Measurement by XPS of the recovery rate of a cosmetic treatment on natural skin

Estimation of the recovery rate concerns many manufacturers: from the coating sector to the world of textiles and cosmetics. The players in these fields are interested in it in order to verify that the treatments applied to different substrates are indeed homogeneous and cover all the areas of interest.

The coverage rate is key data for many manufacturers wishing to control the coverage of a treatment on a substrate.

In this application note, we will take the example of determining the coverage rate of a foundation on natural skin. The methodology followed is very easily transposable to sun protection creams, or other cosmetic treatments. Foundation, an essential base for any make-up, has seen its market evolve in recent years. Social networks reveal, in fact, every day photos of people displaying flawless skin with a smooth texture and an even complexion. Whether it comes from filters, innovative skincare products or sophisticated make-up, this trend has quickly won over and demand has turned to high-coverage foundations to camouflage the slightest imperfection.

To best respond to this growing appetite for a perfect complexion, the coverage rate is important data for the cosmetics industry.

Different models of artificial skins can be used to mimic natural skin in terms of morphology and chemical composition. The stratum corneum or "horny layer" constitutes, by its natural character, the ideal substrate for the evaluation of the effectiveness and the homogeneity of a cosmetic treatment. The stratum is the outermost layer of the epidermis of the skin.

Of a lipid nature rich in nitrogen (ceramides), the stratum corneum plays an important role, in particular for hydration and immune defences. It acts as a barrier, preventing molecules and bacteria from entering or leaving the body thanks, among other things, to its antibacterial properties associated with an acid pH.

X-ray photoelectron spectroscopy (XPS) comes in several acquisition modes. The grid mode is perfectly suited to the statistical characterization of the surface of a treated material.

Giving access to a data matrix spread over an analysis surface of the order of a cm², this mode makes it possible to accurately assess the homogeneous character of a treatment.
In XPS, the surface is irradiated by X photons from a monochromatic source. The atoms of the sample then emit photoelectrons possessing energies specific to each element as well as to its environment. The spectra thus obtained usually show the number of photoelectrons detected as a function of their binding energy (difference between the energy of the X-rays and the kinetic energy of the electrons emitted).
Classically, the surface analyzed by XPS is of the order of a few hundred µm². The grid mode, based on perfectly adapted acquisition software, makes it possible to cover a much larger surface of up to several cm².

TESCAN ANALYTICS has more than 20 years of expertise in the use of XPS on all types of materials, insulators or conductors... With the latest generation instruments, our team of experts works with all industrial sectors.

Figure 1: Overlay of survey spectra of untreated stratum


Figure 2: Overlay of stratum survey spectra after application of cosmetic treatment
 

The recovery rate of the samples of treated stratum can be estimated from the difference in the atomic concentrations of nitrogen, the tracer element of the skin and characteristic of the stratum, before and after application of the cosmetic treatment, from the following formula:
with :
[N]_{average reference}: average atomic % of the nitrogen measured for the untreated stratum
[N]_{treated skin}: average atomic % of nitrogen measured after application of the cosmetic treatment

The superpositions of the 16 overflight spectra acquired per sample are presented in Figures 1 (untreated stratum) and 2 (after application of the cosmetic treatment).


Table 1: Elemental compositions (atomic %)

The elemental analyzes presented above show the detection of nitrogen at 3.7 ± 0.1 atomic % at the extreme surface of the untreated stratum corneum. After application of the cosmetic treatment, the tracer nitrogen of the stratum corneum is no longer detected, giving way to iron, the tracer of the cosmetic product present at 3.0 ± 0.1 atomic %.
These results indicate total coverage (100%) of the stratum corneum by the cosmetic treatment, which has a thickness greater than 10 nm (analysis depth of the XPS).

The low standard deviations of the atomic concentrations measured between the different analysis points of the grid reflect the good homogeneity of the elementary composition of the surface of the two samples, in particular of the treated sample and of the coverage rate of the cosmetic treatment.
 

In this example, the contribution of the grid mode to statistically quantify the coverage of a cosmetic treatment on a sample of stratum corneum has been demonstrated.

Grid mode XPS is the method of choice for statistical monitoring of the coverage rate of a substrate by a treatment.

This methodology can be transposed to many applications in order to determine the effectiveness and homogeneity of a treatment.

In other works, it has been shown the strength of XPS for the evaluation of the homogeneity of a chemical functionalization treatment by atmospheric plasma on polymer films.

For other examples of the application of XPS or our other analysis techniques, ask us.

Other complementary techniques can be used to study the covering of a substrate by a treatment (TOF-SIMS in imaging mode; TEM or SEM depending on the thickness of the treatment).