Mixed solutions of cationic and nonionic surfactants

Mixed solutions of cationic and nonionic surfactants

Mixed Solutions of Cationic and Nonionic Surfactants The surfactants with a similar chemical structure form mixed micelles in aqueous solutions and th...

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Mixed Solutions of Cationic and Nonionic Surfactants The surfactants with a similar chemical structure form mixed micelles in aqueous solutions and the process can be described in terms of Shinoda's relation (1). For ideally behaving systems, Clint (2) has proposed a model in order to predict the CMC of the mixture as well as the concentration of monomeric surfactants above CMC. The mixtures of surfactants differing in both hydrophobic and hydrophilic parts presents important deviation from the ideal behavior predicted by the above-mentioned models. Moroi et aL (3) have proposed a model to deal with these cases, but it requires adjustable parameters. Rubingh (4) treats the micellization of surfactant mixtures in term of regular solution theory which implies the introduction of an interaction parameter between surfactant molecules in the micelles. The micellization of cationic and nonionic mixtures have been relatively less studied: a maximum for micellar weight as function of composition (5) and a pronounced minimum on the CMC of the mixtures vs composition have been reported (6). In the present contribution, the mixtures of cetyltrimethylammonium bromide (abbreviated CTAB) with a series o f polyoxyethyleneglycol dodecyl ethers (C~2EOm) have been studied either at constant molar ratio between components or by keeping constant concentration of nonionic surfactants higher than CMC. The behavior o f the surfactant solutions was studied by surface tension and electrical conductivity measurements.

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~ 42

~38

36



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3/+ 32 -5

-Z- -3'.5 -i

i

-4.5

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log C0ncent ration of CTAB~molII

FIG. 1. Surface tension for mixture of CTAB:CI2EOm of molar ratio a = 0.8: (O) m = 10; (A) m = 20; (0) m = 27. literature data (7, 8). The nonionic surfactants C12H250(CH2CH20)mH had an average ethoxylation degree of 10, 20, and 27, respectively. The surface tension-concentration curves for CTAB and CtzEOm show a single breakpoint from which the CMC values (Table I) have been determined, and a wellmarked plateau above CMC can be observed, at 25°C.

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vE 200

150

MATERIALS Cetyltrimethylammonium bromide, BDH reagent, has a CMC of 9.4 × 10-4 mole/liter as determined from surface tension and a CMC of 8.7 × 10-4 mole/liter from electrical conductivity in good agreement with

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.,4 150

100 o

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TABLE I to

The CMC of Single Surfactants Used and Surface Tension at CMC

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0

&

Surfactant

CMC × 104 (mole/liter)

b3

Surface tension (dyn/em)

/ --

o

CTAB Cl2EOm Cl2EOzto C12EO27

9.4 0.8 1.25 8.0

40 29.5 37.8 41.5

.

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Concentration CTAB,103~mol/I FIG. 2. Dependence of specific conductivity on concentration for CTAB:C~2EO,~ mixture for molar ratio a = 0.8: (O) m = 10; (A) m = 20; (O) m = 27. 247 0021-9797/85 $3.00

Journal of Colloid and Interface Science, Vol. 106, No. 1, July 1985

Copyright © 1985 by Academic Press, Inc. All fights of reproduction in any form reserved.

NOTES

248

TABLE II The Values for Breakpoints for Surface Tension and Electrical Conductivity for Mixture CTAB:C~2EOm at Molar Ratio a = 0.8 at 25°C (CMC)I × 104

(CMC)z × 103

Nonionic surfactant in mixture

Conductivity

Surface tension

Conductivity

Surface tension

r × 101~ (mole/cm~)

A X 1016 (cm 2)

C[2EO1o C12EO2o C12EO27

6.5 4.9 4.0

2.0 2.2 4.66

1.75 1.90 2.00

1.34 1.12 1.57

13 14.6 18.8

124 110 86

The surface tension for mixtures with constant ratio CTAB:Ct2EOm a = 0.8 are presented in Fig. 1 as a function of cationic surfactant concentration, each curve being characterized by two breakpoints designated (CMC)~ and (CMC)2. The specific conductivity vs concentration curves for mixtures of cationic and nonionic surfactants exhibit also two breakpoints (Fig. 2). The first breakpoint in X vs C curves for mixtures with ~ = 0.8 is located at concentration higher than (CMC)~ obtained from surface tension data as shown in Table II. For such mixtures, Birdi (5) found high values for aggregation numbers, an indication for the existence of strongly asymmetrical micelles, so the second breakpoint on surface tension and electrical conductivity curves might be due to transition of spherical to assymmetrical micelles. The surface excess r and the specific surface area of the surfactant molecules near (CMC)I are given in Table II. The (CMCh for CTAB:CI2EO~ mixtures as a function of molar ratio of cationic surfactant exhibit a pronounced minimum below the CMC of the nonionic surfactant (Fig. 3). By using the nonideal mixed micelle theory as proposed by Rubingh (4) in the systems studied gives for the I

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~-~

CTAB:C~2EOt0 mixture with ~ = 0.8 a mole fraction of CTAB in micelle xt = 0.91 and a nonplausible high interaction parameter fl = -185.5, leading to a micellization enthalpy ~ H = -9.5 keal/mole. This value does not correspond to the experimentally determined values of +11 kcal/mole obtained from CMC dependence on temperature (6). These data emphasize that the abovementioned model is limited to systems of surfactants having comparable alkyl chain lengths. The obtained data suggest the possibility of using surface tension and specific conductivity measurements in order to obtain information about mixed micelles of ionic and nonionic surfactants at high molar ratio in ionic surfactant. REFERENCES 1. Shinoda, K., J. Phys. Chem. 54, 541 (1954). 2. Clint, J. H., J, Chem. Soc. Faraday Trans. 1 71, 1327 (1975). 3. Moroi, Y., Nishikido, N., Saito, M., and Matuura, R., J. Colloid Interface Sci. 52, 356 (1975). 4. Rubingh, D. N., in "Solution Chemistry of Surfacrants" (K. L. Mittal, Ed.), Vol. 1, p. 337. Plenum, New York/London, 1979. 5. Birdi, K. S., "Proceedings of the International Conference on Colloid and Surface Science," Vol. 2, p. 473. Akad. Kiadt, Budapest, 1975. 6. Vilcu, R., and Olteanu, M., Rev. Roum. Chim. 24, 509 (1979), 7. Duff, D. G., and Giles, C. H., J. Colloid Interface Sci. 41, 407 (1972). 8. Barry, B. W., and Russell, G. F. Y., J. Colloid Interface Sci. 41, 174 (1972). MIHAELA OLTEANU I. MANDRU

-5

d.z dz 0'.6 0'8 Mole fraction of CTAB~

FIG. 3. The (CMC)I for mixture CTAB:Cx2EOlo vs molar ratio. Journal of Colloid and Interface Science, Vol. 106, No. l,,July 1985

Department of Physical Chemistry Polytechnic Institute Blvd. Republicii Nr. 13 Bucharest, Romania Received October 17, 1983