SUN CARE
illustrate how the esters behave in a perfect world. As shown in Figure 1b, when the new polymers are above the critical concentration of entanglement (c*
),9 they
organise into structures to maximise the overlap of the periphery. Hydrophobic materials can be loaded into the regions of fatty groups surrounding the hydrophobic core (the red overlapping regions on the cartoon).
The dual polarities of the new polymers make them soluble and effective when added into polar oil-based sun care formulations as well as non-polar oil-based sunscreen formulations. The major benefit of these spider esters is that they are capable of ‘encapsulating’ sunscreen filters in the core and ‘shielding’ them from the surrounding environment. This allows the filters to be placed into a wide variety of solvents, while also this ‘shielding’ of the filters can drastically improve their performance. The hydrophilic core will respond to the polar solvent in the opposite manner, and the solvent will cause the core to swell and maximise its contact with the polar solvent. This phenomenon is the basis for the ‘loading’ or encapsulation of small molecules into the core of the spider ester. We have coined this phenomenon the ‘Spider Effect’. To better illustrate the spider effect, a simple experiment was conducted. Spider ester was heated at a constant heating rate of 5.0˚C/min in the presence of avobenzone, a commonly used organic sunscreen. The temperature of the ester was monitored and recorded. As seen in the Figure 2, in the temperature range of 20˚C to 50˚C, the temperature of the spider ester increased in a linear fashion and the avobenzone remained in powder form. Once the temperature reaches 50˚C, the temperature of the spider ester solution remained constant over a 2.5 minute period. During this period, the energy being introduced into the solution is being used to drastically change the ester’s structure and not increase temperature. The core of the spider ester expands and starts to
Table 2: Formulations 3, 4 and 5. Ingredient
Sorbeth 2 Hexaoleate (Spider Ester ESO) Octyldodecyl Citrate Crosspolymer Octocrylene Octisalate
Oxybenzone Avobenzone
C12-15 Alcohols Benzoate Caprylic/Capric Triglyceride
102 PERSONAL CARE April 2012
70 60 50 40 30 20 10
02 4 Figure 2: The spider effect. a
40 30 20 10 0
0
5 10 15 20 25 Sorbeth heaxoleate (%)
b
120 100 80 60 40 20 0
0
5 10 15 20 25 Sorbeth 2 heaxoleate (%)
Figure 3: a) SPF versus concentration of sorbeth 2 hexaoleate, and b) %SPF increase by by the addition of sorbeth 2 hexaoleate.
interact with the hydrophobic periphery. This temperature range (50˚C to 60˚C) represents the ‘loading’ region of this ester. Small molecules can be loaded into the core of the spider ester during this temperature range. This loading zone allows for the solubilisation of the avobenzone and a clear solution is observed. After the ester’s core is expanded, a linear response in temperature is restored. When the solution is cooled to ambient temperature, the avobenzone remains in the core of the spider ester and a clear solution is maintained. The release of the entrapped avobenzone is controlled by the diffusion through the hydrophobic periphery and can be controlled by introducing the loaded spider into different solvents.
% w/w
Formulation 3 Formulation 4 Formulation 5 0.00
0.00 2.90 2.90 1.94 0.98
45.66 45.66
7.58 1.16 2.90 2.90 1.94 0.98
44.26 44.26
22.75 3.50 2.90 2.90 1.94 0.98
34.97 34.97
Boost in SPF
Spider esters have been the subject of multiple studies in sunscreen formulations and articles.10
The use of spider esters in
improving the SPF of sunscreen formulations is focused on making the sunscreen formulations more efficient. SPF sunscreen products work based on the ability of the sunscreen actives to absorb photons in the UVA and UVB range. The absorbance of these actives follows Beer’s law,11
which
shows that the concentration of filters is directly proportional to the absorbance. Beer’s law is the principle behind one of the most common ways to increase SPF, which is increasing the amount of filters in the finished formulation. From the published absorbance of sunscreen actives approved for use in the US it can readily be deduced that many sunscreens utilised actives at levels many times greater than should be necessary to obtain the desired SPF. This can be attributed to several factors but certainly one that is extremely important is the sunscreen active solvent system used in the formulations. Until recently, there have been three major types of sunscreen formulations:12,13 water-in-oil emulsions, oil-in-water emulsions, and alcohol-based formulations. All have their respective advantages and disadvantages. Spider esters used with these major types of sunscreens can drastically improve the SPF.
6 Time (minutes) 8 10 12
SPF
Temperature (˚C)
SPF increase (%)
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