Analysis of the drop rest phenomenon

An alternative to the film-thinning model for coalescence of drops at the horizontal liquid-liquid interface has been proposed and elucidated through analysis of drop rest time distribution, both in the presence and absence of added surfactants. The model is based on the assumption that the drop rests at the interface as a result of the repulsive force generated by a high concentration of adsorbate molecules on the opposite faces of the barrier ring. The repulsive force could be DLVO type (double layer force) or non-DLVO type (hydration/steric). High concentration of the adsorbate builds up at the barrier ring during the approach of the drop at the interface, when the resulting shear displaces the adsorbate on the faces of the entrapped film towards the barrier ring. The repulsive force at the barrier ring decays with time, due to back-diffusion of the adsorbate towards the centre of the film. Coalescence occurs when the repulsive force at the barrier ring is too weak to support the weight of the drop. The major cause of the distribution of drop rest time during the experiment involving sequential addition of drops is the drop-to-drop fluctuation in the surface excess of the adsorbate. This fluctuation is caused by the interfacial disturbance resulting from coalescence of the previously added drops and also by non-uniform distribution of the adsorbate at the barrier ring. Based on these ideas, a mathematical model for the rest time distribution has been developed. The model fits well to our own experimental data and those reported in the literature. Validity of the model is established through comparison of the predicted trends of the rest time distribution, with those observed under a variety of experimental conditions. The evidence, which reveals the shortcomings of the currently accepted film-thinning model, has also been presented

ROLE OF INTERFACIAL REACTION IN HETEROGENEOUS AROMATIC NITRATION

The previously established mechanism of two-phase nitration has been revised by proposing an additional mode of reaction, viz., interfacial nitration. The interfacial mechanism involves adsorption of the nitronium ion at the interface, followed by its reaction with the aromatic substrate present in the interfacial monolayer. The evidence for the proposed mechanism is provided by the results of the experiments, conducted in this work, on nitration of nitrobenzene. Relative magnitudes of the bulk and the interfacial rates during the nitration of nitrobenzene have been determined. The effects of interfacial area, concentrations of sulfuric and nitric acids, and addition of surfactant and inert diluent in the organic phase on the rate of interfacial nitration are studied. On the basis of the proposed mechanism, plausible explanations have been provided to the anomalies in the rates of nitration, observed under the industrially relevant operating conditions

Prediction of energy of interaction among tethered polymer chains confined between two parallel plates

A mean field continuum model has been used to predict the free energy of interaction between the layers of polymer chains grafted on two plane parallel plates. Four cases are considered: (a) a single plate with tethered chains, (b) two plates with equal amount of tethered chains on both plates, (c) tethering only on one plate, the other plate being bare, and (d) tethered loops on two plates. The predicted force-distance profiles are compared with the experimental data, reported in the literature, on interaction of polystyrene chains grafted on mica surface, with toluene as the solvent. The tethered amount is estimated from the regression of the experimental data. The predicted and the experimental profiles are in good agreement except for the case of short tethered loops. Moreover, in the experiment where the tethered amount is reported, the regression estimate closely matches with the actual value. In the cases where the tethered amount is not reported, the estimates of the same show the correct trend. Moreover, the estimated critical distance between the plates at which the tethered layers begin to interact also matches with the experimentally measured values. The present continuum model correctly accounts for the stiffness of the polymer chain and the difference in the size of the Kuhn segment and the solvent molecule. The predictions of the present theory are compared with those of the SCF lattice model of Scheutjens and Fleer

Elucidation of Band Structure of Charge Storage in Conducting Polymers Using a Redox Reaction

A novel technique to investigate charge storage characteristics of intrinsically conducting polymer films has been developed. A redox reaction is conducted on a polymer film on a rotating disk electrode under potentiostatic condition so that the rate of charging of the film equals the rate of removal of the charge by the reaction. The voltammogram obtained from the experiment on polyaniline film using Fe2+/Fe3+ in HCl as the redox system shows five distinct linear segments (bands) with discontinuity in the slope at specific transition potentials. These bands are the same as those indicated by electron spin resonance (ESR)/Raman spectroscopy with comparable transition potentials. From the dependence of the slopes of the bands on concentration of ferrous and ferric ions, it was possible to estimate the energies of the charge carriers in different bands. The film behaves as a redox capacitor and does not offer resistance to charge transfer and electronic conduction

Steady dynamic friction at elastomer-hard solid interface: A model based on population balance of bonds

We present a model for the steady dynamic friction of a block of an elastomer, sliding steadily on a hard surface. The model uses population balance of the bonds between the hard surface and the polymer chains of the elastomer to estimate the force of friction. Although the basic premises of the present model are the same as those of the Schallamach model for dynamic friction (1963), the present formulation is a clearer representation of the phenomena involved. Moreover, the model is not based on the ergodic hypothesis and is therefore more versatile. It also allows us to correct the error in the expression for the force of friction in the Schallamach model. The present model exhibits the same qualitative trends as the Schallamach model. However, there are significant quantitative differences between the two models. We also show that our expression for the force of friction is equivalent to that obtained by the Chernyak and Leonov (1986) model, which is based on the ergodic hypothesis. The model is further modified to account for both the non-Hookean extension of the bonded chains and the viscous retardation effect. The model is validated using the experimental data of Vorvolakos and Chaudhury (2003) on sliding of crosslinked PDMS solid on silane coated silicon wafer. From this analysis, scaling laws, which relate the model parameters to the molecular weight of the polymer chains and the temperature, are derived and justified

Ion-exchange kinetics: Heterogeneous resin-phase model

A new approach to cation-exchange kinetics views ion-exchange resin as an aggregate of uniform cylindrical pores. ion exchange occurs at the pore surface. The counterions dissociate only partially from the fixed sites, and the charge created on the pore surface is balanced by the counterions in the electrical double layer: Thus: any cross section of a pore consists of an annular double-layer region and a central core region. Electroneutrality prevails in the core region through an influx of coions. Diffusion of ions through both regions is considered. The complete model incorporating these ideas for, kinetics of monovalent cation exchange was tested with oui experimental data as well as with the reported data. The pore diffusion coefficients used in the model are free ionic diffusion coefficients corrected for the tortuosity factor of the pores. Unknown parameters of the model are the degree of dissociation of counterions from the fixed sites of the pore (f(sigma)), and the film thickness (delta). Both parameters are insensitive to the type of counterions, solution concentration, and the direction of exchange. Further, f(sigma) appears to be a characteristic property of resin alone. Superiority of the developed model over the previous models is established

Estimation of Equilibrium Capacitance of Polyaniline Films Using Step Voltammetry

The equilibrium charge storage characteristic of polyaniline films has been determined using step voltammetry technique. The step coulogram (plot of charge stored versus potential) consists of five distinct regions, which correspond to the different energy bands exhibited by the film, in keeping with Raman and ESR spectroscopy data. The coulogram is piecewise linear, with constant capacitance in each band. Effect of the type and concentration of the acid on the equilibrium capacitances of the bands has been studied. The polaron band has the highest capacitance, followed by the bipolaron band and then the polaron lattice band. The charge stored at a given potential is found to be proportional to film mass. The film needs to be polarized to a threshold charge before faradaic charging begins. This faradaic threshold is independent of the type and concentration of the acids. The step coulogram is compared with the sweep coulogram obtained from linear sweep voltammetry. At low sweep rates, the sweep coulogram also exhibits five distinct bands. However, with increase in the sweep rate, the bands progressively reduce in number. The capacitance of the polaron band decreases linearly with square root of sweep rate and attains the equilibrium capacitance at zero sweep rate. (C) The Author(s) 2015. Published by ECS. All rights reserved