Ions and ion pairs are the species that lead to CaCO3 nucleation.

A combination of theory, X-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) are used to probe the hydration structure of aqueous Na. The high spatial resolution of the XRD measurements corresponds to Q = 24 Å while the first-reported Na K-edge EXAFS measurements have a spatial resolution corresponding to 2k = Q = 16 Å. Both provide an accurate measure of the shape and position of the first peak in the Na-O pair distribution function, g(r). The measured Na-O distances of 2.384 ± 0.003 Å (XRD) and 2.37 ± 0.024 Å (EXAFS) are in excellent agreement. These measurements show a much shorter Na-O distance than generally reported in the experimental literature (Na-O ∼ 2.44 Å) although the current measurements are in agreement with recent neutron diffraction measurements. The measured Na-O coordination number from XRD is 5.5 ± 0.3. The measured structure is compared with both classical and first-principles density functional theory (DFT) simulations. Both of the DFT-based methods, revPBE and BLYP, predict a Na-O distance that is too long by about 0.05 Å with respect to the experimental data (EXAFS and XRD). The inclusion of dispersion interactions (-D3 and -D2) significantly worsens the agreement with experiment by further increasing the Na-O distance by 0.07 Å. In contrast, the use of a classical Na-O Lennard-Jones potential with SPC/E water accurately predicts the Na-O distance as 2.39 Å although the Na-O peak is over-structured with respect to experiment.

Hydrophobic solid surfaces have been found to promote the formation of gas hydrates effectively and thus help to realize the immense potential applications of hydrates in many sectors such as energy supply, gas storage and transportation, gas separation, and CO2 sequestration. Despite the well-known effectiveness, the molecular mechanism behind the promotion effect has not been thoroughly understood. In this work, we used both simulation and experimental means to gain insights into the microscopic level of the influence of hydrophobic solid surfaces on gas hydrate formation. On one hand, our simulation results show the presence of an interfacial gas enrichment (IGE) at hydrophobic surface and a gas depletion layer at hydrophilic surface. In the meantime, the analysis of water structure near the hydrophobic solid interface based on the molecular trajectories also shows that water molecules tend to get locally structured near a hydrophobic surface while becoming depressed near a hydrophilic surface. On the other hand, the experimental results demonstrate the preferential formation of gas hydrate on a hydrophobic surface. The synergic combination of simulation and experimental results points out that the existence of an IGE at hydrophobic solid surface plays a key role in promoting gas hydrate formation. This work advances the molecular level understanding of the role of hydrophobicity in governing the gas hydrate as well as interfacial phenomena in general.

Gas hydrates are crystalline solids composed of water and gases. They occur abundantly in nature and are potentially significant to industry. Solid surfaces and confined spaces strongly affect the formation of gas hydrates. Research into this particular topic is active, particularly aiming to understand the effects of solid surfaces and confinements on gas hydrate formation and using functional solids for controlling the formation kinetics. Experimental observations appear to vary from one solid to another. The observations demand a knowledge of (1) why the effects vary among the solids and ( 2) what factors are determining. Here, we critically review experimental observations, discuss the underlying mechanisms, and generalize the literature findings for a better understanding of the mechanism. It is inferred that open hydrophobic solids can promote gas hydrate formation via a tetrahedral ordering of water and an increased density of gases at the solid−water interfaces. Open hydrophilic solids hinder gas hydrate formation via a distorted water structure and a depleted density of gases at the solid−water interfaces. Confining solids have rather complex effects due to the complexity of wetting in confined spaces. Therefore, confined media with moderate wettability and partial water saturation might provide optimum conditions for gas hydrate formation.