Preactivation in the nucleation of ice by Langmuir films of aliphatic alcohols

We report a study of preactivation in the heterogeneous nucleation of ice b y Langmuir films adsorbed at the surface Of water droplets. The Langmuir fi lms consisted of aliphatic alcohols with 21, 23, or 25 carbon atoms per mol ecules. We find that the mean freezing temperature T*(f) of the water dropl ets decreases abruptly as the preparation temperature T-prep is increased. T-prep is the highest temperature to which the droplets are heated since th e immediately preceding freezing event. Analysis of the distributions of fr eezing temperatures yields the dependence of thermodynamic parameters on T- prep, allowing us to identify the role of thermal history separately in the energy barrier and the attempt frequency for nucleation. We find that the change in T*(f) is exclusively due to a change in the energy barrier for nu cleation and that all preactivation characteristics are a strong function o f chain length. We find good agreement between our results and those expect ed from models for preactivation which rely on the expected properties of i celike ordered water near the nucleating agent. We also discuss the possibi lity that structural changes in the Langmuir film play a role in preactivat ion. We discuss future experimental directions for this system, now that pr eactivation behavior has been established. (C) 2001 American Institute of P hysics.