AMPHIPHILIC MONOLAYERS OF INSOLUBLE CYCLODEXTRINS AT THE WATER AIR INTERFACE - SURFACE PRESSURE AND SURFACE-POTENTIAL STUDIES

The monolayer properties of amphiphilic cyclodextrins esterified at positions 2 and 3 have been assessed by surface pressure and surface potential measurements at a constant area. The cyclodextrins (CDS) possessing six (alpha), seven (beta), and eight (gamma) linked glucopyranose units with C-14 hydrocarbon chains have been examined. In a second experiment, the modified beta cyclodextrins with the chain length varying from 2 to 14 carbons (beta C-2, beta C-6, beta C-8, beta C-10, beta C-12, and beta C-14) have also been characterized. The beta C-2 system is best considered apart from the other molecules, belonging to the class of amphiphilic cyclodextrins in which the organizational properties are related to the presence of a short impermeable hydrophobic layer parallel to the aqueous surface. The surface pressure (II)-surface density (delta) relationships reveal that beta C-6 is surprisingly the most surface active molecule as compared to other studied CDs, and yet this molecule appears to display the lowest value of the maximum effective surface potential. The sharp differences between this molecule and beta C-8 arise from the different packing at the interface, with beta C-6 in a close packed arrangement (A = 1.8 times the effective diameter of the beta CD area itself); in contrast, beta C-8 packs in an open arrangement (A = 2.8 times the effective diameter of the beta CD area itself). The observed surface properties of modified beta CDs are analyzed in terms of hydrocarbon chain orientation, coordination of water between the molecules, and the interaction of this surface layer with the strata of water molecules extending into the bulk phase. The effective dipole moment of each studied CD was calculated as well as the dipole moment corresponding to one aliphatic chain and one glucopyranose unit. The role of the symmetry of the cyclodextrin molecules upon their surface properties is discussed for the alpha C-14, beta C-14, and gamma C-14 series, in which it is observed that the 7-fold geometry of beta C-14 is unfavorable for packing at the water/air interface.