25 March 2014
The effects of water-soluble phosphates in cosmetic emulsions: complexation
Carephos N sodium polyphosphate shows a good and stable complexation performance in cosmetic emulsions according to tests
By BK Giulini
In the past, phosphates in cosmetic formulations have been limited to use for regulation of pH value and as buffer reagents. The multi-functional action mechanisms of water-soluble phosphates - dispersion, complexation, pH regulator, buffer reagent, bacteriostatic (1) - are rarely used in cosmetics.
In contrast, phosphates already play an important and helpful role in many other areas of our daily lives. Due to their excellent calcium-binding capacity and outstanding dispersion activity, phosphates are used in many laundry detergent and cleaning agent formulations. The dispersive effect of phosphates makes it possible to manufacture water-based paints with inorganic pigments. Phosphates have also been gaining in significance in the food industry.
For this reason, tests have been carried out to determine potential ways of using phosphates in cosmetic emulsions. In the present analysis, three water-soluble phosphates (INCI: Sodium hexametaphosphate), all with different pH value, were used to carry out the testing.
• Carephos N , pH 7.3-7.9 (1 % water)
• Utanit AF, pH 3.0-3.4 (1 % water)
• Phoskadent Pyro, pH 10.0-10.6 (1 % water)
Phosphate complexation action
To detect and document phosphate complexation action, iron ions are used which can be introduced into formulations through tubes, mixers or raw materials, for example. As base formulation, a recipe for sun formulation containing Butyl Methoxydibenzoylmethane is used. Since it already contains traces of iron, this UVA filter forms an intensive, red complex.
Production takes place using the system described in Table 1.
Table 1: Production system of various formulations to achieve complexation effect.
A colour measurement is used to carry out the assessment. To do this, the Minolta Chroma-Meter CR 300 is used to capture the colour conditions of the formulations after six weeks. Numerical values are obtained during the measurement making an objective comparison of the individual formulations possible. Three numerical values are determined during the measurement: L, a and b. The L value describes the light-dark values, where "0" represents the ideal black and "100" the ideal white. The a-value describes the red-green values and the b-value the yellow-blue values. The a- and b-values have different prefixes, because with the present CIE Lab system it is assumed that no colour can be reddish or greenish at the same time, nor yellowish or blueish at the same time. Thus, –a stands for green, +a for red, –b for blue and +b for yellow.
The differences from the "white standard" of the L-, a- and b-values are shown in the measurement. Since red complexes are formed in this particular case, the a-values must be given particular consideration – these are shown in Figure 1.
The higher the a-value, the worse the complexation action. For comparison purposes, the value from the formulation with 0.01% iron-III-chloride-hexahydrate is used.
Figure 1: a-values from the colour measurement
The best complexation performance is achieved by the acidic phosphate Utanit AF. However, the formulations containing Utanit AF do not remain sufficiently stable. It is possible to achieve stability in the formulation by adding the alkaline phosphate Phoskadent Pyro; however, the level of complexation action will be reduced considerably. A complexation performance at a similarly good level to formulations containing Utanit AF is achieved by the phosphate Carephos N. Furthermore, the formulations with Carephos N remain stable. In the present case, Disodium EDTA cannot be used to reduce the red colouring.
Figure 2: Left: Formulation with 0.09% Carephos N, Right: Formulation with 0.09% EDTA
(1) Maier, Scherer, Loessner: Long-chain polyphosphate causes cell lysis and inhibits bacillus cereus septum formation, which is dependent on divalent cations; Applied and Environmental Microbiology; Sept. 1999, p. 3942-3949, Vol. 65, No. 9]