English EditionInternational Journal for Applied Science • Personal Care • Detergents • Specialties

11-2014

A. Thiemann, J. Scholze, M. Salmina-Petersen, J. Jänichen

Wetting Agents: Friends or Enemies of Solubilizers

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Wetting agent

■■ Introduction

Wetting agents belong to the heteroge-neous group of surfactants. In a bi-phase system they concentrate at the interface between both phases e.g. liquids, sol-

ids or air and thereby lower the surface tension between them. Such a reduc-tion of interfacial tension in a cosmetic formulation can macroscopically result in interesting effects, such as improved spreadability and absorption of the for-

mulation. But also microscopically inter-esting effects can be observed. The pre- sence of a wetting agent during an emul-sification process leads, for example, to a more effective conversion of mechani-cal work into micelle formation.A common property of surfactants in general is their amphiphilic character, as result of structural parts with a polar, hydrophilic part and a non-polar, hydro-phobic part in the same molecule. The balance between the polar and the non-polar part is used to divide surfactants into groups and to draw conclusions regarding their cosmetic properties. A way to express the polar-nonpolar bal-ance is the HLB-value devised by W.C. Griffin in 1949 (1). On a scale of 1 to 20, a low HLB-value indicates a low po-lar molecule proportion, whereas a high HLB-value indicates a surfactant whose polar part outweighs its non-polar part. Wetting agents according to Fishman (2) have an HLB between 7 and 9, indicating a slightly higher molecular weight of the non-polar, hydrophobic part. Solubilizers, another group of surfac-tants, usually possess a high HLB-val-ue between 15 and 18 indicating that solubilizer molecules are constructed of a relatively large hydrophilic and a relatively small hydrophobic portion (2). Their cosmetic function is to enable the homogenous incorporation of otherwise non-soluble lipophilic materials into a water phase. Solubilizers achieve this by the formation of water-soluble micelle structures which encase the hydropho-bic material. Due to the small size of the micelle structures, they don’t scatter the light and the formed solubilizates there-fore appear transparent.

a. thiemann, J. scholze, m. salmina-Petersen, J. Jänichen*

Wetting Agents: Friends or Enemies of Solubilizers

Abstract

Wetting agents are surface active materials, which reduce the in-terfacial tension between two immiscible phases. Unlike larger surfactant molecules, however, they do not have the capacity to

form stable micelle dispersions such as emulsions or solubilizates. On the contrary, they rather tend to disrupt organized molecular layers formed by emulsifiers or solubilizers at the interface between the polar aqueous and the non-polar lipid phase. Consequently, the properties of wetting agents do not seem to make them attractive raw materials on the formulator’s in-gredient list for emulsions or solubilizates. In the present study, however, we intended to take a closer look into possible positive effects of wetting agents in such systems.For this purpose, we analyzed the concentration-dependent influence of two commercially available wetting agents on the efficacy of conventional and natural solubilizers. Our results confirmed that at elevated concentra-tions the high surface activity of the wetting agents results in a destabili-zation of the solubilizates. Lower concentrations, however, helped to pro-duce transparent solubilizates and even allowed to reduce the amount of solubilizer needed. Further, the incorporation of a wetting agent bears the potential of improving the sensory profile and appearance of a cosmetic formulation.So the answer to the question whether wetting agents are friends or en-emies of solubilizers is: The dosage makes the difference.

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In personal care products solubilizers are employed to incorporate lipophilic com-ponents such as caring oils, actives or fragrances into e.g. shower gels, tonics or aqueous fragrance products to provide transparent systems. For this purpose a variety of solubilizers exist on the market, some of them being pure raw materials, others synergistic blends of different types of surfactants. An ideal solubilizer should produce transparent solubilizates of high sensory quality for a wide range of oils and for different cosmetic applications. In the real world, however, the solubilization ca-pacity and thereby the amount of needed solubilizer depends on the type of lipo-philic substance and further co-factors in the system. These factors pose challenges to the formulators and restrict them in their creativity and might also pose addi-tional costs and work on the final product. At this point wetting agents could provide useful help.Being useless alone for the production of transparent solubilizates their ability to reduce interfacial tension at certain concentrations might be helpful to im-prove the performance of the solubilizer allowing for a lower surfactant concen-trations or even in an improved sensory profile of a solubilizate.An example in which this concept has been converted successfully into the mar-ket product is symbio®solv clear (INCI:

Caprylyl/Capryl Glycosides; Aqua; So-dium Cocoyl Glutamate; Polyglyceryl-5 Oleate; Glyceryl Caprylate; Citric Acid). In this patent-pending commercially avail-able solubilizer blend the strong wetting agent, Glyceryl Caprylate, has shown to boost the efficacy of a natural surfactant systems to a level which is comparable to PEG-40 Hydrogenated Castor Oil or Poly-sorbate-20, two petrochemical bench-mark solubilizers on the market (3, 4). In order to understand this boosting ef-fect better, we were interested in the following questions: • Howstrongdoesthewettingagent

influence the solubilization efficacy?

• Howdoestheconcentrationofthewetting agent influence the effect?

• Doalsoothersolubilizersbenefitfrom

the addition of a wetting agent?

• Whatkindofwettingagents (structure, HLB value) are compatible?

• Doesthetypeofoilaffectthewet-ting agent’s beneficial behavior in the solubilization process?

• Howdoestheadditionofawettingagent influence the sensory profile of the final formulation?

To address these questions, two com-mercially available wetting agents were evaluated in our study. Glyceryl Caprylate (trade name: dermosoft® GMCY) a mono-ester from glycerin and a medium-chain fatty acid and Caprylyl Glycol (trade name: dermosoft® Octiol) a medium size glycol. Beside the wetting ability, their multi-functional cosmetic properties comprise moisturizing, refatting, viscosity regulat-ing, antimicrobial, and also co-emulsifier activities. Both wetting agents are ap-proved in the EU, US and Japan. Glyc-eryl Caprylate moreover is available from 100 % natural resources and is compli-ant with Ecocert, BDIH, NaTrue and NPA. Fig. 1 illustrates their function-determin-ing amphiphilic structure.

■■ Test Methods and Results

Wetting agents’ concentration- dependent impact on solubilizationsThe surface-active properties of wet-ting agents potentially allow for ben-eficial as well as adverse effects on the efficacy of a solubilizer. To analyze these conflicting forces and to reveal which impact the concentration of the wetting agent has, we performed an analysis with a commercial solubilizer system and the wetting agent Glyceryl Caprylate.

new:symbio®solv clearThe second generation for PEG- and alcohol free natural solubilization

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Wetting agent

In this test we gradually varied the con-centration of the wetting agent in a sol-ubilizer blend and determined the sys-tem’s solubilization efficacy. The starting point was the solubilizer symbio®solv clear, lacking its wetting agent Glyc-eryl Caprylate, normally contained in the blend. For the determination of the solubilization efficacy, we identified the maximum oil concentration, which could be transparently solubilized by a fixed amount of 1 % solubilizer.The results in Fig. 2 demonstrate that rising concentrations of Glyceryl Capry-late steadily improve the solubilization efficacy of the solubilizer. This gradual increase of the solubilization efficacy

stops at a concentration of ca. 10 % of Glyceryl Caprylate. From this point the solubilizer efficacy falls steeply until a concentration of 12 % Glyceryl Capry-late is reached. At this concentration no transparent oil solubilization was ob-servable anymore. We assume that this general curve progression also applies to other solubilizers, wetting agents and oils with differences presumably in total values. This hypothesis was tested in the following experiments.

Solubilizer efficacy testThe composition and the polarity of an oil as well as the structure and the polarity of the solubilizer can influence

the micelle structure and as a conse-quence the solubilization efficacy. To see whether a beneficial effect of the two studied wetting agents can be seen, we conducted experiments with different market-relevant solubilizer systems and oils. The structurally diverse solubilizers originated either from petrochemical or natural sources. The petrochemical solubilizer in this analysis was PEG-40 Hydrogenated Castor Oil, a polyethox-ylated benchmark solubilizer. Besides symbio®solv clear (base lacking Glyceryl Caprylate), the group of naturally-de-rived solubilizers comprised one with the INCI Heptyl Glucoside and one with the INCI D-Sorbitan Oleate Decylglucoside Crosspolymer. The analyzed oils com-prised three perfume oils, three essential oils and three lipophilic actives (Fig. 3).For the determination of the solubiliza-tion efficacy, a fixed amount of oil/lipo-philic substance (either 0.5 % or 0.1 %) was stirred well with a variable amount of solubilizer. After addition of water the solution was checked for transpar-ency. By doing so, for every single solu-bilizer and solubilizer/wetting agent (5 % diluted in solubilizer) blend the minimum amount of solubilizer necessary to achieve a transparent solution was determined. Every preparation had to be done anew, since subsequent addition of solubilizer after the addition of water does not yield in a transparent solution.The results revealed an increase in the solubilization efficacy of most, but not all combinations of solubilizer and lipo-philic substance, caused by the addition of either one of the two wetting agents (Fig. 3). In 66.7 % (for Glyceryl Caprylate) and 58 % (for Caprylyl Glycol) of all solu-bilizates the wetting agent improved the solubilization efficacy of either one of the four solubilizers. In Fig. 4 the average improvements in solubilizer efficacy are illustrated for the two wetting agents, respectively. Here the strongest effect was observed for the Heptyl Glucoside, where Glyceryl Caprylate allowed for a significant (p < 0.05, t-test) average re-duction in necessary solubilizer concen-tration of 36.7 %, and Caprylyl Glycol of 37.2 %. The second strongest improve-ment in solubilizer efficacy was observed for symbio®solv clear (initially lacking Glyceryl Caprylate), where Glyceryl Cap-

Fig. 2 Concentration-dependent influence of a wetting agent (Glyceryl Caprylate) on the solubilization efficacy (maximum amount of oil in a transparent solubiliza-tion) of a solubilizer (symbio®solv clear, initially lacking Glyceryl Caprylate). Fur-ther shown are hazy (a, b, e) and transparent (c, d) solubilizations with 0.3 % oil (red line), 1 % symbio®solv clear and varying concentrations of Glyceryl Caprylate (a: 0 %, b: 0.01 %, c: 0.05 %, d: 0.1 %, e: 0.12 %).

Fig. 1 Glyceryl Caprylate and Caprylyl Glycol. Amphiphilic structure and properties of the two commercially available wetting agents.

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rylate and Caprylyl Glycol allowed for a highly significant (p < 0.01, t-test) average reduction in the solubilizer concentration of 32.7 % and 26.4 %, respectively. The positive effect for the well-performing PEG-40 Hydrogenated Castor Oil was less pronounced with an average, but non-significant solubilizer

reduction of 7.17 % for Glyceryl Capry-late and 5.5 % for Caprylyl Glycol. Also for the D-Sorbitan Oleate Decylgluco-side Crosspolymer the overall reduction of 8.3 % (Glyceryl Caprylate) and 18.9 % (Caprylyl Glycol) in solubilizer concen-tration was not significant. This lack of significance was due to the large vari-

ance of results ranging from excellent to poor effects depending on the combina-tion of solubilizer and oil. For example for Heptyl Glucoside and D-Sorbitan Oleate Decylglucoside Crosspolymer, in four of seven cases the addition of a wetting agent enabled the production of a transparent solution of an otherwise

Fig. 4 Average reduction in necessary solubilizer concentration (in %), due to the implementation of the wetting agents Glyceryl Caprylate and Caprylyl Gylcol, respectively. P-values (paired, two-sided t-test) indicate significant differences between the average solubilizer efficacy of solubilizer with and without either one of the two wetting agents.

Peg-40 Hydrogenated Castor oil symbiosolv clear (without gmCY) Heptylglucoside D-sorbitan oleate Decylglucoside Crosspolymer

+ glyceryl Caprylate + Caprylyl glycol + glyceryl Caprylate + Caprylyl glycol + glyceryl Caprylate + Caprylyl glycol + glyceryl Caprylate + Caprylyl glycol

Solu

biliz

er a

lter

atio

n (%

)

Fig. 3 Solubilization experiments with 3 perfume oils, 3 essential oils, 3 lipophilic actives (from left) and 4 different solubilizers either without or with Glyceryl Caprylate or Caprylyl Glycol (5 % diluted in solubilizer) respectively. Given are the minimum amounts of solubilizer (calculated per 1 % oil) necessary to achieve a transparent solution. Red stars indicate solubilizations where addition of a solubilizer at no time yielded in a transparent solution of the respective oil.

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timum wetting agent concentrations may vary for different oil solubilizer combinations. Therefore, a slight vari-ation in the wetting agent concentra-tion might help in such cases. At the end, the formulator has to test and to adjust his solubilizer/wetting agent-system to the requirements and char-acteristics of each formulation. Comparative sensory profileSo far, we demonstrated that the two wetting agents positively influenced the formation of clear solubilizates for a variety of different solubilizers and oils. This again allows the formulator to lower the solubilizer’s concentration in the final product. Such a decrease not only saves costs but may also act positively on the sensory profile of the formulation. To determine if this de-crease in total surfactant concentra-tion and the wetting agents themselves positively influences the sensory profile of a formulation, a sensory blind test with a panel of 10 experienced subjects was conducted. The panel evaluated

insoluble oil. In some other cases only one or neither of the two wetting agents increased the efficacy of a solubilizer. In 13.9 % of the cases even a reduction in the efficacy could be observed.A possible explanation for this obser-vation lies in the structure of the four solubilizers. Symbio®solv clear’s main component Caprylyl/Capryl Glyco-sides and the Heptyl Glucoside solu-bilizer are quite similar in their linear structure and in addition are much smaller than the widely branched complex molecules of PEG-40 Hydro-genated Castor Oil and D-Sorbitan Oleate Decylglucoside Crosspolymer. Therefore differences in the micelle structure of these two types of solu-bilizers might explain the different overall impact of the two wetting agents. Another possible explanation regards the fixed concentration of the wetting agents implemented in the solubilizer blends. From the previous experiment we know that the ben-eficial effect is concentration depen-dent. We therefore suppose that op-

Wetting agent

Formulation 1 Formulation of a natural caring tonic without (A) and with (B) Glyceryl Caprylate, used in the comparative sensory profile test.

NATurAl CAriNg ToNiCA B

PhASE iNgrEdiENT iNCi SuPPliEr % %

A

dest. water Aqua 95.70 96.90

Glycerin 99.5 % Glycerin 4.00 4.00

dermofeel® PA-3 Sodium Phytate, Aqua, Alcohol Dr. Straetmans 0.10 0.10

Ajidew NL 50 Sodium PCA, Aqua Ajinomoto 0.50 0.50

REFORCYL

Glycerin, Aqua, Glutamine, Decyl Glucoside, Phenethyl Alcohol, Citric Acid, Cistus Incanus Flower/Leaf/Stem Extract, Gynostemma Pentaphyllum Leaf/Stem Extract

Rahn 2.00 2.00

Verstatil® BL non GMO Aqua, Sodium Levulinate, Sodium Benzoate Dr. Straetmans 1.50 1.50

B

Perf. Flowerdream Nat. P0251132 Perfume Frey & Lau 0.20 0.20

symbio®solv clear without Glyceryl Caprylate

Caprylyl/Capryl Glucoside, Aqua, Sodium Coco-yl Glutamate, Polyglyceryl-5 Oleate, Citric Acid Dr. Straetmans 2.50

symbio®solv clear with Glyceryl Caprylate

Caprylyl/Capryl Glucoside, Aqua, Sodium Co-coyl Glutamate, Polyglyceryl-5 Oleate, Glyc-eryl Caprylate, Citric Acid

Dr. Straetmans 1.50

RonaCare Bisabolol Bisabolol Merck 0.10 0.10

100.00 100.00

several sensory criteria of a tonic for-mulation (Formulation 1) including the minimal possible amount of solubilizer with and without Glyceryl Caprylate, respectively. The results from the paired comparison of tonics with and without Glyceryl Cap-rylate in Fig. 5 revealed differences re-garding all analyzed sensory criteria. The tonic with Glyceryl Caprylate showed a faster spreadability and a higher skin absorption. The total surfactant reduc-tion due to Glyceryl Caprylate most likely explains the lower foaming and sticki-ness of the tonic. After the application, the tonic with Glyceryl Caprylate further convinced through a richer and more caring overall skin feel.In summary, the results of the test re-vealed a positive effect on the sensory profile of the formulation, by the addi-tion of a wetting agent. On one hand this sensory enhancement can be attributed to the wetting agents´ inherent charac-teristics, and on the other hand to the economic use of total surfactant con-centrations.

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■■ Conclusion

In this article it was demonstrated on two commercially available examples, Glyceryl Caprylate (dermosoft® GMCY) and Caprylyl Glycol (dermosoft® octiol), that wetting agents according to Fish-man (2) can reveal a positive influence on the performance of solubilizers. This concentration-dependent, beneficial ef-fect of the wetting agents was shown

to follow a curve distribution with an optimal concentration level. In a large series of tests the beneficial effect of both wetting agents was successfully transferred to a variety of structurally diverse solubilizers and a wide range of solubilized oils. Moreover, it has been demonstrated that the sensory profile of a formulation was remarkably improved by the incorporation of a wetting agent and the consequential saving of solu-

bilizer. As a consequence from the re-sults wetting agents, in addition to their wetting and antimicrobial properties, can also be considered as efficient »co-solubilizer« agents from now on, with multiple positive effects on the formu-lation.

references

(1) Griffin W. C. Classification of surface active

agents by HLB. J. Soc. Cosmet. Chem. Vol. 1

(1949).

(2) Fishman H.M. A look at HLB. Happi, April, p.

28, 128 (1991).

(3) PEG-freie Solubilisatoren, Dr. Straetmans

GmbH, EP 2 366 376 A2

(4) Thiemann A., Scholze J., Grandke N., Salmina-

Petersen M., A clear solution. SPC, July (2014).

*Authors´ addresses: Alexander Thiemann

(alexander.thiemann@dr-straetmans.de)Jessica Scholze

(jessica.scholze@dr-straetmans.de)Manuela Salmina-Petersen

(manuela.salmina-petersen@dr-straetmans.de)Jan Jänichen

(jan.jaenichen@dr-straetmans.de)

Dr. Straetmans GmbH Merkurring 90

22143 Hamburg, Germany

■

Wetting agent

Fig. 5 Sensory profile (paired comparison) of a tonic formulation (Formulation 1) with and without Glyceryl Caprylate.