Hard vs. Soft Complex

One major area that cosmetic formulators are interested in is the interaction between anionic and cationic surfactants. Most formulators of two-in-one shampoos understand that indiscriminate mixing of anionic and cationic materials can result in undesired, insoluble solids.

Incompatible anionic and cationic materials that are mixed together are referred to as hard complexes.1 As the expression implies, the cationic and anionic compound possess properties that, when added together, form insoluble complexes or salts.

Alternatively, anionic and cationic compounds that can be mixed over a wide range of ratios and that provide a clear, viscous, high-foaming complex are defined as soft complexes. Optimized soft complexes have many desirable properties including high levels of foam, viscosity build without alkanolamids, conditioning properties and low levels of eye and skin irritation. Proper matching of the complex can lead to the formulation of clear products that have superior conditioning and deposition delivered from a clear formulation.

These terms, used for describing the interaction of cationic surfactants and anionic surfactants, are an adaptation of the work of Pearson used to describe acids and bases. Pearson proposed that “hard acids bind strongly to hard bases and soft acids bind softly to soft bases.”2 The anionic and cationic interactions are exactly analogous.

The structural changes made to cationic molecules can “soften” them, making them more compatible with anionic systems. Silicone added to the molecule can soften it, allowing for use in clear two-in-one conditioners. The compatibility of specific quats with sodium lauryl sulfate (SLS) and sodium laureth-3-sulfate (SLES), the foam properties of the combinations with SLS and SLES, and the substantivity of these combinations with SLS and SLES are key factors in understanding the function of conditioners.

1. T O'Lenick, Anionic/Cationic Complexes, Surfatech company Web site, available at http://surfatech.com/pdfs/Complexes.pdf
     (Accessed Apr 23, 2008)
2.  RG Pearson, J Am Chem Soc, 85 335 (1963)

More in Literature/Data