Solving the puzzle of Solubility
Solubility is a key focus point for Nynas’ researchers as it is one of the main characteristics to consider when choosing process oil for different applications. However, as the established methods for measuring the dissolving capability of oil are not ideal, the researchers are looking for alternatives.
Pär Wedin
Staff Researcher, Nynas Research Team
“To predict the compatibility of mineral oils with different resins, plasticizers, polymers, and solvents is a challenging task as mineral oils are made up of thousands of different hydrocarbon molecules that have different impact on the solubility,” explains Pär Wedin, Staff Researcher.
The dissolving capability of oil can be measured using well-established methods based on the oil’s aniline point, Kauri-butanol number, carbon type distribution, and viscosity-gravity constant. The identification of solubility parameters has been developed based on the understanding of intermolecular interactions. The two most widely used methods are called Hansen and Hildebrand, respectively.
Hansen’s solubility parameters are determined experimentally by mixing a substance with different solvents of known solubility. By analysing which solvents are compatible with the substance, and which are not, the solubility parameters for the substance can be determined. For substances such as resins and polymers, this method is straightforward. Typically, between 35 and 50 carefully selected solvents would be enough.
“However, work at Nynas has shown that over a hundred solvents are required to determine the Hansen solubility parameters for mineral oils. Despite the number of solvents used, some data are still missing, preventing complete accuracy,” says Pär Wedin.
The Hildebrand method derives its parameters from the cohesive energy density of the substance. This works well for non-polar substances, but not so well for predicting the compatibility of polar substances.
“Hildebrand’s parameters are practical in the sense that they can be determined from the heat vaporisation. On the other hand, Hansen’s method provides more useful information about the different types of intermolecular interaction the substance has. But it is labour-intensive, and many of the solvents used are problematic from a health and safety perspective,” he says.
Hence, safer and faster methods for determining solubility parameters are being sought after, and the area is the subject of plenty of international research efforts.
“Binary solvents have been successfully used for certain polymers. Chromatography has been investigated as a potentially faster method. The use of density functional theory together with the conductor-like screening model for real solutions (COSMO-RS) for molecular modelling and structural optimisation could potentially be a useful future tool for predicting material compatibility,” he says.
Elena Minchak, Research Manager, adds: “At Nynas Research Department, we are continuously developing our knowledge of the relationship between the solubility of oils and the degree of interaction between materials, as it is key to the development of new, sustainable base oils.”
Calculating solubility using calorimetry
Hildebrand’s solubility parameters can be calculated using calorimetric methods that measure the heat of vaporisation of the substance. The Figure shows the results of a simultaneous thermal analysis for determining the solubility parameter of a base oil.
The Table shows the Hildebrand solubility for three base oils. The biogenic base oil and the GTL (gas to liquid) base oils have similar chemical composition and contain only iso-paraffins. The naphthenic base oils have stronger intermolecular interactions, resulting in higher solubility parameters than the iso-paraffins, and consequently greater solubility power.
