Stability Study of O/W Cosmetic Emulsions Using <i>Rosmarinus officinalis</i> and <i>Calendula officinalis</i> Extracts

doi:10.4236/ojapps.2012.23020

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Open Journal of Applied Sciences, 2012, 2, 139-145

doi:10.4236/ojapps.2012.23020 Published Online September 2012 (http://www.SciRP.org/journal/ojapps)

Stability Study of O/W Cosmetic Emulsions Using

Rosmarinus officinalis and Calendula officinalis Extracts

Evi-Maria Varka, Eforia Tsatsaroni*, Nikolet a Xristoforidou, An na - Ma ri a Dard a

Department of Chemical Technology and Industrial Chemistry, School of Chemistry, Aristotle University, Thessaloniki, Greece

Email: *[email protected]

Received May 20, 2012; revised June 21, 2012; accepted July 10, 2012

ABSTRACT

Cosmetic emulsions, as all macro emulsions, are inherently unstable systems, from a thermodynamic viewpoint. More

specific eco-friendly oil/water (O/W) cosmetic emulsions are usually less stable than conventional ones as milder

chemicals or less intense (energy consuming) production processes are involved. In this context, two traditional tech-

niques an optical technique and a volumetric one have been used for the assessment of the stability of cosmetic emul-

sions and compared to each other. Eco-friendly cosmetic emulsions were produced with different olive oil/water ex-

tracts (Rosmarinus officinalis and Calend ula officin alis extracts) and emulsifier (Glycerol monostearate, GMS) concen-

trations. Emulsions’ stability was registered simultaneously by 1) Microscopy photos of samples withdrawn at regular

intervals from the test vessel; 2) Global volumetric measurements of the different phases (water/oil/emulsion) inside the

test vessel made at regular intervals for determining the evolution of the location of the phases separation interface.

Analysis of the results of each technique and comparisons among them are presented and discussed in detail.

Keywords: Cosmetic Emulsions; Olive Oil; Stability Studies; Rosmarinus officin alis; Calendula officinalis

1. Introduction

Since antiquity, emulsions have been used in cosmetics.

In contrast to pharmaceutical ointments which must pe-

netrate deep into the skin, cosmetic products are meant

only for the immediate surface of the skin i.e. the epi-

dermis. However, many commercial cosmetic products

contain artificial chemicals that inevitably penetrate the

skin. In the last few years physical products have started

to gain the interest of consumers. Because of the incon-

siderate use of chemical components and the harmful

consequences of them in peoples’ health, scientists

started to look out for alternative findings to surpass

these problems. For this reason herbs’ study has gained

attention [1-4].

Eco-friendly cosmetics—also known as organic cos-

metics-appeared in the early 1900s. The biggest advan-

tage of organic cosmetics is that they are safe for humans

and the environment as they do not contain toxic chemi-

cals, they need no animal testing and their results can be

as pleasing as those achieved by commercial products.

Rosmarinus officinalis L. (Lamiaceae), commonly known

as rosemary, is a household plant used worldwide as a

food-flavouring agent, widely distributed in the Mediter-

ranean region. A preclinical survey confirmed that rose-

mary has powerful anti-inflammatory, antibacterial, anti-

diabetic, antitumor, cytoprotective and hepatoprotective

properties [5-10]. Rosemary has one of the highest anti-

oxidant activities of all the herbs and spices that have

been investigated [11]. The antioxidant activity of rose-

mary justifies its use in a broad range of applications,

including food preservatives [12], cosmetics [13], phy-

tomedicines etc. These properties attributes can be re-

lated to rosemary’s high content of polyphenolic com-

pounds, especially rosmarinic acid [14], which is con-

sidered a chemical marker of this species.

Calendula officinalis L. (Asteraceae), also known as

marigold, is an annual herb native to the Mediterranean

region. It is cultivated for ornamental and medicinal

purposes in Europe and America. More than 35 proper-

ties have been attributed to the decoctions and tinctures

from the flowers, e.g. anti-inflammatory, analgesic, bac-

tericide, tonic and the healing of wounds and skin erup-

tions [15]. Previous phytopharmacological studies on the

extracts of Calendula officinalis flowers have confirmed

the presence of bioactive secondary metabolites, flavonol

glycosides [16,17] and triterpenoids [18].

From the above we can see that traditional herbs pro-

vide an interesting, largely, unexplored source for de-

velopment of potential new cosmetics. In the present

study we tried to combine all this beneficial components

of the herbs in order to make O/W emulsions.

The oil being used was olive oil. Olive oil is natural oil

*Corresponding author.

E.-M. VARKA ET AL.

140

typically used in cosmetics and foods. Apart from being

a natural and hypo allergic way to moisture skin, extra

virgin olive oil has the extra advantage of containing

strong antioxidants, like vitamins A and E. The antioxi-

dants help repairing and renewing skin that has been

damaged from overexposure to sun, air pollution and

other environmental hazards. The selected emulsifier was

GMS which is an eco-friendly emulsifier widely used in

lotions, creams powders etc. as it is safe for humans and

gives skin a soft and smooth appearance, while the water

phase was deionized water.

A series of emulsions was made with different oil or

water phase from the above extracts and their stability

was studied. Emulsions’ stability was registered simul-

taneously by 1) Microscopy photos of samples with-

drawn at regular intervals from the test vessel or deter-

mining the evolution of the local droplet size distribution;

2) Global volumetric measurements of the different pha-

ses (water, oil) inside the glass tubes for determining the

evolution of the phase’s separation.Additionally, pH,

surface/interface tension measurements were also made.

Analysis of the results of each technique and compare-

sons among them are presented and discussed in detail.

2. Materials and Methods

2.1. Materials

The plant materials Rosmarinus officinalis and Calen-

dula officinalis, were collected at the bloom stage at the

end of May. The plant material was dried at room tem-

perature and then milled. The dry plant material was then

packed in paper bag and kept in a dark, dry and cool

place.

Greek olive oil was obtained by “Altis S.A.” whereas

Glycerol monostearate (GMS), an esterification product

of glycerine and stearic acid was obtained from Panreac

(purity > 98%) and used as emulsifier.

2.2. Plant Material Extractions (Oil/Water

Phases)

Plant materials (Rosmarinus officinalis and Calendula

officinalis), were placed separately in a vessel with olive

oil in a proportion of 10%. The material was warmed up

for 5 minutes and then was kept in a dry place for 40

days approximately in order to have a complete extrac-

tion. The remaining oils were used as oily extracts.

Water extracts, where needed, were taken by inserting

herbs (Rosmarinus officinalis and Calendula officinalis)

into boiled water for 10 minutes in a proportion of 10%.

2.3. Emulsification Process

Emulsification was implemented by mixing water or wa-

ter extract with olive oil or oil extract using an impeller

rotating at the central axis of a glass vessel with internal

diameter of 9.5 cm. The impeller was placed 1.0 cm

above the bottom of the vessel and not at the traditional

(for mixing applications) 1/3 of the vessel height. Four

buffles were placed at 90 degrees intervals around the

vessel in order to allow high rotation speeds without

vortexing and air suction. Emulsions were prepared in

the above glass vessel as follows. Initially, proper

amount of GMS (Tables 1, 2) was added to the 20 ml of

oil and then the mixture was heated to 70˚C (GMS is

solid at ambient temperature). The corresponding amount

of aqueous solution 80 ml was heated separately to 70˚C.

Emulsification started by adding the hot aqueous phase

into the hot oil. At that moment heat supply was turned

off and agitation was set at 700 rpm. The impeller was

running at 700 rpm, for a total 1.5 min. This period was

followed by 30 more minutes of agitation at 700 rpm

which was sufficient for the emulsion to cool down to

ambient temperature (25˚C ± 1˚C).

2.4. Creaming Index from Volumetric

Measurements

Direct optical observations were employed to determine

the instantaneous heights of the emulsion and of the aque-

ous phase inside the glass vessel (Figure 1) and to esti-

mate the creaming index according to the formula (Equa-

tion (1)):

100

tot

CI H

 (1)

Table 1. Oil-to-water types of Rosmarinus officinalis emul-

sions and concentration of GMS in the oily phase used for

emulsion preparation.

Type of emulsions 4.5 g GMS 6 g GMS

O/Wa √b √

O/R √ √

R/W √ √

R/R √ √

Table 2. Oil-to-water types of Calendula officinalis emul-

sions and concentration of GMS in the oily phase used for

emulsion preparation.

Type of emulsions 3 g GMS 4 g GMS

O/W √ √

O/C √ √

C/W √ √

C/C √ √

a. O/W: always the first letter denotes oily phase whereas the second one the

water phase; R: Rosmarinus officinalis extracts; C: Calendula officinalis

extracts. b. √: denotes the emulsions being made.

E.-M. VARKA ET AL.

141

Creaming index, CI, represents the instantaneous global

(i.e. the whole liquid volume) volumetric water fraction.

It is the most customary measure of emulsion destabili-

zation reflecting phase separation (Figure 1).

siometer. The platinum plate was partially immersed into

the oil layer, where it remained to fixed position during

the course of the experiment. Thus, the surface tension

decreased as a function of time. When the surface tension

was stabilized then the experiment was stopped. For the

interfacial tension measurement Ring Du Nouy method

was used. A ring from platinum was going down and

immersed into the water phase. Then carefully the oily

phase was being placed from the top of the water one.

The ring was pulling up until the lamella from the inter-

face breaks off. At this point the measured force gives

interfacial tension value. This method has an easy han-

dling, big accuracy and does not depend on wetting con-

tact angle. All measurements were made at 25˚C ± 0.1˚C.

2.5. Droplets Size from Microscope Photos

Several photos were taken by the microscope from dif-

ferent parts of the withdrawn samples until a population

above 300 droplets was collected for each sample in or-

der to ensure statistical significance in the determination

of droplets size. A custom made software capable of

handling even very dense emulsions was employed to

obtain droplet diameter distributions and from them to

acquire droplet size distributions (Figure 2).

2.6. Additional Measurements 3. Results and Discussion

2.6.1. pH Measurements 3.1. Creaming Index

pH measurements were made by dipping into the emul-

sion the pH sensor of a WTW pH 535 microprocessor. Figures 3 and 4 present the computed creaming index for

Rosmarinus officinalis/Calendula officinalis oil-in-water

cosmetic emulsions respectively.

2.6.2. Su rface/I n te rface Tension Me a s u rements The higher the creaming index value is, the more de-

stabilized the emulsion is. In our occasion maximum

The surface tension measurements were taken with the

Wilhelmy plate technique using a KSV Sigma 70 ten-

Aqueous Phase H

total

(a) (b)

Figure 1. (a) Glass vessels being employed for the determination of creaming index value according to scheme (b).

Figure 2. Schematic representation of estimating droplet diameter distributions.

E.-M. VARKA ET AL.

142

value proportion of water is 80%. There are two general

features in the evolution of creaming index: 1) as oil

phase is enriched with oil extract, creaming rate de-

creases; 2) as emulsifier concentration increases, cream-

ing rate decreases.

More specific, Figure 3 represents creaming index

value for Rosmarinus o fficinalis emulsions. For an emul-

sion with 6 g GMS and Rosmarinus officinalis oil extract

no sign of creaming was observed for the first 60 days

after production. Yet, all other examined cases show

some variability with time although none of the emul-

sions reached the value of 80% representing the complete

separation. Probably the emulsions were stabilized after

having a metastable form. Separation of the two phases

was almost complete in less than 20 days in the emul-

sions with no addition of extracts, regardless GMS con-

centration.

Same trends are also observed in Figure 4. Separation

of the two phases was almost complete in less than 5

days in the emulsions with no addition of extracts.

Comparing Figures 3 and 4 the following comments

can be made 1) Emulsions made up with Calendula offi-

cinalis extracts are more stable than those made with

Rosmarinus officinalis due to lower values of creaming

index (results were not changed up to 30 days for Ca-

lendula officinalis emulsions) 2) emulsions having oily

extracts were the most stable in both cases.

3.2. Droplet Size Distributions

Results obtained from the analysis of the microscope

photographs are presented in Figures 5 and 6. The mean

diameter of the emulsion from samples is shown against

the GMS concentration for the two emulsion types. In

accordance with stability results from volumetric meas-

urements the following are concluded: 1) droplet size

decreases as GMS concentration increases and also as

emulsions are enriched with herbal extracts. One would

expect that droplet sizes would be smaller for emulsions

made up with 6 g GMS instead of 3 and 4 g. Although

data points for the two emulsion compositions are close

enough and can be considered comparable given the sta-

tistical lack of confidence (at a 95% level), on the other

hand; 2) different herb components may affect the total

results. In terms of droplet size Calendula officinalis’

emulsions present better stability (smaller droplets) than

CI%

0 20 40 60

t, Days

W 4.5 0

O/R 4.5 0

R/W 4.5 0

R/R 4.5 0

CI%

0 20 40 60 80

t, Days

O/W 6.0

O/R 6.0

R/W 6.0

R/R 6.0

(a) (b)

Figure 3. Creaming index versus time for Rosmarinus officinalis extract with (a) 4.5 g GMS and (b) 6 g GMS (where O: Olive

oil; W: water; R: Rosmarinus officinalis oil and water extract).

0 5 10 15 20

days

O/W(3.0)

C/W(3.0)

O/C(3.0)

C/O(3.0)

CI%

0 5 10 15 20 25 30 35

days

O/W(4.0)

O/C(4.0)

C/W(4.0)

C/C(4.0)

CI%

(a) (b)

Figure 4. Creaming index versus time for Calendula officinalis extract with (a) 3 g GMS and (b) 4 g GMS (where O: Olive oil;

W: water; C: Calendula officinalis oil and water extract).

E.-M. VARKA ET AL. 143

3 3.5 4 4.5 5 5.5 6 6.5

GM S concentration,g

O/W

O/R

R/W

R/R

Figure 5. Effect of GMS concentration in mean droplet diameter in Rosmarinus officinalis emulsions.

2 3 4 5 6

GM S concentration,g

O/W

O/C

C/W

C/C

d,μm

Figure 6. Effect of GMS concentration in mean droplet diameter in Calendula officinalis emulsions.

those of Rosmarinus officinalis’ emulsions. In accor-

dance with creaming index results Calendula officinalis

emulsions are more stable.

3.3. pH Measurement

In Figures 7(a) and (b) the effect of storage on the pH

values of the emulsions is given. As expected, in both

types of emulsions as oil and/or water extract is added,

pH decreases and this due to different active ingredients

of herb extracts. The emulsions with Calendula Offici-

nalis present a remarkable pH stability with time (Fig-

ure 7(b)). The emulsions with Rosmarinus officinalis

extracts are not as pH stable as those with Calendula

officinalis.

3.4. Surface—Interface Tension Measurements

The low values of interfacial tensions between oil ex-

tracts and water extracts (Tables 3-5) are in line with

results obtained from optical monitoring and volumetric

measurements. The lower the interfacial tension is, the

more stable the emulsion will be. The droplet size distri-

bution resulting from the emulsification procedure is the

outcome of the simultaneous processes of coalescence

between droplets and droplet breakage. The presence of

E.-M. VARKA ET AL.

144

0 5 10 15 20 25 30 35

t, Days

O/W 4.5 0

O/W 6 0

O/R 4.5 0

O/R 6 0

R/W 4.5 0

R/W 6 0

R/R 4.5 0

R/R 6 0

(a)

0 5 10 15 20 25

t, Days

8.5

7.5

6.5

5.5

4.5

C/C(3.2)

C/C(3.1)

C/C(3.0)

C/W(3.2)

C/W(3.1)

C/W(3.0)

O/C(3.2)

O/C(3.1)

O/C(3.0)

O/W(3.2)

O/W(3.1)

O/W(3.0)

(b)

Figures 7. Results obtained from pH meter.

Table 3. Surface tensions of olive oil.

Temperature (˚C) Surface Tension (mN/m)

Olive oil 25 ± 0.1 20

H2O 25 ± 0.1 71.8

Table 4. Interfacial tensions olive oil/Rosmarinus officinalis

oils.

Temperature

(˚C)

Interfacial Tension

(mN/m)

Olive oil-H2O 25 ± 0.1 17

Olive oil-Water extract 25 ± 0.1 6

Olive oil extract-H2O 25 ± 0.1 14

Olive oil extract-Water extract 25 ± 0.1 6

Table 5. Interfacial tensions olive oil/Calendula officinalis

oils.

Temperature

(˚C)

Interfacial Tension

(mN/m)

Olive oil-H2O 25 ± 0.1 17

Olive oil-Water extract 25 ± 0.1 5

Olive oil extract-H2O 25 ± 0.1 13

Olive oil extract-Water extract 25 ± 0.1 6

surface active components in herbal extracts reduces the

coalescence process (stabilizing the water film between

colliding droplets) and enhances the breakage process

due to the reduction of the effective interfacial tension of

the droplet. The combination of these effects results in

the production of smaller droplets (more stable emul-

sions), as interfacial tension decreases.

Conclusively, comparison of the two herbs, Rosmari-

nus officinalis and Calendula officinalis, results in the

following: 1) The effect of Calendula officinalis extracts

is stronger than those of Rosmarinus officinalis. Less

amount of GMS result in more stable emulsions; 2) The

emulsion stability evaluated from factors such as Cream-

ing Index, volumetric index for Calendula officenalis

emulsions reveals more stability than those referred to

Rosmarinus officinalis.

4. Conclusion

In the present work, two experimental techniques were

used for the assessment of the stability of oil-in-water

cosmetic emulsions produced with different herbs and

surfactant concentrations. Results from two classical

techniques (the well known volumetric measurement of

phase separation and the optical measurement of droplet

diameters from withdrawn samples) were examined and

compared to each other. The droplet sizes estimated from

optical observations of the droplets were in accordance

with results obtained by interfacial tension measurements

referring that emulsions made up with Rosmarinus offi-

cinalis are less stable than those of Calendula o fficinalis.

The results and their discussion in this work reveal that

the addition of herbs in cosmetic emulsions can improve

emulsions’ stability without further addition of chemical

stabilizers.

5. Acknowledgements

The authors wish to thank Professor T. D. Karapantsio

for his kind permission to use his custom made software

for the estimation of droplet sizes.

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