Viscosity Reduction with Self-Assembly of Cationic Surfactant on Tetra-n-butyl Ammonium Bromide Semi-Clathrate Hydrate Aqueous Slurry

Aims: To utilize hydrate slurry for phase change refrigerants, the rheological properties are essential. In the present study, the viscosity characteristics of hydrate slurry are investigated. Additionally, the effect of cationic surfactant on viscosity is also evaluated.
Study Design: Experimental and analytical study.
Place and Duration of Study: Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, between December 2010 and October 2011.
Methodology: Tetra-n-butyl ammonium bromide (TBAB) semi-clathrate hydrate aqueous slurry was adopted. The viscosity of solutions and slurries was measured at various concentrations of solution and various solid fractions by use of stress-control-type rheometer. The measurements were performed in the both case without and with cationic surfactant. Additionally, the size and shape and aggregation property of hydrate particles in slurry were evaluated by means of direct observation and zeta potential analysis.
Results: The apparent viscosity of both TBAB solution and hydrate slurry increases as the temperature decreases, which is caused by the increase of solid fraction in slurry. The TBAB hydrate slurry behaves like pseudo-plastic fluid. In addition, the effect of minimal surfactant on rheological characteristics of TBAB hydrate slurry was investigated. A very small amount of surfactant drastically reduces the apparent viscosity of TBAB hydrate slurry, and then the morphology of TBAB hydrate particle does not change regardless of the existence of surfactant. From the measurement on zeta potential of the surface of TBAB hydrate particle, the viscosity reduction effect may be caused by the electric repulsion of surfactant-adsorbed hydrate particles.
Conclusion: A very small amount of surfactant drastically reduces the apparent viscosity of TBAB hydrate slurry, which is due to the electric repulsion among surfactant-adsorbed hydrate particles.