Formation and growth of polyelectrolyte-surfactant complexes: An in-depth structural and thermodynamic investigation
The proposed dissertation research project will examine the behavior of polyelectrolytes in the presence of oppositely charged surfactants. When polyelectrolytes and surfactants of opposite charge are mixed at a stoichiometric charge ratio, they self-assemble into highly ordered complexes. These types of assembly processes have been well documented in the past, but a great deal still remains to be understood.
The proposed research will specifically be related to the examination of the thermodynamics associated with polymer/surfactant interactions, the morphological sensitivity to surfactant concentration, and the overall effect of ionic strength on aggregation. We have chosen to study a number of polyanions, of varying hydrophobic character and charge density to determine how they will interact with the cationic surfactant, cetyltrimethylammonium chloride (CTAC). Structural evaluation of complexes will be accomplished using small-angle neutron scattering (SANS) to examine soluble complexes, while insoluble precipitates will be examined through small-angle X-ray scattering (SAXS).
Thermodynamic investigations will be carried out through the use of isothermal titration microcalorimetry in conjunction with potentiometry. The enthalpic data is of much greater value when combined with measured changes in surfactant chemical potential during the binding process. The change in chemical potential can be used to calculate the total change in free energy for the surfactant during the binding process. Combining this information with the free enthalpy change will provide the means necessary to describe the full thermodynamic profile of polyelectrolyte/surfactant binding. We will achieve these measurements through the use of a surfactant selective electrode that directly allows for measurement of the free surfactant concentration.
Both the thermodynamics of binding and the structural evolution will be greatly influenced by the concentration of small-molecule electrolytes present in the solutions. By varying the ionic strength during these experiments, we would also like to be able to quantify the contribution made by counter-ion release to the binding process. In a similar fashion, the effect of polyelectrolyte charge density will also be examined, to address the effect of localized charge density on the driving force for surfactant binding.