Monolayer behavior of [6-deoxy-6-S-phenyl]-alpha-, beta-, and gamma-cyclodextrins at the air-water interface

Characteristics of monolayers of 15 new amphiphilic alpha-, beta-, and gamma-cyclodextrins at the air-water interface are reported. The modification involves substitution at the positions C-6 of the cyclodextrin with S-phenyl, S-(4-bromophenyl), S-(4-n-butoxyphenyl), S-(4-n-pentylphenyl), and S-(4-nitrophenyl). Even simple thiophenol-substituted cyclodextrins exhibit good amphiphilic properties, and the sequence of increasing degree of organization and stability of the monolayers is given. Increasing hydrophobicity of the group attached to the position C-6 results in the increase of the stability of the monolayer. However, for highly substituted cyclodextrins agglomeration of the molecules at the air-water interface was observed. This problem displayed by mean molecular areas smaller than those obtained from the crystal structures can be overcome by using more diluted spreading solutions. The beta-series was found to deviate from other cyclodextrins in that the collapse surface pressure is higher than and almost independent of the substituent. This was ascribed to the 7-fold symmetry of beta-cyclodextrin molecules.