Thermodynamic Model for the Prediction of Janus Particle Morphology and Interfacial Tensions

Composite Janus particles exhibit many unique structural, chemical, electrical, and optical properties owing to their two distinct surfaces. Phase separation of two dissolved polymers within O/W emulsion droplets is a straightforward approach to large-scale synthesis of Janus particles. An array of Janus particle morphologies can be obtained by changing experimental conditions such as surfactant type and concentration, total polymer concentration, solvent evaporation rate, and solvent type. Of the aforementioned factors, it was found that the type and concentration of ionic surfactant have the most significant effect on particle morphology. If kinetic factors (i.e., solvent evaporation rate) are adequately controlled, then the morphology of Janus particles can be predicted based on thermodynamics alone.

The starting point of the model is the creation of a Gibbs free energy equation containing terms for each of the three interfaces: 1) Polymer 1-Water, 2) Polymer 2-Water, and 3) Polymer 1-Polymer 2. The Janus particle is modeled as two overlapping spheres, and the free energy equation is minimized using Lagrange multipliers to ensure constant Polymer 1 and Polymer 2 volumes. The end result is a system of equations that can quantitatively predict Janus particle morphology given interfacial tensions. The model accurately predicted PLGA/PCL Janus particle morphology for three case studies using PVA, SDS, and SDBS surfactants.

In reverse, the model can be applied to predict interfacial tensions based on Janus particle morphology. This is an important feature of the model, as the interfacial tension between two solid polymers cannot be experimentally measured. Results from the PLGA/PCL system were compared to literature values of calculated PLGA-PCL interfacial tensions with excellent agreement.