Chemical structure and surface activity part XXIV. Adsorption of n-octyloligooxypropylene—oligooxyethylene block copolymers at the aqueous solution—air interface

Abstract

Surface active n-octyloligooxypropylene—oligooxyethylene block copolymers C 8H 17 (OCH 2CH(CH 3)) n (OCH 2CH 2) m OH ( n = 1, 2 and 3; m = 2, 5 and 8) having definite chain lengths of oligooxypropylene, as well as oligooxyethylene, were synthesized and equilibrium surface tension—concentration isotherms of aqueous solutions of these individual substances at 20°C were determined. The experimental values of the surface tension were well described by a simplified form of Temkin's isotherm equation. Adsorption behaviour can be described quantitatively by the new empirical adsorption equation which reflects the effect of structural elements of the surfactant molecules on the adsorption process. It is shown that the standard free energy contributions, Δ G o π[-OCH 2CH 2-] and Δ G o π [-OCH 2CH(CH 3)-], to Δ G o π for a given copolymer (I) are not constant. These values depend on the whole structure of the compounds investigated with respect to the nearest environment of the above groups. Moreover, the significant changes in Δ G o π[-OCH 2CH 2-] and Δ G o π [-OCH 2CH(CH 3)-] have been found to be a function of the surface pressure π.