Adsorption Kinetics of a Cationic Surfactant Bearing a Two-Charged Head at the Air-Water Interface

Marcos Fernández Leyes,Ezequiel Cuenca,Santiago Gimenez Reyes,Jhon F Sánchez Morales,Hernán Ritacco

doi:10.3390/coatings10020095

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.

Highlights

The dynamics of adsorption-desorption of soluble surfactants at water-air interfaces play a very important role in many phenomena and technological applications such as wetting, detergency, emulsification, and foaming [1,2,3,4]
By means of dynamic surface tension and time-resolved surface potential, we have identified several processes with their characteristic times
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

Summary

Introduction

The dynamics of adsorption-desorption of soluble surfactants at water-air interfaces play a very important role in many phenomena and technological applications such as wetting, detergency, emulsification, and foaming [1,2,3,4]. It was found that the adsorption dynamics is controlled by diffusion (DLA) at very short adsorption times but it is kinetically limited (KLA), at times close to the equilibrium, by an electrostatic barrier built-up, as the ionic surfactant molecules adsorb onto the interface. In another very recent article, we explain the appearance of negative surface viscosities for a cationic Gemini surfactant, dimethylene-1,2-bis(dodecyldimethylammonium) bromide (G12-2-12), by the asymmetries found in the adsorption-desorption processes and dynamics [11]. The unphysical result (negative viscosities) is just an artifact due to the procedure used for data analysis, which is invalidated by the existence of those asymmetries

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Discussion

Conclusion