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

The monolayer properties of amphiphilic cyclodextrins esterified at po sitions 2 and 3 have been assessed by surface pressure and surface pot ential measurements at a constant area. The cyclodextrins (CDS) posses sing six (alpha), seven (beta), and eight (gamma) linked glucopyranose units with C-14 hydrocarbon chains have been examined. In a second ex periment, the modified beta cyclodextrins with the chain length varyin g 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 c lass of amphiphilic cyclodextrins in which the organizational properti es are related to the presence of a short impermeable hydrophobic laye r parallel to the aqueous surface. The surface pressure (II)-surface d ensity (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 effe ctive surface potential. The sharp differences between this molecule a nd beta C-8 arise from the different packing at the interface, with be ta C-6 in a close packed arrangement (A = 1.8 times the effective diam eter of the beta CD area itself); in contrast, beta C-8 packs in an op en arrangement (A = 2.8 times the effective diameter of the beta CD ar ea itself). The observed surface properties of modified beta CDs are a nalyzed in terms of hydrocarbon chain orientation, coordination of wat er between the molecules, and the interaction of this surface layer wi th the strata of water molecules extending into the bulk phase. The ef fective dipole moment of each studied CD was calculated as well as the dipole moment corresponding to one aliphatic chain and one glucopyran ose unit. The role of the symmetry of the cyclodextrin molecules upon their surface properties is discussed for the alpha C-14, beta C-14, a nd 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.