Roman D. Oparin

In this work, we investigated how the degree of saturation of a nanocrystalline cellulose (NCC) aerogel with mefenamic acid (MA) affects the chemical reaction between MA and carbon dioxide retained in aerogel pores. A combination of low-temperature nitrogen sorption and IR spectroscopy revealed that with increasing aerogel saturation, the probability of forming an amorphous MA phase inside the aerogel pores increases, while the amount of retained CO2 markedly decreases. Consequently, the probability of initiating a chemical reaction between MA and CO2 is significantly reduced. Furthermore, we found that upon release into the scCO2 phase, the conformer population of MA at low temperatures differs substantially from that in a saturated MA/scCO2 solution at the same state parameters. At elevated temperatures, however, the equilibrium approaches that of a saturated solution. We attributed this effect to the combined influence of the aerogel's confined pore geometry and its ability to form specific interactions with MA molecules, which alters their conformational state.

In this work, we studied conformational equilibria of molecules of mefenamic acid in its diluted solution in scCO₂ under isochoric heating conditions in the temperature range of 140–210 °C along the isochore corresponding to the scCO₂ density of 1.1 of its critical value. This phase diagram range totally covers the region of conformational transitions of molecules of mefenamic acid in its saturated solution in scCO₂. We found that in the considered phase diagram region, the equilibrium of two conformers is realized in this solution. In the temperature range of 140–180 °C, conformer I related to the first, most stable polymorph of mefenamic acid prevails. In the temperature range of 200–210 °C, conformer II, which is related to the second metastable polymorph becomes dominant. Based on the results of quantum chemical calculations and experimental IR spectroscopy data on the mefenamic acid conformer populations, we classified this temperature-induced conformational crossover as an entropy-driven phenomenon.

The possibility of using near IR spectroscopy to analyze the effect isobaric heating has on hydrogen bonding in an aqueous solution of LiCl in equilibrium with supercritical carbon dioxide (a LiCl–H ₂ O–scCO ₂ ternary mixture) is demonstrated in a wide range of electrolyte concentrations. It is shown that this approach is highly efficient when studying ion and molecular systems with different types of interparticle interactions. The use of near IR spectroscopy allows distinguishing spectral contributions from hydrogen bonded n -mers of bulk water and water molecules in the solvation shells of ions or in ion–water chains like those formed in solutions with extremely high electrolyte concentrations that do not contain bulk water. It is shown for the studied ternary mixture that raising the concentration of electrolyte completely neutralizes the destructive effect of carbon dioxide on the formation of a hydrogen bonded structure of water. The latter is stabilized under the influence of an ion field, which also substantially weakens the temperature effect.

Two polymethyl methacrylate (PMMA) samples are impregnated with carbamazepine (CBZ) in a supercritical CO₂ (SC-CO₂) medium at temperatures of 75 and 105°C and a constant CO₂ density of 1.3 of its critical value. Before that, to determine the kinetic parameters of CO₂ sorption by the polymer matrix, high-pressure NMR spectroscopy is used. By Raman spectroscopy, it is established that both polymethyl methacrylate samples contain carbamazepine, whose concentration in the polymer is 2.7 times higher for the case of impregnation at 105°C compared to that at 75°C. In addition, it is shown that, in the process of impregnation, CBZ is not crystallized in the polymer matrix, but rather forms a solution in PMMA, which, in turn, agrees well with the results of the X-ray phase analysis. The difference in the conformations of CBZ molecules in the polymer matrices impregnated at two different temperatures is established.