Creating long-range hydrophobic forces by coadsorption of ionic and neutral surfactants

• Main Author: Ravishankar, S. A.
• Other Authors: Materials Engineering Science
• Format: Others
• Language: en
• Published: Virginia Tech 2014
• Subjects: LD5655.V856 1995.R385
• Online Access: http://hdl.handle.net/10919/38100; http://scholar.lib.vt.edu/theses/available/etd-06062008-155250/

A Mark IV surface force apparatus was used to measure surface forces between mica surfaces in dodecylamine hydrogen chloride (DAHCI) solutions at pH 5.7 in the presence and absence of long-chain alcohols. With DAHCI alone, only "short-range" hydrophobic forces were observed with decay lengths of 1.2-1.4 nm. In the presence of octanol, long range hydrophobic forces were observed with decay lengths in the range of 2.0-6.8 nm. In the presence of dodecanol, the decay length became 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 the co adsorption mechanism, in which neutral surfactant molecules adsorb along with dodecylammonium (DAH⁺) ions on mica to form close-packed monolayers. Long-range hydrophobic forces were also measured between mica surfaces in DAHCI solutions at pH 9.5, at which considerable amount of neutral amine (DA) is produced as a result of hydrolysis. The force measurements exhibit long-range hydrophobic forces with decay length of 5.5 nm. Appearance of the long-range hydrophobic force at this pH can be attributed to the coadsorption of DA and DAH⁺ on mica. The pH where the long-range hydrophobic force appears corresponds to the pH of maximum flotation of quartz, which suggests that long-range hydrophobic force may be required for good flotation. The force measurements conducted at 10.1 show that phase-separated amine is formed on the mica surface at a 50-times lower concentration than for the bulk precipitation. Above this concentration, the mica surfaces do not jump into contact with each other, suggesting that the phase-separated amine renders the surface hydrophilic. Furthermore, the force measurements conducted at pH 10.1 show repulsive steric forces, which become stronger with increasing DAHClI concentration. The adhesion force between mica surfaces becomes zero when surface precipitation occurs, while water contact angle is less sensitive. Hydrophobic force increases sharply when advancing water contact angle (θ_α) is greater than 90°, which may be attributed to the formation of domains of close-packed monolayers of hydrocarbon chains. The domains may acquire large dipole moments, which correlate with those of the opposing surface to give rise to long-range hydrophobic forces. Alternatively, microscopic cavitation may occur on the domain, whose microscopic contact angle may be greater than 90° to satisfy the thermodynamic requirement for cavitation. === Ph. D.