Chemical structure and surface activity. Part XIX. The adsorption of alkyl-2-hydroxyethyl sulfides at the aqueous solution—air interface

Abstract

Surface tension measurements were performed for aqueous solutions of alkyl-2-hydroxyethyl sulfides ( 1) at 25°C. The alkyl substituent in these compounds was varied from n-C 4H 9 to n-C 8H 17. From the surface tension values the standard free energies of adsorption, Δ G o π, were calculated. The values of Δ G o π are compared with the values for alkyl monoethers of ethylene glycol ( 2) and normal aliphatic alcohols ( 3). By using the appropriate multiple regression equation a quantitative correlation has been found between Δ G o π and the structure of amphiphilic compounds ( 1) – ( 3) and with the value of the surface tension decrease π. The equations are valid in the whole range of applicability of the Temkin isotherm. The equations made it possible to modify the concept of the effective length of alkyl chain n eff and to use it to evaluate quantitatively the effect of the thioethylene group [-SCH 2CH 2-] upon the standard free energy of adsorption of sulfides at the aqueous solution-air interface. The thioethylene group exhibits a weak hydrophobic character similar to that induced by the insertion of an additional 1.5 [-CH 2-] group between the alkyl chain and hydroxyl group of the alcohol molecule. It means that the sulfur atom itself is of a weak hydrophilic character.