Orientation phase transitions of undissociated n-decanoic acid at the air/solution interface revealed by surface...

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_⊥/ε_s, as a function of chosen adsorption ordinates such as the bulk concentration, c, partial molar area, A, and surface molar fraction of DA, X_DA^s. 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_⊥/ε_s dependences on different adsorption's ordinates (c, A or X_DA^s) revealed consistently three local μ_⊥/ε_s maxima of their height increasing with the adsorption coverage. The μ_⊥/ε_s change was recalculated into inclination angle (α_incl) of the total dipole moment vector (μ→) to the interface, assuming ε_s = 1. The μ_⊥/ε_s which reflects polarization orientation is an order parameter used by us for analysis of 2D phase transitions in the monolayer. The three μ_⊥/ε_s 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_⊥/ε_s maxima 2 and 3 indicates that transition of the L2 into the L2' 2D phase is of the first order. Decrease of the μ_⊥/ε_s above the maximum 3 indicates transition into two phase regime by nucleation of aggregates (possibly in form of lamellas) within the L2' phase.