Modeling and Numerical Simulation of Material Science, 2013, 3, 17-25
doi:10.4236/mnsms.2013.34B004 Published Online October 2013 (
Copyright © 2013 SciRes. MNSMS
Experimental Study of Surface and Solution Properties of
Gemini -conventional Surfactant Mixtures on
Solubilization of Polycyclic Aromatic Hydrocarbon
M. Kamil*, Huma Siddiqui
Department of Petroleum Studies, Aligarh Muslim University, Aligarh-202002, India
Email: *
Received May, 2013
Experimental data are presented on the enhanced solubilities of fluorene (FLR) resulting from solubilization in aqueous
solutions of t wo co nventio nal sur factan ts: catio nic cet yltri me thyla mmonium bromide (CTAB) , anionic sodium dodecyl
sulfate (SDS), nonioinic polyethylene glycol dodecyl ether (Brij35) and a cationic gemini bis (hexadecyldimet hylam-
monium) pentane dibromide (G5). The critical micellar concentration of surfactants was determined by surface tension
meas urement s and aque ous sol ubilitie s of fluor ene co mpound in surfactant solutions were measured spectrophotometr-
ically. Solubilization of PAH compound commenced at the surfactant critical micelle concentration and was propor-
tional to the concentration of surfactant in micelle. The results of the mixed systems were analyzed with the help of
regula r so luti on t heor y, in whi ch the d ev iatio n of CMC exp values for mixed sur facta nt s yst ems fr om C MCideal was mea s-
ured by evaluating the interaction parameter, βm. Negative values of βm were observed in all equimolar binary systems
which show synergism in the mixed micelle. Attraction force between two oppositely charged head groups lead the
strongest synergism effect between cationic gemini and anionic conventional surfacta nt. I n ad dition to mola r sol ubiliz a-
tion ratio (M SR) so lub iliz ation efficiency is also q uantified in terms of micelle-water partition co e fficient (Km).
Keywords: Gemi ni Surfactants; PAH s; Solubil iz ation; Mixed micelle; Solution Prop e r ties
1. Introduction
Hydrophobic organic contaminants (HOCs) are found in
the priority list of hazardous substances as listed by the
EPA and the agency for toxic substances and disease
registry of USA [1]. The hrdrophobicity of these conta-
minants is one of the factors that determine the fate of the
contaminant in the environment. Contamination of soil
and underground water by persistent organic pollutant is
a major environmental concern. Polycyclic aromatic hy-
drocarbons (PAHs) are one class of such pollutant s. T he y
are made of two or more fused benzene rings formed
mainly from the combustion of fossil fuels and are al-
ways found as a mixture o f individual compounds. T hey
are of special interest because (i) they are known or sus-
pected carcinogens or mutagens and (ii) they strongly
adsorb to soil sediments making them persist in the soil
for extended periods of time [2-7]. As a consequence,
remediation of PAHs in soilwater system is often de-
pendent on desorption of the contaminant from the soil
surface and its subsequent incorporation into the bulk
aqueous phase. In addition, owing to their low aqueous
solubility a nd low vap or pr essure, the ir removal from the
environment water and soil still presents a considerable
challenge to researchers involved in separation technol-
ogy. One promising technique, surfactant enhanced sub-
surface remediation (SESR) has emerged in which the
solubility of organic solutes is greatly enhanced by the
presence of surfactant micelles [8-13]. Micelles are
self-association of surfactant molecules with the surfac-
tant hydrophobic portion oriented towards the center of
the aggregates a nd the h ydrophilic p ortions lo cated at the
aggregate surface and facing the solvent molecules. The
central core of the micelle thus constitutes a hydrophobic
pseudo phase that may accommodate a certain amount of
a lipophilic solubilizate, resulting in an enhancement of
its solubilization. To get a better system, mixed micellar
systems have already been used for the significant en-
hancement of water solubility of poorly soluble organic
contaminants. Mixed surfactants improve the perfor-
mance of surfactant enhanced remediation of soils and
sediments by decreasing the applied surfactant level and
thus its cost.
Conventional surfactant molecules are composed of a
long hydrophobic hydrocarbon tail with an ionic or polar
*Corresponding author.
Copyright © 2013 SciRes. MNSMS
hydrophilic head. Gemini surfactants are made up of two
hydrocarbon tails and two ionic head groups connected
by a ‘spacer’ in the sequence: hydrocarbon tail / ionic
group / spacer / ionic group / hydrocarbon tail [14]. The
spacer can be attached directly to the identical ionic
groups, each of which is in turn bonded to an identical
hydrocarbon tail; alternatively, the two identical amphi-
philes are joined midway. Their various surface active
properties are superior to those of corresponding conven-
tional surfactants with one hydrophilic and one hydro-
phobic group. Thus, they have much lower CMC values
and are more efficient in lowering the surface tension of
water. As a result, the Gemini surfactants form larger
micelles than the conventional surfactants and thus
should have a better solubilizing capacity [15-16].
Thus in the open literature, only few studies are re-
ported on solubilization of PAHs in Gemini-conventional
mixed surfact ant s ystems are available. The objectives of
the present study are: (i) to compare the efficiency of few
Gemi ni-conventional mixed surfactants in enhancing the
water solubility of Fluorene and (ii) to have the idea
about the synergistic solubilizatio n of Fluorene by mixed
suefactant systems. This experimental study is aimed to
ascertain if a mixed surfactant solution may be used in
the SER of organic pollutants. In this study, we have
studied interfacial properties and molecular interactions
of individual as well as equimolar binary mixtures of
cati onic gemini and c onventional surfactants.
2. Experimental
2.1. Materials Used
The surfactants cetyltrimethyl ammonium bromide
(CTAB), sodium dodecyl sulphate or sodium lauryl sul-
phate (SDS), and polyethylene glycol dodecyl ether ( Brij
35) were purch ased fr om S igma Al drich an d us ed as re-
ceived. Gemini surfactant pentanediyl-1,5-bis (dimethyl-
cetylammonium bromide), abbreviated as G5 was syn-
thesized in the research laboratory of the Department of
Chemistry, AMU, Aligarh. Synthesis of gemini’s entail
refluxing of 1,6-dibromohexane with N, N- di methylce-
tylamine (molar ratio 1:2.1) in dry ethanol. For maxi-
mum bisquaternization continuous stirring at 80°C is
done for 48 h to ensure as much as possible a complete
bisquaternization. The progress of the reaction was mo-
nitored by using TLC technique. The solvent was re-
moved under vacuum after the completion of the reac-
Fluorene was used as polycyclic aromatic hydrocarbon
in the present work and was also procured by sigma Al-
drich chemical company. Surfactant solutions were pre-
pared in double distilled water. Structures of above
chemicals are shown in Figure 1.
2.2. Methods
2.2.1. Critical Micelle Concentration Determination
by Surfa c e Te nsion Measureme nts
For CMC determination tensiometric experiments were
performed for single as well as for mixed surfactant sys-
tems. The apparatus used for the purpose was Hardson
tensiometer (Hardson make Kolkata, India), which works
on ring detachment method. The vertically hung ring was
initially dipped in the surfactant solution to measure its
surface tension. It was then forced to pull out from the
solution. The required force which was applied to pull
out the ring from the solution is its surface tension. Ex-
periments were repeated twice for each surfactant to en-
sure the reproducibility of the results. The surface tension
versus log [surfactant] plots for individual and mixed
surfa ctant syste ms are s hown in Figures 2 and, 3 respec-
tively. The concentration at which inflexion in curve is
obtained is the CMC of that substance.
Figure 1. Structures of reagents used in the stu dy polyethylen e glycol dodecyl ether (B rij 35), cetyltri methyla mmoni um bro-
mide (CTAB), sodium lauryl sulphate (SDS), gemini sur factant ( G 5), and fl uorene.
Copyright © 2013 SciRes. MNSMS
-6.2 -6.0 -5.8 -5.6-3.6 -3.4 -3.2 -3.0 -2.8
Log [surfactant] (mM)
Surface Tension (mNm-1)
-5.5 -5.0 -4.5-
Surface Tension (mN m -1)
Log [surfactant] (mM )
Figure 2. Surface tension versus. log [surfactant] plots for single surfactant systems.
-5.8 -5.6 -5.4 -5.2 -5.0 -4.8 -4.6 -4.4 -4.2
Surface Tension (mN m -1)
Log [surfactant] (mM)
G5-BR IJ35
Figure 3. Surface tension versus log [surfactant] plots for
G5/conventional mixed surfactant systems.
2.2.2. Solubilization Ex pe riment s
Solubility of Fluorene in surfactant system was deter-
mined by solubilization experiments. Solutions of con-
centration higher than their corresponding CMC were
prepared. These solutions were then filled in borosilicate
screw-capped glass vials of capacity of 5 ml with an
excess amount of Fluorene. Extra amount of Fluorine
was added to ensure maximum solubility in surfactant
solution. These samples are then agitated on magnetic
stirrer for a period of 24 h at 30°C. To ensure good agita-
tion magnetic teflon pieces were also dropped in each
vial. After this, a portion of the samples are collected in
eppendorf tubes and centrifuged at 12000 rpm, using a
high speed micro centrifuge (REMI, RM-12C) to settle
do wn the und issol ved F luore ne. T he co ncentr ation o f the
solubilized Fluorine of centrifuged sample was deter-
mined spectrophotometrically using Shimadzu spectro-
photometer (model UV mini-1240), following by appro-
priate dilution of a sample of the supernatant with the
corresponding surfactant solution. Before taking spectra
baseline correction was done with the surfactant solution
of same concentration.
3. Results and Discussion
3.1. Critical Micelle Concentration
The surfactant concentration at which monomers begin t o
assemble in ordered, colloidal aggregates or micelle is
termed as critical micelle concentration (CMC or cmc).
The CMC values of pure as well as of binary surfactant
mixtures (cmcexp) were evaluated on the basis of tensi-
ometric measurements. Surface tension decreases as the
concentration of the surfactant increases. Surfactant mo-
lecules at low concentrations adsorb at the liquid/air in-
terface until the surface of the solution is completely oc-
cupied. Then the excess molecules tend to self-associ- ate
in the solution to form micelles, and surface tension be-
comes constant. Two opposite effects control micelliza-
tion: the effect of the hydrophobic group is an important
driving force in micellization and the effect of the hy-
drophilic group opposing it. The cmc values were deter-
mined by noting inflections in the surface tension (γ)
versus logarithm of surfactant concentration isotherms
and are given in Table 1. The gemini surfactant has re-
markably low cmc value as compared to the co nventional
surfactants because of its two polar head groups and two
hydrophobic chains which transfer at the same time from
the aqueous phase to micellar phase.
Copyright © 2013 SciRes. MNSMS
3.2. Inter facial P roper ti es
The surface properties such as max
Γ (the maximum
surface excess), Amin (the minimum surface area per mo-
lecule), and thermodynamic par ameters, m
G (Gibbs free
energy of micellization),
(the standard Gibbs
energy of adsorption, Gmin (the free energy of air/water
interface) of individual as well as equimolar binary sur-
factant systems were determined.
The adsorption efficacy of selected surfactants and
their mixtures at the air/solution interface were evaluated
with the help of the Gibbs adsorption equation [17-19].
2.303* **loglogP
whe re ,
is surface tension of the solution, R is the
universal gas constant (8.314 JK1 mol1), T is tempera-
ture in the absolute scale, X is concentration of the sur-
factant in solution, and ‘n’ is a constant, which depends
on the number of species con stituting the surfactant.
The factor
was obtained from the slopes
of the plots of surface tension vs. log [surfactant]. max
values were used to calculate the minimum area per sur-
factant molecule (Amin) at the air/water interface using
the equati on:
min max
where, N = Avogadro’s Number
The minimum area per surfactant molecule was found
to be minimum for Brij 35, and maximum for SDS as
given in Table 2. For the case of gemini surfactants, an
increase in Amin was observed with increase in carbon
number of spacer group. The values of the surface pres-
sure at the CMC (
) were obtained from the fol-
lowing equation:
= γ
o - γCMC (3)
Table 1. Experimental and literature CMC values of surfac tants.
Table 2. Maximum surface excess (
), the minimum surface area per molecule (Amin ), degree of micellar ionization , the
standard Gibbs energy of adsorption (ΔGad), Gibbs free en ergy of micellization (
), the free energ y of air/w ater interface
(Gmin) values.
Copyright © 2013 SciRes. MNSMS
where γo is the surface tension of pure solvent and γCMC
is the surface tension at CMC. The standard Gibbs free
energy of micelliza tion per mole was estimated using the
= R * T * ln CMC * 0.001 (4)
The standard free energy of adsorption at air/water in-
terface can be derived fro m this standard free energy, as:
ad m
∆ =∆−
Table 2 clearly shows that both m
and ad
negative and their magnitudes showed a somewhat im-
pulsive nature of ad
G, which causes surfactant mole-
cules movements toward air/water interface. With this
finding, it was revealed that adsorption is primary and
micelle formation process is secondary process, during
surfactant addition in water. The maximum ad
Gis ob-
served for the G5/Brij 35 combination. Further, Amin of
conventi onal surfact ants follow the order S DS > CT AB >
Brij35 (anionic > cationic > non-ionic). For the mixed
systems the ord er is: G5-Brij35 > G5-CTAB > G5-SDS.
3.3. Surfactant – surfactant Interaction
To determine, whether the binary systems follows ideal
or nonideal behavior, the experimental CMC values of
equimolar binary surfactant systems were compared with
ideal CMC values. The CMC ideal values were calculated
using Clint equation [20]:
ideal1 2
= +
where CMC1, CMC2, α1 and α2 are the critical micelle
concentrations and the mole fractions of component 1
and 2 in mixed surfactant solutions. In Table 1, it is ob-
served that all CMCexp values were less than CMCideal, as
predicted by above equation which shows that the forma-
tion of mixed micelles exhibits a negat ive deviation with
respect to ideal mixture.
From Table 1, it is clear that CMCs of ionic surfac-
tants are muc h higher t han nonionic surfactant. This fact
can be justified as nonionic surfactant molecules show
hydrophobic interaction among hydrocarbon chains,
which are easily separated from the aqueous environment,
whereas ionic surfactants requires higher concentrations
to overcome the electrostatic repulsion between ionic
head groups while aggregating [21]. Moreover it was
also observed that the CMCexp values of binary systems
are lower than their corresponding ideal values, which
indicate synergistic interaction in all mixed systems .
In the light of the re gular sol utio n theor y, the deviatio n
of CMCexp values for mixed surfactant systems from
CMCideal can be measured by evaluating the interaction
parameter, β. This parameter can be calculated with the
help of Rubingh’s equation [22]:
( )( )
12 1122
112 2
lnln lnln
β1X 1X
 
 
 
= =
are the micellar mole fraction of sur-
factant 1 and 2 in the mixed micelles, and CMC1, CMC2
and CMC12 are critical micelle concentrations of surfac-
tants 1 and 2, and C MC o f mi xed surfa cta nt s yste m, co n-
sisting of surfactant 1 and 2, both respectively, α1, and α2
are their corresponding bulk mole fractions. The micellar
mole fraction m
X was calculated with the help of fol-
lowing equation for nonideal binary mixture of surfac-
tants by solvin g iter a tively [22,23].
( )
( )
( )
m2 1 12
m2 1 12
1 12
X lnlnX1
1 Xlnln1 XCMC
 =
A negative value of β shows negative deviation of
CMCexp from CMCid eal, which indicates a reduction in
free energy of micellization over that predicted by the
ideal solution theory [23]. This implies good interaction
between sur facta nts in mi xed sys tem. A po sitive va lue o f
signifies antagonism between components of surfac-
tant combination. Another parameter, activity coefficient
f and 2
f within t he mixed micelles derived from Ru-
bingh equations was equated as :
( )
{ }
fX= −
( )
{ }
The values of β obtained experimentally for the se-
lected surfactants are given below in Table 3.
All negative β value s in Ta ble 3 indicate good interac-
tion between the components of mixed system and dem-
onstrate synergistic effect for all binary equimolar mixed
surfactant systems. The larger negative value of β de-
notes the greater negative deviation of CMCexp from
CMCideal. The order of deviation exhibited through β is
G5-SDS > G5-CTAB > G5-Brij35. The strongest syn-
ergis m effec t is fo und be twee n catio nic gemini and a nio-
nic co n ve nti o nal surfa ctant. T he r ea so n behind t hi s mi g ht
be the attractive forces between oppositely charged head
groups. The least value was for cationic gemini and nonio-
nic conventional surfactant, as Brij35 has polyoxyethylene
(POE) groups with large number of oxygen atoms and a
lone pair o f electro n, thus it ma y have a te ndency to react
coulombically with cationic gemini surfactant, but the
Copyright © 2013 SciRes. MNSMS
existence of long polyoxyethylene head group imposes
some steric constraints due to thermal vibrations, which
causes control on effective head group interactions and
give reason to reduce the value of β [24,25].
Rubingh’s model observes the attractive interaction in
the mixed micelle formation. To analyze interaction be-
tween the amphiphiles in a mixed surfactant system at
air/water interface, Rosen model was used. According to
Rosen model the mole fraction of surfactant 1 at the
mixed adsorbed film can be calculated iteratively as [26]:
( )
( )
( )
σ1 12
σ1 12
Xlnln CX 1
1 Xln1X C
 =
where C12, C1 a nd C2 are t he concentration of mixture at a
fixed surface tension value and the concentrations of
indivi d ua l s ur fac t ant s at a f ixe d sur fac e t e nsion value a nd
α1 was the mole fraction of surfactant 1. From this ex-
pre ssio n t he val ue o f
was obtained, which was then
used to evaluate the interaction parameter,
at air/
water interface, with the help of following equation:
( )( )
12 1122
112 2
lnln lnln
β1X 1X
 
 
 
= =
The values o f
are presented in Ta ble 4.
The negative values of
indicate attractive intera c tion.
The activity coefficients (
) were calculated
through Rosen approach within the mixed micelle with
the help of interaction parameters as given below:
( )
{ }
= −
( )
{ }
From Tab le 4, it was ob served that all the systems e x-
hibits positive value of interaction parameter, which
shows antagonistic effect between components of mixed
surfactant s ystem.
3.4. Solubilization by Single Surfactants
To determine the extent of solubilization of PAH in sur-
factant, absorbance of PAH in surfactant solution (of
known concentration) is checked with the help of spec-
trophoto meter. Before examining the so lubilization power
of binary mixtures, single systems were first studied, to
get an idea about the efficiency of gemini in comparison
with co nve ntiona l surfa ctant s. Gr aphs o f the sol ubili ty o f
FLR as a function of the concentration of surfactant are
plotted in Fig ures 4 and 5. Both plots show that on in-
creasing surfactant concentration, concentration of dis-
solved FLR is also increasing, or its solubility increases
linearly with the increasing surfactant concentrations
above CMC [27 ]. This behavio r indicates that sol ubiliza-
tion is related to micellization. Though, the reduced
CMC value does not absolutely represent the increased
solubilizatio n abili t y. Thus, wa ter so lub i lit y enha nc e me nt
of FLR by selected single and equimolar binary surfac-
tant systems was further to evaluate and compare.
Table 3. Micellar mole fraction (
), interaction parameter (β), activity coefficients (f1and f2) values for gemi-
ni/conventional mixed surfactant systems at 30oC
Table 4. Surface composition at air/water interface (
), interaction parameter (
), activity coefficients (f1
and f2
val ue s for gemini-conventional mixed surfactant systems at 30 oC
Copyright © 2013 SciRes. MNSMS
-1 0123 20 30 40 50 60 70
[Fluorene] (mM )
[Surfactant] (mM)
Figure 4. Variation of solubility of FLR with surfactant
concent ration.
-0.20.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
[Fluorene] (mM )
[G 5 ] (mM)
G5-BR IJ35
Figure 5. Variation of solubility of Fluorene with G5 con-
centration in 1: 1 binary surfactant.
A measure of the effectiveness of a surfactant in solu-
bilizing a given solubilizate is the Molar Solubilization
Ratio, ( MSR) which is given by [25, 28-33].
MSR = {[St] [Scmc]} / {CtCMC} (10)
where Scmc and St are the solub ilities at CMC a nd at total
surfactant concentration Ct respectively. Since (Ct - CMC)
was the concentration of the surfactant in the micellar
form, MSR was equal to the ratio of solubilizate concen-
tration in the micelles to the concentration of surfactant
in the form of micelles. Value of MSR is obtained from
the slope of solubilizate concentration versus surfactant
concentration plot.
In the presence of excess FLR, MSR values of both
single and mixed surfactants can be obtained from the
slope of the linearl y fitted line in which the concentration
was plotted against surfactant concentration above the
CMC (both the concentrations were in mM) given in
Figures 4-5. One can obtain the fact that as on increasing
surfactant concentration, FLR concentration is also risen
up, which provides positive value of MSR. The effec-
tiveness of solubilization can also be expressed with the
help of the partition coefficient Km [17,32] which is
defined as distribution of the mole fraction of FLR be-
tween surfactant micelles and the aqueous phase. It may
be calculated as [32]:
Km= Xm / Xa (11)
where Xm and Xa are the mole fraction of F LR in micelle
phase and mole fraction of FLR in aqueous phase. The
quantity Xm can be expressed in terms of MSR, as
Mole fraction of the solute in the aqueous phase was
approximated for dilute solution by:
Xa = SCMC VW (13)
where SCMC is the total appa rent solubilit y of the sol ute at
CMC and VW is the molar volume of water (1.807 * 10-2
L / mol at 30oC). So, the Km expression can be rear-
ranged as:
( )
As observed from Table 4 the MSR and Km values
were highest for cationic surfactant and lowest for anio-
nic and foll ow the o rder as Brij35 > G5 > CTAB > SDS.
The order of solubilizing power for organic solutes by
inner nonpolar core of micelles has been reported to be
nonionic > cationic > anionic surfactant having same
nonpolar chain length [24, 31-32]. For the case of FLR
our observed data support these findings. The difference
in solubilization capabilities of surfactant is because of
their different structures. Higher solubilization power of
Brij35 than G5 and SDS may b e due to its larger micellar
size helping in more micellar core solubilization [22].
The cationic surfactant exhibited lower MSR than nonio-
nic due to limited solubilization at micelle/water inter-
face and core of micelle.
3.5. Solubilization by Equ imo lar B inar y Mix ed
Surfactant Systems
When MSR values were compared for all the mixed sys-
tems, the order was found as: G5-SDS > G5- Brij35 >
G5-CTAB. MSR and log Km values of cationic-nonionic
surfactants were found higher than for cationic-cationic
mixed surfactant solutions as previously reported by Wei
et al. [30].
This is because nonionic micelle processing higher
micellar core solubilization characteristic interpolates to
the micellewater interface producing greater solubiliza-
tion power towards PAHs. In addition, the solubilization
Copyright © 2013 SciRes. MNSMS
Table 5. Molar solubilization ratio (MSR), log Km, the free energy of solubilization (ΔG0s), R, and, B values for FLR sol ubi-
lized in individual and mixed surfactant sys tems at 30.
of PHE and FLR by mixed gemini/nonionic surfactant
soluti ons i s also higher than those in si ngle no nionic sur-
factant. In the intere st of ascertaining the mixing effect of
mixed surfactants on solubilization for PAHs and seeing
the nature of deviation, the deviation ratio (R) between
the MSRexp and the MSRideal can be determined by the
following equation:
MSRideal is the MSR for organic compounds in mixed
surfactant system at the ideal mixed state and can be es-
timated using the MSR of single surfactant solutions
based on the ideal mixing rule:
MSRideal = MSR1 X1 + MSR2 X2 (16)
where X1, X2, MSR1 and M SR2 are the mole fraction and
the molar solubilization ratio for solute of components 1
and 2 in mixed surfactant solutions, respectively. The
data of parameter R from Table 5 obviousl y indi cate tha t
the MSRexp values have positive deviation from ideal
mixture for the gemini/nonionic and gemini/anionic sur-
factant systems meaning that they have positive mixing
effect on solubilizatio n of FLR. Another parameter Km12,
the partition coefficient of a neutral organic solute be-
tween micelles and aqueous phase in a mixed surfactant
has been used by Treiner et al. [32]. This parameter pro-
vides better understanding of the mixing effect of mixed
surfactant systems on solubilization of solutes.
This partition coefficient’s expression is based on the
regular solution approximation as follows:
ln Km12 = m
X ln Km1 + (1 m
X) ln Km2
+ B m
X (1 m
X) (17)
where Km1, Km2 are the micellewater partition
coefficients of individual surfactant solutes constituting
the mixed micelles and
represents the micellar
mole fraction of a surfactant having the value of Km1. B
is an empirical parameter involving both the surfactant–
surfactant and surfactantsolute interactions. If value of
B becomes 0 it means there would be no mi xing effe ct on
partitioning of a solute between aqueous and micellar
phase. Whereas for B > 0 (<0) implies that Km12 in the
mixed surfactant system is larger (smaller) than predicted
by idea l mi xing r ule. As p rese nted in Table 5 , the B val-
ues are found to be negative for all the equimolar binary
mixe d systems.
4. Conclusions
The experimental results obtained in the present study
may be useful for the selection of appropriate mixed sur-
factant systems. This study would also facilitate the de-
sign and optimization o f new surfactant s ystems for their
better performance. The mixed micelles of gemini sur-
factant G5 with all the conventional surfactants i.e.
Brij35, CTAB and SDS are studied. Highest attracting
interaction in mixed micelle formation is observed in
G5-SDS and lowest attracting interaction G5-Brij35,
which indicates good synergism in mixed micelles.
and ad
values are negative in all systems and
show the spontaneity. The values of
are negative
for all mixed systems, demonstrating the stability of the
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