(419b) Use of Water-Oil-Surfactant System Phase Behavior Data/Model for Emulsion-Based Chemical Product Design

Emulsions are defined as mixtures of two or more liquids that are normally immiscible, where one liquid, the dispersed phase, is dispersed in the other, the continuous phase. These two phases are generally defined as water and oil, where these terms identify any liquid showing affinity with water or alkanes, respectively. This type of mixtures can be formed through mechanical work (most often provided through mixing), since emulsions are naturally unstable. For this reason, surfactants are usually added to the two main constituent of the emulsion to enhance its stability as a macroscopically homogeneous mixture. Therefore, the correct understanding of the phase behavior of water-oil-surfactant systems is necessary in the field of emulsion science and its adoption in a computer-aided product design framework allows a more flexible choice of the optimal formulation, when an emulsified product is required.

Since several variables (the composition of the three components and temperature) are involved in the description of the ternary phase diagram, there are different ways to graphically represent the phase behavior of these systems. One of the common and the most useful with regards to chemical product design, consists of an X-Y diagram, where the surfactant content (usually in weight percentage) is on the X-axis, while temperature is on the Y-axis. The ratio between water and oil is then kept constant. This diagram is sometimes called “fish-diagram”, because of the typical shape of its phase boundaries. Through this diagram, it is possible to recognize three distinct regions: a homogeneous single-phase area, describing the microemulsion domain; a two-phase area, describing the macroemulsion domain (divided into water-in-oil and oil-in-water emulsion domains, depending on the temperature); a three-phase area, describing the coexistence of a microemulsion and a microemulsion. Depending on the specific need, it may be necessary to design formulations that fall in the two-phase area, if an emulsified product is wanted. Or, if a microemulsion is needed, the designed formulation must lie in the single-phase area. The three-phase area, on the other hand, is rarely considered for product purposes, since the stability of the system in these conditions is extremely poor.

To be able to draw the fish-diagram, it is necessary to have available phase boundary data as a function of temperature, measured or predicted with an appropriate model. Unfortunately, this kind of data is not readily available and, therefore, the availability of a model able to describe the phase behavior of the involved systems will allow extrapolation both in terms of temperature and chemical structure of the main constituent of the systems. Consequently, the emulsifying agent (surfactant) can be designed. UNIQUAC, UNIFAC and CPA has been applied to these systems and preliminary results are shown.

The availability of fish-diagrams with different chemicals involved (both oils and surfactants) and at different water-oil ratios, gives fundamental information for chemical-based product design. In particular, the boundaries for the manufacture of water-in-oil and oil-in-water emulsions (the three-phase area need to be entirely below or above the room temperature, respectively), as well as microemulsions (the dependence from the temperature is not crucial, while it is important that the minimum concentration at which the microemulsion domain is identified). The application to reaction engineering is also highlighted, where the identification of the position of the three-phase domain in necessary to detect the optimum conditions for advanced chemical reactions, occurring in a microemulsified environment.