We have investigated with titration microcalorimetry beta-cyclodextrin with 3-alkoxyl-2-hydroxypropyl trimethyl ammonium bromides, which is a new surfactant synthesized in this laboratory, in aqueous solutions at a temperature of 298.15 K. The results are discussed in the light of amphiphilic interaction and the complex formed of water molecules around a hydrophobic surfactant tail. 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 + O-l, increases from 8 to 14. All the complexes are stable with the apparent experimentally determined stability constants of beta(1) = 1.08 (.) 10(3) dm(3) (.) mol(-1), beta(1) = 28.66 (.) 10(3) dm(3) (.) mol(-1) and beta(2) = 141.9 (.) 10(3) dm(6) (.) mol(-2) for n = 8, 12 and 14, respectively. The enthalpy decreases with increasing n of the hydrophobic chain. (c) 2004 Elsevier Ltd. All rights reserved.