The HSP Sphere

HSP are measured using the Sphere technique. A set of solvents with known HSP is used to test the "happiness" of a test material in the solvents then a sphere is constructed with all the "good" solvents inside it and all the "bad" ones outside. The centre of the sphere is the HSP of the test material and the radius defines its limits of "happiness" for when you want to find other solvent systems to work with it.

If the test material is a simple polymer or crystalline solute then instead of the word "happiness" we would use the word "solubility". However, the technique is perfectly general and the definition of "happiness" varies. For nanoparticles and dispersions the definition might be "remains well suspended". For cross-linked polymers it might be "swells significantly". The point is that the formulator knows what they want for a "happy" formulation so can choose their own definition so that the measured HSP and radius are both meaningful for the given application.

Here we show the sphere of solubility of one type of polylactic acid (PLA). The centre of the sphere is a reasonable estimate of the HSP of PLA. The radius is rather large because the specific PLA was of modest MWt and crystallinity. In other (unpublished) work we have used a highly crystalline PLA which gave the same central HSP but a much smaller radius. All the good solvents (blue) are inside the sphere (transparent green), all the bad solvents (red) are outside and PLA itself is the green dot in the middle.

Navigation and Information

1. The default is a conventional 3D view via a Trackball, using your mouse to move around and scroll wheel to zoom in and out.
2. Those who are venturous can choose the option to "fly" through HSP space using the classic game controls listed below. You can also fly with the mouse (select the MouseFly option - left/right click move forward/backward) and/or use the keyboard, keys shown below.
3. You can turn on names or HSP values if you wish, otherwise just point at any solvent and the data appear or, if you wish, click on a solvent, it will change colour to yellow and its name and HSP are shown below.

The data behind the fit are from a public domain dataset taken from HSPiP. The "Score" is simply 0 for insoluble, 1 for soluble, and the RED is the Relative Energy Distance which is HSP Distance/Sphere Radius, with good solvents <=1 and bad solvents >1.