@article{coasne_hung_pellenq_siperstein_gubbins_2006, title={Adsorption of Simple Gases in MCM-41 Materials: The Role of Surface Roughness}, volume={22}, ISSN={["0743-7463"]}, DOI={10.1021/la051676g}, abstractNote={This paper reports the development and testing of atomistic models of silica MCM-41 pores. Model A is a regular cylindrical pore having a constant section. Model B has a surface disorder that reproduces the morphological features of a pore obtained from an on-lattice simulation that mimics the synthesis process of MCM-41 materials. Both models are generated using a similar procedure, which consists of carving the pore out of an atomistic silica block. The differences between the two models are analyzed in terms of small angle neutron scattering spectra as well as adsorption isotherms and isosteric heat curves for Ar at 87 K and Xe at 195 K. As expected for capillary condensation in regular nanopores, the Ar and Xe adsorption/desorption cycles for model A exhibit a large hysteresis loop having a symmetrical shape, i.e., with parallel adsorption and desorption branches. The features of the adsorption isotherms for model B strongly depart from those observed for model A. Both the Ar and Xe adsorption branches for model B correspond to a quasicontinuous pore filling that involves coexistence within the pore of liquid bridges and gas nanobubbles. As in the case of model A, the Ar adsorption isotherm for model B exhibits a significant hysteresis loop; however, the shape of the loop is asymmetrical with a desorption branch much steeper than the adsorption branch. In contrast, the adsorption/desorption cycle for Xe in model B is quasicontinuous and quasireversible. Comparison with adsorption and neutron scattering experiments suggests that model B is too rough at the molecular scale but reproduces reasonably the surface disorder of real MCM-41 at larger length scales. In contrast, model A is smooth at small length scales in agreement with experiments but seems to be too ordered at larger length scales.}, number={1}, journal={LANGMUIR}, author={Coasne, B and Hung, FR and Pellenq, RJM and Siperstein, FR and Gubbins, KE}, year={2006}, month={Jan}, pages={194–202} }
 @article{chennamsetty_bock_scanu_siperstein_gubbins_2005, title={Cosurfactant and cosolvent effects on surfactant self-assembly in supercritical carbon dioxide}, volume={122}, ISSN={["1089-7690"]}, DOI={10.1063/1.1855291}, abstractNote={The impact of alcohol additives on the self-assembly of surfactants in supercritical carbon dioxide is investigated using lattice Monte Carlo simulations. We observe that all studied (model) alcohols reduce the critical micelle concentration. The reduction is stronger the longer the hydrocarbon chain of the alcohol, and the higher the alcohol concentration. Short-chain alcohols are found to concentrate in the surfactant layer of the aggregates, replacing surfactant molecules and leading to a strong decrease of the aggregation number and a large increase of the number of aggregates. On the other hand, only a small number of alcohol molecules with longer chain length are found in the aggregates, leading to a slight increase in the aggregation number. However, structural properties such as size and density profiles of aggregates at the same aggregation number are not influenced markedly. Consequently, short-chain alcohols act as cosurfactants, directly influencing the properties of the aggregates, while alcohols with longer hydrocarbon chains work as cosolvents, altering the properties of the solvent. However, the transition between both extremes is gradual.}, number={9}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Chennamsetty, N and Bock, H and Scanu, LF and Siperstein, FR and Gubbins, KE}, year={2005}, month={Mar} }
 @article{hung_coasne_santiso_gubbins_siperstein_sliwinska-bartkowiak_2005, title={Molecular modeling of freezing of simple fluids confined within carbon nanotubes}, volume={122}, ISSN={["1089-7690"]}, DOI={10.1063/1.1881072}, abstractNote={We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate molecules from the pore wall. The simulations show evidence of a rich phase behavior in confinement; a number of phases, some of them inhomogeneous, were observed for the pore sizes considered. The multiple transition temperatures obtained from the simulations were found to be in good agreement with recent dielectric relaxation spectroscopy experiments for CCl(4) confined within multiwalled carbon nanotubes.}, number={14}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Hung, FR and Coasne, B and Santiso, EE and Gubbins, KE and Siperstein, FR and Sliwinska-Bartkowiak, M}, year={2005}, month={Apr} }
 @article{coasne_hung_siperstein_gubbins_2005, title={Molecular simulation of gas adsorption in realistic models of silica nanopores}, volume={30}, ISSN={["1958-5934"]}, DOI={10.3166/acsm.30.375-383}, abstractNote={Cet article presente une etude par simulation moleculaire de l'adsorption de Xe a 195 K dans deux modeles atomiques des pores de silice MCM-41. Le modele A consiste en un pore cylindrique regulier a section constante. Le modele B presente un important desordre de surface qui reproduit les aspects morphologiques d'un modele obtenu par simulation sur reseau du processus de synthese des pores de MCM-41. L'isotherme d'adsorption pour le modele A comporte une large boucle d'hysteresis, typique de la condensation capillaire dans des nanopores reguliers. Au contraire, le processus d'adsorption/desorption pour le modele B a une allure quasi continue et quasi reversible. A cause de l'importante rugosite de surface du modele B, la chaleur d'adsorption isosterique pour cet echantillon est beaucoup plus grande que celle obtenue pour le pore cylindrique regulier. Dans la mesure du possible, les resultats sont compares a des donnees experimentales.}, number={4}, journal={ANNALES DE CHIMIE-SCIENCE DES MATERIAUX}, author={Coasne, B and Hung, FR and Siperstein, FR and Gubbins, KE}, year={2005}, pages={375–383} }
 @article{siperstein_gubbins_2003, title={Phase separation and liquid crystal self-assembly in surfactant-inorganic-solvent systems}, volume={19}, ISSN={["0743-7463"]}, DOI={10.1021/la026410d}, abstractNote={The behavior of surfactant−inorganic oxide−solvent systems is studied using lattice Monte Carlo simulations. Under no inorganic condensation conditions, these systems phase separate into a liquid crystal phase that contains mainly surfactant and inorganic oxide, in equilibrium with a solvent-rich phase. In the systems studied, the solvent and the inorganic oxide have favorable interactions with the surfactant head, but the inorganic oxide−surfactant interactions are stronger than the solvent−surfactant interactions, which leads to a phase separation, regardless of the oxide−solvent miscibility. The formation of ordered liquid crystal phases is observed in the phase containing a high surfactant concentration, and the structure of this phase depends on the system composition and strength of the interactions. The formation of hexagonal and lamellar structures at different conditions is in qualitative agreement with experimental evidence on the formation of surfactant−silica liquid crystals and the synthesis ...}, number={6}, journal={LANGMUIR}, author={Siperstein, FR and Gubbins, KE}, year={2003}, month={Mar}, pages={2049–2057} }
 @inbook{siperstein_gubbins_2002, title={Influence of synthesis conditions on surface heterogeneity of M41 type materials studied with lattice Monte Carlo}, ISBN={9780444512611}, ISSN={0167-2991}, url={http://dx.doi.org/10.1016/s0167-2991(02)80192-6}, DOI={10.1016/s0167-2991(02)80192-6}, abstractNote={The synthesis of mesoporous silica materials using surfactants as structure-directing agents is studied using lattice Monte Carlo simulations. The surfactant HmTn is modeled as a sequence of m hydrophilic segments or heads (H) followed by n hydrophobic segments or tails (T). Favorable interactions between the surfactant heads and the silica result in the formation of a surfactant-rich silica-rich phase in equilibrium with a dilute phase. Liquid crystal behavior is observed in the phase containing high-surfactant and high-silica concentration, with different structures depending on the overall system composition that are similar to the M41 family. The formation of a silica hexagonal phase is observed at low surfactant/silica ratios and lamellar or perforated lamellar phases are formed at high surfactant/silica ratios. The structure of the silica materials depends on the surfactant chemistry, the surfactant/silica ratios and the temperature. Heats of adsorption of simple gases on model MCM-41 type materials are calculated using Grand Canonical Monte Carlo simulations. Adsorption properties on model materials that were generated through a mimetic synthesis using lattice Monte Carlo simulations are compared with those on smooth cylindrical pores. Energetic heterogeneity in the materials studied is due to surface roughness and structural defects and not to the presence of areas with different chemical composition. This is in agreement with experimentally measured heats of adsorption of simple gases. At low coverage, heats of adsorption of argon and krypton on MCM-41 decrease with coverage, indicating that MCM-41 is not a homogeneous adsorbent even for non-polar spherical gas molecules, where the presence of polar groups in the adsorbent surface should have little effect.}, booktitle={Characterization of Porous Solids VI, Proceedings of the 6th International Symposium on the Characterization of Porous Solids (COPS-VI)}, publisher={Elsevier}, author={Siperstein, Flor R. and Gubbins, Keith E.}, year={2002}, pages={647–654} }