Effect of Inorganic Additives on Solutions of Nonionic Surfactants

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

189, 117–122 (1997)

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Effect of Inorganic Additives on Solutions of Nonionic Surfactants XIV. Effect of Chaotropic Anions on the Cloud Point of Octoxynol 9 (Triton X-100) HANS SCHOTT School of Pharmacy, Temple University, Philadelphia, Pennsylvania 19140 Received October 22, 1996; accepted February 11, 1997

The effect of salts on the nonionic surfactant octoxynol 9 was studied by the changes they produced in its cloud point (CP): CP increases indicate salting in. The temperatures by which the sodium salts of water structure-breaking (chaotropic) anions increased the CP of 2.0% octoxynol solutions were measured as a function of salt concentration. The curves representing changes in CP versus salt molality rose to a maximum in a parabolic fashion, followed by steep decreases. Their ascending branches, corresponding to salting in, were caused by a disruption of the water structure due to the chaotropic effect of the anions combined with the effect of elevated CP temperatures. The descending branches were due to salting out by Na / . The net CP increases due to the chaotropic effect of the anions were calculated at each concentration by subtracting the CP decrease due to Na / from the observed CP increase of the respective Na / salts. With the exception of ClO 0 4 , the plots of CP changes produced by the chaotropic anions rose in a nearly linear fashion to a maximum and then levelled off. The levelling off occurred at the salt concentration and CP temperature leading to the maximum disruption of the water structure of which each anion was capable. The chaotropic anions were ranked in the following order according to their capacity for increasing the CP: SCN 0 ú I 0 ú [Fe(CN)5NO] 20 ú ClO 0 4 ú BF 0 4 . Even though the thiosulfate anion is a very soft Lewis base, it lowered the CP in direct proportion to its molality; i.e., it enhanced the structure of water and promoted salting out at all concentrations. q 1997 Academic Press Key Words: nonionic surfactants—chaotropic salt effects on cloud point; cloud point of nonionic surfactants—chaotropic salt effects; chaotropic salts—effect on cloud point of nonionic surfactants; salt effects on nonionic surfactants’ cloud point.

INTRODUCTION

The capacity of electrolytes to salt nonelectrolytes in or out of aqueous solutions is commonly measured by their effect on the solubility of the nonelectrolytes. This approach is unsuitable for nonionic surfactants (NS), many of which form almost uninterrupted series of solutions or mesophases with water over wide temperature ranges below their cloud points (CP).

As an alternative, salt effects for NS are measured by changes in CMC or in CP. Most NS have small CMC values. Therefore, the increases in CMC through salting in and the decreases through salting out have even smaller absolute values (1–5). The cloud points or lower consolute temperatures of NS, on the other hand, while nearly independent of surfactant concentration between £0.5 and §5% ( 6 ) , are strongly dependent on additives, including electrolytes ( 1, 7 – 11 ) . Electrolytes that raise the CP extend the temperature range in which NS form undersaturated isotropic solutions. This represents salting in. Reductions in CP represent salting out ( 6 ) . The changes in CP produced by individual ions are additive algebraically. Characteristic CP shift values of various anions and cations for several NS have been published (10, 11). All cations examined except Na / , K / , NH 4/ , Rb / , and Cs / salt NS in through complexation with the multiple ether groups (8, 9, 12). The effects of anions in salting NS in or out, raising or lowering their CP, are considerably greater than those of the cations (1, 11). The present study focusses on anions and employs only sodium salts. According to their effects on the structure of water, anions either disrupt or enhance the association of water molecules by hydrogen bonds into flickering clusters, shifting the equilibrium nH2O ` (H2O)n

towards the left or the right, respectively. Structure-making anions are small and/or have multiple negative charges and generate strong electrostatic fields that not only polarize, immobilize, and electrostrict the adjacent water molecules but induce additional order (entropy loss) beyond the first water layer (13). Such anions, e.g., OH 0 , F 0 , SO 20 4 , PO 30 4 , have lyotropic numbers £8 and produce relatively large increases in the viscosity and surface tension of water (13, 14). They are classified as hard Lewis bases, with high electronegativity and low polarizability (15, 16).

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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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Structure-breaking anions, e.g., I 0 and SCN 0 , are generally monovalent and relatively large, with lyotropic numbers §11. Most are soft Lewis bases (15, 16). Because of their low electronegativity, high polarizability, and weak electrostatic fields, they disrupt the structure of the surrounding water in all but the immediately adjacent layer (13, 14). Therefore, these anions are called chaotropic (17). By ‘‘melting’’ the flickering clusters or ‘‘icebergs’’ of water molecules that were self-associated by hydrogen bonds, chaotropic anions increase the concentration of single water molecules, which are capable of forming hydrogen bonds with the ether groups of NS. By ‘‘depolymerizing’’ water, they make it a better solvent. Hence, they salt NS in; i.e., they raise the CP. Most salting-out electrolytes lower the CP of NS in direct proportion to their molal concentrations. Likewise, most electrolytes that salt NS in through cation complexation raise the CP in direct proportion to their molal concentrations (1, 8–11, 18). Even for salts with chaotropic anions, progressively increasing concentrations generally produce monotonic but less than proportional increases in the CP of NS. This behavior was observed until 3-m salt levels or the normal boiling point of the solutions was reached (9, 10, 18). However, there are reports of maxima in the relation between CP and concentration of chaotropic salts (9, 19, 20). There are actually two mechanisms involved in disrupting the structure of water and boosting the CP when increasing concentrations of chaotropic salts are added to NS solutions: The first is the structure-breaking effect of the anions at room temperature discussed above. The second mechanism is the effect on the water structure of the higher temperatures required to reach the ever increasing CPs. In pure liquid water, the median cluster size (the average number of water molecules self-associated by hydrogen bonds into single clusters) is 11.2, 7.8, 6.5, 5.6, and 4.9 at 0, 20, 40, 60, and 907C, respectively (21, 22). Increasing concentrations of chaotropic salts at room temperature reduce the cluster sizes even faster than increasing temperatures do. However, these two mechanisms for breaking the structure of water are self-limiting: As more water structure is disrupted by the parallel increases in chaotropic salt concentration and in the cloud point temperature, less water structure remains. This gradually curtails additional incremental increases in CP due to the further chaotropic action of additional salt. When most or all of the self-association of water molecules has been disrupted, further increases in chaotropic salt concentration cannot produce additional increases in CP. At that point, the salting-out effect of the alkali metal cations should become the dominant effect, and the CP would be expected to drop upon further increases in the concentration of chaotropic salts. The purpose of this study is to extend the concentration

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range of the known chaotropic salts in order to determine whether maxima in the CP–salt concentration relationship, followed by CP decreases at higher concentrations, are common features. The following chaotropic anions were investigated in the form of their sodium salts: iodide, nitroprusside, perchlorate, and thiocyanate. Two additional salts were included for the purpose of determining whether their anions are chaotropic, i.e., cloud point boosters, namely, sodium thiosulfate and sodium tetrafluoroborate. S2O 20 3 is an even softer base than the strongly chaotropic SCN 0 (15, 16); for instance, it is oxidized more readily. Sodium thiosulfate is officially listed as a monograph in the U.S. Pharmacopeia. If it were chaotropic, it would ipso facto be mucolytic but unlike sodium iodide and sodium thiocyanate, it might have dual mucolytic activity: In addition to liquefying mucus indirectly by disrupting the structure of its water, it might also break the disulfide bridges that cross-link glycoprotein molecules. Both actions reduce the gelatinous, viscoelastic nature of mucus (23). The ClO 40 anion is exceptional because it is a CP booster (9) despite being a hard Lewis base (15). Because tetrafluoroborates resemble the corresponding perchlorates in their solubilities and crystal structures (24), NaBF4 was included in this study. EXPERIMENTAL

Materials and Solution Preparation The advantages of studying a normally distributed polyoxyethylated NS rather than one that is homogeneous (monodisperse) were outlined recently (12). The NS selected was octoxynol 9 (Triton X-100) because of its wide chemical and biochemical applications and its extensive published CP data. The number 9 is omitted throughout the text. The anhydrous liquid was supplied by Union Carbide Corp., Lot No. 1S682323. Its CP was 65.67C in 2.0 and 4.0% solutions. All other chemicals were ACS reagent grade except sodium tetrafluoroborate, which was Aldrich 98%, Lot No. 06922MF. Surfactant–salt mixtures were prepared by adding analyzed, concentrated salt solutions and water to 15.0% octoxynol solutions, all weighed out to the nearest milligram. The final surfactant concentration was 2.00%; the percentage of surfactant is based on the amount of water present. The mixtures were stored under nitrogen at 5–87C for §20 hr prior to measuring the CP to ensure that the micelles were fully hydrated and had reached their equilibrium size. Cloud Point Determination Upon heating, the solutions turn somewhat hazy below the CP. Three plots of turbidity versus temperature were determined turbidimetrically by means of a spectrophotometer equipped with a jacketed cell using white light. The

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the temperature was reversed, the solutions were heated above and cooled below the CP by only 17C instead of 3. The original CP was estimated by extrapolation to zero heating time (see below). Sodium nitroprusside produces monotonic but less than linear CP increases when its concentration is increased up to its room-temperature solubility. The concentration range was extended by taking advantage of increases in solubility with temperature: Mixtures containing solid Na2[Fe(CN)5 NO]r2H2O in excess of its room temperature solubility but forming undersaturated solutions at the elevated CP temperatures were prepared with constant 2.00% octoxynol levels in order to search for a maximum in CP at higher concentrations. RESULTS AND DISCUSSION

FIG. 1. Turbidity of a 1.00% octoxynol 9 solution in water as a function of temperature: ( s ) clouding on heating; ( l ) clearing on cooling.

temperature was measured with a thermocouple whose junction was immersed in the surfactant solution. Heating and cooling rates were £0.257C/min. The plots were S-shaped in the vicinity of the CP (see Fig. 1). The curves corresponding to the clouding on heating and clearing on cooling coincided. The temperature interval between incipient and complete phase separation for 1.0 and 2.0% octoxynol solutions was merely ca. 17C. Because of the suddenness and sharpness of the phase separation, visual inspection was sufficiently accurate for routine measurements. For these measurements, the solutions were stirred under nitrogen and illuminated with an intense beam of white light. In the vicinity of the CP, they were heated and cooled at rates £0.57C/min. They were heated to at least 37C above the CP before cooling and cooled at least 37C below the CP before heating. The CP was taken as the temperature at which the immersed portion of the thermometer suddenly became invisible on heating and fully visible on cooling. CP measurements were made in triplicate. The six successive CP values of a given solution measured visually agreed in most instances within 0.27C, indicating the absence of chemical degradation. The CP values of the three solutions that were determined by turbidity measurements agreed with the average CP values determined visually on the same three solutions within 0.27C. The exceptions, where the six successive CP values determined on a given sample showed small but consistent decreases, were solutions containing sodium tetrafluoroborate, which hydrolyzes. The hydrolysis products etch glass and depress the CP (see below). To reduce the duration (and hence the extent) of such hydrolysis, the solid powder was dissolved in 2.00% octoxynol solutions only 30 min before its CP was measured, all in polycarbonate containers. Before

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There are minor lot-to-lot variations in the CP of octoxynol. To compare present with previous CP data, CP changes D rather than absolute CP values are reported: D is the difference between the CP of a solution containing octoxynol plus additive and the CP of the corresponding blank octoxynol solution. Positive D values indicate salting in. Cloud Point Effect of Sodium Tetrafluoroborate Solutions of NaBF4 hydrolyze according to the overall reaction (24) NaBF4 / 3H2O ` H3BO3 / NaF / 3HF.

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The hydrolysis is more extensive at higher temperatures; none occurred at refrigerator temperatures. As F 0 is a strong cloud point depressant (10, 11), progressive hydrolysis continually lowers the CP. In Fig. 2, the observed CP values are plotted against heating time at each of the six NaBF4 concentrations, using the number of successive heating and cooling periods as the time scale. Each heating period (odd numbered) and each cooling period (even numbered) lasted 6 { 1 min. As a result of hydrolysis, the observed CP decreased during the six successive heating and cooling periods by values ranging from 2.57C in 2.00 m NaBF4 to 0.27C in 0.25 m NaBF4 . The original CP values were estimated by extrapolating the initial linear segments (solid lines) to period 0. The fact that the CP versus time plots tend to level off (broken lines) is ascribed to the reversibility of reaction [2] and indicates that equilibrium degrees of hydrolysis were approached at each concentration during the CP measurements. Because of hydrolysis, the pH decreased from 4 for fresh solutions to 2 after the CP measurements. Cloud Point Effects of Sodium Salts of Chaotropic Anions Figures 3–5 contain D versus m plots for five chaotropic salts. Four plots have maxima (see Table 1). No maximum

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FIG. 2. Cloud points of 2.0% octoxynol 9 solutions containing the listed NaBF4 molalities as a function of the number of successive heating and cooling periods.

could be reached with sodium nitroprusside even though its plot tends to level off progressively at higher concentrations. Its CP versus m curve crosses over and drops below its temperature versus solubility curve before the former can reach its presumed maximum.

FIG. 3. Cloud point shift values D of NaNO3 ( j ), NaClO4 ( l ), and ClO 04 ( s ) as a function of their molal concentrations m.

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FIG. 4. Cloud point shift values D of NaSCN ( l ), SCN 0 ( s ), NaI ( m ), and I 0 ( n ) as a function of their molal concentrations m. Na / salts: solid lines, left ordinate; anions: broken lines, right ordinate.

Cloud Point Effects of Chaotropic Anions In order to separate the salting-in effect of the anions from the salting-out effect of Na / , the CP shift value of the latter,

FIG. 5. Cloud point shift values D of Na2[ Fe( CN )5NO ] ( l ) , [ Fe( CN )5NO ] 20 ( s ) , NaBF4 ( m ) , and BF 04 ( n ) as a function of their molal concentrations m . Tetrafluoroborate, left ordinate; nitroprusside, right ordinate.

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TABLE 1 Maximum Cloud Point Increases (Dmax) and the Corresponding Molalities (mmax) of Five Chaotropic Salts and of Their Anions For sodium salt

For anion

Anion

Dmax , 7C

mmax , molal

Dmax , 7C

mmax , molal

SCN0 I0 ClO04 BF04 [Fe(CN)5NO]20

39.1 23.1 16.3 4.0 ú28.5

4.5 4.6 1.33 0.62 ú2.53

99 79 27 9 ú61

8.5 7.5 2.0 1.0 ú2.53

DNa / , must be subtracted from the measured D values of the corresponding Na / salts. The D values of two or more salts are additive, as are the D values of the anion and the cation of a single salt ( 10, 11 ) . The nitrate anion has neither pronounced salting-in nor salting-out tendencies, and DNO 30 is zero ( 25 ) . Therefore, since DI 0 Å DNaI 0 DNa / and since / 0 DNaNO3 Å DNa / / DNO 0 Å DNaI 0 3 Å D Na , D I DNaNO3 , provided that the D values refer to comparable molal concentrations. The nitroprusside anion is divalent. Therefore, D[ Fe( CN )5NO ] 20 Å DNa2[ Fe( CN )5NO ] 0 2DNaNO3 . The D versus m plot for NaNO3 is shown in Fig. 3. It is linear up to 3 m, with a constant slope D /m Å 06.48. At higher concentrations, the plot curves downwards and the absolute value of the negative slope increases gradually. In addition to the D values of the Na / salts, Figs. 3–5 show the D values of their anions as a function of concentration. As postulated in the Introduction, the D versus m plots of the anions I 0 , SCN 0 , and BF 40 level off after reaching the maximum D. This occurs at the concentrations at which the maximum attainable disruption of the water structure has occurred, caused by a combination of chaotropic effect and elevated temperatures. The ascending branches of the D versus m plots of the sodium salts of the chaotropic anions in Figs. 3 and 4 have parabolic shapes; those of the anions themselves are more nearly linear. The nitroprusside anion is divalent whereas all other chaotropic anions are monovalent. The fact that it possesses chaotropic activity is attributed to its large size, which results in a surface charge density no higher than those of the monovalent chaotropic anions. The D versus m plot of ClO 40 (Fig. 3) is different from those of the other anions: It curves strongly downwards after reaching its maximum, rather than levelling off. Thus, the ClO 40 anion appears to disrupt the structure of water at lower concentrations but to promote it at higher concentrations. Unique features that set ClO 40 apart from the truly chaotropic anions are the following: ClO 40 is a hard base (15). Furthermore, ‘‘the tetrahedral arrangement of its oxygen atoms around the chlorine atom very closely resembles the water

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tetrahedron with regard to shape and molecular dimensions. The oxygen atoms may form hydrogen bonds with the surrounding water molecules without greatly disturbing the structural order of the water lattice’’ (13). Finally, ‘‘the structure-breaking effect of ClO 40 (at lower concentrations) depends upon its entropy of dilution’’ rather than on decreases in the average cluster size and in hydrogen bonding among the water molecules (13). Ranking the salting-in anions according to their maximum D values results in the following order: SCN 0 ú I 0 ú [Fe(CN)5NO] 20 ú ClO 40 ú BF 40 . The same order applies to the concentrations corresponding to the maximum D values (see Table 1). This order of the salting-in capacity conforms to the Hofmeister lyotropic series (14). Cloud Point Effects of Sodium Thiosulfate As is seen in Fig. 6, D is negative at all Na2S2O3 concentrations, indicating that Na2S2O3 , as well as S2O 20 3 , salts the surfactant out. The plot of D versus m for Na2SO4 is included for comparison. Both plots are linear. Their slopes D /m are 060.07C/m for Na2S2O3 and 074.9 for Na2SO4 . While S2O 20 3 is a soft base, which should cause it to break the structure of water and promote salting in, it is also divalent, which has the opposite effect. Evidently, the latter factor outweighs the former. The fact that the [Fe(CN)5NO] 20 anion promotes salting in despite being divalent is attributed to its large size: Its molecular weight is nearly twice as large (216 versus 112) and its specific surface as that of S2O 20 3 area (based on equivalent spheres) is at least 13 larger. This results in a commensurately smaller surface charge density. Another factor contributing to the salting-out capacity of Na2S2O3 (and of Na2SO4 , whose anion is a hard base) is

FIG. 6. Cloud point shift values D of Na2S2O3 ( l ) and of Na2SO4 ( s ) as a function of their molal concentrations m.

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that both anions are extensively hydrated. This is demonstrated by the existence of the two stable solid hydrates, Na2S2O3r5H2O and Na2SO4r10H2O, whose water of crystallization is associated mainly with the anions. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Schick, M. J., J. Colloid Sci. 17, 801 (1962). Schick, M. J., J. Phys. Chem. 68, 3585 (1964). Mukerjee, P., J. Phys. Chem. 69, 4038 (1965). Ray, A., and Nemethy, G., J. Am. Chem. Soc. 93, 6787 (1971). Schott, H., and Han, S. K., J. Pharm. Sci. 65, 975 (1976). Schott, H., and Han, S. K., J. Pharm. Sci. 65, 979 (1976). Maclay, W. N., J. Colloid Sci. 11, 272 (1956). Schott, H., J. Colloid Interface Sci. 43, 150 (1973). Schott, H., and Han, S. K., J. Pharm. Sci. 64, 658 (1975). Schott, H., and Royce, A. E., J. Pharm. Sci. 73, 793 (1984). Schott, H., Royce, A. E., and Han, S. K., J. Colloid Interface Sci. 98, 196 (1984). 12. Schott, H., J. Colloid Interface Sci. 173, 265 (1995).

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13. Kavanau, J. L., ‘‘Water and Solute–Water Interactions,’’ pp. 52–60. Holden-Day, San Francisco, 1964. 14. Collins, K. D., and Washabaugh, M. W., Quart. Rev. Biophys. 18, 323 (1985). 15. Pearson, R. G., J. Chem. Ed. 45, 581 and 643 (1968). 16. Pearson, R. G., Science 151, 172 (1966). 17. Hatefi, Y., and Hanstein, W. G., Proc. Natl. Acad. Sci. U.S. 62, 1129 (1969). 18. Durham, K., in ‘‘Surface Activity and Detergency’’ (K. Durham, Ed.), pp. 20–22. Macmillan & Co., London, 1961. 19. Luck, W. A. P., in ‘‘Water Act.: Influences Food Qual.’’ (L. B. Rockland and G. F. Stewart, Eds.), pp. 407–434. Academic Press, New York, 1981. 20. Van de Pas, J. C., Buytenhek, C. J., and Brouwn, L. F., Rec. Trav. Chim. Pays-Bas 113, 231 (1994). 21. Hagler, A. T., Scheraga, H. A., and Nemethy, G., J. Phys. Chem. 76, 3229 (1972). 22. Schott, H., J. Pharm. Sci. 69, 369 (1980). 23. Schott, H., in ‘‘Remington: The Science and Practice of Pharmacy’’ (A. R. Gennaro, Ed.), 19th ed. Chap. 22. Mack, Easton, PA, 1995. 24. Moeller, T., ‘‘Inorganic Chemistry—An Advanced Textbook,’’ pp. 760–761. Wiley, New York, 1952. 25. Schott, H., Colloids Surf. 11, 51 (1984).

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