(311d) Influence of Counterion Solvation on the Properties of Polyelectrolytes and It's Application in Hand Sanitisers

Hand sanitisers are an important tool to combat the spread of infectious diseases. They are typically composed of 80 v/v alcohol and 20 v/v of water at neutral pH. The most common alcohols used are ethanol and 2-propanol. Water-alcohol mixtures are low viscosity liquids, a feature that limits their efficacy. To overcome this problem, polymers can be added but the high alcohol content of hand sanitisers means that many common polymers, including most polyelectrolytes, cannot be used as thickening agents due to their poor solubility in alcohol media.

Here we show that it is possible to solubilise polyelectrolytes in alcohols (and other non-aqueous media) by increasing the affinity of counterions to the solvent. We focus on carboxymethylcellulose (CMC), an ionic derivative of cellulose which is widely employed in aqueous industrial formulations. While it’s sodium salt NaCMC is insoluble in alcohols and other organic solvents, it’s tetrabutylammonium salt (TBACMC) is found to be soluble in many organic media. This holds for other polyelectrolyte systems such as polystyrene sulfonate.

A study of the phase behaviour of TBACMC in over 50 solvents shows that counterion solvation and not counterion entropy is the main parameter determining the solubility of the TBACMC. Further, we investigate the structural and flow properties of this system using small angle scattering and rotational rheometry. TBACMC shows ‘solvophilic’ rheology in lower alcohols such as ethanol but tends gel in higher alcohols such as 1-pentanol or 1-hexanol. We establish the dependence of the overlap concentration, correlation length and solution viscosity as a function of solvent dielectric constant and find that our results strongly deviate from the prediction of scaling and other theories of polyelectrolyte solutions. These findings highlight the importance of accounting for the role of counterion solvation when modelling the behaviour of ion-containing polymers in solution.