Development of User-Friendly Computer Programs to Predict the Bulk Solution Behavior and Interfacial Properties of Single and Mixed Surfactant Systems

As the need for a detailed understanding and prediction of the bulk solution behavior and interfacial properties of surfactant systems increases, the surfactant technologist is faced with the challenge of modeling the complex behavior of these systems. In our research group at MIT, we are committed to the development of a molecular-level understanding of bulk solution behavior and interfacial properties of single and mixed surfactant systems. With this in mind, we are developing molecular-thermodynamic theories which are capable of predicting a wide range of important bulk and interfacial properties of solutions of single surfactants and surfactant mixtures, including commercial surfactants. In order to make our theoretical advances accessible to all those interested in surfactant design, manufacturing, and formulation, we have incorporated our current predictive capabilities into five user-friendly computer programs named PREDICT, MIX2, MIXn, SURF, and DYNAMIC. Below, we describe the predictive capabilities of each computer program (for more details, see: http://web.mit.edu/dbgroup/computer/index.html).Program PREDICT predicts a broad spectrum of bulk solution properties of single nonionic, zwitterionic, or ionic hydrocarbon-based surfactants under a variety of solution conditions, including surfactant concentration, temperature, and salt type and concentration. These properties include: (i) bulk solution characteristics, such as the critical micelle concentration (CMC), (ii) equilibrium micellar characteristics, such as micelle shape, core minor radius, size, and size distribution (including the average micelle hydrodynamic radius and radius of gyration), and (iii) phase behavior characteristics, such as the critical surfactant concentration signaling the onset of phase separation and the crossover surfactant concentration marking the transition from the dilute to the semi-dilute micellar solution regimes.Program MIX2 predicts a wide spectrum of bulk solution properties of binary surfactant mixtures containing any combination of nonionic, zwitterionic, or ionic surfactants as a function of total bulk surfactant concentration and composition. These properties include the mixture critical micelle concentration and the ideal mixture critical micelle concentration, the beta interaction parameter characterizing the non-idealities of the surfactant mixture at the micelle level, the micelle shape and core minor radius, the average micelle aggregation number and the variance of the micelle size distribution, the micelle composition, and the total monomer concentration and composition. All these predictions can be made at various temperatures and salt concentrations for various salt types.Program MIXn predicts the critical micelle concentration and the micelle composition of a surfactant mixture comprising any number of surfactant components, including commercial surfactants. The required inputs are the critical micelle concentrations of the various single surfactants comprising the mixture and their pairwise beta interaction parameters, which can be obtained using programs PREDICT and MIX2, respectively.
Program SURF predicts either the air-water surface tension or the oil-water interfacial tension, as well as the surface concentration and composition, of aqueous solutions of surfactant mixtures comprising any number of surfactant components. The surfactants can be nonionic, zwitterionic, or ionic. A single experimental equilibrium surface or interfacial tension value is required for each surfactant component comprising the mixture.Program DYNAMIC predicts the dynamic air-water surface tension, and the dynamic surface concentration and composition, of aqueous mixtures comprising any number of nonionic surfactants below the critical micelle concentration. The program also predicts the time scale required for each surfactant component in the mixture, as well as for the surface tension, to reach adsorption equilibrium.All our computer programs are designed for easy use by those interested in predicting micellar solution characteristics and interfacial properties of common surfactant types already included in the computer programs, as well as by those who are interested in incorporating new surfactant structures relevant to their specific needs. The computer programs are written and compiled for use on a personal computer, and lead the user through a series of questions to gather the relevant information. The programs require only basic information about the surfactant molecular structure and the solution conditions, thus greatly reducing the level of expertise necessary to predict the bulk solution behavior and the interfacial properties of surfactant systems.The predicted fundamental micellar and interfacial properties compare well with the experimental values, and furthermore, can be correlated with practical and industrially important surfactant performance characteristics, including skin irritation, viscosity, foam stability, detergency, and dispersion ability. The computer programs can also be combined with experimental design methodologies to predict surfactant properties, for example, the critical micelle concentration of ternary surfactant mixtures over the entire range of solution compositions, thus considerably reducing the need for tedious and time consuming trial-and-error experimentation.Use of the new computer programs, combined with an understanding of the relationship between the predicted fundamental surfactant bulk solution and interfacial properties and the practical surfactant performance characteristics, can facilitate the design and optimization of new surfactant products of industrial relevance.