An SFG study of the effects of salts on MIBC adsorption and foamability

Methyl isobutyl carbinol (MIBC) is an alcohol-based surfactant and is widely used as a frother in the industry. Classical studies of MIBC adsorption by tensiometry fail to elucidate the mechanism(s) governing the frothing power of MIBC in the presence of salts. In this paper, sum-frequency-generation vibrational spectroscopy (SFG) is employed to study the adsorption of MIBC at the air-saline water interface and relate it with foamability and surface tension. The interface-specific spectroscopic experiments using non-linear optics SFG allowed detecting the change in the orientation and surface excess of MIBC molecules at the interface versus their bulk concentrations. At a concentration of MIBC of approximately 20 mM, the surface excess of MIBC stops increasing, implying the saturation of adsorption at the topmost monolayer. At this concentration, the antagonistic (salting-down) and synergistic (salting-up) effects of NaCl on the MIBC adsorption can be detected at low and high NaCl concentrations, respectively. At MIBC concentrations being either lower or higher than 20 mM, only a synergistic effect of NaCl is observed but with different underlying mechanisms. The finding is supported by the experimental results of foamability but cannot be explained by the results of continuously decreasing surface tension due to the adsorptions at the topmost monolayer and within the under-monolayer region at the MIBC concentration above 20 mM. This study provides insights into the frother-salt interaction and the role of the under-monolayer adsorption in controlling interfacial properties, stimulating further research and saline water use for a sustainable mineral industry.