Rigidity and Moisture Hysteresis in Gels

Abstract

SEVERAL equations have been suggested for the swelling pressure of a hygroscopic gel, of which the best known is the approximate form due to Katz¹. Porter² gives an exact and general equation for the osmotic pressure of a solution. We may, for simplicity, conform to the usual conceptions of swelling pressure and restrict Porter's equation to the case where the only pressure exerted on the solvent is that of its own vapour; giving Here p is the applied pressure; (p—h₀,) is the osmotic or swelling pressure required to raise the vapour pressure of the solution to saturation h₀ from its initial value h₁, without change in concentration, s is equal to that is, the change in volume V of the solution per unit mass of solvent removed or added at constant pressure, and V is the specific volume of the solvent vapour under the pressure h. By making V represent the volume of solution containing unit mass of dry solute, m becomes identical with the moisture content defined as the mass of solvent per unit mass of dry solute. Katz's equation may be obtained from this by assuming s to be equal to the specific volume of free water and therefore a constant.