Effects of NaCl, NaOH, and HCl concentration on the cloud point of poly(vinyl methyl ether) in water—electrostatic interactions are inevitably involved in the hydrophobic interaction

Abstract

It has been known that OH− and/or H+ ions are adsorbed at any hydrophobic substance/water interface depending on the medium pH. This strongly suggests that the hydrophobic interaction inevitably involves electrostatic interactions. In the present study, we found that it is the case via measurements of the cloud point (T c) of poly(vinyl methyl ether) as a function of the ionic strength. Namely, T c increased with increasing NaCl concentration from 0 to 10−4 M; the order of T c was 10−1 M < 10−2 M < 10−3 M < 0 M < 10−4 M. This unexpected obstruction of the hydrophobic aggregation of the polymer chains observed at the low salt concentration was interpreted as caused by the electrostatic stabilization of the OH− adsorption and the resultant increment in the electrostatic repulsion among the polymer chains. Experimental results on the turbidity obtained as a function of NaCl, NaOH, and HCl concentration were consistently interpreted together with those on the zeta potential measured under the relevant salt conditions.