Equilibria of cyclopentane hydrates with varying HLB numbers of sorbitan monoesters in water-in-oil emulsions

This study investigated the effect of hydrophilic-lipophilic balance (HLB) numbers of sorbitan monoesters on equilibrium conditions of cyclopentane hydrates in water-in-oil emulsions with or without NaCl. HLB numbers of the surfactants (Span 20, 40, 60, and 80), which have the same hydrophilic group, are determined by the length and shape of alkyl chain and the HLB number refers to hydrophobicity of surfactant. The equilibrium state of the hydrate phase can be affected by the HLB of the surfactants whose number is the lowest in Span 80 (4.3) and the highest in Span 20 (8.6). The emulsion size is proportional to the increasing HLB number of sorbitan monoesters. Equilibrium dissociation temperatures of cyclopentane hydrate in the water-in-oil emulsion also rise as the HLB number increases, which was determined in micro differential scanning calorimetry (micro-DSC). In the presence of 3.5 wt % NaCl in the aqueous phase, hydrate equilibrium temperatures were almost constant regardless of the HLB number because the salt ions diminished the interaction between water and sorbitan esters as a thermodynamic hydrate inhibitor. This HLB-dependent behavior provides basic insights into the relationship between clathrate hydrate equilibria and hydrophobicity in the hydrate-forming emulsion system.