(256a) Surfactant Adsorption On Structure II and Semi-Clathrate Hydrates

Clathrate hydrates are supramolecular crystalline compounds, in which small molecules are encaged in a polyhedral framework composed of hydrogen bonded water molecules. Three major structures of clathrate hydrates have been reported: sI, sII, and sH hydrates. The structure of clathrate hydrates is predominantly determined by the size of hydrate formers, e.g. methane forms sI, cyclopentane forms sII, and a mixture of methane and neohexane forms sH. One volume of clathrate hydrates can encage about 170 volumes of gas at STP conditions if all the cavities are singly occupied. Therefore, clathrate hydrates are being considered as a promising storage medium for natural gas and hydrogen. Despite of this potential, one big obstacle to industrial application of the hydrate-based gas storage is unfavorable formation kinetics. Surfactants have been known to accelerate enclathration kinetics. Thus, the interaction between surfactants and hydrates provides insight into the role of surfactants in promoting hydrate formation. This work present the adsorption behavior of sodium dodecyl sulfate (SDS) on cyclopentane (CP) hydrates and its derivative surfactant on tetrabutylammonium bromide (TBAB) hydrates. Cyclopentane (CP) is a hydrophobic former while tetrabutylammonium bromide (TBAB) is a salt that forms semi-clathrate hydrates. The adsorption on these two hydrates was studied by z-potential and pyrene fluorescence measurements. CP hydrates have a negative surface charge in the absence of SDS and it decreases to a minimum as the SDS concentration increases from 0 to 0.17 mM. Then, it increases with further increased SDS concentrations. The adsorption density of DS- on CP hydrates reaches a saturated value at 1.73 mM SDS. The micropolarity parameter of the TBAB hydrate/water interface starts to increase rapidly at 0.17 mM SDS and levels off at 1.73 mM SDS. The presence of Br- in TBAB hydrate suspensions could compete with TBADS (from association of DS- and TBA+) and DS- for the adsorption on the hydrate surface but they have a much stronger affinity for the hydrates than Br-. From the fluorescence measurements, it was found that the micropolarity of the hydrate/water interface is mainly dependent on the polarity of hydrate formers.