(496c) New Theoretical Framework to Design Optimal Nonionic Surfactant Formulations That Exhibit a Desired Dynamic Surface Tension Behavior

Applications like ink-jet printing, pesticide sprays, and foam and emulsion formation, all involve the rapid formation of fluid/fluid interfaces. Surfactants are used in these applications in order to stabilize the freshly-formed interfaces by adsorbing at a desired rate. In these applications, the kinetics of surfactant adsorption is expected to play a significant role in determining the performance and effectiveness of the surfactant formulations.

We propose a novel theoretical framework to design nonionic surfactant formulations that result in a desired surfactant adsorption kinetics behavior specified in the form of a desired dynamic surface tension profile (referred to as the ?design problem'). The new theoretical framework circumvents the more widely used and time consuming experimental trial-and-error surfactant selection approach. Specifically, the proposed new theoretical framework involves using predictive dynamic surface tension (DST) models in conjunction with optimization techniques to identify the surfactant formulation that optimally meets a desired dynamic surface tension behavior. To demonstrate the feasibility of the proposed new theoretical framework, we implement it using: (i) the molecularly-based Mulqueen-Stebe-Blankschtein (MSB) adsorption kinetics model [1], which is applicable to model the DST behavior of nonionic surfactant mixtures when the adsorption kinetics is diffusion-controlled, and (ii) the optimization package SNOPT (Sequential Nonlinear OPTimization) [2]. We formulate the design problem as an optimization problem using the MSB adsorption kinetics model, and developed a framework to find the solution of the formulated optimization problem using SNOPT. We demonstrate the effectiveness of the new theoretical framework by analyzing a representative case study in which we identify the nonionic surfactant formulation that results in an unconventional ?linear' DST profile on the log (t) scale. Using this case study, we demonstrate and conclude that the new theoretical framework can be extremely effective in designing surfactant formulations that meet a required adsorption kinetics behavior in specific applications.

References:

1. M. Mulqueen, K. J. Stebe, and D. Blankschtein. Dynamic interfacial adsorption in aqueous surfactant mixtures: Theoretical study. Langmuir, 17:5196-5207, 2001.

2. P. E. Gill, W. Murray, and M. A. Saunders. SNOPT: An SQP algorithm for large-scale constrained optimization. SIAM Review, 47: 99-131, 2005.