Monte Carlo Simulation Study of Water Adsorption in Activated Carbon

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Monte Carlo Simulation Study of Water Adsorption in Activated Carbon J.-C. Liu and P. A. Monson* Department of Chemical Engineering, UniVersity of Massachusetts, 686 North Pleasant Street, Amherst, Massachusetts 01003

We present a Monte Carlo simulation study of adsorption and desorption for two molecular models of water in activated carbon, focusing on the temperature range for which the system exhibits hysteresis. The activated carbon is modeled using an adaptation of the platelet model developed by Segarra and Glandt (Chem. Eng. Sci. 1994, 49 (17), 2953-2965). The active sites in the carbon are modeled by placing interaction sites at the periphery of the platelets. To model water we have used two model potentials: the simple point charge (SPC) model and adaptation of the primitive model of water developed by Kolafa and Nezbeda (Mol. Phys. 1987, 61 (1), 161-175). We find that both models considered yield quite good qualitative agreement with the experimental data for water in BPL carbon obtained by Levan and co-workers (Ind. Eng. Chem. Res. 1992, 31 (4), 1122-1130). In particular both the shape of the hysteresis loops and their temperature dependence are correctly described by the simulation results. The sensitivity of the results to changes in the model parameters is investigated. I. Introduction Activated carbons are among the most widely used porous materials for many applications and are of particular importance in the development of filter materials for removing organic compounds from vapor streams.1 The affinity for water of activated carbons can reduce their effectiveness as filters under high humidity environments,2-7 and the design of new materials with low affinity for water that maintain a high affinity for organic molecules would be of considerable interest. A significant component in the design of such materials is an understanding of how adsorption of water takes place at the microscopic level. This poses a challenge to molecular modeling because it involves describing the molecular level structure of activated carbons as well as the properties of water under confinement. In this paper we focus on a particular but important aspect of water adsorption in activated carbons: the observation of adsorption/desorption hysteresis. Recent work has indicated that the type H1 and type H2 hysteresis loops observed for adsorption of fluids in mesoporous materials can be understood using statistical mechanics in the grand canonical ensemble applied to molecular models that give a realistic description of the pore structure.8-10 Adsorption and desorption isotherms in qualitative agreement with those from experiments are obtained from computer simulations9 or mean field density functional theory8,10 studies of the models. Hysteresis is seen to arise from the very slow dynamics associated with equilibration of medium density states of the system at low temperatures,10 and the states encountered in the hysteresis region can be identified as local minima of the grand free energy of the system.8,10 Only those states above and below the upper and lower closure points of the hysteresis loop where the isotherm is reversible correspond to true equilibrium states of the system. The current work was partly motivated by the question of whether the same kind of picture might apply to water adsorption in activated carbon. Levan and co-workers2 have made a thorough study of adsorption and desorption of water vapor in BPL carbon at temperatures in the range 298-398 K for pressures up to saturation. The isotherms obtained in this work fit into the type * To whom correspondence should be addressed.

V category of the IUPAC classification.11,12 The adsorption of water remains low up to relative pressures, P/P0 (where P0 is the bulk saturated vapor pressure), of 40% or more and then increases smoothly until P0 is reached. The system exhibits hysteresis between adsorption and desorption, with the width of the hysteresis decreasing with increasing temperature. An important feature of the observed hysteresis loops is that the upper closure point is very close to the saturation pressure, as seen in the type H3 and type H4 hysteresis loops of the IUPAC classification of hysteresis. This contrasts with the type H1 and type H2 hysteresis often encountered in studies of nitrogen adsorption in mesoporous materials.11,12 In those cases the isotherm is usually reversible for a range of pressures between the upper closure point and P0, indicating the formation of an equilibrated dense (liquid) phase in the void space of the porous material. For water in activated carbon the coincidence of the upper closure point with P0 raises the possibility that the system is still out of equilibrium at that point and that the void space of the carbon is not yet filled with liquid. If this were the case the desorption isotherm obtained by starting from the adsorption branch at P0 might lie on a scanning curve linking states of density lower than those that would be achieved by starting from a state where the isotherm was reversible. In these particular experiments2 this possibility is rendered less likely by the fact that the authors studied hysteresis by heating and cooling the system at fixed pressure, rather than by changing the pressure along an isotherm. There have been a large number of computer simulation studies of models of water in carbon adsorbents,13 including the slit pore model5,6,14-22 and fully atomistic models.23-25 In some of these models the effect of activation has been included by placing interaction sites on the pore walls.5,6,15,21,22 Such interaction sites nucleate hydrogen bond networks and promote the formation of a dense state in the system. Without such sites the adsorption of water generally remains low for pressures up to and beyond P0.26 In the present work we have used an adaptation of a model due to Segarra and Glandt (SG)27 in which the carbon material is considered to be a disordered array of graphene planes modeled as platelets. This model occupies something of a middle ground between the slit pore model and the fully atomistic models. It includes the pore connectivity and

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the disorder in pore shape that are not addressed in the slit pore model, while being more computationally efficient than the full atomistic models. Our goal has not been to develop a fully quantitatively accurate model of water in activated carbons (indeed we would argue that given the variability between samples of carbon used in experiments, placing too much emphasis on close quantitative agreement with experiment is inappropriate). We have sought instead to have a model that is qualitatively accurate and can yield information about the physics of adsorption and desorption in these systems. Our paper describes grand canonical Monte Carlo (GCMC) simulations of adsorption and desorption of water in activated carbon using our adaptation of the SG model together with two models of water. We find results in good qualitative accord with those seen experimentally. We see hysteresis that persists up to pressures close to the saturation pressure while the temperature dependence and shape of the hysteresis loops is also captured. The remainder of this paper is organized as follows: In the next section we describe the molecular models and Monte Carlo simulations. In section III, we describe our results for water adsorption isotherms and hysteresis behavior. Finally section IV gives a summary of our results and conclusions. II. Molecular Models and Simulation Details A. Molecular Models for Water. We have used two model potentials for water in this work. The first model used is based on the primitive model of water by Nezbeda and co-workers28-31 and consists of a Lennard-Jones 12-6 potential together with four tetrahedrally coordinated square well association sites. The interactions between these association sites model the hydrogen bonds between the water molecules without the explicit inclusion of electrostatic interactions. We will refer to this as the LJ4SQW model. This model has been used in studies of adsorption of water in the slit pore model by McCallum et al.,32 and we have used the same values for the parameters as these workers. The second potential we used is the simple point charge (SPC) model.33 For both models, the fluid-fluid intermolecular potentials were truncated at 4.5σff, where σff is the LennardJones 12-6 collision diameter. Although we did not include the long-ranged electrostatic contributions for the SPC model, these contributions would not be expected to change the qualitative adsorption/desorption behavior observed. Moreover, our estimates of the bulk vapor-liquid coexistence for the SPC model26 agree well with earlier work using Ewald sums to describe the long-range interactions.34,35 We were originally motivated to study the LJ4SQW model because of the efficiency of calculating the short range interactions used for hydrogen bonding versus electrostatic potentials. However, this advantage is balanced by a lack of smoothness in the orientation dependence of the model that makes it more difficult to sample efficiently in Monte Carlo simulations. B. Molecular Model for Activated Carbon. As we mentioned above, to model activated carbon we have adapted the SG27 model. In this model the carbon is treated as a disordered array of graphene planes represented by platelets. The basic parameters of the model are derived from our recent study of methane adsorption in BPL carbon,36 and we refer the reader to that work for a more detailed description of the model. The principal differences between our implementation of the SG model and the original one are that we use a larger plate diameter, 1.7 nm, and that each platelet has only a single graphite basal plane. As in the original model the platelet configuration comes from a Monte Carlo simulation of an assembly of platelets with hard core interactions. The larger

anisotropy of the platelets in our version of the model increases the orientational correlations between them. In most cases we have used a 32 platelet configuration in periodic boundaries, with a system volume of 64.88 nm,3 for the model carbon, although in some cases 108 particles were used with the same density. The SG potential is obtained by integrating the Lennard-Jones 12-6 potential over the graphene planes associated with each platelet.27 It thus represents a level of approximation similar to the 10-4 potential.37,38 For the interaction of water with graphite there are additional contributions, for example, induction, that are not directly modeled by the short-range repulsions and longrange dispersion interactions in the 12-6 potential. To obtain results that are comparable with experiment we have to use a carbon-oxygen 12-6 well depth that is substantially larger so that the additional interactions are incorporated in at least an effective manner. We use values of co/k ) 77 K (SPC) and co/k ) 79.44 K (LJ4SQW), where k is Boltzmann’s constant, leading to a binding energy for a water molecule with a single platelet of -9.66 kJ/mol. This latter value lies within the (wide) range of values for the water-graphite binding energy that have been estimated from quantum mechanical calculations.39-41 Our value for co was chosen by comparing experimental adsorption isotherms at 298 K with those from GCMC simulations of the SPC model and choosing a value that gave reasonable agreement. The Lorentz-Berthelot combining rule was used to obtain σsf. The difference between the values of co for the SPC and LJ4SQW models comes from the difference in the oxygen collision diameter between the models. To model the effect of activation we include model carbonyl (CdO) groups at the periphery of the platelets. In the original SG model this was done by using a distribution of dipoles around the periphery of the platelets. In this work we use explicit interaction sites. The carbonyl groups are placed so that the CdO axis is parallel to the flat surface of the platelet while normal to the curved surface. The CdO groups are randomly distributed around the platelet periphery. When using the SPC model the parameters for the carbonyl group interaction with water are taken from the OPLS parameter set,42 following earlier work by Brennan et al.43 and Jorge et al.21,22 for other carbon models. These parameters are as follows: oo/k ) 105.76 K, σoo ) 0.296 nm, and the CdO bond length is 0.1214 nm, with the point charges at the oxygen and carbon center given by -0.5e and +0.5e, respectively. For the case of the LJ4SQW model, we model the interaction with a CdO group using a Lennard-Jones sphere with a single square well bonding site, an approach used earlier by McCallum et al.32 The geometry parameters for the oxygen atoms and square well sites are the same as those in the LJ4SQW potential. The CdO bond length is again 0.1214 nm. The energy parameters are set to be comparable to the strength of interactions in the OPLS parameter set for the SPC model: oo/k ) 105.76 K and sf,HB/k ) 2300 K. Experimental estimates for the density of active sites in BPL carbon are in the range of 0.7-2.0 mmol/g, and we have used a value of 1.4 for this parameter. A computer graphics visualization for the model carbon with active sites is shown in Figure 1. For the SPC model we have made some studies of the effect of changing the density of active sites and the charges associated with them, as well as the configuration of platelets. C. Monte Carlo Simulations. We have used the conventional GCMC simulation technique44,45 to study the adsorption behavior of the SPC model of water in model carbon. Hysteresis can occur in the GCMC method with the Metropolis algorithm because of infrequent sampling of the large scale density

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Figure 1. Computer graphics visualization for the model carbon with active sites for the LJ4SQW model.

Figure 2. Saturated vapor pressure of water versus temperature: circles, LJ4SQW model; squares, SPC model; diamonds, experiment.51 The lines through the points are drawn as a guide to the eye.

fluctuations required to equilibrate states of intermediate density at low temperature (i.e., temperatures below the bulk critical temperature).9 This is thought to parallel the adsorption/ desorption hysteresis encountered experimentally.9 For models of fluids in mesoporous glasses two regimes in the relaxation dynamics have been identified.10,46 A short time (“transport”) regime is associated with the mass transfer of fluid to and from the external surfaces of the porous material. A long time (“quasiequilibrium”) regime involves transitions between local minima of the grand free energy. These transitions involve spatial redistribution of dense fluid in the porous material, an intrinsically slow process. It is argued that the “equilibrated” states associated with experimentally determined hysteresis loops correspond to this latter regime.10,46 Additionally a relationship between the relaxation dynamics associated with a GCMC simulation and real dynamics has been established.10 For the case of the LJ4SQW model we have improved the sampling of density fluctuations by using hyper-parallel tempering GCMC (HPTGCMC).47 In HPTGCMC several replicas of the system, corresponding to a range of activities and temperatures, are simulated in parallel in a simulation that includes trial moves with exchange of configurations between different replicas. Parallel tempering methods are designed to sample transitions across energy barriers, and we might expect that the HPTGCMC would eliminate the hysteresis in the system. What we find in this case is that the method facilitates the relaxation

Figure 3. Adsorption/desorption isotherms for water in carbon at five temperatures: (a) 298; (b) 323; (c) 348; (d) 373; and (e) 398 K. The circles denote the simulation results for the LJ4SQW model, and the squares denote the experimental data.2,52 Filled symbols are for adsorption, and the open symbols are for desorption. Lines are fits to the simulation results, drawn as a guide to the eye.

of the system into a local minimum (“transport” regime) of the grand free energy but does not seem lead to complete equilibration of the system. Location of the saturated vapor state for the models of water is a key component of this study, especially in the context of a qualitative comparison with experiment. To obtain the bulk vapor-liquid equilibrium activities for the models, the Gibbs ensemble Monte Carlo (GEMC)48 technique was used. An orientational bias technique45 was also used in both GCMC and GEMC simulations in this work to improve the sampling of orientations. For calculation of the adsorption isotherms using GCMC simulations, we started from an empty model carbon sample and then performed simulations by successively increasing the configurational activity of water defined as λ ) eµc/kT, where µc is the configurational chemical potential. For calculation of the desorption isotherms, we started from the final configurations at the highest activity studied on the adsorption branch and then performed simulations by successively decreasing the activity. Each subsequent simulation was started by employing the final configuration of the previous one. For the simulations of the LJ4SQW model using HPTGCMC, we simultaneously simulated states associated with ranges of activities on each of nine isotherms at equally spaced temperatures from 298 to 398 K. For adsorption, in the first run each replica was initially empty while in the subsequent runs each replica at a given temperature was given the final configuration from the highest activity

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Figure 5. Comparison of the simulation results for adsorption and desorption from the two models with experiment at T ) 298 K plotted versus pressure: (a) LJ4SWQ model; (b) SPC model. The circles donote the simulation results for the model water, and the squares denote the experimental data. Filled symbols are for adsorption, and open symbols are for desorption. Lines are fits to the simulation results, drawn as a guide to the eye. Figure 4. Adsorption/desorption isotherms for water in carbon at five temperatures: (a) 298; (b) 323; (c) 348; (d) 373K; and (e) 398 K. The circles denote the simulation results for the SPC model, and the squares denote the experimental data.2,52 Filled symbols are for adsorption, and the open symbols are for desorption. Lines are fits to the simulation results, drawn as a guide to the eye.

replica at the same temperature from the previous run. For desorption, in the first run each replica at a given temperature was given the final configuration from the highest activity replica at the same temperature in the final adsorption run. For the subsequent desorption runs each replica at a given temperature was given the final configuration from the lowest activity replica at the same temperature in the previous desorption run. The simulations were run for 0.5-1 × 108 configurations for the SPC model and 1-2 × 109 configurations for the LJ4SQW model, with half of the configurations used for equilibration. New configurations were obtained by the attempted translation, rotation, creation, and destruction of a randomly chosen molecule. These moves were chosen with equal probability. For the HPTGCMC simulations, there is also a step of swapping two randomly selected replicas. To obtain the vapor-liquid equilibrium for the models from GEMC simulations, the total volume and number of molecules were chosen so that, after equilibrium, the volume of the liquid phase was larger than that required by the potential truncation radius and the number of molecules in the vapor phase was in the range 50-100. GEMC simulations for the SPC model were run for 8 × 104 to 2 × 105 configurations per molecule, while those for the LJ4SQW model were run for 2 × 106 to 5 × 106 configurations per molecule, again with half of the configurations for equilibration. New configurations were obtained through attempted translation or rotation of a randomly chosen molecule, transfer of a randomly chosen molecule between

phases, and changes of the volume of the subsystems. The relative probabilities for attempting these moves varied with the temperature and were chosen to optimize the rate of equilibration. The Widom test particle method49 was used to obtain the activity at phase equilibrium. We have calculated the saturated vapor pressure for SPC and LJ4SQW models of water at five temperatures by using a combination of GEMC and GCMC simultaions. These results are showed in Figure 2. The uncertainties for the vapor pressures are 2-4% and 5-9% for the SPC and LJ4SQW models, respectively. Uncertainties are largest at the lowest temperatures in each case. III. Results and Discussion A. Adsorption Isotherms and Hysteresis. The simulation results for adsorption and desorption for the LJ4SQW model versus the relative pressure, P/P0, are shown in Figure 3. For comparison we have included the experimental data of Levan and co-workers.2 The qualitative behavior shown in the experiments is reproduced quite well by the model, except for the curvature of the adsorption isotherm in the low to moderate pressure regime and the shift of the hysteresis loop to lower pressure. Low pressure adsorption in the model (and presumably in the real system) occurs at the active sites because the graphene plane is otherwise hydrophobic. In the LJ4SQW model the association interactions at the active sites are short ranged and strongly directional allowing only one water molecule to interact strongly with an active site. If the association interaction was longer ranged then more water molecules could associate with each active site and the low pressure adsorption would increase more smoothly (as we shall see for the SPC model). It is possible and even likely that further adjustment of the model parameters would improve the quantitative agreement with experiment, but that was not the goal of this study.


Figure 6. Computer graphics visualization for the LJ4SQW model of water in activated carbon at T ) 298 K and P/P0 ) 0.26. The bottom picture show the system with the platelets removed for clarity.

The LJ4SQW model captures the shape of the hysteresis loops and their dependence on the temperature. In common with what was seen experimentally2 the upper closure point of the hysteresis loop lies close to P0 at each temperature, again in good agreement with experiment, although the precise location is difficult to determine because of uncertainty in the amount adsorbed. For this model we extended the adsorption calculations to states beyond P0 where the isotherm was reversible within the uncertainty in the amount adsorbed and the sequence of states on desorption was initiated in this region to ensure that our desorption branch was not a scanning curve. The corresponding results for the SPC model are shown in Figure 4. We see that the quantitative agreement with experiment is generally better in this case, which is at least in part associated with the fact that the SPC model is a more accurate representation of water than the LJ4SQW model. The low pressure behavior is noticeably better, reflecting the longer ranged interactions in the SPC model, and the width of the hysteresis is narrower, especially at lower temperature. The difference in the widths of the hysteresis loops may be due to the fact that it is easier to sample the configurations of the SPC model than those of the LJ4SQW model so that the SPC simulations represent an effectively longer sample of the system relaxation dynamics. Up to this point we have shown a comparison of our model results with those from experiment using a relative pressure scale, which reduces the effects of errors in the estimation of the bulk saturated vapor pressure obtained from the models.

Figure 7. Computer graphics visualization for the LJ4SQW model of water in activated carbon at T ) 298 K and P/P0 ) 0.34. The bottom picture shows the system with the platelets removed for clarity.

Figure 5 shows a direct comparison with experiment for the two models using a linear scale for the pressure. This plot again shows superior quantitative agreement from the SPC model for the location of the hysteresis loop due to the more accurate value of the vapor pressure obtained for this model as shown in Figure 2. Similarly, the appearance of hysteresis at even lower pressure with respect to experiment for the LJ4SQW model reflects the lower value of the vapor pressure from this model at this temperature. The mechanism of adsorption in the models is apparent from the computer graphics visualizations of the LJ4SQW model shown in Figures 6 and 7 at 298 K for a low and an intermediate pressure. It is evident that at low pressure the adsorption occurs around the active sites on the periphery of the platelets where association of the water molecules can occur (Figure 6). Further adsorption occurs by the development of hydrogen bond networks that start in these regions and permeate throughout system (Figure 7). B. Parameter Sensitivity. It is worthwhile to investigate the extent to which our results are sensitive to the model parameters used, especially for the model of carbon. For the SPC model at 298 K we carried out additional studies where we considered the following: (i) sample size for the carbon; (ii) configuration of the platelets; (iii) distribution of actives sites; (iv) overall density of active sites; and (v) strength of the water moleculeactive site interaction. Figure 8 shows the effects of the first three of these parameter changes. Figure 8a,b shows the effect of changing the distribution of active sites while keeping the

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Figure 9. Simulation results for the SPC model of water adsorption in platelet model carbon at T ) 298 K. Filled symbols for adsorption and open symbols for desorption. (a) Fas ) 0.8 mmol/g, CO charge ) 0.5e; (b) Fas ) 1.4 mmol/g, CO charge ) 0.37e. The dashed lines in each case show our base case calculation described earlier and shown in Figure 4a for which Fas ) 1.4 mmol/g and CO charge ) 0.5 e.

Figure 8. Simulation results for the SPC model of water adsorption in platelet model carbon at T ) 298 K. Filled symbols for adsorption and open symbols for desorption. (a) Platelet configuration 1, active site distribution 2; (b) platelet configuration 1, active site distribution 3; (c) platelet configuration 2, active site distribution 1; (d) platelet configuration 3, active site distribution 1; (e) 108 platelet system. The dashed lines in each case show our base case calculation described earlier and shown in Figure 4a.

overall density of these sites and the platelet configuration fixed. Figure 8 c,d shows the effect of changing the platelet configuration. Figure 8e shows results obtained from a larger sample of the model carbon with 108 platelets. The sensitivity of the results to these changes is a guide to how well our results represent an accurate realization of the model, and we see that the effect of these changes is small. Figure 9 shows the effect of decreasing the overall density of the active sites and of decreasing the charges on the active sites. In each case the amount adsorbed is lower at a given pressure. However, these quite substantial changes do not change the qualitative behavior of the model. IV. Summary and Conclusions We have presented a Monte Carlo simulation study of two models of water (SPC and LJ4SQW) in activated carbon. The model carbon is an adaptation of the SG platelet model with interaction sites at the platelet periphery to model the effect of activation. This model includes the effects of structural disorder which are missing in the slit pore model while being more computationally efficient than recently developed atomistic models.23,24,43 The focus of the calculations has been a comparison of the model with experimental data from Levan and co-workers2 for water in BPL carbon for temperatures where the system exhibits hysteresis between adsorption and desorp-

tion. We find that both models of water in carbon are able to capture the key qualitative features of the experimental data, especially the overall shape of the isotherms and hysteresis loops. Both models exhibit upper closure points lying close to the saturation pressure, in common with experiment. Visualizations of configurations from the simulations indicate an adsorption mechanism which starts with association of water molecules with the active sites. Further adsorption involves the propagation of hydrogen bond networks starting with the water molecules at the active sites. The desorption process is in general the reverse of this although because we are using periodic boundaries in all directions we cannot see percolation effects that might arise if we included an external surface.9,50 Also at the two lowest temperature studies the LJ4SQW model shows a substantial difference in slope between adsorption and desorption similar to that seen experimentally2 and reminiscent of that seen in a type H2 hysteresis loop.11,12 The hysteresis observed reflects lack of equilibration on both the adsorption and the desorption branches of the isotherm. Our results support the idea that the SG model provides a useful framework for studying water adsorption in activated carbon. On the basis of our work several areas for future work are suggested. The relationship between hysteresis and the possibility of an underlying vapor-liquid transition for confined water8,10 has not been investigated in this work. We suspect that as in the case of fluids in other strongly disordered porous materials such as Vycor,10 the hysteresis seen here does not reflect the occurrence of a true phase transition. It would also be interesting to study the effect of including external surfaces upon the width of the hysteresis and the mechanism of desorption.9,50 Applying the model to water-organic mixtures should provide some insight into how these mixtures behave in carbons, and this could be used to help understand the performance of carbon based filters in high humidity environ-


ments. While further optimization of the model could be made to improve the quantitative agreement with experiment and this may be useful for particular cases, the model is probably most useful as a qualitative description of the behavior of water in carbons. We close our paper by noting that the success of the SG model in describing adsorption/desorption hysteresis for water in activated carbon is yet another illustration of Eduardo Glandt’s deep insights into molecular thermodynamics. Acknowledgment This work was supported by a grant from the U.S. Army Research Office (Grant DAAD19-02-1-0384). The authors are grateful to M. D. Levan for providing tabulations of experimental data and for help in its interpretation. Literature Cited (1) Bansal, R. C.; Donnet, J.-B.; Stoeckli, F. ActiVe carbon; M. Dekker: New York, 1988. (2) Rudisill, E. N.; Hacskaylo, J. J.; Levan, M. D. Coadsorption of Hydrocarbons and Water on BPL Activated Carbon. Ind. Eng. Chem. Res. 1992, 31 (4), 1122-1130. (3) Eissmann, R. N.; Levan, M. D. Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 2. 1,1,2-Trichloro-1,2,2trifluoroethane and Dichloromethane. Ind. Eng. Chem. Res. 1993, 32 (11), 2752-2757. (4) Russell, B. P.; LeVan, M. D. Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 3. Ethane, Propane, and Mixing Rules. Ind. Eng. Chem. Res. 1997, 36 (6), 2380-2389. (5) Slasli, A. M.; Jorge, M.; Stoeckli, F.; Seaton, N. A. Water adsorption by activated carbons in relation to their microporous structure. Carbon 2003, 41 (3), 479-486. (6) Slasli, A. M.; Jorge, M.; Stoeckli, F.; Seaton, N. A. Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content. Carbon 2004, 42 (10), 1947-1952. (7) Rubel, G. O. Water isotherm measurements for microparticles of carbon. Carbon 1992, 30 (7), 1007-1011. (8) Kierlik, E.; Monson, P. A.; Rosinberg, M. L.; Sarkisov, L.; Tarjus, G. Capillary condensation in disordered porous materials: hysteresis versus equilibrium behavior. Phys. ReV. Lett. 2001, 87 (5), 055701. (9) Sarkisov, L.; Monson, P. A. Hysteresis in Monte Carlo and Molecular Dynamics Simulations of Adsorption in Porous Materials. Langmuir 2000, 16 (25), 9857-9860. (10) Woo, H. J.; Monson, P. A. Phase behavior and dynamics of fluids in mesoporous glasses. Phys. ReV. E 2003, 67 (4), 041207. (11) Sing, K. S. W.; Everett, D. H.; Haul, R. a. W.; Moscou, L.; Pierotti, R. a.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57 (4), 603-619. (12) Sing, K. S. W.; Rouqerol, F.; Rouqerol, J. Adsorption by Powders and Solids; Academic Press: London, 1999. (13) Bandosz, T. J.; Biggs, M. J.; Gubbins, K. E.; Hattori, Y.; Iiyama, T.; Kaneko, K.; Pikunic, J.; Thomson, K. T. In Chemistry and Physics of Carbon; Radovic, L., Ed.; Marcel Dekker: New York, 2003; Vol. 28, pp 41-228. (14) Ulberg, D. E.; Gubbins, K. E. Water adsorption in microporous graphitic carbons. Mol. Phys. 1995, 84 (6), 1139-1153. (15) Muller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. Adsorption of Water on Activated Carbons: A Molecular Simulation Study. J. Phys. Chem. 1996, 100 (4), 1189-1196. (16) Striolo, A.; Chialvo, A. A.; Cummings, P. T.; Gubbins, K. E. Water Adsorption in Carbon Slit Nanopores. Langmuir 2003, 19 (20), 85838591. (17) Striolo, A.; Gubbins, K. E.; Chialvo, A. A.; Cummings, P. T. Simulated Water Adsorption Isotherms in Carbon Nanopores. Mol. Phys. 2004, 102 (3), 243-251. (18) Jorge, M.; Seaton, N. A. Predicting Adsorption of Water/Organic Mixtures Using Molecular Simulation. AICHE J. 2003, 49 (8), 2059-2070. (19) Striolo, A.; Gubbins, K. E.; Chialvo, A. A.; Cummings, P. T. The effect of pore connectivity on water adsorption isotherms in non-activated graphitic nanopores. Adsorption 2005, 11, 337-341.

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ReceiVed for reView February 8, 2006 ReVised manuscript receiVed March 15, 2006 Accepted March 17, 2006 IE060162P

Monte Carlo Simulation Study of Water Adsorption in Activated Carbon

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