Partitioning and transport in complex nano-structured systems: Gradient diffusion of ionic micelles in gels and partitioning of hydrophobic aroma compounds

With the overall goal of advancing nano-scale encapsulation technology, two companion problems in the controlled release of small hydrophobic molecules have been studied: equilibrium partitioning and colloidal transport, each within nanostructured matrices.;In the first research focus, a solid phase microextraction (SPME) method was developed to measure air--water--surfactant micelle partitioning of hydrophobic analytes. Vapor--liquid partitioning (Kv ℓ) measurements were performed independently in the headspace (HS-SPME) and via direct immersion in the liquid (DI-SPME), and the results were compared. The method involves varying the total amount of analyte as well as the ratio of vapor to liquid in the closed, static system, such that the need for an external calibration is eliminated. The compounds studied cover four orders of magnitude in Kv ℓ, and agreement between DI-SPME and HS-SPME results was good, showing that these two methods were capable of providing accurate, complementary measurements. Equilibrium partitioning of limonene to sodium dodecyl sulfate (SDS) micelles was also measured using HS-SPME by varying the concentration of SDS. By fitting the data to a simple model, the cmc was accurately measured and the micelle--liquid partition coefficient was determined.;In the second research focus, the diffusion of SDS in solution and in agarose gel was measured and compared with an a priori model for colloidal transport which invokes hydrodynamic and statistical thermodynamic arguments to account for micelle--micelle and micelle--gel fiber interactions. Experimental results show that the concentration effect is enhanced considerably by the strong charge associated with SDS micelles. At high ionic strength, this concentration effect is further enhanced in 1% agarose gel, but in 2% gel the trend reverses, strongly suggesting a decrease in the micellar aggregation number. At low ionic strength, a concentration effect with a pronounced second order dependence is evident. Results show that the diffusion coefficient does not change appreciably with gel concentration up to 2%, and theoretical analysis suggests that this is due to partial canceling between the effects of solute--gel and solute--solute interactions. The pronounced second-order dependence is explained qualitatively by the changing Debye length and its influence on the thermodynamic driving force.