Vijay Mukund Naik

Hydrogen bond thermodynamic properties of water from dielectric constant data

We apply statistical mechanical principles to derive simple expressions relating the hydrogen bond thermodynamic properties to the static dielectric constant of water. The approach followed by us was to develop an expression for the Kirkwood’s structure factor (g) of water, taking into account the dipolar correlations between a central molecule and H-bonded neighbors present in infinite number of shells surrounding the central molecule. The number of H-bonded neighbors in a specific shell was related to the probability P for the various donor/acceptor sites of any given water molecule to be associated. Neglecting cooperativity effects, we evaluated P by focusing only on the correct counting of H-bonds formed between various association sites rather than on the oligomer distribution. The theory yielded an extremely simple expression for the structure factor (g) of the fluid at any given temperature in terms of the enthalpy (H) and entropy (S) changes associated with bond formation. The proposed theory was ...

Structure and rheology of the defect-gel states of pure and particle-dispersed lyotropic lamellar phases

Abstract:We present important new results from light-microscopy and rheometry on a moderately concentrated lyotropic smectic, with and without particulate additives. Shear-treatment aligns the phase rapidly, except for a striking network of oily-streak defects, which anneals out much more slowly. If spherical particles several microns in diameter are dispersed in the lamellar medium, part of the defect network persists under shear-treatment, its nodes anchored on the particles. The sample as prepared has substantial storage and loss moduli, both of which decrease steadily under shear-treatment. Adding particles enhances the moduli and retards their decay under shear. The data for the frequency-dependent storage modulus after various durations of shear-treatment can be scaled to collapse onto a single curve. The elasticity and dissipation in these samples thus arises mainly from the defect network, not directly from the smectic elasticity and hydrodynamics.

Theory of dielectric constant of aqueous solutions

We apply principles of statistical mechanics to derive simple expressions relating the hydrogen bond thermodynamic properties to the static dielectric constant of aqueous solutions. The approach followed by us was to develop an expression for the dipolar correlations between a centrally fixed molecule of a given type and its neighbors present in the surrounding shells, in terms of bonding probabilities, and combine the resulting expression with the Kirkwood–Frohlich equation. We considered only those neighboring molecules which are a part of the H-bonded cluster containing the central molecule. The bonding probabilities were evaluated by assuming a reaction equilibrium model, in which the formation of clusters between different association sites was represented by a series of chemical reactions. To demonstrate the utility of the theory, we provide comparison of the results for the temperature and composition variation of dielectric constant and H-bond stoichiometry of three model systems, methanol+water, ...

Hydrodynamic cavitation: a bottom-up approach to liquid aeration

We report the use of hydrodynamic cavitation as a novel, bottom-up method for continuous creation of foams comprising of air micro-bubbles in aqueous systems containing surface active ingredients, like proteins or particles. The hydrodynamic cavitation was created using a converging–diverging nozzle. The air bubble size obtained using this technique was found to be significantly smaller than that achieved using conventional mechanical entrapment of air via shearing or shaking routes, which are in essence top-down approaches. In addition, the technique provided the possibility of forming non-spherical bubbles due to the high elongational stresses experienced by the bubbles as they flow through the nozzle throat. We show that surface active agents with a high surface elasticity modulus can be used to stabilize the nascent air bubbles and keep their elongated shapes for prolonged periods of time. This combination of the cavitation process with appropriate surface active agents offers an opportunity for creating bubbles smaller than 10 microns, which can provide unique benefits in various applications.

A MULTILAYER THEORY FOR INTERFACIAL PROPERTIES OF SYSTEMS CONTAINING HYDROGEN BONDING MOLECULES

We propose a generalized, lattice-based statistical thermodynamic theory for understanding the interfacial phenomena in systems containing hydrogen bonding molecules (often termed as associating molecules), such as water, amphiphiles, block copolymers and associating solid surfaces. The basic assumption is that the configurational partition function (Q) can be factored into two parts: (i) one term [Q(phys)] arising from the presence of nonassociating, or the “physical,” interactions, for which we adopt the self-consistent-field theory [Scheutjens and Fleer, J. Phys. Chem. 84, 178 (1980)], (ii) the other term [Q(hbond)] arising from the presence of hydrogen bond interactions, for which we propose a new association theory. The focus of the proposed association theory is on the correct counting of the number of H bonds that are formed between various types of donor and acceptor sites that satisfy the proximity and orientational requirements for bond formation. The expression for Q(hbond) is evaluated by acco...

Effect of particulate solids on the rheology of a lyotropic gel medium

Experimental studies of the rheology of concentrated suspensions of particulate solids (marble) in a lyotropic gel phase (potassium palmitate with 40% water) were carried out. Such gels are viscoplastic and are characterized by strong colloidal interactions. Suspensions with lyotropic gels as a continuous phase serve as models for systems such as household detergent products, and processed foods. For comparison, experiments were also carried out on suspensions of the same solids in a viscous Newtonian medium (silicone oil). The steady shear experimental data for both the systems were fitted by the Bingham model. The entrance pressure drop for capillary flow of the gel–marble suspensions was interpreted in terms of an entrance yield stress. The latter when extrapolated to zero extrusion speeds was found to be nearly independent of die geometry, and was found to increase exponentially with solids volume fraction. The Bingham yield stress showed a similar behavior with volume fraction though its magnitude wa...

Electric field induced cloudy–clear transitions in micellar solutions of a block copolymeric amphiphile

We report observations of cloudy–clear transitions, triggered by electric fields, in micellar solutions of a poly(oxy ethylene/propylene)–poly(dimethylsiloxane) based triblock amphiphile dissolved in silicone oil (DC245). Initial studies showed complete clarification of a cloudy 2% solution at ambient temperature when subjected to a uniform electric field of ∼5 kV cm−1. Static Light Scattering measurements showed that the clarification was associated with a significant reduction in the size of the micellar aggregate from ca. 200 nm to 50 nm. Field strength studies at varying temperatures established the potency of the effect at 1 kV cm−1 field equivalent to a 1 K decrease in temperature. The observed effect was qualitatively different from the electric field induced macroscopic pattern formations, or self assembled domain rotations or mesogen alignment phenomena; and quantitatively far more impactful than the minor shifts in phase boundaries reported earlier. Although the mechanistic underpinning of the observed field effect has yet to be established, the optical switchability of the solutions could be exploited for various electro-optical applications such as displays, and optical filters. The work also opens up the possibilities of using electric fields for manipulating micellar solubilization or creating tunable templates for nano-material synthesis.