It has long been believed that the formation of Langmuir monolayers at the air-water interface is limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup, such as OH or COOH, which interacts with the subphase.['] These materials have pressure-area (17-A) isotherms which give an average area per molecule corresponding to monolayer coverage. Recently, isotherm and grazing-incidence X-ray diffraction data have been reported indicating monolayer formation for the non-amphiphilic linear fluorocarbon C,,F,, on water.['] In contrast, the longchain linear hydrocarbons CnHZn+,, 20&n& 30, spread on a water surface give a 17-A isotherm with an average area per molecule far too low to be consistent with a monolayer. They have therefore generally been believed not to form ordered crystalline films at the air-water interface.
The spontaneous formation and coexistence of crystalline polymorphic trilayer domains in amphiphilic films at air-liquid interfaces is demonstrated by grazing incidence synchrotron x-ray diffraction. These polymorphic crystallites may serve as models for the early stages of crystal nucleation and growth, helping to elucidate the manner in which additives influence the progress of crystal nucleation, growth, and polymorphism and suggesting ways of selectively generating and controlling multilayers on liquid surfaces. Auxiliary molecules have been designed to selectively inhibit development of the polymorphs, leading primarily to a single phase monolayer.
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