Adsorption of Methylamine on Amorphous Ice under Interstellar Conditions. A Grand Canonical Monte Carlo Simulation Study

Abstract

The adsorption of methylamine at the surface of amorphous ice is studied at various temperatures, ranging from 20 to 200 K, by grand canonical Monte Carlo simulations under conditions that are characteristic to the interstellar medium (ISM). The results are also compared with those obtained earlier on crystalline ( I) ice. We found that methylamine has a strong ability of being adsorbed on amorphous ice, involving also multilayer adsorption. The decrease of the temperature leads to a substantial increase of this adsorption ability; thus, considerable adsorption is seen at 20-50 K even at bulk gas phase concentrations that are comparable with that of the ISM. Further, methylamine molecules can also be dissolved in the bulk amorphous ice phase. Both the adsorption capacity of amorphous ice and the strength of the adsorption on it are found to be clearly larger than those corresponding to crystalline ( I) ice, due to the molecular scale roughness of the amorphous ice surface as well as to the lack of clear orientational preferences of the water molecules at this surface. Thus, the surface density of the saturated adsorption monolayer is estimated to be 12.6 ± 0.4 μmol/m, 20% larger than the value of 10.35 μmol/m, obtained earlier for I ice, and at low enough surface coverages the adsorbed methylamine molecules are found to easily form up to three hydrogen bonds with the surface water molecules. The estimated heat of adsorption at infinitely low surface coverage is calculated to be -69 ± 5 kJ/mol, being rather close to the estimated heat of solvation in the bulk amorphous ice phase of -74 ± 7 kJ/mol, indicating that there are at least a few positions at the surface where the adsorbed methylamine molecules experience a bulk-like local environment.

Adsorption of Formamide at the Surface of Amorphous and Crystalline Ices under Interstellar and Tropospheric Conditions. A Grand Canonical Monte Carlo Simulation Study

“…Interestingly, this distribution also shows a peak at around -25 kJ/mol even in state I on LDA ice, indicating that adsorbed molecules are not completely isolated from each other even in this state of very low surface density (i.e., when less than 5 formamides are attached, on average, to each of the two ice surfaces in the basic box, see Table 3). This behavior is in a clear contrast with our previous observation concerning the adsorption of methylamine on LDA ice, 70 and stresses the strong lateral interaction between the adsorbed formamide molecules. It should be noted that, by contrast with the LDA ice, no such peak is seen on The distribution of the total binding energy, P(Ub), exhibits a single peak around -100 kJ/mol in states I-III, reflecting the compensation of the decrease of its ice, and increase of its lateral component (in magnitude) with increasing chemical potential, and indicating that adsorbed formamide molecules always prefer to form four hydrogen bonds with their neighbors, irrespectively of whether they are waters or formamides.…”

“…Similar results were obtained earlier concerning the adsorption of methylamine at LDA and Ih ice surfaces. 70 Further, the fact that the obtained max values are considerably larger than the surface densities corresponding to the plateau region of the (prel) isotherms, scattering between about 6 and 8 mol/m 2 (see Fig. 3) stresses again that multilayer adsorption precedes the saturation of the first molecular layer at formamide at the surface of both amorphous and crystalline ice.…”

“…As a consequence, the co-planar alignment with the macroscopic plane of the surface is also preferred here. 70 These possible H-bonding patterns between surface water and first layer formamide molecules are also illustrated in Fig. 5c.…”

“…Similar orientational preferences were obtained earlier at the liquid-vapor interface of water-formamide mixtures. 117 The preference of the formamide molecules for these orientations can be understood by 70 At the surface of LDA ice, these preferences are much less sharp, the corresponding peaks of the orientational map are strongly merged, indicating that surface waters can rather largely deviate from these orientations. As a consequence, the co-planar alignment with the macroscopic plane of the surface is also preferred here.…”

“…Thus, by systematically varying the chemical potential and determining the number of adsorbed molecules as its function, the adsorption isotherm can easily be calculated. Indeed, the GCMC method has successfully been used to simulate the adsorption of various small molecules at the surface of, among others, clay minerals (e.g., kaolinite), 48,49 zeolites, 50-56 various metal oxides, 57-60 crystalline 26,61-69 as well as amorphous ice, 70 various carbonaceous materials, [71][72][73][74][75][76][77] self-assembled aerosol monolayers, 78,79 and covalent organic frameworks, [80][81][82] as well as inside protein crystals 83 and clathrate cages. [84][85][86][87][88][89][90][91] In this paper, we investigate in detail the adsorption of formamide both at the surface of Ih ice (at 200 K) and at the surface of LDA ice (at 200 K, 100 K, and 50 K).…”