Solvent Self-diffusion Research Articles

Effect of ethanol addition upon the structure and the cooperativity of the water H bond network

Density and water self-diffusion in ethanol-water mixtures are measured as a function of the temperature and for several alcohol concentrations in the water rich region. An extension of the five species model for liquid water, in which H bond cooperativity is taken into account, is proposed to interpret the temperature and concentration trends of the mixture density, solvent self-diffusion and near infrared spectra. This approach allowed us to extract some significant parameters characteristic of the solvent water H bond network such as the probability of forming H bonds, the degree of cooperativity, the supercooling and superheating limit temperatures and the distribution of water molecules among the five species. The emerging picture suggests that alcohol acts upon the water H bond network as a structure maker in the normal temperature region and, probably, as a structure breaker in the supercooled one. Moreover, a progressive disappearance of the H bond network cooperativity is observed as the alcohol concentration is increased.

A friction factor analysis of the coupling between polymer/solvent self- and mutual-diffusion: polystyrene/toluene

The Bearman statistical mechanical theory, which couples the mutual-diffusion and self-diffusion coefficients via friction factors, has been applied to polystyrene/toluene solutions with polystyrene molecular weights of 18 kDa and 900 kDa. Toluene and polystyrene self-diffusion coefficients, obtained from the literature and measured here, along with polystyrene/toluene binary mutual-diffusion coefficients and thermodynamic data, were employed to independently calculate the three friction coefficients (ζ12, ζ11, and ζ22) required to describe transport within binary solutions. Results reveal that the frequently used geometric mean approximation (GMA) for relating the friction coefficients consistently underestimates ζ22 and worsens with increasing polymer molecular weight. The GMA is found to be appreciably more accurate than the arithmetic mean approximation. A formalism recently proposed by Vrentas et al. has also been evaluated for this system and is found to describe the concentration dependence of ζ11 very well. The Vrentas model, therefore, can accurately predict mutual-diffusion coefficients from solvent self-diffusion coefficients for polystyrene/toluene solutions. © 1996 John Wiley & Sons, Inc.

Predicting ability of free-volume theory for solvent self-diffusion coefficients in rubbers

The effect of solvent size on the diffusion process is studied for various solvents with natural rubber and polybutadiene in terms of the free-volume theory. The importance of energy effects on the diffusion of penetrants in rubbers is examined. The critical molar volume of the polymer jumping unit is correlated with its glass transition temperature over the range 172 K to 305 K. The correlation shows a linear relationship between these two properties and can be used to predict one of the most sensitive free-volume parameters. Using this parameter in conjunction with the Vrentas-Duda free-volume theory, solvent self-diffusion coefficients in rubbers are then predicted over wide ranges of concentration and temperature. For all the systems, the predictions are comparable with experimental data. © 1996 John Wiley & Sons, Inc.

Predicting ability of free‐volume theory for solvent self‐diffusion coefficients in rubbers

The effect of solvent size on the diffusion process is studied for various solvents with natural rubber and polybutadiene in terms of the free-volume theory. The importance of energy effects on the diffusion of penetrants in rubbers is examined. The critical molar volume of the polymer jumping unit is correlated with its glass transition temperature over the range 172 K to 305 K. The correlation shows a linear relationship between these two properties and can be used to predict one of the most sensitive free-volume parameters. Using this parameter in conjunction with the Vrentas-Duda free-volume theory, solvent self-diffusion coefficients in rubbers are then predicted over wide ranges of concentration and temperature. For all the systems, the predictions are comparable with experimental data. © 1996 John Wiley & Sons, Inc.

Normal and Reverse Vesicles with Nonionic Surfactant: Solvent Diffusion and Permeability

Normal and reverse vesicle systems with nonionic surfactants were investigated. The solvent self-diffusion coefficient was measured using the 1H-NMR Fourier transform pulsed gradient spin-echo technique. In the case of normal vesicles, the water solvent is found to exchange rapidly between the inside and outside of the vesicles on the experimental time scale (≈0.1 s). Here, only an average self-diffusion coefficient can be measured from which the fraction of entrapped water can be determined. In the reverse vesicle case, we observe either a fast or a slow exchange, depending, on the oil and the bilayer composition. In particular we have investigated a semipermeable membrane system, where with a solvent mixture, one of the two types of solvent molecules exchanges fast while the other exchanges slowly on the experimental time scale. Here, the lifetime of a solvent molecule inside the reverse vesicles was found to depend on the composition of the mixed reverse bilayers, leading to an observed transition from fast to slow exchange conditions when varying the bilayer composition. In the slow exchange case, the self-diffusion coefficients of solvent molecules on the outside and inside of the vesicles, where the latter reports on the vesicle self-diffusion coefficient, are in principal resolved. From the bimodal type of decay of the spin-echo amplitude, it is also possible to determine directly the fraction of solvent molecules entrapped inside the vesicles.

Experimental and Theoretical Analysis of Photoinduced Electron Transfer: Including the Role of Liquid Structure

Experimental determinations of the dynamics of photoinduced electron transfer from rubrene to duroquinone in three solvents, dibutyl phthalate, diethyl sebacate, and cyclohexanone are presented. Measurements of the donor (rubrene) fluorescence decays were made with time-correlated single-photon counting. The data are analyzed using recent theoretical developments that include important features of the solvent, i.e., the effects of finite molecular volume on local solvent structure and on the mutual donor−acceptor diffusion rates. Inclusion of the liquid radial distribution function (rdf) in the theory accounts for the significant variation of the acceptor concentration near a donor. Because the concentration of acceptors near a donor is substantially greater than the average concentration used in a featureless continuum liquid model, incorporating the rdf is necessary to properly analyze experimental data. Hydrodynamic effects, which slow the rate of donor−acceptor approach at short distance, are important and are also included in the theoretical analysis of the data. The data analysis depends on a reasonable model of the rdf. A hard-sphere liquid rdf is shown to be sufficiently accurate by comparing model electron-transfer calculations using a hard-sphere rdf and an rdf from neutron-scattering experiments reported in the literature. A method is presented to obtain the hard-sphere parameters needed to calculate the rdf. The method uses a self-consistent determination of the hard-sphere radius and diffusion constant and the solvent self-diffusion constant calculated from the Spernol and Wirtz equation. The Marcus form of the distance-dependent transfer rate is used. For the highest viscosity solvent (dibutyl phthalate), a unique set of the Marcus transfer parameters is obtained. For lower viscosity solvents, the transfer parameters are less well defined, but information on the distance and time dependence of charge separation is still acquired. These experiments, combined with the theoretical analysis, yield the first realistic description of through-solvent photoinduced electron transfer.

Effect of Solvent Size on Solvent Self-Diffusion in Polymer−Solvent Systems

The free-volume theory for solvent self-diffusion coefficients in polymer−solvent systems is modified to include a more general analysis of the effects of solvent size. The predictions of the theory are compared with self-diffusion data for a diverse set of solvents and four polymers, polystyrene, poly(vinyl acetate), poly(methyl methacrylate), and poly(p-methylstyrene). The new prediction of size effects is based on the consideration of the asymmetry of solvent molecules.

PGSE-NMR studies of solvent diffusion in poly(N-isopropylacrylamide) colloidal microgels

The solvent self-diffusion coefficient has been studied in thermoshrinking poly(N-isopropyl acrylamide) microgel dispersions by the pulsed-gradient spin-echo PGSE-NMR technique, as a function of temperature and mass fraction. After suitable corrections for the temperature, the H2O/D2O ratio and the relative volume fractions, all the self-diffusion data obtained over a temperature range of approximately 40 °C and mass fraction (2–12 % wt/wt) could be superimposed with the volume fraction as the universal factor. The observed reduction in the solvent self-diffusion coefficient with volume fraction was greater than that predicted by simple obstruction theory. After correction for-, and the subsequent removal of the obstruction effect, the diffusion of the solvent through the core of the particle is elucidated. As found for other polymer-solvent systems, there were no specific binding effects. The diffusion of the solvent in these dispersions over such temperature and mass fraction ranges could be rationalised assuming a constant solvent self-diffusion coefficient in the core of the particles.

Solvent Self-Diffusion in Glassy Polymer-Solvent Systems

The free-volume theory for solvent self-diffusion in polymer-solvent systems is used to describe the self-diffusion process in glassy polymer-solvent mixtures. General predictions of the theory and comparisons of theoretical predictions with experimental data are considered

Solvent Self-Diffusion in Rubbery Polymer-Solvent Systems

The free-volume theory for solvent self-diffusion in polymersolvent systems is applied to rubbery mixtures at temperatures both above and below the polymer T g . A new parameter evaluation scheme is introduced, and it is shown that the theory provides good predictions for the solvent self-diffusion coefficient over wide temperature and concentration ranges

Nuclear magnetic resonance observation of cyclohexane self-diffusion in concentrated polybutadiene solutions

The coeffiicient of self-diffusion of cyclohexane, in concentrated solutions of polybutadiene, was measured using a pulsed magnetic field gradient method. The polymer volume fraction was varied from 0.2 to 0.95, and the temperature range of observation was 260 K<T<335 K. The concentration and temperature dependences of the solvent self-diffusion were analyzed according to a free volume description. These two dependences were found to be well described by using either the Fujita or WLF equations or by applying the Vrentas-Duda model. The size of the polymer jumping unit, derived from the Vrentas-Duda approach, was determined

Energy effects for solvent self-diffusion in polymer-solvent systems

The combined effects of energy and free volume are considered in the self-diffusion of trace amounts of solvents in amorphous polymers. A method for the determination of the parameters of the free-volume theory of transport is discussed. The ability of the combined energy and free-volume formulation to describe the temperature dependence of the self-diffusion process is evaluated using diffusion data both above and below the polymer glass transition temperature

Water self-diffusion in lyotropic systems by simulation of pulsed field gradient-spin echo nuclear magnetic resonance experiments

In this paper the pulsed field gradient-spin echo nuclear magnetic resonance (PFG-SE) experiment has been performed by computer simulation technique. The diffusive phenomenon is reproduced by letting particles, moving only by Brownian motion, travel among reflecting aggregates; in such a way it is possible to calculate the nuclear magnetic resonance (NMR) echo attenuation due to the solvent self-diffusion in lyotropic systems overcoming the problem of the analytical determination of the probability distribution of the spins. The results obtained on lamellar and inverted hexagonal lyomesophases are in good agreement with the previous functions reported in literature and new results are given for the hexagonal phase treated using this approach; in particular a new interpolation formula for the determination of the obstruction factor as a function of the structural parameters is proposed.

Simulation of Restricted Self-diffusion

Abstract In this paper the simulation of restricted self-diffusion is developed using the formal support of Cellular Automata so that it is described as a local interactions based system with discrete time and space. The main topic concerns the solvent self-diffusion in some lyotropic mesophases. In the model the lyotropic aggregates are assumed immovable, impenetrable and perfectly reflecting. The simulation data presented is in good accord with Pulsed Field Gradient NMR measurements. Finally the simulation results for motion restricted by single lamella are compared with those obtained by a purely theoretical investigation.

Solvent self‐diffusion in crosslinked polymers

AbstractA theory is developed for predicting the solvent self‐diffusion coefficient in a crosslinked, amorphous polymer. The theory is based on a modification of the free‐volume theory of transport for the presence of crosslinks in the polymer. The general predictions of the theory for the variations of the self‐diffusion coefficient with temperature, concentration, degree of crosslinking, and solvent size are compared with general experimental trends.

Self-diffusion of toluene in polystyrene solutions

Abstract : The pulsed-gradient spin-echo (PGSE) NMR technique was used to acquire a broad range of solvent self-diffusion coefficients for the toluene/ polystyrene (MW 270,000) system. Self-diffusion coefficients were determined for samples having polymer weight-fractions ranging from 0.04 to 0.90 over temperature range of 25-115 C. Several different approaches to interpretation of the diffusion data have been considered. The temperature dependence of the data exhibits Arrhenius behavior with energies of activation that increase with concentration from 2.6 to 16.2 kcal/mole. Several theories based on the assumption that the solvent diffusion is slowed by the polymer obstructing its diffusion path are also used to interpret this data. Our results support the hypothesis th the normalized diffusion coefficient is relatively independent of thermodynamic parameters. The data are in qualitative agreement with a standard curve for normalized diffusion coefficients. This approach shows potential as a tool for predicting diffusion rates over a wide concentration range for a variety of polymer solvent systems. The final approach used to interpret this data is that of the free volume theory proposed by Vrentas and Duda. The agreement between the free volume theory and experimental data is improved considerably if two of the needed parameters are adjusted to give optimum fits to the experimental diffusion data.

Free-volume analysis of solvent self-diffusion in polymer solutions

The free-volume theory of transport is used to analyze the concentration dependence of the solvent self-diffusion coefficient for two polystyrene-solvent systems. The agreement between predictions of free-volume theory and the experimental data is good.

Concentration dependence of solvent self‐diffusion coefficients

AbstractThe concentration dependence of solvent self‐diffusion coefficients is examined near the pure solvent limit. It is shown that it is possible to explain an apparently anomalous concentration dependence for the solvent self‐diffusion coefficient at high penetrant mass fractions within the framework of the free‐volume theory of transport.

Solvent self-diffusion in polymer solutions

We compare the solvent self-diffusion coefficients in a variety of aqueous and nonaqueous solutions, including polystyrene (PS) in toluene or cyclohexane, poly(ethylene oxide) (PEO) in water, and various biological materials. Remarkably similar changes are observed in the solvent self-diffusion properties relative to those of the neat solvent for all systems; no chemically specific effects can be resolved. Taken together, the transport data indicate substantial changes in the motional properties of the solvent due to interactions with the polymer. The reduction in transport might be determined by nonspecific kinetic effects associated with the presence of relatively immobile solute molecules.

The influence of polymer size and shape on self-diffusion of polysaccharides and solvents

Self-diffusion data obtained using pulsed-field-gradient nuclear magnetic resonance spectroscopy are presented for the poly- d-glucose molecules, dextran, amylopectin and glycogen in water and dimethyl sulphoxide. A model is developed for the influence of macroparticle shape on the concentration-dependence of polymer self-diffusion. The concentration-dependence of solvent and polymer self-diffusion indicates that amylopectin is planar with an axial ratio in excess of 6. In contrast, the dextran and glycogen data are consistent with random coil and spherical particle models, respectively. Both amylopectin and glycogen exhibit significantly different dimensions in the two solvent used.