Formation and structure of tunable supramolecular complexes
Fajalia, Ankitkumar I.
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The spontaneous organization in aqueous solution of amphiphilic molecules such as surfactants, and their interactions with other macromolecules present in solution is of great fundamental and technological interest. The self-assembly of amphiphiles and their interactions with other molecules is dictated by their inherent properties (e.g., molecular structure, chemical composition) as well as by their surroundings (e.g., solutes, co-solvents, temperature, pH). Such interactions and the effects of solution conditions on them have been addressed in this dissertation. The effects of cyclodextrins (CDs) on the formation and structure of common surfactant molecules, a single-tail surfactant, sodium dodecyl sulfate (SDS) and a double-tail surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been investigated. CDs are known to bind to surfactant molecules (forming inclusion complexes) below the surfactant critical micelle concentration (CMC); however, CD interactions with surfactant micelles (above the CMC) are still not well understood. In particular, direct structural evidence of CD localization in micellar solution is lacking. Direct evidence of CD location in the micelle core using small angle neutron scattering (SANS) contrast matching has been provided here. Furthermore, the effects of cyclodextrin type and concentration on the aggregation behavior of the surfactants, the micelle structure, and the inter-micelle interactions have been discussed. Such results are particularly important in pharmaceutical formulations where CDs are utilized for encapsulation of actives. The dissertation has further explored the effects of polar organic solvents on polymer-surfactant interactions. Mixtures of surfactants and polymers are widely used in aqueous media to provide structure, solubilization domains and/or colloidal stability. The solution behavior and interactions between polymers and surfactants can be profoundly affected by the presence in water of polar organic solvents, which are ubiquitous in pharmaceutical, coating, and home/personal care formulations. Specifically, the effect of ethanol and/or isopropanol on the association behavior of two different water soluble polymers (1) ethyl (hydroxyethyl) cellulose (EHEC) and (2) poly (ethylene oxide) (PEO), in the presence of surfactant sodium dodecyl sulfate were investigated. Addition of alcohol was found to alter both the polymer and the micelle structure and the interactions between them as evidenced by small-angle neutron scattering (SANS) measurements. Finally, the Layer-by-Layer (LbL) technique was capitalized upon to generate stimuli-responsive multifunctional micro-carriers suitable for dual drug delivery applications. Mono-dispersed meso-porous silica spheres with high surface areas and large pores were synthesized and were subsequently loaded with a model drug, aspirin. The aspirin-loaded silica particles were then coated via LbL with polyelectrolyte/polypeptide/cyclodextrins based multilayers, embedding another model drug in form of inclusion complex with the cyclodextrins, thus, proving a dual cargo carrying capability in these particles. The stimuli-responsive release of the encapsulated model drugs from the micro-carriers was successfully demonstrated and their observed release mechanisms have been elucidated.