Adsorption of cationic and anionic surfactants on cyclopentane hydrates

This work presents the adsorption of an anionic surfactant, sodium dodecyl sulfate (SDS), and a cationic one, dodecyl-trimethylammonium bromide (DTAB), on cyclopentane (CP) hydrates. Adsorption isotherms were obtained by liquid-liquid titrations. Also, the adsorption was qualitatively studied by zeta potential measurements. The isotherm of both SDS and DTAB exhibits a two-plateau (L-S) type. The saturated adsorption amount in the first step (Langmuir type) is 0.01 and 0.03 mmol g^-1 for SDS and DTAB, respectively. The Langmuir equilibrium constant is 1.17 mM^-1 for SDS and 0.32 mM^-1 for DTAB. The maximum adsorption amount of SDS in the second step (S type) is 2.5 times higher than that in the Langmuir step. The similar trend is observed for DTAB. SDS adsorption shifts the zeta potential from ∼60 to ∼90 mV in the first step, followed by a further 30 mV decrease as the adsorption approaches the second plateau. For DTAB, the zeta potential increases from ∼60 to 50 mV in the Langmuir step and then it approaches 90 mV in the second step. On the basis of these results, the role of surfactants in enclathration is discussed and the adsorption mechanism of surfactant on clathrate hydrates is also presented.