Polyelectrolytes in alcohol-water mixtures: From counterion solvation to hand sanitisers

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Hand sanitisers are an important tool to combat the spread of infectious diseases. Unlike hand-washing with soap, they can be readily used without the need of a running water source. Hand sanitisers 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 isopropanol. Water-alcohol mixtures are low viscosity liquids, a feature that limits their efficacy. To overcome this problem, polymers could be added, however, the high alcohol content of hand sanitisers means that common polymers cannot be used as thickening agents due to their poor solubility in alcohol media. We propose a simple method to improve the solubility of polyelectrolytes in alcohol-water mixtures by using hydrophobic counterions. Polyelectrolytes are already in wide use as gelling agents in pharmaceutical and food products in water-based formulations. Preliminary experiments show that while the sodium salt of carboxymethyl cellulose is insoluble in practically all non-aqueous solvents, its tetrabutyl-ammonium salt readily dissolves in alcohols and alcohol-water mixtures and other organic media. The ion-exchange approach can in principle be used for any ionic (bio-)polymer and has the advantage that it does not require the synthesis of new polymers. Being applicable to a large number of polymers, including some very widely produced biopolymers, the issue of supply in case of a sudden spike in demand can be tackled with the approach proposed. Carboxymethyl cellulose or alginate are already approved by the Federal Drug Agency and European Medicine Agency as are many quaternary ammonium salts. From the fundamental perspective, solutions of polyelectrolytes with hydrophobic ions in alcohols (and other organic media) are interesting because their thermodynamic properties are dictated by energetic counterion-solvent interactions, instead of counterion entropy, as is common for most systems studied thus far. Recent developments in simulation techniques, particularly by Douglas and co-workers, have highlighted the important role of counterion solvation. Counterion solvation has therefore attracted much attention in the last five years and it is thought to be a crucial factor in understanding some of the key conformational, scattering and rheological properties of polyelectrolytes which classical theories are unable to explain.The key challenges are to understand the influence of counterion entropy, counterion solvation, and polymer-solvent interactions on the phase behaviour, structure and rheology of polyelectrolytes in mixed alcohol-water solutions. While the work is motivated by the potential application of polyelectrolytes in hand sanitisers, the knowledge generated in this proposal will lead to a substantial increase in our fundamental understanding of polyelectrolyte thermodynamics.