Regulation of THF Hydrate Surface Morphology by Surfactants with Different Molecular Structures: Insights from In Situ AFM Characterization

Surfactants are widely used to promote hydrate formation and improve flow assurance, yet their effects on hydrate surface morphology at the microscale remain poorly understood. Here, in situ Atomic Force Microscopy (AFM) was employed to quantitatively characterize the surface microstructure of tetrahydrofuran (THF) hydrate formed in the presence of five distinct surfactants, with particular emphasis on grain area, protrusion distribution, grain-boundary geometry, and surface roughness. Results show that pure THF hydrate exhibits a relatively smooth and regular grain morphology, with narrow and shallow grain boundaries and only limited protrusions. Surfactant addition induces pronounced surface reconstruction, yet the morphological evolution pathways differed markedly among systems. Surfactants I and II were associated with grain refinement, reducing the average grain area to 71.4 and 89.9 μm2, respectively, while protrusions were concentrated mainly near grain boundaries and grain-boundary grooves became deeper, corresponding to a grain-boundary-dominated roughening mode. In contrast, surfactants III–V were associated with grain coarsening, increasing the average grain area to 121.1, 128.2, and 156.0 μm2, respectively, together with more pronounced intragranular protrusions and widened grain boundaries, corresponding to an intragranular-dominated roughening mode. All surfactants increase surface roughness of THF hydrate to different extents. These findings reveal that surfactants regulate hydrate surface architecture through site-selective adsorption, offering a mechanistic framework for the molecular design of surfactant additives in hydrate-based applications.