Abstract
Dissolution studies of fractionated polyvinylpyrrolidone (PVP) in acetone-water (4 : 1) were carried out according to the rotating disk method. It was considered from the anomalous viscosity that PVP of low molecular weight (below 10000) behaved as an polyelectrolyte. Three stages of dissolution were observed with the lapse of time, i.e., the initial, the main and the final ones, as described in a previous paper. The initial stage was shortened with the decrease in molecular weight. Analyzing the dissolution rate of PVP of the main stage, the dissolution rate constant, KT, was expressed as a function of the molecular weight, M, and the absolute temperature, T, as follows : KT=a·exp [[-ln {a/(KT)x}]·M/Mx]·exp {-b (1-M/Mx)/RT} where R is the gas constant ; (KT)x and Mx are the converging dissolution rate constant and the converging molecular weight, respectively, of the plots of log KT at various temperatures against M ; a and b are theoretically the frequency factor and the activation energy, respectively, at M=0. The decrease in viscosity of acetone-water (4 : 1) with addition of PVP was considered due to the destruction of structure and it was concluded that the destruction of acetonewater structure by the dissolved molecule might be an important factor affecting the dissolution rate constant, KT.
