Comparatively Speaking: Amphiphilic vs. Ionic Amphiphilic Molecule

The term amphiphile was coined by Paul Winsor 50 years ago.1 It comes from two Greek roots. First, the prefix amphi, which means "double," "from both sides" and "around," as in amphitheater or amphibian. The term's root, philos, expresses friendship or affinity, as in philanthropist (the friend of man), hydrophilic (compatible with water) or philosopher (the friend of wisdom or science). Amphiphilic materials have one or more of groups present in a molecule that would be insoluble in each other in their pure form including: water-soluble groups, oil-soluble groups, silicone-soluble groups and fluoro-soluble groups.

Ionic amphiphilic groups are materials that meet the definition above, but also possess an ionic group. The ionic group can be anionic, cationic or amphoteric. The formulator must familiarize oneself with ionic amphiphilic groups, as they can interact to provide either an insoluble salt or a complex that has increased viscosity, foam and lower irritation. Capitalizing on the interaction between ionic amphilphilic compounds is one of the interesting ways to formulate more cost-effective products.

Anionic and cationic surfactants can be incompatible; this is expressed, for example, by the white, sticky solid resulting from adding stearylalkonium chloride to sodium lauryl sulfate (SLS). Such material has been termed a hard complex. As the expression implies, the cationic and anionic compounds possess properties that when added together form insoluble complexes (salts). There are other cationic materials with different structures that are soluble in the presence of anionic surfactants. The terms used here for quats and anionic materials 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

On the other hand, unlike the combination of stearalkonium chloride with SLS, the combination of cocamidopropyl trimethyl ammonium chloride with sodium laureth sulfate results in a high foaming, high viscosity (50,000 csp), low irritation mixture that has good foaming attributes and good conditioning.3

The ability to capitalize on the formulation with ionic amphiphilic compounds—anionic surfactants interacting with amphoteric surfactants—is also important. Aminoproprionates do not produce an increase in viscosity when formulated with sodium lauryl ether sulfate-3 (SLES-3), but interactions between cocamido betaine SLES-3 can result in a 40,000 csp clear liquid with good foam.4

Formulators are encouraged to review the interactions between ionic amphiphilic compounds to know the synergies that they offer to maximize the effectiveness of formulations.

1. A Jákli and A Saupe, One- and two-dimensional fluids: Properties of Smectic, Lamellar & Columnar Liquid Crystals, Boca Raton, FL, USA: Taylor and Francis Group of CRC Press (2006)
2. RG Pearson, Hard and soft acids and bases, J Am Chem Soc 85 (22) 3533–3539 (1963)
3. AJ O’Lenick, and L Anderson, Anionic/Cationic Complexes, Cosmet & Toil 120(11) 63 (2005)
4. AJ O’Lenick and L Anderson, Amphoteric Anionic Interactions, Cosmet & Toil, 121(3) 67 (2006)

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