AFM surface force measurements conducted with silica in C_nTACl solutions: Effect of chain length on hydrophobic force

Surface forces were measured using an AFM with silica surfaces immersed in C_nTACl (n = 12-18) solutions in the absence of added salt. The results showed long-range attractive forces that cannot be explained by the DLVO theory. The long-range attractions increased with increasing surfactant concentration, reaching a maximum at the point of charge neutralization (p.c.n.) and then decreased. The long-range forces decayed exponentially, with the decay lengths increasing from 3 to 32 nm as the chain length of the surfactants increased from C-12 to C-18. The measured forces can be fitted to the charged-patch model of Miklavic et al. [S.J. Miklavic, D.Y.C. Chan, L.R. White, T.W. Healy, J. Phys. Chem. 98 (1994) 9022-9032] by assuming patch sizes that are much larger than the values reported in the literature. It was found that the decay length decreases linearly with the effective concentration of the CH₂/CH₃ groups of the C_nTACl homologues raised to the power of -1/2, which is in line with the Eriksson et al.'s hydrophobic force model derived using a mean-field approach. It appears, therefore, that the long-range attractions observed in the present work are hydrophobic forces originating from changes in water structure across the thin surfactant solution film between the silica surfaces. It is conceivable that hydrocarbon chains in solution disrupt the surface-induced water structure and cause a decrease in hydrophobic force. This observation may also provide an explanation for the very long-range forces observed with silylated, LB-deposited, and thiol-coated surfaces.