Solving the Solubility Puzzle

Solubility is one of the most important characteristics to consider when selecting process oils for different applications, and it is a subject on which Nynas researchers have focused specifically. However, since the existing methods used to measure the solvent power of oils are not adequate, researchers are looking for alternatives. Researcher Pär Wedin explained, "Because mineral oils are composed of thousands of different hydrocarbon molecules with varying effects on solubility, measuring the compatibility of mineral oils with different resins, plasticizers, polymers and solvents is not an easy task." The solvent power of oil can be measured using well-designed methods based on the oil's aniline point, Kauri-butanol number, carbon type distribution and viscosity-gravity constant. The determination of solubility parameters was developed based on understanding intermolecular interactions. The most commonly used methods are Hansen and Hildebrand, respectively. Hansen's solubility parameters are determined experimentally by mixing a substance with different solvents of known solubility. Solubility parameters for a substance can be determined by analyzing which solvents are compatible with the substance and which are not. For substances such as resins and polymers, this is a simple and easily applicable method. Generally, 35 to 50 carefully selected solvents will be sufficient. Pär Wedin states, "However, work conducted at Nynas has shown that over one hundred solvents are required to determine Hansen solubility parameters for mineral oils. Despite the number of solvents used, some data is still missing, which prevents reaching a complete and accurate result." The Hildebrand method derives its parameters from the cohesive energy density of the substance. While this method works well for non-polar substances, it is not suitable for measuring the compatibility of polar substances. Wedin said, "Hildebrand parameters are practical because they can be determined from the heat of vaporization. On the other hand, Hansen's method provides more useful information on different types of intermolecular interactions in the substance. However, this method requires intensive labor, and most of the solvents used present health and safety concerns." For this reason, safer and faster methods for determining solubility parameters are being researched, and much work needs to be done internationally in this field.  

Measuring Solubility by Calorimetry

Hildebrand's solubility parameters can be calculated using calorimetric methods that measure the heat of vaporization of a substance. In the figure below, the results of simultaneous thermal analysis for determining the solubility parameter of a base oil can be seen. The table shows the Hildebrand solubility of three base oils. Bio-based base oil and GTL (gas-to-liquid) base oils have similar chemical composition and contain only isoparaffins. Naphthenic base oils have stronger intermolecular interactions, which means they have higher solubility parameters than isoparaffins and therefore greater solvent power. Wedin said, "Binary solvents are being successfully used for certain polymers. The potential of chromatography is being investigated as a faster method. The use of density functional theory together with COSMO-RS (conductor-like screening model for real solvents) for molecular modeling and structural optimization could be a useful tool for measuring material compatibility." Research Director Elena Minchak added, "As the Nynas Research Department, we are continuously increasing our knowledge about the relationship between the solubility of oils and the level of interaction between materials because we believe this is of key importance for the development of new, sustainable base oils."