<Abstarte>

The effects of a water-soluble quaternary ammonium surfactant (called DA 50) on

cyclopentane (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.
<全文連結>