Temperature of maximum density behaviour of non-electrolytes in water

Abstract

The constants a and b in the empirical equation Δθ_str=ax₂+bx²₂ describing the structural part of the change in the temperature of maximum density (TMD) of water as a function of mole fraction (x₂) of dissolved solutes, have been assigned exact meaning based upon straightforward derivations using defining equations. Constant a is shown to be equal to the ratio of the limiting partial expansibility of the solute to that of pure water at 3 °C. Constant b depends on the temperature coefficient of the solute–solute interaction. Good agreement is obtained between a values calculated on the basis of accurate volumetric data and those from TMD measurements in the case of alcohols and amines. It has been shown that the equation obtained by Garrod and Herrington for the change in TMD as a polynomial in the molality of the solute is formally equivalent to those used in the present work in the limit of low concentration. Although there is a broad agreement on sign and magnitude of b values from volumetric and TMD work for some compounds in some other cases there is a difference in sign of b evaluated by these two methods. The implication of these results on the nature of solute–solute interaction in the case of alcohols is discussed. The constant b is related to a partition function that can be evaluated. Hydrolysis effects preclude the precise definition of b in the case of aliphatic amines.