(382j) Calculation of Hansen Solubility Parameters Based on Solvatochromic Dye

J.Hidebrand defined the solubility parameterin 1950, which considers the cohesion energy of different materials in contact with each other. In many solvents with high polarity such as alcohols, there are many cases where actual phenomenon differs from the actual phenomenon considering the cohesive energy of the substance collectively. Hansen defined the Hansen solubility parameter (HSP), which separates the cohesion energy of mixtures into the London dispersion forces, dipole forces, and hydrogen bonding forces.

The Hansen solubility parameter (HSP) is one of the index for affinity of materials with some solvents. The basic principle of HSP has been ‘like dissolves like’. HSP values over 1200 kinds of pure solvents have been reported by Hansen in the data base of HSPiP program 2004. Furthermore, HSP value is used for various research fields, such as an evaluation of the solubility of solid in solvent, a compatibility and an affinity of polymers in solvent and a dispersibility of fine particles in solvent. The other side, one of the methods of HSP for a liquid or its mixture is able to measure from physical properties of the solvent (such as refractive index, surface tension, permittivity and dipole moment).

The combination of each parameter is considered to be a three-dimensional vector. Materials having similar vectors easily dissolve each other, hence the term “like dissolves like.” The HSP has been developed into an index for evaluating the affinity between materials.

There are three methods for calculating the HSP, namely: (1) evaluating the affinities of substances by dissolution experiments to calculate the Hansen solubility sphere; (2) calculating the properties of each substance and estimating correlations of each parameter; (3) estimating the contributions of certain groups in the chemical structures in the material.

Method of (1) has the highest in precision; however, we have also previously examined methods to calculate properties of matter (2) and shown effective correlation of each parameter. Notably, the dissolution experiments performed in method (1) are time consuming and require many different solvents. Hence, the calculation method (3) is attractive because it does not depend on experiments and is superior to method (1) or (2) in terms of calculation speed and convenience. However, there remain questions about the suitability of calculation-based approaches for treating real phenomenon. Considering the above, the method (2) is a well-balanced calculation method from the viewpoint of convenience of only measuring physical property values and guarantee of accuracy that experimental results are used.

There were some problems to calculate HSP based on physical property values. The parameters δ_t, δ_d, and δ_p are components of the HSP, calculated from the surface tension, refractive index, and permittivity, respectively. It is common that δ_h is calculated with backward calculation with almost no report. Since calculation is performed by back calculation, there is a problem in that the error of each parameter is concentrated to δ_h. Therefore a method to directly calculate δ_h is highly desired, to improve the accuracy of predictions. So, when I was looking for something that correlated with δ_h, I came across a Solvatochromic pigment ^[10] famous for the ionic liquid field. In the ionic liquid field, solvent polarity can be evaluated from various parameters derived by the pigment. The general polarity is the sum of the hydrogen bonding between dipole forces in the HSP. Hence, δ_h might correlate with parameters determined from behaviors of Solvatochromic pigments.

Here, we aimed to investigate the effectiveness of estimating correlations of δ_h with the solvent polarity parameter (E_T^N) and the Hansen solubility parameter, based on measurements of Reichardt’s dye, which is a Solvatochromic pigment. We measured the E_T^N for 41 organic solvents, having various functional groups, based on Reichardt’s dye. The difference between the estimated value and the literature value was compared and the practicality was verified.