The properties of ionized monolayers. Part 4. - Surface fugacities and standard heats of absorption for three alkyl sodium sulphates

It is shown that for ionic surfactants in solutions containing excess indifferent electrolyte, the surface fugacities may be obtained graphically from the variation in surface tension with concentration in solution. From the temperature variation of the fugacities, standard integral partial molar heats of adsorption may be calculated. The method of obtaining surface fugacities proposed by Ward and Tordai¹ based on two-dimensional equations of state is believed to be of limited applicability. The calculations of standard heats of adsorption, and of the equilibrium heats given in parts 1, 2 and 3,^8-10 are discussed in the light of recent criticisms by Parsons.² The fugacity coefficients and standard heats of adsorption for octyl, decyl and dodecyl sodium sulphates are presented, and are compared with the heats calculated on the basis of the ionic double-layer theories of Derjaguin and Landau³ and Verwey and Overbeek.⁴ The values of the standard entropy of adsorption indicate the importance of the surface desolvation of ions.