Phase solubility studies of pure (-)-alpha-bisabolol and camomile essential oil with beta-cyclodextrin.

(-)-Alpha-bisabolol was found to form an inclusion complex with beta-cyclodextrin (beta-CD) in solution as well as in the solid state. To investigate molecular associations of beta-CD with pure (-)-alpha-bisabolol or (-)-alpha-bisabolol as a component of camomile essential oil, phase solubility studies were undertaken. A B(s) type solubility with an apparent complex constant of 273 M(-1) for the pure (-)-alpha-bisabolol and 304 M(-1) for (-)-alpha-bisabolol as a constituent of the essential oil were obtained. The two curves in the phase solubility diagram reach their plateau at different concentrations of (-)-alpha-bisabolol, 7.04 x 10(-4) M for the pure substance and 2.88 x 10(-4) M for the substance as a component of the essential oil. Although the shapes of the curves are almost similar, the intrinsic solubility's of pure (-)-alpha-bisabolol (4.85 x 10(-4) M) and (-)-alpha-bisabolol as a component of the essential oil (1.82 x 10(-4) M) differ significantly. An inclusion complex having a stoichiometric composition of 2:1 (beta-CD: drug) was obtained. A mechanism of complexation has been proposed on the basis of the stability constant calculated from phase solubility data and the stoichiometric ratio of the solid state complexation. Copyright 2003 Elsevier Science B.V.