Thermal and Structural Properties of Ethylammonium Chloride and Its Mixture with Water

Abstract

The temperature dependence of ethylammonium chloride structure has been investigated by in situ laboratory parallel-beam X-ray powder diffraction. A polymorphic transition from a monoclinic LT phase to a tetragonal HT phase has been observed at 358 K. Such transformation has a reconstructive character. The thermal expansion of both polymorphs is small and anisotropic as a consequence of their organization through an anisotropic interaction network. The high temperature (HT) phase (possible space group P4/n or P4/nmm, a = 5.05 Å, c = 9.99 Å) has an excess volume of ~11% as compared with the low temperature (LT) one. The HT polymorph's structure has been solved by direct methods using powder diffraction data. In the absence of clear indications, it has been refined in P4/nmm. The structural properties of an ethylammonium chloride/water mixture at ambient conditions were also studied by using an integrated approach, which combines X-ray diffraction measurements and molecular dynamics simulations carried out with both the SPC/E and TIP5P water models. By refining a single interaction potential, very good agreement between the theoretical and experimental diffraction patterns was obtained, especially in the case of the TIP5P simulation. A complex structural behavior in which cations and anions do not possess a completely closed hydration shell of their own has been highlighted. Conversely, "solvent-shared ion pairs" are formed, in which one or more water molecules act as a bridge between the chloride and ethylammonium ions. Moreover, a strong water-water correlation is found, indicating that the water molecules in the mixture tend to aggregate and form water clusters.

Crystal Polymorphism of Propylammonium Chloride and Structural Properties of Its Mixture with Water

“…2 As can be seen from Figure 7a, the I(Q)'s for the PAC/water and EAC/water mixtures are very similar, and only slight differences between the two X-ray diffraction patterns are found in the Q range between 5 and 9 Å À1 . The observed differences can be better interpreted from the comparison of the Diff(r) functions in distance space (see Figure 7b).…”

“…The structural properties of an EAC/water mixture have also been studied by combining X-ray diffraction measurements and classical molecular dynamics (MD) simulations. 2 By refining a single interaction potential, a very good agreement between the theoretical and experimental diffraction patterns was obtained. A complex structural behavior has been highlighted, in which cations and anions do not possess a completely closed hydration shell of their own and strong cationÀanion and waterÀwater correlations are present.…”

“…In a second step, to improve the description of the experimental data, the two MD simulations with the SPC/E and TIP5P water models have been performed using the same force field parameters with the exception of one Lennard-Jones parameter for the ClÀwater interaction (σ ClÀOW ), for which the value previously optimized for an EAC/water mixture was used. 2 The initial configurations were constructed by positioning the ions and the water molecules on selected lattice positions within a very large cubic simulation box. Extensive equilibration runs have been performed, comprising 10 000 steps of initial energy minimization, a short NPT run at 500 K and 10 atm, aimed at randomizing the system followed by another NPT run at 300 K and very high pressure (100 atm) to compress the box volume until the experimental density was reached (1.01 g/cm 3 ).…”

“…2 In a recent work by this research group, the temperature dependence of an ethylammonium chloride (EAC) structure has been investigated by in situ laboratory parallel-beam X-ray powder diffraction. 2 A polymorphic transition, with a reconstructive character, from a monoclinic low-temperature (LT) phase to a tetragonal high-temperature (HT) phase has been observed at 358 K. The thermal expansion of both polymorphs has been found to be small and anisotropic as a consequence of their organization through an anisotropic interaction network. Moreover, the HT phase (possible space group P4/n or P4/nmm, a = 5.05 Å, c = 9.99 Å) was shown to have an excess volume of ca.…”

Brominated carbon black: An EDXD study

“…Scattering intensities of the sample were obtained by merging several measurements performed in correspondence of eight scattering angles θ = 24°, 15.5°, 5°, 10.5°, 8°, 3°, 1°, .5°. The larger number of angles than normally used in this technique, where five angles are enough to yield I(q) spectra (see below) ranging from 0.12 to 19.56 Å -1 [21], was necessary because bromine fluorescence lines fall amid the bremsstrahlung, so that the usable portion of the radiation is beyond bromine Kβ line. The presence of the very sharp bromine fluorescence lines is clearly visible in the diffracted intensity vs energy pattern registered, for instance, at 15.5 degrees, see Fig.…”