Characterization of ternary phase diagrams by means of thermal and rheological analyses

Abstract

Context: Mixtures made of oil, water and surfactants give rise to a wide range of structure with different characteristics and phase manifestations.

Objective: Aim of this paper is to build up and understand the phase diagram of a model ternary system (Water, Polysorbate 80 and isopropyl myristate) by the use of common techniques such as thermal analysis and rheology, in comparison with visual assessment and polarized light microscopy.

Methods: Different ternary systems were prepared and analyzed by means of DSC and rheology in order to highlight the state of water (free, interphasal, bound water) and the samples structural characteristics.

Results: The resultant phase diagram is divided into four different zones. Bound water zone is predominant at elevated surfactant/oil ratios, while as the surfactant/oil ratio decreases, DSC reveals the presence of free water. Interphasal water prevails at intermediate water and surfactant content which corresponds with gels systems. Mechanical spectra allow to discern between cubic (true gel) and lamellar mesophases (weak gel), while flow curves allow to distinguish among microemulsions, emulsions or lamellar mesophases.

Discussion: A deeper characterization of a model ternary phase diagram is possible, with respect to the simple visual inspection, by the use of thermal analysis and rheology. The state of water molecules and the viscoelastic characteristics of the system allow to obtain important structural considerations.

Conclusions: In conclusion, the knowledge of the state of water and of the viscoelastic characteristics of the systems allow a deeper understanding of the structural features of the ternary phase diagram.

Keywords::

• Ternary phase diagram
• DSC
• Rheology
• Free/bound water