Insights on the amphiphillicity of cellulose: competitive wetting by polar and apolar solvents

Due to its molecular structure, cellulose shows both hydrophilic and lipophilic behaviour. However, the relative importance of both affinities and the extend to which cellulose can be considered amphiphillic remains the subject of conflicting and opposite views. We address this question by considering the behaviour of cellulose when exposed to immiscible solvents, water and apolar solvents (toluene or decane). We have performed molecular dynamics (MD) simulations of the competition between water/toluene and water/decane to cellulose. We consider two cellulose surfaces (from different crystalline planes) with different roughness and different accessibility to hydrophilic and hydrophobic groups. We also use bacterial cellulose to perform wetting and solvent exchange experiments. Both MD and experimental results show that cellulose has high affinity for all solvents considered. The MD simulations for the studied cellulose surfaces predict a clear preference for water over toluene. Toluene can be replaced by water but a water hydration layer cannot be replaced by toluene, an effect that can be traced back to the molecular features of cellulose. This preference is confirmed experimentally by solvent exchange experiments. In the case of water-decane mixtures, MD simulations indicate that the competition between decane and water is more complex, due to the high surface tension of the water-decane interface.