Langmuir 1988, 4 , 90-93
90 and
apj"
o = - aa J+ 1
If one assumes that, for i > J,apio/aaJ= a p J o / a a J , then (A8a) reduces to (A8) for j = J also. In the model we have developed in the text, the properties of the cylindrical part of a rodlike micelle do not vary with ita length. Therefore, the part of pjo 0'> J)which depends on a is simply proportional to j - J . It then follows from (A9) that
+O'-J)/W j > J
(A10)
This is the second member of (7). Equations A8 and A10 show that the determination of the degree of dissociation by minimization of the free energy reduces to minimizing the quantities $' + jajpM for spherical micelles 0'5 J)and P," + [ J ~ J+ 0 - J ) a b M for k z e r rodlike micelles 0' > Thus the derivation is complete. Registry No. Sodium dodecyl sulfate, 151-21-3; sodium
4.
chloride, 7647-14-5.
Cloud Point of Mixed Ionic-Nonionic Surfactant Solutions in the Presence of Electrolytes Leszek Marszall Pharmacy No. 09068, 86-1 70 Nowe, Poland Received March 11, 1987. In Final Form: June 25, 1987 The cloud point of mixed ionic-nonionic surfactants (sodium dodecyl sulfate (SDS)-Triton X-100) is drastically lowered by a variety of electrolytes at concentrations that are considerably lower than those affecting the cloud point of nonionic surfactants used alone. The results indicate that the factors affecting the clouding phenomena of mixed surfactants at very low concentrations of ionic Surfactants and electrolytes are primarily electrostatic in nature. The change in the original charge distribution of mixed micelles at a fixed SDS-Triton X-100 ratio (one molecule per micelle), as indicated by the cloud point measurements as a function of electrolyte concentration, depends mostly on the valency number of the cations (counterions) and to some extent on the kind of the anion (co-ion)and is independent of the type of monovalent cation (counterion).
Introduction One of the characteristic features of the nonionic surfactants is their cloud point, T,. The concentration dependence of the cloud point defines a phase boundary between the one-phase and two-phase systems, and the minimum temperature can be identified as a lower critical solution temperature.l The cloud point of a dilute nonionic surfactant solution increases upon addition of ionic surfactant.2+ To explain this phenomenon, various mechanisms have been suggested including formation of mixed micelles, solubilization, or complex f ~ r m a t i o n .The ~ incorporation of ionic surfactant into the nonionic micelles introduces electrostatic repulsion between the micelle^,^ thus hindering the coacervate phase formation and raising the cloud point.6 Similarly, the small decrease of the surfactant self-diffusion coefficient upon addition of sodium dodecyl sulfate (SDS) to nonionic micelles was interpreted as being mainly due to an enhanced micellemicelle repulsion.8 However, the
relatively small effect in the observed self-diffusion coefficient upon addition of very small amounts of SDS, as compared to significant changes in cloud point for relevant systems, shows the independence of the self-diffusion coefficient of critical effects. Recently, Valaulikar and Manohar9 have interpreted a linear rise in the clouding temperature of Trition X-100 upon addition of minute amounts (about M) of SDS in terms of the increase in the surface charge of the micelle. At such small concentrations (far below the cmc of SDS) the SDS molecules occurred either as monomers or as mixed micelles with Triton X-100. At the same time, the authors noted the insensitiveness of the cloud point of Triton X-100 to NaCl over the same concentration range. According to the authors, this added gurface charge increases the repulsion between the micelles, thus supporting the view that the cloud point is a critical phenornenon1+l6 rather than micellar g r o ~ t h . ' ~ J * In a recent article we demonstrated with the example of NaCl and NaSCN addition, using cloud point mea-
(1) Degiorgio, V. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland Amsterdam, 1985;p 303. (2)Maclay, W. N. J. Colloid Sci. 1956,11, 272. (3)Kuriyama, K.;Inoue, H.; Nakagawa, T. Kolloid 2.2.Polym. 1962, 183, 68. (4)Nishikido, N.; Akisada, H.; Matuura, M. Mem. Fac. Sci., Kyushu Univ., Ser. C 1977,IO, 91. (5)Schott, H.; Royce, A. E. J. Pharm. Sci. 1984,73, 793. (6)Scamehorn, J. F. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311;American Chemical Society: Washington, DC, 1986;p 1. (7)De Salvo Sonzo, L.; Corti, M.; Cantu, L.; Degiorgio, V. Chem. Phys. Lett. 1986,131, 160. (8)Nilsson, P.-G.; Lindman, B. J. Phys. Chem. 1984,88,5391.
(9)Valaulikar, B.S.;Manohar, C . J. Colloid Interface Sci. 1985,108, 403. (10)Corti, M.; Degiorgio, V. J. Phys. Chem. 1981,85, 1442. (11)Hayter, J. B.;Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (12)Triolo, R.; Magid, L. J.; Johnson,J. S., Jr.; Child, H. R. J. Phys. Chem. 1982,86,3689. (13)Zulauf, M.; Rosenbusch, J. R. J. Phys. Chem. 1983, 87, 856. (14)Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984,88,309. (15)Magid, L.J.; Triolo, R.; Johnson,J. S., Jr. J. Phys. Chem. 1984, 88,5730. (16)Zulauf,M.; Weckstrom, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J . Phys. Chem. 1985,89,3411. (17)Balmbra, R. R.; Clunie,J. S.; Corkill, J. M.; Goodman,J. F. Trans. Faraday SOC.1962,58,1661; 1964,60,979. (18) Attwood, D. J. Phys. Chem. 1968, 72, 339
0743-7463/88/2404-0090$01.50/00 1988 American Chemical Society
Langmuir, Vol. 4, No. 1, 1988 91
Cloud Point of Ionic-Nonionic Surfactants
t 7 4
/
/
65
65 I
I
10-5
10-3
10-4
SD 5
07
I
I
10-1
10-5
Electrolyte [ m o l e / l l
Figure 1. Cloud point of a 1%solution of Triton X-100 as a function of the molar concentrations of added SDS (e),NaSCN (A),NaI (X), '[zN@04 (A),and NaC1, NaBr, LiC1, KCl, '/&aC12, '/zCa(N03)2, /2MPclz, '/&fg(NOa)2, ' / d C b , '/3A1(N03)3(0). surements, that the original charge distribution of such mixed micelles was swamped and the corresponding repulsions are screened, depending on the electrolyte concentration, and the effect was greatly affected by the SDS-Triton X-100 ratio.l9 This study was aimed at evaluation of how the charge of mixed micelles varies with the specific properties and valence number of the cation (counterion) and anion (co-ion) and how it affects the cloud point.
Experimental Section Materials. Triton X-100 (a polydisperse preparation of (1,1,3,&tetramethylbutyl)phenyl]poly(oxyethylene) with an average of 9.5 oxyethylene units per molecule)of scintillation grade and sodium dodecyl sulfate (Texapon L loo), 99%, were kindly donated by Riedel-de Haen and Henkel, West Germany, respectively, and were used without further purification. The electrolytes used in all experiments were of analytical grade and used without further purification. Demineralized and distilled water was used to prepare the sample solutions. Methods. Cloud points were determined visually by noting the temperature at which a solution heated above the clouding temperature lost ita turbidity on cooling. The temperature at which the scale of the thermometer was visible in the solution was recorded as the cloud point. Heating and cooling were regulated to about 1OC/min around the cloud point. The cloud point of a 1% Triton X-100 solution was 66 "C. Results and Discussion Figure 1 shows the cloud point measurements in 1% Triton X-100 solutions as a function of SDS and of electrolyte concentrations. The linear rise in the cloud point of Triton X-100 upon addition of SDS supports the findings of Valaulikar and Manohar.p Data presented in Figure 1 suggest that the ionic-nonionic micelles composed mainly of nonionic surfactant are charged. This leads to electrostatic repulsion between the micelles, thus increasing the cloud point. Rathman and Scamehorn20.21have investigated fractional counterion binding on the mixed micelles of ionicnonionic surfactants based on the electrostatic model. As a general phenomenon they observed that at low ionic surfactant mole fraction in the mixed micelle the counterion binding fell rapidly with decreasing ionic content. (19) Marszall, L. Colloids Surf. 1987, 25, 279. (20) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984,88,5807. (21) Rathman, J. F.; Scamehorn, J. F. Langrnuir 1986,2,354.
10-4
E Lectro 1 3 t e
III
I
lo-*
10-2
[lnole/lJ
Figure 2. Cloud point of a 1% solution of Triton X-100 in the presence of SDS (one molecule of SDS per micelle of Triton X-100) as a function of the molar concentrations of added LiCl ( O ) , NaCl (e), and KC1 (X). a5-
00-
m
u p
u
-
'Is.
-
-
70-
65I
I
I
I
, , / , , I
(22) Kuriyama, K. Kolloid 2.2.Polym. 1962, 181, 144. (23) Schick, M. J. J. Colloid Sci. 1962, 17,801. (24) Dorm, A.; Goldfarb,J. J. Colloid Interface Sci. 1970, 32, 67. (25) Shinoda, K.; Takeda, H. J. Colloid Interface Sci. 1970, 32,642. (26) Schott, H. J. Colloid Interface Sci. 1973, 43, 150. (27) Schott, H.; Han, S. K. J.P h r m . Sci. 1975,64,658; 1977,66,165. (28) Deguchi, K.; Meguro, K. J. Colloid Interface Sci. 1975,50, 223. (29) Balaaubramanian, D.; Mitra, P. J.Phys. Chem. 1979,83, 2724. (30) Marazall, L. Tenside Deterg. 1981, 18, 25.
Marszall
92 Langmuir, Vol. 4, No. 1, 1988
t
- ,i r)
k V
7 5 1
"C
85
I
I
10-5
I 10-4
10-3
10-2
ELectro L y t e [mote i l l
Figure 5. Cloud point of a 1% solution of Triton X-100in the presence of SDS (one molecule of SDS per micelle of Triton X-100)as a function of the molar concentrationsof added NaN03 ( O ) , '/2Mg(N03)2 (A),'/2Ca(N03)2 (Oh and '/~Al(N03)3( X I .
tration below 0.01 M, most of the electrolytes have no effect on the cloud point and NaSCN, NaI, and NazS04 affect only slightly the cloud point above the 0.001 M level as shown in Figure 1. However, when electrolytes are added to the Triton X-100 solution in the presence of SDS, the cloud point curve changes significantly even below the 0.01 M concentration of the electrolyte as shown in Figures 2-5. The effect of electrolytes is shown in Figures 2-5 at a SDS concentration corresponding to one molecule of SDS per micelle of Triton X-100. For calculation of the ratio we used the equationg N = N/[(c - cmc)/NA] (1) where R is the number of SDS molecules per micelle of Triton X-100, Nand c are the molar concentrations of SDS and Triton X-100, respectively, and NAand cmc are the aggregation number (150) and the critical micelle conM) of Triton X-100, respectively. centration (3.2 X Figure 2 shows the effect of monovalent electrolytes (LiC1, NaC1, and KC1) on the cloud point of the SDSTriton X-100 solution. (31) Ward, A. J. I. J.Pharm. Pharmacol. 1982, 34, 612. (32) Schott, H.;Royce, A. E.;Han, S . K. J. ColloidZnterface Sci. 1984, 98,196. (33) Weckstrom, K.; Zulauf, M. J. Chem. Soc., Faraday Tram. 1 1986, 81, 2947.
As mentioned before, the uncompensated charge on the micellar surface gives rise to repulsion between the micelles. The range of the repulsion will vary according to the composition of the solution. When electrolyte is added to the solution, the original charge distribution is swamped and the corresponding repulsions are screened. This results in a dramatic cloud point lowering. It can be seen from Figure 2 that the effect depends only on the concentration of the monovalent electrolyte added. The type of the monovalent cation, linked to the same anion, has practically no effect on the shape of the function. Figure 3 shows the cloud point measurements in SDSTriton X-100 mixtures in the presence of NaC1, NaBr, NaI, NaSCN, and Na2S04. In this case the surface charge is compensated not only in a ratio depending on the bulk concentration of electrolytes but also on the change in their anion (co-ion) species. At higher electrolyte concentrations (above 0.0005 M) the effect on depressing the cloud point is closely related to the lyotropic series of anions. Anions with lower lyotropic numbers (e.g., S042-,C1-) reduce the cloud point of SDS-Triton X-100 mixtures more effectively than do those with higher lyotropic numbers (e.g., SCN-, I-). No theory of salting in and salting out appears to adequately describe the effects of electrolytes on the mixed ionie-nonionic surfactants dissolved in water observed in our experiments. Both the ions which destroy the structure of water and reduce the hydrogen bonding among the water molecules and those which build up the structure and self-association of water depress the cloud point of mixed surfactants in a similar way. However, the results of our investigations suggest that the factors responsible for variations in the clouding phenomenon of mixed surfactants at such low concentrations of electrolytes and SDS are primarily electrostatic in nature. On the basis of electrostatic model, Rathman and Scamehorn20developed a localized and a mobile adsorption model to describe the binding of counterions. For ionicnonionic micelles composed mainly of nonionic surfactants, a localized adsorption model is preferred, whereby the counterions adsorb or bind onto the charged hydrophilic groups in the micelle. Although the exact distinction between the bound and free counterions is difficult, and depends much on the technique and adsorption model used, it seems appropriate to interpret the differences in the effect of anions in our study in terms of the change of the cation (counterion) binding. The position of an anion in a lyotropic series can be correlated with its polarizability and hydrated r a d i ~ s . ~Thus, - ~ ~ it can be hypothesized that anions with higher lyotropic number probably hinder the counterion binding of Na+ or affect the closeness of attachment of the counterion to the mixed micelle, thus making these swamping electrolytes less effective as compared to those with lower lyotropic numbers. Figures 4 and 5 show the effect of mono-, di-, and trivalent electrolytes on the cloud point of SDS-Triton X-100 mixtures. The electrolytes of higher charge (di- and trivalent) suppressed the cloud point of mixed ionic-nonionic surfactants at much lower concentrations than did the monovalent ones. Comparison of Figures 2-5 shows that specific properties of the cations (counterions; Figure 2) and of the anions (co-ions; Figure 3) play only a minor role as compared to the effect of the valency number of the counterions. (34)Larsen, J. W.; Magid, L. J. J. Am. Chem. SOC.1974, 96,5774. (35) Schott, H. Colloids Surf. 1984, 11, 51. (36) Schick, M. J. J. Phys. Chem. 1964,68, 3585.
Langmuir 1988,4, 93-96 At higher electrolyte concentrations, when the number of changes of the swamping electrolytes per ionic surfactant molecule is higher than 1(above 0.0002 M), the mono-, di-, and trivalent electrolytes depress the cloud point as expected. The cation of a higher charge is more effective in the reduction of the cloud point of mixed surfactants than one of the lower charge. The most striking feature in Figures 4 and 5 is the enormous variation of the cloud point with electrolyte concentration when the number of charges of the swamping electrolyte per SDS molecule is lower than 1 (below 0.O001 M). In this case the cloud point of a mixed ionic-nonionic surfactant mixture is depressed most effectively by divalent electrolytes (Mg2+,Ca2+). It is interesting to note that such a behavior was noted both for chlorides (Figure 4) and for nitrates (Figure 5). Our in-
93
terpretation of this finding is that polyvalent cations readily hydrolyze, frequently incorporating anions other than hydroxyl in the complex solute. Consequently, the aqueous solution’containsspecies of various composition and changes, which are not always well-defined. Their effect becomes more evident upon lowering electrolyte concentration.
Acknowledgment. I thank Prof. Dr. R. Piekos for many helpful discussions and for linguistic correction of this manuscript. Registry No. SDS,151-21-3;NaCl, 7647-14-5;NaEir, 7647-15-6; NaI, 7681-82-5; NaSCN, 540-72-7; Na2S04, 7757-82-6; CaC12, 10043-52-4; Ca(N03)2,10124-37-5;MgC12,7786-30-3;Mg(N03)2, 10377-60-3;AlC13,7446-70-0;Al(NOd3,13473-90-0;LiC1,7447-41-8; KC1, 7447-40-7; Triton X-100, 9002-93-1.
The Close Analogy between the Preferential Solvation of Polymers in Mixed Solvents and Adsorption from Liquid Mixtures at Solid/Liquid Interfaces M.Nagy Department of Colloid Science, Lordnd E6tv6s University, 1088 Budapest, Hungary Received November 3, 1986. In Final Form: June 26, 1987 In this work a comparison of some phenomenological aspects of sorption and solvation, the role of the sorbent as a component, and a critique of the traditional plot of isotherms are discussed. It was pointed out that a sorbent-sorbate system can be characterized in a correct way only by a set of isotherms involving a limiting case when the mass of sorbent tends to zero. The treatment was extended to cases when more than one sorbent or more than two mixture components are present in the system.
Introduction In classical and modern colloid and surface science, phenomena occurring at different types of interfaces have always played a central role. There are at least two reasons for this. First, interactions between surfaces or between colloid particles are decisively affected by the extent, structure, and other properties of adsorbed layers, and what is also essential, the stability of colloid systems besides other factors is controlled mainly by interparticular interactions. Second, the adsorption-desorption equilibria and processes are of great practical importance; therefore they have widespread scientific and industrial applications. In one of the commonly used considerations, adsorption interaction occurs at a geometrical surface which divides the phases in contact a t the molecular level into two or more parts, and crossing this borderline is accompanied by a jumplike change in many properties, e.g. in density, polarity, etc. In terms of classical colloid chemistry this is called disc0ntinuity.l From a geometrical point of view the discontinuities can be of a point, a line, or surface type, and for most real solid sorbents, due to their geometrical and energetic irregularities, one can usually count with “mixed” discontinuities. More precisely, if a solid surface is in contact with a liquid or a solution, the molecules of the mobile phase surround the surface elements with different coordination numbers. Furthermore, sorbents with high specific surface area contain channels, holes, i.e. (1) B w h , A. Colloid Systems, 1st ed.; The Technical Press: London: 1937; Chapter 11.
inner surface elements resulting in an energetic spectrum of the interaction of sorbate molecules with the solid. That is why it is somewhat more appropriate to speak of sorption phenomena instead of adsorption in case of solid/ liquid and solid/gas, or vapor, interfaces. However, there is another essential point. It is long known that in mixed solvents macromolecules behave like sorbents, so that an enrichment of one of the mixture components may occur in their solvation Even the more, the same phenomenon can be observed when small ions are present in a mixed solvent instead of macromolecules.s It follows from these experimental facts that the preferential binding of one of the mixture components is not merely a question of being a discontinuity in a classical sense present in the system, but it may frequently emerge when “discontinuities”brought about by intermolecular interactions perturb locally the random spatial distribution of molecules in these multicomponent (2) Flory, P. J. Principles of Polymer Chemistry, 1st ed.; Cornell University Press: Ithaca, N.Y., 1953; Chapter XIII. (3) Ewart, R. H.; Roe, C. P.; Debye, P.; McCartney, J. M. J. Chem. Phys. 1946, 14,687. (4) Strazielle, C.; Benoit, H. J. Chim. Phys. Phys.-Chim. Biol. 1961, 58, 675. (6) Read, B. E. Trans. Fargday Soc. 1960,56, 382. (6) %my, A.; Pouchly, J.; Solc, K. Collect. Czech. Chem. Commun. 1967,32,2753. (7) Nagy, M.; Wolfram, E.; GyBrfy-Szemerei,A. J. Polym. Sci., Part C 1972,39, 169. (8)Grunwald, E. In Eletrolytes; Pesce, B., Ed.; Pergamon: Oxford, London, New York, Paris, 1962; p 62.
1988 American Chemical Society
