Orientation phase transitions of undissociated n-decanoic acid at the air/solution interface revealed by surface pressure and electric potential

Abstract

The surface pressure (Π) and electric surface potential (ΔV) vs. concentration (c) isotherms of n-decanoic acid (DA) in 1 × 10−3 mol/dm3 HCl were measured at the air/solution interface - the Π with a du Noüy ring, the ΔV with the vibrating plate (named also the dynamic condenser) method. The DA solutions fulfilled criterion of surface-chemical purity. The complementary Π-c and ΔV-c isotherms were jointly evaluated to obtain dependence of quotient of the effective dipole moment to the interface's permittivity, μ⊥/εs, as a function of chosen adsorption ordinates such as the bulk concentration, c, partial molar area, A, and surface molar fraction of DA, XDAs. The crucial point for the analysis is knowledge of the surface excess (Γ) dependence on concentration (c). Since, experimental determination of a Γ-c course is problematic, so far, we used Γ-c courses calculated basing on different adsorption models (Gibbsian, the classical Frumkin’ model and the Lunkenheimer’ and Hirte’ two state approach). Despite, the Γ-c courses determined basing on the different adsorption models differ significantly, the μ⊥/εs dependences on different adsorption's ordinates (c, A or XDAs) revealed consistently three local μ⊥/εs maxima of their height increasing with the adsorption coverage. The μ⊥/εs change was recalculated into inclination angle (αincl) of the total dipole moment vector (μ→) to the interface, assuming εs = 1. The μ⊥/εs which reflects polarization orientation is an order parameter used by us for analysis of 2D phase transitions in the monolayer. The three μ⊥/εs local maxima are ascribed by us to three 2D mono-phases, one transferring into the next one of the higher order in the sequence: liquid expanded (L1) → the liquid condensed tilted (L2) → the liquid condensed untilted (L2′). One inflection point appearing in the Π-A isotherm within the region between the μ⊥/εs maxima 2 and 3 indicates that transition of the L2 into the L2’ 2D phase is of the first order. Decrease of the μ⊥/εs above the maximum 3 indicates transition into two phase regime by nucleation of aggregates (possibly in form of lamellas) within the L2′ phase.