Search results for "dynamics"

Excess free energy, enthalpy and entropy of surfactant-surfactant mixed micelle formation

1996

Enthalpies of dilution and osmotic coefficients of sodium decylsulfate (NaDeS)-dodecyldimethylamine oxide (DDAO) mixtures in water were determined at 298 and 310 K, respectively. From the enthalpies of dilution, the apparent and then the partial molar relative enthalpies of the surfactant mixtures were calculated. From the osmotic coefficients, calculated at 298 K, the non-ideal free energies were derived. The latter were combined with the partial molar relative enthalpies to obtain the non-ideal entropies. From the apparent molar properties, using a previously reported approach, the excess thermodynamic properties for the surfactant-surfactant mixed micelle formation in water were evaluate…

Monte Carlo simulation of the glass transition in polymeric systems: Recent developments

1995

Abstract The bond fluctuation model on square and s.c. lattices is used as a coarse-grained model for flexible polymers in dense melts. Using an energy that favours long bonds, a conflict is created between the tendency of the bonds to stretch at low temperatures and packing constraints. This simple concept of ‘geometric frustration’ leads to glass transition. Both static and dynamic properties of this model are investigated by Monte Carlo simulations, paying attention to effects found by varying the cooling rate and the chain length N of the polymers. In two and three spatial dimensions an effective (cooling-rate dependent) glass transition temperature T g can be defined, where the system …

Molecular dynamics of supercooled polymer films

2000

We present results of molecular dynamics simulations for a supercooled polymer melt confined between two smooth and purely repulsive walls. The thickness D of the film is about 7 times the bulk radius of gyration. For all temperatures studied, a significant increase of the monomer and chain mobilities with respect to the bulk is observed. Preliminary results suggest that structural relaxation times exhibit a power-law behavior in the vicinity of a critical temperature T c (D) 0.39 (in Lennard-Jones units). This estimate of T c (D) is about 14% smaller than the corresponding bulk value. Despite this significant decrease the time dependence of various mean-square displacements seems to be una…

Determination of the adiabatic compressibility of bovine serum albumen in concentrated solution by a new ultrasonic method

2000

Abstract New data are presented for the adiabatic compressibility of bovine serum albumen (BSA) over the temperature range 2–85°C and at pH 5, 6, 7 and 8. The relative impact of intrinsic and hydration contributions to the observed compressibility for BSA is estimated and shown to agree very well with general values for globular protein and with previous results for BSA. The compressibility manifests a maximum value in the regions where BSA is most stable, exhibiting a parabolic dependence on temperature with lowest values where it is least stable. These data are obtained with a novel technique that employs a commercial, low cost and automated pulse echo apparatus for the measurement of the…

Understanding Noncovalent Interactions of Small Molecules with Carbon Nanotubes

2017

We combine experimental methods, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations in the quantitative analysis of noncovalent interactions between (6,5)-enriched single-walled carbon nanotubes (SWNTs), as hosts, and a set of pyrene derivatives with different electronic properties and surface areas, as guests. The experiments and calculations were carried out in two solvents with markedly different polarities, namely 1,1',2,2'-tetrachloroethane (TCE) and N,N-dimethylformamide (DMF). Our results show that dispersion forces govern the supramolecular association of small molecules with (6,5)-SWNTs, with negligible contributions from ground-state charge-trans…

Protein Flexibility and Preorganization in the Design of Enzymes. The Kemp Elimination Catalyzed by HG3.17

2015

A recently designed enzyme, HG3.17, obtained by directed evolution, has shown a catalytic activity close to natural enzymes. Hybrid QM/MM molecular dynamics simulations for the Kemp elimination in this new enzyme have provided a deep insight into the origin of its catalytic efficiency. In this case, we have first demonstrated the presence of different conformations with significantly different reactivity. The larger reactivity is related with a better electrostatic preorganization of the environment that creates a more favorable electrostatic potential for the reaction to proceed. In HG3.17, efforts to improve the catalytic properties must be focused in possible mutations increasing the pre…

THEORETICAL INSIGHTS ON O2 AND CO ADSORPTION ON NEUTRAL AND POSITIVELY CHARGED GOLD CLUSTERS

2006

With the aim of understanding the elementary steps governing the oxidation of CO catalyzed by dispersed or supported gold nanoclusters, the adsorption of molecular species, such as O2 and CO, on model neutral and positively charged clusters (Au(n)(m+) n = 1, 9, and 13; m = 0, 1, and 3) has been studied using an ab initio approach. The computed structural and thermodynamic data related to the binding process show that molecular oxygen interacts better with neutral clusters, acting as an electron acceptor, while CO more strongly binds to positively charged species, thus acting as an electron donor.

Thermodynamics of copolymer solutions: how the pair interactions contribute to the overall effect.

2014

Vapor pressure measurements were performed for solutions of poly(methyl methacrylate-ran-tert-butyl methacrylate) with different weight fractions of tert-butyl methacrylate units, and their parental homopolymers in chloroform at 323 K, over a large domain of concentrations. The Flory–Huggins interaction parameters obtained from these experimental investigations show complex dependences of the Flory–Huggins interaction parameter on concentration and copolymer composition. This behavior can be modeled by taking into account an approach which considers the ability of the polymers to rearrange in a response to changes in their molecular surroundings [Adv. Polym. Sci. 2011, 238, 1–66]. According…

Liquid/Gas and Liquid/Liquid Phase Equilibria of the System Water/Bovine Serum Albumin

2013

The thermodynamic behavior of the system H2O/BSA was studied at 25 °C within the entire composition range: vapor pressure measurements via head space sampling gas chromatography demonstrate that the attainment of equilibria takes more than one week. A miscibility gap was detected via turbidity and the coexisting phases were analyzed. At 6 °C the two phase region extends from ca. 34 to 40 wt % BSA; it shrinks upon heating. The polymer rich phase is locally ordered, as can be seen under the optical microscope using crossed polarizers. The Flory-Huggins theory turns out to be inappropriate for the modeling of experimental results. A phenomenological expression is employed which uses three adju…

1988

An apparatus for turbidimetric measurements of demixing temperatures under shear flow is presented, and the results of experiments with trans-decahydronaphthalene/polystyrene (TD/PS) solutions, investigating molecular weights ranging from 100 kg/mol to 1770 kg/mol, are compared with viscometric data. It is found that the sign and magnitude of shear effects depend on molecular weight (Mw), polymer concentration (c2), and shear rate (). For the first time, it was possible to study a solution which exhibits shear dissolution at low shear rates but shear demixing at high shear rates.