Emulsion & Foam Stability in Aerosol Products Montfort A. Johnsen, Ph.D. Most of us don’t realize that about 78% of U.S. aerosols contain from 15% to 95% water, mostly due to economic and regulatory pressures. Emulsion and foaming aerosols obviously are water-based; however air fresheners, disinfectant-deodorant sprays and many hair sprays also contain water, but less obviously. Not all emulsions foam, and not all foams are produced by emulsions. Some emulsions and foams are anhydrous. Sea water can foam on beaches, not because of the salt content, but due to impurities that include micron-sized particulates. A 2.0v/v% solution of 2-methylbutanol in water produces a foam that finally collapses about 40 minutes after initial shaking. An emulsion has generally been defined as a relatively stable suspension of liquid particles in an immiscible liquid. By “relatively stable” it is meant from about two minutes to two years. In most emulsions, the particles are about 0.2 to 50 microns in diameter. They are white and opaque. Technically, they are called as macro-emulsions. When the particles are 0.01 to 0.20 microns, they are termed micro-emulsions, and when the particles get down to 0.05 micron or smaller they become transparent. The “natural emulsions,” called soaps, consist of salts of fatty carboxylic acids—such as sodium stearate (from lard) and potassium laurate (from coconuts). They have been used for cleaning purposes since time immemorial. The situation is different for “synthetic emulsions,” often called syndets, which were first produced in Germany in 1917. A typical compound was propyl-napthalene sulfate or “Nekol”. It had limited popularity. Then, about 1928, sodium lauryl sulfate (SLS) and kindred products were developed. They were white, odorless and soluble in water in all proportions. Still, the growth of syndets was very slow at first, with most people Table 1. preferring the venerable bars of facial, body and laundry soap, as well as laundry and industrial powders and emulsions. However, gradually the syndets became recognized for their superior cleaning power, better foaming qualities and compatibility with low pH and hard water (see Table 1). Types of Surfactants Anionic surfactants are the most popular, with about 80 types, 70 of which are either sulfates or sulfonates. Non-ionics are next and almost all are condensate products of ethylene oxide and hydrophobic (water immiscible) compounds. In general, at least six ethylene oxide (EtO) groups per molecule are required before it becomes water soluble. Cationic surfactants are used less frequently. They are usually amines or quaternary ammonium salts. They often exhibit poor detergency but are important as germicides and fabric softeners. They are more costly than anionic and nonionic surfactants. As a rule, they are incompatible with anionics, excepting amine oxides. Finally, there are the amphoteric surfactants, including the zwitterons as a special type. Typically, they consist of fatty acid condensates with technicAl eDitor Polenghi-coltivia chocolate Mousse offers convenient portion control, enhanced decoration and minimal waste. Valves and actuators are from lindal. 34 Spray June 2017 U.S. SALES (1,000 TONS) YEAR SOAP SYNDETS 1940 1,410 4.5 1980 550 5,010 1990 465 5,620 Table 2. U.S. RELATIVE SALES OF SURACTANTS CLASSIFICATION ALL SURFACTANTS SYNDET SURFACTANTS ONLY Soap 7% --- Anionics 65 70% Non-ionics 19 21 Cationics 6 6 Amphoterics 3 3
Spray June 2017
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