The crystallization kinetics and morphology of nitric acid trihydrate

Abstract

Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.

The mid‐IR Absorption Cross Sections of α‐ and β‐NAT (HNO3 · 3H2O) in the range 170 to 185 K and of metastable NAD (HNO3 · 2H2O) in the range 172 to 182 K

“…[] were the first to point out two distinctly different structures for NAT, namely, the low‐temperature and metastable α ‐NAT and the thermodynamically stable β ‐NAT using Fourier transform infrared (FTIR) absorption in transmission of 2–4 µm thick films of nitric acid hydrates. Subsequently, unambiguous proof for the structure of β ‐NAT by Grothe and coworkers [ Tizek et al ., ; Grothe et al ., ] has been given by combining X‐ray diffraction (XRD) and FTIR in transmission. Recently, Weiss et al have obtained the structure of α ‐NAT using XRD in conjunction with neutron scattering and supporting FTIR absorption in transmission (Weiss et al, manuscript in preparation, 2015).…”

“…Koehler et al [1992] were the first to point out two distinctly different structures for NAT, namely, the low-temperature and metastable α-NAT and the thermodynamically stable β-NAT using Fourier transform infrared (FTIR) absorption in transmission of 2-4 μm thick films of nitric acid hydrates. Subsequently, unambiguous proof for the structure of β-NAT by Grothe and coworkers Grothe et al, 2006] has been given by combining X-ray diffraction (XRD) and FTIR in transmission.…”

Metastable Nitric Acid Trihydrate in Ice Clouds

“…Interestingly,alpha-NAT has not received much attention by atmospheric scientists,a nd most papers in the field just focus on the thermodynamically stable beta-NAT.H owever, under the conditions of the lower stratosphere (LS) and the upper troposphere (UT), alpha-NAT can persist for several hours,a nd when present as am ixture with ice,i tc an even persist for more than one day. [14] Unfortunately,the crystalline structure of alpha-NAT has not been reported, which has hampered its spectroscopic identification. Spectroscopic data from the laboratory are essential to determine the composition of ice clouds as their analysis relies solely on remote sensing.…”

Characterisation using environmental scanning electron microscopy (ESEM)

“…The simplest case to consider is that where the water vapour pressure is altered to allow not simply hydration to occur, but some change in state. The growth and morphology of nitric acid trihydrate grown in the ESEM at cryo temperatures under nitrogen gas were studied by Grothe et al (2006). The growth of polymeric films at an air±water interface was followed in real time by Miller and Cooper (2002).…”