ORIGINAL ARTICLE

Effect of Hydrophobicity and Temperature on the Interactionsin the Mixed Micelles of Triblock Polymers [(EO76PO29EO76)and (EO19PO69EO19)] with Monomeric and Gemini Surfactants

Tarlok Singh Banipal • Ashwani Kumar Sood

Received: 10 October 2013 / Accepted: 29 May 2014

� AOCS 2014

Abstract The interactions in the mixed micelles of two

triblock polymers, F68 (EO76PO29EO76) and P123 (EO19-

PO69EO19), with a series of monomeric (dodecyltrimethyl

ammonium bromide, tetradecyltrimethyl ammonium bro-

mide and cetyltrimethyl ammonium bromide) and gemini

{dimethylene bis(alkyldimethyl ammonium bromide), m-2-

m, where m = 10, 12 and 14} cationic surfactants were

studied by surface tension and viscosity measurements in

aqueous solutions at different temperatures. The mixed

micellar and interfacial properties of the binary mixtures

were analyzed using Clint, Rubingh, Rosen and Maeda

approaches. Both F68 and P123 show weak interactions

with the studied cationic surfactants at 298.15 K which

become favorable (synergistic) at higher temperatures.

Further, the synergistic interactions are more in mixtures of

P123 than F68 at higher temperatures. A comparison of the

effects of number of EO and PO blocks in triblock poly-

mers on various physicochemical parameters of the mixed

micelles has also been made. The unfavorable enthalpy

changes are compensated by favorable entropy changes as

a result of the hydrophobic effect. The relative viscosity

(gr) studies show that the size of the micelles formed by

pure P123 is smaller than those of F68. The values of gr for

the mixed micelles of F68 show significant variation with

chain length of gemini surfactants whereas no such effect is

seen in mixtures with P123.

Keywords Triblock polymer � Monomeric surfactant �Gemini surfactant � Surface tension � Mixed micelles �Relative viscosity

Introduction

The mixed micellar solutions containing more than one type

of surfactant are important due to their better performance in

some applications than single surfactant systems [1–6]. The

mixtures having a triblock polymer (TBP) as one of the

constituents form aggregates with ionic surfactants depend-

ing upon their molecular weight, the block size and the

temperature [7]. A variation in the number of EO or PO

blocks of TBP induces significant changes in the properties

of the mixed micelles. For instance, the increase in molecular

weight of PO blocks provides greater hydrophobicity with

the result that various micellar parameters such as critical

micelle concentration (CMC), cloud point (CP) etc. shift to

lower concentrations [8]. Further, due to the presence of

oxyethylene groups both in PO and EO, these blocks acquire

a certain degree of hydration which is quite significant for

EO rather than PO blocks. Therefore, a change in the

hydration of these groups with temperature influences the

hydrophobicity of TBP micelles and their stability.

Most of the industrial formulations involve non-ionic sur-

face active molecules (including TBP) along with monomeric

ionic surfactants [9–17]. Nowadays, the use of gemini sur-

factants, having two hydrophilic heads and two hydrophobic

tails connected by a spacer, provides superior micellar and

surface properties in comparison to conventional monomeric

ionic surfactants [18–20]. Thus, the mixtures of TBP with

monomeric as well as gemini surfactants form interesting

systems for investigations. Although, there are a few studies

on the mixed micellar behavior of some TBP with ionic

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11743-014-1607-0) contains supplementarymaterial, which is available to authorized users.

T. S. Banipal (&) � A. K. Sood

Department of Chemistry, Guru Nanak Dev University,

Amritsar 143 005, Punjab, India

e-mail: tsbanipal@yahoo.com

123

J Surfact Deterg

DOI 10.1007/s11743-014-1607-0

surfactants using different techniques [9–11, 13, 15, 17], but

no report is available on the surface tension measurements on

these systems as a function of temperature. The CMC of TBP

changes significantly with temperature which in turn affects

the mixed micellar properties of these polymers [21, 22]. In

view of the widespread applications of mixed systems, the

knowledge of interfacial and thermodynamic properties of

such mixtures is of fundamental significance. Therefore, the

interactions in the mixed micelles formed by two TBP, namely

F68 and P123, with series of monomeric and gemini surfac-

tants were studied by surface tension and viscosity measure-

ments at different temperatures. The purpose of choosing these

two TBP is that they posses large difference in hydrophobic/

hydrophilic ratio (0.19 and 1.79 for F68 and P123, respec-

tively). An attempt has also been made to evaluate the effect of

hydrophobic/hydrophilic ratio of TBP and chain length of

ionic surfactants on the physicochemical properties of these

mixtures. The results of the present study have also been

compared with the previous reports on similar systems.

Materials and Methods

Materials

The triblock polymers, with trade names Pluronic� F68

(EO76PO29EO76) and P123 (EO19PO69EO19) used in the

present study with the general formula H[OCH2CH2–]n–

[OCH(CH3)CH2–]m–[–OCH2CH2–]nOH or PEO–PPO–PEO

having average molecular weights of 8,400 and 5,750

respectively, were obtained from BASF India Ltd. Mumbai,

as gift samples. The detailed specifications of these TBP are

given in Table 1. The sources of monomeric and procedure

for the synthesis of gemini surfactants were described in our

earlier study [23]. The monomeric surfactants and TBP were

used as such without any further purification. All the solutions

were prepared in deionized double distilled and degassed

water having specific conductance in the range 1–5 lS cm-1.

Methods

Surface Tension Measurements

The surface tension (c) measurements were carried out by

ring detachment method using a DuNouy tensiometer

(Sumeet Instruments and Chemicals, Kolkata) at different

temperatures [23]. The temperature of the solution was

maintained constant by circulating water through the jacket

surrounding the glass container using thermostat (MV 25F

Julabo, Germany) within ±0.1 K. The tensiometer was

calibrated using double distilled water and the surface

tension value (71.8 ± 0.1 mN m-1) at 298.15 K agreed

well with the literature [14]. The platinum ring used for the

purpose was cleaned using hot chromic acid solution,

rinsed repeatedly with double distilled water and then

heated in an alcohol flame. The concentration of the

solution was varied by aliquot addition of a stock surfactant

solution of known concentration to a known volume of

solvent in the container using a micropipette (Finnpipette

Labsystems, Finland). The surface tension was then mea-

sured after thorough mixing and equilibration. Each read-

ing was taken in triplicate to ensure the reproducibility and

accuracy of measurements.

Viscosity Measurements

An Ubbelohde type suspended level capillary viscometer

was used to determine the relative viscosity (gr) of sur-

factants and their mixtures in aqueous solutions. The

temperature of the viscometer was kept constant by cir-

culating water through the glass jacket surrounding the

viscometer using a thermostat. The ratio of the efflux time

of test solution, t, to that of the reference solvent, to(224.26 s at 298.15 K), was utilized to calculate gr by

considering that the density of dilute solutions remains

constant [14, 16, 22, 24]. For each solution, the average of

three readings was used to calculate gr.

Results and Discussion

Surface Tension Studies

Mixed Micellar Interactions

The CMC values of pure F68 and P123, and their mixtures

with the studied ionic surfactants in aqueous solutions were

obtained from the break in c versus ln[surfactant] plots as

shown in Fig. 1. The observed values of CMC of both F68

and P123 agree well with the literature values [8, 12]. The

Table 1 Molecular characteristic of Pluronic� triblock polymers

Pluronic� MWa Structure Hydrophobic/

hydrophilic ratio

HLBa CPa (1 %) CMC (mM)

F68 8,400 EO76PO29EO76 0.19 [24 >100 �C 1.40 [12]

P123 5,750 EO19PO69EO19 1.79 [12 90 �C 0.09 [8]

a Average molecular weights (MW), HLB and cloud point (CP) values are as per specifications by the manufacturer

J Surfact Deterg

123

CMC values of pure monomeric/gemini surfactants were

taken from our earlier study [23]. The ideal CMC of the

binary mixtures (Ci) was calculated on the basis of the

Clint equation [25] at different mole fractions of mono-

meric/gemini surfactant (a) as follows:

1=Ci ¼ a=C1 þ 1� að Þ=C2 ð1Þ

where C1 and C2 represent the CMC of monomeric/gemini

and TBP, respectively. The experimental (Cm) and ideal

(Ci) CMC thus determined for the mixtures of F68 with

monomeric/gemini surfactants at 298.15 K only are given

in Table 2. The CMC data at higher temperatures and for

the mixtures with P123 at different temperatures are

included in Supplementary Information (Tables S1 to S10).

In mixtures of DTAB with F68, the lower Cm in compar-

ison to Ci at 298.15 K shows favorable interactions

(Cm \ Ci) between the two components. This can be

related to the reduction in head group repulsions of the

DTAB molecule as a result of incorporation of F68 because

the hydrophilic PEO chains longer than PPO chains in F68

induce higher hydration in the stern layer and thus reduce

polar head group repulsions [26–28]. In mixtures with

longer chain length monomeric surfactants, i.e. TTAB and

CTAB, although the interactions are favorable at low mole

fractions but become unfavorable slightly (Cm [ Ci) at

higher mole fractions of F68 (Table 2). The excessive

hydration at higher concentration of F68 causes the longer

hydrophobic tail of the monomeric surfactant to be inclu-

ded in a medium more polar than that of the hydrophobic

micellar core of CTAB and TTAB. Moreover, both tri-

methyl ammonium head groups and bromide counterions

are immersed in a medium less polar than pure water as a

consequence of orientation of the water molecules in the

vicinity of oxygen atoms in the PEO chains. Brigante and

Schulz [17] have reported similar observations in mixtures

of long chain cationic surfactant cetyltrimethyl ammonium

tosylate with F68 at higher mole fractions of F68. The

mixed micelles of F68 with 10-2-10/14-2-14 exhibit almost

similar mixing behavior whereas unfavorable mixing was

observed in the case of 12-2-12 throughout the mole fac-

tion range.

The mixtures of P123 and monomeric surfactants indi-

cate unfavorable mixing behavior except at a = 0.903 in

the case of CTAB (Table S5, Supplementary Information).

Similar results are obtained for gemini surfactants 10-2-10/

12-2-12, although a favorable mixing is observed with

longer chain 14-2-14. This shows that P123, having much

larger PPO than PEO chain, induces incompatibility with

smaller hydrophobic chain cationic surfactants in mixed

micellar core, resulting in unfavorable mixing [28]. How-

ever, an increase in temperature reduces the unfavorable

mixing or enhances the favorable mixing as the case may

be, for different mixtures with both TBP. It is due to

dehydration of TBP molecules which strengthens the

hydrophobic environment for mixed micelle formation

[22]. Besides, a much steeper fall in the CMC of P123 than

that of F68 with an increase in temperature can be

explained on the basis of number of EO in comparison to

PO blocks in each case. The former having a lower number

of EO blocks, induces less hydration of micelles than latter,

resulting in greater dehydration even with small increase in

temperature [21].

The quantitative interpretation of these results can be

made by evaluating the mixed micellar mole fraction of

monomeric/gemini surfactant (X1) and interaction param-

eter (b) from Regular Solution theory [29]

X21 ln Cma=C1X1ð Þ

� �=f 1� X1ð Þ2ln Cm 1� að Þ=C2 1� X1ð Þð Þg ¼ 1

ð2Þ

b ¼ ln Cma=C1X1ð Þ= 1� X1ð Þ2 ð3Þ

The activity coefficients f1 and f2 of surfactants 1 and 2 are

related to b as

f1 ¼ exp b 1� X1ð Þ2n o

ð4Þ

f2 ¼ expfbX21g ð5Þ

The magnitude of b is a measure of the interactions

between the two components in the mixed micelles. Neg-

ative b values indicate attractive interactions (synergism)

while positive values show repulsive interactions (antago-

nism) between the two components. The b values for all

the studied mixtures were calculated at different a values

(data not given). However, in order to understand the effect

of hydrophobicity and temperature on these mixed micellar

systems, the average values of b (bavg) have been reported

[21, 22]. The negative magnitude of bavg values in mixtures

of F68 and DTAB at 298.15 K indicates weak synergistic

interactions (Fig. 2). These synergistic interactions do not

exhibit any significant variation (within the experimental

20

30

40

50

60

70

-10 -9 -8 -7 -6 -5 -4

α = 0.000 = 0.156 = 0.356 = 0.480 = 0.625 = 0.787 = 0.903 = 1.000

γ/ m

N m

-1

ln[surfactant]

Fig. 1 Plot of surface tension (c) vs ln[surfactant] for F68 and 10-2-

10 binary mixtures at different mole fractions of 10-2-10 (a) at

298.15 K

J Surfact Deterg

123

Table 2 Experimental and ideal CMC (Cm, Ci), surface excess

concentration (Cmax), experimental and ideal values of minimum area

per molecule (Amin, Aminid ), CMC and surface concentration ratio

(Cm/C20) and surface pressure at CMC (PCMC) of the binary mixtures

of F68 with monomeric/gemini surfactants as a function of mole

fraction of monomeric/gemini surfactant (a) at 298.15 K

a Cm

(mmol dm-3)

Ci

(mmol dm-3)

Cmax 9 106

(mol m-2)

Amin

(A2 molecule-1)

Aminid

(A2 molecule-1)

Cm/C20 PCMC

(mN m-1)

DTAB

0.000 1.240 (1.40)a 1.240 0.93 178.07 2.21 29.59

0.156 1.315 1.446 1.87 088.77 170.83 3.75 30.65

0.356 1.647 1.754 2.86 057.96 167.25 4.33 30.89

0.480 2.041 2.234 4.05 041.00 162.24 4.84 31.65

0.625 2.638 2.909 4.50 036.91 160.09 5.67 31.45

0.787 3.714 4.472 3.35 049.51 152.21 6.04 32.20

0.903 6.128 7.671 2.87 057.12 145.76 6.45 30.98

1.000 16.441b 16.441 1.56 106.37 2.69 29.35

TTAB

0.000 1.240 1.240 0.93 178.07 2.21 28.64

0.156 1.448 1.381 1.23 134.13 169.01 3.00 28.95

0.356 1.517 1.552 1.60 103.22 168.48 3.25 29.97

0.480 1.751 1.827 1.86 089.17 163.19 3.36 30.28

0.625 1.984 2.089 1.92 086.96 156.85 2.97 31.75

0.787 2.417 2.545 1.74 095.33 149.45 3.24 29.01

0.903 2.867 3.034 1.55 106.81 142.05 3.17 29.84

1.000 3.632b 3.632 1.32 125.13 2.70 29.59

CTAB

0.000 1.240 1.240 0.93 178.07 2.21 28.64

0.156 1.194 1.163 1.12 148.02 165.98 2.83 28.99

0.356 1.114 1.097 1.50 110.29 160.20 2.36 30.36

0.480 1.037 1.027 1.11 149.43 153.53 3.08 29.82

0.625 0.901 0.976 1.18 139.77 149.52 2.34 29.30

0.787 0.815 0.927 1.33 123.95 144.63 1.84 30.80

0.903 0.792 0.894 1.16 142.17 141.07 1.42 30.20

1.000 0.867b 0.867 1.24 133.51 2.64 27.29

10-2-10

0.000 1.240 1.240 0.93 178.07 2.21 29.59

0.156 1.587 1.417 1.41 117.58 167.22 6.87 30.88

0.356 1.925 1.652 1.84 090.26 165.68 9.25 30.67

0.480 2.246 2.046 1.97 084.09 159.52 8.41 31.65

0.625 2.417 2.560 2.01 082.40 150.28 8.91 30.26

0.787 2.998 3.458 1.72 096.18 139.50 9.86 30.99

0.903 3.967 4.726 1.76 094.00 131.03 11.14 31.80

1.000 6.960b 6.960 1.64 101.00 14.90 31.13

12-2-12

0.000 1.240 1.240 0.93 178.07 2.21 28.64

0.156 1.453 1.176 2.42 068.60 154.48 3.88 29.78

0.356 1.346 1.116 2.62 063.32 143.93 3.93 30.32

0.480 1.221 1.056 2.95 056.28 132.58 3.80 30.36

0.625 1.097 1.012 2.07 080.01 123.66 4.15 30.46

0.787 1.010 0.965 2.17 076.23 116.36 4.74 30.78

0.903 0.947 0.935 1.71 096.93 111.49 5.44 29.80

1.000 0.910b 0.910 1.38 120.06 7.33 30.95

J Surfact Deterg

123

uncertainties) with the increase in chain length (m) of

monomeric surfactants whereas the mixtures of 10-2-10/

12-2-12 gemini surfactants show almost ideal behavior.

The synergistic interactions in mixtures of monomeric

surfactants are due to effective neutralization of the polar

head group repulsions upon intercalation of F68. However,

replacing monomeric by gemini surfactants induces steric

hindrance in the stern layer of the mixed micelles which

reduces the degree of synergism [28]. It has been reported

that the presence of a supporting electrolyte such as KCl

enhances these synergistic interactions [30].

On the other hand, the synergistic interactions are

comparatively stronger in mixtures of more hydrophobic

P123 and gemini than with monomeric surfactants at

298.15 K (Fig. 3). It indicates that the hydrophobic inter-

actions play a dominant role in mixed micelles of TBP with

larger PPO than PEO chains. Similar results were obtained

for the interactions of P103 and L64 (hydrophobic/hydro-

philic ratio = 2.30 and 1.15 respectively) with ionic

surfactants [28, 31]. Therefore, in order to better under-

stand the effect of hydrophobicity of TBP on their mixed

micellar behavior, bavg obtained from the present and

previous studies on similar systems have been plotted in

Fig. 4 as a function of the hydrophobic/hydrophilic ratio of

TBP. When the ratio is less than unity, the interactions with

monomeric surfactants are synergistic and show a mini-

mum whereas those of gemini surfactants do not exhibit

any particular trend. At ratios greater than unity, these

interactions shift from synergistic to antagonistic with the

increase in ratio for all the mixtures except 14-2-14. Fur-

ther, the smaller the chain length of the studied ionic sur-

factant, the higher is the antagonism. The interactions in

the mixed micelles involving gemini surfactant (14-2-14),

having longer hydrophobic chains, are synergistic irre-

spective of the hydrophobicity of the TBP.

Although both F68 and P123 show weak interactions

with ionic surfactants at 298.15 K, these interactions are

Table 2 continued

a Cm

(mmol dm-3)

Ci

(mmol dm-3)

Cmax 9 106

(mol m-2)

Amin

(A2 molecule-1)

Aminid

(A2 molecule-1)

Cm/C20 PCMC

(mN m-1)

14-2-14

0.000 1.240 1.240 0.93 178.07 2.22 28.64

0.156 0.741 0.597 1.28 129.21 128.94 2.66 30.80

0.356 0.352 0.386 1.39 118.89 121.80 3.48 30.83

0.480 0.255 0.278 1.33 124.50 113.77 3.22 30.58

0.625 0.218 0.226 1.89 087.79 109.31 3.96 30.38

0.787 0.170 0.187 1.98 083.86 104.85 7.08 29.99

0.903 0.151 0.165 1.95 084.92 095.04 8.88 30.61

1.000 0.151b 0.151 1.87 096.89 10.06 29.98

a Ref. [12]b Ref. [23]

-6

-5

-4

-3

-2

-1

0

1

2

10 12 14 16m

βav

g

Fig. 2 Variation of bavg values with chain length for the binary

mixtures of F68 with monomeric (dotted lines) and gemini (full lines)

surfactants at (filled diamonds) 298.15, (filled squares) 308.15 and

(filled triangles) 318.15 K

-14

-12

-10

-8

-6

-4

-2

0

2

10 12 14 16m

βav

g

Fig. 3 Variation of bavg values with chain length for the binary

mixtures of P123 with monomeric (dotted lines) and gemini (full

lines) surfactants at (filled diamonds) 298.15, (filled squares) 308.15

and (filled triangles) 318.15 K

J Surfact Deterg

123

relatively more synergistic in the case of the former than

the latter as explained earlier. Further, the mixed micellar

core of TBP and ionic surfactants consist of weakly

hydrated PPO chains of TBP and the hydrophobic part of

the ionic component. As the temperature of the system is

increased, it causes dehydration of PPO chains, making the

core more hydrophobic [21]. This results in an increase in

synergistic interactions among all the studied mixtures. In

addition, the synergism becomes more in mixtures of TBP

having large PPO chains, i.e. P123 as compared to F68

(Figs. 2, 3). However, the synergistic interactions decrease

with increases in m for the mixtures with P123, whereas in

the case of F68, these interactions decrease with an

increase in m from 10 to 12 and no specific trend is fol-

lowed with further increases in m.

In order to evaluate the contribution of each surfactant

in the mixed micelles, the mole fraction of the ionic sur-

factant in its ideal state (Xi) was also calculated using the

Motomura equation [32].

Xi ¼ aC2= aC2 þ 1� að ÞC1f g ð6Þ

In mixtures of F68 with DTAB at 298.15 K, the higher

X1 as compared to Xi indicate that mixed micelles are

richer in the DTAB component (Fig. 5a). The difference

between X1 and Xi decreases as shown in Fig. 5b, c

indicating that the relative contribution of ionic com-

ponent decreases (especially in ionic component rich

region) or that of F68 increases with the increase in the

hydrophobic chain length of the ionic surfactant in the

mixed micelles. Similar results have been reported in

mixtures of P105 and L64 with these surfactants [23,

31]. For P123 and 10-2-10/DTAB/TTAB mixtures, X1

values could not be calculated. However, X1 values

were more than Xi for the remaining mixtures except in

a mixture with 14-2-14 for a [ 0.625 (figures not

shown). The contribution of F68 in the mixed micelles

decreases (Fig. 6) whereas that of P123 remains almost

the same at higher temperatures. An increase in the

contribution of P105 in mixed micelles with these ionic

surfactants with temperature has also been reported

[23]. These observations indicate that the mole fraction

of TBP in the mixed micelles depend upon the hydro-

phobicity as well as temperature of the mixture.

-6

-4

-2

0

2

4

6

0 0.5 1 1.5 2 2.5

Ratio

βav

gDTAB

TTAB

CTAB

10-2-10

12-2-12

14-2-14

Fig. 4 Effect of hydrophobic/hydrophilic ratio on bavg for the mixed

micelles of TBP with monomeric (dotted lines) and gemini surfac-

tants (full lines) at 298.15 K. The bavg at ratios 0.46, 0.76, 1.15 and

2.23 were taken from Ref. [23, 28, 31]

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

α

X1,

Xγ 1,

Xi

(a)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

α

X1,

Xγ 1,

Xi

(b)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

α

X1,

Xγ 1,

Xi

(c)

Fig. 5 Variation of (filled diamonds) X1, (filled squares) X1c and

(filled triangles) Xi vs a for the binary mixture of F68 with a DTAB,

b TTAB and c CTAB at 298.15 K

J Surfact Deterg

123

Interfacial Interactions

The values of the interfacial molecular interaction param-

eter (bc) and the mixed micellar mole fraction of mono-

meric/gemini surfactant at interface (X1c) were obtained by

Rosen equation [33]

fXc21 ln C12a=Co

1Xc1

� �g=f 1�X

c1

� �2ln C12 1�að Þð =Co

2 1�Xc1

� ��g¼1

ð7Þ

where C1o, C2

o and C12 are the molar concentrations in the

solution phase of surfactant 1 (monomeric/gemini), sur-

factant 2 (TBP) and their mixtures, respectively. The val-

ues of X1c can be used to calculate the interfacial molecular

interaction parameter (bc) as

bc ¼ ln C12a=Co1X

c1

� �= 1� X

c1

� �2 ð8Þ

In mixtures of F68 with DTAB at 298.15 K, the X1c, X1 and

Xi increase with a and follow the order X1c [ X1 [ Xi

(Fig. 5a). Further, the higher mole fraction of ionic com-

ponent than that of nonionic TBP can be explained by

considering the relative area occupied by each component

at the interface. The ionic surfactant, having a single

hydrophilic head group, will be adsorbed almost vertically

on the interface and will occupy a smaller area, whereas in

the case of TBP, the hydrophobic portion (PO) of the

molecule between the hydrophilic groups (EO) tend to lie

flat at the interface and hence, the area occupied is much

higher. Thus the concentration of ionic surfactant will be

more than that of TBP at the interface. Similar results were

obtained in all the mixtures involving P123 (except with

14-2-14). The contribution of DTAB increases with tem-

perature as shown in Fig. 6 and is more in mixtures

involving P123 than F68. With the increase in the chain

length of the monomeric surfactant in the mixture, the

positive deviation of X1c and X1 with reference to Xi

decreases and become negative in the case of CTAB at

higher a values (Fig. 5c).

The calculated values of bc (Eq. 8) are negative indi-

cating synergism in mixed monolayer formation. The

average values of bc (bavgc ) are more negative in compari-

son to bavg (Fig. S1, Supplementary Information) sug-

gesting that the interactions at the air/solution interface are

stronger than in mixed micelles. It is due to greater diffi-

culty of accommodating the hydrophobic part of the ionic

component in the interior of a curved micelle as compared

to a planar interface [34]. This results in a larger reduction

in electrostatic repulsion energy at the interface than in the

micelles. Moreover, bavgc among the studied TBP mixtures

follow the order: P123 [ P105 [23] [ F68.

The extent of adsorption at the air–water interface or the

surface excess concentration (Cmax) and minimum area per

molecule (Amin) were calculated as

Cmax ¼ �1=nRT dc=d ln surfactant½ �ð Þ ð9Þ

Amin ¼ 1020=NACmax ð10Þ

where the symbols have their usual meaning [18]. The

value of n is taken as 2 for monomeric/gemini surfactants

and their mixtures with TBP and n = 1 for pure TBP [23].

The lower Cmax for pure F68 than for P123 is because F68

being bigger in size, occupies more surface area and

therefore, would be present at a lower concentration. The

Cmax values for the mixed micelles of ionic surfactants with

F68 are greater than that of pure F68 indicating that these

mixtures possess a greater adsorption tendency at the

interface. Alternately, the lower Amin is the result of a

reduction in electrostatic repulsions among the ionic head

groups due to intercalation of nonionic component and as a

consequence, the adsorbed monolayer attains closer

molecular packing. Besides, the lower Cmax in mixtures of

monomeric surfactants with P123 than that in mixtures

with gemini surfactants indicates comparatively less com-

pact mixed micelles [34]. With the increase in the hydro-

phobic chain length of the ionic surfactant, the

hydrophobicity of the molecule increases and a larger

number of molecules tend to be adsorbed at the interface,

thus increasing Cmax (decreasing Amin) [35]. The effect of

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1α

X1,

Xγ 1,

Xi

(a)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1α

X1,

Xγ 1,

Xi

(b)

Fig. 6 Variation of (filled diamonds) X1, (filled squares) X1c and

(filled triangles) Xi vs a for the binary mixture of F68 with DTAB at

a 308.15 and b 318.15 K

J Surfact Deterg

123

temperature on Cmax can be explained by taking into

consideration two thermally controlled factors. Firstly, the

dehydration of the hydrophilic head group and secondly the

thermal motion of the adsorbed molecules. The relative

magnitude of these two opposing effects determines whe-

ther Cmax increases or decreases with temperature. The

Cmax values for pure F68, P123 and their mixtures with the

studied surfactants increase whereas that of a pure ionic

surfactant decreases with temperature. This indicates that

in mixed micelles possessing nonionic TBP, the dehydra-

tion effect shrinks the head group size and increases the

hydrophobicity of the molecules which overshadows the

effect of thermal motions in the adsorbed molecules [36].

Therefore, the adsorbed molecules become closely packed

in the monolayer and hence Cmax increases with

temperature.

The surface area per head group at the ideal mixing

conditions (Aminid ) is given by Eq. (11)

Aidmin ¼ X

c1A1 þ 1� X

c1

� �A2 ð11Þ

where A1 and A2 are the minimum surface areas of pure

monomeric/gemini surfactant and TBP respectively. Aminid

values in the mixtures with F68 decrease with an increase

in a whereas an increase is observed in the case of P123.

Moreover, the Amin values are less than Aminid for all the

mixtures with P123 except DTAB and TTAB showing that

the area occupied by these mixed micelles is smaller than

expected ideally. Similar results were observed for all the

mixtures with F68 but the difference between Aminid and

Amin is much larger compared to that in P123. Also, this

difference decreases with increases in chain length of ionic

surfactants in mixtures with F68 although no significant

variation is observed in mixtures with P123. An increase in

the difference of chain length in mixtures with P105 has

also been reported [23]. This comparison shows that the

hydrophobicity of the TBP plays important role in mixed

monolayer formation at the interface. With the increase in

temperature, the difference decrease in mixtures with F68

(Tables S1 and S2, Supplementary Information), however,

in mixtures with P123, it decreases in the P123 rich region

and increases in the ionic surfactant rich region probably

owing to electrostatic repulsions [37].

The relative effect of some structural or micro envi-

ronmental factors on micellization and adsorption can be

determined by Cm/C20 ratio, where C20 is the concentration

of the surfactant in the bulk phase that produces a reduction

of 20 mN/m in the surface tension of solvent. The higher

values of Cm/C20 in mixtures with gemini surfactants

indicate relative easier adsorption at the air-solution

interface. Also, the higher Cm/C20 ratio in mixtures with

F68 than P123 is probably due to the fact that the mixed

micellar core of the mixtures containing F68 is less

hydrophobic than that of P123 resulting in higher Cm val-

ues. The decrease in this ratio with increasing temperature

is because the size of the hydrophobic group decreases as a

result of dehydration.

Surface pressure at the CMC (PCMC) has also been

evaluated using PCMC = co - cCMC, where co and cCMC

are the surface tension values of the solvent and the mix-

ture at the CMC respectively. The PCMC values (Table 2)

are higher in mixtures with F68 than in pure F68 whereas

lower values are observed in mixtures with P123 than pure

P123. In pure F68, the presence of large hydrophilic groups

containing electronegative oxygen result in repulsive for-

ces at the aqueous solution-air interface causing close

packing of hydrophobic groups at the interface (small

Cmax) and relatively small Cm/C20 ratio, thus producing

smaller PCMC values. These values are not affected by the

increase in chain length of ionic surfactants. PCMC values

show a slight decrease with temperature in mixtures with

P123 whereas these remain almost same in mixtures with

F68.

Thermodynamics of Adsorption and Mixed

Micellization

The standard Gibbs energy of micellization (DGmo ) and

standard free energy of adsorption (DGadso ) were calculated

as:

DGom ¼ RT � ln XCMCð Þ ð12Þ

DGoads ¼ DGo

m �PCMC=Cmax ð13Þ

where XCMC is the CMC in the mole fraction units. The

more negative values of DGmo in mixed micelles of P123

(Table S6, Supplementary Information) in comparison to

those in F68 (Table 3) shows that the hydrophobicity of the

TBP plays an important role in micellization. The magni-

tude of these values decrease slightly with increases of a in

mixtures with monomeric surfactants although remain

almost constant in the case of gemini surfactants. In

addition, DGmo decreases with increases in the hydrophobic

chain length of monomeric/gemini surfactants as a result of

a decrease in the CMC of these mixtures. Further, the more

negative DGadso as compared with DGm

o for all the mixtures

implies that the adsorption of mixed systems at the air/

water interface is more spontaneous than micelle formation

due to the hydrophobic portion of the interacting species,

which leads them towards the air/water interface [14]. The

difference between these values indicates that the work

needed to transfer the surfactant molecule from the surface

to the mixed micellar core. As the temperature increases,

the work required decreases and hence the process of

mixed micelles formation becomes more favorable.

J Surfact Deterg

123

The enthalpy and entropy of micellization, DHmo and

DSmo were calculated as follows:

DHom ¼ �RT2d=dT ln XCMCð Þ½ � ð14Þ

DSom ¼ DHo

m � DGom

� �=T ð15Þ

The positive values of DHmo in all the binary mixtures

indicate that the process of mixed micellization involving

TBP is endothermic in nature. The destruction of the high

degree of hydrogen bonding in the icebergs around the

alkyl chains results in positive enthalpy change [36]. These

Table 3 Standard free energy

(DGmo ), enthalpy and entropy

(DHmo , DSm

o ) of micellization,

adsorption and excess free

energy (DGadso , DGex

o ), Maeda’s

interaction parameter and free

energy (B1, DGMaedao ) of binary

mixtures of F68 with

monomeric/gemini surfactants

as a function of mole fraction of

monomeric/gemini surfactant

(a) at 298.15 K

a DGmo

(kJ mol-1)

DHmo

(kJ mol-1)

DSmo

(J mol-1 K-1)

DGadso

(kJ mol-1)

DGexo

(kJ mol-1)

B1 DGMaedao

(kJ mol-1)

DTAB

0.156 -26.40 095.93 410.30 -40.10 -0.49 -5.01 -17.65

0.356 -25.84 101.69 427.76 -36.62 -0.15 -3.56 -17.19

0.480 -25.31 104.21 434.40 -33.12 -0.29 -3.46 -18.06

0.625 -24.67 104.95 434.75 -31.66 -0.32 -3.57 -18.95

0.787 -23.82 99.55 413.81 -33.42 -0.50 -3.44 -19.93

0.903 -22.58 83.29 355.11 -39.05 -0.52 -5.01 -17.65

TTAB

0.156 -26.16 98.04 416.56 -49.55 – -1.07 -16.58

0.356 -26.04 93.12 399.69 -44.67 – -1.07 -16.58

0.480 -25.69 92.06 394.93 -41.95 -0.10 -1.28 -17.38

0.625 -25.38 89.42 385.06 -44.88 -0.17 -1.37 -17.77

0.787 -24.89 81.29 356.16 -42.44 -0.17 -1.36 -18.23

0.903 -24.46 66.51 305.16 -43.66 -0.18 -1.44 -18.71

CTAB

0.156 -26.63 94.60 406.64 -52.48 – 0.35 -16.58

0.356 -26.81 89.43 389.86 -46.98 – 0.35 -16.58

0.480 -26.98 82.03 365.67 -53.82 – 0.35 -16.58

0.625 -27.33 68.29 320.73 -52.00 -0.28 -0.17 -16.27

0.787 -27.58 52.84 269.76 -50.58 -0.40 -0.56 -16.31

0.903 -27.65 32.52 201.83 -53.51 -0.46 -1.12 -16.30

10-2-10

0.156 -25.93 103.47 434.05 -47.80 – -1.72 -16.58

0.356 -25.45 107.16 444.81 -42.12 – -1.72 -16.58

0.480 -25.07 105.68 438.57 -41.10 0.15 -0.65 -16.69

0.625 -24.89 96.82 408.21 -39.91 -0.12 -1.98 -17.77

0.787 -24.35 98.29 411.38 -42.30 -0.40 -2.38 -18.78

0.903 -23.66 84.99 364.43 -41.66 -0.49 -2.55 -19.61

12-2-12

0.156 -26.15 99.03 419.88 -49.32 – 0.31 -16.58

0.356 -26.34 90.16 390.77 -48.05 0.47 1.33 -15.92

0.480 -26.58 80.56 359.35 -49.35 0.65 1.36 -15.56

0.625 -26.84 67.25 315.63 -42.95 0.54 1.31 -15.52

0.787 -27.05 49.25 256.08 -42.60 0.51 1.77 -15.44

0.903 -27.21 33.63 204.06 -42.45 0.23 1.75 -15.64

14-2-14

0.156 -27.82 75.38 346.16 -51.50 0.97 3.67 -13.00

0.356 -29.66 47.00 257.20 -46.67 -0.35 1.36 -13.08

0.480 -30.46 33.48 214.48 -48.08 -0.37 0.97 -12.58

0.625 -30.85 26.31 191.72 -45.96 -0.25 0.87 -12.08

0.787 -31.47 16.09 159.52 -42.82 -0.39 0.31 -12.29

0.903 -31.76 10.35 141.24 -42.85 -0.43 -0.28 -12.22

J Surfact Deterg

123

values decrease with increase in chain length of ionic

surfactant although increase with temperature. The increase

in DHmo with temperature suggests that large numbers of

hydrogen bonds with water molecules are broken during

mixed micellization. Further, these values follow the

sequence P105 [23] [ P123 [ F68 among the studied

binary mixtures of TBP. In mixed micelles involving TBP,

the unfavorable enthalpy effect is overcome by an even

stronger entropy effect given by a large positive DSmo as a

result of the hydrophobic effect [7]. The higher DSmo values

in the case of P123 in comparison to F68 mixtures is

probably a result of the organization of a greater number of

F68 molecules from randomly oriented monomers to well-

organized micelle structures.

The values of the excess free energy of micellization,

DGex, were calculated using Eq. (16)

DGex ¼ X1 ln f1 þ 1� X1ð Þ ln f2½ � � RT ð16Þ

where the activity coefficients f1 and f2 of monomeric/

gemini and TBP are given by Eqs. (4) and (5) respectively.

The negative DGex indicate thermodynamic stability of

mixed micelles whereas positive values show unstable

mixed micelles (Table 3). The more negative DGex in

mixtures with F68 with respect to P123 at 298.15 K (Table

S6, Supplementary Information) is due to a greater

reduction in ionic head group repulsions in the former as

discussed earlier. These values decrease with increasing

temperature indicating enhanced stability.

As per Maeda [38], the stability of mixed micelles is

contributed to not only by b but also by another parameter

B1. The free energy of micellization (DGMaedao ) as a func-

tion of ionic component in the mixed micelle (X1) is

therefore given by Eq. (17)

DGoMaeda ¼ RT B0 þ B1X1 þ B2X2

1

� �ð17Þ

where B0 = ln C2 B1 ? B2 = ln C1/C2 B2 = -b B1 and

DGMaedao values thus calculated are given in Table 3. The

parameter B1 consists of two contributions: interactions

between hydrophilic head groups and interactions between

hydrophobic chains. The negative magnitudes of B1 values

are due to a reduction in head group repulsions as a result

of intercalation of TBP and stronger hydrophobic chain–

chain interactions which help in the net stability of mixed

micelles. The positive values of B1 in mixtures of F68 with

12-2-12/14-2-14 at 298.15 K can be explained by the rigid

structure of these gemini surfactants causing a lower

reduction in head group repulsions leading to weakening of

chain–chain interactions. The positive values of B1 have

also been attributed to a contribution from short range

interactions [38]. These B1 values increase with the chain

length of the ionic component in the mixture. Moreover,

the more negative B1 in mixtures with P123 (Table S6,

Supplementary Information) in comparison to F68 shows

that the hydrophobicity of the TBP has a significant effect

on these interactions. The negative magnitude of B1

increases with temperature due to enhanced hydrophobic

interactions. Since DGMaedao values were obtained by con-

sidering the head group repulsions as well as hydrophobic

attractions, they are less negative in comparison with DGmo

[39]. The negative magnitude of DGMaedao increase with

temperature for all the mixtures and is unaffected by the

increase in the chain length of the ionic component.

Viscosity Studies

In order to further explore the influence of hydrophobicity

of TBP on mixed micelle formation, the viscometric

studies of all the mixtures were also carried out. Apart

from this, these studies are also expected to throw light

on the effect of chain length of ionic surfactant on mixed

micellar behavior. Therefore, we measured the relative

viscosity (gr) of all the binary mixtures at the same mole

fractions as were used during surface tension studies. The

extent of interactions between TBP and the studied ionic

surfactants accounts for the variation in gr of the mixed

micelles formed. For instance, the gr values of pure F68

and its mixed micelles are more than that in P123. This

indicates that the size of the micelles formed is affected

by the PEO chain length of the TBP. The aggregates

formed by mixtures containing F68 may be attributed to

two reasons: Firstly, the aggregates containing longer

PEO chains may have larger hydrophilic domains. Sec-

ondly, less compact aggregates are formed due to steric

resistance of the longer PEO chains [40]. The gr values in

mixed micelles of F68 with DTAB are almost same as

that of pure F68. This shows that the presence of nonionic

component reduces the electrostatic repulsions among the

head groups of DTAB thus keeping the gr constant.

Similar variation in gr with a has been observed in

mixtures with TTAB. However, in mixtures with CTAB,

the gr values remain almost same in the F68 rich region

(a\ 0.40) but start decreasing at higher a values indi-

cating a decrease in size of the micellar aggregates (Fig.

S2, Supplementary Information). In case of mixtures with

gemini surfactants, the gr for F68 with 10-2-10 mixtures

at different temperatures show two mixing regions

(Fig. 7a). The first one occurs at around a & 0.38 where

the hydrophobic interactions take place between the two

surfactants. The second is at around a & 0.80 where

electrostatic repulsions originate due to coulombic inter-

actions between the charged head groups [41]. The effect

due to electrostatic repulsions occurs at lower a values

with the increase in chain length of gemini surfactant i.e.

12-2-12 (Fig. S3, Supplementary Information). However,

J Surfact Deterg

123

in mixtures with 14-2-14, the hydrophobic interactions

dominate the head group repulsions; hence gr values are

much lower than that in pure F68 micelles.

In mixtures of P123 with DTAB, the gr are smaller

than those of either of the two pure components

(Fig. 7b). It indicates that the size of the mixed micelles

formed is smaller due to repulsive interactions between

the two components causing an increase in fluidity [42].

The origin of such behavior can be ascribed on the basis

of large number of PO blocks which causes poor solu-

bility of P123 in the aqueous phase and at the same

time, it is also expected that these blocks may create

greater steric hindrance in the mixed state. Similar

variation in gr was observed in mixtures of P123 with

TTAB/10-2-10/12-2-12. However, in mixtures with

longer chain CTAB/14-2-14, the gr at all mole fractions

are much lower than that of pure P123 although quite

close to pure CTAB/14-2-14 indicating dominance of

hydrophobic interactions. These observations are in line

with the results from surface tension studies. Moreover,

the gr for all the studied mixtures decreases with

increases in temperature due to dehydration of the PPO

chains making the core more compact.

References

1. Chavda S, Bahadur P, Aswal VK (2011) Interaction between

nonionic and gemini (cationic) surfactants: effect of spacer chain

length. J Surfact Deterg 14:353–362

2. Parekh P, Varade D, Parikh J, Bahadur P (2011) Anionic–cationic

mixed surfactant systems: micellar interaction of sodium dodecyl

trioxyethylene sulfate with cationic gemini surfactants. Colloids

Surf A 385:111–120

3. Naous M, Aguiar J, Ruiz CC (2012) Synergism in mixtures of n-

octyl-b-D-thioglucoside and different n-alkyltrimethylammonium

bromides: effect of the alkyl chain length. Colloid Polym Sci

290:1041–1051

4. Cirin DM, Posa MM, Krstonosic VS (2012) Interactions between

sodium cholate or sodium deoxycholate and nonionic surfactant

(Tween 20 or Tween 60) in aqueous solution. Ind Eng Chem Res

51:3670–3676

5. Martın VI, Rodrıguez A, Lopez-Cornejo P, Moya ML (2013)

Role of the spacer in the non ideal behavior of alkanedyil-

bis(dodecyldimethyl ammonium) bromide-MEGA10 binary

mixtures. Colloids Surf A 418:139–146

6. Nandni D, Mahajan RK (2013) Micellar and interfacial behavior

of cationic benzalkonium chloride and nonionic polyoxyethylene

alkyl ether based mixed surfactant systems. J Surfact Deterg

16:587–599

7. Naskar B, Ghosh S, Moulik SP (2012) Solution behavior of

normal and reverse triblock copolymers (Pluronic L44 and 10R5)

individually and in binary mixture. Langmuir 28:7134–7146

8. Bakshi MS, Bhandari P (2007) Fluorescence studies on the non-

ideal mixing in triblock copolymer binary mixtures under the

effect of temperature: a block hydration effect. J Photochem

Photobiol A Chem 186:166–172

9. Singh PK, Kumbhakar M, Ganguly R, Aswal VK, Pal H, Nath S

(2010) Time-resolved fluorescence and small angle neutron

scattering study in pluronics-surfactant supramolecular assem-

blies. J Phys Chem B 114:3818–3826

10. Thummara AD, Sastry NV, Verma G, Hassan PA (2011) Aque-

ous block copolymer–surfactant mixtures—Surface tension, DLS

and viscosity measurements and their utility in solubilization of

hydrophobic drug and its controlled release. Colloids Surf A

386:54–64

11. Torcello-Gomez A, Maldonado-Valderrama J, Jodar-Reyes AB,

Foster TJ (2013) Interactions between pluronics (F127 and F68)

and bile salts (NaTDC) in the aqueous phase and the interface of

oil-in-water emulsions. Langmuir 29:2520–2529

12. Lopes JR, Loh W (1998) Investigation of self-assembly and

micelle polarity for a wide range of ethylene oxide-propylene

oxide-ethylene oxide block copolymers in water. Langmuir

14:750–756

13. Nambam JS, Philip J (2012) Effects of interaction of ionic and

nonionic surfactants on self-assembly of PEO–PPO–PEO triblock

copolymer in aqueous solution. J Phys Chem B 116:1499–1507

14. Kabir-ud-Din, Al-dahbali GA, Naqvi AZ, Akram M (2012)

Surface and solution properties of amphiphilic drug-nonionic

surfactant systems. J Surfact Deterg 15:777–786

15. Patel V, Chavda S, Aswal VK, Bahadur P (2012) Effect of a

hydrophilic PEO–PPO–PEO copolymer on cetyltrimethyl

ammonium tosylate solutions in water. J Surfact Deterg 15:

377–385

16. Sastry NV, Kumar A, Thummar D, Punjabi SH (2013) Mixed

micelles of trisiloxane based silicone and hydrocarbon surfactants

systems in aqueous media: dilute aqueous solution phase dia-

grams, surface tension isotherms, dilute solution viscosities,

critical micelle concentrations and application of regular solution

theory. J Surfact Deterg 16:829–840

1.25

1.45

1.65

1.85

2.05

0 0.2 0.4 0.6 0.8 1α

ηr

(a)

1.00

1.10

1.20

1.30

1.40

0 0.2 0.4 0.6 0.8 1α

ηr

(b)

Fig. 7 Variation of gr vs a for the binary mixtures of a F68 and

b P123 with 10-2-10 at (filled diamonds) 298.15, (filled squares)

308.15 and (filled triangles) 318.15 K

J Surfact Deterg

123

17. Brigante M, Schulz PC (2011) Aggregation and adsorption at the

air-solution interface of the cetyltrimethyl ammonium tosylate

with two poly(oxyethylene)–poly(oxypropylene)–poly(oxyethyl-

ene) block copolymers aqueous mixtures. J Surfact Deterg

14:439–453

18. Khan IA, Khanam AJ, Sheikh MS, Kabir-ud-Din (2011) Analysis

of mixed micellar behavior of cationic gemini alkanediyl-a,x-

bis(dimethylcetyl ammonium bromide) series with ionic and

nonionic hydrotropes in aqueous medium at different tempera-

tures. J Phys Chem B 115: 15251–15262

19. Banipal TS, Sood AK, Singh K (2011) Micellization behavior of

14-2-14 gemini surfactant with some conventional surfactants at

different temperatures. J Surfact Deterg 14:235–244

20. Sood AK, Singh K, Kaur J, Banipal TS (2012) Mixed micelli-

zation behavior of m-2-m gemini surfactants with some con-

ventional surfactants at different temperatures. J Surfact Deterg

15:327–338

21. Bakshi MS, Sachar S (2006) Influence of temperature on the

mixed micelles of Pluronic F127 and P103 with dimethylene-bis-

(dodecyldimethyl ammonium bromide). J Colloid Interface Sci

296:309–315

22. Bakshi MS, Sharma P, Kaur G, Sachar S, Banipal TS (2006)

Synergistic mixing of L64 with various surfactants of identical

hydrophobicity under the effect of temperature. Colloids Surf A

278:218–228

23. Banipal TS, Sood AK (2013) Mixed micellar and interfacial

interactions of a triblock polymer (EO37PO56EO37) with a series

of monomeric and dimeric surfactants. J Surfact Deterg

16:881–891

24. Bakshi MS, Sachar S, Yoshimura T, Esumi K (2004) Association

behavior of poly(ethylene oxide)–poly(propylene oxide)–

poly(ethylene oxide) block copolymers with cationic surfactants

in aqueous solution. J Colloid Inter Sci 278:224–233

25. Clint JH (1975) Micellization of mixed nonionic surface active

agents. J Chem Soc Faraday Trans 71:1327–1334

26. Nakashima K, Bahadur P (2006) Aggregation of water-soluble

block copolymers in aqueous solutions: recent trends. Adv Col-

loid Interface Sci 123–126:75–96

27. Thummara AD, Sastry NV, Verma G, Hassan PA (2011) Aque-

ous block copolymer–surfactant mixtures—Surface tension, DLS

and viscosity measurements and their utility in solubilization of

hydrophobic drug and its controlled release. Colloids Surf A

386:54–64

28. Bakshi MS, Sachar S (2006) Influence of hydrophobicity on the

mixed micelles of Pluronic F127 and P103 plus cationic surfac-

tant mixtures. Colloids Surf A 276:146–154

29. Rubingh DN (1979) Mixed micelle solutions. In: Mittal KL (ed)

Solution chemistry of surfactants, vol 1. Plenum, New York,

p 337

30. Mahajan RK, Kaur N, Bakshi MS (2005) Cyclic voltammetry

investigation of the mixed micelles of cationic surfactants with

pluronic F68 and Triton X-100. Colloids Surf A 255:33–39

31. Bakshi MS, Sachar S, Singh K, Shaheen A (2005) Mixed micelle

behavior of Pluronic L64 and Triton X-100 with conventional and

dimeric cationic surfactants. J Colloid Interface Sci 286:369–377

32. Motomura K, Aratono M (1993) Mixed surfactant systems. In:

Ogino K, Abe M (eds) Surfactant science series 46. Marcel

Dekker, New York

33. Li F, Rosen MJ, Sulthana SB (2001) Surface properties of cat-

ionic gemini surfactants and their interaction with alkylglucoside

or maltoside surfactants. Langmuir 17:1037–1042

34. Yang J, Xie J, Chen G, Chen X (2009) Surface, interfacial and

aggregation properties of sulfonic acid-containing gemini sur-

factants with different spacer lengths. Langmuir 25:6100–6105

35. Noori S, Naqvi AZ, Ansari WH, Akram M, Kabir-ud-Din (2014)

Synthesis and investigation of surface active properties of

counterion coupled gemini surfactants. J Surfact Deterg 17:

409–417

36. Islam MN, Kato T (2003) Thermodynamic study on surface

adsorption and micelle formation of poly(ethylene glycol) mono-

n-tetradecyl ethers. Langmuir 19:7201–7205

37. Mata J, Joshi T, Varade D, Ghosh G, Bahadur P (2004) Aggre-

gation behavior of a PEO–PPO–PEO block copolymer ? ionic

surfactants mixed systems in water and aqueous salt solutions.

Colloids Surf A 247:1–7

38. Maeda H (1995) A simple thermodynamic analysis of the sta-

bility of ionic/nonionic mixed micelles. J Colloid Interface Sci

172:98–105

39. Kabir-ud-Din, Sharma G, Naqvi AZ (2011) Micellization and

interfacial behavior of binary surfactant mixtures based on cat-

ionic geminis and nonionic Tweens. Colloids Surf A 385: 63–71

40. Hu C, Li R, Yang H, Wang J (2011) Properties of binary sur-

factant systems of nonionic surfactants C12E10, C12E23, and

C12E42 with a cationic gemini surfactant in aqueous solutions.

J Colloid Interface Sci 356:605–613

41. Li Y, Bao M, Wang Z, Zhang H, Xu G (2011) Aggregation

behavior and complex structure between triblock copolymer and

anionic surfactants. J Mol Struct 985:391–396

42. Ganguly R, Kuperkar K, Parekh P, Aswal VK, Bahadur P (2012)

Phenol solubilization in aqueous Pluronic solutions: investigating

the micellar growth and interaction as a function of Pluronic

composition. J Colloid Interface Sci 378:118–124

Tarlok Singh Banipal is a professor of physical chemistry at Guru

Nanak Dev University, Amritsar. He received his Ph.D. degree from

the same University. He also worked as scientist at the Indian Institute

of Technology, New Delhi for five years. His research interests are:

physicochemical investigations on aqueous solutions of biopolymer

model compounds, protein stability, studies on micellar solutions,

nanomaterials.

Ashwani Kumar Sood is an assistant professor and pursuing his

Ph.D. in the Department of Chemistry, Guru Nanak Dev University,

Amritsar. He received his M. Tech. degree in Material Science from

the National Institute of Technology, Jalandhar. Presently he is

working on the physicochemical properties of surfactant mixtures.

J Surfact Deterg

123