Long-Range Hydrophobic Forces between Mica Surfaces in Dodecylammonium Chloride Solutions in the Presence of Dodecanol

Abstract

Hydrophobic forces were measured using a Mark IV surface force apparatus with mica surfaces in equilibrium with dodecylamine hydrochloride (DAHCl) solutions at pH 5.7. With DAHCl alone, only “short-range” hydrophobic forces were observed with decay lengths of 1.3 nm. In the presence of dodecanol, long-range hydrophobic forces were observed with the decay length as large as 9.0 nm, which represents the strongest ever recorded hydrophobic force with soluble single-chain surfactants. The appearance of the long-range hydrophobic forces in the presence of neutral surfactants can be attributed to thecoadsorptionmechanism, by which a close-packed mixed monolayer of dodecylammonium ions and dodecanol is formed on the mica surface. The decay lengths of the hydrophobic forces obtained in the present work and reported in the literature have been plotted vs advancing water contact angles (θa). The results show that long-range hydrophobic forces appear at θa≥ 90°, which may be attributed to the formation of domains of close-packed hydrocarbon chains. These domains may have large dipole moments (or charges), which correlate with those of the opposing surface to give rise to long-range hydrophobic forces. The domains may also serve as sites for microscopic cavitation, as the local contact angle may exceed 90° and thereby satisfy the thermodynamic requirement for cavitation.