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

HYPOTHESIS: The adsorption kinetics of heptadecafluoro-1-nonanol (C9 H2 F17 OH) 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 C9 H2 F17 OH was measured and compared with the theoretical γ(t) curves predicted from phase transition model.

EXPERIMENTS: A video-enhanced pendant bubble tensiometer was used to measure the γ(t) data of aqueous C9 H2 F17 OH 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.

FINDINGS: 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 C9 H2 F17 OH 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 C9 H2 F17 OH onto a clean air-water interface is of mixed-control.