Controlling packing structure of hydrophobic alkyl tails of monolayered films of ion-paired macrocyclic amphiphiles as studied by sum-frequency generation spectroscopy

Two kinds of monolayered films of ion-paired macrocyclic amphiphiles formed from anionic surfactant 1,2-bis[(dodecyloxy)carbonyl]ethane-1-sulfonate and tetracationic macrocycles with different molecular sizes were prepared on hydrophilic silica plates by the Langmuir-Blodgett technique. Vibrational sum-frequency generation (SFG) spectroscopy was applied to investigate the packing structure of the alkyl chains of the films. The SF intensity ratio of the CH symmetric stretching mode of the methylene group to that of methyl group in the alkyl chains increased when the occupied area of the macrocycle was larger than the sum of the cross-sectional areas of the hydrophobic alkyl tails. The appearance of a methylene CH band in the SFG spectra implies the presence of gauche conformation in the alkyl chains. This indicates that the molecular size of the cyclic tetracation hydrophilic head exclusively governs the packing structure of the alkyl group hydrophobic tails in the monolayer state. The results show that monolayer fabrication using ion-paired macrocyclic amphiphiles has advantages in allowing control over the packing structure of the outermost alkyl chains of solid surfaces through the choice of the size of the tetracation.