Effect of ion structure on the physicochemical properties and gas absorption of surface active ionic liquids

TL;DR

This study explores how the molecular structure of surface active ionic liquids influences their physical properties and gas absorption capabilities, revealing structure-property relationships crucial for their application development.

Contribution

It provides detailed insights into how different ionic structures affect physicochemical properties and gas absorption in SAILs, filling a significant knowledge gap in their characterization.

Findings

Linear alkyl chains lead to more compact packing and smaller molar volumes.

SAILs are about 100 times more viscous than conventional ionic liquids.

Larger non-polar domains enhance gas absorption capacity.

Abstract

Surface active ionic liquids (SAILs) combine useful characteristics of both ionic liquids (ILs) and surfactants, hence are promising candidates for a wide range of applications. However, the effect of SAIL ionic structures on their physicochemical properties remains unclear, which limits their uptake. To address this knowledge gap, in this work we investigated the density, viscosity, surface tension, and corresponding critical micelle concentration in water, as well as gas absorption of SAILs with a variety of cation and anion structures. SAILs containing anions with linear alkyl chains have smaller molar volumes than those with branched alkyl chains, because linear alkyl chains are interdigitated to a greater extent, leading to more compact packing. This interdigitation also results in SAILs being about two orders of magnitude more viscous than comparable conventional ILs. SAILs at the liquid-air interface orient alkyl chains towards the air, leading to low surface tensions closer to n-alkanes than conventional ILs. Critical temperatures of about 900 K could be estimated for all SAILs from their surface tensions. When dissolved in water, SAILs adsorb at the liquid-air interface and lower the surface tension, like conventional surfactants in water, after which micelles form. Molecular simulations show that the micelles are spherical and that lower critical micelle concentrations correspond to the formation of aggregates with a larger number of ion pairs. $\mathrm{CO_{2}}$ and $\mathrm{N_{2}}$ absorption capacities are examined and we conclude that ionic liquids with larger non-polar domains absorb larger quantities of both gases.