The counterdiffusion of HCl and NH₃: An experimental and modeling analysis of topochemistry, diffusion, reaction, and...

Vapor-phase ammonia, NH_3(g), and hydrochloric acid, HCl_(g), undergo a series of complex reactions, including nucleation and growth, to form solid ammonium chloride, NH₄Cl_(s). The counterdiffusional experiment, whereby HCl_(g) and NH_3(g) diffuse from opposite ends of a tube and react to form spatiotemporally complex patterns, has a rich history of study. In this paper, we combine experimental data, molecular simulations, and analysis and simulations of a partial differential equation model to address the questions of where the first unobserved vapor product NH₄Cl_(g) and visually observable precipitate NH₄Cl_(s) form and how these positions depend on experimental parameters. These analyses yield a consistent picture which involves a moving reaction front as well as previously unobserved heterogeneous nucleation, wall nucleation, and homogeneous nucleation. The experiments combined with modeling allow for an estimate of the heterogeneous and homogeneous nucleation thresholds for the vapor-to-solid phase transition. The results, synthesized with the literature on this vapor-to-particle reaction, inform a discussion of the details of the reaction mechanism, including the role of water, which concludes the paper.