Effect of molecular weight of polyvinylpyrrolidone on the skin irritation potential and properties of body wash cosmetics in the coacervate form

Material

Raw materials used in the commercial cosmetics were used to develop the body wash gels: Sodium Laureth-2 Sulfate (SLES, trade name Brensurf 25; supplier Brenntag, Poland), Cocamidopropyl Betaine (Dehyton K, BASF, Germany), Citric Acid (Citric Acid, Chempur, Poland), Polyvinylpyrrolidone (pharmaceutical purity); average molecular weight 10 000, 40 000, 1 600 000 g/mol (Luviskol K17, K30, K90 BASF, Germany), Sodium Chloride (Sodium Chloride, Chempur, Poland), Sodium Benzoate and Potassium Sorbate Euxyl Schulke and Mayr, Schulke Poland), MiliQ water. In the physico-chemical tests were used: zein from corn (Zein, Sigma Aldrich, USA), Albumin (Albumin Bovine fraction V, Bioshop, Canada), potassium sulfate (Chempur, Poland), copper sulfate pentahydrate (Chempur, Poland), sulfuric acid 98% (Chempur, Poland), Tashiro indicator (Chempur, Poland), sodium hydroxide, citric acid (Chempur, Poland). All reagents were analytical grade.

Methods

Analyzed of coacervation phases

The process of coacervation was observed in all four starting formulations as a result of sodium chloride addition. Table 2 lists values of the optimum NaCl concentration necessary for coacervation to take place in each of the samples under analysis and volumes of the (surfactant-rich) coacervate phase and the (diluted, surfactant-poor) aqueous phase formed during the process.

In each of the cases analyzed, the coacervation process produced biphasic systems in which coacervate was the top phase (the dye introduced to the system passes into the enriched phase, i.e. coacervate). The coacervation process occurred for different test samples at varying NaCl concentrations. The lowest salt concentration (12.5 g of NaCl per 100 g of the starting formulation) was needed for coacervate formation in the baseline formulation. Introducing PVP into the system contributes to an increase in the content of the electrolyte, for which the process of coacervation is observed. Samples containing the polymer with molecular weights of 10 000 and 40 000 g/mol required the addition of 13.5 g of NaCl for coacervation to occur. An increase in molecular mass caused an increase in the optimum electrolyte concentration. Accordingly, for the starting formulation containing PVP with a molecular mass of 1 600 000 g/mol the process of coacervation took place after adding 14.5 g of NaCl to 100 g of the starting formulation. Polyvinylpyrrolidone in systems containing anionic surface active agents shows an ability to create complexes with surfactants occurring both in the form of monomers and micellar aggregates formed in solutions after the CMC is exceeded (Fig. 1) [47], [48], [49], [50], [51], [52].

Fig. 1:

Interactions of surfactants with polymers.

The emergence of polymer-surfactant complexes of this type may have a direct effect on the observed differences in NaCl concentrations determining the occurrence of the process of coacervation and the formation of the coacervate phase. The addition of polymer (PVP) has an impact on increasing the system’s stability. The transformation of micellar shapes from spherical to cylindrical and then lamellar droplets, the flocculation of which is essential for the process of coacervation, may be markedly more difficult in the presence of PVP which is capable of their stabilization. However, as the molecular mass of PVP grows, system stabilization increases, and consequently inducing the process of coacervation requires a higher concentration of NaCl to occur.

The study also found that the addition of PVP had an impact on the volume of phases arising after the separation of systems due to the coacervation process. The phase separation ratio after coacervation will affect the content of active substances in the coacervate phase and determine its concentration, and ultimately the functional properties of body wash products in the coacervate phase. Consequently, the concentration of NaCl was selected in such a way as to make it optimal and to achieve the highest possible concentration of surfactants in the separated coacervate phase. The highest percentage content of the coacervate phase (60%) was noted in the baseline sample. Consequently, this sample can be treated as the sample with the lowest concentration of surfactants. PVP-enriched samples were characterized by a lower percentage content of the coacervate phases (i.e. higher degrees of concentration), at a level of approximately 55%. However, the molecular mass of PVP was found to have no effect on the volumes of separated coacervate phases (differences within the margin of error).

In the next stage of the study, the content of active substances (surfactants+PVP+dye) in the separated coacervate phases was evaluated. To this end, gravimetric assessment was performed for each coacervate obtained from the starting formulation to determine the content of dry matter and sodium chloride. Based on the difference between the values, the content of active substances contained in the phases separated during the process of coacervation was evaluated. The results are listed in Table 3.

The results show that the addition of PVP to the starting formulations for obtaining body wash cosmetics in the coacervate form produces more concentrated coacervates than that obtained for the baseline sample. The coacervate phase arising from the coacervation process in the baseline sample contains approximately 13.1% of active substances. The content of active substances in the samples P1, P2 and P3 containing PVP with different molecular masses was in the similar range of approximately 15.8–15.9%. The molecular mass of the polymer had no effect on the content of active substances in the obtained coacervate phases. The differences identified between the baseline sample and the samples P1, P2 and P3 are attributable to a difference in the compositions of starting formulations from which coacervate is obtained (in addition to surfactants present in the baseline sample, the samples P1, P2 and P3 additionally contained PVP) and a difference in the volume of obtained phases.

Irritant potential

In the next stage of the study, the coacervate phases obtained from the starting formulations were preserved, and their pH was adjusted to the value of 5.5±0.1. The coacervate samples thus obtained, representing model body wash cosmetics, were tested to evaluate their skin irritation potential. The zein number was determined, and an incubation test with bovine serum albumin was performed. The results are shown in Figs. 2 and 3.

Fig. 2:

Irritant potential (zein value) of model body wash cosmetics in coacervate form.

Fig. 3:

Increase of pH of BSA with model body wash cosmetics in coacervate form.

As pointed out in the introductory section, one of the main disadvantages of using body wash cosmetics is their potential to cause skin irritation. This applies in particular to cosmetics based on anionic surfactants such as sodium laureth sulfate or sodium lauryl sulfate. Following their adsorption on the skin surface, they interact through electrostatic interactions with the surface proteins. The process leads to changes in the structure of proteins and their denaturation. Water used for skin washing exhibits no ability to dissolve and wash away proteins from the skin. In contrast, surfactant solutions through their protein-binding ability may result in proteins being washed away from the skin, which manifests itself through skin irritation. The literature indicates that the process outlined above is the basic mechanism underlying skin irritations induced by washing substances. It has been proposed that the skin irritation potential of body wash cosmetics should be reduced by limiting the surfactant-induced process of washing proteins from the skin. Results of multiple studies show that the main factors affecting interactions between surfactants and skin surface proteins are the type, concentration and duration of skin exposure to surfactants. The strongest interactions are observed at concentrations below the critical micelle concentration (CMC) and for surfactants containing 12 carbon atoms in the hydrophobic chain. At concentrations below the CMC, the binding of surfactants to proteins rises significantly along with the increase in surfactant concentration. At surfactant concentrations above the CMC, the interactions are not directly proportional, and their nature depends on the type of the surface active agent. Studies [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] show that irritant effect is attributable primarily to monomers, i.e. individual surfactant molecules (the form occurs in solutions below the CMC). On account of their small size, monomer molecules are able to easily penetrate through the skin and interact with proteins. The process is much more difficult for micelles, which are larger in size. Aggregates arising in micellar systems are thermodynamically unstable and, as a result, they constantly disintegrate, releasing free monomers into the volume phase of solutions. Consequently, stabilization of micelles formed in solutions, coupled with a decrease in the number of free monomers contained in them, may be the best solution for reducing the adverse interactions of surfactants with the skin and the development of skin irritations [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23].

Studies often use a test imitating interactions taking place between surfactants and proteins, which is based on zein number determination. Results obtained for analyzed coacervates show indisputably that the incorporation of PVP into the system markedly enhances the safety of using body wash cosmetics in the coacervate form. The result achieved for the baseline sample (approx. 270 mgN/100 mL) shows that the coacervate has an irritant effect on the skin. The addition of PVP to the formulations of model body wash cosmetics contributes to a decrease in the skin irritation potential, and the higher the polymer’s molecular mass, the more the skin irritation effect is reduced. The value obtained for the sample containing PVP with the lowest molecular mass is approximately 215 mgN/100 mL, and for the coacervate containing PVP with the highest molecular mass it is about 180 mgN/100 mL. Consequently, after PVP is added, the skin irritation potential of coacervates is reduced from approximately 20% (sample P1) to approximately 30% (sample P3) in relation to the baseline sample. The results were confirmed by performing an incubation test with BSA solution. Imokawa et al. [55] and Tavss et al. [57] proposed investigating changes in the pH level of the BSA solution as a method of predicting the irritant activity of the surfactant solution added to it. The authors demonstrated the suitability of the method for evaluating the irritant potential of anionic surfactants and their mixtures with other surfactants. Nonionic, cationic and amphoteric surfactants used alone have a very minor effect on BSA. The results obtained by Imokawa and Tavss corresponded to the results of the zein test and collagen swelling test, which demonstrates the suitability of the method for the estimation of irritant activity. An increase in the pH of the solution observed in such systems is caused by the neutralization of cationic BSA groups by anionic surfactants, and the adsorption of protons from the solvent (water) by negatively charged groups in the protein molecule [56], [57].

For the baseline sample, the pH increase was about 25%. In the PVP-enriched coacervates the increase was much lower and diminished along with increasing molecular mass of the polymer used. In the sample containing PVP with the lowest molecular mass it was approximately 20%, and in the model coacervate enriched with PVP with the highest molecular mass the increase in the pH of the BSA solution was just 15%. The results of both tests confirm the effectiveness of PVP as a substance that strongly reduces the skin irritation effect of anionic surfactants. The effectiveness of the polymer in the stabilization of structures formed in micellar systems was found to rise in proportion to increases in its molecular mass. Furthermore, longer polymer chains, which have more active centers capable of “capturing” free monomers, are able to decrease the adverse effect of surfactants on the skin proteins more powerfully.

Stability and turbidity

In the process of product formulation it is vital to ensure that the desired improvement in product quality, and the selection of ingredients and product form to enhance a particular parameter, will not adversely affect other product quality determinants. As shown, the addition of PVP to the model body wash formulations, and the use of coacervate (a new physiochemical product form) contribute significantly to a reduction in the irritant effect. As the next stage of the study, an attempt was undertaken to evaluate the basic quality parameters in order to determine the effect of the solutions implemented in the model products on viscosity, stability and foaming properties. The stability of the coacervates obtained in the study was assessed on the basis of COLIPA Guidelines [58]. Thermal and mechanical load tests found the products to be stable. Also, there were no signs of microbiological instability or visual changes in the samples during a period of 6 months after product preparation. These findings were confirmed by turbidity tests which, after 6 months, yielded results that differed only in a statistically insignificant way from the values noted immediately after coacervate formation. In addition, the molecular mass of PVP was not found to have any effect on the measured values (Fig. 4).

Fig. 4:

Turbidity of model body wash cosmetis in coacervate form.

Foaming properties

Furthermore, the tests found that the formulated model body wash cosmetics in the coacervate form exhibited very good foaming properties, and the foam they produced achieved very high values of the foam stability index (Fig. 5). In addition, there was an increase in the foaming ability of the products and a slight increase in foam stability index along with increasing molecular mass of PVP used in the formulations. The foaming ability in the sample P1 was about 10% higher than in the baseline sample, and in the sample P3, containing the polymer with the highest molecular mass, the value was approximately 20% higher.

Fig. 5:

Foaming properties of model body wash cosmetis in coacervate form.

The results obtained in the study can be interpreted by referring to surfactant–polymer interactions. The findings are consistent with reports available in the literature. Micelles arising in polymer–surfactant systems initially, i.e. at low surfactant concentrations, have the form of aggregates bound to polymer chains and emerge at a specific concentration which is referred to as the critical aggregation concentration (CAC). An increase in surfactant concentration causes gradual saturation of polymer chains with micelles. After all the active centers on the chains are saturated, free (polymer-unbound) micelles start occurring in the volume phase. Higher foaming ability and foam stability values are attributed, as Folmer and Kronberg claim [53], [54] a higher viscosity of the film separating gas bubbles in the foam structure. Higher viscosity reduces the dehydration of the intervesicular film, which leads to an increase in stability and volume of foam produced in polymer-containing systems, as compared to systems based exclusively on surfactants.

Viscosity properties

As the literature and our results (Fig. 6) show, the viscosity of coacervates increases as a result of adding water to them. Viscosity measurements performed for the model body wash cosmetics in the coacervate form as a function of added water are shown in Fig. 5.

Fig. 6:

Influence of water mass introduced into coacervates on their dynamic viscosity.

This is a very interesting property of the coacervate, particularly as a novel form of body wash cosmetics, because consumers view the viscosity of body wash products as the main determinant of quality (highly viscous products are regarded as products of top quality). Based on this property, coacervates can be seen as concentrated body wash products, and hence products with a high ecological appeal. A coacervate with a low viscosity level which is diluted in water acquires the consistency of a traditional product as a result of increasing viscosity. Consequently, consumers are able to control product quality according to their individual needs (coacervate dilution with water at different ratios, for example 1:1 or 1:2, makes it possible to obtain a product with more potent or more delicate washing abilities, etc.). In addition, the possibility of product distribution in small packages translates into a reduction of the size and volume of waste generated in households.

An increase in coacervate viscosity achieved through the addition of water is due to changes in the structure of micellar aggregates present in the coacervate. As mentioned above, the coacervate phase contains primarily bilayered lamellar structures in the form of the so-called lamellar vesicles [7], [8], [15], [16], [17]. Following the addition of water, the product becomes diluted and the structures transform into cylindrical micelles (an increase in viscosity). An increase in the concentration of added water increases viscosity up to a certain maximum value, after which viscosity begins to drop as a result of formation of flattened spherical micelles, followed by spherical micelles, in the system [7], [8], [9], [10], [11].