j. Cosmet. Sci.,55, 309-325 (July / August2006)

Microemulsionsof triglyceride-basedoils: The effect of co-oil and salinity on phasediagrams NAPAPORN

KOMESVARAKUL,

MONICA

D. SANDERS,

ERIKA SZEKERES, EDGAR J. ACOSTA, JAMES F. FALLER, TONY MENTLIK,

LOUIS B. FISHER, GREGG NICOLL,

DAVID A SABATINI, andJOHN F. SCAMEHORN, University of Oklahoma, School of Chemical, Biological, and Material Engineering and theInstitute for AppliedSurfactant Research, Satkeys EnergyCenter,1O0 E. Boyd,Norman,OK 73019 (N.K., M.D.S., E.S.,J.F.S.), University of Toronto, Chemical Engineering andAppliedChemistry, 200 College Street,Toronto,Ontario,CanadaM5S3E5 (E.J.A.), Mary Kay Inc., 1330 RegalRow, Dallas, TX 75247 (J.F.F., T.M., L.B.F., G.N.), and University of Oklahoma, Civil Engineering and Environmental Science Department and theInstitute for AppliedSurfactant Research, 202 W. Boyd,Norman,OK 73019 (D.A.S.). Accepted for Publication March2, 2006. Synopsis

Microemulsification of triglyceride-based oil is challengingdue to the formationof undesirable phasessuch as macroemulsions, liquid crystals,or spongephases.This researchevaluatesthe formationof artificial sebummicroemulsions usinglinker molecules, with the additionof co-oilto help enhancesebumsolubilization. The microemulsionconsistsof a lipophilic linker (sorbitanmonooleate),a hydrophiliclinker (hexylglucocide), a main surfactant(sodiumdioctyl sulfosuccinate), a co-oil, and artificial sebum.The effect of addingco-oilto the phasebehaviorand the microstructure of the resultingmicroemulsion is described. The effectof severaltypesof co-oilis alsostudied;the co-oilsevaluatedherearesqualene,squalane,isopropyl myristate,and ethyl laurate.The effectof salinityon the microemulsion phasebehavioris alsopresented. Fishdiagramsareobtainedby plottingtotal surfactant/linker concentration asa functionof sebumfraction in the oil mixture (co-oil + sebum).Different microemulsiontypes(Winsor TypesI, II, III, and IV) are formed,dependingon the total surfactant/linker concentration and the fractionof co-oilin the oil mixture. WinsorTypeIV (single-phase) microemulsions areobserved at high surfactant/linker concentrations. These single-phase, isotropic,andlow-viscous fluidsareparticularlyusefulfor cleansinganddeliveryof functional ingredientsin skincareproducts.Saltadditionshiftsthe fishdiagramtowardsmorehydrophobic oil systems and higher surfactant/linkerconcentrations.

The currentaddressof NapapornKomesvarakulis Unilever Home and PersonalCare-North America,40 Merritt Blvd, Trumbull, CT 06611. The currentaddressof Erika Szekeresis CloroxServiceCompany,7200 JohnsonDrive, Pleasanton,CA 94588.

Addressall correspondence to David A. Sabatini. 309

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INTRODUCTION

Microemulsions are transparentand thermodynamically stablemixturesof oil and water stabilizedby surfactants.Microemulsionscontainextremelyhigh oil/water interfacial areas,offeringultra-low interfacialtension(lessthan 0.1 mN/m). Practicalapplications of microemulsionsystemsinclude enhancedoil recovery(EOR), drug delivery, nanoparticlesynthesis, food,andcosmetics (1-3). The transparency of microemulsions makes them especiallyattractivefor cosmeticformulationsas they give the perceptionof a "clean"system.The ultralow interfacialtensionbetweenoil and water facilitatesthe penetrationof the productinto nanoscale poresof humanskin, makingmicroemulsions a candidatefor deep-cleansing products. A Type I microemulsion(O/W microemulsion)is conceptualizedas swollenmicelles surroundedby water wheresurfactantmiceliescoexistwith excessoil. A Type II microemulsion(W/O microemulsion)is conceptualizedas swollenreversemiceliessurroundedby oil wherethe reversemiceliescoexistwith excess water.A Type III microemulsionconsistsof an oil, water, and a "middle" bicontinuousmicroemulsion phase coexistingin a three-phaseequilibrium. A Type IV microemulsionis defined as a single-phase microemulsionsystemwhereboth oil and water are completelysolubilized in the surfactantmicroemulsionphase.Microemulsiontransitioncan be achievedin severalways,dependingon the type of surfactants.For example,for ionic surfactant systems,a Type I-III-II transitioncanbe obtainedby increasingthe electrolyteconcentration, whereasincreasingthe temperaturecanachievethe sametransitionfor nonionic surfactantsystems.The optimum conditionis definedasthe conditionat whichan equal volumeof oil and water is solubilizedin the bicontinuous phase(Type III). The electrolyte concentration requiredat the optimum conditionis called"optimumsalinity"or S*. The solubilizationparameter(SP),which is definedby the amountof oil solubilized in the middlephaseper unit massof surfactant,at this optimumconditionis knownas the optimumsolubilization parameter(SP*);maximizingthisparameterfor triglyceride oils is a goal of this work.

Two importantparameters that describethe ability and effectiveness of a surfactantto form microemulsions arethe sizeandcurvatureof the microemulsion and the flexibility of the surfactant film it forms (4). An elastic and flexible surfactant film favors the

formationof a microemulsion,whereasa lameliarphaseis formedwith a morerigid or stiff film. The flexibility of the film also dependson the molecularstructureof the surfactant.Cosolventssuchas short chain alcoholscan improve the film's flexibility (5-8). While microemulsionphasebehaviorcan be describedin variousways,the "fish diagram"is one of the most common.A fish diagramis typically plotted betweenthe surfactantconcentration and a scanor tuning parameter(e.g., salt or hydrophobicityof the system),as shownin Figure 1. Ro is radiusof the oil dropletand RW is the radius of the water droplet.The curvatureof the oil and water dropletsis then equalto 1/Ro and 1/Rw, respectively.The scanparameterdirectly affectsthe curvatureof the surfactant membrane,which is a very important factorfor a surfactantto form microemulsions, as mentioned above.

Electrolyteaddition to ionic surfactantsystemsincreasesthe hydrophobicityof the surfactantsystemand decreases the surfactantfilm curvature(seeFigure 1). Therefore, whenthe surfactant systemhasrelativelylow hydrophobicity or is at low salinity,a Type I microemulsion (O/W microemulsion) occurs.At high hydrophobicity,wherethe cur-

MICROEMULSIONS

OF TRIGLYCERIDE-BASED

OILS

311

IV: Single-phasemicroemulsion

Type

Type Iß

Bicontinuous Type II:

Micelies

Reverse

micelies

©© Ro

Salinity

Curvature(H) =I/R

o ReducingCurvature

Figure 1. Fishdiagramshowingphasebehaviorandchanges in curvaturewith surfactantconcentration and formulationhydrophobicityas adjustedby a scanningvariable(salinity).

vature decreases,a Type II microemulsion(W/O microemulsion)exists. When the hydrophobicityis intermediatebetweenthesetwo conditions,and at lower surfactant concentration,a three-phasemicroemulsionor Type III microemulsionoccurs,with a net zero curvature.When the surfactantconcentrationincreaseswithin the Type III region,a Type IV microemulsioncan be obtained.The minimum surfactantconcentration for completesolubilizationof the water and the oil is wherethe three-phaseand one-phaseregions(Type IV) meet, which appearsat relativelyhigh surfactantconcentrations.Fishdiagramswith similarbehaviorhavebeenreportedelsewhere(1,5,7,9-12). For example,JakobsetaL (9) obtainedthe well-knownfish diagramfor water-,-decanenonionicsurfactantsystemsby plotting surfactantconcentrationversusthe scanning variable of temperature.Von Corswantand co-workers(5,7) plotted fish diagrams betweenalcoholconcentration asa functionof surfactantconcentrations for triglyceride microemulsification.

Formationof microemulsionsystemswith short-chainoils or alkaneshas been extensivelystudied(9,13-15); a large rangeof surfactantsand additivescanbe usedto control their microstructuralproperties.However,microemulsification of triglycerides,and in particularlong-chaintriglycerides,is very challenging(5-8,10,16-33). Thesetriglycerideshaveminimum solubilityin microemulsion systems and thustend to form liquid crystalmesophases. This is due to the fact that the hydrocarbonchain portionsof the surfactantfilms presentat triolein-waterinterfaceshavedifficultypenetratingthe large triolein molecules.Therefore,higher temperaturesare required to induce sufficient disorderof the triglyceride-surfactant film, which generallycontainslong hydrocarbon

chai•s,straightandof uniformlength.Thisdisorder permitssignificant amounts of

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triolein to be solubilizedin suchfilms (34). In other words, if the surfactantfilms were

lessordered(e.g.,if the hydrocarbon chainswereof nonuniformlengthand/orbranched), solubilization

of triolein

would be facilitated.

Addition of short-chainalcoholsis known to mitigate formationof undesirableliquid crystalphases(5,7,18,19,23).Joubranand coworkers (18) havealsodemonstrated that the formationof triglyceridemicroemulsion can be achievedby incorporatingsucrose anda short-chainalcohol.The synergisticinteractionsamongthe alcoholandthe sucrose moleculesresult in the destabilizationof the liquid crystallinemesophases and thus facilitate the triglyceride microemulsionformation. They have also studied a micro-

emulsionof soybean oil, polyxyethylene (40) sorbitanhexaoleate, andwater--ethanol and found that the microemulsionformationstronglydependson temperatureand that the systemsrequire large amountsof surfactantsand alcohol(19). Von Corswantand coworkers(5,7) observedthe presence of lameliarliquid crystalor Lc phasesin long-chain triglyceridesystems. They explainedthat the Lc is destabilizedby incorporatingwater when a short-chainalcoholis present.Adding the short-chainalcoholscan increasethe flexibility of the surfactantfilm, sincethe Lc phaseis destabilizedin favorof a microemulsionphasedue to the increaseof the curvature(short-chainalcoholsdecrease the polarity of the aqueousphase).In addition,they found that addingco-oil into microemulsionsystemshelpsreducethe surfactantconcentrationand the amountof alcohol required'to form a microemulsion (7). Huang andLips(11) alsofoundthat microemulsificationof triglyceridesrequireshigh temperaturesand surfactantconcentrations, whereasMinana-Parezand coworkers (35,36) havepreviouslyreportedthe useof alkyl sulfateswith oxyethyleneand oxypropylene groups,or so-called"extendedsurfactants," for formingefficientmicroemulsions of the bicontinuous type (Type III microemulsion) with polar oil, includingtriglycerides;however,high salt concentrations (up to 7% wt NaC1) were required. The solubilizationat optimum formulationwith conventionalsurfactants(commercialized non-extendedsurfactants)hasbeenfound to reachvaluesas high as 30 ml/ml or ml/g surfactantwith short-chainalkanes,and ashigh as 10 ml/g with hexadecane, while it canbe lessthan 4 ml/g with mono-chainpolaroils and much lesswith triglyceride

oils(35). Graciaaandcoworkers (37,38)first introduced the lipophiliclinkerconceptto help enhancesurfactant-oilinteractionand thus improvethe solubilizationcapacityof hydrocarbonand polar oils. Lipophilic linkers, suchas a long-chainalcohol,tend to segregate nearthe oil sideof the oil-water interface,closeto the tailsof the surfactants (39), as depictedin Figure 2. In this schematic,the surfactant,sodiumdihexyl sulfosuccinate,adsorbsat the oil-water interface.The lipophilic linker, dodecanol,is shown to adsorbat the palisadelayerof the interface(oil sideof the surfactantlayer),promoting the local order and increasingthe interactionbetweenthe surfactanttail and the oil phase.In contrastto the surfactantand the lipophilic linker, sodiummono- and dimethyl naphthalenesulfonate(SMDNS) is a hydrophiliclinker that segregates nearthe water side of the oil-water interface;this hydrophiliclinker moleculeis believedto increasethe total interfacialareaandthe overallinteractionbetweenthe surfactantlayer and the aqueousphase(40). For certainoils, addinga lipophilic linker aloneto the microemulsion giveslimited solubilizationenhancement. Acostaandcoworkers (39-41) foundthat hydrophiliclinkerscanhelpimprovesolubilization abilitybecause theyallow more room for lipophilic linkersto segregateand further enhancethe solubilization ability.

MICROEMULSIONS

Water Surfactant

OF TRIGLYCERIDE-BASED

OILS

313

side of the interface

SDHS

00 :••0 •.,_.., ,.

0

Oil side"i'i"int j

Lipophilic linker Dodecanol

Combined

linker

SDHS = sodiumdihexylsulfosuccinate SMDNS = sodiummono-anddimcthylnaphalene sulfonate Figure 2. Schematicof the linker concept,showingthe surfactant(e.g., sodiumdihexylsulfosuccinate or SDHS), lipophilic linker (e.g., dodecanol),and hydrophilic linker (e.g., sodium mono- and dimethylnaphthalenesulfonateor SMDNS).

There are only limited choicesof surfactants/ingredients that can be usedin cosmetic formulationsand that form microemulsions with triglyceridessuccessfully. The main benefitof alcohol-freeformulationsstemsfrom a consumerperceptionthat alcoholsare drying and potentiallyirritating, and so alcohol-freeformulationswould be usefulfor sensitive-skinconsumers. This paperdescribesour successful useof linker conceptsto formulatealcohol-freemicroemulsionswith artificial sebum(human oil) using biocompatible and cosmeticallyfriendly ingredientsat ambient temperatureand low salt concentrations.We haveinvestigatedthe opportunityto tune the curvatureof surfactant film by addingco-oilto the triglyceridein orderto helptriglyceridemicroemulsification as well as to minimize the surfactantrequired to form single-phasemicroemulsions without adding alcohol.

EXPERIMENTAL

PROCEDURES

MATERIALS

The following materialswere obtainedfrom Aldrich (Milwaukee, WI) at the concentrationsshownand were usedwithout further purification:sorbitanmonooleate(Span 80, 99%), squalene(98%), squalane(99%), isopropylmyristate (IPM, 98%), ethyl laurate (99%), and sodium chloride (99%). Sodium dioctyl sulfosuccinate(AOT, -100%) waspurchased from FisherScientific(Fair Lawn,NJ). Hexylpolyglucocide AG 6206TM, donatedby Akzo Nobel (Chicago,IL), was receivedas a 75 wt% aqueous

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JOURNAL OF COSMETIC SCIENCE Table

I

Compositionof Artificial Liquid Sebum(25øC) Used in This Study Component

%

Lauric acid Oleic acid Isostearic acid

11.73 11.73 5.86

Tricaprin

11.73

Triolein

11.73

Glycerol triisostearate Oleyl oleate Myristyl myristate Isostearylisostearate Squalene

5.86 10.6 10.6 4.13 12.23

Cholesterol

1.53

Cholesterol oleate

2.27

solutionand usedwithout furtherpurification.Artificial sebumwaspreparedby Mary Kay Inc., with the compositionshownin Table I; the sebumwasliquid at the experimental temperatureof 25øC. Propertiesof the co-oilsare shownin Table II. METHODS

Phasebehaviorstudieswereperformedusingequalvolumesof waterand oil (or sebum/ co-oil mixtures),giving a water/oil ratio (WOR) equal to one. Preliminarystudieswere conductedto determinean optimum formulationand optimum salinity.The optimum

formulationof the aqueousphasewasfoundto be a surfactantmixtureof 4% AOT + 5.13% sorbitanmonooleate+ 5.06% hexylglucocide by weight;the optimumsalinity Table

II

SelectedPropertiesof the Co-Oils Used in This Study Co-oil

Squalene

Squalane

EACN

24a

-24

MW (g/mole)

Molecular formula

410

422

L%O (CH2}t3 CH(CH2) 3 CHCH 2CH 2-- 2

270

CHa(CH2) • •CH2--C --O CHCH3

I[

Isopropylmyristate (IPM)

13b

o ii

Ethyllaurate(EL)

From ref. 39.

From ref. 12. From this work.

< 13c

224

CHa(CH=)9CH =-- C --OCH2CHa

MICROEMULSIONS

OF TRIGLYCERIDE-BASED

OILS

315

(S*) for this compositionis 0.5% NaC1. Stocksolutionsof AOT, hexylglucocide, and sorbitanmonooleate at the selectedweight ratioswerepreparedat differenttotal surfactant/linkerconcentrations rangingfrom 14.19 to 56.06 wt %. The phasestudieswere carried out in 16 x 125 mm flat-bottomed tubes; 2.5 ml of surfactant solution was

added,followedby the additionof co-oil,then sebumoil. The total volumeof co-oiland sebumoil requiredis equivalentto the amountof water presentin the 2.5 ml of surfactantsolution;this is to keepthe WOR equalto one.The fractionof sebumoil in the oil mixtures is varied from zero (100% vol. co-oil) to one (100% vol. sebum oil). The

preparedsamplesweregentlyshakenoncea day for threedaysandleft to equilibrateat room temperature(25øC) for two weeks.Phasediagrams(fish diagrams)were constructedby plotting the total surfactant/linker concentration asa functionof the sebum fractionin oil. The microemulsionphases(TypesI, II, III, and IV) were obtainedby visual observation.The effectof salinity (0.5 wt %, 1.5 wt%, and 3 wt % NaC1) and co-oil on the phasebehaviorwas investigated.

RESULTS EACN

AND

OF CO-OiL

DISCUSSION AND

OPTIMUM

SALINITY

Formulatingmicroemulsions requiresthe right combinationof variablesthat will provide an optimummiddle-phase microemulsion. Salagereta/. (42) proposeda semiempirical equationthat relatesthe differentformulationvariables:

In(S*) = k(EACN)+ f(A) - tr + Or.rAT

(1)

whereS* is the optimumsalinity,or electrolyteconcentration; k is a constant,normally between0.1 to 0.17; and EACN is the equivalentalkanecarbonnumberfor nonlinear hydrocarbon (e.g., triglycerides).For linear alkanehydrocarbons, the alkanecarbon number (ACN) is applied. The EACN is estimated basedon the optimum salinity obtainedin our formulationstudies;the higher the optimum salinity required,the higherthehydrophobicity or EACN of the oil. The effectof alcoholor additivesis noted byf(A), (r is a functionof the typeof the surfactant, (xisa constant,T is the temperature

of the system, andAT is the difference in temperature betweenthe temperature of the systemand an arbitraryreferencetemperature.However,in this study, alcoholis not includedand the temperatureof the systemis constant. Acostaet aL (12,39) determinedthe EACN of squaleneand isopropylmyristate(IPM) as shownin Table II (24 for squaleneand 13 for IPM). The EACN for squalaneis expectedto be closeto the valuefor squalene(-24). Table III showsthe optimum salinity of oil mixtures(co-oil and sebummixtures).The optimum salinity of pure isopropylmyristateis 3.5% NaC1, whereasthe optimum salinity is lower when the amountof sebumoil is increased (e.g., the optimum salinityfor the oil mixture of 20% vol. IPM and 80% vol. sebumis lessthan 0.5%). This suggeststhat IPM hasa higher EACN or is more hydrophobicthan sebumoil. The optimum salinity of pure ethyl laurate(EL) is 1-1.5% NaCI, which is closerto the optimum salinityof the 20% vol. EL and 80% vol. sebummixture, indicatingthat EL hasan EACN closerto sebumoil thanIPM does(EACNseb• m < EACN•L < EACNiPM).This is a veryimportantfinding in formulatingcleansing productsbecause the amountof sebumin humanskin canbe differentdependingon skin types.The ideal objectiveis to be able to formulatea

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JOURNAL OF COSMETIC SCIENCE Table

III

Optimum Salinitiesfor Oil Mixtures

Isopropylmyristate(IPM)-sebum mixtures % IPM

% Sebum

20

80

40 60

60 40

80

20

0

100

S* (%)