Effects of a Free Adsorbate Boundary on the Description of an Argon Adsorbed Film on Graphite below the Bulk Triple Point.

Monte Carlo simulations have been carried out to study argon adsorption on graphite at temperatures below the bulk triple point temperature, Ttr(bulk) = 83.8 K. Two models for graphite have been used to investigate the effects of an adsorbate patch with a free boundary on the layering temperatures, the two-dimensional (2D)-triple point and the 2D-critical point for the three adsorbate layers on the surface. The first model (S-model) has a planar surface of infinite extent in the two directions parallel to the surface, and the second is a finite (2D-patch model). Although simulations using both models describe the characteristic temperatures, only the 2D-patch model can represent the experimental isotherms accurately, and the condensation pressures at which first-order transitions occur, while simulations with the S-model yield many unphysical substeps that are not observed experimentally in the first layer adsorbate, which leads to a poor description of higher adsorbate layers. These results support the interpretation that boundary growth of an adsorbate patch is the mechanism for argon adsorption at temperatures below the bulk triple point temperature. Combining the results derived from this simulation study for temperatures below the bulk triple point temperature, with results reported in the literature for temperatures above Ttr(bulk) and experimental data, we have constructed a generic pattern for the adsorption isotherms of simple gases on graphite at temperatures ranging from well below the bulk triple point temperature up to the bulk critical temperature, a comprehensive description not widely recognized in the literature.

Transcriptome analysis of blastoderms exposed to prolonged egg storage and short periods of incubation during egg storage.

BACKGROUND: Cool temperature egg storage prior to incubation is a common practice in the broiler industry; however, prolonged egg storage causes increased embryonic mortality and decreased hatchability and growth in surviving chicks. Exposing eggs to short periods of incubation during egg storage (SPIDES) reduces the adverse consequences of prolonged storage. SPIDES increases blastodermal cell viability by reducing apoptosis, though the counteracting mechanisms are unclear. To define the impact of prolonged storage and SPIDES, transcriptome analysis compared gene expression from blastoderms isolated from eggs exposed to the following treatments: control (CR, stored at 17 °C for 4 days), prolonged storage (NSR, stored at 17 °C for 21 days), SPIDES (SR, stored at 17 °C for 21 days with SPIDES), and incubated control (C2, stored at 17 °C for 4 days followed by incubation to HH (Hamburger-Hamilton) stage 2, used as the ideal standard development) (n = 3/group). Data analysis was performed using the CLC Genomics Workbench platform. Functional annotation was performed using DAVID and QIAGEN Ingenuity Pathway Analysis. RESULTS: In total, 4726 DEGs (differentially expressed genes) were identified across all experimental group comparisons (q < 0.05, FPKM> 20, |fold change| > 1.5). DEGs common across experimental comparisons were involved in cellular homeostasis and cytoskeletal protein binding. The NSR group exhibited activation of ubiquitination, apoptotic, and cell senescence processes. The SR group showed activation of cell viability, division, and metabolic processes. Through comparison analysis, cellular respiration, tRNA charging, cell cycle control, and HMBG1 signaling pathways were significantly impacted by treatment and potential regulatory roles for ribosomal protein L23a (RPL23A) and MYC proto-oncogene, BHLH transcription factor (MYC) were identified. CONCLUSIONS: Prolonged egg storage (NSR) resulted in enriched cell stress and death pathways; while SPIDES (SR) resulted in enriched basic cell and anti-apoptotic pathways. New insights into DNA repair mechanisms, RNA processing, shifts in metabolism, and chromatin dynamics in relation to egg storage treatment were obtained through this study. Although egg storage protocols have been examined through targeted gene expression approaches, this study provided a global view of the extensive molecular networks affected by prolonged storage and SPIDES and helped to identify potential upstream regulators for future experiments to optimize egg storage parameters.

Characterization of non-graphitized carbon blacks: a model with surface crevices.

Experimental isotherms for argon and nitrogen adsorption on two non-graphitized carbon substrates, Carbopack B and Cabot BP280, do not obey Henry's Law in the range of pressures accessible to the most sensitive MKS pressure transducers. At high pressures, close to the bulk coexistence pressure (P0), the isotherms at temperatures below the bulk triple point temperature cross the P0 axis at a finite loading, a behaviour which is interpreted as incomplete wetting. It was found that the adsorbed density at P0 for Cabot BP280 is lower than that for Carbopack B which is, in turn, only slightly lower than that for the highly graphitized Carbopack F, suggesting that there is a long-range effect of the surface structure in non-graphitized carbon blacks, in the accumulation of higher layers, especially for Cabot BP280. We have carried out extensive Monte Carlo simulations to compare experimental observations with a molecular model for substrate surfaces decorated with crevices of molecular dimensions. From the analysis of the experimental data, it was found that the typical width of crevices is of the order of 0.65-0.9 nm. In the high pressure region, the crossing of the P0 axis by isotherms at temperatures below the bulk triple point temperature can be explained by an adsorbate structure which is less dense and more disordered than the fcc structure of the bulk crystal, with a consequent raising of the coexistence pressure between the adsorbate and the gas phase above P0. Adsorbate loading at the point where the isotherm crosses the P0 axis for Cabot BP280 is lower than for Carbopack B which can be attributed to a higher concentration of crevices leading to a lower adsorbate density and an irregular arrangement of atoms at the interface separating the adsorbed phase and the gas phase. This results in weaker gas-adsorbate interactions which supresses the build-up of higher layers. We suggest that the use of the adsorbed density at the bulk coexistence pressure, at temperatures below the bulk triple point temperature, can be a useful tool for assessing the presence and concentration of surface crevices on non-graphitized carbon black.

A simulation study of the low temperature phase diagram of the methane monolayer on graphite: a test of potential energy functions.

We have used molecular simulation with two intermolecular potential models, TraPPE-UA and TraPPE-EH, the latter of which accounts for the tetrahedral shape, to study the effects of shape on methane adsorption on graphite. Both models give good descriptions of the vapour-liquid equilibria in the bulk phase, but adsorption on graphite is better described by the TraPPE-EH model. Molecular configurations in the monolayer, show the variation with temperature of the registry sites for the carbon and hydrogen atoms of the methane molecules. At temperatures below 70 K, the centre of mass (COM) of the molecules is in registry with the centre of the carbon hexagons. For temperatures above 70 K, a commensurate monolayer is initially formed as at low temperatures, then as the loading is increased the first layer remains in registry, but the COM of the methane molecules in the first layer shifts to the top of the graphite carbon atoms with the C-H bond pointing to carbon atoms in the second shell of a C-hexagon. At temperatures above 93 K, the first adsorbate layer goes through these two commensurate states and then undergoes a transition to an incommensurate solid. Finally, for temperatures greater than 110 K methane behaves like a pseudo spherical molecule.

On the transition from partial wetting to complete wetting of methanol on graphite.

The transition from partial wetting to complete wetting for methanol adsorbed on a highly graphitized thermal carbon black, Carbopack F, over a range of temperature from the triple point at 185 K to 298 K, was investigated using Monte Carlo simulation and high-resolution experiments. At 190 K, (above the triple point) both the experimental and simulated adsorption isotherms cut the P/P0 axis at a finite loading; a feature of partial wetting that has not been recognized previously in the literature. This occurs because most O- and H-atoms in the second layer of the adsorbate point towards the adsorbent surface to form hydrogen bonds with molecules in the first layer and therefore the interface between the bilayer adsorbed film and the gas phase consists mainly of methyl groups, preventing the system from forming higher layers. At temperatures above 263 K, methanol adsorption increases with pressure and wets the surface as the pressure approaches the bulk coexistence pressure P0. This is because the O-H and O-CH3 bonds of methanol in the region above the second layer have random orientation, and adsorption in higher layers takes place via hydrogen bonding. From extensive simulations of methanol adsorption on adsorbents of different strength over a wide temperature range, a parametric map has been constructed which identifies the regions of non-wetting, partial wetting and complete wetting. For a given surface strength, wetting is favoured at higher temperatures, and at a given temperature there is a transition from non-wetting on weakly adsorbing substrates to either partial wetting or to complete wetting on strong adsorbents at temperatures below or above the roughening temperature Tr of 260 K.

On the canonical isotherms for bulk fluid, surface adsorption and adsorption in pores: A common thread.

Kinetic Monte Carlo simulated isotherms calculated in the canonical ensemble, at temperatures below the critical temperature, for bulk fluid, surface adsorption and adsorption in a confined space, show a van der Waals (vdW) loop with a vertical phase transition between the rarefied and dense spinodal points at the co-existence chemical potential, µco. Microscopic examination of the state points on this loop reveals features that are common to these systems. At state points with chemical potentials greater than µco the microscopic configurations show clusters, which coalesce to form two co-existing phases along the vertical section of the loop (the coexistence line). As more molecules are added, the dense region expands at the expense of the rarefied region, to the point where the rarefied region becomes spherical (cylindrical for 2D-systems) with a curvature greater than that of the coexisting phases. This results in a decrease of chemical potential from µco to the liquid spinodal point where the rarefied region disappears. With a further increase in loading, the chemical potential and the density increase. The existence of a vdW loop is the microscopic reason for the hysteresis observed in the grand canonical isotherm, where the adsorption and desorption boundaries of the hysteresis loop are first-order transitions, enclosing the vertical section of the vdW loop of the canonical isotherm. However, a first-order transition is rarely observed in experiments where transitions are usually steep, but not vertical. From our extensive simulations, we provide two possible reasons: (1) the finite extent of the system and (2) the existence of high energy sites that localize the clusters. In the first case, the desorption branch, and in the second case the adsorption branch, either comes close to, or collapses onto the coexistence line. When both occur, the hysteresis loop disappears and the isotherm is reversible, as often observed experimentally.

On the growth of argon clusters on a weak adsorbent decorated with patches.

Much attention has been paid to understanding the clustering mechanism of water adsorbed on carbonaceous adsorbents. Adsorbed water forms clusters around strong sites, such as functional groups and surface defects, and these clusters then coalesce if the strong sites are sufficiently close to each other. Simulations of water adsorption are notoriously time consuming because of the slow relaxation of the strongly-directional hydrogen bonds. Our objective in this paper is to gain a better insight into clustering and coalescence of water, without incurring large computing overheads. To this end we have chosen argon as an adsorbate, and a substrate that is a very weak adsorbent for argon. To mimic functional groups, the substrate surface is decorated with strongly adsorbing patches. The adsorbate forms nano-clusters with convex surfaces at pressures greater than the saturation vapour pressure. When these clusters are sufficiently close to each other, they coalescence to form larger fused clusters, and there is a decrease in the equilibrium pressure. The relationship between the radius of curvature of the developed nano-clusters and the equilibrium pressure follows the functional form of the Kelvin equation, but the energy parameterγvM is smaller than the bulk value, implying that the clusters have a smaller cohesive energy.

Nonwetting/Prewetting/Wetting Transition of Ammonia on Graphite.

Simulations of ammonia adsorption on graphite were carried out over a range of temperatures to investigate the transition from nonwetting to wetting. The process is governed by a subtle interplay between the various interactions in the system and the temperature. At temperatures below the bulk triple point, the system is nonwetting; above the triple point, we observed continuous wetting, preceded by a prewetting region in which the so-called thin-to-thick film transition occurs. This system serves as an excellent example of wetting/nonwetting behavior in an associating fluid as a function of temperature because the heat of sublimation (or condensation) is greater than the isosteric heat of adsorption at zero loading. The nonwetting-to-wetting transition (NW/W) is also strongly affected by the adsorbate-adsorbate interaction, which becomes important when this contribution to the isosteric heat is of a similar magnitude to the heat of condensation. An appropriate indicator of a NW/W transition at a given loading is therefore the difference between the isosteric heat and the heat of sublimation (or condensation). Our simulation results show the "thin-to-thick" film transition in the temperature range between 195 and 240 K, which has not been previously explained. Above 240 K, continuous wetting occurs. This study provides a basis for a better understanding of adsorption in a range of systems because ammonia is an intermediate between simple molecules, such as argon, and strongly associating fluids, such as water.

The role of adsorbate size on adsorption of Ne and Xe on graphite.

We have carried out an extensive grand canonical Monte Carlo simulation to investigate the adsorption of neon and xenon on graphite. The adsorbate collision diameters of neon and xenon are smaller and greater respectively, than the commensurate graphite lattice spacing λ=3×3R300 of 0.426 nm. Simulated isotherms and isosteric heats were obtained using a graphite model that has been shown to describe successfully the adsorbate transitions for krypton, methane and nitrogen by Prasetyo et al. (2017), which have collision diameters close to λ. Neon does not exhibit commensurate (C) packing because the gain in the intermolecular potential interactions in the incommensurate (IC) packing when molecules move away from carbon hexagon centres, does not compensate for the increase in the solid-fluid potential energy. Xenon, on the other hand, exhibits IC packing because its molecular size is greater than λ. Nevertheless, at a sufficiently high chemical potential, the first layer of xenon changes from the IC to C packing (in contrast to what is observed for krypton, nitrogen and methane). This transition occurs because the decrease in the xenon intermolecular interactions is sufficiently compensated by the increase in the solid-fluid interaction, and the increase in the fluid-fluid interactions between molecules in the first layer and those in the second layer. This finding is supported by the X-ray diffraction study by Mowforth et al. (1986) and Morishige et al. (1990).

Water adsorption on carbon - A review.

Water adsorption on carbonaceous materials has been studied increasingly in the recent years, not only because of its impact on many industrial processes, but also motivated by a desire to understand, at a fundamental level, the distinctive character of directional interactions between water molecules, and between water molecules and other polar groups, such as the functional groups (FGs) at the surfaces of graphene layers. This paper presents an extensive review of recent experimental and theoretical work on water adsorption on various carbonaceous materials, with the aim of gaining a better understanding of how water adsorption in carbonaceous materials relates to the concentration of FGs, their topology (arrangement of the groups) and the structure of the confined space in porous carbons. Arising from this review we are able to propose mechanisms for water adsorption in carbonaceous materials as the adsorbate density increases. The intricate interplay between the roles of FGs and confinement makes adsorption of water on carbon materials very different from that of other simple molecules.

On the microscopic origin of the temperature evolution of isosteric heat for methane adsorption on graphite.

Understanding methane adsorption is fundamental to understanding gas storage and gas separation technologies. Detailed analyses of methane adsorption on non-porous substrates are pivotal for understanding the intrinsic interactions between the methane molecule and the adsorbent. In this paper, we particularly address the isosteric heat, which is a crucial parameter that characterizes the energetics of such systems. We have used grand canonical Monte Carlo simulations to study methane adsorption on graphite over a range of temperatures (from 50 K to 110 K). Our simulation results show good agreement with experimental data for the 2D phase transition, the 2D triple and critical points in the first layer obtained from low energy electron diffraction, neutron scattering and heat capacity measurements. On the basis of this agreement, we present a detailed microscopic picture of isosteric heat and its evolution with temperature. Our results show that the origin of the cusp and spike in the isosteric heat curve and their shift with temperature are associated with the balance of entropic and enthalpic contributions between the first and second layer.

Computational methodology for determining textural properties of simulated porous carbons.

We have refined and improved the computational efficiency of the TriPOD technique, used to determine the accessible characteristics of porous solids with a known configuration of solid atoms. Instead of placing a probe molecule randomly, as described in the original version of the TriPOD method (Herrera et al., 2011), we implemented a scheme for dividing the porous solid into 3D-grids and computing the solid-fluid potential energies at these grid points. We illustrate the potential of this technique in determining the total pore volume, the surface area and the pore size distribution of various molecular models of porous carbons, ranging from simple pore models to a more complex simulated porous carbon model; the latter is constructed from a canonical Monte Carlo simulation of carbon microcrystallites of various sizes.

Alcohol and healthy ageing: a challenge for alcohol policy.

OBJECTIVES: This paper presents findings of a qualitative study of older people's use of alcohol during retirement and identifies ways that an improved understanding of older people's drinking can inform policy approaches to alcohol and active and healthy ageing. STUDY DESIGN: Qualitative semi-structured interviews conducted with a self-selecting sample of retired people. METHODS: Participants were recruited from three geographical locations in the West of Scotland. A quota sampling design was used to ensure a broad spread of participants in terms of socio-economic position, age and gender. In total 40 participants were interviewed and the data analysed thematically using Braun and Clarke's (2006) approach. RESULTS: Amongst those who used alcohol, it was most often framed in terms of pleasure, relaxation, socialising and as a way to mark the passage of time. Alcohol was often associated with social occasions and interactions both in private and in public spaces. There were also many examples of the use of imposed routines to limit alcohol use and of a decreasing volume of alcohol being consumed as participants aged. This suggests that older people are often active in constructing what they regard as 'healthier' routines around alcohol use. However, processes and circumstances associated with ageing can lead to risk of social isolation and/or increased alcohol consumption. Such processes include retirement from paid work and other 'biographical disruptions' such as caring for a partner, bereavement and/or loss of social networks. CONCLUSIONS: These findings highlight processes that can result in changes in drinking habits and routines. Whilst these processes can be associated with a reduction or cessation of alcohol use as people age, they can also be associated with increased risk of harmful alcohol consumption. Fractured or disrupted routines, particularly those associated with bereavement or the burden of caring responsibilities, through increasing the risk of loneliness and isolation, can construct increased risk of harmful alcohol consumption. These findings reframe the pathway of risk between ageing and alcohol-related harm by highlighting the vulnerability to harmful drinking practices brought by fracture or sudden change of routine. The findings point to a role for public health in supporting the reconstruction of routines that provide structure and meaning and can be used to actively manage the benefits and harms associated with drinking.

Texture Evolution during Isothermal, Isostrain and Isobaric Loading of Polycrystalline Shape Memory NiTi.

In situ neutron diffraction was used to provide insight into martensite variant microstructures during isothermal, isobaric, and isostrain loading in shape memory NiTi. Results show variant microstructures were equivalent for the corresponding strain and more importantly, the reversibility and equivalency was immediately evident in variant microstructures that were first formed isobarically but then reoriented to a near random self-accommodated microstructure following isothermal deformation. Variant microstructures formed isothermally were not significantly affected by a subsequent thermal cycle under constant strain. In all loading cases considered, the resulting variant microstructure correlated with strain and did not correlate with stress. Based on the ability to select a variant microstructure for a given strain despite thermomechanical loading history, the results demonstrated here can be obtained by following any sequence of thermomechanical loading paths over multiple cycles. Thus for training shape memory alloys (repeating thermomechanical cycling to obtain the desired variant microstructure), optimal paths can be selected so as to minimize the number of training cycles required thereby increasing the overall stability and fatigue life of these alloys in actuator or medical applications.

A GCMC simulation and experimental study of krypton adsorption/desorption hysteresis on a graphite surface.

Adsorption isotherms and isosteric heats of krypton on a highly graphitized carbon black, Carbopack F, have been studied with a combination of Monte Carlo simulation and high-resolution experiments at 77K and 87K. Our investigation sheds light on the microscopic origin of the experimentally observed, horizontal hysteresis loop in the first layer, and the vertical hysteresis-loop in the second layer, and is found to be in agreement with our recent Monte Carlo simulation study (Diao et al., 2015). From detailed analysis of the adsorption isotherm, the latter is attributed to the compression of an imperfect solid-like state in the first layer, to form a hexagonally packed, solid-like state, immediately following the first order condensation of the second layer. To ensure that capillary condensation in the confined spaces between microcrystallites of Carbopack F does not interfere with these hysteresis loops, we carried out simulations of krypton adsorption in the confined space of a wedge-shaped pore that mimics the interstices between particles. These simulations show that, up to the third layer, any such interference is negligible.

Structure-activity correlation in transfection promoted by pyridinium cationic lipids.

The efficiency of the transfection of a plasmid DNA encoding a galactosidase promoted by a series of pyridinium lipids in mixtures with other cationic lipids and neutral lipids was assessed in CHO-K1 cells. We identify key molecular parameters of the lipids in the mixture - clog P, lipid length, partial molar volume - to predict the morphology of the lipid-DNA lipoplex and then correlate these same parameters with transfection efficiency in an in vitro assay. We define a Transfection Index that provides a linear correlation with normalized transfection efficiency over a series of 90 different lipoplex compositions. We also explore the influence of the same set of molecular parameters on the cytotoxicity of the formulations.

On the explanation of hysteresis in the adsorption of ammonia on graphitized thermal carbon black.

We present a Monte Carlo simulation and experimental study of ammonia adsorption on graphitized thermal carbon black. Our new molecular model for the adsorbent is composed of basal plane graphene surfaces with ultrafine pores grafted with hydroxyl groups at the junctions between graphene layers. The simulated adsorption isotherms and isosteric heats are in good agreement with the experimental data of Holmes and Beebe, and the simulations reproduce the unusual experimental hysteresis of ammonia adsorption on an open graphite surface for the first time in the literature. The detailed mechanisms of adsorption and desorption, and the origin of hysteresis, are investigated by the microscopic analysis of the adsorbate structures to show that restructuring occurs during adsorption. The main results from this work are: (i) at the triple point, ammonia adsorbs preferentially around the functional groups to form clusters in the ultrafine pores and spills-over onto the basal plane as the loading is increased; followed by a 2D condensation on the graphite surface to form a bilayer adsorbate; (ii) at the boiling point, adsorption occurs on the basal plane due to the increasing importance of thermal fluctuations (an entropic effect); (iii) the isosteric heat is very high at zero loading due to the strong interaction between ammonia and the functional groups, decreases steeply when the functional group is saturated, and eventually reaches the heat of condensation as the fluid-fluid interaction increases.

On the 2D-transition, hysteresis and thermodynamic equilibrium of Kr adsorption on a graphite surface.

The adsorption and desorption of Kr on graphite at temperatures in the range 60-88K, was systematically investigated using a combination of several simulation techniques including: Grand Canonical Monte Carlo (GCMC), Canonical kinetic-Monte Carlo (C-kMC) and the Mid-Density Scheme (MDS). Particular emphasis was placed on the gas-solid, gas-liquid and liquid-solid 2D phase transitions. For temperatures below the bulk triple point, the transition from a 2D-liquid-like monolayer to a 2D-solid-like state is manifested as a sub-step in the isotherm. A further increase in the chemical potential leads to another rearrangement of the 2D-solid-like state from a disordered structure to an ordered structure that is signalled by (1) another sub-step in the monolayer region and (2) a spike in the plot of the isosteric heat versus density at loadings close to the dense monolayer coverage concentration. Whenever a 2D transition occurs in a grand canonical isotherm it is always associated with a hysteresis, a feature that is not widely recognised in the literature. We studied in details this hysteresis with the analysis of the canonical isotherm, obtained with C-kMC, which exhibits a van der Waals (vdW) type loop with a vertical segment in the middle. We complemented the hysteresis loop and the vdW curve with the analysis of the equilibrium transition obtained with the MDS, and found that the equilibrium transition coincides exactly with the vertical segment of the C-kMC isotherm, indicating the co-existence of two phases at equilibrium. We also analysed adsorption at higher layers and found that the 2D-coexistence is also observed, provided that the temperature is well below the triple point. Finally the 2D-critical temperatures were obtained for the first three layers and they are in good agreement with the experimental data in the literature.

Adsorption on ordered and disordered duplex layers of porous anodic alumina.

We have carried out systematic experiments and numerical simulations of the adsorption on porous anodic aluminum oxide (AAO) duplex layers presenting either an ordered or a disordered interconnecting interface between the large (cavity) and small (constriction) sections of the structured pores. Selective blocking of the pore openings resulted in three different pore topologies: open structured pores, funnel pores, and ink-bottle pores. In the case of the structured pores having an ordered interface, the adsorption isotherms present a rich phenomenology characterized by the presence of two steps in the condensation branch and the opening of one (two) hysteresis loops during evaporation for the ink-bottle (open and funnel) pores. The isotherms can be obtained by summing the isotherms measured on uniform pores having the dimensions of the constrictions or of the cavities. The numerical analysis of the three different pore topologies indicates that the shape of the junction between the two pore sections is only important for the adsorption branch. In particular, a conic junction which resembles that of the AAO pores represents the experimental isotherms for the open and funnel pores better, but the shape of the junction in the ink bottle pores does not matter. The isotherms for the duplex layers with a disordered interface display the same general features found for the ordered duplex layers. In both cases, the adsorption branches coincide and have two steps which are shifted to lower relative pressures compared to those for the ordered duplex. Furthermore, the desorption branches comprise hysteresis loops much wider than those of the ordered duplex layers. Overall, this study highlights the important role played by morphologies where there are interconnections between large and small pores.

Existence of ultrafine crevices and functional groups along the edge surfaces of graphitized thermal carbon black.

Adsorption of different gases on graphitized thermal carbon black (GTCB) has been studied with a new molecular model to examine the consequences of micropore crevices and functional groups at the junctions between adjacent basal planes. Adsorption was simulated in the Grand Canonical Monte Carlo ensemble and the theoretical Henry constants were calculated by Monte Carlo volume integration over the Boltzmann factor of the solid-fluid potential. The simulation results are in good agreement with high-resolution experimental isotherms for argon on mineralogical graphite measured by Lopez-Gonzalez et al.1 From detailed inspection of the argon isotherms at extremely low coverages, we find two distinct Henry law regions, separated by a plateau (suggesting saturation of the stronger sites) that spans over a few decades of pressure. The first Henry law region is attributed to adsorption in the ultrafine crevices at the junctions between two adjacent basal planes, and the second region corresponds to adsorption on the basal plane, as confirmed by the theoretical Henry constant. The simulated isosteric heat and snapshots of molecular configurations show that argon adsorbs preferentially in the ultrafine crevices where there is a deep potential well due to overlap from the opposite pore walls. Similar behavior was found for other nonassociating fluids (Ar, N2, and CO2); however, for associating fluids (NH3 and H2O), the strong sites for adsorption and nucleation come from the combined effects of functional groups and ultrafine crevices, since the latter cannot alone account for the observed adsorption.