A method for performing molecular dynamics simulation in the grand canonical ensemble is developed. The molecular dynamics, with coupling to an external bath, simulation method of [Berendsen et al., J. Chem. Phys. 81, 3684 (1984)] is extended for this purpose. Here the physical system of interest consists of real indistinguishable particles plus one fractional particle, whose potential energy of interaction with the rest of particles is scaled by a coupling parameter, ranging dynamically between zero and one. This coupling changes the number of particles in the system gradually and dynamically, depending on the target values of the excess chemical potential, temperature, and volume. A nonlinear scaling scheme has been adopted to scale the potential energy of interaction of the fractional particle with the rest of the system. The method has been employed to predict the density of compressed Lennard-Jones fluid, compatible with the target values of temperature and the excess chemical potential, over a wide range of temperatures and densities. The method has further been applied to do molecular dynamics simulation in the grand canonical ensemble for water and to predict its vapor-liquid phase coexistence point. The results obtained using this method are in complete agreement with previously reported results in the literature.
A series of biopolymer-based superabsorbent hydrogels based on carboxymethyl cellulose has been prepared by free-radical graft copolymerization of acrylamide and 2-acrylamido-2-methylpropan sulfonic acid (AMPS) in aqueous solution using methylenebisacrylamide as a crosslinking agent and ammonium persulfate as an initiator. The effect of variables on the swelling capacity such as: acrylamide/AMPS weight ratio, reaction temperature, and concentration of the initiator and crosslinker were systematically optimized. The results indicated that with increasing the amount of AMPS, the swelling capacity is increased. FT-IR spectroscopy and scanning electron microscope analysis were used to confirm the hydrogel structure. Swelling measurements of the synthesized hydrogels in different salt solutions indicated considerable swelling capacity. The absorbency under load of the superabsorbent hydrogels was determined by using an absorbency under load tester at various applied pressures. A preliminary swelling and deswelling behaviors of the hydrogels were also studied.
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