Activity and activity coefficient of water in aqueous solutions and their relationships with solution structure parameters

The water activity and the activity coefficient of water (gamma(W)) were calculated from the freezing point depression by using the Hildebrand and Scott's equation for various solutions of electrolytes, sugars, alcohols, amide, and urea, The water activity measured from the freezing point depression was very close to that at room temperature, showing that the effect of temperature on the water activity is neglegible. The activity coefficient of water was described well as a function of the solute mole fraction (X(S)) by the following equation. gamma(W)=exp(alpha X(S)(2)+beta X(S)(3)) The parameter alpha in this equation correlated well with the B-coefficient of viscosity for single-valent electrolytes, with the hydration parameter for nonelectrolytes, and with the number of equatorial-OH groups for sugars suggesting that the solution structure was reflected in this parameter.