Effect of a Hydrophilic Cationic Surfactant on Cyclopentane Hydrate Crystal Growth at the Water/Cyclopentane Interface

The effects of a water-soluble quaternary ammonium surfactant (called DA 50) on cydopentane (CP) hydrate growth were studied at the water/CP interface. Microscopic observations were made without and with DA 50 at concentrations of 0.01, 0.1, and 1 wt % (in the aqueous phase). In addition, the effect of NaCl at different concentrations (0 to 4 wt %) was also investigated, in the presence and absence of the surfactant. Systems with 0, 0.1, or 1 wt % DA 50 and 0 or 2 wt % NaCl, as well as those with 0.01 wt % DA 50 and 2 or 3 wt % NaCl, all led to the formation of a hydrate layer, composed of an assembly of smooth and/or striated plates, at the water/CP interface. With 0.01 wt % DA 50 and without NaCl, hydrate needles formed at the interface before aggregating into a thick unconsolidated layer in the aqueous phase. For systems without DA 50 and with 3 or 4 wt % NaCl, and for the one with 0.01 wt % DA50 and 4 wt % NaCl, a few hexagonal, triangular, and needle-like crystals grew very slowly at the interface, and most of the interface remained free of hydrate crystals for several hours after the onset of crystallization. Spectacular changes in the hydrate growth pattern and morphology were observed for the systems with 3 or 4 wt % NaCl and 0.1 or 1 wt % DA 50, where small individual crystals, in the shape of step pyramids with their vertex pointing to the CP phase, formed at the interface. Results of interfacial tension measurements showed that the adsorption kinetics of the surfactant molecules and the amount of surfactant adsorbed on the water/CP interface increased significantly with NaCl concentration. A formation mechanism of the pyramidal hydrate crystals is proposed.