Peter Monson - Academia.edu (original) (raw)
Papers by Peter Monson
Langmuir : the ACS journal of surfaces and colloids, Jan 14, 2015
The reaction ensemble Monte Carlo method was used to model the self-assembly and structure of sil... more The reaction ensemble Monte Carlo method was used to model the self-assembly and structure of silica nanoparticles found in the initial stages of the clear-solution synthesis of silicalite-1 zeolite. Such nanoparticles, which comprise both silica and organic structure directing agents (OSDAs), are believed to play a crucial role in the formation of silica nanoporous materials, yet very limited atomic-level structural information is available for these nanoparticles. We have modeled silica monomers as flexible tetrahedra with spring constants fitted in previous work to silica bulk moduli, and OSDAs as spheres attracted to anionic silica monomers. We have studied one-step and two-step formation mechanisms, the latter involving initial association of silica species and OSDAs driven by physical solution forces, followed by silica condensation/hydrolysis reactions simulated with reaction ensemble Monte Carlo. The two-step process with pre-association was found to be crucial for generatin...
Langmuir : the ACS journal of surfaces and colloids, Jan 13, 2015
We apply dynamic mean field theory to study relaxation dynamics for lattice models of fluids conf... more We apply dynamic mean field theory to study relaxation dynamics for lattice models of fluids confined in linear pores with side streams and with ink bottle structures. Our results show several mechanisms for how the pore structure affects the dynamics, and these are amplified in longer pores. An important conclusion of this work is that features such as side streams and ink bottle segments can substantially slow the equilibration of fluids confined in long pore systems where the pore lengths can be more than 100 micrometers, such as in porous silicon. This may make it difficult to properly equilibrate these systems for states close to those where the pores should be completely filled with liquids. The presence of trapped bubbles in the system may change the desorption characteristics of the system and the shape of the hysteresis loops.
Langmuir : the ACS journal of surfaces and colloids, Jan 29, 2005
Grand canonical Monte Carlo simulations using both Glauber dynamics and Kawasaki dynamics have be... more Grand canonical Monte Carlo simulations using both Glauber dynamics and Kawasaki dynamics have been carried out for a recently developed lattice model of a nonwetting fluid confined in a porous material. The calculations are aimed at investigating the molecular scale mechanisms leading to mercury retention encountered during mercury porosimetry experiments. We first describe a set of simulations on slit and ink-bottle pores. We have studied the influence of the pore width parameter on the intrusion/extrusion curve shapes and investigated the corresponding mechanisms. Entrapment appears during Kawasaki dynamics simulations of extrusion performed on ink-bottle pores when the system is studied for short relaxation times. We then consider the more realistic and complex case of a Vycor glass building on recent work on the dynamics of adsorption of wetting fluids (Woo, H. J.; Monson, P. A. Phys. Rev. E 2003, 67, 041207). Our results suggest that mercury entrapment is caused by a decrease ...
Langmuir : the ACS journal of surfaces and colloids, Jan 20, 2004
We consider mercury porosimetry from the perspective of the statistical thermodynamics of penetra... more We consider mercury porosimetry from the perspective of the statistical thermodynamics of penetration of a nonwetting liquid into a porous material under an external pressure. We apply density functional theory to a lattice gas model of the system and use this to compute intrusion/extrusion curves. We focus on the specific example of a Vycor glass and show that essential features of mercury porosimetry experiments can be modeled in this way. The lattice model exhibits a symmetry that provides a direct relationship between intrusion/extrusion curves for a nonwetting fluid and adsorption/desorption isotherms for a wetting fluid. This relationship clarifies the status of methods that are used for transforming mercury intrusion/extrusion curves into gas adsorption/desorption isotherms. We also use Monte Carlo simulations to investigate the nature of the intrusion and extrusion processes.
International Reviews in Physical Chemistry, 2014
ABSTRACT We present a perspective on the molecular modelling of nanoporous silica material synthe... more ABSTRACT We present a perspective on the molecular modelling of nanoporous silica material synthesis. We focus on two classes of materials: microporous zeolite materials in their all-silica forms, and ordered mesoporous silica materials. Several approaches have provided insight into the synthesis processes. These approaches range from quantum chemistry modelling of silica polymerisation to molecular simulations of ordered mesoporous silica assembly, and consider physical and chemical phenomena over several lengths and time scales. Our article focuses on models of porous silica material formation based on the assembly of corner-sharing tetrahedra, which we illustrate with applications to silica polymerisation, the formation of microporous crystals and the formation of ordered mesoporous materials. This is a research area where theoretical developments must closely align with experimentation. For this reason, we also devote a significant component of the present review to a survey of key developments in the experimental synthesis and characterisation of these materials. In particular, recent experiments have bracketed length scales of zeolite nuclei in the 5–10 nm range. On the other hand, recent molecular modelling work has accomplished the in silico self-assembly of both zeolitic and mesoporous materials within a unified modelling format. Our article serves to demonstrate the substantial progress that has been made in this field, while highlighting the enormous challenges and opportunities for future progress, such as in understanding the interplay of thermodynamics and kinetics in silica nanopore formation.
We present a lattice model describing the formation of silica nanoparticles in the early stages o... more We present a lattice model describing the formation of silica nanoparticles in the early stages of the clear-solution templated synthesis of silicalite-1 zeolite. Silica condensation/hydrolysis is modeled by a nearest-neighbor attraction, while the electrostatics are represented by an orientation-dependent, short-range interaction. Using this simplified model, we show excellent qualitative agreement with published experimental observations. The nanoparticles are identified as a metastable state, stabilized by electrostatic interactions between the negatively charged silica surface and a layer of organic cations. Nanoparticle size is controlled mainly by the solution pH, through nanoparticle surface charge. The size and concentration of the charge-balancing cation are found to have a negligible effect on nanoparticle size. Increasing the temperature allows for further particle growth by Ostwald ripening. We suggest that this mechanism may play a role in the growth of zeolite crystals.
We present Monte Carlo simulation studies of coarse-grained models of some recently developed com... more We present Monte Carlo simulation studies of coarse-grained models of some recently developed comb polymers for anhydrous proton transport applications. Our models of the comb polymers incorporate the chain architecture but require only a single χ-parameter for segment-segment interactions. We have studied these models with Monte Carlo simulations using the single chain in mean field (SCMF) method developed by Muller, de Pablo, and co-workers. In these simulations we determine the mesoscale structure and relate it to the polymer architecture in the models. The calculations reveal spontaneous selfassembly of the model comb polymers into mesoscale structures with lamellar or cylindrical symmetries that are similar to those seen in the experiments. The results are also consistent with the picture of the conducting groups concentrating within one of the mesophases, proposed on the basis of the experimental studies. Our calculations also suggest an alternative explanation for the disordered morphologies found in experiments for some of the polymers in terms of the effect of chain branching upon the location of the order-disorder transition.
We present Monte Carlo simulations of a lattice model describing silica polymerization with an em... more We present Monte Carlo simulations of a lattice model describing silica polymerization with an emphasis on the transition between gel states and nanoparticle states as the pH and silica concentration are varied. The pH in the system is controlled by the addition of a structure-directing agent (SDA) of the type SDA(+)(OH(-)). The silica units are represented by corner-sharing tetrahedra on a body-centered cubic lattice and the SDA(+) species by single sites with near-neighbor repulsions. We focus on two systems: one with a low silica concentration with composition comparable to that of the clear solution silicalite-1 zeolite synthesis and a high silica concentration system that leads to gel states. In the dilute system, clusters have a core-shell structure, with the core predominantly comprised of silica with some SDA(+) cations, surrounded by a shell of only SDA(+) cations. Moreover, the average cluster size gradually decreases from 2 to 1.6 nm with increasing pH. The concentrated system forms a gel that remains stable to increasing pH up to about 9.2. At pH values in the range of 9.2-10, the gel transforms to nanoparticles of size around 1.0 nm, surprisingly smaller than those in the dilute system. We also study the evolution of the Q(n) distribution (a measure of the silica network structure) for both systems and obtain good agreement with (29)Si NMR data available for the concentrated system.
Langmuir : the ACS journal of surfaces and colloids, Jan 14, 2015
The reaction ensemble Monte Carlo method was used to model the self-assembly and structure of sil... more The reaction ensemble Monte Carlo method was used to model the self-assembly and structure of silica nanoparticles found in the initial stages of the clear-solution synthesis of silicalite-1 zeolite. Such nanoparticles, which comprise both silica and organic structure directing agents (OSDAs), are believed to play a crucial role in the formation of silica nanoporous materials, yet very limited atomic-level structural information is available for these nanoparticles. We have modeled silica monomers as flexible tetrahedra with spring constants fitted in previous work to silica bulk moduli, and OSDAs as spheres attracted to anionic silica monomers. We have studied one-step and two-step formation mechanisms, the latter involving initial association of silica species and OSDAs driven by physical solution forces, followed by silica condensation/hydrolysis reactions simulated with reaction ensemble Monte Carlo. The two-step process with pre-association was found to be crucial for generatin...
Langmuir : the ACS journal of surfaces and colloids, Jan 13, 2015
We apply dynamic mean field theory to study relaxation dynamics for lattice models of fluids conf... more We apply dynamic mean field theory to study relaxation dynamics for lattice models of fluids confined in linear pores with side streams and with ink bottle structures. Our results show several mechanisms for how the pore structure affects the dynamics, and these are amplified in longer pores. An important conclusion of this work is that features such as side streams and ink bottle segments can substantially slow the equilibration of fluids confined in long pore systems where the pore lengths can be more than 100 micrometers, such as in porous silicon. This may make it difficult to properly equilibrate these systems for states close to those where the pores should be completely filled with liquids. The presence of trapped bubbles in the system may change the desorption characteristics of the system and the shape of the hysteresis loops.
Langmuir : the ACS journal of surfaces and colloids, Jan 29, 2005
Grand canonical Monte Carlo simulations using both Glauber dynamics and Kawasaki dynamics have be... more Grand canonical Monte Carlo simulations using both Glauber dynamics and Kawasaki dynamics have been carried out for a recently developed lattice model of a nonwetting fluid confined in a porous material. The calculations are aimed at investigating the molecular scale mechanisms leading to mercury retention encountered during mercury porosimetry experiments. We first describe a set of simulations on slit and ink-bottle pores. We have studied the influence of the pore width parameter on the intrusion/extrusion curve shapes and investigated the corresponding mechanisms. Entrapment appears during Kawasaki dynamics simulations of extrusion performed on ink-bottle pores when the system is studied for short relaxation times. We then consider the more realistic and complex case of a Vycor glass building on recent work on the dynamics of adsorption of wetting fluids (Woo, H. J.; Monson, P. A. Phys. Rev. E 2003, 67, 041207). Our results suggest that mercury entrapment is caused by a decrease ...
Langmuir : the ACS journal of surfaces and colloids, Jan 20, 2004
We consider mercury porosimetry from the perspective of the statistical thermodynamics of penetra... more We consider mercury porosimetry from the perspective of the statistical thermodynamics of penetration of a nonwetting liquid into a porous material under an external pressure. We apply density functional theory to a lattice gas model of the system and use this to compute intrusion/extrusion curves. We focus on the specific example of a Vycor glass and show that essential features of mercury porosimetry experiments can be modeled in this way. The lattice model exhibits a symmetry that provides a direct relationship between intrusion/extrusion curves for a nonwetting fluid and adsorption/desorption isotherms for a wetting fluid. This relationship clarifies the status of methods that are used for transforming mercury intrusion/extrusion curves into gas adsorption/desorption isotherms. We also use Monte Carlo simulations to investigate the nature of the intrusion and extrusion processes.
International Reviews in Physical Chemistry, 2014
ABSTRACT We present a perspective on the molecular modelling of nanoporous silica material synthe... more ABSTRACT We present a perspective on the molecular modelling of nanoporous silica material synthesis. We focus on two classes of materials: microporous zeolite materials in their all-silica forms, and ordered mesoporous silica materials. Several approaches have provided insight into the synthesis processes. These approaches range from quantum chemistry modelling of silica polymerisation to molecular simulations of ordered mesoporous silica assembly, and consider physical and chemical phenomena over several lengths and time scales. Our article focuses on models of porous silica material formation based on the assembly of corner-sharing tetrahedra, which we illustrate with applications to silica polymerisation, the formation of microporous crystals and the formation of ordered mesoporous materials. This is a research area where theoretical developments must closely align with experimentation. For this reason, we also devote a significant component of the present review to a survey of key developments in the experimental synthesis and characterisation of these materials. In particular, recent experiments have bracketed length scales of zeolite nuclei in the 5–10 nm range. On the other hand, recent molecular modelling work has accomplished the in silico self-assembly of both zeolitic and mesoporous materials within a unified modelling format. Our article serves to demonstrate the substantial progress that has been made in this field, while highlighting the enormous challenges and opportunities for future progress, such as in understanding the interplay of thermodynamics and kinetics in silica nanopore formation.
We present a lattice model describing the formation of silica nanoparticles in the early stages o... more We present a lattice model describing the formation of silica nanoparticles in the early stages of the clear-solution templated synthesis of silicalite-1 zeolite. Silica condensation/hydrolysis is modeled by a nearest-neighbor attraction, while the electrostatics are represented by an orientation-dependent, short-range interaction. Using this simplified model, we show excellent qualitative agreement with published experimental observations. The nanoparticles are identified as a metastable state, stabilized by electrostatic interactions between the negatively charged silica surface and a layer of organic cations. Nanoparticle size is controlled mainly by the solution pH, through nanoparticle surface charge. The size and concentration of the charge-balancing cation are found to have a negligible effect on nanoparticle size. Increasing the temperature allows for further particle growth by Ostwald ripening. We suggest that this mechanism may play a role in the growth of zeolite crystals.
We present Monte Carlo simulation studies of coarse-grained models of some recently developed com... more We present Monte Carlo simulation studies of coarse-grained models of some recently developed comb polymers for anhydrous proton transport applications. Our models of the comb polymers incorporate the chain architecture but require only a single χ-parameter for segment-segment interactions. We have studied these models with Monte Carlo simulations using the single chain in mean field (SCMF) method developed by Muller, de Pablo, and co-workers. In these simulations we determine the mesoscale structure and relate it to the polymer architecture in the models. The calculations reveal spontaneous selfassembly of the model comb polymers into mesoscale structures with lamellar or cylindrical symmetries that are similar to those seen in the experiments. The results are also consistent with the picture of the conducting groups concentrating within one of the mesophases, proposed on the basis of the experimental studies. Our calculations also suggest an alternative explanation for the disordered morphologies found in experiments for some of the polymers in terms of the effect of chain branching upon the location of the order-disorder transition.
We present Monte Carlo simulations of a lattice model describing silica polymerization with an em... more We present Monte Carlo simulations of a lattice model describing silica polymerization with an emphasis on the transition between gel states and nanoparticle states as the pH and silica concentration are varied. The pH in the system is controlled by the addition of a structure-directing agent (SDA) of the type SDA(+)(OH(-)). The silica units are represented by corner-sharing tetrahedra on a body-centered cubic lattice and the SDA(+) species by single sites with near-neighbor repulsions. We focus on two systems: one with a low silica concentration with composition comparable to that of the clear solution silicalite-1 zeolite synthesis and a high silica concentration system that leads to gel states. In the dilute system, clusters have a core-shell structure, with the core predominantly comprised of silica with some SDA(+) cations, surrounded by a shell of only SDA(+) cations. Moreover, the average cluster size gradually decreases from 2 to 1.6 nm with increasing pH. The concentrated system forms a gel that remains stable to increasing pH up to about 9.2. At pH values in the range of 9.2-10, the gel transforms to nanoparticles of size around 1.0 nm, surprisingly smaller than those in the dilute system. We also study the evolution of the Q(n) distribution (a measure of the silica network structure) for both systems and obtain good agreement with (29)Si NMR data available for the concentrated system.