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Micellization study of sodium dodecyl sulfate in water and microemulsion systems by conductivity and counterion-activity measurements
Micellization Study of Sodium Dodecyl Sulfate in Water and Microemulsion Systems by Conductivity and Counterion-Activity Measurements JOSEPH GEORGES
AND J I A N - W E I C H E N
Laboratoire de Chimie Analytique 3 - U.A. CNRS 04 0435, Universit£ Claude Bernard, Lyon L 69622 Villeurbanne Cedex, France
Received September 16, 1985; accepted December 9, 1985 The micellization process of sodium dodecyl sulfate (SDS) involved in water/SDS, water/SDS/1pentanol, and water/SDS/1-pentanol/dodecane systems has been followed by means of pNa and conductivity measurements in a wide SDS concentration range lying from concentrations below the critical micelle concentration up to the boundary of the realm of existence of the isotropic monophasic media investigated. In all cases, the plots of free counterion activities and conductivities against the logarithm of SDS concentration show two characteristic breakpoints. The first breakpoint agrees well with the critical micelle concentration of SDS and the second breakpoint, corresponding to an obvious increase of the degree of counterion dissociation c~ is interpreted as proceeding from a change in the micelle structure and a different distribution between free and bound counterions. In pure SDS solutions, the slopes of the pNa curve above the first and the second breakpoints give a values equal to 0.21 and 0.39, respectively. The addition of 1-pentanol and dodecane acts upon both micellization steps by decreasing the concentration of the breakpoints and by changing the amount of associated counterions. In all systems, the electrical conductivity of the solutions is ensured essentially by the free sodium counterions. The addition of NaC1 is followed by a marked increase of associated Na ÷ counterions in the water/SDS solutions but the effect is lowered in the ternary and quaternary systems. © 1986AcademicPress,Inc. INTRODUCTION A m p h i p h i l i c molecules or surfactants which combine pronounced hydrophilic and hydrophobic properties within one molecule may lead to various k i n d s o f isotropic solutions inc l u d i n g binary, ternary, a n d q u a t e r n a r y systems. I n water, the a m p h i p h i l i c ions or m o l ecules, at t h e critical micelle c o n c e n t r a t i o n (CMC), aggregate s p o n t a n e o u s l y to f o r m m i celles. O n e o f the m o s t i m p o r t a n t p r o p e r t i e s o f these s u r f a c t a n t - w a t e r systems is t h e i r ability to solubilize a third c o m p o n e n t (1). Typical t e r n a r y systems are o b t a i n e d w h e n the solubilizate is a w e a k l y p o l a r substance as a m e d i u m - o r l o n g - c h a i n alcohol. As the solubility o f the a l c o h o l in w a t e r is small, it is i n c o r p o r a t e d into the surfactant aggregates to f o r m " s w o l l e n " micelles. A t high a l c o h o l contents, the a l c o h o l b e c o m e s the solvent a n d t h e syst e m s changes f r o m L1 to L2 structures. T h e
a d d i t i o n o f a f o u r t h c o m p o n e n t , generally a h y d r o c a r b o n solvent, m a y lead, w h e n the c o m p o u n d s are in a d e q u a t e a m o u n t s , to the so-called W i n s o r IV solutions (2) o r m i c r o e m u l s i o n s (3, 4). I n the case o f ionic surfactants, the electrostatic r e p u l s i o n forces b e t w e e n t h e ions o f t h e s a m e charge at the m i c e l l a r surface are w e a k e n e d b y the b i n d i n g o f c o u n t e r i o n s to t h e m i celle (5). So a m o n g the p a r a m e t e r s characterizing the m i c e l l i z a t i o n process a n d the p r o p erties o f ionic micelles, c o u n t e r i o n b i n d i n g is with the C M C o n e o f the m o s t studied. T h e p u r p o s e o f this w o r k was to follow the micellization process o f s o d i u m d o d e c y l sulfate (SDS) i n v o l v e d in three k i n d s o f solutions: a q u e o u s m i c e l l a r solutions, t e r n a r y s o l u t i o n s c o m p o s e d o f water, SDS, a n d n - p e n t a n o l , a n d q u a t e r n a r y solutions c o m p o s e d o f water, SDS, n-pentanol, a n d n-dodecane. O u r e x p e r i m e n t s were p e r f o r m e d b o t h b y direct m e a s u r e m e n t 143 0021-9797/86 $3.00
Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.
144
GEORGES AND CHEN
of counterion activity using an ion-specific electrode and by conductivity. Unlike most investigations where informations are almost exclusively derived from measurements at surfactant concentrations just above the CMC, the present study was carded out in a large concentration range, from concentration below the CMC up to the boundary of the realm of existence of the isotropic monophasic media.
tion obtained by adding agar-agar to a 3 M NH4C1 electrolytic solution (6). With this electrochemical cell, the response of the electrode against NaC1 was linear in the concentration range studied (10 -3 to 5 × 10 -1 M) with a slope equal to 56 mV. The conductivity measurements were carried out wtih a Tacussel-Solea CD 7N conductivity meter equipped with a CM 02-55 G cell. In all cases, the measuring cell was thermostated with a water bath at 25 ___0.2°C. EXPERIMENTAL Samples and methods. The experiments Materials. Sodium dodecyl sulfate was sup- were done on samples whose composition is plied by Merck or BDH Ltd. Due to its high indicated on Figs. 1 and 2 which represent the purity (>199%) further purification was not ne- mass phase diagrams of the ternary system cessary. Yet to be sure of our results a sample water/SDS/1-pentanol and the quaternary of SDS was purified by a double recrystalli- system water/SDS/1-pentanol/dodecane with zation from ethanol; as no significant change a weight ratio of surfactant to alcohol k = ½. appeared in the results, most of the experi- These diagrams are well known and given ments were done with commercial SDS. The elsewhere (7-9) but they were redrawn for our n-pentanol (>/99%) and the n-dodecane work because some experiments had to be (>/99%) were purchased from Merck and used performed with 0.1 MNaC1 in water. Our diawithout any further purification. Sodium grams show little differences with the pubchloride was supplied by PROLABO. Water lished ones with respect to the boundaries of was deionized and distilled in glass; its con- the monophasic areas but no significant ductivity was 2-3 #S cm -1. change appears along the composition paths Apparatus. Potentiometric measurements of studied in this work. Therefore the composicounterion activity were performed with a tion paths indicated on Figs. 1 and 2 are valid digital reading pH-meter millivoltmeter (So- for the experiments made with or without lea-Tacussel). The sodium selective electrode electrolyte in water. Four sets of experiments were carried out was a glass electrode manufactured by Corning. The glass membrane (NAS 11-18) exhibits on the lines A, B, C, and D (Figs. 1 and 2): a high specificity to sodium ions. The electrode binary systems water/SDS, ternary systems response to sodium ions is unaffected by pH water/SDS/pentanol along two composition as long as pH is at least 2 units higher than paths (weight ratio of SDS to pentanol = 2.33 pNa. The reference electrode was a saturated and 0.5) and quaternary systems with a weight KCI calomel electrode with a double junction ratio of dodecane to active blend equal to (Tacussel RDJ/C8). The junction between the 0.176. reference system (saturated KC1) and the sampNa and conductivity experiments were ple solution was chosen so that: (i) the leakage performed step-by-step on the four lines A, B, of the solution into the sample results in the C, D. A concentrated solution of SDS or a minimum contamination of the sample; (ii) concentrated microemulsion (composition the bridge solution does not react with the located near the upper boundary of the monosample because precipitation would block the phasic area on the studied lines) was first prejunction; and (iii) there is no variation of the pared and used as a mother solution. Then junction potential during an experiment. The two operating modes were used. Low concenbest results were obtained with a gel,like junc- tration samples were obtained by addition of Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE
145
SDS
PENTANOL
WATER+O.1 M NaCI
~G. 1. Brine/sodium dodecyl sulfate/pentanol system: partition of the realm of existence of the monophasic ternary micellar solutions. A, B, and C are the composition paths followed in the experiments (see Experimental).
the mother solution to water or water + O. 1 M NaC1 with a microsyringe directly in the measuring cell; high concentration samples
were obtained by dilution o f the mother solution with water or water + 0.1 M NaC1. For the experiments carried out with NaC1, the
SDS
PENTANOL
WATER +0.1 M NaCI
=¢ 1 2
DODECANE
FIG. 2. Mass pseudoternary phase diagram of the quaternary system brine/SDS/pentanol/dodecane. Weight ratio of surfactant to pentanol: k = ½. D is the composition path studied (see Experimental). Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
146
GEORGES AND CHEN
mother solution contained also 0.1 M NaC1 in the aqueous phase. RESULTS AND DISCUSSION
1. Micellization of SDS in Pure Water The activity of free sodium counterions and the conductivity were measured in aqueous solutions of SDS in a large concentration range from 5 X 10 -4 to approximately 1 M (~29% w/v). The plots of the electrode potential and of the conductivity as a function of the logarithm of SDS concentration (Fig. 3) are very similar in shape. Except for concentrations below the CMC, the plots follow a curve from which appear three almost linear parts intersecting in two characteristic points. At low concentration, the linear relationship between the electrode potential and log[SDS] has a slope close to that of the calibration curve obtained with NaC1; it indicates that the concentration of the free sodium counterions is equal to the concentration of added SDS. Above the first break, the SDS undergoes micellization and a great part of Na + ions are
associated to the micelle surface causing a reduction of free sodium ions. If the concentration of SDS is increased further, a second change of the slope occurs but with an upward break located between 5 and 7 X 10-2 M according to the experiments and quality of SDS used. The conductivity curve shows approximately the same variations and the same breakpoints, which would indicate that the electrical conductivity of the solution is ensured essentially by the free sodium ions. Critical micelle concentration. If obviously the first break at 7.4 X 10-3 M in the pNa curve and 7.9 X 10-3 M in the conductivity curve corresponds to the CMC of SDS, the interpretation of the second break is not easy. Many workers have studied micellar growth in SDS solutions with high NaC1 content or in presence of organic additives. Quasielastic light scattering experiments in SDS solutions have demonstrated a continuous transition in micellar size and shape from spherical miceUes to rode-like structures (10); only within the limits of high temperature (>50°C), low NaC1
0
-25
-5o °
-~5
-IO0
-I
-125
-3
-21.5
I
-2
109
I
-1.5
i
-1
i
-.5
[SOS]
FIG. 3. Variations of the potential of the pNa electrode (O) and of the electrical conductivity a (MS cm -l) (O) along the composition path A, without NaC1. Journal of Colloid and Interface Science, Vol. 113, No, 1, September 1986
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE concentration (<0.4 M), or low surfactant concentration (near the CMC), do the SDS micelles have a minimum size. Other workers (11) have shown, by small-angle X-ray scattering techniques, that a change in the micellar form of SDS from spheres to rods occurs within a range of concentration whose limits are 7 X 10 -2 and 2.5 X 10-~ M. On the other side, some workers studying SDS solutions without any additive, had talked about a "second CMC" at 8.5 × 10 -2 M ( 1 2 ) or 7 × 10 -2 M (13). More recently, in the study of fluorescence intensity ratio of excimer to monomer of pyrene as a function of the concentration of SDS, Song et al. (14) showed a very distinct breakpoint at 7 × 10 -2 M, which was interpreted as a phase transition from sphereshaped micelles to hemicapped rod-like micelles. These values are close to our second breakpoint and even if the term of "second CMC" is incorrect, it surely corresponds to a change in the micelle structure. Counterion binding. The decrease in pNa and conductivity values above the CMC is explained by the binding of the counterion to the micelle. The degree of counterion binding [3 or the degree of dissociation a = 1 - [3 may be estimated from the slope of the pNa curve below and above the CMC. Many equations have been proposed in the literature (5), but the simplified Botr6's equation (15) which assumes that the concentration of the free amphiphilic ions remains constant above the CMC gives a good approximation of a or/3: (Na +) = 7Na+(CMC + a(CT -- CMC)) where (Na +) is the activity of free counterion, YNa+ the activity coefficient of free counterion above the CMC, and CT the surfactant concentration. If ~'Na* is assumed to be a constant, one obtains 1 ot =
d(Na +) X
YNa+
-
-
dCT
where d(Na÷)/dCT is the slope above the CMC. Assuming "YN~+= 0.9(16), one obtains a = 0.21. In fact d(Na÷)/dCT is not really a constant and the value given for a is a mean value.
147
Above the second breakpoint, a is almost a constant because [SDS] ~> CMC and the same calculation gives a = 0.39. These results compare favorably with values given by other workers (Table I). Indeed, it appears that two groups of values, one around 0.25 and the other near 0.4, would lead to the assumption that a depends greatly on the concentration of SDS at which the determination was made. Electrical conductivity. The decrease of the conductivity above the CMC is also the result of micelle counterion binding. If the concentration of monomeric amphiphilic ions is assumed to remain constant above the CMC, the change in specific conductivity is due to the free Na + counterions not associated to the micelles and to the ionic micelles. The conductivity of each species is proportional to the product of its concentration, its charge, and its mobility. The mobility being proportional to the ratio of the charge to the radius of the species, the conductivity of the species is proportional to the product (square of the charge X concentration) divided by the radius. For the free Na ÷ ions, the concentration is Ct -/3(CT -- CMC) and the charge is unity; for the micelles, the concentration is (CT -- CMC)/ n and the charge - n a , where n is the aggregation number. At low surfactant concentration (CT), the conductivity of the micelles is much smaller than that of the free counterions. When CT >> CMC, the product (square of the charge X concentration) is aCT for the free counterion and na2Ct for the micelles; so, even taking into account the difference in size of the two species, the conductivity of the micelles may not become negligible, which may explain the great slope of the conductivity curve above the second breakpoint. In spite of that, the increase in conductivity above the second breakpoint demonstrates the liberation of Na ÷ ions shown also by the pNa curve. Interpretation. According to the different models proposed to describe the ionic micelles (21-25), the counterion binding is described in terms of a continuous radial distribution function corresponding to a high counterion concentration close to the micelle and continJournal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
148
GEORGES
AND CHEN
TABLEI DegeesofCoun~fion
Dissociafiona ~r MicellesofSDS:ComparisonofOurValues withThoseFoundintheL~ure Other works
Present work
a
0.21 0.39
(5)
(16)
(20)
(17)
(18)
(19)
0.22-0.27
0.21
0.25 to 0.32 a
0.38
0.4
0.36
a Values m e a s u r e d for SDS c o n c e n t r a t i o n s lying between 1 a n d 2 X 10 -2 M.
uously decaying with increasing distance from the micelle (18). So the distinction between bound and free counterions is difficult and would greatly depend on the technique used to study counterion binding. In addition, some workers have described the micelle structure with an ill-defined Stern layer (24) or even in the case of SDS an "open" structure (25) with a not well-ordered phase boundary between the Stern layer and the hydrocarbon core. In both cases, the incorporation of water between the polar head groups would be important. This structure is probably true at low surfactant content, but as the surfactant concentration increases, the spherical micelles are assumed to form larger aggregates or rod-like micelles in which the area per head group at the surface of the cylindrical portion of the micelle would be less than in the case of spherical micelles (26). In absence of electrolyte, some of the spherical micelles may, before or instead of transforming into rod-like miceUes, associate into other types of aggregates where the spherical micelles would tend to link with one another to form microgel particles (27). In all cases, the increase in SDS concentration would induce an exit of the water incorporated between the polar head groups, leading to a different distribution between free and bound counterions or at least a different reading of the pNa electrode.
2. Influence of Additives Figures 4, 5, and 6 show the comparative plots of the electrode response to free Na + and Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
of the conductivity with the logarithm of SDS concentration, along the composition paths B, C, and D indicated on Figs. 1 and 2. As in the case of SDS, the pNa and conductivity curves follow the same variations and present two breakpoints. The various slopes and the values of the breakpoints are compared in Table II and III.
Ternary systems: water/SDS/1-pentanol. Below the CMC, the electrode response is not changed by the presence of the alcohol (Figs. 4 and 5) and the solution behaves as an ordinary electrolyte solution while the pentanol is dissolved in water. When the concentration of (pentanol + SDS) increases, with constant ratio, the curves first break downward at a SDS concentration less than in pure water, indicating that the alcohol induces the micelle formation. The CMC obtained from potential data agree well with those obtained from conductivity data (Tables II and III). At the same time, the slope of the curves above the CMC increases in the presence of 1-pentanol, indicating an increase of the degree of counterion dissociation a as the alcohol is incorporated into the micelles. The CMC lowering and counterion liberation effect increase as the ratio 1-pentanol/SDS increases (lines B and C). The increase in a with increasing alcohol concentration may be explained by a greater partitioning of the alcohol into the micelle phase. The alcohol penetration into the micelle produces a "tightening" of the SDS micelle (28); the alcohol binds to the micelle with its hydroxy groups between the surfactant head groups and so decreases the repulsion between
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE
149
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-1~
-1~
I
-3
I
-2. 5
I
I
-2 -1.5 lo9 [SOS]
I
I
-1
-. 5
FIG. 4. Variations of the potential of the pNa electrode (©) and of the electrical conductivity a (mS cm -~) (O) in the ternary system along the composition path B defined by a SDS-to-pentanol weight ratio of 2.33, without Na~l. I
I
0
I
I
o.." ./,~"
-25
~
..o
-50
0 °
-75
8~
-100
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-125
l
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-2.5
l
l
-2
-1.5
l
-1
l
-.5
Io9 [SDS] FIG. 5. AS in Fig. 4. Composition path C defined by a SDS-to-pentanol weight ratio of 0.5, without NaC1. ]?he arrow on the graph indicates the boundary of the L1 monophasic area.
Journal of Colloid and Interface Science,
Vol.113,No. 1, September1986
150
GEORGES AND CHEN
o
-2
-7:
-IIX
-1~
. . . . .
I
I
I
I
I
I
-3
-2. 5
-2
-1.5
-1
-. 5
lo 9 [sos]
FIG. 6. Variations of the potential of the pNa electrode (O) and of the electrical conductivity a (mS cm -1) (e) in the quaternary system along the composition path D defined by a SDS-to-pentanol and a dodecaneto-active blend weight ratio of, respectively, 0.5 and 0.176, without NaC1.
them, leading consequently to liberation of counterions. It is meaningful to compare the concentration of free Na ÷ measured at [SDS] = 0.1 M (Table II). Along both composition paths B and C, the curves show a second breakpoint with an increase of the slope, as in the absence of pentanol, but at a SDS concentration which is also lowered. The decrease of the concentration of the second breakpoint
and the invariance of the various curves above this second breakpoint seem to indicate that the alcohol effect acts essentially at low SDS content and decreases the concentration at which the change in the micelle structure occurs.
Quaternary systems: water/SDS/1-pentanol/dodecane. With the addition of dodecane (line D) the micellization process of SDS (Fig.
TABLE II Parameters of the pNa Curves for the Samples on Composition Paths A, B, C, and D (Figs. 1 and 2) without Electrolyte
SDS line
sl (mY)
CMC ( 10-3 M)
s2 (mY)
bp2 ( 10-2 M)
s3 (mV)
A B C D
52 51 53 56
7.4 6.4 5.2 4.6
28 30 33 25
5-7 3.6 1.5 1.6
58 60 60 59
Free Na + (M) at C r = 0.1 M
3.3 4.6 8.3 6.3
× × X ×
10-2 10-2 10-2 10-2
Note. CMC corresponds to the first breakpoint; bp2 is the second breakpoint; sl, s2, and s3 are the slopes of the curves, respectively, below the CMC, between the two breakpoints, and above the 2nd breakpoint; free Na + was estimated from the calibration curve obtained with NaC1. Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE
151
TABLE IU Parameters of the Conductivity Curves for the Samples on Composition Paths A, B, C, and D (Figs. 1 and 2) without Electrolyte~ SDS line
A B
C D
sl (mS cm -1)
CMC (10 -3 M )
s2 (mS cm -I)
bp2 (10-2 M)
s3 (mS cm -I)
8.7 9.1 8.7 9.1
7.9 6.3 5.2 4.4
3.3 4.1 4 4.4
5.6 3.3 1.4 1.4
12.5 11.2 11 10.5
~ (mS crn -1) at CT = 0.1 M
2.9 4
4.8 4.5
a Same indications as in Table II.
6) is nearly the same as in the corresponding ternary system (line C). The counterion dissociation is rather less because a part of the alcohol m a y dissolve with dodecane in the hydrophobic core rather than in the palisade layer. At higher SDS content, the conductivity curve shows a m a x i m u m and then decreases. At this stage in the discussion, it is interesting to compare the variation of the electrical conductivity versus water weight fraction along the various composition paths A, B, C, D. In the water-SDS system, the conductivity increases almost linearly until high values when the water content decreases; when pentanol is added to the system, the plot incurvates when one reaches the boundary of the monophasic area and the m a x i m u m in conductivity decreases when the pentanol to SDS weight ratio increases (lines B and C). This feature anticipates the further change in the structure of the micelles from monophasic solutions of pentanol in water (L1) to monophasic solutions of water in pentanol (L2), change which occurs at lower SDS concentration when the a m o u n t of pentanol increases. In the case of the microemulsions (Figs. 6 and 9), the conductivity curve reaches soon the m a x i m u m value which is interpreted as corresponding to the beginning of the phase transition zone between O/ W and W / O structure (29, 30). The comparison of Figs. 1 and 2 and these results show that the configuration and the properties of the pseudoternary system issue directly from those of the water/SDS/pentanol system (9).
Influence of NaCl. As the addition of an electrolyte in the water phase buffers the solutions, the influence of added NaC1 will be discussed only from the conductivity experiments. The curves obtained with and without NaC10.1 M are compared in Figs. 7-9. In the water/SDS system (Fig. 7), the addition of NaC1 does not lead to a large variation of the high conductivities except of course for the
[I
.5
.6
.7
.8
.9
W0t~ weight fr~ti0~
FIG. 7. Variations of the electrical conductivity of the water/SDS system (line A) with ([~) and without (m) 0.1 M NaCI in water. Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
152
GEORGES AND CHEN
less concentrated solutions. The difference between the two curves decreases markedly when the SDS concentration increases, meaning that the presence of NaCl increases the degree of counterion association/3; this results in a relative decrease of the electrical conductivity due to an increase of associated Na + ions and to a decrease of the micelle charge. In the ternary system (Fig. 8), the difference between the two curves remains more important along the whole composition path. Indeed, the incorporation of l-pentanol in the SDS micelles has an effect opposite to that of NaC1, but the resuiting effect is also an increase of/3. In the quaternary system, the addition of NaCl leads to a greater increase of the electrical conductivity in the realm of existence of O/W microemulsions (Fig. 9). As shown in the figure, the presence of the electrolyte affects the position of the conductivity maximum, which means that the phase transition zone between
T
~
N
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2,
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FIG. 9. Variations of the electricalconductivity of the waterfSDS/pcntanolJdodccane system (line D) with (D) and without (m) 0.I M NaCl in water. The dotted partsof the curves indicatethe discontinuitydue to the proximity of the mesophasic area (sce text).
! the O/W and W/O structures begins at a greater water content along the composition path studied. When the water content is less than 55%, the curves join and the added NaC1 has no influence on the electrical conductivity ofbicontinuous or W/O microemulsions. The discontinuity in the curves before the maximum originates from the fact that the studied composition path goes along and even, without added NaC1, into the mesophasic area located on the water-active mixture side (Fig. 2). REFERENCES
.5
,6
~
.7
,8
.8
feint froctim
l~o, 8. Variations of the electricalconductivity of the watcr/SDS/pcntanol system (lineB) with (D) and without (m) 0.I M NaCl in water.The arrow indicatesthe boundary of the LI monophasic area. Journal of Colloid andlnterface Science,
Vol.113,No.1,September1986
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MICELLIZATION STUDY OF SODIUM DODECYL SULFATE 5. Nishikido, N., J. Colloid Interface Sci. 92, 588 ( 1983). 6. Chen, J. W., and Georges, J., Anal. Chim. Acta 177, 231 (1985). 7. Clausse, M., Heil, J., Peyrelasse, J., and Boned, C., J. Colloid Interface Sci. 87, 584 (1982). 8. Zradba, A., Th~se 3~me cycle, Universitg de Pau, 1983. 9. Clausse, M., Heil, J., Zradba, A., and Nicolas-Morgantini, L., XVI Jornados del Comite Espafiol de la Detergencia, Tensioactivos y Afines, Barcelona, March 13-15, 1985, p. 497. 10. Missel, P. J., Mazer, N. A., Carey, M. C., and Benedek, G. B., in "Solution Behavior of Surfactants" (K. L. Mittal and G. J. Fendler, Eds.), Vol. 1, p. 373. Plenum, New York, 1982. 11. Reiss-Husson, F., and Luzzati, V., J. Phys. Chem. 68, 3504 (1964). 12. Kodama, M., J. Sci. Hiroshima Univ., Ser. A. 37, 53 (1973). 13. Masaji, M., Bull. Chem. Soc. Japan 45, 428 (1972). 14. Kim, J. S., Kim, C. K., Song, P. S., and Lee, K. M., J. Colloid Interface Sci. 80, 294 (1981). 15. Botre, C., Crescenzi, V. L., and Mele, A., J. Phys. Chem. 63, 650 (1959). 16. Backlund, S., and Rundt, K., Acta Chem. Scand. A. 34, 433 (1980). 17. Baumuller, W., Hoffman, H., Ulbricht, W., Tondre, C., and Zana, R., J. Colloid Interface Sci. 64, 418 (1978). 18. Lindman, B., and Wennerstr6m, H., "Topics in Cur-
19. 20. 21.
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27. 28. 29.
30.
153
rent Chemistry," Vol. 87, p. 1. Springer-Veflag, Berlin/Heidelberg/New York, 1980. Shirahama, K., Bull. Chem. Soc. Japan 47, 3165 (1974). Jain, A. K., and Singh, R. P. B., J. Colloid Interface Sci. 81, 536 (1981). Hartley, G. S., in "Micellization, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. 1, p. 23. Plenum, New York, 1977. Stiger, D., and Mysels, K. J., J. Phys. Chem. 59, 45 (1955). Grahame, D. C., Chem. Rev. 41, 441 (1947). Menger, F. M., Jerkunica, J. M., and Johnston, J. C., J. Amer. Chem. Soc. 100, 4676 (1978). Russel, J. C., and Whitten, D. G., J. Amer. Chem. Soc. 103, 3219 (1981). Mazer, N. A., Carey, M. C., and Benedek, G., in "Micellization, Solubilizafion and Microemulsions" (K. L. Mittal, Ed.), Vol. 1, p. 359. Plenum, New York, 1977. Hayashi, S., and Ikeda, S., J. Phys. Chem. 84, 744 (1980). Russel, J. C., and Whitten, D. G., J. Amer. Chem. Soc. 104, 5937 (1982). Clausse, M., Peyrelasse, J., Boned, C., Heil, J., NicolasMorgantini, L., and Zradba, A., in "Surfactants in Solution" (K. L. Mittal and B. Lindman, Eds.), Vol. 3, p. 1583. Plenum, New York, 1984. Chen, J. W., Dipl6me d'Etudes Approfondies, University of Lyon, 1984.
Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
AND J I A N - W E I C H E N
Laboratoire de Chimie Analytique 3 - U.A. CNRS 04 0435, Universit£ Claude Bernard, Lyon L 69622 Villeurbanne Cedex, France
Received September 16, 1985; accepted December 9, 1985 The micellization process of sodium dodecyl sulfate (SDS) involved in water/SDS, water/SDS/1pentanol, and water/SDS/1-pentanol/dodecane systems has been followed by means of pNa and conductivity measurements in a wide SDS concentration range lying from concentrations below the critical micelle concentration up to the boundary of the realm of existence of the isotropic monophasic media investigated. In all cases, the plots of free counterion activities and conductivities against the logarithm of SDS concentration show two characteristic breakpoints. The first breakpoint agrees well with the critical micelle concentration of SDS and the second breakpoint, corresponding to an obvious increase of the degree of counterion dissociation c~ is interpreted as proceeding from a change in the micelle structure and a different distribution between free and bound counterions. In pure SDS solutions, the slopes of the pNa curve above the first and the second breakpoints give a values equal to 0.21 and 0.39, respectively. The addition of 1-pentanol and dodecane acts upon both micellization steps by decreasing the concentration of the breakpoints and by changing the amount of associated counterions. In all systems, the electrical conductivity of the solutions is ensured essentially by the free sodium counterions. The addition of NaC1 is followed by a marked increase of associated Na ÷ counterions in the water/SDS solutions but the effect is lowered in the ternary and quaternary systems. © 1986AcademicPress,Inc. INTRODUCTION A m p h i p h i l i c molecules or surfactants which combine pronounced hydrophilic and hydrophobic properties within one molecule may lead to various k i n d s o f isotropic solutions inc l u d i n g binary, ternary, a n d q u a t e r n a r y systems. I n water, the a m p h i p h i l i c ions or m o l ecules, at t h e critical micelle c o n c e n t r a t i o n (CMC), aggregate s p o n t a n e o u s l y to f o r m m i celles. O n e o f the m o s t i m p o r t a n t p r o p e r t i e s o f these s u r f a c t a n t - w a t e r systems is t h e i r ability to solubilize a third c o m p o n e n t (1). Typical t e r n a r y systems are o b t a i n e d w h e n the solubilizate is a w e a k l y p o l a r substance as a m e d i u m - o r l o n g - c h a i n alcohol. As the solubility o f the a l c o h o l in w a t e r is small, it is i n c o r p o r a t e d into the surfactant aggregates to f o r m " s w o l l e n " micelles. A t high a l c o h o l contents, the a l c o h o l b e c o m e s the solvent a n d t h e syst e m s changes f r o m L1 to L2 structures. T h e
a d d i t i o n o f a f o u r t h c o m p o n e n t , generally a h y d r o c a r b o n solvent, m a y lead, w h e n the c o m p o u n d s are in a d e q u a t e a m o u n t s , to the so-called W i n s o r IV solutions (2) o r m i c r o e m u l s i o n s (3, 4). I n the case o f ionic surfactants, the electrostatic r e p u l s i o n forces b e t w e e n t h e ions o f t h e s a m e charge at the m i c e l l a r surface are w e a k e n e d b y the b i n d i n g o f c o u n t e r i o n s to t h e m i celle (5). So a m o n g the p a r a m e t e r s characterizing the m i c e l l i z a t i o n process a n d the p r o p erties o f ionic micelles, c o u n t e r i o n b i n d i n g is with the C M C o n e o f the m o s t studied. T h e p u r p o s e o f this w o r k was to follow the micellization process o f s o d i u m d o d e c y l sulfate (SDS) i n v o l v e d in three k i n d s o f solutions: a q u e o u s m i c e l l a r solutions, t e r n a r y s o l u t i o n s c o m p o s e d o f water, SDS, a n d n - p e n t a n o l , a n d q u a t e r n a r y solutions c o m p o s e d o f water, SDS, n-pentanol, a n d n-dodecane. O u r e x p e r i m e n t s were p e r f o r m e d b o t h b y direct m e a s u r e m e n t 143 0021-9797/86 $3.00
Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.
144
GEORGES AND CHEN
of counterion activity using an ion-specific electrode and by conductivity. Unlike most investigations where informations are almost exclusively derived from measurements at surfactant concentrations just above the CMC, the present study was carded out in a large concentration range, from concentration below the CMC up to the boundary of the realm of existence of the isotropic monophasic media.
tion obtained by adding agar-agar to a 3 M NH4C1 electrolytic solution (6). With this electrochemical cell, the response of the electrode against NaC1 was linear in the concentration range studied (10 -3 to 5 × 10 -1 M) with a slope equal to 56 mV. The conductivity measurements were carried out wtih a Tacussel-Solea CD 7N conductivity meter equipped with a CM 02-55 G cell. In all cases, the measuring cell was thermostated with a water bath at 25 ___0.2°C. EXPERIMENTAL Samples and methods. The experiments Materials. Sodium dodecyl sulfate was sup- were done on samples whose composition is plied by Merck or BDH Ltd. Due to its high indicated on Figs. 1 and 2 which represent the purity (>199%) further purification was not ne- mass phase diagrams of the ternary system cessary. Yet to be sure of our results a sample water/SDS/1-pentanol and the quaternary of SDS was purified by a double recrystalli- system water/SDS/1-pentanol/dodecane with zation from ethanol; as no significant change a weight ratio of surfactant to alcohol k = ½. appeared in the results, most of the experi- These diagrams are well known and given ments were done with commercial SDS. The elsewhere (7-9) but they were redrawn for our n-pentanol (>/99%) and the n-dodecane work because some experiments had to be (>/99%) were purchased from Merck and used performed with 0.1 MNaC1 in water. Our diawithout any further purification. Sodium grams show little differences with the pubchloride was supplied by PROLABO. Water lished ones with respect to the boundaries of was deionized and distilled in glass; its con- the monophasic areas but no significant ductivity was 2-3 #S cm -1. change appears along the composition paths Apparatus. Potentiometric measurements of studied in this work. Therefore the composicounterion activity were performed with a tion paths indicated on Figs. 1 and 2 are valid digital reading pH-meter millivoltmeter (So- for the experiments made with or without lea-Tacussel). The sodium selective electrode electrolyte in water. Four sets of experiments were carried out was a glass electrode manufactured by Corning. The glass membrane (NAS 11-18) exhibits on the lines A, B, C, and D (Figs. 1 and 2): a high specificity to sodium ions. The electrode binary systems water/SDS, ternary systems response to sodium ions is unaffected by pH water/SDS/pentanol along two composition as long as pH is at least 2 units higher than paths (weight ratio of SDS to pentanol = 2.33 pNa. The reference electrode was a saturated and 0.5) and quaternary systems with a weight KCI calomel electrode with a double junction ratio of dodecane to active blend equal to (Tacussel RDJ/C8). The junction between the 0.176. reference system (saturated KC1) and the sampNa and conductivity experiments were ple solution was chosen so that: (i) the leakage performed step-by-step on the four lines A, B, of the solution into the sample results in the C, D. A concentrated solution of SDS or a minimum contamination of the sample; (ii) concentrated microemulsion (composition the bridge solution does not react with the located near the upper boundary of the monosample because precipitation would block the phasic area on the studied lines) was first prejunction; and (iii) there is no variation of the pared and used as a mother solution. Then junction potential during an experiment. The two operating modes were used. Low concenbest results were obtained with a gel,like junc- tration samples were obtained by addition of Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE
145
SDS
PENTANOL
WATER+O.1 M NaCI
~G. 1. Brine/sodium dodecyl sulfate/pentanol system: partition of the realm of existence of the monophasic ternary micellar solutions. A, B, and C are the composition paths followed in the experiments (see Experimental).
the mother solution to water or water + O. 1 M NaC1 with a microsyringe directly in the measuring cell; high concentration samples
were obtained by dilution o f the mother solution with water or water + 0.1 M NaC1. For the experiments carried out with NaC1, the
SDS
PENTANOL
WATER +0.1 M NaCI
=¢ 1 2
DODECANE
FIG. 2. Mass pseudoternary phase diagram of the quaternary system brine/SDS/pentanol/dodecane. Weight ratio of surfactant to pentanol: k = ½. D is the composition path studied (see Experimental). Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
146
GEORGES AND CHEN
mother solution contained also 0.1 M NaC1 in the aqueous phase. RESULTS AND DISCUSSION
1. Micellization of SDS in Pure Water The activity of free sodium counterions and the conductivity were measured in aqueous solutions of SDS in a large concentration range from 5 X 10 -4 to approximately 1 M (~29% w/v). The plots of the electrode potential and of the conductivity as a function of the logarithm of SDS concentration (Fig. 3) are very similar in shape. Except for concentrations below the CMC, the plots follow a curve from which appear three almost linear parts intersecting in two characteristic points. At low concentration, the linear relationship between the electrode potential and log[SDS] has a slope close to that of the calibration curve obtained with NaC1; it indicates that the concentration of the free sodium counterions is equal to the concentration of added SDS. Above the first break, the SDS undergoes micellization and a great part of Na + ions are
associated to the micelle surface causing a reduction of free sodium ions. If the concentration of SDS is increased further, a second change of the slope occurs but with an upward break located between 5 and 7 X 10-2 M according to the experiments and quality of SDS used. The conductivity curve shows approximately the same variations and the same breakpoints, which would indicate that the electrical conductivity of the solution is ensured essentially by the free sodium ions. Critical micelle concentration. If obviously the first break at 7.4 X 10-3 M in the pNa curve and 7.9 X 10-3 M in the conductivity curve corresponds to the CMC of SDS, the interpretation of the second break is not easy. Many workers have studied micellar growth in SDS solutions with high NaC1 content or in presence of organic additives. Quasielastic light scattering experiments in SDS solutions have demonstrated a continuous transition in micellar size and shape from spherical miceUes to rode-like structures (10); only within the limits of high temperature (>50°C), low NaC1
0
-25
-5o °
-~5
-IO0
-I
-125
-3
-21.5
I
-2
109
I
-1.5
i
-1
i
-.5
[SOS]
FIG. 3. Variations of the potential of the pNa electrode (O) and of the electrical conductivity a (MS cm -l) (O) along the composition path A, without NaC1. Journal of Colloid and Interface Science, Vol. 113, No, 1, September 1986
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE concentration (<0.4 M), or low surfactant concentration (near the CMC), do the SDS micelles have a minimum size. Other workers (11) have shown, by small-angle X-ray scattering techniques, that a change in the micellar form of SDS from spheres to rods occurs within a range of concentration whose limits are 7 X 10 -2 and 2.5 X 10-~ M. On the other side, some workers studying SDS solutions without any additive, had talked about a "second CMC" at 8.5 × 10 -2 M ( 1 2 ) or 7 × 10 -2 M (13). More recently, in the study of fluorescence intensity ratio of excimer to monomer of pyrene as a function of the concentration of SDS, Song et al. (14) showed a very distinct breakpoint at 7 × 10 -2 M, which was interpreted as a phase transition from sphereshaped micelles to hemicapped rod-like micelles. These values are close to our second breakpoint and even if the term of "second CMC" is incorrect, it surely corresponds to a change in the micelle structure. Counterion binding. The decrease in pNa and conductivity values above the CMC is explained by the binding of the counterion to the micelle. The degree of counterion binding [3 or the degree of dissociation a = 1 - [3 may be estimated from the slope of the pNa curve below and above the CMC. Many equations have been proposed in the literature (5), but the simplified Botr6's equation (15) which assumes that the concentration of the free amphiphilic ions remains constant above the CMC gives a good approximation of a or/3: (Na +) = 7Na+(CMC + a(CT -- CMC)) where (Na +) is the activity of free counterion, YNa+ the activity coefficient of free counterion above the CMC, and CT the surfactant concentration. If ~'Na* is assumed to be a constant, one obtains 1 ot =
d(Na +) X
YNa+
-
-
dCT
where d(Na÷)/dCT is the slope above the CMC. Assuming "YN~+= 0.9(16), one obtains a = 0.21. In fact d(Na÷)/dCT is not really a constant and the value given for a is a mean value.
147
Above the second breakpoint, a is almost a constant because [SDS] ~> CMC and the same calculation gives a = 0.39. These results compare favorably with values given by other workers (Table I). Indeed, it appears that two groups of values, one around 0.25 and the other near 0.4, would lead to the assumption that a depends greatly on the concentration of SDS at which the determination was made. Electrical conductivity. The decrease of the conductivity above the CMC is also the result of micelle counterion binding. If the concentration of monomeric amphiphilic ions is assumed to remain constant above the CMC, the change in specific conductivity is due to the free Na + counterions not associated to the micelles and to the ionic micelles. The conductivity of each species is proportional to the product of its concentration, its charge, and its mobility. The mobility being proportional to the ratio of the charge to the radius of the species, the conductivity of the species is proportional to the product (square of the charge X concentration) divided by the radius. For the free Na ÷ ions, the concentration is Ct -/3(CT -- CMC) and the charge is unity; for the micelles, the concentration is (CT -- CMC)/ n and the charge - n a , where n is the aggregation number. At low surfactant concentration (CT), the conductivity of the micelles is much smaller than that of the free counterions. When CT >> CMC, the product (square of the charge X concentration) is aCT for the free counterion and na2Ct for the micelles; so, even taking into account the difference in size of the two species, the conductivity of the micelles may not become negligible, which may explain the great slope of the conductivity curve above the second breakpoint. In spite of that, the increase in conductivity above the second breakpoint demonstrates the liberation of Na ÷ ions shown also by the pNa curve. Interpretation. According to the different models proposed to describe the ionic micelles (21-25), the counterion binding is described in terms of a continuous radial distribution function corresponding to a high counterion concentration close to the micelle and continJournal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
148
GEORGES
AND CHEN
TABLEI DegeesofCoun~fion
Dissociafiona ~r MicellesofSDS:ComparisonofOurValues withThoseFoundintheL~ure Other works
Present work
a
0.21 0.39
(5)
(16)
(20)
(17)
(18)
(19)
0.22-0.27
0.21
0.25 to 0.32 a
0.38
0.4
0.36
a Values m e a s u r e d for SDS c o n c e n t r a t i o n s lying between 1 a n d 2 X 10 -2 M.
uously decaying with increasing distance from the micelle (18). So the distinction between bound and free counterions is difficult and would greatly depend on the technique used to study counterion binding. In addition, some workers have described the micelle structure with an ill-defined Stern layer (24) or even in the case of SDS an "open" structure (25) with a not well-ordered phase boundary between the Stern layer and the hydrocarbon core. In both cases, the incorporation of water between the polar head groups would be important. This structure is probably true at low surfactant content, but as the surfactant concentration increases, the spherical micelles are assumed to form larger aggregates or rod-like micelles in which the area per head group at the surface of the cylindrical portion of the micelle would be less than in the case of spherical micelles (26). In absence of electrolyte, some of the spherical micelles may, before or instead of transforming into rod-like miceUes, associate into other types of aggregates where the spherical micelles would tend to link with one another to form microgel particles (27). In all cases, the increase in SDS concentration would induce an exit of the water incorporated between the polar head groups, leading to a different distribution between free and bound counterions or at least a different reading of the pNa electrode.
2. Influence of Additives Figures 4, 5, and 6 show the comparative plots of the electrode response to free Na + and Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
of the conductivity with the logarithm of SDS concentration, along the composition paths B, C, and D indicated on Figs. 1 and 2. As in the case of SDS, the pNa and conductivity curves follow the same variations and present two breakpoints. The various slopes and the values of the breakpoints are compared in Table II and III.
Ternary systems: water/SDS/1-pentanol. Below the CMC, the electrode response is not changed by the presence of the alcohol (Figs. 4 and 5) and the solution behaves as an ordinary electrolyte solution while the pentanol is dissolved in water. When the concentration of (pentanol + SDS) increases, with constant ratio, the curves first break downward at a SDS concentration less than in pure water, indicating that the alcohol induces the micelle formation. The CMC obtained from potential data agree well with those obtained from conductivity data (Tables II and III). At the same time, the slope of the curves above the CMC increases in the presence of 1-pentanol, indicating an increase of the degree of counterion dissociation a as the alcohol is incorporated into the micelles. The CMC lowering and counterion liberation effect increase as the ratio 1-pentanol/SDS increases (lines B and C). The increase in a with increasing alcohol concentration may be explained by a greater partitioning of the alcohol into the micelle phase. The alcohol penetration into the micelle produces a "tightening" of the SDS micelle (28); the alcohol binds to the micelle with its hydroxy groups between the surfactant head groups and so decreases the repulsion between
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE
149
-2
-1~
-1~
I
-3
I
-2. 5
I
I
-2 -1.5 lo9 [SOS]
I
I
-1
-. 5
FIG. 4. Variations of the potential of the pNa electrode (©) and of the electrical conductivity a (mS cm -~) (O) in the ternary system along the composition path B defined by a SDS-to-pentanol weight ratio of 2.33, without Na~l. I
I
0
I
I
o.." ./,~"
-25
~
..o
-50
0 °
-75
8~
-100
-1
-125
l
-3
I
-2.5
l
l
-2
-1.5
l
-1
l
-.5
Io9 [SDS] FIG. 5. AS in Fig. 4. Composition path C defined by a SDS-to-pentanol weight ratio of 0.5, without NaC1. ]?he arrow on the graph indicates the boundary of the L1 monophasic area.
Journal of Colloid and Interface Science,
Vol.113,No. 1, September1986
150
GEORGES AND CHEN
o
-2
-7:
-IIX
-1~
. . . . .
I
I
I
I
I
I
-3
-2. 5
-2
-1.5
-1
-. 5
lo 9 [sos]
FIG. 6. Variations of the potential of the pNa electrode (O) and of the electrical conductivity a (mS cm -1) (e) in the quaternary system along the composition path D defined by a SDS-to-pentanol and a dodecaneto-active blend weight ratio of, respectively, 0.5 and 0.176, without NaC1.
them, leading consequently to liberation of counterions. It is meaningful to compare the concentration of free Na ÷ measured at [SDS] = 0.1 M (Table II). Along both composition paths B and C, the curves show a second breakpoint with an increase of the slope, as in the absence of pentanol, but at a SDS concentration which is also lowered. The decrease of the concentration of the second breakpoint
and the invariance of the various curves above this second breakpoint seem to indicate that the alcohol effect acts essentially at low SDS content and decreases the concentration at which the change in the micelle structure occurs.
Quaternary systems: water/SDS/1-pentanol/dodecane. With the addition of dodecane (line D) the micellization process of SDS (Fig.
TABLE II Parameters of the pNa Curves for the Samples on Composition Paths A, B, C, and D (Figs. 1 and 2) without Electrolyte
SDS line
sl (mY)
CMC ( 10-3 M)
s2 (mY)
bp2 ( 10-2 M)
s3 (mV)
A B C D
52 51 53 56
7.4 6.4 5.2 4.6
28 30 33 25
5-7 3.6 1.5 1.6
58 60 60 59
Free Na + (M) at C r = 0.1 M
3.3 4.6 8.3 6.3
× × X ×
10-2 10-2 10-2 10-2
Note. CMC corresponds to the first breakpoint; bp2 is the second breakpoint; sl, s2, and s3 are the slopes of the curves, respectively, below the CMC, between the two breakpoints, and above the 2nd breakpoint; free Na + was estimated from the calibration curve obtained with NaC1. Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
MICELLIZATION STUDY OF SODIUM DODECYL SULFATE
151
TABLE IU Parameters of the Conductivity Curves for the Samples on Composition Paths A, B, C, and D (Figs. 1 and 2) without Electrolyte~ SDS line
A B
C D
sl (mS cm -1)
CMC (10 -3 M )
s2 (mS cm -I)
bp2 (10-2 M)
s3 (mS cm -I)
8.7 9.1 8.7 9.1
7.9 6.3 5.2 4.4
3.3 4.1 4 4.4
5.6 3.3 1.4 1.4
12.5 11.2 11 10.5
~ (mS crn -1) at CT = 0.1 M
2.9 4
4.8 4.5
a Same indications as in Table II.
6) is nearly the same as in the corresponding ternary system (line C). The counterion dissociation is rather less because a part of the alcohol m a y dissolve with dodecane in the hydrophobic core rather than in the palisade layer. At higher SDS content, the conductivity curve shows a m a x i m u m and then decreases. At this stage in the discussion, it is interesting to compare the variation of the electrical conductivity versus water weight fraction along the various composition paths A, B, C, D. In the water-SDS system, the conductivity increases almost linearly until high values when the water content decreases; when pentanol is added to the system, the plot incurvates when one reaches the boundary of the monophasic area and the m a x i m u m in conductivity decreases when the pentanol to SDS weight ratio increases (lines B and C). This feature anticipates the further change in the structure of the micelles from monophasic solutions of pentanol in water (L1) to monophasic solutions of water in pentanol (L2), change which occurs at lower SDS concentration when the a m o u n t of pentanol increases. In the case of the microemulsions (Figs. 6 and 9), the conductivity curve reaches soon the m a x i m u m value which is interpreted as corresponding to the beginning of the phase transition zone between O/ W and W / O structure (29, 30). The comparison of Figs. 1 and 2 and these results show that the configuration and the properties of the pseudoternary system issue directly from those of the water/SDS/pentanol system (9).
Influence of NaCl. As the addition of an electrolyte in the water phase buffers the solutions, the influence of added NaC1 will be discussed only from the conductivity experiments. The curves obtained with and without NaC10.1 M are compared in Figs. 7-9. In the water/SDS system (Fig. 7), the addition of NaC1 does not lead to a large variation of the high conductivities except of course for the
[I
.5
.6
.7
.8
.9
W0t~ weight fr~ti0~
FIG. 7. Variations of the electrical conductivity of the water/SDS system (line A) with ([~) and without (m) 0.1 M NaCI in water. Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986
152
GEORGES AND CHEN
less concentrated solutions. The difference between the two curves decreases markedly when the SDS concentration increases, meaning that the presence of NaCl increases the degree of counterion association/3; this results in a relative decrease of the electrical conductivity due to an increase of associated Na + ions and to a decrease of the micelle charge. In the ternary system (Fig. 8), the difference between the two curves remains more important along the whole composition path. Indeed, the incorporation of l-pentanol in the SDS micelles has an effect opposite to that of NaC1, but the resuiting effect is also an increase of/3. In the quaternary system, the addition of NaCl leads to a greater increase of the electrical conductivity in the realm of existence of O/W microemulsions (Fig. 9). As shown in the figure, the presence of the electrolyte affects the position of the conductivity maximum, which means that the phase transition zone between
T
~
N
.,o
"
"
2,
I
I
.5
,6
.'~
.8
.9
¥0t~rNi~t fr0cti0n
FIG. 9. Variations of the electricalconductivity of the waterfSDS/pcntanolJdodccane system (line D) with (D) and without (m) 0.I M NaCl in water. The dotted partsof the curves indicatethe discontinuitydue to the proximity of the mesophasic area (sce text).
! the O/W and W/O structures begins at a greater water content along the composition path studied. When the water content is less than 55%, the curves join and the added NaC1 has no influence on the electrical conductivity ofbicontinuous or W/O microemulsions. The discontinuity in the curves before the maximum originates from the fact that the studied composition path goes along and even, without added NaC1, into the mesophasic area located on the water-active mixture side (Fig. 2). REFERENCES
.5
,6
~
.7
,8
.8
feint froctim
l~o, 8. Variations of the electricalconductivity of the watcr/SDS/pcntanol system (lineB) with (D) and without (m) 0.I M NaCl in water.The arrow indicatesthe boundary of the LI monophasic area. Journal of Colloid andlnterface Science,
Vol.113,No.1,September1986
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Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986