**5. Summary and Conclusions**

We studied the adsorption dynamics of a cationic surfactant bearing two charges, Gemini 12-2-12, onto the air-solution interface. By means of dynamic surface tension and time-resolved surface potential, we have identified several processes with their characteristic times. For the more dilute solutions, well below the cmc, and at very short adsorption times, the adsorption is consistent with a process limited by diffusion (DLA). The final stage of the adsorption dynamics, close to equilibrium, follows the exponential law, derived from the existence of a surface potential, built up by the charged molecules as they adsorb onto the interface. This behavior for ionic surfactants were observed previously [10,20,21,29]. Between the initial diffusion controlled stage and the final kinetically controlled (KLA) adsorption process, several processes have been identified with their characteristic times. The number of processes observed depends on surfactant concentration. In this respect, the observed behavior is consistent with several processes and characteristic times, which are observed in step-compression surface rheology experiments on these systems [11], which help to explain them. Based on the surface potential results, it is clear that those processes are related to a charge reorganization in the interfacial region. The reorganization may include the formation of aggregates at the interface, condensation of counter-ions onto those aggregates, and further surfactant adsorption, as well as phase transitions [26,30]. For the surfactant concentrations above the cmc, some of the processes observed could be related to the presence of micelles and their dynamics (adsorption-desorption, aggregation-disaggregation [27,28]). The redistribution of charges within the interfacial region, as observed in the surface potential experiments, seems to be responsible for the existence of the intermediate relaxations, well-described by exponentials, which are observed on the dynamic surface tension curves. Finally, the existence of phase transitions or surfactant aggregates at the interface, mentioned above, remains to be probed experimentally.

**Author Contributions:** Conceptualization, H.R.; Methodology, M.F.L., E.C. and S.G.R.; Experimental realization M.F.L., S.G.R., E.C. and J.F.S.M.; Formal analysis, H.R., M.F.L and E.C; Writing—original draft preparation, H.R.; Writing—review and editing, M.F.L., E.C., J.F.S.M., H.R; Supervision, H.R. and M.F.L.; Project administration, H.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Universidad Nacional del Sur (UNS, Argentina) under grant PGI-UNS 24/F080; by Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT, Argentina) under grant PICT-2016-0787 and by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, Argentina) under grant PIP-GI 2014 Nro 11220130100668CO.

**Conflicts of Interest:** The authors declare no conflicts of interest.
