Solvation of apolar compounds in protic ionic liquids: the non-synergistic effect of electrostatic interactions and hydrogen bonds

Abstract

The solvation properties of protic ionic liquids such as alkylammonium salts are still virtually uncharacterized. Both electrostatic interactions between charged particles and hydrogen bond networks in a solvent are known to hinder the solubility of apolar species. Protic ionic liquids can be a priori expected to dissolve hydrocarbons worse than aprotic ionic liquids which do not form hydrogen bonds between the ions. We measured the limiting activity coefficients of several alkanes and alkylbenzenes in propylammonium and butylammonium nitrates at 298 K. Surprisingly, we observed the tendency of higher solubility than for the same compounds in aprotic ionic liquids with a similar molar volume. The calculations of the excess Gibbs free energies using test particle insertions into the snapshots of molecular dynamics trajectories reproduced lower values in protic rather than in aprotic ionic liquids for both methane molecules and hard sphere solutes. This can be explained by the favorable solvation of apolar species in the apolar domain of nanostructured PILs. For the first time, we point out at the essential difference between the solvation properties of two types of ionic liquids and prove that it arises from the cavity formation term.

Exploring the Gas-Permeation Properties of Proton-Conducting Membranes Based on Protic Imidazolium Ionic Liquids: Application in Natural Gas Processing

“…However, here we do observe a tendency of higher CH 4 solubility in comparison with similar aprotic ionic liquids. Sedov et al [ 17 ] explain this behaviour by a preferential cavity formation and favorable solvation of hydrocarbons in the apolar domain of nanostructured protic ionic liquids. Consequently, the higher CH 4 permeability values measured for SILM S prepared from protic ionic liquids would be expected.…”

“…The solubilities of CO 2 , CH 4 , C 2 H 6 , N 2 and O 2 in several aprotic ILs have been studied extensively [ 12 , 13 , 14 , 15 ]. On the contrary, the thermodynamic properties of protic ionic liquids, i.e., those comprising proton-donor and proton-acceptor centers in their molecules, have been investigated in lower extent [ 16 , 17 , 18 , 19 ].…”

Protic ionic liquids with primary alkylamine-derived cations: the dominance of hydrogen bonding on observed physicochemical properties

“…Extensive research has been performed to understand its fundamental properties. [5][6][7][8] Ludwig et. al.…”

Room Temperature Ionic Liquids from Purine and Pyrimidine Nucleobases

“…It has been shown that the PILs exhibit more interesting properties due to H‐bonding abilities of cations or anions and hence may have more potential applications in drug formulations . Physicochemical studies of PILs and other ionic liquids such as amino acid ionic liquids indicated that the cooperative H‐bonding is the main factor responsible for their observed high hydration numbers than those expected from the H‐bond donor acceptor sites available …”