Dynamics of Two-Dimensional Self-Aggregation: Pressure and pH-Induced Structural Changes in a Fluorocarbon Amphiphile at Liquid-Air Interfaces. An X-ray Synchrotron Study

In order to provide, on the molecular level, information on crystal nucleation of monolayers at air-water interfaces, the self-aggregation of 1H,1H,2H,2H-perfiuorododecyl aspartate CF₃(CF₂)₉(CH₂)₂OCOCH₂CH(NH₃⁺)CO₂^- (PFA) over water subphases at various pH values was studied using synchrotron X-ray grazing incidence diffraction (GID), including Bragg rods (BR) and reflectivity (XR) measurements. Two-dimensional crystalline domains with coherence lengths exceeding 1500 Å were detected for low surfactant surface densities and zero surface pressure. GID measurements reveal structural changes with subphase pH and composition. Structural models are proposed at high, neutral, and low pH. For water subphases containing KOH at pH ≥ 11.2, the diffraction is consistent with molecules arranged in a hexagonal net and vertically aligned. Over pure water and acidic subphases containing HC1 at pH = 1.5, the molecules pack in a distorted hexagonal net with the fluorocarbon chains tilted from the vertical. The growth in time of the uncompressed crystallites over aqueous glycine solutions was directly monitored by GID. Compression and subsequent decompression of the monolayers over pure water and HC1 (pH = 1.5) subphases, for which the fluorocarbon chains are originally tilted, were found to reduce the crystallinity of the system considerably. By contrast, over KOH at pH ≥ 11.2, the hexagonal net with vertically aligned molecules is preserved at all surface pressures and the crystalline order of the system is reduced upon compression but increases again upon release of pressure. Estimates of the degree of crystallinity of the monolayer were made over water for various states of compression and over KOH at pH ≥ 11.2 in the uncompressed state. The packing characteristics and the dynamics involved in the formation and partial destruction of the crystallites can be understood in terms of interaction between the hydrophilic ionic head groups of the monolayer and, if present, the attached molecules or ions (water, K⁺ or Cl^-). Additional support for the packing arrangements proposed at high, neutral, and low pH was obtained from studies of the oriented growth of sodium chloride under PFA monolayers.