31 Dec 2014 Who needs solubility science?
I've just finished creating 7 YouTube videos on various aspects of solubility using Hansen Solubility Parameters and the HSPiP software. The aim is to make it as easy as possible for scientists to cope with the day-to-day solubility issues that we all face
The particular technique I tend to use, HSP, is rather simple science - some might say "simplistic". But amazingly there are few alternatives. I'm a great fan of the COSMO-RS approach from Prof Andreas Klamt and his team at COSMOlogic. The Abraham Parameter approach from Prof Mike Abraham is favoured by some. And especially in the pharma industry there is some liking of NRTL-SAC. Apart from the mighty UNIFAC industry (that most of us can't afford to access) that's about it.
Why is solubility science so under-represented? Part of the reason comes to our education. I remember being taught at Oxford that predicting solubility was essentially impossible so let's not waste time on solubility theory. Most of us have a shaky grasp of basic ideas such as activity coefficients and very few seem to have heard of Ideal Solubility (Van't Hoff)- though without knowing that crystalline solubility depends on MPt and enthalpy of fusion, much of what we do is incomprehensible.
I use HSP on a regular basis because it gives me the right balance of power and simplicity for the sorts of issues I come across in my work. What interests me is that many of its limitations are not so much because of the approximations behind it. Rather it's because there are plenty of solubility-related issues that defy any current solubility theory. The word "hydrotrope" opens up a bizarre world of confusion because that one word covers many different phenomena which in turn have other terms such as "solvosurfactant" which are too ill-defined to be of much help. Whether you are adding urea as a classical small-molecule hydrotrope, using Tween-80 as a "solubilizer", using a Dowanol as a "solvosurfactant" or using a genuine microemulsion to solubilize something, there are "solubility" effects that go beyond anything that "solubility theory" can currently explain.
My current attempts to understand and explain "solubility" are in my Practical Solubility site and attempts to explain "hydrotropes" are in my Practical Hydrotropes site. I'm planning to revise the Hydrotropes site shortly thanks to work with my colleague Dr Seishi Shimizu at U. York who is using a fundamental approach (Kirkwood-Buff theory) to revolutionise the understanding of what is going on.
There are also interesting things happening in the worlds of organogelators and Metal Organic Frameworks where "solubility" thinking (and, especially, HSP) is starting to become part of the way to bring some order to the current chaos of lots of data with little explanatory or predictive power. Yet there are clearly other issues beyond solubility. The trick is to work out where solubility ends and other effects take over. A paper I coauthored with Prof Joachín Coronas at U Zaragoza Using Hansen solubility parameters to study the encapsulation of caffeine in MOFs gives some idea of the challenge and opportunities with respect to MOFs.
I thrive on feedback, positive or negative, on the contents of these sites. If you have views on solubility, hydrotropes etc. I'd love to hear from you!