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“Give me matter, and I will construct a world out of it!” —Immanuel Kant
Maybe you develop skin care products, shampoo or fragrance. In reality, you develop mixtures of chemicals that interact with skin, hair and the environment. Because of this, being able to characterize the molecules you work with every day is increasingly important. With thousands of ingredients at product developers’ disposal—and hundreds being added each year—it is essential to be able to predict as much as possible about their behavior. For skin care actives, this means a way to predict skin penetration. For green products, it means being able to assess environmental impact. When creating a formula, it helps knowing where ingredients will go and how they will interact.
Solubility is typically viewed in terms of oil and water, or, alternatively, as either polar or nonpolar. This is a simple, black-and-white view of nature—a view that needs to be refined into a graduated scale of values. The solubility parameter does this by assigning numbers based on contributions from different parts of the molecule. There also are other approaches, such as ClogP, that rely on the physical partitioning between water and oil.1
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Traditional chemistry is usually concerned with strong forces: covalent or polar bonds. Solubility depends on weak forces, which are ignored much of the time. An example of strong and weak forces is gravity versus magnetism. Gravity is much weaker—a magnet can hold a piece of metal against the entire gravity of the earth, but gravity holds us down and makes the earth move around the sun. Sometimes strong forces are in charge, but other times, weak forces rule.
The van der Waals force is loosely used to describe all the weak intermolecular forces. Understanding these weak forces allows many valuable predictions to be made on how materials used in the creation of beauty products will act in many vital ways. Evaporation rates, compatibility of ingredients, properties of nanoparticles and much more can be understood more precisely when the underlying properties are understood as well.
The Importance of a Solubility Parameter
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.
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.
HSP also sheds light on nanotechnology. For example, carbon nanotubes are often considered insoluble or poorly soluble in organic solvents. In fact, carbon nanotubes can be often be solubilized in suitably chosen solvents, where their solubility optimization can be discovered and understood in terms of the HSPs of the constituents.
Because product developers work with thousands of chemicals intended for use on human skin and hair, as well as impact the environment, you need to know as much about their behavior as possible. Data is always best, but theoretical constructs and computer models are increasingly relied upon. For many important situations, from the partitioning of an ingredient in a cosmetic formulation to the compatibility of solvents with polymers to a predicted degree of skin permeation, the HSP is an essential tool. Areas as seemingly diverse as nanotechnology and aroma perception can be illuminated by the application of solubility parameters, making solubility parameters invaluable in understanding how materials behave and interact.
- S Herman, Smells great! What’s the ClogP?, GCI mag, 164 3 22–25 (1999)
- S Abbott, CM Hansen and H Yamamoto, Hansen Solubility Parameters in Practice, 3rd ed (2010)
- Sloan, B Kenneth, et al., Use of Solubility Parameters of Drug and Vehicle to Predict Flux Through Skin, J Invest Dermatol 87 244–256 (1986)
Steve Herman is president of Diffusion LLC, a consulting company specializing in regulatory issues, intellectual property, and technology development and transfer. He is a principal in PJS Partners, offering formulation, marketing and technology solutions for the personal care and fragrance industry. He is an adjunct professor in the Fairleigh Dickinson University Masters in Cosmetic Science program and is a Fellow in the Society of Cosmetic Chemists.