Reversible liquid-liquid transitions in the early stages of monolayer self-assembly

Time-resolved atomic force microscopy, contact angle analysis, and X-ray ph otoelectron spectroscopy were employed to study the effect of cyclic heatin g and cooling of a partial octadecyltrichlorosilane (OTS) monolayer on the oxidized Si(100) surface. A reversible structural change of the partial mon olayer between high density and low density two-dimensional liquid phases w as observed as a function of temperature at constant coverage. These studie s show that the monolayer exists in a highly mobile hydrogen-bonded state a kin to the equilibrium state of Langmuir films at the air-water interface. The lifetime of the mobile state was measured to be on the order of several minutes, after which grafting and cross-linking immobilize the monolayer. The results show that the mobile state permits large scale rearrangements o f the molecules within the monolayers.