Poisson-Boltzmann Theory of Charged Colloids: Limits of the Cell Model for Salty Suspensions

Thermodynamic properties of charge-stabilised colloidal suspensions are commonly modeled by implementing the mean-field Poisson-Boltzmann (PB) theory within a cell model. This approach models a bulk system by a single macroion, together with counterions and salt ions, confined to a symmetrically shaped, electroneutral cell. While easing solution of the nonlinear PB equation, the cell model neglects microion-induced correlations between macroions, precluding modeling of macroion ordering phenomena. An alternative approach, avoiding artificial constraints of cell geometry, maps a macroion-microion mixture onto a one-component model of pseudo-macroions governed by effective interactions. In practice, effective-interaction models are usually based on linear screening approximations, which can accurately describe nonlinear screening only by incorporating an effective (renormalized) macroion charge. Combining charge renormalization and linearized PB theories, in both the cell model and an effective-interaction (cell-free) model, we compute osmotic pressures of highly charged colloids and monovalent microions over a range of concentrations. By comparing predictions with primitive model simulation data for salt-free suspensions, and with predictions of nonlinear PB theory for salty suspensions, we chart the limits of both the cell model and linear-screening approximations in modeling bulk thermodynamic properties. Up to moderately strong electrostatic couplings, the cell model proves accurate in predicting osmotic pressures of deionized suspensions. With increasing salt concentration, however, the relative contribution of macroion interactions grows, leading predictions of the cell and effective-interaction models to deviate. No evidence is found for a liquid-vapour phase instability driven by monovalent microions. These results may guide applications of PB theory to soft materials.Comment: 27 pages, 5 figures, special issue of Journal of Physics: Condensed Matter on "Classical density functional theory methods in soft and hard matter

Substituents modulate biphenyl penetration into lipid membranes

Electrochemical impedance techniques and fluorescence spectroscopic methods have been applied to the study of the interaction of ortho (o)-, meta (m)- and para (p)-Cl-, o-, m- and p-HO-, p-H3CO-, p-H3C-, p-NC- and p-O3− S- substituted biphenyls (BPs) with Hg supported dioleoyl phosphatidylcholine (DOPC) monolayers and DOPC vesicles. Non-planar o-substituted BPs exhibit the weakest interactions whereas planar p-substituted BPs interact to the greatest extent with the DOPC layers. The substituted BP/DOPC monolayer and bilayer interaction depends on the effect of the substituent on the aromatic electron density, which is related to the substituents’ mesomeric Hammetts constants. Substituted BPs with increased ring electron density do not increase the DOPC monolayer thickness on Hg and penetrate the DOPC vesicle membranes to the greatest extent. Substituted BPs with lower ring electron density can cause an increase in the monolayer’s thickness on Hg depending on their location and they remain in the interfacial and superficial layer of the free standing DOPC membranes. Quantum mechanical calculations correlate the binding energy between the substituted BP rings and methyl acetate, as a model for the –CH2-(CO)O-CH2- fragment of a DOPC molecule, with the location of BPs within the DOPC monolayer

Estimating thermodynamic expectations and free energies in expanded ensemble simulations systematic variance reduction through conditioning

International audienceMarkov chain Monte Carlo methods are primarily used for sampling from a given probability distribution and estimating multi-dimensional integrals based on the information contained in the generated samples. Whenever it is possible, more accurate estimates are obtained by combining Monte Carlo integration and integration by numerical quadrature along particular coordinates. We show that this variance reduction technique, referred to as conditioning in probability theory, can be advantageously implemented in -emph{expanded ensemble} simulations. These simulations aim at estimating thermodynamic expectations as a function of an external parameter that is sampled like an additional coordinate. Conditioning therein entails integrating along the external coordinate by numerical quadrature. We prove variance reduction with respect to alternative standard estimators and demonstrate the practical efficiency of the technique by estimating free energies and characterizing a structural phase transition between two solid phases

Challenges in molecular simulation of homogeneous ice nucleation

NoWe address the problem of recognition and growth of ice nuclei in simulation of supercooled bulk water. Bond orientation order parameters based on the spherical harmonics analysis are shown to be ineffective when applied to ice nucleation. Here we present an alternative method which robustly differentiates between hexagonal and cubic ice forms. The method is based on accumulation of the maximum projection of bond orientations onto a set of predetermined vectors, where different terms can contribute with opposite signs with the result that the irrelevant or incompatible molecular arrangements are damped out. We also introduce an effective cluster size by assigning a quality weight to each molecule in an ice-like cluster. We employ our cluster analysis in Monte Carlo simulation of homogeneous ice formation. Replica-exchange umbrella sampling is used for biasing the growth of the largest cluster and calculating the associated free energy barrier. Our results suggest that the ice formation can be seen as a two-stage process. Initially, short tetrahedrally arranged threads and rings are present; these become correlated and form a diffuse ice-genic network. Later, hydrogen bond arrangements within the amorphous ice-like structure gradually settle down and simultaneously `tune-up¿ nearby water molecules. As a result, a well-shaped ice core emerges and spreads throughout the system. The process is very slow and diverse owing to the rough energetic landscape and sluggish molecular motion in supercooled water, while large configurational fluctuations are needed for crystallization to occur. In the small systems studied so far the highly cooperative molecular rearrangements eventually lead to a relatively fast percolation of the forming ice structure through the periodic boundaries, which inevitably affects the simulation results.EPSR

Communication: Conformation state diagram of polypeptides: A chain length induced α - β transition

By using a generic coarse grained polypeptide model, we perform multicanonical molecular dynamics simulations for determining the equilibrium conformation state diagram of a single homopolypeptide chain as a function of the chain length and temperature. The state diagram highlights the thermal regimes of stability for various conformational patterns in polypeptides, including swollen, random and collapsed coils, globular structures, extended and bended helices, and compact bundles. Remarkably, at low temperatures we observe a sharp transition from extended helix to compact bundles as the chain length increases. This finding indicates that the chain length is one of the intrisic factors that can trigger - transformations in a broad class of polypeptides. © 2011 American Institute of Physics

Phase phenomena in supported lipid films under varying electric potential

We model cyclic voltammetry experiments on supported lipid films where a non-trivial dependence of the capacitance on the applied voltage is observed. Previously, based on a mean-field treatment of the Flory–Huggins type, under the assumption of strongly screened electrostatic interactions, it has been hypothesized that peaks in the capacitance-vs.-voltage profiles correspond to a sequence of structural or phase transitions within the adsorbed film. To examine this hypothesis, in this study we use both mean-field calculations and Monte Carlo simulations where the electrostatic effects due to the varying electric potential and the presence of salt are accounted for explicitly. Our main focus is on the structure of the film and the desorption–readsorption phenomena. These are found to be driven by a strong competition for the progressively charged-up (hydrophobic) surface between lipid hydrocarbon tails and the electrode counterions (cations). As the surface charge density is raised, the following phenomena within the interface are clearly observed: (i) a gradual displacement of the monolayer from the surface by the counterions, leading to complete monolayer desorption and formation of an electric double layer by the surface, (ii) a transformation of the monolayer into a bilayer upon its desorption, (iii) in the case of zwitterionic (or strongly polar) lipid head groups, the desorption is followed by the bilayer readsorption to the electrodevia interaction with the electric double layer and release of the excess counterions into the bulk solution. We argue then that the voltammetry peaks are associated with a stepwise process of formation of layers of alternating charge: electric double layer – upon film desorption, triple or multi-layer – upon film readsoption

Free energy calculations from adaptive molecular dynamics simulations with adiabatic reweighting

International audienceWe propose an efficient procedure for estimating free energies along an external parameter in adaptive Monte Carlo methods. Using Bayes formula, the conditional expectation used to adapt the biasing force given the external parameter is replaced by an average involving the conditional probability of the external parameter given the sampled configurations. We apply the procedure to a structural transition in a cluster and to the migration of a crystalline defect along a reaction coordinate. We observe that the proposed average speeds up the convergence of the adaptive biasing force towards the mean force along the external parameter, compared to simulations using the standard conditional expectation. The proposed procedure is also able to iteratively construct a biasing potential fully compensating the free energy along the reaction coordinate with two practical advantages over standard adaptive procedures: (i) the second derivatives of the reaction coordinate are not needed, (ii) no biasing potential is differentiated to generate the dynamics

Problems of Safety, Occupational Hygiene and Control Over Infections in Fighting with Occupational Diseases of Healthcare Workers with COVID-19 in Treatment Facilities of Ukraine

The objective of the publication was to assess the safety of treatment facilities, occupational health and infection control in Kiev, Zhytomyr and Zhytomyr region to enhance risk management of SARS-CoV-2 infection of healthcare workers and reduce occupational illness and mortality for COVID-19. Bibliosemantic, hygienic, questionnaire, statistical methods and methods of comparative and system analysis have been used. The work of doctors involved in overcoming COVID-19 pandemic is classified as dangerous (extreme). In addition to the SARS-CoV-2 virus, the working conditions of medical workers are influenced by physical, chemical factors and high physical and neuro-emotional stress. Among medical workers of the Zhytomyr region who were diagnosed with an acute occupational disease COVID-19, nurses prevailed (38.57%). Junior nurses (26.1%) – the second COVID-19 incidence, paramedics (5.31%) occupy the third place. The doctors' incidence was ranked in the following sequence: doctors of GPFM – 4.85%, surgeons – 4.16%, anesthesiologists – 2.54%, infectious disease doctors – 2.08%, radiologists – 1.85%. This distribution of medical professions is observed for all Ukraine regions. Chance of becoming infected with SARS CoV 2 for healthcare workers in October was by 3.8 times higher than the general population. Risk of dying from COVID-19 in healthcare workers is greater by 1.5 times than the general population. The high level of occupational morbidity of COVID-19 in Ukrainian medical personnel is determined by personal negligence, incomplete staffing of TF with medical workers, of medical workers with PPE; dis-use of PPE if available, absence or poor-quality instruction on labor protection; shortage of epidemiologists, hygienists and occupational pathologists