Volumes of polar additives in aqueous solutions of the poly(ethylene oxide)13-poly(propylene oxide)30-poly(ethylene oxide)13 triblock copolymer at 293 and 301 K

Density measurements of poly(ethylene oxide)(13)-poly(propylene oxide)(30)-poly(ethylene oxide)(13) (L64)-water and alcohol-L64-water systems were carried out at 293 and 301 K. The alcohols studied are propanol to pentanol and 2,2,2 trifluoroethanol (F3EtOH) to 2,2,3,3,4,4,4-heptafluorobutanol (F7BuOH). From the experimental data of the water-L64 binary system as functions of L64 concentration (m(C)), the partial molar volumes of L64 in the standard state and in the aqueous and micellar phases were calculated. At both temperatures L64 micelle is formed by a core of pure polypropylene oxide units and a hydrated shell of poly(ethylene oxide) units. In the case of the ternary systems, the apparent molar volumes of alcohol (V-Phi,V-,(R)), at a fixed concentration, as functions of m(C) were determined. At 293 K, only the premicellar region was studied because the viscosity of the solutions in the micellar region did not permit us to obtain reliable density values. For hydrogenated alcohols as well as for F3EtOH, V-Phi,V-R as functions of m(C) are concave curves while for F7BuOH the curve is convex. At 301 K, V-Phi,V-R of hydrogenated alcohols slightly depends on m(C) up to the cmc beyond which it increases monotonically with the concentration. The trends of V-Phi,V-R vs m(C) for the fluorinated alcohols are peculiar since at approximate to 0.07 mol kg(-1) (micellar region) they show anomalies which are more pronounced the more hydrophobic the alcohol is. The latter were ascribed to structural changes of the aggregates. On the basis of an equation previously reported, from the V-Phi,V-R data as functions of m(C), the partial molar volumes of alcohol in the aqueous and the micellar phases and the distribution constant of alcohol between the two pseudophases were derived.