Binding studies of cyclodextrin-surfactant complexes
Wilson, Lee Douglas
This dissertation deals primarily with a comprehensive study of the complexes formed between cyclodextrin (host) and surfactant (guest) systems in aqueous solution (cf. Scheme 1).* Scheme 1. The formation of an host-guest inclusion complex where Ki (i = 1:1) is the equilibrium binding constant and i denotes the host-guest stoichiometry. Thecyclodextrin compounds studied are: α-CD, β-CD, 6-O-(2-hydroxypropyl) β-CD (HP-β- CD), 2,6-di-O-methyl β-CD (DM-β-CD), 2,3,6-tri-O-methyl β-CD (TM-β-CD), and randomly methylated β-CD (RAMEB). The hydrocarbon (hc) and fluorocarbon (fc) guest systems consisted of a homologous series of sodium alkyl carboxylate salts [CxH2x+1CO2Na, x = 5, 7, 9, 11, 13] and a series of sodium perfluoroalkyl carboxylate salts [CxF2x+1CO2Na, x = 1, 3, 6-9]. The use of thermodynamic and spectroscopic techniques has provided information about the magnitude of the binding constant, the type of host-guest stoichiometry, and the host-guest inclusion mode for the complexes studied here. In all cases,quantitative analysis of the data involved the use of different models to represent complexed and uncomplexed species according to the host-guest stoichiometry such as 1:1, 1:1 plus 2:1, and 1:1 plus 1:2 complexes. This study has contributed to a further understanding of the factors that govern the stability of CD-surfactant complexes and the calculation of binding constants from different physical techniques. The main source of complex stability is the hydrophobic effect as shown by the dependence of Ki on alkyl chain length of the surfactant, the greater binding affinity of fc surfactants relative to hc surfactants with a common host, and the linear relation observed between -log K1:1 and log CMC of the surfactants. The various types of host-guest stoichiometry and inclusion modes illustrate that complex formation can maximize hydrophobic and hydrophilic interactions within the CD interior and at the host-guest interface while minimizing unfavourable hydrophobic hydration of the surfactant alkyl chain. The results indicate that desolvation and solvation processes play a significant role in the formation of CD-S complexes, as expected for processes involving hydrophobic hydration and hydrophobic interactions. The significance of the results in this study can be extended to numerous fields that involve the exploitation of noncovalent host-guest interactions, e.g.; (1) quantification for biochemical model systems, (2) development of drug delivery systems, (3) selective phases for affinity chromatography, (4) new analytical and diagnostic procedures, (5) development of molecular sensors and switches, and (6) catalysis and synthesis in water as a solvent. *Please refer to dissertation for diagrams.