Monte carlo simulations of self-assembled surfactant aggregates

Monte Carlo simulations provide some insight into the self-assembly of amphiphiles in aqueous environment. A rather simple solvent-free model, with only two adjustable parameters in the effective pair potential, allows one to describe the formation of micelles, stable curved membranes, and metastable vesicles. Characteristic features of the self-assembled aggregates, such as the distribution of the micelle size and the value of the curvature elastic constant for membranes, can be obtained from simulated data. The capability of the simple approach was demonstrated for a surfactant model with three spherical segments. The extension of the simulation to molecules with more segments and branched amphiphiles is straightforward.     

Monte carlo simulation of many-arm star polymers

The statistics of many-arm star polymers in dilute solution is studied by means of the enrichment algorithm [K. Ohno and K. Binder, J. Stat. Phys. 64 , 781 (1991)] using a Monte Carlo simulation technique. In this study, typically 50,000 samples of star polymers with 12–32 arms each of which has 50 bonds are generated. The results for the configuration-number exponent γ(f) are γ(12) = -3.4 ± 0.3, γ(18) = -8.9 ± 0.2 and γ(32) = -29 ± 2.     

Monte carlo simulation of three-body ion-ion recombination

The extent to which the use of models in discussing spectroscopic measurements of molecular reorientation in isotropic fluids is necessary, and possibly misleading, is discussed by considering the basic conditions imposed by molecular symmetry on reorientation.     

Monte carlo data on ion-hydration shell characteristics

Ufa Aviation Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 5, pp. 160–162, September–October, 1988.     

水-甲醇体系的Monte Carlo分子膜拟

本文应用Monte Carlo分子膜拟方法对水、甲醇水-甲醇的1:1混合物、甲醇无限稀释时的水溶液和水无限稀释时的甲醇溶液等五个体系进行了研究。采用TIP分子位能函数, 得到了上述体系的热力学性质、原子径向分布函数、分子氢键配位数分布。并以Monte Carlo分子模拟获得的结构函数与X射线衍射实验结果进行了比较。     

Monte carlo simulation of self-assembled ordered hybrid materials

Lattice Monte Carlo simulations were performed to study the structure of hybrid organic-inorganic materials. Several cases were modeled where the composition corresponds to high surfactant concentration phases similar to that obtained from the synthesis of hybrid materials resulting from a phase separation. When using hybrid inorganic precursors, comparable to organosilicas, we observe that the organic segment is well mixed with the inorganic precursor and surfactant heads and no preferential location of the organic groups is observed. We show that the behavior of surfactant/hybrid precursor systems is analogous to those where co-surfactants or co-solvents are used, and that the lack of ordering in some cases can be explained by the change in solvent quality when using hybrid precursors. A comparison of structural characterization of the different phases using several tools, such as aggregate size distribution, density profiles, and pair radial distribution function is presented.     

Monte carlo simulation of carboxylic acid phase equilibria

Configurational-bias Monte Carlo simulations were carried out in the Gibbs ensemble to generate phase equilibrium data for several carboxylic acids. Pure component coexistence densities and saturated vapor pressures were determined for acetic acid, propanoic acid, 2-methylpropanoic acid, and pentanoic acid, and binary vapor-liquid equilibrium (VLE) data for the propanoic acid + pentanoic acid and 2-methylpropanoic acid + pentanoic acid systems. The TraPPE-UA force field was used, as it has recently been extended to include parameters for carboxylic acids. To simulate the branched compound 2-methylpropanoic acid, certain minor assumptions were necessary regarding angle and torsion terms involving the -CH- pseudo-atom, since parameters for these terms do not exist in the TraPPE-UA force field. The pure component data showed good agreement with the available experimental data, particularly with regard to the saturated liquid densities (mean absolute errors were less than 1.1%). On average, the predicted critical temperature and density were within 1% of the experimental values. All of the binary simulations showed good agreement with the experimental x-y data. However, the TraPPE-UA force field predicts saturated vapor pressures of pure components that are larger than the experimental values, and consequently the P-x-y and T-x-y data of the binary systems also deviate from the measured data.     

Monte carlo simulation of macromolecular ionization by nanoelectrospray

Electrospray ionization (ESI) is commonly used in macromolecular mass spectrometry, yet the dynamics of macromolecules in ESI droplets are not well understood. In this study, a Monte Carlo based model was developed, which can predict the efficiency of electrospray ionization for macromolecules, i.e., the number of macromolecular ions produced per macromolecules electrosprayed. The model takes into account ESI droplet evaporation, macromolecular diffusion within the droplet, droplet fissions, and the statistical nature of the ESI process. Two idealized representations of macromolecular analytes were developed, describing cluster prone, droplet surface inactive macromolecules and droplet surface active macromolecules, respectively. It was found that surface active macromolecules are preferentially ionized over surface inactive cluster prone macromolecules when the initial droplet size is large and the analyte concentration in solution is high. Simulations showed that ESI efficiency decreases with increasing initial droplet size and analyte molecular weight, and is influenced by analyte surface activity, the properties of the solvent, and the variance of the droplet size distribution. Model predictions are qualitatively supported by experimental measurements of macromolecular electrospray ionization made previously. Overall, this study demonstrates the potential capabilities of Monte Carlo based ESI models. Future developments in such models will allow for more accurate predictions of macromolecular ESI intensity.     

Monte carlo study of the dynamics of star-branched polymers

A uniform star-branched polymer model with f = 3 arms based on a simple cubic lattice was studied by means of the dynamic Monte Carlo method. The model chain is athermal with excluded-volume interactions and it is flexible. A new type of local micromodification was introduced to make the branching point movable. Static properties of the star polymer are in accordance with other theoretical predictions and experimental evidence. Scaling of the self diffusion constant and the terminal relaxation times is close to those of the Rouse theory and to simulation results of linear chains.     

Monte carlo simulation of polymers in steady potential flows

This work presents a study of the configurational properties of bead-spring chains in steady-state potential flows. These properties are obtained from off-lattice Monte Carlo simulations. The results of our simulations compare favourably with theoretical results for phantom chains. We investigate the effects of excluded volume and hydrodynamic interaction on the configurations of the chains, and compare our results to the predictions of scaling arguments. The scaling law observed here for the latter case deviates from theoretical predictions but is in agreement with findings reported from molecular dynamics simulations.     

Monte carlo simulation of ethane in dilute aqueous solution

A Monte Carlo simulation of a dilute aqueous solution of ethane both fixed and flexible conformation runs shows that energetically and entropically the eclipsed conformation of ethane is preferred to the staggered conformation in solution which is a reversal in the gas phase.     

流体混合物热力学性质的分子模拟研究-逐级取样法

本文对三个Lennard-Jonese二元混合物进行了逐级取样模拟, 获得各体系的超额Helmholtz自由能, 超额内能及超额压力等性质, 所得结果与Nakanishi通过伞形取样法的结果一致, 表明本文方法能够有效地对混合物各类超额性质进行研究。     

Monte carlo study of a simple model for micelle structure

A simple lattice model for a micelle is formulated and studied by Monte Carlo numerical methods. Results are presented for aggregates of 20, 30, and 50 chain molecules. The density profiles are much broader than is usually assumed in conventional pictures of micelle structure and the average shape found to be distinctly aspherical.     

Monte carlo calculations of second virial coefficients of chain molecules

Monte Carlo calculations have been performed for different types of chain molecules whose units interact through Lennard-Jones potentials. From the averaged Mayer function, we have evaluated the intermolecular two-body cluster integral, obtaining results for second virial coefficients. We have investigated the following points: a) the site modelization of alkanes by comparison of our results with gas phase data of different linear and branched alkanes and their mixtures. b) the prediction of interpenetration factors for flexible linear and star polymer chains in a good solvent (or excluded volume conditions). c) the determination of the theta point for a model of flexible polymer chains and the comparison of data for finite chains with theoretical predictions.     

Monte carlo simulations of polydisperse polymers grafted on spherical surfaces

This article presents effects of polydispersity in polymers grafted on spherical surfaces on grafted polymer chain conformations, grafted layer thickness, and free‐end monomer distribution within the grafted layer. At brush‐like grafting densities, as polydispersity index (PDI) increases, the scaling exponent of radius of gyration of grafted chains approaches that of a single chain grafted on the same nanoparticle, because polydispersity alleviates monomer crowding within the brush. At high PDI, the chains shorter than the number average chain length, N_n, have more compressed conformations, and the chains longer than N_n overall stretch less than in the monodisperse case. As seen in polydisperse flat brushes at high grafting densities, the grafted layer thickness on spherical nanoparticle increases with PDI. Polydispersity eliminates the region near the surface devoid of free‐end monomers seen in monodisperse cases, and it reduces the width of free‐end monomer distribution and shifts the free‐end monomer distribution close to the surface. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Monte carlo simulation of diffusion-limited drug release from finite fractal matrices

How fast can drug molecules escape from a controlled matrix-type release system? This important question is of both scientific and practical importance, as increasing emphasis is placed on design considerations that can be addressed only if the physical chemistry of drug release is better understood. In this work, this problem is studied via Monte Carlo computer simulations. The drug release is simulated as a diffusion-controlled process. Six types of Menger sponges (all having the same fractal dimension, d _f = 2.727, but with different values of random walk dimension, d _w ∈ [2.028, 2.998]) are employed as models of drug delivery devices with the aim of studying the consequences of matrix structural properties (characterized by d _f and d _w ) on drug release performance. The results obtained show that, in all cases, drug release from Menger sponges follows an anomalous behavior. Finally, the influence of the matrix structural properties on the drug release profile is quantified.     

A quantum monte carlo simulation of the two-dimensional H2 molecule

The problems connected with the simulation by the diffusion quantum Monte Carlo method of atomic and molecular systems in the two-dimensional (2D ) space have been investigated on the 1 ²S and 2 ²S states of the hydrogen atom and on the 1 ¹S and 1 ³S states of the helium atom, assuming r^?1 Coulomb interactions between the particles. The potential surface of the ¹Σ_g⁺ state of the 2D hydrogen molecule has been calculated in the range 0.1–1.5 bohr: A dissociation energy of 1.2703 Hartrees is found at the equilibrium distance of 0.3639 bohr. © 1994 John Wiley & Sons, Inc.     

Monte carlo method in calculation of the melting temperatures of silicate glasses

Numerical constants for the various oxides for calculating the melting temperature of silicate glasses are re-determined by using a simple Monte Carlo method. These constants are used to calculate the melting temperatures of 60 glass compositions; the deviations of the calculated values from the practical melting temperatures are small compared with those obtained with an earlier method.     

Monte carlo simulation and thermodynamic perturbation theory for mixtures of diatomic molecules

The thermodynamic properties and site—site distribution functions of mixtures of non-spherical molecules are obtained by Monte Carlo simulation. A non-spherical reference-system perturbation theory based on the RISM equation is developed to predict these results. The agreement between theory and simulation for the thermodynamic properties is encouraging. Important differences in the relative peak heights of the site—site distribution functions from theory and simulation are attributed to the role of attractive forces in determining local structure in the fluid mixtures, where the volumes of the components are similar but the well depths differ.