(54e) Marangoni Synergism in Binary Surfactant Mixtures

A surface tension gradient along a fluid interface drives a flow, known as Marangoni flow, from low surface tension to high surface tension regions. When the surface tension gradient is caused by compositional gradients, the corresponding flow is termed solutal Marangoni flow, and it is manifested in numerous technological, biomedical and natural processes. Solutal Marangoni flows influence emulsification processes, uniformity of coatings, dispersal of therapeutic agents in lungs, and even propulsion of certain insects along water surfaces. Marangoni flow is often initiated by localized placement of a surfactant monolayer or localized deposition of a surfactant solution on a liquid surface. The resulting radial Marangoni spreading proceeds in an axisymmetric manner. A significant experimental and theoretical literature has established important characteristics of the fluid flow during Marangoni spreading. Spreading proceeds until the driving surface tension gradient has dissipated. Emphasizing single-component surfactant systems, the prior literature has established the importance of the liquid subphase depth, density and viscosity and the “spreading parameter”, which is the difference between the initial surface tension of the surfactant deposit and the clean liquid surface, as the primary factors controlling Marangoni spreading dynamics. Motivated by the ubiquity of multicomponent surfactant systems in technologically significant complex fluid formulations, this presentation will establish new controlling factors that emerge in Marangoni spreading driven by localized deposition of binary surfactant solutions.

The phenomenon of surface tension synergism is well established. This synergism is conventionally defined such that a surfactant mixture produces the same surface tension reduction at a lower total concentration than the concentration required of either of the component surfactants acting alone. This is equivalent to stating that a mixture will produce a larger surface tension reduction than either single component surfactant would at the same total concentration. While subphase properties exert important influences on Marangoni spreading dynamics, the value of the spreading parameter is commonly taken to be the primary driver of Marangoni spreading. Thus, one may expect that a binary surfactant mixture that exhibits surface tension synergism would produce some synergistic effect on Marangoni spreading. Here, in parallel with the fixed concentration perspective for surface tension synergism, Marangoni synergism will be defined as the situation where a surfactant mixture produces either a greater extent of spreading or a greater spreading velocity than either of the component surfactants do when acting alone, at the same total concentration. Through both experimentation and transport modeling, we establish that the conditions under which a binary surfactant system produces surface tension synergism are not the same as those producing Marangoni synergism. From a high level point of view, this reflects the fact that surface tension synergism is a strictly thermodynamic phenomenon, whereas Marangoni spreading couples the thermodynamic surface tension driving force and a set of dynamic properties, including surfactant intrinsic adsorption and desorption kinetics. The research to be presented here is organized according to the strength of the mutual attractive interaction in binary surfactant pairs. This is because strongly attractive surfactant pairs are more likely to produce surface tension synergism.

Spreading experiments based on the motion of surface tracer particles that track the location of the moving surfactant front were conducted with anionic sodium octylsulfate (SOS), cationic octyltrimethylammonium bromide (OTAB) and zwitterionic N,N-dimethyldecylamine N-oxide (DeDAO) surfactants. The SOS/OTAB pair is the most strongly attractive, followed by SOS/DeDAO. The OTAB/DeDAO pair is the least attractive. The occurrence of surface tension synergism in these systems conformed to expectations. Among these systems, Marangoni spreading synergism was only observed when conditions also yielded surface tension synergism. However, surface tension synergism did not ensure that Marangoni synergism would occur. Furthermore, different binary surfactant mixtures prepared with different ratios of the two surfactants that nevertheless had the same surface tension (thus the same spreading parameter) generally did not produce the same Marangoni spreading velocity or extent. Even when different compositions yielded surface tension synergism and the same spreading parameter, one composition could yield Marangoni synergism while the other did not. Additionally, whereas by definition surface tension synergism cannot be time-dependent, Marangoni synergism occurred within finite time ranges during the spreading event.

A transport model was developed to fully describe Marangoni spreading dynamics, coupling the Navier-Stokes, advective-diffusion, and surface material balance equations with an extended Frumkin type binary surface equation of state with mutually consistent expressions for surfactant adsorption and desorption fluxes. The model makes it possible to separately manipulate system thermodynamic properties, including the strength of the binary surfactant mutual attraction, and dynamic properties, especially the intrinsic kinetic rate constants. The model produced each of the experimental behaviors. It indicated that surface tension synergism does not ensure Marangoni synergism. Within the parameter space examined, it also produced results whereby Marangoni synergism would not occur without surface tension synergism.

The details of the temporal evolution of the Marangoni spreading velocity were revealing. At the earliest times the surfactant front velocity was entirely controlled by the spreading parameter, the thermodynamic driving force. Thus, any system that exhibited surface tension synergism would exhibit Marangoni synergism at the earliest times. As spreading proceeded, the velocity began to depend on the intrinsic adsorption rate constants, and that would determine whether or not a system was capable of sustaining Marangoni synergism.

In summary, this work has established the occurrence of Marangoni synergism in surfactant mixtures and conditions under which it occurs. It established that those conditions overlap, but are not identical to, surface tension synergism conditions. Since the performance of formulated complex fluid products frequently results from synergistic effects in multicomponent systems, this work should aid in the interpretation of fluid product performance and in future design of new complex fluid formulations.