TOWARD A DETERMINATION OF THE CRITICAL SIZE OF ICE NUCLEI - A DEMONSTRATION BY GRAZING-INCIDENCE X-RAY-DIFFRACTION OF EPITAXIAL-GROWTH OF ICE UNDER THE C31H63OH ALCOHOL MONOLAYER

The structure of an uncompressed monolayer of C31H63OH, which induces freezing of supercooled water drops at similar to-2 degrees C, was studied by grazing incidence X-ray diffraction in the temperature range +6 degrees C to freezing of the water subphase. As the water was cooled, the hydrocarbon chains underwent a change in tilt angle. The monolayer preserved its structure on the epitaxially-grown surface of hexagonal ice (after freezing) and, after the ice was melted, subsequent to the temperature being raised above 0 degrees C. The coherence length of the observed ice crystallites parallel to the interface was about 25 Angstrom, which is in agreement with the extent of match between the lattice of the monolayer and the ab lattice of ice. This yields some information on the critical size of the ice nucleus bound to the monolayer, just below 0 degrees C.