Surfactants are molecules that compatibilize gases, liquids, and solids. Common classes include foaming agents (gas-liquid), emulsifiers (liquid-liquid), and dispersants (solid-liquid). While classical surfactants are based on polar head groups, which can be anionic, cationic, or non-ionic, with non-polar tails, polymers or nanoparticles often have excellent properties. Different lignin types have been explored for use as foaming agents and emulsifiers, but they generally have low interfacial activities and result in only modest changes in air-water surface tension and are not especially effective at stabilizing oil-water emulsions at neutral pH.
However, it was shown that grafting polymers onto silica nanoparticles significantly enhances their effectiveness in forming Pickering emulsions, and recent studies in the Washburn Lab have shown that similar dramatic improvements in performance are possible with polymer-grafted lignin nanoparticles. Cyclohexane-in-water emulsions with droplet size that depends on polymer grafting density can be formed from kraft lignin grafted with polyacrylamide, and these emulsions are stable against coalescence for months.
Images of cyclohexane-in-water emulsion prepared using kraft lignin grafted with polyacrylamide having degree of polymerization 100 and grafting densities of 2/lignin particle (left) and 16/lignin particle (right).
Particles as surfactants—similarities and differences. B. P. Binks, Current Opinion in Colloid & Interface Science 2002; 7: 21–41 (2002).
Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces. N. J. Alvarez, S. L. Anna, T. Saigal, R. D. Tilton, and L. M. Walker, Langmuir 2012; 28: 8052–8063.
Polymer-Grafted Lignin Surfactants Prepared via Reversible Addition—Fragmentation Chain-Transfer Polymerization. C. Gupta and N. R. Washburn, Langmuir 2014; 30: 9303–9312.