The Clathrate–Water Interface Is Oleophilic

Abstract

The slow nucleation of clathrate hydrates is a central challenge for their use in the storage and transportation of natural gas. Molecules that strongly adsorb to the clathrate-water interface decrease the crystal-water surface tension, lowering the barrier for clathrate nucleation. Surfactants are widely used to promote the nucleation and growth of clathrate hydrates. It has been proposed that these amphiphilic molecules bind to the clathrate surface via hydrogen bonding. However, recent studies reveal that PVCap, an amphiphilic polymer, binds to clathrates through hydrophobic moieties. Here we use molecular dynamic simulations and theory to investigate the mode and strength of binding of surfactants to the clathrate-water interface and their effect on the nucleation rate. We find that the surfactants bind to the clathrate-water interface exclusively through their hydrophobic tails. The binding is strong, driven by the entropy of dehydration of the alkyl chain, as it penetrates empty cavities at the hydrate surface. The hydrophobic attraction of alkyl groups to the clathrate surface also results in strong adsorption of alkanes. We identify two regimes for the binding of surfactants as a function of their density at the hydrate surface, which we interpret to correspond to the two steps of the Langmuir adsorption isotherm observed in experiments. Our results indicate that hydrophobic attraction to the clathrate-water interface is key for the design of soluble additives that promote the nucleation of hydrates. We use the calculated adsorption coefficients to estimate the concentration of sodium dodecyl sulfate (SDS) required to reach nucleation rates for methane hydrate consistent with those measured in experiments. To our knowledge, this study is the first to quantify the effect of surfactant concentration in the nucleation rate of clathrate hydrates.

How Do Surfactants Control the Agglomeration of Clathrate Hydrates?

“…The mobility of dodecanol at the clathrate–alkane interface, however, is orders of magnitude faster than at the clathrate–water interface, to which dodecanol binds through insertion of the alkyl tail into half-cages exposed at the surface. 68 We find that a dodecanol molecule takes less than 1 ns to move between two consecutive cages at the clathrate–pentane interface, while the same surfactant at the clathrate–water interface does not move between adjacent cages within microsecond simulations. 68 This difference arises from the distinct mode of binding of dodecanol to the clathrate–oil and clathrate–-water interfaces: the alkyl end of the surfactants is adsorbed deeply into the half-cages at the clathrate–water interface, 68 resulting in a high barrier to move from cage to cage.…”

“…Surfactants can bind to both clathrate–water 9,12,13,68,83,86,87 and clathrate–oil 21,37,83 interfaces. Binding of surfactants to clathrate–water interfaces promotes the nucleation 68 and could stall the growth 40,88 of clathrate hydrates. Binding of surfactants to clathrate–oil interfaces is used to prevent the agglomeration of clathrate particles and their coalescence with water droplets.…”

“…The enthalpy-driven binding of dodecanol to the clathrate–alkane interface contrasts with the binding of the same surfactant to the clathrate–water interface, which is almost equally strong but driven by the entropy of dehydration of the alkyl groups as they adsorb into the half-cages at the clathrate surface. 68,86,92 We expect that adsorption of quaternary ammonium surfactants to the clathrate–alkane interface in low water-cut environments is also driven by enthalpy, dominated by the enthalpy of hydration of the ions at the clathrate surface.…”

The effect of surfactants on hydrate particle agglomeration in liquid hydrocarbon continuous systems: a molecular dynamics simulation study

“…To identify and quantify the surfactant mechanism at the molecular level, we report single surfactant molecule adsorption onto a hydrate seed and the interaction of the surfactant molecule with the capillary liquid bridge using a classical molecular dynamics (MD) simulation, which has been widely and successfully used in previous investigations of hydrates, including AAs. 15,17,21,24,36 In this paper, we used the general Amber force eld 37 to describe three different surfactant molecules: (1) 1-phenylacetic acid, (2) 2-napthylacetic acid and (3) 1pyreneacetic acid. Using this approach, atom types of each surfactant were taken from the AMBER library.…”

“…[14][15][16][17][18][19][20][21] In addition, some reports have shown that AAs could also potentially affect hydrate formation and growth processes. [22][23][24][25] To evaluate the effectiveness of AAs, one frequently used method is to measure model (cyclopentane, CyC5) hydrate interparticle cohesive forces in the presence of AAs using an ambient pressure micromechanical force (MMF) apparatus, 26 because the capillary liquid bridge between hydrate particles was proposed to be another essential feature dominating hydrate particle cohesion in oil continuous systems. [27][28][29] Interestingly, one crucial result of MMF studies of surfactants indicated that polynuclear aromatic carboxylic acids can be highly effective in reducing hydrate cohesion forces by up to 87 AE 9% for a four-ring conjugated hydrophobic group, 16,30 thereby subsequently preventing hydrate particles from agglomerating.…”