A study of hydrophobically modified polysaccharides adsorbed onto hydrophobic surfaces
The use of a non-ionic polymer as a primary dispersion stabilizer has been shown theoretically to be feasible, yet experimentally has not been accomplished. This research is designed to study the effects of polymer molecular weight, backbone flexibility and hydrophobe length and percent on the ability of the polymer to stabilize an emulsion/dispersion. Dextran polymers were chosen for the intended research and were hydrophobically modified under homogeneous conditions with aliphatic isocyanates of variable lengths. Two molecular weights were studied. Several experiments were conducted on adsorbed polymers at hydrophobic surfaces. Determination of c* and solution rheology measurements provided information about the polymers in solution. Phase diagrams, interfacial tension experiments and rheological studies of emulsions were conducted at the oil/water interface. At the solid interface, adsorption isotherms were constructed, the bound fraction determined and rheological studies conducted for polymers adsorbed onto polystyrene latices. With regards to the hydrophobe length, it was determined that the longer hydrophobe formed stronger associations in solution as well as at the interface whereas the C8 hydrophobes formed weaker anchors. Polymers modified with the C18 hydrophobe filled the interface at lower concentrations and showed a lower bound fraction. The C18 polymers also showed a lower packing fraction with the latices and a thicker adsorbed layer at that packing fraction. Phase diagrams showed that C18 polymers were only better at low polymer concentrations and oil percentages. However, the longer hydrophobe is necessary to produce a dispersed emulsion. Lower molecular weight polymers were not capable forming a dispersed emulsion and showed lower viscoelastic moduli. The flexibility of the polymer backbone appeared to affect the region of emulsion stability. Data suggests that the flexible dextran polymers form micelle structures in solution and possibly at the interface, with the C18 modified polymers forming tighter micelles than the polymers with the C8 hydrophobes. We believe the C18 modified polymers possess a stronger drive to associate, thereby adopting a kinetic conformation instead of a more thermodynamically favorable conformation.