Effect of Salt on Antiagglomerant Surface Adsorption in Natural Gas Hydrates

Abstract

We used molecular dynamics simulations to examine the surface adsorption of a model antiagglomerant (AA) molecule binding to an sII methane–propane hydrate in environments of different salinities. From our simulation data, we identified the preferred binding sites on the hydrate surface and characterized the equilibrium binding configurations. In addition, for a subset of these binding configurations, we calculated the standard binding free energy in different concentrations of brine using potential of mean force free-energy calculations. We demonstrate that in higher salinity environments, the surface adsorption of the AAs is enhanced through two distinct mechanisms. First, the salt decreases the solubility of the AA in the solution, which increases the thermodynamic driving force for surface adsorption. Second, the salt ions create a negatively charged interfacial layer close to the hydrate surface that effectively solvates the cationic head of the AA molecule. Quantitatively, we found that the presence of 3.5 and 10 wt % NaCl decreases the standard binding free energy of the long hydrocarbon tail binding configuration by 0.8 and 1.4 kcal/mol, decreases the standard binding free energy of the cationic head binding by 1.5 and 3.3 kcal/mol, and decreases the standard binding free energy of simultaneous head and tail binding by 1.9 and 4.3 kcal/mol, respectively.

Molecular properties of interfaces relevant for clathrate hydrate agglomeration

“…In other words, the monomers of those KHIs that were known to be effective in practical applications were found to be strongly attracted to the hydrate surface (more negative potential of mean forces compared to values far from the surface). Building from that work, several AAs were simulated at the hydrate-water interface [43,44]. Conducted for one surfactant at the interface, these atomistic studies revealed preferential adsorption sites, adsorption mechanisms on the hydrate surfaces, and AAs conformational properties.…”

“…Conducted for one surfactant at the interface, these atomistic studies revealed preferential adsorption sites, adsorption mechanisms on the hydrate surfaces, and AAs conformational properties. For example, Mehrabian et al [44] quantified the free energy of adsorption for ndodecyl-tri(n-butyl)-ammonium chloride at the sII hydrate -water interface at 277K and 100 bar and provided molecular explanations for the experimental observation that increasing salt content enhances AAs performance (see Figure 3) [23]. As the NaCl concentration increased from 0 to 3.5 and 10 wt% the free energy of adsorption decreased by 1.9 and 4.3 kcal/mol, respectively.…”

Surfactants in Upstream E&P

“…Furthermore, Bui et al studied how antiagglomerants could either enhance or impair hydrate formation [107]. Similarly, other groups studied how sodium chloride might influence the adsorption behavior of anti-agglomerants [207,208], surfactants like SDS, and hydrocarbons [209]. In another study by Bui et al [105], they were able to reproduce micromechanical force experiments using equilibrium molecular dynamics.…”

Desorption energy of soft particles from a fluid interface

“…One approach uses the free energy perturbation method to create alchemical transformations (see e.g. 23,24 ), while another one constructs the free energy profile along a geometrical reaction coordinate using methods such as thermodynamic integration or umbrella sampling [25][26][27] . In this study, we use the thermodynamic integration method to compute the free energy of binding.…”