Lower consolute boundaries of a poly(oxyethylene) surfactant in aqueous solutions of monovalent salts

Abstract

Lower consolute boundaries up to 50 vol% surfactant have been determined for noctylpenta(oxyethy1ene glycol) (C,E,) in the presence of the monovalent salts NaF, LiC1, NaCl, KC1, CsCl, NaBr and NaI. We find that the lower consolute boundaries are shifted, with only small changes in shape, to lower or higher temperatures (salting-out or salting-in, depending on the salt used). The amount and sign of the miscibility shift is determined almost solely by the anion, and the shift is related to the surface charge density of the ion. The shifts in the lower consolute boundary cannot be explained by structural changes in the bulk water structure due to the addition of salts. We have applied the Flory-Huggins lattice theory to relate the shifts of the lower consolute boundaries to changes in micelle-solvent interactions. This analysis shows that small changes in the free energy of interaction between oxyethylene and water can explain the shifts in temperature of the lower consolute boundary. We discuss the use and limitations of the Flory-Huggins theory, as applied to micellar phase separation. The salt effects are explained in terms of salt-deficient or salt-rich regions around the oligo-oxyethylene chains.

The Effects of Salts on the Lower Consolute Boundary of a Nonionic Micellar Solution

“…For a given salt, the steeper the T c vs critical Figure 8 presents the LCST, T c , as a function of critical salt molarity plot, the greater the salting-out effectiveness of salt molarity for the salts NaF, NaCl, NaBr, and NaI using that salt. Experimental evidence (1,(4)(5)(6)(7)(8)(9)(10) shows that the method two, in which the cation and anion diameters are set salting-out effectiveness of the salts considered in Fig. 7 fol-equal to d Table 1 for values of d , which fit the salt-water theoretical of Na 2 SO 4 compared to the other 1:1 salts (NaF, NaCl, NaBr, osmotic coefficients to experimental data, allows us to indiand NaI); however, the theoretical results also indicate that rectly incorporate some of the effects due to ion-solvent the salting-out effectiveness of the other sodium salts is almost interaction into our model.…”

“…In the first method we set d 1 , and e d are used to obtain coexis-fects which cause the effective ion size to increase due to tence curves for our model C 8 E 5 micelle-salt-water mix-the bound water surrounding it. We also notice that the value ture.…”

“…They and tant, which in turn affect the polyoxyethylene-water free others attributed the salting-in phenomena exhibited by urea energy of interaction. (9)(10)(11)(12) and salts containing SCN 0 anions (9,10,14), and A decade ago Blankschtein and co-workers (17)(18)(19) de-I 0 anions (1,10,12,14) to the fact that they act as structure veloped a theory to explain phase separation in micellar breakers of the bulk water and consequently enhance the solutions based on a Gibbs free energy model which takes hydration of the ether oxygen in the polyoxyethylene head into account the molecular interactions between the monogroups. The salting-in effect exhibited by salts such as meric species, the entropy of the aggregates, and the interacCaCl 2 , Ca(NO 3 ) 2 , and Al(NO 3 ) 3 is attributed to the forma-tions between the micellar aggregates.…”

“…Weckssalt ions. Schott and co-workers (11)(12)(13)(14) carried out an tröm and Zulauf (1) propose that salt-induced miscibility extensive study of the salting-in effect (increase in cloud shifts result from the presence of salt-rich and salt-deficient point temperature) of a number of inorganic salts on solu-regions around the poly(oxyethylene) chains of the surfactions of polyoxyethylated nonionic surfactants. They and tant, which in turn affect the polyoxyethylene-water free others attributed the salting-in phenomena exhibited by urea energy of interaction.…”

Influence of salts on the coexistence curve and protein partitioning in nonionic aqueous two-phase micellar systems

“…In fact, it is well known that salts added at rather high concentrations depress or elevate the cloud point of nonionic surfactant solutions, depending on the saltingout or salting-in nature of the salt species (Weckström, Zulauf, 1985;Koshy et al, 1996;Schott, 1997). In the same way that salts may interfere with the cloud point of nonionic surfactants, these electrolytes can influence biomolecule partitioning in nonionic ATPMS.…”