Computational assessment of the crystallization tendency of 1-ethyl-3-methylimidazolium ionic liquids

Molecular Dynamics Simulation of the Influence of External Electric Fields on the Glass Transition Temperature of the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

We present the results of molecular dynamics simulations of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁im][NTf₂] in the presence of external electric fields (EEFs) of varying strengths to understand the effects of EEFs on the glass transition temperature T_g. We compute T_g with an automated and objective method and observe a depression in T_g when cooling the IL within an EEF above a critical strength. The effect is reversible, and glasses prepared with EEFs recover their original zero-field T_g when heated. By examining the dynamics and structure of the liquid phase, we find that the EEF lowers the activation energy for diffusion, reducing the energetic barrier for movement and consequently T_g. We show that the effect can be leveraged to drive an electrified nonvapor compression refrigeration cycle.

Does Explicit Polarizability Improve Molecular Dynamics Predictions of Glass Transition Temperatures of Ionic Liquids?

Molecular dynamics simulations are used for predictions of the glass transition temperatures for a test set of five aprotic ionic liquids. Glass transitions are localized with the trend-shift method, analyzing volumetric and transport properties of bulk amorphous phases. A classical nonpolarizable all-atom OPLS force-field model developed by Canongia Lopes and Pádua (CL&P) is employed as a starting level of theory for all calculations. Alternative approaches of charge scaling and the Drude oscillator model, accounting for atomic polarizability either implicitly or explicitly, respectively, are used to investigate the sensitivity of the glass transition temperatures to induction effects. The former nonpolarizable model overestimates the glass transition temperature by tens of Kelvins (37 K on average). The charge-scaling technique yields a significant improvement, and the best estimations were achieved using polarizable simulations with the Drude model, which yielded an average deviation of 11 K. Although the volumetric data usually exhibit a lesser trend shift upon vitrification, their lower statistical uncertainty enables to predict the glass transition temperature with lower uncertainty than the ionic self-diffusivities, the temperature dependence of which is usually more scattered. Additional analyses of the simulated data were also performed, revealing that the Drude model predicts lower densities for most subcooled liquids but higher densities for the glasses than the original CL&P, and that the Drude model also invokes some longer-range organization of the subcooled liquid, greatly impacting the temperature trend of ionic self-diffusivities in the low-temperature region.

Decay of hydrogen bonding in mixtures of aliphatic heptanols and bistriflimide ionic liquids

Hydrogen bonding in liquids of the constitution isomers of heptan-1-ol mixed with 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs), [C_nmim][NTf₂], is investigated using both computational and experimental techniques. All-atom non-polarizable molecular-dynamics (MD) simulations predict that the hydrogen bonds gradually decay with increasing temperature. This decay is more pronounced for the branched alcohols and in the presence of the ionic liquids. The primary and linear isomer, heptan-1-ol, and its tertiary and bulky analogue 3-ethylpentan-3-ol are identified as the opposite extremes of the spectrum of hydrogen bonding stability in the bulk liquid. While neat heptan-1-ol exhibits strong hydrogen bonding at 350 K, 3-ethylpentan-3-ol is prone to hydrogen bonding decay already at 300 K. The presence of ionic liquids is found to affect the hydrogen bonding comparably as a 50 K temperature increase. Since the heat capacities of the associating liquids are very sensitive to any variation in hydrogen bonding strength and to the character of the hydrogen-bonded clusters in the bulk liquid, the calorimetric effort provides useful experimental data to confirm the results predicted by MD simulations. In this work, excess heat capacity is measured for equimolar single-phase mixtures of alcohols and ILs, and it differs largely in its sign and magnitude for individual heptanol isomers. Temperature trends of the excess heat capacities suggest that the stability of hydrogen bonding for individual heptanol isomers is temperature-shifted, based on their capability of hydrogen bonding. The predicted hierarchy of hydrogen bonding in individual alcohols and its impact on the excess heat capacity trends are qualitatively confirmed via thermodynamic modelling of the associative contribution to the excess heat capacities. These terms are found to predetermine the observed non-monotonous excess heat capacity trends.

Structure and Glass Transition Temperature of Amorphous Dispersions of Model Pharmaceuticals with Nucleobases from Molecular Dynamics

Glass transition temperature (Tg) is an important material property, which predetermines the kinetic stability of amorphous solids. In the context of active pharmaceutical ingredients (API), there is motivation to maximize their Tg by forming amorphous mixtures with other chemicals, labeled excipients. Molecular dynamics simulations are a natural computational tool to investigate the relationships between structure, dynamics, and cohesion of amorphous materials with an all-atom resolution. This work presents a computational study, addressing primarily the predictions of the glass transition temperatures of four selected API (carbamazepine, racemic ibuprofen, indomethacin, and naproxen) with two nucleobases (adenine and cytosine). Since the classical non-polarizable simulations fail to reach the quantitative accuracy of the predicted Tg, analyses of internal dynamics, hydrogen bonding, and cohesive forces in bulk phases of pure API and their mixtures with the nucleobases are performed to interpret the predicted trends. This manuscript reveals the method for a systematic search of beneficial pairs of API and excipients (with maximum Tg when mixed). Monitoring of transport and cohesive properties of API-excipients systems via molecular simulation will enable the design of such API formulations more efficiently in the future.