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American Journal of Analytical Chemistry, 2015, 6, 957-964
Published Online November 2015 in SciRes. http://www.scirp.org/journal/ajac
http://dx.doi.org/10.4236/ajac.2015.612091
How to cite this paper: George, M.J. and Motsamai, T. (2015) The Study of a Simple Pine-Oil Based Laboratory Prepared
and Commercial Detergents Using Conductivity Measurements. American Journal of Analytical Chemistry, 6, 957-964.
http://dx.doi.org/10.4236/ajac.2015.612091
The Study of a Simple Pine-Oil Based
Laboratory Prepared and Commercial
Detergents Using Conductivity
Measurements
Mosotho J. George*, Ts’ukulu Motsamai
Department of Chemistry and Chemical Technology, National University of Lesotho, Roma, Lesotho
Received 8 October 2015; accepted 16 November 2015; published 19 November 2015
Copyright © 2015 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Abstract
Detergents are very important substances in everyday life as they are used in laundry services. This
manuscript reports the study of the commercial and laboratory prepared pine oil-based detergents
employing the critical micelle concentration (CMC) phenomenon using conductivity measurements.
The two samples showed the CMC values of 0.0725 g/cm3 and 0.0920 g/cm3 for laboratory and com-
mercial samples respectively. The effect of ionic strength was investigated using NaCl and it demon-
strated a drop of CMC value of about CMC by 40% (laboratory sample) and 70% (commercial sam-
ple) while the equi-molar naphthalene increased the CMC values by about 50% (laboratory sam-
ple) and 12% (commercial sample) relative to their corresponding values under distilled water.
The combined effect of equi-molar NaCl and naphthalene lowered the CMC by 5% (laboratory sam-
ple) and 30% (commercial sample). These differences could signify the superiority of the laboratory
sample in that it is somehow buffered against drastic changes in the CMC under different conditions.
The relationship between conductivity and the CMC values does not show sufficient linearity (R2 <
0.8403) suggesting different mechanisms of interactions between NaCl and naphthalene. Overall, the
results are gratifying to the small-scale manufacturer who supplied the preparation protocol for la-
boratory sample preparation, in two respects: they inspire some degree of confidence in his product
as well as enabling the manufacturer to employ the same protocol for his quality control practices as
such improve product consistency and hence profitability.
Keywords
Detergent, Pine Gel, Critical Micelle Concentration, Conductivity, Ionic Strength, Small-Scale
*
Corresponding author.
M. J. George, T. Motsamai
958
Manufacturer
1. Introduction
Detergents and soaps are chemicals used in laundry to aid the removal of dirt or foreign matter from surfaces.
Their formulations commonly comprise essential constituents (surface active agents) and subsidiary constituents
(builders, boosters, fillers and auxiliaries) that wet the fabric, matter, solubilize grime (accumulation of dirt),
keep the soil in suspension, freshen, removing odour as well as brightening the fabrics as they clean. Generally
these are products of the reactions between long chain fatty acids with alkali metals. Soaps are made from fats
and oils, or their long chain fatty acids, by treating them chemically with a strong alkali [1]. Their properties
depend on the chain length of fatty acids in the blend, amount of unsaturation, formulation and the soap’s in-
tended applications [2] [3]. The detergents on the other hand use the sulfonated acid instead of a normal carbox-
ylic acid moiety resulting in a slightly harsher “soap” called a detergent, which is mostly used in multi-purpose
cleaning more than in bathing. Higher molecular weight detergents are claimed to give superior detergency
compared to the lower molecular weight counterparts [4] [5].
Several approaches have been reported for estimating or determining the detergency of some simple deter-
gents, namely, the hydrophilic-lipophilic balance (HLB) approach [6], the phase inversion temperature (PIT)
approach [7], as well as the Critical Micelle Concentration (CMC) approaches. It was not clear from literature
when CMC was firstly reported unlike the other two with the earliest report being 1971 [8]. The HLB and CMC
approaches are the most commonly used approaches probably due to the fact that PIT requires that analytes are
temperature stable, an attribute surfactants do not have [9]. The HLB was developed as an empirical scale where
oleic acid and its salt, sodium oleate were used as standards with values of 1 and 20 respectively, and other
compounds (analytes) were compared and placed accordingly on this scale. It has, however, been argued lately
that the HLB approach only gives approximate values since some arbitrary scale is deployed and the values ob-
tained in the original experiments by Griffin are not experimental but rather calculated from theoretical formulae
[10].
Critical micelle concentration on other hand measures the maximum concentration that monomers of the
surfactant can exist before they aggregate into micelles, vesicles, micro-emulsions, bilayers, membranes and
liquid crystals [11]. Micelles are the most predominant aggregate structures in surfactant solutions and form
over a narrow range of surfactant concentration—the CMC. These are spontaneously formed aggregated units
with the hydrophobic ends forming the core while the hydrophilic ends form a surface hydrated by water
molecules [12] [13]. Due to this behaviour, surfactants have a higher degree of ionization below the CMC—
thus behave as “strong electrolytes”, while above the CMC, they are partially ionised due to micelles forma-
tion reducing ion mobility. This micellarisation, therefore, results in the change in most physical properties
such as electrical conductivity, surface tension, osmotic pressure, density and light scattering or refractive in-
dex [14].
This change in the properties has enabled this technique to be applicable to the use of CMC approach on de-
termining the strength of the detergency of ordinary surfactants used as detergents or soaps [15] [16]. Different
approaches have been reported for determination of CMC of detergents. However, conductivity is by far the
easiest, most affordable and quite versatile as demonstrated by its application in deducing the mechanism of
metal uptake by solid adsorbents [17]. In principle, the increase in electrical conductance below the CMC is
usually higher than after the CMC due to the presence of freely moving ions of both the monomers and the salt
ions before the CMC, while after the CMC, there is less conductivity since the organic moieties are bound into
the micelles, and as such they are not free to move, thus reducing conductivity [12] [18]. Though this approach
(CMC) seems to perform satisfactorily, it is criticised for being too subjective since it requires visual inspection
of the point at which the change in the physical property takes place.
This manuscript reports the study of the detergency of a simple laboratory prepared pine oil based detergent
compared with the commercial brand using conductivity measurements. The laboratory sample was prepared
following a protocol obtained from a local small-scale detergents manufacturer while a commercial brand was
obtained from a local grocery store just outside of the main University campus in Roma, Lesotho. Since the
chemical composition of the commercial detergent was known, the one that looks similar to the laboratory pre-
M. J. George, T. Motsamai
959
pared was used for the comparison.
2. Experimental
2.1. Materials and Methods
The industrial grade reagents, namely, 96% dodecyl-benzenesulphonic acid (CAS No. 25155-30-0), pine oil, so-
dium hydroxide and “green dye” were obtained from LEFCHEM—a local small-scale manufacturer in Lesotho.
Besides these, all other reagents used were of analytical grade: methanol, naphthalene, sodium chloride and so-
dium hydroxide were obtained from ACE (Johannesburg South Africa). Deionized water was prepared in-house
using simple water still. The commercial Pine Gel (trade name) detergent was obtained locally from a grocery
store. Conductivity measurements were performed using a HANNA conductometer HI 8033 and HANNA
membrane pH-meter HI 8314.
2.2. Preparation of 200 mL In-House Pine Gel Detergent (the Manufacturer’s Protocol)
To a 24 mL of dodecyl-benzenesulphonic acid in a beaker, was added 72 mL of cold water, followed by 41 mL
of 2 M NaOH added gradually while the mixture was being stirred continuously and the pH was checked using
litmus. Once the solution was neutral, 52 mL of water were added and stirred appropriately. Appropriate amount
of dye was added to get the appropriate visual appearance. The resulting detergent was left to cure to a semi-
solid gel. The appearance of the laboratory sample was compared with the commercial sample for appearance
and consistency (see Figure 1).
2.3. Characterisation of the Produced Detergent by CMC Determination
A detergent stock solution of 5 mM concentration was prepared by dissolving the appropriate mass in small vo-
lume of methanol and diluted with distilled water. A test solution was prepared by adding 50 mL of distilled
water in to an Erlenmeyer flask, to which 1.0 mL aliquots of the detergent stock solution were added at the time
and stirred continuously using a magnetic stirrer as the resultant conductivity measured. Meanwhile a plot of
conductivity against the volume of the detergent solution added was being continuously carried out. This proce-
dure was repeated until the concentration of the detergent was higher than the CMC level (after a break in the
linearity—see Figure 2).
To determine the effect of different additives on the CMC, the same procedure was repeated for different
fresh detergent solutions in the presence of equi-molar NaCl, naphthalene and the naphthalene-NaCl mixture,
respectively.
3. Results and Discussions
3.1. Preparation and Modification a Laboratory Pine Gel Detergent
Figure 1 shows the photograph of the commercial and laboratory samples of the detergent. The laboratory pro-
(a) (b)
Figure 1. A picture of the detergents in the commercial packaging: (a) The
laboratory prepared sample; and (b) The commercial brand.
Commercial Sample La bo rat ory Sa mple
M. J. George, T. Motsamai
960
Figure 2. Variation of specific conductivity against the detergent concentration (labo-
ratory sample).
duct matched the commercial product both in colour, texture and consistency.
As it can be seen from Figure 1, the two samples looked similar. It was not only their visual appearances, but
also their feel and consistencies were comparable.
3.2. Characterisation of the Two Detergent Samples by Critical Micelle Concentration
Approach
The critical micelle concentrations of the two detergents were determined using conductivity measurement
method. The plot of conductivity versus surfactant concentration shows two straight lines with different slopes
(Figure 2).
The initial part (in the range 0 - 0.07) of the plot in Figure 2, represents the conductivity of detergent surfac-
tant before CMC (about 0.0725 g/cm3) where only monomers of the surfactant exist in solution, while the later
section (above the 0.0725 g/cm3) shows the conductivity of the surfactant after the formation of micelles. The
intersection of the two lines is taken as the CMC of the detergent as it has been discussed in the introduction.
The explanation for this behaviour is that, as a principle, at low concentrations of the surfactant, below the
CMC, an ionic surfactant is completely dissociated, as such there is a linear relationship between the specific
conductivity () of the surfactant solution and its concentration as the surfactant monomers behave as normal
electrolyte, Kohlrausch’s law is obeyed (Equation (1)). On the other hand, at concentrations above the CMC,
specific conductivity is independent of surfactant concentration as the micelles behave like a weak electrolyte
(partially dissociate into ions) and this is explained by (Equation (2)).
Equation 1:
( )
[ ]
cation anion surfactant
κλ λ
′
= +
Equation 2:
( )
[ ] [ ]
cation anion cation micelle
cmc surfactant micelle
κλ λ λα λ
′′
=++ +
where
cation anion
λλ
+
,
[ ]
surfactant ′
,
micelle
λ
,
[ ]
surfactant ′′
, and
α
represent conductivity of the cation, an-
ion, concentration of surfactant below CMC, conductivity of a micelle, fractional micellar ionization, and degree
of counter-ion dissociation respectively [19].
3.3. Determination of the Effect of Electrolyte (NaCl) on the CMC
Figure 3 presents the plot of specific conductivity versus concentration of surfactant in the presence of 5 mM
sodium chloride to increase the ionic strength of the mixture.
As expected the CMC value dropped in the presence of NaCl (from 0.0725 to 0.0520). This trend is consistent
with the available literature in that when a salt is added to a micellar solution, the CMC decreases due to the ef-
fect of the electrolyte counter-ion [20]. The main reason of lowering of CMC upon addition of an electrolyte is
the “salting out” effect. Once the electrolyte is added, the ionic power of the solution increases, the CMC de-
creases and the transition from spherical to cylindrical shape along with increase of the micelle aggregation
number occurs. The hydrophilic part of surfactant releases counter (molecular negatively charged) ions and
these ions decrease the CMC value. These counter-ions create Coulombic shielding of the electrostatic repul-
sions among charged hydrophilic parts of ionic molecules on the micelle surface. CMC of the ionic surfactant is
0
500
1000
1500
2000
2500
00.025 0.05 0.075 0.1 0.125 0.15
(𝜇𝑆/cm)
Detergent concentration (g/cm
3
)
M. J. George, T. Motsamai
961
Figure 3. Variation of specific conductivity with detergent concentration in the pre-
sence of sodium chloride.
also decreased by electrolyte dehydration activity. Solutions of electrolytes remove the lyosphere of surfactants
and, as a consequence, ions become strongly hydrated [21] [22].
3.4. Effect of the Presence of an Organic Compound (Naphthalene) on the CMC
Following the determination of the effect of the ionic strength, a fresh mixture was prepared, to which naphtha-
lene, an organic compound with relatively high hydrophobicity, was added. Figure 4 shows the effect of naph-
thalene addition to the CMC of the detergent. The value of the CMC increased from 0.0725 to 0.1109 (53% in-
crease).
When naphthalene is added to the micellar solution, it increases the hydrophobicity of surfactant molecules.
Naphthalene molecules infiltrate between the surfactant monomers’ tails, increasing the radius of the micelle
formed hence the increase in the CMC. A higher number of surfactant monomers together with naphthalene
molecules will aggregate themselves to form larger micelle molecules. This shows that at low concentration of
surfactant less number of micelle molecules are formed and most of the monomers are freely floating in the so-
lution. As the number of monomers is increased, there is more association leading to micelle formation although
much fewer in number than in the case of NaCl which acts in the opposite in that it draws away the water parti-
cles away from the organic surfactants thus forcing them to come together into the micelles.
3.5. Effect of the Combination of Organic Compound (Naphthalene) and an Electrolyte
(NaCl) on the CMC
Whereas sodium chloride decreases the CMC of detergent while the converse is true with naphthalene, when the
two compounds are added together in the detergent solution, the value of CMC lies between the CMC values of
the two compounds when added to the detergent solutions individually. But the value of CMC in this case is still
greater than when only water is added to the detergent solution (about 35%). This shows that naphthalene has
stronger effect on CMC than sodium chloride, that is; the magnitude at which naphthalene increases CMC is
greater than the magnitude at which sodium chloride decreases CMC value using water-surfactant solution as
the reference point.
Figure 5 shows the four plots combined to show the CMC values for detergent solutions in different condi-
tions, namely, in distilled water (a), in NaCl (b), in naphthalene (c) and in a mixture of NaCl and naphthalene
(d).
The analytical parameters demonstrated by the plots in Figure 5 are summarised in Table 1 showing the co-
ordinates for the CMC values.
As can be seen from the values of the slopes before the CMC compared to those higher than the CMC, clearly
conductivity increases much faster at concentrations below the CMC than higher at the concentrations higher
than the CMC as has been discussed prior. Despite the seeming correlation between critical micelle concentra-
tion and the conductivity values in Table 1, this relationship is not significantly linear (R2 = 0.8403) confirming
the existence of different types of interactions of the detergent surfactant with naphthalene and sodium chloride.
0
500
1000
1500
2000
2500
00.025 0.05 0.075 0.1 0.125 0.15
(𝜇𝑆/cm)
Detergent concentration (g/cm
3
)
M. J. George, T. Motsamai
962
Figure 4. Variation of specific conductivity with detergent concentration after add-
ing naphthalene.
Figure 5. The comparison of the variation in specific conductivity in the presence
of different additives.
Table 1. A summary of CMC values and the corresponding conductivities in different media.
Solution composition Correlation coefficient, R2 Slope CMC (g/cm3) Conductivity κ
(μS/cm)
Before CMC After CMC Before CMC After CMC
Distilled water 0.9951 0.9924 14694 8669 0.0725 1430
NaCl 0.9966 0.9972 17104 11714 0.0520 959
Naphthalene 0.9984 0.9926 12749 4802 0.1109 1511
Naphthalene + NaCl 0.9952 0.9927 14694 8669 0.0693 1330
Figure 6 shows the comparison of the two samples, laboratory prepared and the commercial sample, upon
different media (in the absence and presence of NaCl, naphthalene).
Clearly the two products do not seem to be comparable. The major difference that seems significant is the
CMC value without any additives with the CMC values of 0.0725 g/cm3 (laboratory sample) and 0.0920 g/cm3
(commercial sample)—accounting for a difference of about 25% relative to the laboratory sample. However,
since: i) too few (3) replicate measurements were made, ii) the other parameters are comparable, iii) the linearity
of the curves before and after the CMC were satisfactory (minimum R2 of 0.9915 for commercial sample after
the CMC for the distilled water case), then it could be argued that this deviation was just a result of an experi-
mental error; hence the results could be improved by taking several replicate measurements and using multiple
samples as well.
0
500
1000
1500
2000
2500
00.025 0.05 0.075 0.1 0.125 0.15
(𝜇𝑆/cm)
Detergent concentration (g/cm
3
)
0
500
1000
1500
2000
2500
00.025 0.05 0.075 0.1 0.125 0.15
(𝜇𝑆/cm)
Detergent concentration (g/cm
3
)
(b)
(a) (d)
(c)
M. J. George, T. Motsamai
963
Figure 6. A comparison of the CMC values for the laboratory and com-
mercial samples under different conditions.
4. Conclusions
The results presented in this project have shown that indeed critical micelle concentration approach can indeed
be used for the determination of the detergency of different products. However, due to the fact that no statistics
was performed, the results cannot be validated beyond presenting just a bird’s eye view of the approach. Since
the two formulations exhibited different magnitudes of shifts under different conditions, it could be argued that
they are probably not identical although they look and feel the same. This is interesting given that the small-
scale manufacturer that supplied the protocol claimed to have received it from one recognisable and experienced
manufacturer in the neighbouring South Africa.
The results further demonstrated that the effect of NaCl decreased the CMC by 40% (laboratory sample) and
70% (commercial sample). Naphthalene on the other hand increased the CMC values for both samples by just
over 50% (laboratory sample) and about 12% for the (commercial sample). A combination of equi-molar NaCl
and naphthalene resulted in the decrease of the CMC by about 5% (laboratory sample) and 30% for the com-
mercial sample. The decrease in CMC for the combined additives indicates that the interactions of the naphtha-
lene molecules are weaker than those from NaCl ions and hence the result could be the salting out of the deter-
gent in the form of too many tiny micelles, thus could reduce the detergency of the products. While it is a good
attribute to have a detergent with a lower CMC, this has to be considered with care lest the detergent loses activ-
ity completely by salting out of the washing solution.
With the application of the conductivity measurements confirmed, more work will be carried out to determine
the effect of other several parameters amenable to laundry services such as effect of temperature on the CMC,
with a hope of characterising the laboratory sample for the small-scale manufacturer as a way of contributing to
building his business further in an effort to practice the so-called “third mission” of higher education—incuba-
tion and mentorship of small business. Certainly, conductometer is a very simple and easy gadget; hence the
manufacturer can easily afford it so that they can put some CMC data on the label to improve confidence of the
customers in their product.
Acknowledgements
The authors gratefully acknowledge National University of Lesotho and LEFCHEM for supporting this project.
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