Interfacial Behaviors of Giant Amphiphilic Molecules Composed of Hydrophobic Isobutyl POSS and Hydrophilic POSS Bearing Carboxylic Acid Groups at the Air-Water Interface

The behavior of giant amphiphilic molecules at the air-water interface has become a subject of concern to researchers. Small changes in the molecular structure can cause obvious differences in the molecular arrangement and interfacial properties of the monolayer. In this study, we have systematically investigated the interfacial behaviors of the giant amphiphilic molecules with different number of hydrophobic BPOSS blocks and one hydrophilic ACPOSS block ((BPOSS)_n-ACPOSS (n = 1-5)) at the air-water interface by the surface pressure-area (π-A) isotherm, Brewster angle microscopy (BAM), compression modulus measurement, and hysteresis measurement. We found that both the number of BPOSS blocks and the compression rate can significantly influence the interfacial behaviors of giant molecules. The π-A isotherms of giant molecules (BPOSS)_n-ACPOSS (n = 2-5) exhibit a “cusp” phenomenon which can be attributed to the transition from monolayer to multilayer. However, the cusp is dramatically different from the “collapse” of the monolayer studied in other molecular systems, which is highly dependent on the compression rate of the monolayer. In addition, the compression modulus and hysteresis measurements reveal that the number of BPOSS blocks of (BPOSS)_n-ACPOSS plays an important role in the static elasticity, stability, and reversibility of the Langmuir films.