Spontaneous Polarization of Cryo-Deposited Films for Five Normal Saturated Monohydroxy Alcohols, CnH2n+1OH, n = 1-5.

Abstract

Vapor-deposited films of neutral polar molecules on cold surfaces exhibit spontaneous electric polarization across the faces of the film. We deposited known amounts of the first five members of the normal monohydroxy alcohols at substrate temperatures in the range of 30-130 K. It is known that voltages measured are proportional to film thickness and that the effect is substrate-independent. The main trend observed was that the voltage per molecule surface density grows with molecule size and drops with deposition temperature. With the exception of some high-temperature methanol depositions, the films had a negative voltage on the vacuum side. Furthermore, the heavier members (1-butanol and 1-pentanol) showed reduced voltages for the lowest temperatures, suggesting that all members of the series could follow this pattern at temperatures below what was experimentally accessible. After deposition, film voltage was monitored during a temperature ramp showing first an increase in the voltage measured, then gradual decay before film evaporation. Initial film voltages are in general agreement with the concept of surface mobility of vapor-deposited molecules and indicate a negative correlation with dipole moment size.