Non-Newtonian viscous shear thinning in ionic liquids

Abstract

We have studied the shear thinning of several protic and aprotic ionic liquids (ILs) by stress-controlled rheometry, which indicated that some ILs exist as liquid phase aggregates that can be disrupted or broken at high shear rates. Increasing the temperature shifted the onset of shear thinning to higher frequency and indicated that a smaller or less cohesive aggregate was formed. However, upon application of a sufficiently high temperature, the sample resumed the characteristics of a Newtonian fluid. (HOEt)2NH.AcOH was selected as an example of a protic ionic liquid (PIL) to probe concentration effects on liquid ordering. At water concentrations lower than one equivalent per PIL component pair, the viscosity rapidly decreased, although the onset frequency of shear thinning remained constant. The shear thinning and low viscosity regime remained until approximately 3–4 equivalents of water were added and the PIL behaved as a Newtonian fluid. The effect of water on the shear thinning of (HOEt)2NH.AcOH indicated that small dilution volumes did not alter the aggregate structure, but instead reduced the number of aggregates. Both the temperature and additive water strongly suggested that the IL aggregates were due to intermolecular hydrogen bond networks.