The Effect of Butanol on the Micellar Properties of SodiumDodecyl Sulfate in Aqueous Electrolyte Solutions D. ATTWOOD, V. MOSQUERA,* AND V. PEREZ-VILLAR* Department of Pharmacy, University of Manchester, Manchester M13 9PL, United Kingdom, and *Departmento de Fisica de la Materia Condensada, Facultad de Fisica, Universidad de Santiago, Santiago do Compostela, Spain Received January 12, 1988; accepted March 18, 1988 The effect of butanol on the micellar properties of sodium dodecyl sulfate in aqueous solutions containing a range of concentrations of sodium chloride has been investigated by quasi-electric light scattering. Butanol addition up to 1 M at constant electrolyte concentration caused a progressive decrease in hydrodynamic radius. Interpretation of diffusion data using the Derjaguin-Landau-Verwey-Overbeek theory showed a decrease of surface charge with increase in concentration of added butanol. © 1989Academic Press, Inc.
The effect of medium chain-length alcohols (propanol to heptanol) on the properties of the micelles of sodium dodecyl sulfate (SDS) has been extensively studied, often with the objective of determining the role of these alcohols as cosurfactants in microemulsion systems. These alcohols are partitioned between the micellar and aqueous phases and several papers dealing with their distribution in SDS/ water systems have been published in recent years (1-9). Several workers have demonstrated a decrease of the micellar aggregation number of SDS following the addition of medium chainlength alcohols (10-12) and an accompanying decrease of the critical micelle concentration (CMC) (2, 5, 9, 13, 14). Recent studies on the properties of micelles using the technique of quasi-elastic light scattering have demonstrated the value of this technique, not only in the determination of the hydrodynamic radius of surfactant micelles, but also in the study of interactions between them. Corti and Degiorgio (I 5, 16) have interpreted quasielastic light scattering data for aqueous SDS solutions containing added electrolyte using
the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability. This, or similar, approaches were subsequently applied to other ionic micellar solutions (17-20). In essence, this method of data analysis assumes that the micellar properties are independent of the surfactant concentration over the concentration range under investigation. The concentration dependence of the measured diffusion coefficient in the presence of a range of concentrations of electrolyte is then interpreted in terms of micellar interaction using the pair interaction potential of the DLVO theory. The approach adopted by Nicoli and Dorshow and Athanassakis et aL (I 9, 20) differs in treating in this way only systems containing low salt concentrations where the diffusion coefficient increases linearly with surfactant concentration; the negative gradients exhibited at high salt concentrations are assumed by these workers to be indicative of micellar growth and are excluded from the analysis. We have adopted a similar approach in our analysis of quasi-elastic light scattering data from SDS/butanol/water systems conraining added electrolyte and have utilized the DLVO theory to determine the influence of
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Journal of ColloM and Interface Science, Vol. 127, No. 2, February 1989
EFFECT OF BUTANOL ON SDS MICELLES
increasing quantities of butanol on the micellar charge. EXPERIMENTAL
Materials. Sodium dodecyl sulfate (BDH) was specially purified grade (purity >I 99%) and was used as received. Butan-1-ol (BDH) and sodium chloride (BDH) were of AnalaR grade. Anilinonaphthalenesulfonate (Sigma) was used as the ammonium salt. In the preparation of dilutions of the SDS/butanol/NaC1 (aq) systems, the alcohol and electrolyte were treated as cosolvents of the surfactant in the usual way; i.e., only the surfactant concentration was progressively decreased. Light scattering measurements. Quasi-elastic light scattering measurements were made at 25 + 0.1 °C using a Malvern 7027 digital autocorrelator equipped with a 3-W argon ion laser (Coherent Innova 90) operating at 488 nm on the single clipped homodyne mode with 60 linearly spaced channels. Solutions were clarified by ultrafiltration through 0.1~tm filters. RESULTS AND DISCUSSION
Effect of Butanol on Micellar Size and Charge Apparent diffusion coefficients, D, of SDS in aqueous solutions containing 0.5 Mbutanol and 0.1 to 0.5 M sodium chloride are presented in Fig. 1 as a function of SDS molality, C. Similar plots were obtained in the presence of 0.35 and 1 M butanol. Experimental data have been fitted with the linear function D = D0[1 + kb(C-CMC)],
where Do is the limiting diffusion coefficient at the critical micelle concentration. Preliminary determinations of the CMC by the measurement of the concentration dependence of the fluorescence from solubilized anilinonaphthalenesulfonate (ANS) (21) indicated an approximate CMC value of 1.5 × 10 -5 mole kg -1 in systems containing 0.5 M butanol in the presence of 0.1 M NaC1. Although this
technique was unsuitable for precise measurement at such low concentrations it was clear that butanol addition caused a lowering of the CMC (the CMC of SDS in 0.1 M NaC1 in the absence of butanol is 14.9 × 10 -4 mole kg -3 (22)), in agreement with the reported effects of this and other medium chain-length alcohols on the CMC of SDS (2, 5, 9, 13, 14). Consequently it was considered that the CMCs in the systems studied here were sufficiently low for extrapolation to be performed to zero concentration without the introduction of significant error. The limiting diffusion coeffidents and the hydrodynamic radii, r~, derived from the Stokes-Einstein relationship, rh = kBT/(61r~Do), where kB is the Boltzmann constant, T is temperature, and ~ the solvent viscosity, are given in Table I. Values of 71for these systems were calculated from the mole fraction, x, of component in the system using
(23) log n = XA1Og~A ~- xalog ~B.
Subscripts A and B of Eq.  refer to aqueous electrolyte and butanol, respectively. Values of nA and nB were literature values. Table I shows a tendency for an increase of hydrodynamic radius as a function of added salt concentration at constant concentration of added butanol and a decrease of rh with increase of butanol at constant ionic strength. This latter decrease occurs despite the increased amounts of alcohol which are partitioned into the micelles at the higher alcohol concentrations and it indicates a decreased number of surfactant molecules per micelle. Candau and Zana (24) have reported a similar decrease of rh of tetradecyltrimethylammonium bromide (TTAB) micelles as a function of the molarity of added butanol. A decrease of aggregation number following the addition of significant amounts of the water-soluble alcohols to aqueous solutions of SDS has been reported ( 10-12). In order to relate the changes of slope of the diffusion plots to changes in the interactive Journal of Colloid and Interface Science, VoL 127,No. 2, February1989
A T T W O O D , MOSQUERA, A N D PEREZ-VILLAR 2.0 10
c (r,,o, k~')
I~G. 1. Diffusion coefficient, D, plotted as a function o f S D S molality at various concentrations o f added NaCI in the presence of 0.5 M butanol.
forces between the miceUes, the data were analyzed according to the treatment proposed by Corti and Degiorgio (15, 16). Expressing the data in terms of the volume fraction, 9, of the micelles rather than their concentration involves the correction of kb as determined TABLE! Limiting Diffusion Coefficient, Do, Hydrodynamic Radius, rh, Experimental and Theoretical Slopes, kD, as a Function of Butanol and Sodium Chloride Molarities kD
kD = 1.56 +
dxF(x) x [1 -
[BuOH l (mole dm -a)
[NaCI] (mole dm -a)
lOJ°Do (m 2 s-t)
0.1 0,2 0.3
1.02 1.00 0.97
2.37 2.40 2.45
19.3 11.8 6.4
18.8 10.7 7.8
0.1 0.2 0.3 0.4 0.5
1.06 1.03 1.01 0.97 0.94
2.28 2.33 2,37 2.43 2,50
19.5 8.9 5,0 0.0 -9.5
18.7 10.3 7.4 ---
0.1 0.2 0.3 0.4 0.5
1.16 1.13 1.10 1.05 0.96
2.05 2,09 2,13 2.2 ! 2.38
20.9 9.4 6.6 0.0 -2.9
19.6 10.3 7.0 ---
from the slopes of these plots, according to k o = k b / ~ m where Vmis the partial molar volume of the micelles as derived from density measurements. kD may be related to the pair interaction potential, V ( x ) , between rigid spherical partieles of radius a (equated to the hydrodynamic radius rh) using expressions proposed by Felderhof (25),
Journal of Colloid and lmerface Science, Vol. 127,No. 2~February 1989
where x = ( R - 2 a ) / 2 a , R is the distance between the centers of two particles, and F ( x ) is given by
F(x) = 12(I + x ) - 15/8(1 + x) -2 + 27/64(1 + X) -4 -~- 75/64(1 + X) -5. 
Two analytical expressions are available for estimation of the repulsive Coulombic interaction, VR(X), relating to the limiting cases Ka ,~ 1 and Ka ,> 1. For the systems under investigation Ka > 2 and the data were ana-
EFFECT OF BUTANOL ON SDS MICELLES
lyzed assuming the latter limiting case to apply. The relevant equation is VR(x) = (~a92/2)ln[1 + exp(-2Kax)],  where ~is the dielectric constant of the solvent, xI,0 is the surface potential, and Kis the reciprocal Debye-Hfickel length. The attractive London-van der Waals interaction term, VA(X), was determined using VA(X) = - ( A / 1 2 ) [ ( x 2 + 2x) -l +(X z + 2 x +
e~o/kBT [BuOH] (mole dm -3)
0.00 ~ 0.35 0.50 1.00
37 27 24 18
4.5 5 8 12
2.93 2.56 2.44 2.34
2.30 1.96 1.83 1.74
1 . 9 2 1 . 6 3 1.38 1.60 - 1 . 5 0 1 . 2 8 1.10 1 . 4 4 1.18 0.94
where A is the Hamaker constant. It is important to recognize that the validity of this treatment is limited by the concentration of added electrolyte. At low salt concentration the range of the potential VR(X) may become comparable with the interparticle distance, while at high salt concentration the attractive interparticle potential may promote micellar association leading to a polydispersity of particle sizes. The values of the unknown parameters A and xg0 were derived from the positive slope regions only, of the diffusion plots on the assumption that both of these parameters were independent of salt concentration. The computational procedure involved the iteration of values of A and xIt0to give the best fit of computed and experimental values of kD over the range of electrolyte concentration for each butanol molarity. Agreement between computed and experimental kD values was reasonable in view of the assumptions inherent in these calculations (see Table I). The miceUar charge, q, is related to the surface charge, 90, by the expression (26) ~o = (2kBT/e) × sinh -~[2~re~-lqe/(47ra2~kBT)].
Micellar Charge, q, Hamaker Constant, A, and Electric Potential at Shear Surface, x~o, of SDS Micelles as a Function of Butanol and Sodium Chloride Molarities
Data from Ref. (16).
1) -1 + 2 1 n ( x 2 + 2 x ) / (x z + 2 x + 1)1,
Values ofq derived from Eq. [7 ] and Hamaker constants from Eq. [ 6 ] are compared in Table II with values derived by Corti and Degiorgio (t6) for SDS in the absence of butanol. The
ratios eXYo/kBT of this table were derived as a function of salt concentration using the values of q, with the assumption that q is independent of concentration of added electrolyte. It is clear from Table II that butanol addition causes a progressive decrease in charge per unit area of surface of the SDS micelles. The increase of the Hamaker constant reflects the increased attractive forces which result from this effect. Intercalation of butanol between the head groups of SDS has been shown by recent spin-echo modulation studies (27) to open up the surface region and hence from a purely geometric viewpoint it would be expected to lead to a decrease of surface charge density. Opposing this effect is the increased micellar ionization, which has been noted from electromotive force and conductivity measurements for SDS micelles on the addition of pentanol (28) and for TTAB micelles following solubilization of medium chainlength alcohols (29). Such increases of ionization are thought to be a consequence of the decreased polarity at the palisade layer due to the presence of the solubilized alcohol, causing increased repulsions between the ionic head groups and a resultant dissociation of surfactant ions, in an attempt to reduce the repulsive forces. An additional factor which tends to reduce the surface charge is the possible decrease in the number of surfactant monomers per micelle as a consequence of solubilization of medium chain-length alcohol. Our results Journal of Colloid and InterfaceScience, Vol. 127, No. 2, February 1989
ATTWOOD, MOSQUERA, AND PEREZ-VILLAR
show that those factors which act to reduce surface charge outweigh the effect of any increased ionization of the head groups. Similar conclusions have been made by Grieser (30) from a study of the nitrite quenching of luminescence from terbium bound to SDS micelles in the presence of added butanol. Almgren and Swamp (12) have shown from calculations based on a knowledge of the micellar aggregation number and the alcohol/water partition coefficient of the SDS/butanol/water system that the charge density decreases as a linear function of the mole fraction of butanol in the system, again in quantitative agreement with our results. ACKNOWLEDGMENT One of the authors (V.M.)thanks the Xunta de Gallicia for financial support. REFERENCES 1. Holland, H., Ljosland, E., and Backlund, S., J. Colloid Interface Sci. 101,467 (1984). 2. Hayase, K., and Hayano, S., Bull. Chem. Soc. Japan 50, 83 (1977). 3. Hayase, K., Hayano, S., and Tsubota, H., J. Colloid Interface Sci. 101, 336 (1984). 4. Abuin, E. B., and Lissi, E. A., J. ColloidlnterfaceSci. 95, 198 (1983). 5. Hayase, K., and Hayano, S., J. Colloid Interface Sci. 63, 446 (1978). 6. Manabe, M., Shirahama, K., and Koda, M., Bull. Chem. Soc. Japan 49, 2904 (1976). 7. Almgren, M., Greiser, F., and Thomas, J. K., J. Chem. Soc. Faraday Trans. 1 75, 1674 (1979). 8. St]lbs, P., J. ColloidlnterfaceSci. 87, 385 (1982). 9. Rao, I. V., and Ruckenstein, E., or. Colloid Interface Sci. 113, 375 (1986).
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