Department of Chemical and Biomolecular Engineering, National University of Singapore , Singapore 117576, Singapore.
Langmuir. 2017 Oct 31;33(43):11956-11967. doi: 10.1021/acs.langmuir.7b02711. Epub 2017 Oct 19.
Microsecond simulations have been performed to investigate CH hydrate formation from gas/water two-phase systems between silica and graphite surfaces, respectively. The hydrophilic silica and hydrophobic graphite surfaces exhibit substantially different effects on CH hydrate formation. The graphite surface adsorbs CH molecules to form a nanobubble with a flat or negative curvature, resulting in a low aqueous CH concentration, and hydrate nucleation does not occur during 2.5 μs simulation. Moreover, an ordered interfacial water bilayer forms between the nanobubble and graphite surface thus preventing their direct contact. In contrast, the hydroxylated-silica surface prefers to be hydrated by water, with a cylindrical nanobubble formed in the solution, leading to a high aqueous CH concentration and hydrate nucleation in the bulk region; during hydrate growth, the nanobubble is gradually covered by hydrate solid and separated from the water phase, hence slowing growth. The silanol groups on the silica surface can form strong hydrogen bonds with water, and hydrate cages need to match the arrangements of silanols to form more hydrogen bonds. At the end of the simulation, the hydrate solid is separated from the silica surface by liquid water, with only several cages forming hydrogen bonds with the silica surface, mainly due to the low CH aqueous concentrations near the surface. To further explore hydrate formation between graphite surfaces, CH/water homogeneous solution systems are also simulated. CH molecules in the solution are adsorbed onto graphite and hydrate nucleation occurs in the bulk region. During hydrate growth, the adsorbed CH molecules are gradually converted into hydrate solid. It is found that the hydrate-like ordering of interfacial water induced by graphite promotes the contact between hydrate solid and graphite. We reveal that the ability of silanol groups on silica to form strong hydrogen bonds to stabilize incipient hydrate solid, as well as the ability of graphite to adsorb CH molecules and induce hydrate-like ordering of the interfacial water, are the key factors to affect CH hydrate formation between silica and graphite surfaces.
已进行微秒级模拟，分别研究了二氧化硅和石墨表面上的气/水二相体系中 CH 水合物的形成。亲水性二氧化硅和疏水性石墨表面对 CH 水合物的形成具有显著不同的影响。石墨表面吸附 CH 分子形成具有平面或负曲率的纳米气泡，导致水相中的 CH 浓度较低，并且在 2.5 μs 的模拟过程中没有发生水合物成核。此外，在纳米气泡和石墨表面之间形成有序的界面水双层，从而防止它们直接接触。相比之下，羟基化的二氧化硅表面更喜欢被水水合，在溶液中形成圆柱形纳米气泡，导致水相中的 CH 浓度高，在体相区域发生水合物成核；在水合物生长过程中，纳米气泡逐渐被水合物固体覆盖并与水相分离，从而减缓生长速度。二氧化硅表面上的硅醇基团可以与水形成强氢键，水合物笼需要与硅醇基团的排列相匹配，以形成更多的氢键。在模拟结束时，水合物固体被液态水与二氧化硅表面分离，只有几个笼与二氧化硅表面形成氢键，主要是由于表面附近的 CH 水浓度较低。为了进一步探索石墨表面之间的水合物形成，还模拟了 CH/水均相溶液体系。溶液中的 CH 分子被吸附到石墨上，在体相区域发生水合物成核。在水合物生长过程中，吸附的 CH 分子逐渐转化为水合物固体。研究发现，石墨诱导的界面水的类似水合物有序化促进了水合物固体与石墨之间的接触。我们揭示了二氧化硅上硅醇基团形成强氢键以稳定初生水合物固体的能力，以及石墨吸附 CH 分子并诱导界面水类似水合物有序化的能力，是影响二氧化硅和石墨表面之间 CH 水合物形成的关键因素。
