Chemical structure and surface activity: VIII. Statistical evaluation of the influence of alkyl monoethers of polyoxyethylene glycols structure on their adsorption at the aqueous solution-air interface

Abstract

Surface tension isotherms of the individual alkyl monoethers of polyoxyethylene glycols C n H 2 n+1 (OCH 2CH 2) z OH ( n from 4 to 8; z from 1 to 5) at the aqueous solution-air interface have been determined. By using the appropriate multiple regression equations a quantitative correlation has been found between the change of standard free energy of adsorption, Δ G o, and the structure of the studied monoethers and of aliphatic alcohols as well. It has been found that within the studied range of surface tension decrease, π from 0 to 20 mN m −1, the change of standard free energy of adsorption, Δ G o, for the monoethers cannot be regarded as a sum of increments Δ G o[ X i ] corresponding to the definite fragments, X i , of monoether molecules. It has been shown, that not only methylene groups, -CH 2-, of the hydrocarbon chain, but the -OCH 2CH 2- groups of the polyoxyethylene chain have the hydrophobic character as well. The interaction of both these fragments is statistically significant and has an influence on the total value of Δ G o . However, the hydrophobic character of the -OCH 2CH 2- grouping is not constant. For the group of compounds studied it decreases with the increasing length of hydrocarbon chain and with an increase in π. For example, for π = 20 mN m −1, Δ G o[-OCH 2CH 2-] for the series of butyl and octyl derivatives is −0.32 and −0.13 RT, respectively. By assuming a simplified additive model which has been most often used to describe the relationships between Δ G o values and the chemical structure of surfactants at interfaces, we have calculated the Δ G π = 20 o[-CH 2-] and Δ G π = 20 o[-OCH 2CH 2-] values for the monoethers using multiple regression equations. These values are −1.07 and −0.25 RT, respectively. The Δ G π = 20 o[-CH 2-] value for aliphatic alcohols is −1.29 RT. By comparing the increments Δ G π = 20 o[-OH] for monoethers and alcohols (ΔΔ G π = 20 o[-OH] = −0.5 RT) it can be seen that the -OH group at the end of the polyoxyethylene chain seems to be less hydrated in the adsorption layer. This is probably caused by the participation of the mentioned hydroxyl group in inter- and intramolecular hydrogen bonds.