Accurate simulation estimates of phase behaviour in ternary mixtures with prescribed composition

This paper describes an isobaric semi-grand canonical ensemble Monte Carlo scheme for the accurate study of phase behaviour in ternary fluid mixtures under the experimentally relevant conditions of prescribed pressure, temperature and overall composition. It is shown how to tune the relative chemical potentials of the individual components to target some requisite overall composition and how, in regions of phase coexistence, to extract accurate estimates for the compositions and phase fractions of individual coexisting phases. The method is illustrated by tracking a path through the composition space of a model ternary Lennard-Jones mixture.Comment: 6 pages, 3 figure

Chain length dependence of the polymer-solvent critical point parameters

We report grand canonical Monte Carlo simulations of the critical point properties of homopolymers within the Bond Fluctuation model. By employing Configurational Bias Monte Carlo methods, chain lengths of up to N=60 monomers could be studied. For each chain length investigated, the critical point parameters were determined by matching the ordering operator distribution function to its universal fixed-point Ising form. Histogram reweighting methods were employed to increase the efficiency of this procedure. The results indicate that the scaling of the critical temperature with chain length is relatively well described by Flory theory, i.e. \Theta-T_c\sim N^{-0.5}. The critical volume fraction, on the other hand, was found to scale like \phi_c\sim N^{-0.37}, in clear disagreement with the Flory theory prediction \phi_c\sim N^{-0.5}, but in good agreement with experiment. Measurements of the chain length dependence of the end-to-end distance indicate that the chains are not collapsed at the critical point.Comment: 13 Pages Revtex, 9 epsf embedded figs. gzipped tar file. To appear in J. Chem. Phy

Absence of simulation evidence for critical depletion in slit-pores

Recent Monte Carlo simulation studies of a Lennard-Jones fluid confined to a mesoscopic slit-pore have reported evidence for ``critical depletion'' in the pore local number density near the liquid-vapour critical point. In this note we demonstrate that the observed depletion effect is in fact a simulation artifact arising from small systematic errors associated with the use of long range corrections for the potential truncation. Owing to the large near-critical compressibility, these errors lead to significant changes in the pore local number density. We suggest ways of avoiding similar problems in future studies of confined fluids.Comment: 4 pages Revtex. Submitted to J. Chem. Phy

Wetting of a symmetrical binary fluid mixture on a wall

We study the wetting behaviour of a symmetrical binary fluid below the demixing temperature at a non-selective attractive wall. Although it demixes in the bulk, a sufficiently thin liquid film remains mixed. On approaching liquid/vapour coexistence, however, the thickness of the liquid film increases and it may demix and then wet the substrate. We show that the wetting properties are determined by an interplay of the two length scales related to the density and the composition fluctuations. The problem is analysed within the framework of a generic two component Ginzburg-Landau functional (appropriate for systems with short-ranged interactions). This functional is minimized both numerically and analytically within a piecewise parabolic potential approximation. A number of novel surface transitions are found, including first order demixing and prewetting, continuous demixing, a tricritical point connecting the two regimes, or a critical end point beyond which the prewetting line separates a strongly and a weakly demixed film. Our results are supported by detailed Monte Carlo simulations of a symmetrical binary Lennard-Jones fluid at an attractive wall.Comment: submitted to Phys. Rev.

Coexistence Curve Singularities at Critical End Points

We report an extensive Monte Carlo study of critical end point behaviour in a symmetrical binary fluid mixture. On the basis of general scaling arguments, singular behaviour is predicted in the diameter of the liquid-gas coexistence curve as the critical end point is approached. The simulation results show clear evidence for this singularity, as well as confirming a previously predicted singularity in the coexistence chemical potential. Both singularities should be detectable experimentally.Comment: 9 pages Revtex, 3 figures. To appear in Phys. Rev. Let

Interfacial tension of the isotropic--nematic interface in suspensions of soft spherocylinders

The isotropic to nematic transition in a system of soft spherocylinders is studied by means of grand canonical Monte Carlo simulations. The probability distribution of the particle density is used to determine the coexistence density of the isotropic and the nematic phases. The distributions are also used to compute the interfacial tension of the isotropic--nematic interface, including an analysis of finite size effects. Our results confirm that the Onsager limit is not recovered until for very large elongation, exceeding at least L/D=40, with L the spherocylinder length and D the diameter. For smaller elongation, we find that the interfacial tension increases with increasing L/D, in agreement with theoretical predictions.Comment: 8 pages, 7 figures, and also 1 tabl

Phase behaviour of a symmetrical binary fluid mixture

We have investigated the phase behaviour of a symmetrical binary fluid mixture for the situation where the chemical potentials $\mu_1$ and $\mu_2$ of the two species differ. Attention is focused on the set of interparticle interaction strengths for which, when $\mu_1=\mu_2$, the phase diagram exhibits both a liquid-vapor critical point and a tricritical point. The corresponding phase behaviour for the case $\mu_1\ne\mu_2$ is investigated via integral-equation theory calculations within the mean spherical approximation (MSA), and grand canonical Monte Carlo (GCMC) simulations. We find that two possible subtypes of phase behaviour can occur, these being distinguished by the relationship between the critical lines in the full phase diagram in the space of temperature, density, and concentration. We present the detailed form of the phase diagram for both subtypes and compare with the results from GCMC simulations, finding good overall agreement. The scenario via which one subtype evolves into the other, is also studied, revealing interesting features.Comment: 22 pages, 13 figure

Formation of a cultivated spodosol in east-central Finland

The processes involved in Spodosol (Podzol) formation are still being debated. The pedogenic processes in a Spodosol, 10,700 years-old, at Sotkamo that has been cultivated for about 50 years were studied by characterizing the morphology and analyzing the major chemical properties, texture and mineralogy. Before cultivation, organic acids produced by decomposition of organic matter from pine litter in O and A horizons had weathered primary minerals in A and E horizons releasing Al and Fe. Percolating waters moved the organo-metallic complexes from A and E horizons to Bhsm and Bs horizons where the complexes coated and bridged sand grains eventually forming cemented ortstein. Because of the high biotite content of the parent material, the index of accumulation of Fe and Al in the Bhsm horizon (Al + 0.5 Fe = 4.1%) was the highest reported in Spodosols of Finland. The data support the theory of downward movement of Al and Fe as organo-metallic complexes with formation of some ferrihydrite but little or no formation of imogolite type materials. Little, if any, podzolization has likely occurred since the initiation of cultivation because, after agricultural liming and consequent increase of pH in the Ap horizon, organic compounds are likely to chelate Ca and Mg rather than Al and Fe.;Karkeille hietamaille syntyneet podsolit ovat Suomen kehittyneimpiä maannoksia. Niitä tutkimalla saadaan uutta tietoa tämän koko pohjoisella havumetsävyöhykkeelläyleisen maannostyypin kehittymiseen johtaneista prosesseista, joista edelleenkin vallitsee erilaisia käsityksiä. Tämän tutkimuksen kohteena oli Sotkamossa karkealla hietamaalla oleva noin 50 vuotta viljelty maa, joka on ollut kuivillaan noin 10 700 vuotta. Muokkauskerroksen alapuolella oli huuhtoutumiskerros (valkomaa), joka sisälsi lähes pelkkää kvartsihiekkaa. Sen alapuolella oli noin 10 cm paksu rautapalsi eli iskostunut horisontti, johon ylempää orgaanisina kompleksiyhdisteinä huuhtoutuneet rauta ja alumiini ovat saostuneet. Mikroskoopilla voidaan nähdä, miten nämä saostuneet ainesosat peittävät kvartsihiekan jyväset ja sitovat ne yhteen. Tässä horisontissa oli erittäin runsaasti heikosti kiteytynyttä rautaoksidia, joka on uutettavissa ammoniumoksalaattiliuoksella, kun taas valkomaassa tällaista rautaa oli erittäin vähän. Rikastumiskerroksen alumiinista valtaosa oli pyrofosfaattiin uuttuvassa, oletettavasti orgaanisen aineksen sitomassa muodossa, mikä viittaa aineiden kulkeutuneen tähän horisonttiin nimenomaan kelaatteina eikä epäorgaanisina kolloideina. Syvä kyntö on nostanut valkomaata ja kappaleita rikastumiskerroksen iskostumasta myös muokkauskerrokseen. Rikastumiskerroksen alapuolella kvartsihiekkajyvästen pinnoilla ei ollut paljonkaan rautasaostumia, mutta mikroskoopilla näkyi runsaasti rapautumatonta biotiittia. Rikastumiskerroksen rauta lienee suureksi osaksi peräisin juuri biotiitista, joka on kokonaisuudessaan rapautunut pintamaasta. Vähemmän biotiittia sisältäviin maihin ei todennäköisesti kehity näin vahvaa rikastumiskerrosta maan pienemmän rautapitoisuuden takia. Podsoloituminen on luultavasti pysähtynyt sen jälkeen, kun maa on otettu viljelyyn ja sen pintaosien pH on kalkituksen seurauksena noussut

Free energies of crystalline solids: a lattice-switch Monte Carlo method

We present a method for the direct evaluation of the difference between the free energies of two crystalline structures, of different symmetry. The method rests on a Monte Carlo procedure which allows one to sample along a path, through atomic-displacement-space, leading from one structure to the other by way of an intervening transformation that switches one set of lattice vectors for another. The configurations of both structures can thus be sampled within a single Monte Carlo process, and the difference between their free energies evaluated directly from the ratio of the measured probabilities of each. The method is used to determine the difference between the free energies of the fcc and hcp crystalline phases of a system of hard spheres.Comment: 5 pages Revtex, 3 figure

Modelling colloids with Baxter's adhesive hard sphere model

The structure of the Baxter adhesive hard sphere fluid is examined using computer simulation. The radial distribution function (which exhibits unusual discontinuities due to the particle adhesion) and static structure factor are calculated with high accuracy over a range of conditions and compared with the predictions of Percus--Yevick theory. We comment on rigidity in percolating clusters and discuss the role of the model in the context of experiments on colloidal systems with short-range attractive forces.Comment: 14 pages, 7 figures. (For proceedings of "Structural arrest in colloidal systems with short-range attractive forces", Messina, December 2003