Monomer Diffusion Coefficient Research Articles

Adsorption behavior of non-conventional eco-friendly tyrosine glycerol ether surfactants

Due to the increasing demand for environmentally friendly surfactants, a novel non-conventional glycerol ether surfactant with 12 carbon atoms in the hydrophobic alkyl chain and the amino acid tyrosine in the hydrophilic head (TyrGE12) is synthesized. Measurements of dynamic surface tension by maximum bubble pressure tensiometry and drop profile tensiometry are carried out for alkali aqueous solutions over a wide concentration range and for surface ages spanning from milliseconds to hours. Critical Micelle Concentration (CMC) value, standard free energy of adsorption and other physicochemical properties are calculated based on static (equilibrium) surface tension measurements. Dynamic surface tension data are analyzed using approximate solutions of the Ward and Tordai model to estimate diffusion coefficients for short and long adsorption times. For concentrations above the CMC, the effective diffusion coefficient (D*) of monomers is also estimated. Diffusion coefficients for short adsorption times lie in a physically reasonable range whereas for long adsorption times the computed very low values indicate the presence of an adsorption barrier. Interfacial dilatational storage modulus and loss modulus measurements indicate a dominant elastic character of the adsorption layer for all the examined concentrations and oscillation frequencies. Local peaks in the values of both moduli, below and above CMC, imply that structural changes in the adsorption layer might be partially affected by the presence of micelles.

Surface-induced assembly of apolipoprotein A-I: Implications for symmetry-driven non-cooperative clustering

In condensed matter physics the geometry of a crystal is determined by the mechanism of condensation. In biology, the link between clustering mechanisms and the shape of a protein crystal is not well defined. To gain more insight into the problem, we studied clustering of apolipoprotein A-I (apo A-I) on a solid surface using AFM. The amyloidogenic protein apo A-I is the main protein component of high density lipoprotein and thus reduces the risk of atherosclerosis. We found that apo A-I clustered to form nano-scale, symmetrical clusters on mica. Statistical analysis of size distribution for several thousand clusters suggested that the clustering reaction followed a non-cooperative kinetic scheme characterized by a single equilibrium constant of 0.92·106M−1 and a change in free energy (ΔG) of −0.03kJmole−1/residue. This is close to ΔG of−0.04kJmole−1/residue for apo A-I binding to phospholipid membrane; and 30-fold smaller than ΔG for β-amyloid formation by apo A-I. The high symmetry of the clusters is consistent with an isotropic diffusion coefficient of protein monomers on the surface of the substrate. This previously unrecognized link between protein clustering mechanism and the symmetry of the growth pattern may have important implications in medicine, pharmaceutics and polymer science.

Molecular diffusion and slip boundary conditions at smooth surfaces with periodic and random nanoscale textures

The influence of periodic and random surface textures on the flow structure and effective slip length in Newtonian fluids is investigated by molecular dynamics (MD) simulations. We consider a situation where the typical pattern size is smaller than the channel height and the local boundary conditions at wetting and nonwetting regions are characterized by finite slip lengths. In the case of anisotropic patterns, transverse flow profiles are reported for flows over alternating stripes of different wettability when the shear flow direction is misaligned with respect to the stripe orientation. The angular dependence of the effective slip length obtained from MD simulations is in good agreement with hydrodynamic predictions provided that the stripe width is larger than several molecular diameters. We found that the longitudinal component of the slip velocity along the shear flow direction is proportional to the interfacial diffusion coefficient of fluid monomers in that direction at equilibrium. In case of random textures, the effective slip length and the diffusion coefficient of fluid monomers in the first layer near the heterogeneous surface depend sensitively on the total area of wetting regions.

Consistent determinations for the diffusion and reaction rate coefficients of photoactive monomers under holographic exposure

Consistent determinations for the diffusion and reaction rate coefficients of photoactive monomers under holographic exposure

Computational Oral Absorption Simulation for Low‐Solubility Compounds

Bile micelles play an important role in oral absorption of low-solubility compounds. Bile micelles can affect solubility, dissolution rate, and permeability. For the pH-solubility profile in bile micelles, the Henderson-Hasselbalch equation should be modified to take bile-micelle partition into account. For the dissolution rate, in the Nernst-Brunner equation, the effective diffusion coefficient in bile-micelle media should be used instead of the monomer diffusion coefficient. The diffusion coefficient of bile micelles is 8- to 18-fold smaller than that of monomer molecules. For permeability, the effective diffusion coefficient in the unstirred water layer adjacent to the epithelial membrane, and the free fraction at the epithelial membrane surface should be taken into account. The importance of these aspects is demonstrated here using several in vivo and clinical oral-absorption data of low-solubility model compounds. Using the theoretical equations, the food effect on oral absorption is further discussed.

Adsorption Layer Characteristics of Mixed SDS/CnEOm Solutions. 3. Dynamics of Adsorption and Surface Dilational Rheology of Micellar Solutions

The dynamic surface tensions of mixed SDS/C(12)EO(5) and SDS/C(14)EO(8) micellar solutions measured over a wide time range (0.1 ms to 10,000 s) at various mixing ratios are described satisfactorily by a theoretical model for the kinetics of adsorption of surfactant mixtures using the surfactant adsorption parameters obtained for premicellar mixed solutions. Additional relations used for the description of the adsorption kinetics from micellar solutions were expressions of the effective diffusion coefficient of monomers accounting for the disintegration of micelles. The modeled dynamic surface tensions agree well with the experimental data for all studied surfactant mixtures. The rheological behavior of the same mixtures--the dependencies of the viscoelasticity modulus and phase angle--were studied by using the bubble profile method at harmonic bubble surface area oscillations. The theoretical approach employed for data analysis was the same as for the dynamic surface tension behavior. Again, satisfactory agreement between the experimental data and theoretical calculations of the dilational rheological parameters was found.

Diffusion-controlled adsorption kinetics of aqueous micellar solution at air/solution interface

For the micellar surfactant solutions, the diffusion equation, in which the effect of the micelles on the monomer diffusion was taken into account, was solved under the initial and boundary conditions by means of Laplace transformation. And a general equation for the dynamic surface adsorption Г(t) as a function of surface lifetime t, critical micelle concentration (cmc), monomer diffusion coefficient D1, and the demicellization constant \( {\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\leftarrow}$}}{k}} \) was derived. To determine cmc, the equilibrium surface tensions γeq of aqueous hexaethylene glycol monododecyl ether (C12E6) solutions at temperature 25 °C were measured by means of Wilhelmy plate method and the measured cmc is 8.4 × 10−5 mol 1−1. To study adsorption kinetics, the dynamic surface tensions γ(t) of aqueous submicellar and micellar solutions were measured by means of maximal bubble pressure method and analyzed. Both of the monomers diffusion coefficient D1 and the demicellization constant \( {\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\leftarrow}$}}{k}} \) were calculated.

Ab-Initio Modeling of Point Defects, Impurities and Diffusion in Silicon

We have revisited the first-principles calculation of the diffusion coefficient for nitrogen monomers and dimers, both within the dissociative and the direct pair-diffusion picture, using density-functional theory and harmonic transition-state theory. From our current understanding, it seems that the results for the dissociative mechanism are in better agreement with experiment than those for the direct pair-diffusion mechanism. Based on a monomer diffusion coefficient of D(T) = 7.3e-3 exp[-0.53 eV/(kT)] cm^2/s, we calculate for the dissociation diffusion parameter of the dimer P = 1.6e9 exp[-2.4 eV/(kT)] cm^(1/2)s.

Adsorption and aggregation properties of amino acid-based N-alkyl cysteine monomeric and [formula omitted]-dialkyl cystine gemini surfactants

An amino acid-based gemini surfactant derived from cystine (2C n Cys, where n represents the hydrocarbon chain lengths of 8, 10, and 12) was synthesized by reacting cystine with n-alkyl bromide, and its adsorption and aggregation properties were characterized by measurements of equilibrium and dynamic surface tension and dynamic light scattering. The properties of 2C n Cys were compared with those of an amino acid-based monomeric surfactant derived from cysteine (C n Cys). For n = 8 and 10, when compared to C n Cys, 2C n Cys exhibited excellent surface activities, such as a lower critical micelle concentration (cmc), greater efficiency in lowering the surface tension of water, and smaller area occupied per molecule. Adsorption rate at air/water interface decreased with an increase in hydrocarbon chain length, chain number, and concentration of respective compounds. Further, the kinetics were discussed using the monomer diffusion coefficient obtained from short and long time scales in dynamic surface tension plots. In addition, the aggregation properties of 2C n Cys for n = 8 and 10 differed from those in the case of n = 12 . In other words, relatively larger micelles with diameters of approximately 7 nm were formed by 2C n Cys for n = 8 and 10 in comparison to those formed by C n Cys (2–3 nm). On the other hand, for a 0.832 mmol dm −3 2C 12Cys solution, the aggregation structure investigated by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) revealed the coexistence of small unilamellar vesicles and small rods.

Effects of Denaturants on the Dynamics of Loop Formation in Polypeptides

Quenching of the triplet state of tryptophan by close contact with cysteine has been used to measure the reaction-limited and diffusion-limited rates of loop formation in disordered polypeptides having the sequence cys-(ala-gly-gln) j-trp ( j = 1–9). The decrease in the length-dependence of the reaction-limited rate for short chains in aqueous buffer, previously attributed to chain stiffness, is not observed at high concentrations of chemical denaturant (6 M GdmCl and 8 M urea), showing that denaturants increase chain flexibility. For long chains, both reaction-limited and diffusion-limited rates are significantly smaller in denaturant and exhibit a steeper length dependence. The results can be explained using end-to-end distributions from a wormlike chain model in which excluded volume interactions are incorporated by associating a 0.4–0.5 nm diameter hard sphere with the end of each virtual peptide bond. Fitting the data with this model shows that the denaturants reduce the persistence length from ∼0.6 nm to ∼0.4 nm, only slightly greater than the length of a peptide bond. The same model also describes the reported length dependence for the radii of gyration of chemically denatured proteins containing 50–400 residues. The end-to-end diffusion coefficients obtained from the diffusion-limited rates are smaller than the sum of the monomer diffusion coefficients and exhibit significant temperature dependence, suggesting that diffusion is slowed by internal friction arising from barriers to backbone conformational changes.

A transport model for the adsorption of surfactant from micelle solutions onto a clean air/water interface in the limit of rapid aggregate disassembly relative to diffusion and supporting dynamic tension experiments

A transport theory and supporting experiments are detailed for the adsorption of surfactant from micelle solutions onto a clean air/water interface. The study focuses on adsorption from very dilute solutions consisting of globular aggregates. In the usual mechanism conceived for the adsorption process, when a clean interface is created in the solution of monomer and globular aggregates, monomer initially kinetically adsorbs from the sublayer onto the surface, depleting the sublayer concentration of monomer. This depletion, as it perturbs the micelle/monomer equilibrium, is followed by the disassembly of aggregates in the sublayer (to replenish monomer). Co-currently, aggregates and monomer diffuse from the bulk to the depleted sublayer. Thus in this classical picture, while the micelles provide a reservoir for monomer adsorption by their disassembly – and thereby enhance the adsorption rate – they provide no direct route for adsorption. Using this picture as a starting point, in this study an asymptotic transport theory is constructed for the case in which the time scale for the diffusion of the monomer is much slower than the time scale for the kinetic breakdown of the aggregates. With this separation of time scales, disassembly of micelles takes place only along a boundary separating regions without micelles (micelle-free zones) from regions containing micelles. In the regions containing micelles, the larger rate of disassembly relative to the rate of diffusion of monomer maintains the monomer concentration at the critical aggregate concentration ( C CAC). Two regimes arise: in the first, at low aggregate concentrations, micelle diffusion cannot keep up with the kinetic surface adsorption of monomer, and a micelle-free zone emerges from the surface and moves into the bulk. Micelles diffuse from the bulk to the boundary of the zone, break-up and monomer diffuses through the micelle-free zone and kinetically adsorbs onto the surface. In the second regime, at higher aggregate concentrations, the micelle diffusive flux is commensurate with the kinetic rate of monomer surface adsorption, and a micelle-free zone does not form. Micelles diffuse directly to the surface, and break-up in the sublayer; monomer then kinetically adsorbs onto the surface. The adsorption rate is limited only by the kinetics of monomer adsorption since the monomer sublayer concentration is equal to the C CAC, and represents the fastest rate that surfactant can adsorb onto the surface in the absence of diffusion barriers. Numerical solutions are obtained for a polyethoxylated surfactant C 14E 6 whose micelle disassembly time scale satisfies the requirement that it be faster than the diffusion time scale of the monomer. For C 14E 6 the monomer adsorption parameters and diffusion coefficient of monomer are obtained from separate dynamic tension measurements below the C CAC; the micelle diffusion coefficient is estimated from the literature. A critical total surfactant concentration of 4.25 C CAC is computed, below which a micelle-free zone forms and above which the zone does not form, and surface concentrations and tension reductions as a function of time are simulated for both regimes. Experiments are undertaken in which dynamic tensions are measured for adsorption of surfactant onto an initially clean interface from C 14E 6 micelle solutions using the pendant bubble method. These experiments show that below the critical value of 4.25 C CAC, the theory predicts the experimental tension relaxations in excellent agreement and no adjustable constants. Above the critical value, relaxations continue to accelerate with bulk concentration, and become faster than the kinetically-dictated fastest relaxation. This implies an alternate route to surfactant adsorption; since micelles are present in the sublayer in the second regime (and are not in the first regime), we suggest that they directly supply monomer to the surface.

Marangoni Effects in Liquid Jets of Non-Ionic Surfactants

The adsorption of nonionic surfactants in the CnE8 family at the air–water interface has been studied on the millisecond timescale in a free liquid jet. The amount of adsorbed surfactant was measured by ellipsometry. The rates of adsorption are compared with a diffusion-controlled adsorption model. In the case of C10E8, which is below its cmc, the monomer diffusion coefficient provides a good fit to the experimental data. For n = 12, 14, and 16, micelles control the mass transport. The best fit diffusion coefficients are close to, but not identical with, the literature values for the micellar diffusion coefficients. Laser Doppler velocimetry was used to measure the change in surface velocity arising from adsorption of the surfactant, for n = 12, 14, and 16. There was a qualitative correlation between the retardation of the surface velocity and the surface tension gradients.

Generation of shallow nitrogen-oxygen donors as a method for studying nitrogen diffusion in silicon

The nitrogen concentration distribution C(z) obtained after diffusion of nitrogen from the bulk of a sample toward the surface is determined with a high accuracy through subsequent heat treatment at moderate temperatures (for example, at 650°C). This process leads to the formation of shallow thermal donors (nitrogen-oxygen complexes) with a concentration distribution over the depth z of the sample that corresponds to the nitrogen concentration profile C(z) and, therefore, makes it possible to calculate this profile. The proposed method is used to determine the nitrogen concentration profiles C(z) after homogenizing annealing at 950, 1000, and 1050°C. At high oxygen concentrations, the nitrogen transport is promoted by high-rate dissociation of nitrogen dimers. On the other hand, the nitrogen transport is slightly hindered by partial oxidation of nitrogen monomers. The results obtained indicate that the latter effect is not very strong, because the dimeric nitrogen species predominate over the monomeric nitrogen species in the sample and the diffusion profile is determined by the product D1K1/2, where D1 is the diffusion coefficient of nitrogen monomers and K is the dissociation constant.

The mutual diffusion coefficient for (meth)acrylate monomers as determined with a nuclear microprobe.

The value of the mutual diffusion coefficient DV of two acrylic monomers is determined with nuclear microprobe measurements on a set of polymer films. These films have been prepared by allowing the monomers to diffuse into each other for a certain time and subsequently applying fast ultraviolet photo-polymerization, which freezes the concentration profile. The monomer diffusion profiles are studied with a scanning 2.1 MeV proton microprobe. Each monomer contains a marker element, e.g., Cl and Si, which are easily detected with proton induced x-ray emission. From the diffusion profiles, it is possible to determine the mutual diffusion coefficient. The mutual diffusion coefficient is dependent of concentration, which is concluded from the asymmetry in the Cl- and Si-profiles. A linear dependence of the mutual diffusion coefficient on the composition is used as a first order approximation. The best fits are obtained for a value of b=(0.38+/-0.15), which is the ratio of the diffusion coefficient of 1,3-bis(3-methacryloxypropyl)-1, 1,3,3-tetramethyldisiloxane in pure 2-chloroethyl acrylate and the diffusion coefficient of 2-chloroethyl acrylate in pure 1,3-bis(3-methacryloxypropyl)-1,1,3,3-tetramethyldisiloxane. Under the assumption of a linear dependence of the mutual diffusion coefficient DV on monomer composition, it follows that DV = (2.9+/-0.6)10(-10) m(2)/s at a 1:1 monomer ratio. With Flory-Huggins expressions for the monomer chemical potentials, one can derive approximate values for the individual monomer diffusion coefficients.

An investigation of dynamic surface tension, critical micelle concentration, and aggregation number of three nonionic surfactants using NMR, time-resolved fluorescence quenching, and maximum bubble pressure tensiometry

Several physicochemical properties have been determined for N-dodecyllactobionamide (LABA), maltose 6′- O-dodecanoate (C 12-maltose ester), and tetra(ethylene oxide) dodecyl amide (TEDAd). The increase in the flexibility of the sugar headgroup, enabling more possible molecular conformations, reduces the minimum area/molecule at the liquid–vapor interface obtained at the critical micelle concentration (cmc). The obtained cmc's were 0.35 mM (LABA), 0.3 mM (C 12-maltose ester), and 0.5 mM (TEDAd). The monomer diffusion coefficient decreased with the molecular weight and increasing headgroup flexibility of the sugar headgroup, and values were in the range from 3.1×10 −10 to 3.6×10 −10 m 2/s. The micelle diffusion coefficients (0.46×10 −10 to 0.68×10 −10 m 2/s) indicated that the TEDAd micelles deviated most from spherical shape. The micelle aggregation numbers determined by time-resolved fluorescence quenching (TRFQ) were estimated to be 120±10 (LABA), 90±10 (C 12-maltose ester), and 130±10 (TEDAd). The dynamic surface tension measurements show that the adsorption of TEDAd onto the liquid–vapor interface at short surface lifetimes is diffusion-limited, whereas an adsorption barrier is present for the sugar surfactants. The analysis of the dynamic surface tension data above the cmc shows that the rate of demicellization is faster for TEDAd than for the two sugar-based surfactants.

Chain Length Dependence of Radical−Radical Termination in Free Radical Polymerization: A Pulsed Laser Photolysis Investigation

A new method for analyzing the chain length dependence of termination rate coefficients (kt(i,j) where i and j are the chain lengths of the reacting radicals) in low conversion free radical polymerizations is described. The method involves comparing experimental molecular weight distributions of polymers formed in pulsed laser photolysis experiments with those predicted by kinetic simulation. The method is enabled by direct measurements of the concentration of radicals generated per laser pulse. Knowledge of the radical concentrations should mean that the only unknowns in the simulations are the termination rate coefficients. The analysis demonstrates the need for chain length dependent termination rate constants in describing polymerization kinetics. Polymerization kinetics were analyzed in terms of two models which purport to describe the functional form of kt(i,j). Use of a Smoluchowski model for kt(i,j), in conjunction with independently measured monomer diffusion coefficients, provides molecular weight distributions that closely match those obtained experimentally. However, the finding that reasonable fits can also be obtained using a geometric mean model to describe kt(i,j) indicates molecular weight distributions alone do not contain sufficient information for unambiguous model discrimination.

Modelling of binder removal from a (fibre+powder) composite pre-form

A diffusion-controlled ‘shrinking core’ model was established to simulate the removal process of organic binder in a fibre+powder composite pre-form of plate geometry with binder removal essentially in one direction. The influences of compact shrinkage and fibre orientation are manifested via the effective particle volume fraction, the monomer diffusion coefficient, and hence polymer decomposition rate. Two shrinkage modes and three types of fibre orientation were considered. Simulation results were in agreement with experimental ones. It is shown that compact shrinkage leads to slower binder removal. The effect of fibres depends on their aligned orientation to the removal direction and a higher fibre volume fraction has greater effect on the burnout kinetics.

Non-Gaussian dynamics from a simulation of a short peptide: Loop closure rates and effective diffusion coefficients

Intrachain contact formation rates, fundamental to the dynamics of biopolymer self-organization such as protein folding, can be monitored in the laboratory through fluorescence quenching measurements. The common approximations for the intrachain contact rate given by the theory of Szabo, Schulten, and Schulten (SSS) [J. Chem. Phys. 72, 4350 (1980)] and Wilemski–Fixman (WF) [J. Chem. Phys. 60, 878 (1973)] are shown to be complementary variational bounds: The SSS and WF approximations are lower and upper bounds, respectively, on the mean first contact times. As reported in the literature, the SSS approximation requires an effective diffusion coefficient 10 to 100 times smaller than expected to fit experimentally measured quenching rates. An all atom molecular dynamics simulation of an eleven residue peptide sequence in explicit water is analyzed to investigate the source of this surprising parameter value. The simulated diffusion limited contact time is ≈6 ns for a reaction radius of 4 Å for solvent viscosity corresponding to that of water at 293 K and 1 atm (η=1.0 cP). In analytical work, the polymer is typically modeled by a Gaussian chain of effective monomers. Compared to Gaussian dynamics, the simulated end-to-end distance autocorrelation has a much slower relaxation. The long time behavior of the distance autocorrelation function can be approximated by a Gaussian model in which the monomer diffusion coefficient D0 is reduced to D0/6. This value of the diffusion coefficient brings the mean end-to-end contact time from analytical approximations and simulation into agreement in the sense that the SSS and WF approximations bracket the simulated mean first contact time.

Diffusion mobility of reactants, phase equilibrium, and specific features of radical copolymerization kinetics in the nonyl acrylate/2‐methyl‐5‐vinyltetrazole system

AbstractCombined analysis of experimental data on monomer diffusion, phase equilibrium, and copolymerization kinetics in systems of monomers (nonyl acrylate and 2‐methyl‐5‐vinyltetrazole) as well as their copolymers and homopolymers was carried out. The composition of the mixture in the vicinity of the growing macroradical can differ significantly from the average composition in the whole reactor volume because of consumption of the more‐reactive reactant 2‐methyl‐5‐vinyltetrazole. Nonyl acrylate exhibited limited compatibility with copolymers enriched in 2‐methyl‐5‐vinyltetrazole and its homopolymer. Phase diagrams were obtained for the latter homopolymer. The concentration plots of the diffusion coefficients of both monomers in their copolymers of various compositions were determined. Microphase separation was observed at specific conversions in the reaction system where the composition of a copolymer and its concentration in a monomer solution approached the binodal of the phase diagram. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1383–1389, 2002

Photopolymer holographic recording material

Photopolymers are promising materials for use in holography. They have many advantages, such as ease of preparation, and are capable of efficiencies of up to 100%. A disadvantage of these materials is their inability to record high spatial frequency gratings when compared to other materials such as dichromated gelatin and silver halide photographic emulsion. Until recently, the drop off at high spatial frequencies of the material response was not predicted by any of the diffusion based models available. It has recently been proposed that this effect is due to polymer chains growing away from their initiation point and causing a smeared profile to be recorded. This is termed a non-local material response. Simple analytic expressions have been derived using this model and fits to experimental data have allowed values to be estimated for material parameters such as the diffusion coefficient of monomer, the ratio of polymerisation rate to diffusion rate and the distance that the polymer chains spread during holographic recording. The model predicts that the spatial frequency response might be improved by decreasing the mean polymer chain lengths and/or by increasing the mobility of the molecules used in the material. The experimental work carried out to investigate these predictions is reported here. This work involved (a) the changing of the molecular weights of chemical components within the material (dyes and binders) and (b) the addition of a chemical retarder in order to shorten the polymer chains, thereby decreasing the extent of the non-local effect. Although no significant improvement in spatial frequency response was observed the model appears to offer an improved understanding of the operation of the material.