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Water and Oil

By: Steve Herman
Posted: June 7, 2011, from the June 2011 issue of GCI Magazine.

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The interaction of materials is complicated, and consequently the value of the solubility parameter depends on the assumptions used to calculate it. The Hildebrand solubility parameter (SP) is the square root of the cohesive energy density, a measure of all the forces holding things together. The Hansen solubility parameter (HSP) uses three parameters: dispersion forces, intermolecular bonds and hydrogen bonding.

There are three sources for HSP values: the literature, online databases and computer programs. The literature should contain reliable data, but unfortunately, this isn’t always the case. A government study2 found a large amount of data errors published on the properties of DDT. Accuracy, reproducibility and citations are all questionable. If the literature on DDT is rife with error, it should make product developers cautious about the accuracy and methodology of all published data.

Hansen Solubility Parameter in Practice3 (HSiP) is a computer program that creates 3-D images of solubility regions. The closer two materials are in the space, the more likely that they will be compatible. It was designed originally to optimize solvents for polymers, which in itself is useful for beauty product applications. HSPiP software creates a graph in which a green ball within the graph represents the compatible volume in three dimensions, and databases have been added to the program for cosmetic and fragrance materials, further enhancing its value for beauty product development.

As an aid to using this program, a guide4 is included that gives several examples pertinent to the beauty industry. One subject considered is the prediction of skin penetration. Obviously human skin is too complicated a structure to have a simple HSP, but some general values can be assumed. In considering the penetration potential of molecules, factors such as molecular size and shape are important, in addition to the solubility parameter. Nonetheless, the HSP is universally regarded as a useful tool in evaluating drug delivery. An example is found in the work of Sloan,5 who looked at the importance of the solubility parameter of a drug used for respiratory problems and discovered clear correlations. Obviously, the efficacy of cosmetic actives can also involve and benefit from the same optimization as effective drug delivery.

Further Uses

HSP can also help design electronic noses (e-noses) and understand human ones.6 The CalTech e-nose uses an array of conducting polymers to precisely categorize an odor. Solubility parameters can be used to predict the response of chemicals to the e-nose. While human noses are more complicated in their response than electronic ones, it has been suggested the HSP still provides some indication of why molecules smell as they do, and SC Johnson has assisted the effort by making data on 144 chemicals pertinent to understanding the connection of the HSP to an odor response publicly available.