Enhanced flocculation of oil-in-water emulsions by hydrophobically modified chitosan derivatives

Flocculation efficiency of hydrophobically modified (HM) chitosan derivatives in comparison with commercial cationic polyacrylamide flocculant and unmodified (UM) chitosans varying in molecular weights and acetylation degrees has been evaluated in sodium dodecyl sulfate (SDS)-stabilized and surfactant-free oil-in-water emulsions at pH 4-9. It was found that due to the synergetic effect of cationic and hydrophobic functionalifies HM-chitosans provide superior phase separation in SDS-stabilized systems at twice- and four-fold lower doses than UM-chitosan and cationic polyacrylamide, respectively. For this type of emulsions, pH had little influence on the efficiency of phase separation. In surfactant-free emulsions with low surface charge density of oil droplets, hydrophobic substitution had weaker effect on chitosan flocculation performance. Complete phase separation was reached only at pH > 7.5, and at UM and HM chitosan doses considerably higher than it was required for surface charge neutralization. This suggests that in surfactant-free emulsions hydrophobic interactions could not compensate a dramatic decrease of electrostatic attraction between polymers and droplet surface, thus, flocculation occurred mainly through the sweep floc process in contrast to the charge patch mechanism in SDS-stabilized systems. (c) 2005 Elsevier B.V. All rights reserved.