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ing of chloride and sodium ions to mixed anionic/cationic micelles was measured using ion-specific electrodes. Counterion binding was found to be strongly ...

Colloid & Polymer Science

Colloid Polym Soi 266:954-957 (1988)

Counterion binding on mixed anionic/cationic micelles N. Filipovi~-Vincekovi8 and D. Skrti8 Ruder Bo~kovi8 Institute, Zagreb, Yugoslavia

Abstract: Measurements of counterion binding in mixtures of surfactant aqueous solutions have been performed to study the structure of the anionic/cationic mixed micelle/ solution interface. The mixtures studied were SDS/DDAC and STS/TDPC. The binding of chloride and sodium ions to mixed anionic/cationic micelles was measured using ion-specific electrodes. Counterion binding was found to be strongly dependent on the molar ratio of surfactants present. The mixed micelle/solution interface includes the headgroups of both surfactants and counterions of surfactant in excess. The addition of oppositely charged surfactant caused an increasing dissociation of counterions.

Key words:S_urfactantaqueous solutions; mixtures of surfactants; structure of interfaces; counterion binding.

Introduction There are numerous reports on mutual interactions in mixed surfactant systems which provide information on the formation of mixed adsorption layers at air/ water interfaces and interactions in solution leading to mixed micelle formation. Recently, a significant amount of attention has been paid to modelling and understanding mixed micelles containing anionic and cationic surfactants [1-7]. However, there is an extreme paucity of data on the key parameter in mixed anionic/cationic surfactant systems: the structure of the mixed micelle/solution interface and the binding of counterions. At the micelle/solution interface many interactions occur which affect the energetics of the micellization process and thereby the critical micelle concentration (CMC) and the distribution of micelle size and shape [8]. These interactions include those among surfactant headgroups, between headgroups and counterions and between charged surface and surrounding medium with its diffuse double layer. In order to gain further insight into the structure of the anionic/cationic mixed micelle/solution interface, the systems with constant anionic and varying cationic surfactant concentration were investigated by direct measurements of counterion binding using ion-specific electrodes. GL 459

Experimental The mixed micelles were formed by mixing the cationic surfactant dodecyl ammonium chloride (DDAC) with sodium dodecyl sulfate (SDS) and tetradecylpyridinium chloride (TDPC) with sodium tetradecyl sulfate (STS). The pure surfactants SDS and TDPC (obtained from BDH Chemicals Ltd.), STS (obtained from Henkel KGaA) and prepared DDAC [9] were purified by repeated recrystallization. The purity of the surfactants was checked by surface tension measurements (Lecomte du No/.iy interfacial tensimeter). The systems were prepared at a constant anionic surfactant concentration and varying concentration of cationic surfactant. The anionic/ cationic molar ratio decreases with increasing cationic surfactant concentration (from 10 to 0.1). The concentration of free counterions (counterions not bound to mixed rnicelles) was measured with chloride and sodium ion-specific electrodes and an Orion Research Inc. digital pH/mV meter. The following cells were used [12]:

Saturated I 2mo 'm 3NNO3 I Sample I selective Ag-AgC1 2 % agar bridge solution electrode

electrode

and

ca,o.

electrode

12mo, m 3N4NO3 rsolution Smie i selective 2 % agar bridge electrode

Binding of counterions to mixed micelles was calculated from EMF measurements. The results are presented as the change in concentration of bound chloride or sodium ions, c~r:, (ci~ = ct - cf, where c denotes concentration, and the subscripts total (t) and free

Filipovi& Vincekovig and ~dkrtid, Counteffon binding on mixed anionic~cationic micelles (/) chloride (I-) or sodium (I+) ion concentration), vs. total concentration of cationic surfactant. The degree of counterion dissociation, cr of anionic/cationic mixed micelles was calculated from the slopes of ct vs. cationic surfactant concentration curves [11]. All measurements were performed at 298 K.

Results and discussion Anionic and cationic surfactants form a series of mixed micelles of varying composition, size and shape, depending on the total concentration and molar ratio of the surfactants [4-7]. Electrophoretic measurements have shown that mixed micelles formed in systems with anionic surfactant in excess are negatively charged and vice versa [7]. The presence of oppositely charged headgroups in the micelle/solution interface cause a high degree of charge neutralization and permits the mixed micelles to attain a large size [13]. In the equimolar concentration range, in most mixtures a slow precipitation of soluble salts occurs [7,14]. We performed a series of experiments to study the dependence of counterion binding on composition of the mixed micelles composed of SDS/DDAC and STS/TDPC. In both cases, sodium and chloride ions act as counterions. In Fig. 1 the EMF measurements for

I

I

,o

5,16

O

Ne§

the series of mixed surfactant systems, SDS/DDAC, are presented as the change in the bound counterions vs. concentration of cationic suffactant. With increasing DDAC concentration, the binding of the sodium ion decreases and, in the equimolar concentration range, becomes unmeasurable (the concentration of free sodium ion corresponds to the concentration of anionic surfactant added). The binding of chloride ions to anionic/cationic mixed micelles starts above the equimolar concentration range and increases with increasing cationic surfactant concentration. It is evident that anionic/cationic mixed micelles prepared in the presence of excess anionic surfactant bind only sodium ions as counterions. Micelles formed with cationic surfactants in excess bind only chloride ions. At the equimolar concentration range of anionic and cationic surfactants, binding of counterions was unmeasurable. The degree of chloride ion dissociation from mixed micelles was calculated from plots of the concentration

(~01 --

jo

/ 0.005

//

T/K = 298

?

e"

o/~ 0.001 -

I

I

0.ooi

\~ "

9 C["

SDS ,, DDAC

[ SDSl/moI drr';3 = 0.001

I

SDS,,,DDAC T/K = 29B [SDS]Imo[ d ~ 3 : 0.001

955

1.10~

[ S D S ] / m o l dm 4 : 0.0001

E

|'104

~176

O

E

,c_>,

[SDS] / tool dm"] = 0.0001 0 5.1(i~

1.104--

S "10~

Na*

9 C1"

O/~

__/l.i 41/

10" 1(~-~

~,, .... .7 I

-5

/o

./

O~o"~O"~,.O

1.1o~

I o.01

0.005

I -~.

I

-3

tog {[DDAC]) / m o l drfi 3

Fig. 1. Concentration of bound counterions, Aq~/mol dm -3 (where I+ denotes sodium and I- chloride ions) vs. DDAC concentration. Concentration of SDS and temperature are indicated

I 10"~

I 5.10 "~

I 0.001

[DDAC]/mol. dm "3 Fig. 2. Concentration of free chloride ions, cf/mol dm -3, vs. concentration of DDAC. Concentration of SDS and temperature are indicated

Colloid and Polymer Science, Vol. 266. No. i0 (1988)

956

I

I

I

STS § TDPC

[ STS] I tool dm4 = 0.0005

_/t/

0.003

"E '13

o.ool

Table 1. Chloride ion dissociation, a, vs. anionic surfactant concentration System

G~ionlc/moldm -3

a

SDS/DDAC

0 0.0001 0.0010

0.15a) 0.85 0.90

STS/TDPC

0 0.0001 0.0005

0.57b) 0.90 0.92

1.104

E

a) Ref. [15]; b) Ref. [7] [STS]/mo[ dr63 = 0.0001

.q 0.003

0.001

i.I0-~

- ~.#/o lip.f II I 1.1~ o.ooI

I o.oo5

[TOPC]/moI drn3 Fig. 3. Concentration of free chloride ions, q/tool dm -3, vs. concentration of TDPC. Concentration of STS and temperature are indicated

of free chloride ions, cI, vs. the concentration of cationic suffactant (Figs. 2 and 3). The increase of a-values in the presence of anionic surfactant (SDS or STS) in comparison with pure cationic micelles (DDAC or TDPC) indicates a decline in counterion binding on mixed micelles (Table 1). The effect is proportional to the anionic surfactant concentration. A similar behaviour has already been observed in some anionic/ nonionic surfactant systems [16,17]. For a qualitative explanation of results obtained, the effect of several parameters on a-values must be considered: i. Addition of anionic surfactant reduces the electrostatic repulsion of the cationic surfactant headgroups at the mixed micelle/solution interface, enhancing a-values for chloride ions. ii. It has been shown that different micelle geometries (size and shape) are associated with different surface charge density; an increase of a has been attributed to a decrease of the charge density at the micelle/ solution interface [18,19]. A comparison of the previously obtained light scattering and microelectrophoretic data [7] shows a sig-

nificant decrease in size when mixed micelles are formed in excess of one of the components. This implies that, besides the electrostatic factors, the change of size and/or shape can influence the g-values. Rathman and Scamehorn [20] have developed models based on electrostatic interactions for binding of counterions in ionic/nonionic and ionic/ionic mixed micelles. Both models describe the counterion binding. On physical grounds, the localized model is preferred, particularly for mixed ionic/nonionic micelles with nonionic surfactants in excess. A similar situation is seen in mixed anionic/cationic micelles with a small excess of one of the components at which there are only a small number of sites onto which counterions my be bound on the micelle/solution interface. In conclusion, binding of counterions in anionic/ cationic mixed micelle/solution interface depends on the surfactant molar ratio.

References 1. Rodakiewicz-Nowak J (1982)J Colloid Interface Sci 85:586 2. Goralczyk D (1980) J Colloid Interface Sci 77:68 3. Cortdll JM, Goodman JF, Harrold SP, Tate JR (1967) Trans Faraday Soc 63:247 4. Mukhayer GI, Davies SS (1975)J Colloid Interface Sci 53:224 5. Barry BW, Gray GMT (1975) J Colloid Interface Sci 52:314 6. Barry BW, Gray GMT (1978) J Colloid Interface Sci 66:110 7. Filipovi&Vincekovi~ N (1986)J Radioanalyt Nucl Chem 99:89 8. BeunenJA, Ruckenstein E (1983)J Colloid Interface Sci96:469 9. Kertes AS (1965) J Inorg Nucl Chem 27:209 10. Larsen JW, Tepiey LB (1974)J Colloid Interface Sci 49:113 11. Brunn TS, H611and H, Vikingstadt E (1978)J Colloid Interface Sci 63:590 12. Koshinuma M (1983) Bull Chem SocJpn 56:2341 13. Malliaris A, Binana-Limbele W, Zana R (1986) 110:114

Filipovi&Vincekovid and Skrti~, Counterion binding on mixed anionic~cationic micelles 14. Hoyer HW, Marmo A, Zoellner AH (1961) J Phys Chem 65:1804 15. Botr~ C, Crescenzi VL, Mele A (1959)J Phys Chem 63:650 16. Tokiwa F, Moriyama N (1969) J Colloid Interface Sci 30:338 17. Jansson M, Rymd~n R (1987)J Colloid Interface Sci 119:185 18. Zana R (1980) J Colloid Interface Sci 78:330 19. Gunnarsson G, J6nsson B, Wennerstr6m H (1980) J Phys Chem 84:3114 20. Rathman FJ, Scamehorn JF (1984)J Phys Chem 88:5807

957 Received December 10, 1987; accepted May 25, 1988

Authors' address: N. Filipovi&Vincekovi8 Ruder Bo~kovi8Institute Zagreb, Yugoslavia

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