On the determination of critical micelle concentration of partially dissociated ionic surfactants using contact angle data

The contact angle (CA) data has been utilized for determining the critical micelle concentration (cmc) of fully dissociated surfactant SDS, but not for nonionic surfactants Triton X-100 and C₁₂E₆ [Thu et al., J.Mol.Liq (2019) 111582]. Could this approach work for a partially dissociated ionic surfactant such as dodecyltrimethylammonium bromide (DTAB)? To verify this, this work studied the evaporation process of DTAB_(aq) solution drops on parafilm. The relaxations of CA, wetting diameter, and drop volume were recorded during evaporation to determine the advancing/receding (θ_a and θ_r) CA's. The data revealed that the breakpoints of θ_a and θ_r matched well with the cmc from the ST data; thereby, confirming that the aforementioned approach is viable for a partially dissociated ionic surfactant. Additional examination of the relaxation data and drop profiles revealed that the movement of a solution drop's triple line (pinning-depinning behavior) during the evaporation process is primarily dependent on the localized surface morphology.