Water Cluster Growth in Hydrophobic Solid Nanospaces

Abstract

The growth mechanism of water clusters in carbon nanopores is clearly elucidated by in situ small-angle X-ray scattering (SAXS) studies and grand canonical Monte Carlo (GCMC) simulations at 293-313 K. Water molecules are isolated from each other in hydrophobic nanopores below relative pressures (P/P(0)) of 0.5. Water molecules associate with each other to form clusters of about 0.6 nm in size at P/P(0)=0.6, accompanied by a remarkable aggregation of these clusters. The complete filling of carbon nanopores finishes at about P/P(0)=0.8. The correlation length analysis of SAXS profiles leads to the proposal of a growth mechanism for these water clusters and the presence of the critical cluster size of 0.6 nm leads to extremely stable clusters of water molecules in hydrophobic nanopores. Once a cluster of the critical size is formed in hydrophobic nanopores, the predominant water adsorption begins to fill carbon nanopores.

Water diffusion in zeolite membranes: Molecular dynamics studies on effects of water loading and thermostat

“…Therefore the peak at 6 Å suggests that the water molecules tend to form stable clusters, and the lengths of these clusters are about 6 Å. This phenomenon, to some extent, is consistent with the finding from the study mentioned earlier in which water molecules have a 6 Å size assembly as a starting clustering length in the 11 Å width slit-pore, water clusters with this size seem very stable [19]. Integration of g pore (r) 20 to 6 Å gives a value of 6.5 (it can be higher than 6 due to the approximation for the pore shell volume made in equation (2)), indicating that all the 6 water molecules tend to be clustered together most of the time.…”

“…As noted above, clustering of water in hydrophobic nanopores at moderate loading has been experimentally observed in [19,21] by in situ SAXS and in situ XRD. In simulations, this effect can be qualitatively observed through direct visualisation of the water molecules within the pore, and can be quantitatively characterised by calculation of the radial distribution function g(r) that for two particles of the same species is,…”

“…Both experimental and simulation results have identified that water in hydrophobic and/or nanoporous structures forms clusters at some loadings [19][20][21]. Small-angle x-ray scattering (SAXS) experiments and grand canonical Monte Carlo (GCMC) studies have shown that water molecules begin to form clusters that are 6 Å in diameter at 0.6 relative pressure (P/P 0 ) within a 11 Å width slit-pore and the cluster size becomes bigger with increasing the pressure (higher water loading) due to combination of clusters [19].…”

“…Small-angle x-ray scattering (SAXS) experiments and grand canonical Monte Carlo (GCMC) studies have shown that water molecules begin to form clusters that are 6 Å in diameter at 0.6 relative pressure (P/P 0 ) within a 11 Å width slit-pore and the cluster size becomes bigger with increasing the pressure (higher water loading) due to combination of clusters [19]. Also, it has been found that water assemblies of 15 Å diameter are formed and the structure has ice-like order in slit-pores of 16.3 Å in width at P/P 0 = 0.6 using x-ray diffraction (XRD) measurements and reverse Monte Carlo simulations [21].…”

Water as a potential molecular probe for functional groups on carbon surfaces

“…Nevertheless, simulation studies from the 1980s onwards, have shed much new light on the mechanism of water adsorption on carbons [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Early simulation studies concentrated on the direct interaction between water and a pure graphene surface [23][24][25][26][27], but since graphene is hydrophobic, in the sense that the water-graphene interaction is much weaker than the intermolecular interaction between water molecules, water only adsorbs at pressures greater than the saturation vapor pressure. This theoretical observation contradicts the experimental data, where isotherms show that there is a strong uptake of water in carbonaceous adsorbents in the reduced pressure range between 0.3 and 0.8, depending on pore size.…”