Polymer conformations of gas-hydrate kinetic inhibitors: A small-angle neutron scattering study

We have used small-angle neutron scattering to characterize the polymer con formations of four nonionic water soluble polymers: poly(ethylene oxide), p oly(N-vinyl-2-pyrollidone), poly(N-vinyl-2-caprolactam), and an N-methyl, N -vinylacetamide/N-vinyl-2-caprolactam copolymer. The last three of these ar e able to kinetically suppress hydrate crystallization, and their inhibitor activity ranges from moderate to very effective. This attribute is of sign ificant commercial importance to the oil and gas industry, but the mechanis m of the activity is unknown. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows little difference among the four polymers: the majority of the scattering is that expected for a po lymer in a good solvent. Each solution also exhibits some additional low-q scattering which we attribute to aggregates. In the presence of a hydrate-c rystal/liquid slurry, the three inhibitor polymers significantly change the ir conformation. Utilizing results from our previous contrast variation stu dy, we show that this arises from polymer adsorbed to the hydrate-crystal s urface. Furthermore, we find a strong correlation between the scattering in tensity at low q values and the effectiveness of the inhibitor polymer. We suggest this is an indication that as surface adsorption increases, the inh ibitor's blocking of growth sites increases. Also measured for one of the k inetic-inhibitor polymers was the dilute-solution polymer conformation in a hydrate-forming propane/water fluid (hydrate crystal free). This system sh ows additional low-q scattering, possibly indicating a polymer-propane inte raction prior to crystal formation. This may affect hydrate nucleation beha vior and offer a second mechanism for kinetic hydrate inhibition. (C) 2000 American Institute of Physics. [S0021-9606(00)70605-2].