Diffusion behaviors of liquid ammonia in the cellulose based on molecular dynamics simulation

Abstract

Purpose In the textile industry, liquid ammonia treatment is an important way to modify the structure of natural fibers. The purpose of this paper is to reveal the diffusion behaviors of liquid ammonia in cellulose. Design/methodology/approach To analysis the diffusion behaviors of liquid ammonia in cellulose, the cellulose model and the system of ammonia and cellulose are built. Infrared spectrum is carried out to test the model of cellulose, which is found to agree with experiment. Diffusion coefficients, free volume and hydrogen bonds are discussed to explain diffusion behaviors. Findings The results demonstrate that diffusion coefficients and free volume of systems rise with increasing temperature. The diffusion coefficients of ammonia are larger than those of water, a result in agreement with free volume. To understand the mechanism of diffusion, the numbers of hydrogen bonds are tracked. It is found that without ammonia, intrachain hydrogen bonds decrease with the increase of temperature, which indicate that the structural stability of cellulose is deteriorated. And the increased interchain hydrogen bonds show that swelling properties of cellulose become better with the increase of temperature. After ammonia treatment, the numbers of intrachain hydrogen bonds remain stable, indicating that the structure stability of cellulose chain is maintained. But, there is a substantial rupture of interchain hydrogen bonds, ammonia molecule destroys the hydrogen bond network between the original cellulose molecular chains, which intensifies the activity of cellulose molecular chains and enlarges the distance between cellulose molecular chains, showing good swelling properties. Originality/value The research findings give a detailed information about the diffusion behaviors of liquid ammonia in cellulose, which provide the theoretical evidence for liquid ammonia treatment.

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