Characteristic two-step phase transitions in chloro(dimethyl)(octadecyl) silane monolayers following Ostwald's step rule

For fundamental understanding of spread monolayers of chloro(dimethyl)(octa decyl) silane (CDMOS), the effects of temperature and of compression strain rate on the properties and morphologies of the monolayers have been system atically studied with Brewster angle microscope (BAM). pi-A isotherms and B AM images clearly demonstrate that the CDMOS monolayers undergo characteris tic two-step phase transitions during compression at a very narrow temperat ure range around 20.0 degrees C, but above and below that temperature range only one-step phase transitions appear. This unusual two-step phase transi tion is explained by the well known Ostwald's step rule. When the compressi on strain rate becomes ten times slower, so-called solid film regions on th e isotherms disappear, indicating that the shape of the pi-A isotherms are determined by the balance between the relaxation times of the molecular pro cesses in the monolayers during compression and the times of observation (t (ob)). Change of condensed phase domain size is considered in relation to t he degree of supersaturation during compression. (C) 1999 Elsevier Science B.V. All rights reserved.