Adsorption of hydrophobically modified polyelectrolytes at the n-octane/water interface

Abstract

The interfacial activity of polyelectrolytes carrying alkyl side chains of different length has been studied. Potassium salts of poly(maleic acid-co-1-olefins), PA-nK2 with n=12, 14, 16, 18, were synthesized, and the interfacial tension at the aqueous solution/n-octane interface was measured as a function of the length of the alkyl side chain. The results show that the interfacial tension lowering, the limiting excess concentration Γm, and the efficiency of adsorption pC20 depend on the number of methylene groups in the alkyl side chain. According to Rosen (Surfactants and Interfacial Phenomena, Wiley, New York, 1989) the last two parameters define two different contributions to the standard free energy of adsorption: one arises from the distribution of the polymer between the bulk of the solution and the interface ΔGdist0, and another comes from the configuration adopted at the interface ΔGint0. These free energies were plotted as a function of the number of carbon atoms in the alkyl side chain and a linear relation was found for both of them. From these plots contributions of 0.83 and −0.58 per methylene group were determined for ΔG0dist and ΔG0int, respectively. The positive value for the incremental free energy of distribution is attributed to the formation of a polymer micelle which is stabilized by longer alkyl side chains. On the other hand, the negative value for ΔG0int indicates that at the interface the polymer adopts a configuration where the hydrocarbon tail is interacting with the octane molecules.