Very few properties of ingredients are more important to a formulator than polarity, and no concept has helped me more over the years to solve formulation problems. Polarity can affect solubility, wetting, dispersability, SPF, sunscreen stability, skin penetration, formulation stability, and emulsification. It is defined as the unequal sharing of electrons between atoms in a molecule in which one has larger electronegativity than another, resulting in a dipole or "bent" structure.
Polarity is dependent on the electronegativity differences in the molecule and its symmetry, and can often be estimated by simply studying the molecule. Polar molecules are typically linear asymmetric molecules, meaning molecules with a single hydrogen, a single OH at one end, or with a single nitrogen at one end. Nonpolars include most carbon compounds and diatomic molecules of the same atoms.
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Polarity can be calculated or determined empirically in the lab using Hildebrand solubility parameters (SP). SP is defined as the total sum of cohesive/interactive molecular forces responsible for solubility or the concept that like molecules are attracted to each other. SP can be calculated using the Fedor’s group substitution method which uses the summation of theoretical heats of vaporization and molar volumes of constituent groups making up a molecule, or there is the Hildebrand method which uses boiling points and molecular weight. Both methods produce similar results.
The values of most cosmetic ingredients range from 5 (very nonpolar) to 16 (very polar). SP has been historically used to predict package compatibility problems, solubility, required HLB, plasticization of polymers, surface adhesion, partitioning of materials, miscibility, and polymer swelling.
There also are several simple techniques for measuring SP. The density of non-halogen or silicone-containing materials correlates well and increases with polarity. The solubility of a 50/50 mixture of an ingredient in SDA 40 alcohol can also give you a quick approximation. A clear solution usually has an SP >8, a smooth dispersion which separates 7-7.7, and a poor dispersion which separates immediately <7. Or the simplest way to quantify SP is to measure the dielectric constant or relative permittivity using a Brookhaven BI-870 Dielectric Constant Meter or a similar type of device. Readings are quick, taking only 1-2 minutes and are very reproducible if the temperature is controlled. The Hildebrand SP can be calculated from the dielectric constant data using the following equation (SP= .2005x(DEC)+7.5507, r²=.92). Required HLB of an ingredient can be calculated from SP as well (required HLB=[ (SP+7) /8]4).
Vaughan CD, Using solubility parameters in cosmetics formulations, J. Soc. Cosmet. Chem., 36, 319-333 (September/October 1985).
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