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Water and Oil

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

“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

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