Equilibrium and dynamic surface tension in relation to diffusivity and foaming properties: Effects of surfactant type and structure

- Foamability of all studied surfactants can be represented by the term of (∂γt∂t)maxwhich was obtained from the dynamic surface tension data.
- Foam stability increased with increasing alkyl chain length of MESx series and the number of EO units of C12EOn series.
- An addition of NaCl provided a negative effect on the surfactant diffusivity because the co-adsorption of Na+ counterion obstructs the transport of MES molecules from bulk liquid to foam surface.

The main objectives of this study were to investigate the effects of molecular structure of surfactant on surface properties and foam characteristics and to correlate surface tension to foam properties. Two series of methyl ester sulfonate with different alkyl chain lengths (MESx, x = 14, 16, and 18) and polyoxyethylated dodecyl alcohol with different head group sizes (C12EOn, n = 3, 5, 7, and 9) were measured for equilibrium and dynamic surface tension as well as foamability and foam stability. The equilibrium experimental data were well fitted with the Langmuir adsorption isotherm. The dynamic surface tension data were used to calculate diffusivity values of all studied surfactants by using Word-Tordai equation. The longer the alkyl group of MESx, the lower the diffusivity value. The larger the EO head group size, higher the diffusivity. For both studied surfactant groups, the higher the maximum rate of surface tension reduction, the higher the diffusivity and foaming properties in terms of foamability and foam stability.

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