**Marcos Fernández Leyes, Santiago Gimenez Reyes, Ezequiel Cuenca, Jhon F. Sánchez Morales and Hernán Ritacco \***

Instituto de Física del Sur (IFISUR), Departamento de Física, Universidad Nacional del Sur (UNS), CONICET, Av. L. N. Alem 1253, B8000CPB—Bahía Blanca, Argentina; mfernandezleyes@uns.edu.ar (M.F.L.); sgimenezreyes@gmail.com (S.G.R.); ezequiel.qnk@gmail.com (E.C.); jhon.sanchez@uns.edu.ar (J.F.S.M.) **\*** Correspondence: hernan.ritacco@uns.edu.ar

Received: 19 December 2019; Accepted: 19 January 2020; Published: 22 January 2020

**Abstract:** We studied the dynamics of adsorption at the air-water interface of a cationic surfactant bearing two charges, Gemini 12-2-12, at concentrations below and above the critical micelle concentration (cmc). We used maximum bubble pressure and Wilhelmy plate techniques in order to access all time scales in the adsorption process. We found that the adsorption dynamics are controlled by diffusion at the initial stage of the adsorption process (milliseconds) and it is kinetically controlled by an electrostatic barrier (minute) approaching the equilibrium surfactant surface concentration. Between these two extremes, we found several relaxation phenomena, all following exponential decays with characteristic times spanning from one to hundreds of seconds. By means of time-resolved surface potential measurements, we show that these processes involve charge redistribution within the interfacial region. The surface tension data are analyzed and interpreted in the framework of the free energy approach.

**Keywords:** cationic surfactants; Gemini 12-2-12 surfactant; dynamic surface tension; maximum bubble pressure; surface potential
