J. Chem. Thermodynamics 71 (2014) 205–211
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J. Chem. Thermodynamics
journal homepage: www.elsevier.com/locate/jct
Effect of temperature and additives on the critical micelle concentration
and thermodynamics of micelle formation of sodium dodecyl benzene
sulfonate and dodecyltrimethylammonium bromide in aqueous
solution: A conductometric study
S. Chauhan ⇑, Kundan Sharma
Department of Chemistry, Himachal Pradesh University, Shimla 171005, India
a r t i c l e
i n f o
Article history:
Received 24 September 2013
Received in revised form 28 November 2013
Accepted 17 December 2013
Available online 24 December 2013
Keywords:
Amino acids
Critical micelle concentration
Hydrophobic interactions
Thermodynamic parameters
a b s t r a c t
Specific conductance of (0.3 to 3.0) mmol kg1 sodium dodecyl benzene sulfonate (SDBS) and (3.0 to
30.0) mmol kg1 dodecyltrimethylammonium bromide (DTAB) has been determined in water and in
the presence of (0.01, 0.05 and 0.10) mol kg1 aqueous solution of glutamine/histidine/methionine at
T = (293.15, 298.15, 303.15, 308.15 and 313.15) K. From the conductivity data, the critical micellar
concentration (CMC) and thermodynamic parameters of micellization (DGom ; DHom and DSom ) have been
computed by applying the mass action model. Enthalpy–entropy compensation effect has also been
observed. The effect of amino acid on the micellar properties of SDBS and DTAB depends upon their nature, concentration, as well as on temperature and has been used to study the interactions present in the
micellar systems. There occurs a gradual increase in the value of CMC with temperature in case of SDBS
while in case of DTAB, it passes through a broad minimum and then tends to increase with increase in
temperature. Increase of amino acid concentration is found to decrease CMC in both the surfactants.
The DGom values are negative and the feasibility of the micellization is found to increase with rise in
temperature. The magnitude of hydrophilic and hydrophobic dehydration determines whether the
CMC values increase or decrease with rise in temperature.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The present study has been undertaken to understand how
micelles get affected in an aqueous environment containing amino
acids. The addition of amino acids to the solvent may affect the
micellization process of a surfactant as a result of changes in
solvent characteristics like hydrogen bond formation capacity,
dielectric constant, density, viscosity and degree of ionization [1].
The properties of amino acids themselves may also affect micellization including surface lattice aggregation, polar/non-polar or
zwitterions character, dimerization and hydrophobicity, etc. [2].
These properties determine the tendency of non-polar groups to
associate in aqueous solutions.
Among amino acids, glutamine (Gln), a polar uncharged amino
acid serves as ammonia transporter to the liver and kidney for urea
synthesis which is a small, non-toxic compound excreted via urine
[3,4]. Methionine (Met), is sulfur containing non-polar amino acid.
Its necessity is to provide the methyl group (CH3) to acceptor
molecules in one-carbon metabolism which is important in the
⇑ Corresponding author. Tel.: +91 177 2830803; fax: +91 177 2830775.
E-mail address: chauhansuvarcha@rediffmail.com (S. Chauhan).
0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jct.2013.12.019
production of red blood cells, white blood cells and platelets. Histidine (His) is positively charged basic amino acid at a pH of
approximately 6 or below. It is the precursor molecule to histamine, the compound that causes many allergic reactions and which
may be blocked by the use of anti-histamines. Because of this,
many people who have itching-related health problems may be
prescribed a drug like doxepin which has both histamine
antagonistic properties and anxiolytic properties. On the other
hand, the surfactants have been used in a similar way in biological
system as are employed in technical systems e.g. to overcome
solubility problems, as emulsifiers, as dispersants and to modify
surfaces, etc. The ionic surfactants, SDBS and DTAB have been chosen because of their strong interaction with protein and a regular
ingredient used in industry [3,4].
In recent years, there has been a growing interest in the
interactions present between amino acid and surfactant due to
their many applications in biosciences, foods and cosmetics, drug
delivery, detergency, and biotechnological processes [5,6]. Using
various numbers of tools and techniques, these interactions have
been studied and published in the past few years [7–12]. Taking
into consideration, the diversity of such molecules (surfactants
and amino acids), we intend to design such system which could
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S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211
prove its efficacy in every field including food, pharmaceutical and
biological industry, etc.
Although a number of studies on the interaction of surfactants
with amino acid molecules have been reported in literature [13–
19], but to the best of our knowledge, very little is known about
the intriguingly of the present system containing amino acids
and surfactants. Keeping these considerations in mind, we employed simple and promising technique, in particular, conductance
to substantiate the interactions present between the anionic surfactant (SDBS) and cationic surfactant (DTAB) with amino acids
(glutamine, histidine and methionine) at different compositions
and temperatures. The resulting data have been discussed in terms
of the interactions operating in surfactant–amino acid–H2O systems including the effect of amino acids on the micellization of
these surfactants. The structure of both, surfactants and amino
acids, which have been attempted in this work, have been shown
in figures 1 and 2, respectively.
2. Experimental
2.1. Materials and method
Glutamine and histidine were obtained from MERCK (Germany)
and methionine from S.D. Fine-Chem Ltd. (India), all were of A.R.
grade and were used as received. SDBS and DTAB were also of
A.R. grade obtained from HIMEDIA (India) and S.D. Fine-Chem
Ltd., respectively. However, a pure sample of SDBS and DTAB was
obtained by giving the additional treatment as reported in literature [20,21]. Aqueous solution of surfactants (SDBS and DTAB) of
different molal concentration in the range (0.3 to 3.0) mmol kg1
for SDBS and (3.0 to 30.0) mmol kg1 for DTAB were prepared by
the addition of small aliquots of concentrated solution of the surfactant to 10 mL of (0.01, 0.05 and 0.10) mol kg1 amino acid
solution prepared as a solvent medium. The solutions so obtained
were gently stirred on magnetic stirrer before subjecting to measurements. A sample of distilled water was collected from the Millipore Elix distillation unit which was subjected to further
distillation on acidified KMnO4 over a long fractionating column
operating at 750 torr. Different fractions of distilled water were
collected having j and pH values in the range (1 to 3) 106 S cm1 and (6.75 to 6.95), respectively. The sample of purified
water so obtained was not used after two days. A high precision
water thermostat fitted with a digital temperature controlled device used for all experimental measurements supplied by NSW–
New Delhi. The temperature of thermostat was maintained within
(0.1 K over the entire temperature range studied. Conductivity
measurements were carried out with digital conductivity meter
Cyberscan CON-510. The temperature of the solution was
FIGURE 1. Chemical structure of SDBS and DTAB.
(Gln)
(His)
(Met)
FIGURE 2. Chemical structure of glutamine (Gln), histidine (His) and methionine
(Met).
maintained to ±0.1 K by circulating water from thermostat through
a double walled vessel containing the solution. The accuracy of the
conductance measurement was well within ±0.4%. The provenance
and purity of the sample used have been provided in table 1.
3. Results and discussion
3.1. Critical micelle concentration and micellization
To design and interpret the amino acid–surfactant interaction
studies, it is necessary to know the critical micelle concentration
(CMC) of the two studied surfactants (SDBS and DTAB) in aqueous
solutions in the absence and in the presence of amino acids. The
conductivity data for aqueous solution of SDBS and DTAB at
T = (293.15, 298.15, 303.15, 308.15, 313.15) K have been summarized in table SM1 (Supplementary material) and the corresponding plots have been shown in figure 3. Each plot showed a linear
variation in the j values with respect to increased surfactant concentration both in the pre-micellar and post-micellar regions. The
abrupt change in conductivity (j) at a certain concentration of surfactant produces sharp break point in the plots. This break point
between the two straight lines gives the value of critical micelle
concentration (CMC) which has been converted into their mole
fraction unit, XCMC before subjecting them to determine the thermodynamic parameters of micellization. The CMC and corresponding, XCMC values for aqueous SDBS and DTAB have been reported in
table 2 which reveal that the CMC values for the surfactant, SDBS
and DTAB in water are not very large as compared to the values reported in literature; as 1.3 mmol kg1 for SDBS [22] and
15.6 mmol kg1 in case of DTAB [23,24], respectively. The temperature dependence of XCMC values for both SDBS and DTAB provides
information about the inhibitory effect of temperature.
Since, the variation in the specific conductance of a surfactant is
quite linear before and after the break, a comparison among the
monomeric and the micellar species over the whole concentration
range can be made by computing the pre-micellar (S1) and postmicellar (S2) slopes. Both S1 and S2 values were determined from
the linear regression analysis of the conductivity data with a correlation factor always much better than 0.997. The slope in the premicellar region has always been found greater than that in the
post-micellar region. Also, there are some examples [25,26] of similar binary ionic surfactant combinations in which the breakpoint
in the conductivity curve is not sharp. This can be generally explained on the basis of two situations: first, when instead of an
instantaneous micelle formation process, stepwise micellization
occurs as in the case of all bile salts [27] or in the presence of organic additives [28]; second, when apart from the ordinary spherical micelles, bilayer assembly [29] or insoluble salt formation
takes place, for example, as in the case of binary combinations of
oppositely charged ionic surfactants.
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S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211
TABLE 1
Specification and mass fraction purity of chemical samples.
a
Chemical name
Source
Purification method
Mass fraction purity
Glutamine (Gln)
Histidine (His)
Methionine (Met)
Sodium dodecyl benzene sulfonate (SDBS)
Dodecyltrimethylammonium bromide (DTAB)
Merck
Merck
S.D. Fine
HIMEDIA
S.D. Fine
None
None
None
Recrystalized
Recrystalized
0.99a
0.99a
0.98a
0.97
0.98
Declared by supplier.
2200
(a)
2000
1800
1600
κ / μS cm−1
1400
1200
1000
800
600
400
200
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
103 · mSDBS / mol · kg−1
(b)
1800
1600
1400
κ / μS · cm−1
1200
1000
800
600
400
200
0
0
5
10
15
20
25
30
103 · mDTAB / mol · kg−1
FIGURE 3. Conductivity vs. concentration plots for aqueous solution of (a) SDBS and
(b) DTAB at T = 293.15 K: j, 298.15 K: N, 303.15 K: d, 308.15 K: ., 313.15 K:
temperatures.
TABLE 2
CMC and corresponding, XCMC, values in aqueous solution of SDBS and DTAB.
T/K
293.15
298.15
303.15
308.15
313.15
SDBS
DTAB
103 CMC
104 XCMC
103 CMC
104 XCMC
1.20
1.28
1.36
1.42
1.51
0.216
0.230
0.245
0.256
0.272
15.1
15.3
15.5
15.8
16.1
2.72
2.75
2.79
2.84
2.90
The uncertainty in the CMC and corresponding XCMC measurements are:
±(0.01 103 and 0.002 104) in case of SDBS and ±(0.1 103 and 0.02 104) in
case of DTAB, respectively.
3.1.1. Effect of additives
In this section, we studied the effect of additives i.e., amino
acids on the critical micelle concentration (CMC) of SDBS and DTAB
in order to examine more closely the manner in which amino acids
affect the micellization of surfactant in their aqueous solutions.
The conductivity data for SDBS and DTAB in aqueous amino acids
have been summarized in tables SM2–SM7. The dependence of j
on surfactant concentration is shown in figures SM1–SM6 and
the corresponding XCMC values have been presented in table SM8.
Additives, on the basis of their influence on the micellization
process, can be classified in two main categories: electrolytes and
non-electrolytes [30]. Electrolytes generally facilitate the formation of ionic micelles, primarily by lowering the coulombic Gibbs
energy of the interface, resulting in decreased CMC, so that at high
ionic strength, huge surfactant aggregates are formed [31]. On the
other hand, non-electrolyte organic additives, which can be further
classified as polar and non-polar, affect micellization in different
ways depending on the nature of the additives as well as its concentration [32,33].
Furthermore, if we compare the XCMC values of these surfactants, as presented in table SM8, it is clear that at the given concentration of amino acid, it decreases in the order: His > Gln > Met in
case of SDBS while in case of DTAB, the order is: Gln > Met > His.
This trend can be explained on the basis of ion-pair formation between oppositely charged amino acid and head group of surfactant
leading to solubilization of respective amino acid. The effect occurs
to maximum extent in case of histidine (positively charged) with
SDBS which has negatively charged head group, thus delaying micellization. However, in case of DTAB, the head group is positively
charged, hence micellization in case of histidine seems to be more
facilitated [20]. Thus, it can be suggested that the nature of amino
acid has a special bearing on the XCMC value of both the surfactants.
3.1.2. Temperature dependence of XCMC (or CMC)
The effect of temperature on XCMC values of SDBS and DTAB
have been presented in figures 4–6 indicating a linear increase
with rise in temperature for SDBS, while in case of DTAB, XCMC values pass through a broad minimum at around T = (298.15 to
308.15) K, thus both the surfactants behave differently. In general,
the effect of temperature on the XCMC value of surfactant in aqueous medium is complex [34] and is analyzed in terms of hydrophobic and hydrophilic hydrations. In monomeric form of surfactant,
both the hydrophobic as well as hydrophilic hydrations are possible whereas only hydrophilic hydration is possible for micellized
surfactant system. Both types of hydrations are known to decrease
with increase in temperature [35]. At lower temperature, a hydrophilic dehydration favors the micelle formation while with the increase in temperature; hydrophobic dehydration disfavours the
micelle formation [36,37]. Thus the magnitude of these two factors
determines whether the CMC (XCMC) values increase or decrease
over a particular temperature range.
In case of micellization of DTAB, the gradual decrease of XCMC
values at lower temperature and gradual increase of XCMC values
at higher temperature may be due to the dominating effect of first
and second factors, respectively. However, in case of SDBS micellization, the gradual increase of XCMC values with temperature may
be due to the dominance of second factor only. Therefore, as the
temperature increases, the effect of hydrophobic groups begins
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3.2
(a)
3.4
(a)
3.1
3.0
3.2
2.8
105 · Xcmc
105 · Xcmc
2.9
2.7
2.6
2.5
3.0
2.8
2.6
2.4
2.4
2.3
2.2
290
295
300
305
310
290
315
295
300
3.0
2.9
(b)
310
315
305
310
315
(b)
2.8
104 · Xcmc
2.9
104 · Xcmc
305
T/K
T/K
2.8
2.7
2.6
2.7
2.5
290
295
300
305
310
290
315
295
FIGURE 4. Plots of XCMC vs. temperature in aqueous solution for (a) SDBS and (b)
DTAB containing 0.01 mol kg1: j, 0.05 mol kg1: N, 0.10 mol kg1: d, concentrations of glutamine.
to exert its influence and finally predominates as the XCMC reaches
a minimum value and finally increases with temperature. The existence of a minimum CMC in the XCMC temperature curve is thus an
outcome of these two opposing effects. In most ionic and several
non-ionic surfactants, minimum in CMC–temperature profile has
been the usual trend [38,39] and the factors affected by the change
of temperature like surfactant solubility, de-solvation, changed solvent structure, etc. play important role in this respect [40].
From another point of view, it has been observed that with increase in temperature, the thermal motions of surfactant and solvent molecules enhance so that the formation of ordered micelle
structures becomes difficult, i.e., the thermal motions may be more
important than the breakage of water structure at high temperatures. The increase of temperature further makes the kinetic energies enhance and the ordered micellar structures destroy, causing
decrease in the micelle aggregation number but the XCMC value increases. Therefore, higher the temperature, greater is the disaggregation degree of micelle, consequently higher is the XCMC.
3.2. Thermodynamics of micelle formation
The temperature dependence of XCMC can be employed to compute the thermodynamic parameters of micellization for amphiphiles in aqueous solution. For ionic surfactants, the standard
enthalpy of micellization DHom is given by the equation [41,42];
DHom ¼ RT 2 ð2 aÞ½dðln X CMC Þ=dT;
300
T/K
T/K
ð1Þ
FIGURE 5. Plots of XCMC vs. temperature in aqueous solution for (a) SDBS and
(b) DTAB containing 0.01 mol kg1: j, 0.05 mol kg1: N, 0.10 mol kg1: d,
concentrations of histidine.
where, dðln X CMC =dTÞ was determined as the slope of the straight
line obtained by plotting ln XCMC against T and subjecting the data
to a least-squares treatment. Here, a is the degree of counter-ion
dissociation, which was calculated from equation (2) [43];
a ¼ S2 =S1 ;
ð2Þ
where, S1 and S2 are the slopes in pre- and post-micellar regions
determined from the conductivity plots (table 3 and table SM9).
The standard Gibbs energy of micellization, DGom and entropy of
micellization, (DSom ) have been estimated from the following equations [44,45];
DGom ¼ ð2 aÞRT lnðX CMC Þ;
ð3Þ
DSom ¼ ðDHom DGom Þ=T:
ð4Þ
The values of DHom , DGom and DSom for aqueous solution of SDBS
and DTAB have been summarized in table 3. On investigating the
data, we found that DHom values for aqueous solution of SDBS and
DTAB are negative over the entire temperature range studied. This
observation appears to suggest that the micellization of these
surfactants, which is found to be spontaneous process, over the entire temperature range, is energy driven. However, since DHom in
case of DTAB is less negative than found in SDBS, this difference
in behavior can be attributed to the fact that micellization of DTAB
is relatively more entropy driven than SDBS. This may be
interpreted to mean that transferring of DTAB molecule to micellar
region is accompanied with a greater disruption of the solvent
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S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211
3.2
-8.6
(a)
-9.0
3.0
-9.2
ΔHom / kJ · mol−1
2.9
2.8
105, Xcmc
(a)
-8.8
3.1
2.7
2.6
2.5
-9.4
-9.6
-9.8
-10.0
-10.2
2.4
-10.4
2.3
-10.6
2.2
290
295
300
305
310
315
290
295
300
T/K
3.0
310
90
(b)
315
(b)
85
ΔSom / kJ · mol −1
2.9
104 · Xcmc
305
T/K
2.8
80
75
2.7
70
2.6
290
295
300
305
310
315
65
290
295
300
T/K
FIGURE 6. Plots of XCMC vs. temperature in aqueous solution for (a) SDBS and
(b) DTAB containing 0.01 mol kg1: j, 0.05 mol kg1: N, 0.10 mol kg1: d,
concentrations of methionine.
TABLE 3
Standard thermodynamic parameters of micellization (DGom , DHom and DSom ) for
aqueous solution of SDBS and DTAB at different temperatures.
T/K
DGom /kJ mol1
DHom /kJ mol1
DGom /J K1 mol1
a
73.0
71.7
70.4
69.2
67.9
0.812
0.815
0.818
0.821
0.824
88.6
87.8
87.0
86.3
85.4
0.241
0.244
0.245
0.246
0.248
SDBS
293.15
298.15
303.15
308.15
313.15
30.71
30.98
31.24
31.53
31.78
9.33
9.62
9.92
10.23
10.54
293.15
298.15
303.15
308.15
313.15
34.76
35.25
35.73
36.25
36.71
8.79
9.08
9.38
9.68
9.99
305
310
315
T/ K
DTAB
The uncertainty in the temperature and thermodynamic measurements for SDBS
and DTAB are: 0.01 K in temperature, ±0.03 kJ mol1 in DHom , ±0.02 kJ mol1 in
DGom and ±2 J K1 mol1 in DSom , respectively. The uncertainty w.r.t. a in case of
SDBS and DTAB are ±0.02.
structure, explaining DSom > 0. As depicted in figure 7, we find that
DHom and DSom both decrease with rise in temperature indicating
that micellization tend to be energy driven at higher temperature,
and thus compensate the contribution due to enthalpy and entropy
making DGom < 0 practically independent of temperature.
Further, the XCMC data reported in table SM8 in respect of DTAB
were subjected to the treatment of a second degree polynomial
[46] in the form as described by equation (5) as
FIGURE 7. Plot of (a) DHom and (b) DSom vs. temperature for aqueous SDBS: j, DTAB:
N.
ln X CMC ¼ a þ bT þ cT 2 ;
ð5Þ
where, the coefficients a, b and c are determined by a least-squares
regression analysis. The DHom values were then calculated by
substituting equation (5) into equation (1). However, DGom has been
calculated as mentioned below;
DHom ¼ RT 2 ð2 aÞðb þ 2cTÞ;
ð6Þ
DGom ¼ RTð2 aÞða þ bT þ cT 2 Þ:
ð7Þ
The thermodynamic parameters of micellization of SDBS and
DTAB determined using the above formulations have been summarized in tables SM10–SM12, and their temperature dependence
behavior in case of glutamine has been presented in representative
figure 8. The most interesting trend that we observe in the data reported in tables SM10–SM12, is that in case of DTAB, DHom is positive up to T = 298.15 K, and becomes negative over and above
303.15 K in all compositions of amino acids irrespective of the nature and the concentration of amino acid. This sequence is similar to
that observed by Chen et al. [47].
For different amino acid-containing solutions, the standard
enthalpy and entropy of micellization decrease with increase in
temperature. This behavior can be justified as follows: at low
temperatures, the reduction of the hydrophobic hydration is
responsible for the observed increase in the value of DSom . However,
with rise in temperature, the structure and size of water molecule
aggregates decreases, consequently, DHom becomes more exothermic, and this effect becomes predominant [48]. Positive, DHom
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-10.0
-10.4
For amphoteric and ionic surfactants, DGom has been reported to
be in between (23 and 42) kJ mol1 at T = 298.15 K [48].
We now turn to DHom and DSom values of SDBS reported in tables
SM10–SM12. Interestingly, the magnitude of both these parameters is found to be practically independent of temperature as well
as the amino acid concentration, presumably suggesting that the
hydrophobic interaction is augmented in the presence of amino
acid. Therefore, we can treat the micellization of SDBS in aqueous
solutions of amino acid as a normal micellization process. The
effective compensation obtained between DHom and DSom values of
SDBS are reflected in the DGom < 0 values which remain constant
over the entire concentration and temperature range studied. This
means that the London–dispersion interactions as an alternative
force contribution for SDBS micellization [53].
(a)
-10.8
ΔHom / kJ · mol−1)
-11.2
-11.6
-12.0
-12.4
-12.8
-13.2
-13.6
-14.0
290
295
300
305
310
315
T/K
3.3. Enthalpy–entropy compensation for SDBS and DTAB micellization
74
(b)
According to the viewpoints of Lumry and Rajender [54] for the
compensation phenomenon, the micellization can be described as
consisting of two-part process: (a) the ‘de-solvation’ part, i.e., the
dehydration of the hydrocarbon tail of surfactant molecules, and
(b) the ‘chemical’ part, i.e., aggregation of the hydrocarbon tails
of surfactant molecules in the formation of micelle. In general,
the compensation phenomenon between DHom and DSom in the various processes can be described as follows;
72
ΔSom / J · K−1 · mol−1
70
68
66
64
62
60
DHom ¼ DHm þ T c DSom ;
ð8Þ
58
56
290
295
300
305
310
315
T/K
FIGURE 8. Sample plot for (a) DHom and (b) DSom of SDBS vs. temperature in aqueous
solution of 0.01 mol kg1: j, 0.05 mol kg1: N, 0.10 mol kg1: d glutamine.
values can demonstrate the importance of hydrophobic interactions, whereas negative DHom values can taken as evidence that
London – dispersion interactions represent the major attractive
force for micellization [30]. With increase in temperature, the
enthalpic contribution to the Gibbs energy increases, meaning
thereby, the hydrogen bond between water molecules start
diminishing and therefore less energy is required to break up the
water cluster. Thus, DHom becomes more significant at higher
temperatures [49].
The entropy change in all cases is positive which confirms that
aggregation of surfactant is favored entropically. Since micelle formation is a structure formation from monomeric surfactant molecules; hence, the entropy change is expected to be negative.
However, its positive value indicates the melting of iceberg clusters around the hydrocarbon tails of the surfactant monomer and
the increased randomness of the hydrocarbon chains in the micellar core [50]. The values of DSom are decreasing with increasing temperature as seen from tables which may be that self – aggregation
becomes poorer at higher temperatures because of enhanced
molecular motion at higher temperature [51].
The DGom value is the sum of the enthalpic (DHom ) and entropic
(T DSom ) contributions. The result in tables SM10–SM12 show that
negative values of DGom are mainly due to the large positive value of
DSom especially at low temperatures; become more negative at
higher temperatures, indicating a larger driving force for
micellization [52]. Different arrangements of solvent molecules
are expected to differ in enthalpy and entropy in a mutually compensating manner, so that DGom value is not significantly affected.
where Tc in DHom vs. DSom curve, known as compensation temperature, can be interpreted as a characteristic of solute–solvent interactions, i.e., proposed as a measure of the ‘de-solvation’ part of the
process of micellization. The intercept DHm characterizes the solute–solute interaction, i.e., considered as an index of the ‘chemical’
part of the process of micellization. Tc value generally lies in the
range T = (270 to 300) K has been used as a diagnostic test for the
participation of water in the solution [55,56]. Note that the DHom
stands for the enthalpy effect under the condition DSom = 0. The increase in the DHom thus corresponds to a decrease in the stability
of the structure of micelles. In the present study, we found that in
all cases there exist a good correlation between DHom and DSom values
of SDBS and DTAB with the correlation coefficient lying near 0.999
and Tc of magnitude in the range (290 to 300) K. Similar enthalpy–
entropy compensation have been observed in case of SDS in aqueous solutions of various amino acids [20].
4. Conclusions
On examining the results, we found that XCMC values of both
SDBS and DTAB decrease with increase in concentration of amino
acid in the solution. This decrease can be interpreted in terms of
interactions of the amino acid with surfactant molecules. On one
hand, it may be proposed that increase in amino acid concentration
may cause partial destruction of the hydration shell around the
alkyl chain of the surfactant monomer, and on the other hand,
addition of amino acid molecules may result in decreased thickness of the solvation layer around the ionic heads of surfactant.
So, the hydrophilicity of the surfactants is decreased, that is, its
surface activity is enhanced, with the result, molecules aggregate
easily on the surface and in the solution; consequently, the value
of CMC decreases. Further, the result indicates the presence of both
electrostatic and hydrophobic interactions at lower surfactant concentration and temperature but the contribution of hydrophobic
interaction becomes dominant at higher temperature.
S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211
Acknowledgements
Kundan Sharma thanks, UGC, New Delhi for the award of Basic
Scientific Research fellowship (No. F. 4-1/2006 (BSR)/7-75/2007
(BSR)).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jct.2013.12.019.
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JCT 13-566