Surface potentials of monomolecular films of n-long chain alcohols at different temperatures and surface pressures

Abstract

Surface potentials (ΔV) of monomolecular films of n-long chain alcohols CH₃(CH₂)_n-1-OH designated as C_n-OH (n = 16, 18, 20, 22) were measured by the ionizing electrode method between 14°C and 25°C as a function of surface pressure (π) and area/molecule (A). The results have been represented as ΔV-π, ΔV-A, µ_⊥ -A, and π-A isotherms, whereµ_⊥ is the normal component of the apparent dipole moment per molecule. The data provide valuable information as regards molecular interaction and orientation of various films at different pressures and also reveal the two-dimensional phase transformations that are observed in π-A and µ (surface viscosity)-π curves. The transition pressures observed in ΔV-π curves agree well with those of π-A and µ-π curves. In all the films studied ΔV increases as the pressure (π) increases or as the area/molecule decreases. It also increases as the chain length increases from C₁₆ to C₂₂ at all temperatures. The surface dipole moment (µ_⊥ ) generally decreases as the area/molecule decreases except in C₂₂-OH at Δ14°C. In all the alcohols the values of ΔV and µ_⊥ decrease from 14° to 19°C but increase at about 24.5°C. Diffused first order change has been observed for C₂₀-OH at 19.2°C and 24.6°C and for C₂₂-OH at 14.4°C.