The hydrophobic effect of hydrate
formation well explains the enhancement
effect of a hydrophobic solid surface on hydrate formation and might
be extended to the hydrophobic traditional surfactant, amino acids,
and oligosaccharides. However, in situ observations are still lacking
to support such extension. In this study, we investigated the effect
of the addition of 0–1.0 wt % β-cyclodextrins (β-CD)
on methane hydrate formation by microscopic observations and in situ
Raman quantitative technique. Our hydrate nucleation experiments observed
the self-association of β-CD near the gas–liquid interface,
confirming that the presence of β-CD aggregates greatly increases
(41.8–43.56%) the local methane concentration and that β-CD
aggregates may act as hot spots for hydrate nucleation, which is consistent
with the description of the hydrophobic effect. In addition, the diffusion
coefficient and the concentration gradient distribution of methane
in the liquid phase during hydrate growth were measured. The results
indicate that the diffusion coefficient of methane increased by 14.47–17.11%
in the aggregate-free β-CD solutions. However, the diffusion
coefficient of methane in the solution section containing β-CD
aggregates is only 0.21–1.96% of that in aggregate-free β-CD
solutions. The numerical model based on one-dimensional hydrate growth
experiments shows that the equivalent hydrate growth rate (diffusion
flux of methane) increases more in the 0.5 wt % β-CD (18.78%)
solution than in the 1.0 wt % β-CD solution (2.77%). The excessive
β-CD addition may lead to the formation of surplus aggregates,
which occupy the whole gas–liquid interface, block the diffusion
of methane, and reduce the mass transfer efficiency of methane during
hydrate growth. This work not only supports the extension of the hydrophobic
effect on amphiphilic surfactants but also may help us better understand
the optimal concentration of surfactant additions, while excessive
surfactant addition may impair the relative efficiency of the system.