Similarity

One of the key barriers to the adoption of HSP is the middle word "solubility". Many people think that HSP only apply to issues involving solvents, so are unaware that the science behind HSP applies to interactions involving polymers, particles, APIs, plasticisers etc. etc.

So let's use a different "S". How about Hansen Similarity parameters? Similarity really does capture the essence of the HSP approach because in general we are not so much interested in any specific HSP values but their relationship, via the HSP Distance, to another HSP set. The Distance might be between solvent and polymer, or polymer and nanoparticle, or nanoparticle and dispersant, or API and skin or ... just about anything where we can measure usable HSP values.

Examples

We have plenty of examples on this site were "similarity" might be more natural than "solubility". For example:

• Carbon Nanotubes: Although there is a good argument to say that nanoparticles really are "soluble" others might prefer to say that CNT are "similar" to a given solvent or polymer. A similar argument is applied to TiO2 nanoparticles.
• Ceramics: When it comes to formulating a complex ceramic, controlled "similarity" between solvents, dispersants and ceramic particles is crucial.
• Skin Permeation: Whether you do (for pharma/cosmetics) or do not (safety) want skin permeation, the similarity of the chemical to the skin is an important fact behind the permeation rate. Exactly the same considerations apply to Glove Safety.
• Adhesion: Similarity between polymers can make all the difference between good and bad adhesion.

HSP will always be Solubility Parameters

HSP have been Solubility Parameters for more than 50 years and are the formal scientific name used in the literature. We are not proposing Similarity Parameters as a formal name change.

But we encourage everyone to use phrases such as "If you are uncomfortable thinking of HSP as Solubility parameters, think of them as Similarity parameters". This will help to lower the barriers to adoption of HSP, because everyone recognises that "similarity" is resonant with "like is compatible with like".