A microcalorimetric study of host-guest complexes of α-cyclodextrin with alkyl trimethyl ammonium bromides in aqueous solutions

Interactions between alpha-CD and three alkyl trimethyl ammonium bromides, a homologues series of surfactants, in aqueous solutions have been investigated with titration microcalorimetry at 298.15 K. The results are discussed in the light of the amphiphilic interaction and the iceberg structure of water molecules existing around the hydrophobic tail of the surfactant. The stoichiometry of the host - guest complex changes from 1: 1 to 2: 1, as the number of carbon atoms (n) in the hydrophobic chain, CnH2n+1, increases from 8 to 14. All the complexes are quite stable, with the apparent experiential stability constants being beta(1) = 2.65 x 10(3) dm(3)-mol(-1), beta(2) = 4.85 x 10(6) dm(6)- mol(-2), beta(2) = 6.50 x 10(6) dm(6)- mol(-2), respectively for n = 8, 12, 14. All the complexation processes are shown to be enthalpy driven, and the standard enthalpy effect (-Delta H-0) increases while standard entropy change (Delta S-0) decreases with elongation of the hydrophobic chain.