Adsorption kinetics of heptadecafluoro-1-nonanol: Phase transition and mixed control

The adsorption kinetics of heptadecafluoro-1-nonanol (C₉H₂F₁₇OH) onto a clean air-water interface at low surfactant concentrations (equilibrium surface tension, γ(C) > 65 mN/m) has been reported, and the controlling mechanism was found to be mixed diffusive-kinetic controlled (Kuo et al., JCIS 402 (2013) 131). However, it remains to be determined what the adsorption kinetics are at higher concentrations. Hence, the dynamic surface tension, γ(t) of C₉H₂F₁₇OH was measured and compared with the theoretical γ(t) curves predicted from phase transition model.

A video-enhanced pendant bubble tensiometer was used to measure the γ(t) data of aqueous C₉H₂F₁₇OH solutions at higher concentrations (C > 7.7 × 10^-9 mol/cm³). A new generalized Frumkin-Langmuir phase transition model was built up to simulate the γ(C) and γ(t) data.

At higher surfactant concentrations, a constant-γ region at 64.8 mN/m was observed for one hundred to a few thousand seconds during the γ(t) relaxation. This constant-γ region implies the existence of a phase transition of the adsorbed surfactant monolayer at air-water interface. The γ(t) data of C₉H₂F₁₇OH can be simulated perfectly using this mixed-controlled phase transition model with the adsorption rate constants β₁ = 1.0 ± 0.5 and β₂ = 13 ± 4 (10⁵ cm³/mol·s). It is therefore concluded that the adsorption process of C₉H₂F₁₇OH onto a clean air-water interface is of mixed-control.