Poly(ethylene oxide)13-poly(propylene oxide)30-poly(ethylene oxide)13 electrolyte interactions in aqueous solutions at some temperatures

The apparent molar volumes (V-Phi,V-C) of poly(ethylene oxide)(13)-poly(propylene oxide)(30)-poly(ethylene oxide)(13) (Pluronic L64) in aqueous electrolyte solutions were determined as functions of L64 concentrations at some temperatures. The electrolytes studied are sodium perchlorate, sodium decyl sulfate (NaDeS), sodium decanoate (NaDec), and decyltrimethylammonium bromide (DeTAB). Sodium perchlorate plays the same role as temperature on V-Phi,V-C data because of its destructuring solvent effect. At a given temperature, sodium decyl sulfate, in both the dispersed and micellized states, strongly affects not only the V-Phi,V-C vs m(C) profile but also their values. In fact, V-Phi,V-C decreases with m(C) to the critical micelle concentration (cmc), beyond which it increases, tending to level off. The experimental data are consistent with the idea that the addition of dispersed L64 to the micellar NaDeS solutions leads to the formation of NaDeS miceaes with aggregation numbers lower than those in pure water; this likely results from the solubilization of copolymer hydrophobic segments in the micellar core and hydrophilic loops at the micellar surface. When the L64 cmc is reached, V-Phi,V-C increases with me because of the cooperative effect between L64 and NaDeS in forming highly charged mixed micelles richer in L64 content. The effect of the nature of the polar head of the dispersed surfactants also was studied at 301 K. The V-Phi,V-C vs m(C) trends in the presence of NaDeS, NaDec, and DeTAB 0.015 mol kg(-1) superimpose on that in water at high m(C). In the premicellar region, the V-Phi,V-C vs m(C) Slope is positive for DeTAB and NaDec, being larger for NaDec and negative for NaDeS. The difference in the behavior of NaDec and NaDeS is due to the reduction of hydrophilic hydration of the polar head caused by the interaction with L64, which is more important for NaDec, as expected from the solute-solvent interaction contribution to the volume for the -COONa and -SO4Na groups.