A novel rheological strategy to determine the critical solution temperature

This work presents a novel rheological strategy to determine more accurately and precisely the critical solution temperature of a mixture. The strategy uses the ratio between the loss and the elastic modulus to determine the inflection point, which signifies a considerable change in rheological behaviour. Such a change occurs when the system changes from monophasic to biphasic, hence at the critical solution temperature. Most literature reports the critical solution temperature with a precision of one Kelvin, however the strategy presented in this work can determine this temperature with a precision of 0.1 Kelvin. The methodology has been applied to a system incorporating a switchable deep eutectic solvent consisting of lidocaine and oleic acid together with a pure aqueous phase. Apart from this pure aqueous phase, aqueous metal salt solutions were also used, which incorporated kosmotropic or chaotropic anions to study the Hofmeister effect. This effect influences the point where a critical solution temperature is present. • Rheology measurements were used to more accurately determine the critical solution temperature of a DES. • The switch of the system from biphasic to monophasic can be monitored accurately through changes in rheological behaviour. • The Hofmeister effect influences the critical solution temperature. • High efficiency copper extraction to the DES was observed.