Activity coefficients of binary methanol alcohol mixtures from cluster weighting

Abstract

The hydrogen bond network of different small alcohols is investigated via cluster analysis. Methanol/alcohol mixtures are studied with increasing chain length and branching of the molecule. Those changes can play an important role in different fields, including solvent and metal extraction. The extended tight binding method GFN2‐xTB allows the evaluation and geometry optimization of thousands of clusters built via a genetic algorithm. Interaction energies and geometries are evaluated and discussed for the neat systems. Thermodynamic properties, such as vaporization enthalpies and activity coefficients, are calculated with the binary quantum cluster equilibrium (bQCE) approach using our in‐house code peacemaker 2.8. Combined distribution functions of the distances against the angles of the hydrogen bonds are evaluated for neat and mixed clusters and weighted by the equilibrium populations achieved from bQCE calculations.

Hydrogen Bonding and Vaporization Thermodynamics in Hexafluoroisopropanol‐Acetone and ‐Methanol Mixtures. A Joined Cluster Analysis and Molecular Dynamic Study

“…Using the bQCE method, we can thus calculate properties such as the enthalpy and entropy of vaporization and as simple difference of the liquid phase and a gas phase reference. Already in earlier works we were able to establish our procedure of using a so‐called QCE 0 calculation, wherein a mf is set to 0 removing all inter‐cluster interactions, as gas phase reference [27,32,72] . Table 4 compares the calculated vaporization enthalpies and entropies of the neat systems to their experimental reference values at room temperature and the boiling point.…”

“…As a first step, to find clusters, we performed a global minimum structure search for each cluster size and composition by running the genetic optimization algorithm OGOLEM [50,51] at the force field level of theory, using the AMBER 2016 program package [52] and the GAFF force field [53] implemented therein. The AMBER/OGOLEM combination is optimized to screen a great number of individual clusters [27,32,38] . The number of individuals per generation was varied between 80 and 240 in accordance to the cluster size to adjust for the increasing complexity.…”

“…wherein b xv is the empirical exclusion volume parameter to correctly scale the cluster volume , which is sensitive to the choice of the volume method and the atomic radii used therein. In previous works, b xv was treated as temperature independent [27,32,33,35,38] . Here, we introduce a linear temperature dependence of b xv …”

“…However, since their experimental detection and analysis can still be challenging and expensive, computational tools may be valuable for this kind of investigation [24–26] . We were able to calculate the activity coefficients of methanol in its binary mixtures with a set of small alcohols, which allowed some insight into how the chain length and branching modify the hydrogen bond network and consequently affect their behavior [27] . Since it is fundamental to study the hydrogen bond network in binary organic solvents, it is interesting to investigate the behavior of HFIP with both an hydrogen bond acceptor (acetone) and a compound able to work both as hydrogen bond acceptor and donor (methanol).…”

“…Boltzmann and bQCE weighting were compared in the past by our group in the field of the computational calculation of vibrational circular dichroism spectra of bulk phase, showing the advantages of our method over Boltzmann weighting [29] . For this purpose we use our in‐house code Peacemaker 2.8, which has proven to be a valuable tool to investigate the thermodynamic properties of both neat and mixed systems [27,30–38] . The standard bQCE method involves optimization of two empirical parameters by fitting them to a set of experimental data such as boiling points or isobars; however, even for simple mixtures of organic solvents, these data are often unavailable in the literature.…”