Despite a long history of study andapplication for water-in-oil emulsions,there has been little research on water-in-oil quick breaking emulsions.

Quick breaking here means instantrelease of the main part of the waterphase when applied. From a texture point of view, two methods contribute to such kind of effect: one depends onaqueous gel network systems formed bywater-soluble polymers (e.g. carbomer). If applied, such a gel network system will break down by the salts on the skin.Another method is relying on very highwater content water-in-oil emulsions. Ifapplied on the skin, this emulsion typebreaks and the inner aqueous phase is instantly released.

The difference of these two products is the shape of released water on the skin,and only the second method could giveround shaped small water droplets. As the inner phase content must be very high,a stable formula is not easy to achieve. In this case, an effective emulsifier is compulsory for emulsifying a largeproportion of water. A US patent gives the method of emulsifying around 80% by weight of water, but these emulsions have no breaking effect and no data onstability.1 The silicone derived emulsifierABIL EM 90 can be a feasible solution as its efficacy comes from its oligomericcharacter, its dimethicone backbone, thealkyl and PEG/PPG branches.2-5

Raw materialsAll samples were formulated using ABILEM 90 (cetyl PEG/PPG-10/1 dimethicone),ABIL Wax 9801 (cetyl dimethicone),TEGOSOFT DEC (diethylhexyl carbonate),ABIL 350 (dimethicone), Dow Corning345 (cyclopentasiloxane &cyclohexasiloxane), Aerosil R812S (silica silylated), ABIL Wax 9810P (C24-28 alkyl methicone), and ethylhexylmethoxycinnamate (UvinulMC 80).

EquipmentAll formulas were made with a mixer, theIKARW 20. All were viscosity-tested by aBrookfield DV-I viscometer and checked

under a microscope (Nikon Eclipse E200).All formula samples were put into aHeraeus oven at 50˚C for one month to test the heat stability. Samples of the formulas were also put into a Haierrefrigerator to run a cold stability test (-15˚C for 1-4 weeks) or a cycle test (24 hours -15˚C/-25˚C, 24 hours roomtemperature). A centrifugation test wasalso used to check the stability with aSaitexiangyi TDZ4-WS centrifuge at 3,000 rpm for 30 minutes.

Breaking effect can be expressed bywater droplet forming effect, which was

tested by applying 1 gram of sample onthe back of the hand, gentle rubbing for 5 seconds, and then evaluated by adefined scoring system from 1 to 5. In thissystem, 5 means water beads form veryeasily indicating a very good breakingeffect; 1 means no bead forms, i.e. nobreaking effect. If the score is less than 3,it will be categorised as unacceptable.

Inner water phase contentSix formulas with different inner waterphase content were prepared according toTable 1, from F1 to F6. All these formulas

March 2008 PERSONAL CARE 45

Hai Zhou Zhang, Jinx Ya Dai, Phoeby Wong – Evonik Degussa (China) EMULSIONS

Moving ahead with W/Oquick-breaking emulsions

ABSTRACT

Water-in-oil quick breaking emulsions are becoming an interesting topic nowadaysbecause they provide the consumer with an extraordinary skin sensation and reveal a good cooling effect at the same time.

Due to the necessary high phase/volume ratio of the inner water phase it is verydifficult to achieve long-time stable formulas of this type. In this article, some feasibleand stable formulas which use cetyl PEG/PPG-10/1 dimethicone (ABIL EM 90) arediscussed.

Many different factors may affect the stability and quick-breaking effect. Thesefactors include aqueous phase content, emulsifier quantity, inner phase particle size,thickening wax content, polyalcohol content, processing speed and processingtechniques. All these parameters and their influence on the stability of the final systemwere tested and discussed. Two possible procedures for this formula type are alsoproposed and compared in this article.

Table 1: Breaking effect of different inner water phase content.

Component Formulation (wt %)

F1 F2 F3 F4 F5 F6

A ABIL EM 90 0.8 0.8 0.8 0.8 0.8 0.8

ABIL Wax 9801 1.6 1.6 1.6 1.6 1.6 1.6

TEGOSOFT DEC 2.1 3.5 4.95 6.35 7.8 10.65Cyclopentasiloxane & cyclohexasiloxane 4.15 7.05 9.9 12.75 15.6 21.3

ABIL 350 1.05 1.75 2.45 3.2 3.9 5.35

Magnesium stearate 0.3 0.3 0.3 0.3 0.3 0.3

B Water 83.95 78.95 73.95 68.95 63.95 53.95

Glycerin 5 5 5 5 5 5

Sodium chloride 1 1 1 1 1 1

Bronopol 0.05 0.05 0.05 0.05 0.05 0.05

Water phase content (%) 88.95 83.95 78.95 73.95 68.95 63.95

Breaking effect 5 3 2 1 1 1

were done by the same process: slowlyadding phase B into phase A within 15 minutes at stirring speed of 200 rpm.When finished, the stirring speed wasincreased to 1,100 rpm and kept for 2 minutes.

The test results show a closerelationship between the breaking effectand the inner water phase content: thehigher the content of the inner phase, thebetter the breaking effect. The aqueousphase content must be higher than 85%to achieve a good breaking effect. It mighteven be worth aiming for a water contentof more than 90%. However, with an innerwater phase proportion of over 90%,emulsion formation becomes more andmore difficult as the phase B adding stepwill become longer and longer and finallyfail to result in an emulsion. As acompromise, the optimal water phasecontent should be close to 90%.

Emulsifier contentGenerally the emulsifier quantity is animportant factor to be considered becauseit may affect the stability, sensory propertyand cost of the emulsion and in this casethe breaking effect. Thus 5 formulas aredesigned in order to check the influenceof the emulsifier (ABIL EM 90) in differentquantity. Formulas listed in Table 2 arehandled by the same process as used for the tests in Table 2.

According to the test results in Table 2,the viscosity obviously depends on theemulsifier content. The viscosity increaseswith the content of ABIL EM 90 becausethe inner particle size becomes smaller,which could be demonstrated by themicroscope photos (10 x 10) (Fig. 1).

When the particle size decreasesgradually from F7 to F11 in Table 2, the

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46 PERSONAL CARE March 2008

Table 2: Influence of different ABIL EM 90 quantities.

Component Formulation (wt %)

F7 F8 F9 F10 F11

A ABIL EM 90 0.4 0.8 1.2 1.6 2

ABIL Wax 9801 1.6 1.6 1.6 1.6 1.6

TEGOSOFT DEC 2 2 2 2 2

Cyclopentasiloxane & cyclohexasiloxane 4 4 4 4 4

ABIL 350 1 1 1 1 1

Magnesium stearate 0.3 0.3 0.3 0.3 0.3

B Water 84.7 84.3 83.9 83.5 83.1

Glycerin 5 5 5 5 5

Sodium chloride 1 1 1 1 1

Bronopol 0.05 0.05 0.05 0.05 0.05

24 h Viscosity (mPas) SP93, 4 rpm, RT 98,000 125,000 149,000 180,000 189,000

Heat stability (50˚C 1 month) Pass Pass Pass Pass Pass

Cycle test (-15˚C - RT) Break Pass Pass Pass Pass

Cold stability -15˚C (4 weeks) Break Break Pass Pass Pass

Breaking effect 5 5 4 4 3

Figure 1: Particle size and emulsifier concentration.

Microscope photo of F7. Microscope photo of F9. Microscope photo of F11.

Table 3: Cold stability affected by different concentrations of glycerin.

Component Formulation (wt. %)

F12 F13 F14 F15

A ABIL EM 90 0.4 0.4 0.4 0.4

ABIL Wax 9801 2.5 2.5 2.5 2.5

TEGOSOFT DEC 2.0 2.0 2.0 2.0

Cyclopentasiloxane & cyclohexasiloxane 4.0 4.0 4.0 4.0

Magnesium stearate 0.6 0.6 0.6 0.6

B Water 81.0 71.0 61.0 51.0

Glyerin 8.0 18.0 28.0 38.0

Sodium chloride 1.5 1.5 1.5 1.5

Cycle test (-15˚C - RT) 3 times Break Break Pass Pass

Cycle test (-25˚C - RT) 2 times Break Break Break Pass

interface becomes larger. As a result, thefriction between each particle increasesgradually which leads to the increase ofviscosity. The cold stability and breakingeffect is sensitive to the ABIL EM 90content. Higher content of emulsifierleads to better cold stability and worsebreaking effect, which could be explainedas the interface film is strengthened bythe emulsifier. To achieve both breakingeffect and stability, the optimal emulsifiershare is around 1% to 0.8%. Maybe even 0.4% ABIL EM 90 is feasible if cold stability could be improved by othermethods.

Cold stabilityWater-in-oil emulsions are more sensitiveto freezing temperatures, ascrystallisation of the inner water phasemight destroy the interface film andfinally make the whole system unstable.In conclusion, cold stability is supposedto be improved by at least two methods.First, increase of the emulsifier quantityto strengthen the interface film. It wasdemonstrated by the tests in Table 2.Second, decrease of the freezing point ofthe water phase by adding some solute(e.g. sodium chloride; polyglycol; alcohol

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March 2008 PERSONAL CARE 47

Table 4: Stability effect of different stabilisers.

Component Formulation (wt %)

Base Mg stearate ABIL Wax 9801 Aerosil R812S Beeswax ABIL Wax 9810P

F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26 F27

A ABIL EM 90 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4

TEGOSOFT DEC 3 3 3 3 3 3 3 3 3 3 3 3

Cyclopentasiloxane &cyclohexasiloxane 6 6 6 6 6 6 6 6 6 6 6 6

Mg stearate 0.3 0.6

ABIL Wax 9801 0.8 1.6 2.5

Aerosil R812S 0.5 1.0

Beeswax 0.5 1.0

ABIL Wax 9810P 0.5 1.0

B Water 84.05 83.75 83.45 83.25 82.45 81.55 83.55 83.05 83.55 83.05 83.55 83.05

Propylene glycol 5 5 5 5 5 5 5 5 5 5 5 5

Sodium chloride 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

Bronopol 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

Appearance Lotion Lotion Lotion Lotion Lotion Lotion Cream Cream Lotion Lotion Lotion Lotion

Viscosity (mPas) 7,080 5,600 6,320 6,160 5,840 5,400 23,200 Too 11,120 18,040 15,960 BreakSP05, 10 rpm thick

Centrifugation Break Break Break Break Pass Pass Pass Pass Break Break Break Break

Heat stability Little Little Little Pass Pass Little Pass Pass Little Break Break Break(50˚C, 1 month) break break break break break

Cycle test Break Break Break Break Break Break Break Pass Break Break Break Break(-15˚C - RT, 3 times)

Cold stability Break Break Break Break Break Break Break Pass Break Break Break Break(-15˚C, 4 weeks)

Table 5: Effect of different concentrations of DEC and MC80.

Component Formulation (wt. %)

TEGOSOFT DEC Ethylhexyl methoxycinnamate

F28 F29 F30 F31 F32 F33

A ABIL EM 90 0.8 0.8 0.8 0.8 0.8 0.8

ABIL Wax 9801 1.6 1.6 1.6 1.6 1.6 1.6

TEGOSOFT DEC 2 4 6

Uvinul MC80 2 4 6

Cyclopentasiloxane & cyclohexasiloxane 4 2 0 4 2 0

ABIL 350 1 1 1 1 1 1

Magnesium stearate 0.3 0.3 0.3 0.3 0.3 0.3

B Water 84.3 84.3 84.3 84.3 84.3 84.3

Propylene glycol 5 5 5 5 5 5

Sodium chloride 1 1 1 1 1 1

Bronopol 0.05 0.05 0.05 0.05 0.05 0.05

Appearance Cream Cream Cream Cream Fail Fail

Viscosity 63,800 62,400 65,700 91,100 – –(mPas, SP93, 10 rpm)

Heat stability Pass Pass Pass Pass – –(50˚C, 1 month)

Cycle test Pass Pass Pass Pass – –(-15˚C - RT, 3 times)

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48 PERSONAL CARE March 2008

or some water soluble polymers likecarbomer or xanthan gum). To make itsimple, only glycerin was investigated, thusfour formulas with different concentrationsof glycerin were tested (Table 3). Thesefour formulas were prepared by the sameprocess: Phase B was introduced intophase A within 15 minutes at stirringspeed of 200 rpm and when finished, thestirring speed was increased to 700 rpmand kept for 2 minutes.

The results in Table 3 confirm that thecold stability is glycerin concentrationrelated, the higher the better. As discussedabove, there are some other raw materialswhich could decrease the freezing point of water. If combined, they are supposed to have synergistic effects, and theconcentration of glycerin can be lowered or glycerin can even be totally substituted.

Heat and centrifugationstabilityStabilisers are often used to make theformulas more long-term stable. For water-in-oil emulsions, waxes, alkaline-earthmetal soaps, hydrophobic polymers orhydrophobic fumed silica can be used asstabilisers.6,7 By increasing the viscosity ofthe outer oil phase or forming an oleogelnet system, they block the process ofseparation and lead to the improvement of high temperature and centrifugationstability. A series of tests was conducted to compare the efficacy of some typicalstabilisers: magnesium stearate, ABIL Wax 9810, Aerosil R812S, Beeswax and ABIL Wax 9810P (Table 4). All theseformulas are essentially the same and are distinguished only by the stabilisers.Moreover, they were made by the sameprocess: slowly adding phase B into phaseA with stirring at 200 rpm for 15 minutesand, when finished, increasing the stirringspeed to 500 rpm for 2 minutes.

“Break” in Table 4 means phaseseparation. Formula F16 is a base formula

without any tested stabiliser. Comparedwith the stability properties of F16, onlyF20, F22 and F23 are improved. Thisresult shows that hydrophobic fumedsilica (Aerosil R812S) is very effective in improving the stability, and in higherconcentration it can also improve the cold stability whereas it also extremelyincreases the viscosity (it exceeds the test range of viscometer in 1% by weight).ABIL Wax 9801 can improve the stabilitywithout increasing the viscosity but it onlyhas its best efficacy at 1.6% by weight.

Oil phase componentABIL EM 90 is very tolerant towards many different kinds of oil phases andconsiderable amounts of silicone oils can be incorporated into the oil phasebecause of the cetyl branch in itsmolecular structure. When the total oilphase proportion is very small (less than10%), different types of oil componentsmay influence the whole system.TEGOSOFT DEC and ethylhexylmethoxycinnamate (Uvinul MC80) weretested respectively to compare their

influence on the stability property of thewhole formula. Six formulas with threedifferent weight proportions of TEGOSOFTDEC or Uvinul MC80 were designedaccording to Table 5 and these formulasare done by the same process as used for the tests of Table 1.

According to the results in Table 5,formulas F28, F29, F30 and F31 pass the stability test while formulas F32 andF33 do not due to the failure to form anemulsion. It demonstrates that this systemdoes not tolerate too high ethylhexylmethoxycinnamate content. Generally thequick breaking emulsion concept may be limited for UV protection products.

Process discussionNormally a water-in-oil emulsion is moresensitive to the formation process due tothe high inner phase content than an oil-in-water emulsion. As previously discussed,the main process can be divided into twosteps: first slowly adding the water phaseinto the oil phase and second increasingthe stirring speed to homogenisation speed.

There are two ways of adding the waterphase into the oil phase (step 1), which arereflected in Figure 2, processes A and B.In process A, the stirring speed is graduallyincreased and in process B the samespeed of stirring is kept when adding thewater phase. Process B is recommendedfor two reasons. First, slowing down thestirring speed leads to lower viscosity andeasier dispersion. Second, increasing thestirring speed leads to smaller particle sizewhich also means larger interface – then itwill consume more emulsifier and possiblylead to insufficient emulsifier concentratedon the interface when the remaining waterphase is added. Process B is also easier tohandle in a real large-scale production asthe process is simpler and requires lessenergy.

The second homogenisation step will be

Table 6: Effect of homogenisation stirring speed on the stability.

F34 F35 F36 F37 F38 F39

Stirring speed (rpm) 700 900 1,100 1,300 1,500 1,800

Viscosity 63,000 96,000 118,000 139,000 183,000 228,000(mPas, SP93, 10 rpm)

Centrifugation Pass Pass Pass Pass Pass Pass

Heat stability Pass Pass Pass Pass Pass Little50˚C, 1 month break

Cycle test(-15˚C - RT) 3 times Pass Pass Pass Pass Pass Pass

Cold stability Failed Failed Pass Pass Pass Pass(-15˚C) after after 1 week 3 weeks 3 weeks 3 weeks

1 week 1 week

Breaking effect 5 5 5 5 5 5

W O

W+O

W O

W+O

Figure 2: Two possible emulsifying procedures.

Process A Process B

450 rpm (5 min)200 rpm (16 min)

550 rpm (6 min)

650 rpm (9 min)

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March 2008 PERSONAL CARE 49

discussed as well. For this formula F8 inTable 2 was tested under 6 different shearrates from 700 rpm to 1,800 rpm in orderto compare their influence on the viscosityand stability (Table 6).

The results of Table 6 show thatviscosity increases when the stirring speedin the homogenisation step is increased.This is due to a smaller particle size, whichis demonstrated in the microscope photos(10 x 10), Figure 3.

The particle size gradually decreasesfrom F34 to F39. Compared with themicroscope photos of F36 and F38, F39 is less homogeneous although most particles are much smaller. Thisphenomenon may result from the over-homogenisation and leads to the problemof stability (slight break of formula F39 at50˚C after one month). There are othertwo interesting findings: the cold stabilitywill improve with smaller particle size whilebreaking effect is not particle size related.As a result, the proper homogenisationspeed in this case is between 1,100 rpmand 1,500 rpm.

Two example creamsTable 7 shows details of two examplecreams with a good breaking effect andstability property.

ConclusionsW/O quick breaking emulsions areattractive because of their instant andvisible transformation when applied to the skin. Water beads are formed andthere is a fresh feeling. Therefore the quick breaking concept is an ideal supportfor skin moisturising claims.

In order to achieve a good breakingeffect, the weight proportion of the waterphase should be near 90% and in thiscase, ABIL EM 90 is a recommendedemulsifier. However, when high quantitiesof the emulsifier are applied it will have a Figure 3: Particle size and homogenisation.

Microscope photo of F38. Microscope photo of F39.

Microscope photo of F34. Microscope photo of F36.

Table 7: Two example creams with a good breaking effect and stability property.

A % w/w B % w/w

A ABIL EM 90 0.8 1.0

ABIL Wax 9801 1.6 1.6

TEGOSOFT DEC 2 6

Cyclopentasiloxane & cyclohexasiloxane 4

ABIL 350 1 1

Magnesium stearate 0.3 0.3

B Water 68.7 68.6

Glycerin 20

Propylene glycol 20

Sodium chloride 1 1

C Preservative & perfume q.s. q.s.

Formula B is richer from a sensory point of view compared with Formula A.

negative influence on the breaking effect.A recommended process for

emulsification is a two-step method: slowlyadding the water into the oil phase at alow stirring speed (e.g. 200 rpm) for about15 minutes and then homogenising at astirring speed of 1,100 rpm to 1,500 rpm.In this process, viscosity of the final

formula increases with more emulsifier ormore stabiliser (e.g. fumed silica) or higherhomogenising speed.

Cold stability can be improved by higherglycerin or emulsifier contents or smallerparticle size. Heat and centrifugationstability can be improved with certainstabilisers such as hydrophobic fumedsilica (Aerosil R 812 S) or silicone waxes(ABIL Wax 9801).

References1 Patent US 7138128 B2, (2006), Beiersdorf AG

Corporation.2 Schaefer D. Silicone surfactants, Tenside

Surfactants Detergents, 27, (1990), p154-158.3 Silicones, Chemistry and Technology, Vulkan

Publishing, Essen (1991), p 117-118.4 Grüning B., Hameyer P., Weitemeyer Chr. Tenside

Surfactants Detergents, 29, (1992), p78-83.5 Jenni K., Hameyer P. Parfümerie & Kosmetik, 79,

(1998), p22-28. 6 Capman I. D. The effect of the emulsifying agent

on the dielectric properties of water-in-oilemulsions. Journal of Physical Chemistry 75 (4),p537-541 (1971).

7 Patent US 5523091, (1996), L’OrealCorporation.

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