A characteristic effect of pressure on the inclusion complexation of modified β-cyclodextrins with 4-substituted phenols

The equilibrium constants for the inclusion complexation of two kinds of modified β-cyclodextrins (heptakis-(2,6-di-O-methyl)-β-cyclodextrin (DM-βCD) and heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-βCD)) with 4-substituted phenols were determined at high pressures. A characteristic pressure effect was found in the inclusion equilibria of modified β-CDs. As the external pressure increases, the inclusion equilibrium constants for the phenols/TM-βCD system increased remarkably, while the effect of pressure for the phenols/DM-βCD system was small. The reaction volumes for the CD inclusion complexation with 4-substituted phenols were estimated from the pressure dependence of the inclusion equilibria: ΔV = -14.9 ∼ -33.4 cm³ mol^-1 for the phenols/TM-βCD system and -2.7 ∼ 8.6 cm³ mol^-1 for the phenols/DM-βCD system. Analysis of the effect of pressure on the inclusion complexation with DM- and TM-βCDs revealed that there is a pressure-dependent competing complexation between included phenols and water molecules, and the number of water molecules included in the DM- and TM-βCD cavities in water was estimated.