Interfacial Dynamics and Its Relations with “Negative” Surface Viscosities Measured at Water–Air Interfaces Covered with a Cationic Surfactant

Abstract

We studied the dynamics of a cationic surfactant monolayer, Gemini 12-2-12, at the air−water interface for surfactant aqueous solutions at concentrations below the critical micelle concentration. We present surface rheology experiments performed in a Langmuir trough by the oscillatory barrier technique. From these, we found negative surface viscosities at certain frequencies. We demonstrate that this unphysical result is a consequence of an unconsidered surfactant dynamics within the interfacial region. By surface pressure relaxation experiments, after a sudden modification of the interfacial area and by dynamic surface tension and surface potential measurements, several relaxation phenomena and relaxation times were identified. We found that surfactant adsorption and desorption processes are asymmetric: the characteristic times and the number of processes involved in the mechanisms of adsorption and desorption are different. This asymmetry invalidates the usual data analysis procedure that leads to the negative viscosities. Similar mechanisms could be at the origin of the negative viscosities reported in other systems, a possibility that remains to be explored.

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

“…Note that the number of processes increases with surfactant concentration until the cmc. The observation of several processes, each with its characteristic time, is consistent with what was found in surface rheology experiment on the same surfactant system [11]. The origin of the intermediate relaxations, between τ D and τ k , seems to be related to charge redistribution as observed from surface potential results on Figures 6 and 7.…”

“…This last process lasts from hundreds to thousands of seconds. The characteristic times obtained from fittings with Equation (8), labelled as τ end in Figures 6 and 7, span from 700 to 3500 s. It is worth mentioning here that these results help to explain the behavior of the surface step-compression rheology observed in these systems [11].…”

“…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.…”

Surface phase transitions in foams and emulsions

“…In another study, Cuenca et al [40] resolved the previous puzzling reports about the measurements of apparently negative surface viscosity in some surfactant systems. The authors found that the surfactant adsorption-desorption processes might be highly asymmetric, with different characteristic times and mechanisms.…”

“…It is worth mentioning that several recent studies [38][39][40] warned about possible problems with some of these experimental methods. Thus, it was shown that Langmuir trough-based techniques may give unreliable results for the reversibility of the surface phase transitions, when reaching low surface tensions, due to the leakage of material [38].…”