S

ND

a

ARRAA

KMCCF1

1

atsdpbmooTpsmamt

ebi

h0

Fluid Phase Equilibria 368 (2014) 51–57

Contents lists available at ScienceDirect

Fluid Phase Equilibria

jou rn al h om epage: www.elsev ier .com/ locate / f lu id

tudies of mixing behavior of cationic surfactants

eelima Dubey ∗

epartment of Chemistry, Kurukshetra University, Kurukshetra 136 119, India

r t i c l e i n f o

rticle history:eceived 27 September 2013eceived in revised form 7 February 2014ccepted 10 February 2014vailable online 17 February 2014

eywords:

a b s t r a c t

The behavior and properties of mixed surfactant systems are discussed in the context of experimentaltechniques. The aggregation behavior of tetradecyltrimethylammonium bromide (TTAB), hexade-cyltrimethylammonium bromide (CTAB), and hexadecyltriphe-nylphosphonium bromide (HTPB) andtheir mixtures in aqueous medium was investigated using conductance, fluorescence and NMR tech-niques. The critical micelle concentration (cmc), counterion binding, thermodynamic parameters ofmicellization, aggregation number (Nagg) etc. have been quantitatively estimated. Results were analyzed

ixed micellar systemsationic surfactantsonductanceluorescenceH NMR

using regular solution theory (RST) to obtain the composition of the mixed micelles and the interactionparameter, ˇm, to evaluate the type and strength of interactions of surfactants in the mixed micelle.Activity coefficients and excess free energy of mixing were also determined. 1H NMR studies suggestedthat when mixed with CTAB or TTAB, the bulky head group of HTPB induce steric hindrance. 1H NMRresults suggest that CTAB + HTPB produced compact mixed micelles.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Surfactant mixtures are frequently used in many applicationss they show synergism and provide desirable properties tohe formulated products. These synergistic interactions in mixedurfactant systems often result in enhanced interfacial activity,etergency, and emulsifying/solubilizing characteristics. Micellarroperties of cationic surfactants in bulk aqueous systems haveeen extensively studied [1–5]. A literature survey shows that innu-erable publications have been devoted since long to the world

f surfactants in order to understand the physicochemical aspectsf surfactant systems in its single as well as mixed states [3–10].he critical micelle concentration and aggregation number are twoarameters on which the practical applications of amphiphile sub-tances largely depend. Both these parameters depend upon theolecular characteristics of surfactants, presence of electrolytes

nd non electrolytes and on temperatures, pressure and pH. Treat-ents based on regular solution theory use interaction parameters

o measure the interactions between surfactants in mixed micelles.In my previous communications [11–14], I have reported the

ffect of alcohols (medium to long chain) on the micellizationehavior of some of the ionic surfactants. Although, the behav-

or depends on the nature of the surfactant, a reduction of critical

∗ Tel.: +91 1744 239835; fax: +91 1744 238277.E-mail address: drneelimadubey@gmail.com

ttp://dx.doi.org/10.1016/j.fluid.2014.02.007378-3812/© 2014 Elsevier B.V. All rights reserved.

micelle concentration (cmc) and elevation of aggregation number(Nagg) upon the addition of alcohols is generally observed.

The aim of the present investigation is to explore the ther-mophysical properties of mixed cationic surfactants. In this work,conductance and fluorescence techniques are utilized along withNMR approach to investigate the process and mechanism of themixed surfactant micelle formation. The surfactants chosen arecationic quaternary conventional surfactants, tetradecyltrimethy-lammonium bromide (TTAB) and hexadecyltrimethylammoniumbromide (CTAB) and hexadecyltriphenylphosphonium bromide(HTPB), which has three phenyl rings in the head group region.The intension behind the selection of these cationic surfactants isto study the influence of head group compatibility on the mixedmicelles formation. While CTAB and TTAB are well explored sur-factants, limited literature is available on solution properties ofHTPB [3,15–18]. Theories proposed by Clint [19] and Rubingh[20], have been used to analyze and compare experimental resultsto reveal the synergistic and antagonistic behavior of the sur-factant mixtures. Conductance method has been used to obtaincmc and degree of counterion dissociation at the studied tem-perature range. Fluorescence spectroscopic technique has beenapplied to further analyze the aggregation behavior of mixedsurfactants. Nuclear magnetic resonance, NMR, spectroscopy has

unique advantages of not only providing microscopic informationat molecular levels but also offering the advantages of being ableto observe independently the behavior of the surfactants in themixture.

5 Equilibria 368 (2014) 51–57

2

2

wcANoiAbCbup

2

2

wcwtscvd

2

5p1ctstaanAamits

2

rpfcmfmtstom

H3C-(CH 2)12-CH 2-N+(CH3)3 Br-

a Tb Tc Td

TTAB

H3C-(CH 2)14-CH 2-N+(CH3)3 Br-

a Cb Cc Cd

CTAB

H3C-(CH2)14-CH2-P+(C6H5)3 Br-

a Hb Hc Hd

HTPB

tant mixture of TTAB + CTAB and HTPB + TTAB have been presentedin Figs. 3 and 4 whereas, cmc value have been reported in Table 2.However, cmc value obtained from conductometric method for the

109876543210

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

κ / m

S.cm

-1

2 N. Dubey / Fluid Phase

. Experimental

.1. Materials

TTAB and CTAB of purity > 97% was purchased from Merck andere dried under vacuum and stored over P2O5 in vacuum desic-

ators. HTPB with purity 98+% was obtained from Lancaster, Alfaesar Research Chemicals business unit of Johnson Matthey. ForMR spectral studies, deuterium oxide (D2O), supplied by Aldrichf 99.9% isotopic purity was used. In fluorescence studies, the chem-cals were used without any purification. Pyrene was received fromldrich, cetylpyridinium chloride (CPC) from Loba Chemie, Mum-ai, India and methanol (99.9% purity) was obtained from Ranbaxyhemicals. For the experimental measurements, deionized, dou-le distilled water of conductance 1 × 10−6 S cm−1 at 298.15 K wassed. For weighing, an electronic balance (Afcoset-R120A) with arecision of 0.0001 g was used.

.2. Apparatus

.2.1. Conductivity measurementsThe conductivity measurements of pure surfactants in water

ere performed in a thermostatic glass cell coupled to digitalonductivity meter of Systronics (306). Instrument was calibratedith KCl solution. The temperature of the cell was kept constant

o within ±0.01 K by circulating thermostated water. The cmc ofurfactant in an aqueous solution was taken as the surfactant con-entration at the break point in the plot of specific conductanceersus surfactant concentration in mol dm−3. The accuracy in con-uctance measurements is ±1%.

.2.2. Fluorescence measurementsFluorescence measurements were performed using a RF-

301PC Spectrofluorometer (Shimadzu) using pyrene as theolarity probe. The pyrene solution of approximate concentration0−6 mol dm−3 was prepared in methanol. The ratio of the fluores-ence intensity of the highest energy vibrational band to that of thehird highest energy vibrational band, i.e. (I1/I3) has been used totudy the formation of the surfactant micelles. The emission spec-rum of pyrene was recorded in the wavelength range 350–600 nmt a selected excitation wavelength of 334 nm with excitationnd emission slit widths of 3.0 nm. To determine the aggregationumber, Nagg, surfactant and CPC solutions were freshly prepared.queous surfactant solutions of the pyrene were prepared takingppropriate aliquots of the probes in different vials and evaporatingethanol. Aqueous surfactant of desired concentration was added

n to the vials to achieve final probe concentration and the solu-ion was kept for stirring for about 6 h. Fluorescence spectra of theolutions with different quencher concentrations were recorded.

.2.3. 1H NMR spectroscopic studiesBruker Avance NMR (300 MHz) spectrophotometer was used to

ecord 1H NMR spectra. Deuterated water (D2O) was used for thereparation of the solutions of NMR to weaken the water signalor all solutions. The ability of the alcohols to affect the chemi-al shift of different proton and carbon signals of the surfactantolecules was utilized. 1H and 13C spectra of 0.1 mol kg−1 sur-

actant solution were recorded. The internal reference in NMReasurements is tetramethylsilane (TMS). In the present study,

he chemical shift differences were only considered. The chemical

hift measurements of various resonance peaks of studied surfac-ants have been given on the ı scale in parts per million (ppm)f the applied frequency. Further information about the experi-ental techniques has been provided in literature [14]. Chemical

Scheme 1. Formulae and proton labeling of TTAB, CTAB and HTPB molecules.

structures and proton numberings of TTAB, CTAB and HTPB areshown in Scheme 1.

3. Results and discussion

3.1. Conductometric studies

Conductance was measured as a function of concentration ofsurfactant at the temperature 298.15, 303.15, 308.15 and 313.15 K.Plots of specific conductivity, �, against aqueous CTAB at studiedtemperatures are reported in my previous communication [11]whereas, that of TTAB and HTPB are presented in Figs. 1 and 2respectively. The break point in the plot of specific conductanceversus surfactant concentration was taken as the cmc of surfactantin an aqueous solution. For each temperature, an increase in elec-trical conductivity with concentration of surfactant is seen with agradual decrease in slope. The slope change at cmc is due to an effec-tive loss of ionic charges because a fraction of the counterions arebelieved to be confined to the micellar surface. The comprehensiveresults are presented in Table 1 along with the available literaturereports. The cmc’s of the pure surfactants and their binary mixturesagree with literature values.

The plots of � against various concentration of equimolar surfac-

M . 103 / mol .dm-3

Fig. 1. Conductivity, �, of aqueous TTAB: �, 298.15 K; �, 303.15 K; �, 308.15 K; and�, 313.15 K.

N. Dubey / Fluid Phase Equilibria 368 (2014) 51–57 53

Table 1Values of cmc, ı, and various thermodynamic parameters of micellization of TTAB and HTPB at different temperatures.

Surfactant T (K) cmc (mM) ı �Gm (kJ mol−1) �Hm (kJ mol−1) T�Sm (kJ mol−1)

TTAB 298.15 3.74 0.32 −40.0 −19.5 20.5303.15 3.82 0.34 −40.3 −23.3 17.0308.15 3.95 0.37 −40.6 −33.5 7.1313.15 4.0 0.40 −40.7 −34.4 6.3

HTPB 298.15 0.194 0.39 −50.1 −22.2 27.9303.15 0.197 0.40 −50.6 −42.1 8.5308.15 0.200 0.42 −50.7 −44.7 6.0313.15 0.220 0.43 −50.8 −43.5 5.3

Table 2Values of cmc, ı, Nagg and �Gm for pure surfactants and their binary mixtures.

System cmc (mM) Nagg ı �Gm (kJ mol−1)

� Fluo

TTAB 3.74 (3.6)a 3.28 59 ± 6 (55)a 0.32 −40.0CTAB 0.96 (0.90)a 0.92 73 ± 3 (62)a 0.31 −45.9HTPB 0.194 (0.198)b 0.20 18 ± 4 (15)b 0.39 −50.1TTAB + CTAB (1:1) 1.04 (1.01)a 1.13 (1.44)d 51 ± 3 (50)a 0.24 −48.0HTPB + TTAB (1:1) 0.36 0.39 (0.38)d 12 ± 5 0.29 −50.6HTPB + CTAB (1:1) 0.32 (0.295)c 0.35 (0.33)d 12 ± 3 – –HTPB + CTAB (1:3) – 0.30 (0.33)d 26 ± 2 – –HTPB + CTAB (3:1) – 0.24 (0.26)d 9 ± 5 – –

a Ref. [4].

b[eoc

satTpsm

F�

ciation of hydrocarbon chain of the monomer, is sufficient enough

b Ref. [16].c Ref. [7].d Values obtained from Clint equation.

inary mixture HTPB + CTAB (1:1) has been taken from literature7] and is reported in Table 2. The cmc and the cmc-derived param-ters depend on the methodology adopted [21]. Usually, the cmc’sf mixed surfactants fall in between the cmc’s of the individual pureomponents.

The degree of counterion binding, ı, of the pure and mixedurfactants has been evaluated from the ratio of the postmicellarnd premicellar slopes obtained from the plots of specific conduc-ance of the surfactant solution at different concentrations [11].he results of ı for pure surfactant at studied temperature are

resented in Table 1 whereas that for studied mixed surfactantystems is reported in Table 2. Generally, the values of ı of theixed surfactants are lower than the pure components indicate

3.63.43.23.02.82.62.42.22.01.81.61.41.21.00.80.60.40.20

5

10

15

20

25

30

35

40

κ / μ

S.c

m-1

M .104 / mol.d m-3

ig. 2. Conductivity, �, of aqueous HTPB: �, 298.15 K; �, 303.15 K; �, 308.15 K; and, 313.15 K.

that there is lowering of effective charge density in the mixedsurfactants [4].

3.1.1. Thermodynamics of micellizationThe temperature dependence of cmc values are utilized to obtain

thermodynamic parameters of micellization for surfactants in pureas well as in binary mixtures. The cmc values are generally ana-lyzed in terms of the phase separation or equilibrium model formicelle formation. When the energy, which results from the asso-

to overcome the electrical repulsion between the ionic head group,the micellization takes place. As in CTAB solution reported previ-ously [11], the increase in temperature results in an increase in

876543210

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

κ / m

S.cm

-1

M.103 / mol.dm-3

Fig. 3. Conductivity, �, of equimolar aqueous TTAB + CTAB: �, 298.15 K; �, 303.15 K;and �, 308.15 K.

54 N. Dubey / Fluid Phase Equilibria 368 (2014) 51–57

4.54.03.53.02.52.01.51.00.50.00.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

κ / m

S.cm

-1

M . 103 / mol.dm-3

F3

cket

�

Icfb

�

[

�

wTntmip

�tTsalibmc

twnme

1.51.00.50.0-0.5-1.0

1.25

1.30

1.35

1.40

1.45

1.50

1.55

Pyre

ne I 1/

I 3

log [Surfactant] / mM

ig. 4. Specific Conductivity, �, of equimolar aqueous HTPB + TTAB: �, 298.15 K; �,03.15 K; and �, 308.15 K.

mc values in aqueous TTAB and HTPB surfactants also because theinetic energy of monomer has been raised. The standard Gibbsnergy of micellization for ionic surfactants (�G

◦m) is calculated by

he relation

G◦m = (2 − ˛)RT ln Xcmc (1)

n Eq. (1), the cmc was expressed in mole fraction unit and thehosen standard state was the hypothetical ideal state of unit moleraction. On the other hand, Enthalpy of micellization (�H

◦m) can

e obtained by applying the Gibbs–Helmholtz equation;

H◦m = −RT2

[(2 − ˛)

d ln Xcmc

dt+ ln Xcmc

d(1 − ˛)dT

](2)

Entropy of micellization (�S◦m) has been obtained from Eq. (3)

22],

G◦m = �H

◦m − T�S

◦m (3)

here R is the gas constant and T is absolute temperature.hese relationships are believed to be correct only when there isegligible variation of aggregation number of the micelles withemperature [10]. For pure surfactants TTAB and HTPB, the ther-

odynamic parameters derived from Eqs. (1)–(3) are summarizedn Table 1. However, for binary surfactant mixtures (1:1) �G

◦m are

resented in Table 2.From Table 1, it is clear that in all cases �G

◦m is negative. The

H◦m values are found to be negative indicating that the micelliza-

ion is an exothermic process whereas, T�S◦m values are positive.

he positive �S◦m values are attributed to the disruption of water

tructure around the hydrocarbon part of these additive moleculess they transfer from the aqueous bulk phase to non-aqueous micel-ar interior. Thus, in the micellization process, the change in entropys the driving force, which is caused by the transfer of the hydropho-ic group of the surfactant from the solvent to the core of theicelle. It can be seen that in the studied systems, �G

◦m < 0 and

ompensation between �H◦m and �S

◦m values are noticed.

Similarly, for the mixed surfactants in their equimolar concen-ration, �G

◦m is negative and its value remains almost constant

ith increasing temperature. The enthalpy values are found to beegative and �S

◦m values are positive (not shown in the table). It

ay be concluded that micellization is driven by both enthapic andntropic processes.

Fig. 5. Plots of pyrene I1/I3 versus log [surfactant]. �, TTAB; �, HTPB.

3.2. Fluorescence behavior and micellar aggregation

Fluorescence probe analysis technique provides a versatilemethod for the study of aggregated systems like micelles [4,10,23].The ratio of the first and third vibronic peaks, i.e. I1/I3, in the pyrenefluorescence emission spectrum can be a measure of the polarityof the micellar interior (pyrene dissolves in the core of the normalmicelles). Normally, low and high values of I1/I3 should indicate theenvironment to be nonpolar and polar, respectively. Certain emis-sion bands are unaffected but others show variations in intensitydue to the interaction with the solvents.

3.2.1. Determination of critical micelle concentrationIn the present study pyrene is used as a fluorescence probe to

obtain cmc of the studied surfactants as well as their binary mix-tures. The Plot of pyrene I1/I3 versus log [CTAB] has been reported inmy previous publication [11] whereas, that for TTAB and HTPB arereported in Fig. 5. The curves represent simple sigmoidal expres-sion. A significant change in I1/I3 indicates an aggregation process,i.e. the micelle formation and hence the cmc. The cmc values ofstudied surfactants obtained from the fluorescence studies arereported in Table 2. The cmc of HTPB was found to be significantlylower than the rest of the representatives. Fig. 6 represents the plotsof pyrene I1/I3 versus log of the concentration of the first compo-nent of 1:1 mixture of CTAB + HTPB, TTAB + HTPB and TTAB + CTAB.A close perusal of Fig. 6 shows a single aggregation process of mixedmicelle formation of studied surfactant mixtures in pure water.However, Fig. 7 depicts the I1/I3 curves of CTAB+ HTPB in 1:3, 1:1and 3:1 ratio as a function of log of CTAB concentration. The end ofthe reversed sigmoid curve gives the value of cmc and is presentedin Table 2 along with that of the pure components. As reported inthe literature [21], usually the cmc of the mixed surfactants fallin between the cmc’s of the individual pure components. In thepresent case also the cmc’s of all the three binary mixtures lie inbetween the cmc’s of parent components.

3.2.2. Determination of aggregation number

The aggregation number (Nagg) of aqueous surfactant and their

binary mixtures are obtained from fluorescence quenching ofpyrene using cetylpyridinium chloride (CPC) according to the

N. Dubey / Fluid Phase Equilib

1.00.50.0-0.5-1.0-1.5-2.01.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

pyre

ne I 1/

I 3

log [sur factant] / mM

Fm

f

l

wiowmofNtcam

FH

ig. 6. Plots of pyrene I1/I3 versus log of first component of 1:1 binary surfactantixture. �, CTAB + HTPB; �, TTAB + HTPB; �, TTAB + CTAB.

ollowing equation:

n

(I0IQ

)= Q4micelle

[surf]total= [CPC]micelle

[surf]total

= [CPC]micelle

[Nagg

[surf]total − cmc

](4)

here I0 and IQ represent the fluorescence intensities of pyrenen the absence and presence of quencher CPC, respectively. Qmiceller [CPC]micelle and [surf]total are concentrations of quencher CPCithin the micellar phase and total concentration of single orixed surfactants. The Nagg is calculated from Eq. (4) using the cmc

btained earlier and reported in Table 2. The Nagg of aqueous sur-actants are compared with that reported in the literature [3,4]. Inagg also, HTPB has shown characteristic difference from rest of the

wo studied surfactants. From Table 2, it is clear that Nagg of pureomponents follow the order CTAB > TTAB > HTPB. TTAB and CTABre members of the same homologous series in which the cmc nor-ally decreases logarithmically with the number of carbon atoms

1.00.50.0-0.5-1.0-1.5-2.01.35

1.40

1.45

1.50

1.55

1.60

I 1/I3

log [C TAB ] / mM

ig. 7. Plots of pyrene I1/I3 versus log [CTAB] in different ratio of binary mixture ofTPB + CTAB. �, 1:1; �, 3:1; �, 1:3.

ria 368 (2014) 51–57 55

in the chain. However, in the case of HTPB, as the surfactant has abulky head group, lower Nagg has been reported. For binary mix-tures (equimolar), Nagg values lie between the parent components.However, CTAB-HTPB mixture in 3:1 ratio shows Nagg value higherthan that containing the surfactant ratio 1:3.

3.3. Interactions of surfactants in micelles

The mixing behavior between TTAB or CTAB and HTPB isexpected to be non ideal in view of differences in their head groups.Clint’s equation [19] can be used to predict the cmc values of idealityin the mixed micelle formation (cmc*).

1cmc∗ =

k∑i=1

˛i

cmci(5)

where ˛i is the mole fraction of surfactant ith component in thesolution and cmci is critical micelle concentration of that in purewater. The deviations of the experimentally determined cmc’s fromthose of the calculated ones indicate the degree of nonideality.

Rubingh [20] modified Clint’s equation by introducing the activ-ity coefficients (� i) for the component i in the micelle to accountfor the non-ideal behavior. The modified equation is as follows:

1cmc∗ =

k∑i=1

˛i

�icmci(6)

Rubingh also proposed that the composition of the mixedmicelle and the interaction parameter, (ˇm), can be calculatedemploying the following equations:

X21 ln

[cmc ∗ ˛1

cmc1X1

]= (1 − X1)2 ln

[cmc ∗ (˛2)

cmc2X2

](7)

and

ˇm = ln[(cmc ∗ ˛1)/(cmc1X1)]

(1 − X1)2(8)

In the above equations, X1 represent the mole fraction of compo-nent 1 in the mixed micelle. A non-ideal mixing process is indicativeof interactions between the surfactant chains inside the mixedmicelles. Normally, the attractive interactions are characterizedby both a negative ˇm value and negative deviation of cmc* fromthe calculated values from Clint’s equation. The composition of themixed micelles is computed by solving Eq. (7) and employing Eq.(8), the interaction parameter, (ˇm), is obtained. The values of X andˇm for systems studied are presented in Table 3. A close perusal ofTable 3 reveals that the surfactant mixtures either 1:1 or in differ-ent ratio show appreciably different mole fractions in the micellarsolution. The X values show dependence on the technique of cmcdetermination. The ˇm values are mostly negative except for theequimolar binary systems HTPB + TTAB and HTPB + CTAB where ˇm

values obtained by conductance as well as fluorescence methodsare positive. According to Moulik et al. [4], among similarly chargedcationic surfactants, repulsive interaction is expected and ˇm val-ues should be positive. According to Hoffmann and Possnecker [10],the error in the interaction parameter arises due to the errors in cmcvalues, the micellar ratio and the degree of the interaction betweenthe surfactant components. In all binaries, the mixed micelle forma-tion is accompanied by favorable interactions between the chains,

as indicated by the ˇm values. This observation is not surprisingas the HTPB has a higher tendency to form micelles because ofits longer chain length and possesses much lower cmc values ascompared to TTAB or CTAB.

56 N. Dubey / Fluid Phase Equilibria 368 (2014) 51–57

Table 3Micellar composition and interaction parameter for binary mixed surfactants obtained by Rubingh’s theory using conductance and fluorescence data.

System X (ˇm) (cond) X (ˇm) (fluo) �1 (�2) (cond) �1 (�2) (fluo) �Gmex (J mol−1)

CTAB + TTAB (1:1) 0.67 (−1.93) 0.69 (−1.23) 0.81 (0.419) 0.888 (0.557) −1060HTPB + TTAB (1:1) 0.91 (−0.81) 0.97 (0.89) 0.993 (0.51) 1.0 (0.844) −168

0.998 (0.959) 1.0 (1.437) −21– 0.677 (0.548) −1200 (fluo)– 0.995 (0.679) −102 (fluo)

ra

�

�

a

�

m

mmfamists

3

siotfwotsbtTotrsti

T1

x

Table 51H chemical shifts (ppm) of mixed TTAB (T) + HTPB (H).

x HTPB 0.00 0.25 0.50 0.75 1.00

0.743 0.796 0.762 0.7291.092 1.195 1.143 1.1013.215 3.322 3.313 3.2623.018 3.073 3.039 3.022

Ha 0.796 0.762 0.729 0.869Hb 1.195 1.143 1.101 1.264Hc 1.563 1.490 1.459 1.593

tions were found for the binary system HTPB + TTAB as presented inFig. 9. However, TTAB produces relatively greater shielding in HTPBprotons in comparison to CTAB.

0.075

0.100

0.125 Ca Cb Cc Cd

HTPB + CTAB (1:1) 0.83 (−0.06) 0.87 (0.48)

HTPB + CTAB (1:3) – 0.55 (−1.95)

HTPB + CTAB (3:1) – 0.90 (−0.47)

The activity coefficients �1 and �2 of the components 1 and 2espectively and Gibbs energy of mixing �Gm

ex in the mixed micellere given by the relations

1 = exp [ˇm(1 − x)]2 (9)

1 = exp[ˇmx2] (10)

nd

Gmex = RT(x1 ln �1 + (1 − x1) ln �2) (11)

In Table 3 the �1, �2 and �Gmex values for the binary surfactant

ixtures are presented.The results of the �1 and �2 by conductance and fluorescence

ethods maintain a similar trend but at some places they differ inagnitude. The values of � indicate the deviation of the surfactants

rom ideality in the mixed micelles. For the systems HTPB + CTABnd HTPB + TTAB at equimolar concentration, HTPB show higherole fraction as well as higher activity coefficient (closer to unity),

ndicating that HTPB in the mixed micelle is close to its standardtate [24]. In case of HTPB + CTAB (3:1) the value of x is higherhan that in the mixture (1:3). �Gm

ex values are negative for all theystems indicating favorable interaction in the mixed state.

.4. 1H NMR studies

1H NMR spectroscopy is one of the most convenient methods forimultaneous monitoring of changes in aggregate morphologies ofnteracting components. For the present studies the concentrationf interacting components in all the spectra is kept higher thanheir cmc values, 1H signals of pure components and mixed sur-actant systems of HTPB with TTAB and CTAB were studied overhole mole fraction range. The formulas and proton numberings

f TTAB, CTAB and HTPB are shown in Scheme 1. The total surfac-ant concentration was kept 10 mM in the studied binary mixtureo that the observed chemical shifts are those of aggregated assem-lies of cationic surfactants under consideration. The variations ofhe 1H NMR chemical shifts for the binary mixture HTPB + CTAB andTAB + HTPB are listed in Tables 4 and 5 respectively. A close perusalf various 1H signals in the case of pure components and a shift inhe position of these signals upon mixing will help to deduce the

elative arrangement of unlike surfactant monomers in the mixedtate. The observed chemical shift changes are not originated fromhe change in cmc as the extent of the change in the chemical shifts different for different protons in the same molecule.

able 4H chemical shifts (ppm) of mixed CTAB (C) + HTPB (H).

x HTPB 0.00 0.25 0.50 0.75 1.00

Ca 0.719 0.811 0.789 0.779Cb 1.148 1.214 1.211 1.159Cc 3.200 3.292 3.301 3.278Cd 3.024 3.075 3.048 3.040Ha 0.811 0.789 0.779 0.869Hb 1.214 1.211 1.159 1.264Hc 1.600 1.507 1.498 1.593Hd 7.757 7.728 7.710 7.800

, concentration in mole fraction unit.

Hd 7.758 7.659 7.692 7.800

x, concentration in mole fraction unit.

In Fig. 8, the variation in the chemical shifts of various protonsof HTPB and CTAB upon mixing in different proportion is shown.From Fig. 8 it is obvious that incorporation of CTAB in HTPB micellesbrings about upfield shift of all the HTPB protons. The variations inthe chemical shifts of head group as well as hydrocarbon protonsof monomers indicate that protons of HTPB undergo shielding. TheHTPB micelles are basically less compact because of the presenceof bulky head group and experience strong shielding. It indicatesthe formation of compact mixed micelles with reduced steric hin-drances However, due to the compact nature of CTAB micelles, allthe protons including head group protons and end methyl protons(Ca and Cd) experience downfield shifts in the presence of HTPB. Itmeans the incorporation of HTPB in CTAB micelles shows deshield-ing of the CTAB protons. The deshielding of Cd protons of CTAB canbe attributed to the intercalation of bulky phenyl groups of HTPBbetween the head groups of CTAB which is compactly arranged;as a consequence steric incompatibility arises. Similar observa-

0.750.50.250

-0.100

-0.075

-0.050

-0.025

0.000

0.025

0.050

Δδ

[HTPB]/ mol fraction

Ha Hb Hc Hd

Fig. 8. 1H chemical shifts (ppm) for HTPB + CTAB in their mixture at different con-centrations of HTPB.

N. Dubey / Fluid Phase Equilib

0.750.50.250

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

Δδ

[HTPB] / mol fraction

Ta Tb Tc Td Ha Hb Hc Hd

Fc

Tbsp(eIbrmtsN

4

sameta

[[[[[[

[

[

[[[

[[22] T.F. Tadros, Applied Surfactants: Principles and Applications, WILEY-VCH Ver-

lag GmbH & Co. KGaA, Weinheim, 2005.

ig. 9. 1H chemical shifts (ppm) for HTPB + TTAB in their mixture at different con-entrations of HTPB.

The results indicate that the mixed-micelle formation betweenTAB/CTAB with HTPB is greatly influenced by the presence ofulky phenyl group of HTPB. As indicated by the interactiontudies discussed earlier, the mixed micelles undertaken for theresent investigations are rich in HTPB rather than CTAB or TTABin their respective surfactant mixtures) and they would experi-nce shielding which will bring synergism to the mixed micelles.t is expected that trimethylammonium head groups intercalateetween the triphenylphosphonium head groups which in turneduces the steric hindrance as a result, mixed micelles becomeore favorable. In case of the mixture of CTAB + TTAB, because of

he structural similarity of the parent components, they exhibitimilar spectra and most of their proton peaks overlap in the 1HMR spectra.

. Conclusions

The present work deals with physicochemical study of cationicurfactants (TTAB, CTAB and HTPB) and their binary mixtures inqueous medium. The data provide much insight into the nature of

ixtures of these surfactants. The cmc values obtained by differ-

nt methods agree satisfactorily and in the binary mixed systems,he counterion binding capacity of the micelles is lowered. Theggregation number depends on the chain length of the surfactants.

[[

ria 368 (2014) 51–57 57

The cmc and Nagg values obtained for the pure components are ingood agreement with the literature. The negative values of interac-tion parameter (ˇm) suggest that the mixed micelle formation takeplace due to the favorable interactions of amphiphilic molecules.The negative values of �G

◦m show that the micelle formation of sur-

factant at the air/water interface is energetically favorable, whilea negative value of �Gm

ex ensures the stability of mixed micelles.The cmc of the mixed micelles and the activity coefficients of thecomponents in the mixed states clearly indicate nonideality. The1H NMR studies reveal that head-group modification significantlyinfluences the nature of mixed-micelle formation.

Acknowledgements

Author gratefully acknowledges the financial support for theproject under Women Scientist Scheme A (WOS-A) (Grant No.SR/WOS-A/CS-35/2009) by the Government of India through theDepartment of Science and Technology, New Delhi.

References

[1] P.M. Holland, D.N. Rubingh, Mixed Surfactant Systems, American ChemicalSociety, Washington, DC, 1992.

[2] S.D. Christian, J.F. Scamehorn, Solubilization in Surfactant Aggregates, MarcelDekkar, New York, 1995, pp. 55.

[3] M.S. Bakshi, I. Kaur, Colloid Polym. Sci. 281 (2003) 935–944.[4] S.P. Maulik, M.E. Haque, P.K. Jana, A.R. Das, J. Phys. Chem. 100 (1996) 701–708.[5] A. Lainez, P. del Burgo, E. Junquera, E. Aicart, Langmuir 20 (2004) 5745–5752.[6] C.C. Ruiz, L. Diaz-Lopez, J. Aguiar, J. Colloid Interface Sci. 305 (2007) 293–300.[7] M.S. Bakshi, I. Kaur, Colloid Surf. A 227 (2003) 9–19.[8] M.S. Bakshi, I. Kaur, R. Sood, K. Singh, Colloid Polym. Sci. 281 (2003) 771–776.[9] X. Cui, Y. Jiang, C. Yang, X. Lu, H. Chen, S. Mao, M. Liu, H. Yuan, P. Luo, Y. Du, J.

Phys. Chem. B 114 (2010) 7808–7816.10] H. Hoffmann, G. Possnecker, Langmuir 10 (1994) 381–389.11] N. Dubey, J. Mol. Liq. 184 (2013) 60–67.12] N. Dubey, A. Pal, J. Mol. Liq. 172 (2012) 12–19.13] N. Dubey, J. Chem. Eng. Data 54 (2009) 1015–1021.14] N. Dubey, J. Chem. Eng. Data 56 (2011) 3291–3300.15] M. Prasad, S.P. Moulik, A. Macdonald, R. Palepu, J. Phys. Chem. B 108 (2004)

355–362.16] S. Kolay, K.K. Ghosh, A. Macdonald, J. Moulins, R.M. Palepu, J. Solution Chem.

37 (2008) 59–72.17] M.S. Bakshi, S. Sachar, N. Mahajan, I. Kaur, G. Kaur, N. Singh, P. Sehgal, H. Doe,

Colloid Polym. Sci. 280 (2002) 990–1000.18] A. Al-Wardian, K.M. Glenn, R.M. Palepu, Colloids Surf. A (2004) 115–123.19] J.H. Clint, J. Chem. Soc. Faraday Trans. 71 (1975) 1327–1334.20] D.N. Rubingh, in: K.L. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1,

Plenum, New York, 1979, p. 337.21] D. Attwood, H.K. Patel, J. Colloid Interface Sci. 129 (1989) 222–230.

23] K. Behera, Hari Om, S. Pandey, J. Phys. Chem. 113 (2009) 786–793.24] S.B. Sulthana, P.V.C. Rao, S.G.T. Bhat, T.Y. Nakano, G. Sugihara, A.K. Rakshit,

Langmuir 16 (2000) 980.