94 SUN CARE
Potassium cetyl phosphate in natural formulations
Swetlana Vogt, Yana Bykov, Maximilian Franke, Martina Spiegel, Martin Husmann - Schill+Seilacher, Germany
Simple rules of science are sometimes in conflict with experimental results. Well- known models do not always describe the mechanisms of real systems. Potassium cetyl phosphate (PCetP) is such
a material in which several rules are broken when it is used as emulsifier. HLB estimation methods1
would not predict that this
anionic surfactant is an effective emulsifier. Macroemulsions based on it are remarkable in terms of their long-term stability and they show an extraordinary degree of robustness. PCetP is neither water- nor oil-soluble
as it tends to build-up natural membranes at interfaces. A synergism can be achieved when using long chain fatty alcohols as a partner. Valuable gel-like emulsions can be obtained in a natural way due to the formation of lamellar structures. This is similar to what is known about the essential role of phospholipids for living cells. As shown in Figure 1, common anionic
surfactants, like sodium lauryl ether sulfate, sodium lauryl sulfate and potassium oleate, are water-soluble and have an HLB value of above 20. Hence, a typical anionic surfactant is very far from being an ideal O/W emulsifier. PCetP has a similar chemical structure.
However, its solubility in water is very low and its HLB value, we have determined experimentally, is about 10.5. This is a typical value for an O/W emulsifier and differs considerably from the estimated value calculated by incremental methods. This calculation is applicable for classical emulsifiers, but is not suitable for PCetP.
Potassium Cetyl Phosphate HLB=10.5*
shows unusual low HLB for anionic surfactant *HLB by laboratory experiments
5 Behenyl Alcohol/Rice Bran Wax Figure 1: HLB ranges & values of different anionic surfactants
Liquid crystal stabilisation A combination of PCP and long chain alcohols such as behenyl alcohol shows outstanding emulsifying properties and is commercially available under the trade name Pair2Phase1, hereafter referred to as P2P emulsifier. This is a COSMOS- and NaTrue-certified, easy-to- handle universal emulsifier, which imparts a large number of benefits when formulating creams and lotions. To explain the attributes of P2P emulsifiers,
the existence of lamellar networks and hence the presence of liquid crystals (LCs) should be considered. The interaction of the alkyl chain of
PCetP and a long-chain alcohol, such as behenyl alcohol, lead to a structure similar to that of membrane lipids. As shown in Figure 2, single oil droplets are
surrounded by a monolayer of surfactants. In addition, multiple bilayers shield the droplets with a much more pronounced stabilising effect. Due to the network formation, a viscosity increase is observed and can range from a thin lotion to a robust cream. This model also indicates that the system should show gel-like flow behaviour which is typical in reality. It has to be noted, that the lamellar
structures are probably induced by shear when 0 spreading agents
Emulsifiers (water in water)
Antifoaming agents Wetting and
Sodium Lauryl Ether Sulfate HLB=42 Sodium Lauryl Sulfate Potassium Oleate
20
HLB=40 HLB=20
15 Solubilizers
Emulsifiers (oil in water)
Detergents
Water
Oil
Figure 2: Emulsion stabilisation promoted by Pair2Phase1 Note: Lamellar LC-domains shield the oil droplets inside the O/W emulsion (red: PCP, blue: long chain
alcohol) PERSONAL CARE November 2021
Figure 3: Optical micrograph of an emulsion of sunflower oil in water
with P2P emulsifier Note: The oil droplets are not perfectly spherical due to the encapsulation by a lamellar network
www.personalcaremagazine.com
Hydrophilic (oil souble)
dispersible
Water
(water souble)
Hydrophilic
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