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

3-2015

A. Thiemann, N. Grandke, S. Gröne, M. Salmina-Petersen, J. Jänichen

Wetting Agents – Their Concentration-Dependent Effects on the Energy Demand in the

Formation of Stable Emulsions

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■■ Introduction

Mixing immiscible oil and water leads to a bi-phase system which tries to adopt the smallest possible interface between the two phases. This behaviour is owed to differences in the attracting molecular forces within the isolated phases which

as a consequence create a high surface tension. The production of an emulsion is connected with a multiple extension of the interface between the aque-ous and the lipid phase. Consequently, these systems are thermodynamically unstable. Energy in form of mechanical work, specifically shear force, has to be

applied to the system in order to build these large interfaces in form of drop-lets and vesicles. As the surface tension between two immiscible phases is also a function of the temperature, addi-tionally such emulsification processes are usually performed at elevated tem-peratures.

Abstract

a. thiemann, n. grandke, s. gröne, m. salmina-Petersen, J. Jänichen*

Wetting Agents – Their Concentration-Dependent Effects on the Energy Demand in the Formation of Stable Emulsions

Wetting agents are amphi-philic surface active ma-terials which, like emulsi-

fiers, reduce the interfacial tension between an immiscible water and oil phase. However, unlike larger emulsifier molecules, they do not have the capacity to form stable emulsions. On the contrary, wet-ting agents tend to disrupt orga-nized structures formed by emulsi-fiers at the interface between the polar aqueous and the non-polar lipid phase, thereby lowering the viscosity and finally the stability of the emulsion. Consequently, the properties of wetting agents do not seem to make them attractive raw materials for emulsions on the formulator’s ingredient list. In the

present study, however, we take a closer look at possible positive effects of wetting agents in such systems.We therefore analyzed the con-centration-dependent influence of three commercially available wetting agents on the efficacy of different types of oil in water emulsifiers. Our results confirm, that high concentrations of wetting agents resulted in a destabilization of emulsions. Lower concentrations however, helped to produce stable emulsions with fine and homoge-neous droplets of the dispersed phase independent on the type of emulsifier that was investigated. Moreover, in the production of stable emulsions low concentra-

tions of wetting agents allowed for a saving of thermal and kinetic en-ergy during the emulsification pro-cess and enabled the realization of difficult to achieve applications, e.g. self-emulsifying formulations. Further, the implemented low con-centrations of wetting agents ben-eficial to the emulsions, revealed no measurable negative impact on the sensory profile of the finished cosmetic formulations. In summary, wetting agents pos-sess significant co-emulsifier prop-erties, beside their already well described antimicrobial activities, which e.g. allow for the significant reduction of preservatives or even their total elimination from the in-gredient list.

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The thermodynamically unstable sys-tem tries to reduce the high surface tension and internal energy by coales-cence of the droplets. The oil droplets creep together and form a separated phase which, depending on the den-sity, is on the top or the bottom of the system. Factors promoting coalescence are creaming or sedimentation, driven by different densities of water and oil, and Ostwald ripening, driven by vapor pressure differences of smaller and larger droplets. According to Stokes’ law for the frictional force exerted on spherical objects, the velocity of creaming or sedimentation, and there-fore the stability of the interfaces, is partly influenced by the droplet di-ameters and emulsion viscosity. There-fore small and homogeneous droplets of the dispersed phase together with a high viscosity are strong indicators for a stable emulsion.Stabilizing the formed interfaces, as ev-ery formulator knows, is the function of the emulsifier. They belong to the group of surface active substances, the surfactants. As all surfactants emulsi-fiers possess an amphiphilic molecule structure with a polar, hydrophilic part and a non-polar, lipophilic part. The hydrophilic-lipophilic balance value, or HLB-value, devised by W.C. Griffin

in 1949 is a way to categorize surfac-tants and to draw conclusions regard-ing their cosmetic properties (1). On a scale of 1 to 20, a low HLB-value indi-cates a low polar molecule proportion, whereas a high HLB-value indicates a surfactant whose polar part outweighs its non-polar part. Oil in water emul-sifiers (O/W-emulsifiers) according to Fishman (2) have an HLB between 8 and 18, indicating a tendency towards a higher molecular weight of the polar, hydrophilic part. In the range of skin care products O/W-emulsions are playing a dominant role. Consequently, a variety of emulsifiers are present on the market, promising stable emulsions of high sensorial qual-ity for different applications. A factor that is usually not taken into account in the evaluation of emulsifiers is the energy and mechanical work which has to be introduced into the system in or-der to overcome the interfacial tension and to obtain a homogeneous and fine emulsion. However, the performance of an emulsifier, regarding its energy de-mand, is not an insignificant cost fac-tor which needs to be considered by the formulator.At this point, wetting agents with their high surface activity, as a part of the emulsifier system can help to

reduce the interfacial tension and sub-sequently the amount of mechanical work and heat energy which is needed during the production process to ob-tain a fine and homogeneous emul-sion. Under the term wetting agent, amphiphilic surfactants are summa-rized, which possess a significant sur-face activity but a molecular weight much lower than the one of classical O/W-emulsifiers. Wetting agents have an HLB between 6 and 9, indicating a slightly higher molecular weight of the non-polar, hydrophobic part. In con-trast to typical emulsifier structures wetting agents do not have any emul-sifying properties by themself that al-low to create stable emulsions. At el-evated concentrations wetting agents are known to rather disrupt micelle structures and reduce the viscosity of emulsions, thereby destroying them rapidly. The presence of low concen-trations of wetting agents during the manufacturing of emulsions, however, may support the formation of very fine and homogeneous systems by a more effective conversion of mechanical work into micelle formation.In order to check for this assumption, three commercially available wetting agents were evaluated in this study. Glyceryl Caprylate (trade name: der-

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mosoft® GMCY) and Glyceryl Caprate (trade name: dermosoft® GMC) two glycerin monoesters, and Caprylyl Glycol (trade name: dermosoft® oc-tiol) a medium size glycol. Beside the wetting ability, their multifunctional cosmetic properties comprise moistur-izing, refatting, viscosity regulating, antimicrobial, and also co-solubilizer (3) activities. Glyceryl Caprylate and Glyceryl Caprate moreover are available from 100 % nat-ural resources and are compliant with Ecocert, BDIH, NaTrue and NPA. Fig. 1 illustrates their function-determining amphiphilic structure.

■■ Test Methods and Results

Concentration-Dependent Effects of a Wetting Agent on EmulsionsThe stability of an emulsion depends on the emulsifier and on parameters like droplet size and droplet homoge-neity of the dispersed phase and also on the viscosity of the emulsion. These parameters influence the coalescence velocity of the oil droplets in an O/W-emulsion. A reduction of the interfa-cial tension during the emulsification process could lead to a more effective conversion of mechanical work into a finer and therefore more stable mi-celle/droplet structure. A high inter-facial tension reduction, however can also disrupt the organized structures of an emulsion. Therefore, the surface-active properties of wetting agents po-

tentially allow for beneficial as well as adverse effects on the stability of an emulsion. For a better understanding of these antagonistic forces of wetting agents in emulsions, we examined the concen-tration-dependent effects of a wetting agent on the efficacy of an emulsifier. For this purpose the emulsifier system symbio®muls rich (composed of Poly-glyceryl-3 Stearate, Behenyl Alcohol, Glyceryl Stearate Citrate, Helianthus Annuus (Sunflower) Seed Cera (Wax), Glyceryl Caprate) was selected. Its nat-urally-derived components comprise a main emulsifier, a co-emulsifier, two consistency agents and the wetting agent Glyceryl Caprate. Together these components allow for a rich skin feel, suitable for nurturing creams. In order to determine the effect of the wetting agent Glyceryl Caprate, a component of symbio®muls rich, its concentration was gradually varied and afterwards the droplet size distribution and vis-cosity of the emulsion was measured. The stability of the emulsions was tested immediately after production by centrifugation test (1725 x g) for 30 min and on long term by storage for 2 week at 50°C, and 6 weeks at 4°C, 21°C and 40°C.The results in Fig. 2 reveal, that the droplet size and the droplet size dis-tribution of the emulsions gradually decreased with increasing wetting agent concentrations. This positive trend peaked at a concentration of 0.3 % Glyceryl Caprate with a highly

significant average droplet size reduc-tion of 21 % compared to the emul-sion without Glyceryl Caprate (t-test, p<0.01). With further increasing con-centrations of the wetting agent the droplet size and variance started to increase again indicating that the dis-ruptive effects of the wetting agent on the emulsion predominates. At a con-centration of 0.75 % Glyceryl Caprate the emulsion appeared homogeneous after the emulsification process, but turned out to be unstable after 30 min of centrifugation and finally, at a con-centration of 1 % Glyceryl Caprate the production of a homogeneous emul-sion was not possible anymore. The viscosity of the emulsions was only varied marginally by the wetting agent. Interestingly, at first the viscosity of the emulsion even increased due to Glyc-eryl Caprate and then decreased only slightly after further addition of wet-ting agent. This initial increase, which was observed for a variety of emulsi-fiers and wetting agents yet, might be explained by a denser packaging of the smaller droplets inside the emulsion. With increasing concentrations how-ever, this initial viscosity-enhancing ef-fect might be overturned by the general viscosity-reducing effect of the wetting agents. This general reduction of the viscosity is caused by the wetting agent at higher concentrations, interrupting the arranged structure of the two main emulsifiers of symbio®muls rich at the interface between the oil and the water phase.

Wetting agents

Fig. 1 Glyceryl Caprylate, Glyceryl Caprate and Caprylyl Glycol. Amphiphilic structure (4) and properties of the three commer-cially available wetting agents. (HLB values according to Griffin (1))

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In summary the beneficial effect of the wetting agent followed a func-tion with an optimal concentration level. Low concentrations of wetting agents in the production of emul-sions supported the formation of stable systems with very fine and ho-mogeneous droplets and even slight-ly higher viscosities (Fig. 2), whereas at elevated concentrations, the high surface activity of the wetting agent resulted in a destabilization of the emulsion. We assume that this con-centration-dependent progression also applies to other emulsifiers and wetting agents. This hypothesis was tested amongst others in the follow-ing experiments.

Temperature-Dependent Effects of Wetting Agents The temperature during the emulsi-fication process plays a critical role in the formation of fine dispersed, small droplets and therefore in the stability of emulsions. Consequently, emulsifica-tion processes are conducted usually at 70-80°C. To determine if wetting agents allow for a reduction of the applied thermal ener-gy in the emulsification process, emul-sions with and without wetting agents were produced within a hot-process (78°C) and a cold-process (40°C). Ex-periments were conducted with a nat-urally-derived emulsifier, symbio®muls GC (INCI: Glyceryl Stearate Citrate, Ce-

tearyl Alcohol, Glyceryl Caprylate), and a petrochemically-derived, polyethox-ylated benchmark emulsifier, PEG-100 Stearate. Symbio®muls GC is a highly versatile emulsifier system containing a strong wetting agent, Glyceryl Cap-rylate. The blend allows the formation of stable and fine O/W-emulsions with high sensorial quality and which, due to the presence of a wetting agent, are easy to preserve. The analyzed wetting agents comprised Glyceryl Caprylate, Glyceryl Caprate and Caprylyl Glycol. O/W-emulsions with either one of the two emulsifiers and one or neither of the wetting agents were produced, mi-croscopic images were taken and the stabilities of the emulsions were tested by centrifugation and long-time stor-age at different temperatures (see above). The results in Fig. 3 show, that the wet-ting agent positively influenced the droplet size distribution of the final emulsion. In the case of symbio®muls GC (without Glyceryl Caprylate), initially lacking its wetting agent Glyceryl Cap-rylate, the subsequent addition of the different wetting agents resulted in two of three cases in a more stable, and in ev-ery case a more macroscopically homo-geneous emulsion after a cold-process (40°C) emulsification. This result there-fore indicates that the implementation of wetting agents can save thermal en-ergy during the emulsification process. It further confirms our previous assump-tion that the concentration-dependent beneficial effect of Glyceryl Caprate also applies to other emulsifiers and wetting agents, though, depending on the com-bination, with slight differences in the performance.

Kinetic Energy-Dependent Effects of Wetting AgentsWe already demonstrated that low con-centrations of wetting agents lower the thermal energy required to form homog-enous emulsions. In order to see whether wetting agents also allow for a reduction of the kinetic energy needed for a suc-cessful emulsification, we conducted ex-periments on the self-emulsifying prop-erties of a naturally-derived emulsifier. Formulations with self-emulsifying prop-erties are e.g. desired in applications like

Fig. 2 Concentration-dependent influence of a wetting agent (Glyceryl Caprate) on the emulsification process (78°C). The emulsifier was symbio®muls rich (in this experiment initially lacking Glyceryl Caprate (yellow box plot)). Shown are boxplots of the droplet sizes of the respective emulsions, depicting the full range of variation in form of quartiles (25 %) and the median of the values. Further given are the variances of the droplet sizes, the viscosities of the emulsions and an information if the emulsions were stable after centrifugation and/or storage. The emulsion containing 1 % Glyceryl Caprate was the only emulsion separated already after emulsification.

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bath oils or facial cleansing oils (makeup remover). Self-emulsifiying oils are able to sponta-neously emulsify in water at room tem-perature and low agitation. The emulsi-fication is often achieved with the aid of suitable surfactants like Polyglyceryl-3 Palmitate (dermofeel® PP) as the main O/W-emulsifier in combination with the wetting agent Glyceryl Caprylate. The lat-ter prevents the separation of polar com-pounds and is able to interact between the polar O/W-emulsifier dermofeel® PP and the non-polar oil providing clear ho-mogeneous self-emulsifying oils.In this experiment, dermofeel® PP alone, or in combination with Glyceryl Capry-late (3:5 mixture of Glyceryl Caprylate/dermofeel® PP) was added in a total concentration range between 2-16 % to sunflower oil. The resulting oil-emulsifi-er-blend was then added to water (1:4 mixture of oil phase/water) and was in-verted for five times. Pictures were taken immediately after self-emulsification and also after 20 min at room tempera-ture. Because dermofeel® PP without Glyceryl Caprylate was hard to solubilize, oil phases were heated to 35°C before the addition of water. The results in Fig. 4 reveal that while 7 % dermofeel® PP was necessary to spontaneously produce a homogeneous emulsion (0 min) the presence of Glyc-eryl Caprylate resulted in the formation of a homogeneous emulsion already at a concentration of 6 % dermofeel® PP/Glyceryl Caprylate. Thus, in order to achieve similar results the partial re-placement of emulsifier with Glyceryl Caprylate allows the formulator to use smaller total amounts of surfactants. However, with more extensive mechani-cal work (e.g. homogenizer), 6 % of der-mofeel® PP was sufficient in creating an initially homogeneous emulsion (not shown). Based on this observation it was concluded, that the wetting agent seems to lower the kinetic energy necessary for a successful emulsification. After 20 minutes, at a concentration of 10 % dermofeel® PP and lower, emul-sions without Glyceryl Caprylate sepa-rated into two phases. Only a high con-centration of 16 % dermofeel® PP was able to stabilize the emulsion for 20 minutes at room temperature. In con-

Fig. 3 Emulsification experiments at 40°C (cold process) or 78°C (hot process) with two emulsifiers (symbio®muls GC without Glyceryl Caprylate and PEG-100 Stearate either with or without 0.25 % Caprylyl Glycol, Glyceryl Caprylate or Glyceryl Caprate respectively. Shown are the microscopic images of the respective emulsions together with an information about their macroscopic appearance (homogeneous or inhomogeneous) and stability after centrifugation and storage at different temperatures (see text).

Fig. 4 Images of emulsions directly after (0 min) or 20 minutes after (20 min) the self-emulsification process with varying concentrations of emulsifier (dermofeel® PP, upper boxes) or emulsifier-wetting agent blend (dermofeel® PP/Glyceryl Caprylate (5:3), lower boxes) (for details refer to text). Grey boxes show unstable, macroscopically inhomogeneous emulsions (separated), and green boxes show stable homogeneous emulsions at the moment of observation (not separated).

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tinguishable from each other (p=0.44). A graphical representation of the compar-ative sensory profile is shown in Fig. 5.

Formulation 1 Formulation of a nurturing O/W-emulsion without (A) and with (B) Glyceryl Caprate, used in the sensory triangle test. (Recipe No.: L071-9.1/3-1214)

BASiC RiCh EMulSioN

PhASE iNGREDiENT iNCi (PCPC) SuPPliERA B % %

A

dest. water Aqua (Water) ad 100 ad 100Glycerol 99.5 % Glycerin 4.0 4.0dermofeel® PA-3 Sodium Phytate, Aqua (Water), Alcohol Dr. Straetmans 0.1 0.1Verstatil® PC Phenoxyethanol, Caprylyl Glycol Dr. Straetmans 1.0 1.0

A1 Keltrol CG-RD Xanthan Gum CP Kelco 0.2 0.2

B

symbio®muls rich (without Glyceryl Caprate)

Polyglyceryl-3 Stearate, Behenyl Alcohol, Glyceryl Stearate Citrate, helianthus Annuus (Sunflower) Seed Cera (Wax), Ascorbyl Palmitate, Tocopherol, helianthus Annuus (Sunflower) Seed oil

Dr. Straetmans 6.7 6.7

dermosoft® GMC Glyceryl Caprate Dr. Straetmans 0 0.3dermofeel® MCT Tricaprylin Dr. Straetmans 0.7 0.7 Sunflower Oil Helianthus Annuus (Sunflower) Seed Oil Gustav Heess 8.0 8.0Tegosoft DC Decyl Cocoate Evonik Industries 5.0 5.0dermofeel® Toco 70 non-GMO

Tocopherol, Helianthus Annuus (Sunflower) Seed Oil Dr. Straetmans 0.2 0.2

100.00 100.00

Fig. 5 Sensory profile (paired comparison with triangle test) of a basic rich emulsion (see Formulation 1) with (blue line) and without (orange line) Glyceryl Caprate.

The triangle test confirmed the test`s null hypothesis (H0), that the sensory profile of the two emulsions was indis-

trast, emulsions with Glyceryl Caprylate were stable at a total surfactant con-centration of 8 % and higher. Glyceryl Caprylate therefore clearly improved the stability of the emulsions formed in this experiment.

Comparative Sensory ProfileSo far, we demonstrated that wetting agents at low concentration successfully increased the stability of emulsions and allow the formulator to either lower the total amount of surfactant or the ther-mal or kinetic energy necessary during the emulsification process. To determine the potential influence of a wetting agent at its effective use concentrations on the sensorial profile of a cosmetic formulation, emulsions with and without 0.5 % Glyceryl Cap-rate were compared in a sensory tri-angle test with a panel of 10 experi-enced subjects. In this test the subjects were confronted with three emulsions based on symbio®muls rich, of which two were identical and one was dif-ferent (either with or without Glyceryl Caprate) (Formulation 1). The subjects were asked to point out the emulsion they think was different.

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It shows the principal consensus between the sensory profile of the emulsion with and without Glyceryl Caprate.

■■ Conclusion

Multifunctional wetting agents, like Glyceryl Caprylate (dermosoft® GMCY), Glyceryl Caprate (dermosoft® GMC) and Caprylyl Glycol (dermosoft® octiol) are real cosmetic all-rounders. They can be used in many kind of cosmetic applica-tions like emulsions, solubilisations, and surfactants. In our experiments we could confirm our assumption that low con-centrations of wetting agents contribute to a stabilization of emulsions by pro-moting the formation of homogenous and fine droplet size emulsions through the reduction of the interfacial tension. This beneficial effect of the wetting agents is concentration dependent and shows individual optima for the specific systems. Moreover wetting agents can help to reduce the amount of mechani-

cal work and the amount of heat energy which needs to be introduced into the emulsification process to obtain a fine and homogeneous stable emulsion. These positive effects make wetting agents a perfect choice for hard to realize for-mulations, e.g. self-emulsifying formu-lations. In addition, the antimicrobial properties of wetting agents add further valuable function to cosmetic products. AcknowledgmentWe thank Prof. Dr. Stefan Hoth from the Biocenter Klein Flottbek (University of Hamburg) for the provision of the con-focal microscope and Benjamin Möller for the technical support.

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) Thiemann A. et al. Wetting agents: Friends

or Enemies of Solubilizers. SOFW, p. 46, 140

(11-2014).

(4) Bolton et al. PubChem3D: a new resource for

scientists. J Cheminform. 3(1):32 (2011).

*Authors´ address:Dr. Straetmans GmbH

Merkurring 9022143 Hamburg

Germanywww.dr-straetmans.de

Dr. Alexander Thiemann (alexander.thiemann@dr-straetmans.de)

Nadja Grandke (nadja.grandke@dr-straetmans.de)

Sabrina Gröne (sabrina.groene@dr-straetmans.de)

Manuela Salmina-Petersen (manuela.salmina-petersen@dr-straetmans.de)

Dr. Jan Jänichen (jan.jaenichen@dr-straetmans.de)

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