Phase equilibria and interfacial tension between coexisting phases for the system water/2‐propanol/poly(acrylic acid)

The cloud point curve, tie‐lines and interfacial tensions were measured for solutions of 50 mol‐% neutralized (counter‐ion Na ⊕ ) poly(arylic acid) (mass‐average molar mass M̄ w = 9700 g/mol; polydispersity index M̄ w /M̄ n = 2,0) in the mixed solvent water/2‐propanol at 30°C. Due to the non‐uniformity of the polymer, the end‐points of the tie‐lines are not situated on the cloud‐point curve. The interfacial tension σ — determined by means of the spinning drop method — increases in sigmoidal manner with R tl , the length of the tie‐line, expressed in terms of weight fractions. An evaluation of these data in a double‐logarithmic plot of σ · M −0,5 versus R tl yields a critical exponent of 4,23 as compared with 3,85 in the case of binary systems. The dependence of σ on w w , the over‐all weight fraction of water in the mixture, can also be expressed as a power‐law choosing ( w cw − w w )/ w cw as the concentration variable ( w cw : critical weight fraction of water); for the present system the exponent of the resulting equation amounts to 2,33 as compared with 2,0 for toluene/polyisobutylene/poly(dimethylsiloxane).