glycolic acid ethoxylate
lauryl ethers with average molecular weights of 460 (GE-460) and 690
(GE-690), were obtained from Aldrich. Relevant properties of these
surfactants are given in Table 3. T-S-5, T-S-7, and T-S-Mix surfactants
have the same hydrophobic hydrocarbon chains, but differ in EO
numbers. Likewise, T-S-7 and N-7 surfactants have almost the same
molecular weight, HLB number, and carbon number along their
hydrophobic hydrocarbon chains, but only differ in molecular configu-
rations. Namely, the ethoxylated alcohol group is located in the end
position of the N-7 surfactant molecule, whereas T-S-7 has a secondary
one. More importantly, these surfactants are readily biodegradable. All
chemicals and surfactants were used as received. Deionized water with
resistivity greater than 18.2 MΩ‚cm was used in sample preparations.
Apparatus. A Kru ¨ss DSA-10 tensiometer was used to determine
the CMCs of the surfactants. The CMC was estimated by plotting the
surface tension data against the surfactant concentration as the abscissa.
The concentrations of fatty acids were measured by using a gas
chromatograph (Perkin-Elmer Autosystem XD), equipped with a flame-
ionization detector and an HP-Innowax column (30 m × 0.25 mm ×
0.25 µm). Helium was used as the carrier gas with a flow rate of 40
sccm (standard cubic centimeters per minute) and a split ratio set at
50:1. The detector and injection temperatures were set at 300 and 250
°C, respectively. All the concentrations of free fatty acids given in this
study were the average from at least quadruplicate measurements on
each sample in, at least, three different batches of experiments under
the same conditions. The error for each reading was ensured within
5% from the mean values.
Equilibrium Solubilization. The equilibrium solubilities of palmitic,
oleic, and linoleic acids in aqueous solutions of T-S-Mix, T-S-7, N-7,
GE-460, and GE-690 were determined using screw-capped glass culture
vials of 15 mL. Surfactant solutions above CMCs, from 0.5 to 4.0 wt
%, were prepared. For the solubilization of palmitic acid, each vial
was filled with granular palmitic acid in excess and then with 10 mL
of these surfactant solutions at different concentrations. The vials were
then put on a rotary mixer for 5 days to reach equilibrium in a
temperature-controlled room (ca. 23 °C), since equilibrium solubilization
could be achieved after 5 days of mixing as indicated from preliminary
For equilibrium solubilization studies of oleic and linoleic acids,
excess amounts of liquid acids were filled into screw-capped glass vials
containing 10 mL of surfactant solutions. All the solubilization
experiments were performed with a minimum mixing period of 7 days
in a temperature-controlled room. All sample vials were inverted for 2
days to achieve complete separation of the unsolubilized oil phase from
the aqueous phase. Samples from the aqueous phase below the oil phase
were withdrawn to determine the concentrations of oleic and linoleic
acids by gas chromatography (GC).
Dissolution Kinetics for Palmitic Acid. Solid palmitic acid was
chosen as the contaminant used in the dissolution study to quantify
the impact of Tergitol 15-S-7 surfactant on its dissolution rate. To
Table 2. Selected Properties of FFAs
FFA molecular formulaMWlog Kow
linoleicCH3(CH2)4CHdCHCH2CHdCH(CH2)7COO H 280.44
aData obtained from ref 27.bMeasured value in this work at 25 °C.
Table 3. Selected Properties of Surfactants
GE-460 CH3(CH2)11-13O(CH2CH2O)4.5CH2COOH 460
aValue given by the manufacturer.bCalculated from an equation using HLB
) degree of ethoxylation expressed in weight percent divided by 5.cMeasured
value.dMeasured by dynamic light scattering in 1 wt % surfactant solution at 25
°C; averaged micellar size of T-S-Mix, 17.3 nm.
quantify the mass-transfer coefficient of palmitic acid from its solid
surface into water or micellar solutions, an experimental system similar
to that described by Grimberg et al. (13, 14) was developed. A 500
mL Erlenmeyer flask, schematically shown in Figure 1, was modified
by welding a short cylindrical glass well (38 mm internal diameter
and 5 mm height) onto its bottom. Approximately 2 g of palmitic acid
(melting point 64 °C) was melted in a water bath at 70 °C, then pipetted
into the well, and allowed to cool at room temperature. After the melt
solidified, the flask was rinsed with methanol and water to remove
condensed palmitic acid from the glass wall and to simultaneously
smooth the surface of the palmitic acid. Tergitol 15-S-7 micellar solution
of 200 mL was added into a flask placed on an orbital shaker at 150
rpm at ca. 23 °C. Palmitic acid concentration in the bulk liquid was
monitored over time by withdrawing 1 mL of the micellar solution.
The specific surface area for each sampling interval was determined
by dividing the surface area by the corrected liquid volume, i.e., initial
minus sample volume taken.
RESULTS AND DISCUSSION
Solubilization of Free Fatty Acids by Nonionic Surfac-
tants. The equilibrium solubilization curves for palmitic, oleic,
and linoleic acids in the micellar solutions of T-S-Mix, T-S-7,
N-7, GE-460, and GE-690 at ambient temperature (ca. 23 °C)
are shown in Figures 2-4. It is evident that the relationship
between the apparent solubilities of these FFAs and the
surfactant concentrations above the CMC is linear. These linear
enhancements in solubility above the CMC are consistent with
the solubilization data reported for other hydrophobic organic
compounds of environmental concern (11, 15).
There are a few common expressions to quantify the
solubilization capacity of the surfactant, one of which is the
micelle-water partition coefficient Kmthat indicates the dis-
tribution of organic molecules between the micellar pseudophase
and the aqueous phase. Two others are the mass solubilization
ratio (WSR) and the molar solubilization ratio (MSR). The WSR
is defined as the weight of the FFA solubilized by a unit mass
of surfactant above its CMC. Similarly, the MSR is described
as the number of moles of FFA solubilized by each mole of
surfactant above its CMC.
The micelle-water partition coefficient Kmcan be expressed
where Xmand Xaare the mole fractions of FFA in the micellar
pseudophase and in the micelle-free aqueous phase, respectively.
Moreover, Xmcan be calculated in terms of MSR (15):
In addition, Xais often approximated in dilute solutions by
where C*ais the aqueous solubility of solute and Vw,molis the
molar volume of water at the experimental temperature.
Explicitly, the MSR could be written as follows (15):
where CFFAand CFFA,CMC(M) are the apparent solubilities of
FFA in molarity in the micellar solutions having a surfactant
Figure 1. Flow diagram and a schematic figure showing the modified
Erlenmeyer flask used for experiments in dissolution kinetics.
Figure 2. Solubilization of palmitic acid by ethoxylated surfactants (WSR).
Figure 3. Solubilization of oleic acid by ethoxylated surfactants (WSR).
Figure 4. Solubilization of linoleic acid by ethoxylated surfactants (WSR).
Xm) MSR/(1 + MSR)(2)
MSR )CFFA- CFFA,CMC
concentration equal to Csurf(M) and the CMC of the surfactant.
The MSR and WSR can be obtained from the slope of the
solubilization curve in proper units. Here, the term “apprarent
solubility” is used since the solubilities of FFAs contain both
the contribution directly from molecular solubilization and that
made by micelles.
Interestingly, the maximum MSRs for each FFA among these
surfactants differ. For example, Figure 5 gives the solubilization
capacity in terms of MSR and log Km, and indicates that the
MSR of palmitic acid decreases as the HLB of nonionic
surfactants increases. For example, T-S-Mix has the lowest HLB
number among nonionic surfactants used in this work and, thus,
is regarded as the most hydrophobic surfactant among them.
T-S-Mix can solubilize palmitic acid ca. 50-100% more than
T-S-7 and N-7. This could be attributable to the fact that the
nonpolar saturated palmitic acid is preferentially solubilized in
the hydrophobic cores of the nonionic micelles. Moreover,
values of log Kmfor palmitic acid decrease slightly in surfactants
with larger HLB numbers (Figure 5).
For two anionic surfactants, GE-460 and GE-690, used in
the study, consistent results were obtained for the three fatty
acids. GE-690 has a higher solubilization capacity, in terms of
MSR or log Km, than GE-460, though they share similar
molecular structures. They have the same hydrophobic hydro-
carbon chains and one carboxylic group in the end position,
but only differ in the units of ethylene oxides (EOs). Their
hydrocarbon chains may give them the same degree in the van
der Waals attraction between the hydrocarbon chains of sur-
factants and FFAs. Meanwhile, the carboxyl groups on FFAs
will favorably interact with the carboxyl groups of surfactant
molecules. However, more EO units give GE-690 more polar
nature and, consequently, more van der Waals attraction with
However, for oleic and linoleic acid, the maximum solubi-
lization was achieved when GE-690 and N-7 were used,
respectively. Solubilization is a partition process of organic
substances between an aqueous and a micellar phase. This
process can be affected by many factors, including the molecular
properties of surfactants and solubilizates, such as the loci of
solubilizates in micelles and the bending elasticity of surfactant
films in micelles, and some environmental factors (16). In
general, the most important characteristics of a surfactant in
terms of solubilization power are its HLB number and molecular
structure (15, 17, 18).
Micelles of surfactants having lower HLB values from the
same homologous series can provide a more hydrophobic
environment preferentially for hydrophobic solubilizates. In
addition, surfactants with a lower HLB number tend to have
greater micellar core volumes compared with surfactants of the
homologue, but with a higher HLB value (16, 19). This idea
has been used to explain the greater solubilization capacity of
dodecyl alcohol ethoxylates for nonpolar hydrophobic organic
compounds including dodecane, decane, hexane, and cyclohex-
ane (18). Mackay (20) also reported that a decrease of
solubilization capacity is generally expected with a decrease in
micellar size. Tanford (21) has come up with a good empirical
equation to estimate the hydrophobic core volume of a micelle
from its aggregation number and number of carbon atoms of
the surfactant lipophile. In general, surfactants having lower
HLB numbers favor solubilization of nonpolar hydrocarbons.
However, with a decreasing HLB number, aqueous solubility
of the surfactant also diminishes and it may form other
aggregates, such as liquid crystalline phases, instead of micelles,
As aforementioned, the molecular structure of a surfactant
also plays a vital role in the solubilization capacity. Pennell et
al. (22) reported that Witconol 2722 (Tween 80) could solubilize
dodecane and PCE (tetrachloroethylene) 2 or 3 times as much
as Tergitol NP-15 and Witconol SN-120, even though these
surfactants have similar HLB values and aggregation numbers,
except that Witconol 2722 micelles have the largest core volume.
Figure 5b exhibits the logarithmic values of the micelle-
water partition coefficient Kmof these three FFAs by different
surfactants. It shows that the effect of surfactant on log Km
values for each FFA is not as significant as that on MSR.
However, among FFAs, for a certain surfactant, palmitic acid
has a greater log Kmthan the other two acids. This arises from
the more hydrophobic and less polar nature of palmitic acid, as
shown in its low aqueous solubility among these three FFAs.
For a hydrophobic compound, the hydrophobic affinity between
the compound and the hydrophobic micellar core is the primary
driving force for solubilization (11, 15). Therefore, the solubi-
lization behavior of these compounds is more predictable than
that of the polar ones.
Valsaraj and Thibodeaux (23) obtained theoretically and
empirically a linear relationship between log Kmand logarithmic
values of the octanol-water partition coefficient, log Kow, for
11 organic compounds solubilized by sodium dodecyl sulfate.
Subsequently, Edwards et al. (15) also reported a linear
relationship between log Kmand log Kowfor the solubilization
of several polycyclic aromatic hydrocarbons (PAHs) by a few
Figure 5. MSR and log Kmas a function of log Kow: (a) MSR; (b) log Km.
nonionic surfactants. Diallo et al. (18) studied the solubilization
of several alkanes and aromatic hydrocarbons by dodecyl alcohol
ethoxylates and had a similar observation. A linear relationship
between log Kmand log Kowwas also reported for PAHs when
perfluorinated surfactants and linear alcohol ethoxylates were
used (11, 24). Instead, owing to the higher polarity of oleic and
linoleic acids, the linear relationship between log Kmand log
Kowdoes not hold in this work (Figure 5).
In addition, Jafvert et al. (25) have proposed the following
approximate equation to relate Kowto Km:
where a and b are fitted parameters, Ncis the number of carbons
in the hydrophobic group of the surfactant, and Nhis the number
of hydrophilic groups of the surfactant. As a result, eq 5 fails
to describe the solubilization behavior of the FFAs by nonionic
surfactants used in this work.
For polar compounds, unlike nonpolar ones, solubilization
preferably occurs at the shallow palisade of micelles, or even
at the polar micelle-water interface. That is, more space in the
same micelle is available to solubilize the polar substances. For
example, slightly polar solubilizates, benzene for example, that
partition both in the interior of micelles and at the micelle-
water interface have been reported (26). This may explain why
higher solubilization capacity measured by MSR could be
achieved in some surfactants for oleic and linoleic acid than
However, interesting results were found for solubilization of
oleic and linoleic acids in nonionic micellar solutions (Figure
5 and Table 2). Oleic acid (log Kow) 7.64) (27) is slightly
more hydrophobic than linoleic acid (log Kow) 7.05) (27). A
larger portion of oleic acid would tend to reside in the micelles.
Consequently, the Kmof oleic acid is thought to be larger than
that of linoleic acid. Instead, the solubility of oleic acid in aqua
or micellar solutions is larger than that of linoleic acid. Though
oleic acid is more hydrophobic than linoleic acid, it is more
polar, which may attribute to the uncommon findings in their
solubilities. For example, the dielectric constant (28) and dipole
moment (29) of oleic acid are 2.336 at 293.2 K and 4.8033 ×
10-30C‚m, compared to 2.754 at 293.2 K and 4.0628 × 10-30
C‚m for linoleic acid.
If only considering the factor of hydrophobicity, oleic acid
has to be solubilized in the core of the micelles or in the deeper
palisade, compared to linoleic acid. However, higher polarity
makes oleic acid favorable to be solubilized in the shallow
palisade of the micelles as well. Hence, the space for solubi-
lization of oleic acid in a micelle is enlarged and the solubility
is increased, accordingly. A similar result was also reported by
Pennell et al. (22). In their system, tetrachloroethylene (log Kow
) 2.88) and 1,2-dichlorobenzene (log Kow ) 3.38) were
solubilized in micellar solutions of Witconol 2722 (Tween 80),
Witconol SN-120, and Tergitol NP-15. Though 1,2-dichlo-
robenzene is more hydrophobic, it still can be solubilized more
efficiently in terms of MSR.
Solubilization of oils by micellar solution is quite complicated.
Even the governing mechanism could be either by diffusion or
interfacially (30, 31). Among surfactants used in this study, N-7
has the largest micellar volume. Nonetheless, it does not always
give the highest solubilization capacity for different oils. Chen
et al. (30, 31) have shown that preferential or synergistic
solubilization in the mixtures of triolein and FFAs or long-chain
hydrocarbons by micellar solutions would occur. They have
shown that the bigger solubilizates, such as triolein, could be
solubilized faster by more flexible micelles, even though those
solubilizates have smaller saturation concentrations in those
micellar solutions. For instance, micelles of T-S-7, more elastic
than those of N-7 in their system (30, 31), can solubilize triolein
more quickly. Evidence is also seen in the lower cloud-point
temperature of T-S-7 (37 °C) than N-7 (50 °C) at 1 wt %, which
implies more elastic T-S-7 micelles require lower energy for
the uptake of the solubilizate (30, 31).
Cosurfactants, usually very polar or amphiphilic, are often
formulated with surfactants to weaken the interbilayer forces
of surfactant aggregates to form stable microemulsion phases,
which could have significant solubilization capacity of hydro-
phobic compounds, such as FFAs, oils, and lipids (19). The
common cosurfactants include short-chain alcohols, e.g., 1-pen-
tanol, 1-hexanol, and even 1-decanol. In addition to including
cosurfactants, an alternative to form a microemulsion phase is
to use the double-chain surfactants, for example, T-S-Mix and
T-S-7 in this work, other than straight-chain surfactants, e.g.,
N-7 (19, 31).
Furthermore, the concept of “optimum curvature” of a
surfactant monolayer has been frequently adopted to quantify
the solubilization capacity of surfactants through the Helfrich
curvature free energy (19). Near the solubilization limit of oil,
the oil-containing swollen micelles and the microemulsion
droplets would have a radius equivalent to the optimum
curvature, also called “spontaneous curvature”, of surfactant
monolayers (32). For example, if the reciprocal of the radius of
the hydrocarbon core of an oil-in-water (O/W) microemulsion
droplet, assumed to be covered with a surfactant monolayer, is
much less than spontaneous curvature, a significant portion of
oil must be expelled from the microemulsion droplet to the bulk
phase to decrease the curvature free energy (19).
Dissolution Kinetics of Palmitic Acid in the Micellar
Solutions of Nonionic Surfactants. A dissolution process
includes direct molecular dissolution of solute into the bulk
phase, and mass transfer of the same solubilizate from the
interface of the solubilizate-surfactant solution to the bulk
solution through a stagnant boundary layer by micelles. In a
well-mixing system, the concentration gradient exists solely
in this thin film. For a constant interfacial area, a first-order
mass-transfer model applies for the dissolution process (13,
where C denotes the total solute concentration at time t, mg/L,
C* stands for the solubility of solute in micellar solution or
water, mg/L, a is the specific interfacial area, m-1, and k
represents the observed mass-transfer coefficient, which is a
combined function of the aqueous-phase mass-transfer coef-
ficient kaand the mass-transfer coefficient in the micellar phase
kmic, cm/min. In this work, solid palmitic acid is used as the
model solubilizate. Moreover, the observed mass-transfer coef-
ficient could be regarded as a contribution from two parts as
where R denotes the contribution from molecular mass transfer.
Its value should fall between 0 and 1. If no surfactant is added,
R is equal to unity, since no mass transport by micelles could
possibly happen. In concentrated-enough micellar solutions R
will approach zero, meaning that the solubilizate-solution
interface is almost covered by surfactants. Noticeably, at higher
Km) Kow(aNc- bNh)(5)
dt) ka(C* - C)(6)
k ) Rka+ (1 - R)kmic
surfactant concentrations, the rate-limiting step in our model is
the diffusion of solute-saturated micelles across the boundary
Figure 6 shows the dissolution of palmitic acid in micelle-
free water and micellar solutions with various surfactant
concentrations. All observed mass-transfer coefficients and
apparent saturation concentrations of palmitic acid were obtained
by using the nonlinear regression with eq 6. The observed mass-
transfer coefficient is shown in Figure 7 as a function of T-S-7
The aqueous mass-transfer coefficient of palmitic acid in pure
water is around 0.146 cm/min. The observed mass-transfer
coefficient decreases significantly with addition of surfactant
and then approaches asymptotically a constant value with a
further increase in surfactant concentration. Similar results have
been reported in the literature as well (13, 14, 34). Interestingly,
the overall mass-transfer rate actually increases due to the
enhanced solubility of solute C*, viz., the larger concentration
differences (C* - C).
Furthermore, the observed mass-transfer coefficient k can be
expressed as a combination of kaand kmicin the following form:
where C*ais the saturation concentration of solute in the aqueous
phase, viz., its aqueous solubility. Parameters including ka, WSR,
C*a, and CMC can be determined independently. Thus, the
only unknown parameter is kmic. The aqueous solubility of
palmitic acid at the experimental temperature (25 °C) is 7.74
mg/L, which is close to the value of 7.2 mg/L (28) at 20 °C.
WSR obtained from the solubilization curve in Figure 2 is
equal to 0.048, and kaequals 0.148 cm/min. Using eq 8, the
values of kmicfor palmitic acid were calculated and are given
in Figure 7, which indicates that the values of kmicare almost
The values of R in eq 7 at different surfactant concentrations
could be easily converted from Figure 7. Initially, R drops
steeply at first and then smoothly with increasing surfactant
concentration. Thus, the dissolution model, which accounts
for direct dissolution of bulk organic molecules into micelles,
could adequately describe the observed effect of nonionic
surfactant T-S-7 on the observed mass-transfer coefficient of
On the basis of the values of k obtained, the maximum
dissolution rate of palmitic acid, r ) kaC*, i.e., C ) 0 in eq 6,
was calculated at different surfactant concentrations (Figure 8).
The maximum dissolution rates increase linearly with the
surfactant concentration. Moreover, the aqueous solubility of
solubilizate in the aqueous phase, C*, is known to be linearly
proportional to the surfactant concentration in micellar solu-
tions (11, 12, 18, 22-25). At much higher surfactant con-
centrations, the mass-transfer coefficient k will be attributable
Figure 6. Dissolution of palmitic acid as a function of time.
Figure 7. Observed mass-transfer coefficient for palmitic acid dissolution
as a function of surfactant concentration for Tergitol 15-S-7.
Figure 8. Maximum dissolution rate of palmitic acid as a function of
to kmic only, which approaches a constant value (Figure 7).
Consequently, the linear relationship between the maximum
dissolution rate r and surfactant concentration, exhibited in
Figure 8, could be extended to a higher surfactant concentra-
tion, until it reaches the upper limit of the surfactant micellar
Though it has been mentioned previously, it is worth pointing
out that the dissolution rate is indeed attributed to two competing
factors: enhanced solubility and decreased mass-transfer co-
efficient. Increasing surfactant concentration decreases to
some extent the mass-transfer coefficient, but increases the
solubility of the solute. Consequently, the dissolution rate
becomes larger at higher surfactant concentrations, although the
overall mass-transfer coefficient k is reduced in the presence
of surfactant. For example, for a Tergitol 15-S-7 concentration
of 3000 mg/L, the apparent saturated concentration of palmitic
acid is increased by about 20-fold compared to that of palmitic
acid in the absence of surfactant (see Figure 2). In the meantime,
the mass-transfer coefficient k is only reduced by a factor of
Conclusions. The solubilities of the free fatty acids, viz.,
palmitic, oleic, and linoleic acids, are linear functions of
surfactant concentrations above CMC. For palmitic acid, due
to its higher hydrophobicity, the hydrophobic affinity of its
molecules to the micelle core governs its solubilization. In
contrast, for the other two acids, because of their higher polarity,
solubilization both within the micellar core and at the micelle-
water interface may occur. A first-order dissolution model,
which accounts for the direct uptake of palmitic molecules into
the surfactant micelles at the solid-water interface, describes
well the dissolution of palmitic acid. Due to the slower diffusion
of surfactant micelles, the presence of surfactant reduces the
observed mass-transfer coefficient. However, the overall mass-
transfer rate was enhanced because of the much larger driving
force contributed by solubilization.
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Received for review December 15, 2004. Revised manuscript received
March 31, 2005. Accepted April 7, 2005. We thank the National
University of Singapore and National Science Council of Taiwan for
providing the financial support.