xanthan gum dispersions

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described by the popular '*house of cards" model for the aqueous colloidal structure of smectites. Once the MAS has bee•n well dispersed and exists as discrete ...
j. Soc. Cosmet. Chem., 32, 275-285(September/October 1981)

Rheological properties of magnesium aluminumsilicate/xanthan gumdispersions PETERA. CIULLO,R. T. VanderbiltCompany, Inc.,30 IVinfield Street,Norwalk, CT 06855

Received January12, 1981.Presented at theSCCAnnualMeeting, New YorkCity,December 12, 1980.

Synopsis

The abilityto modifythe rheologyof magnesium aluminumsilicate(MAS) with variousORGANIC THICKENERS has long been known and used to advantage. The widely used MAS/ carboxymethylcellulose combination, for example,has beenthe standardstabilizing systemin liquid makeups fora numberof years.Useof xanthangumwithMAS isnowshownto evenfurtherextendthe versatility of thissmectite mineral.Comparative evaluations demonstrate howminorinclusions of xanthan gumin MAS dispersions providesynergism in both viscosity andyieldvalue.The abilityto modifythe THIXOTROPIC natureof MAS isalsoshown,allowingpseudoplastic behavior throughsmalladditions of xanthan gum.Forthebestbalance of properties, themostgenerally usefulrangeof MAS:xanthan gum ratiosin practicewill be9:1to 2:1.Thiscombination shouldprovideformulaswithgoodviscosity stability and smoothflow characteristics. The excellentyield valuepossiblesuggests superiorstabilization of suspensions andemulsions. The combination of theseproperties makesthe MAS/xanthangum system especially wellsuitedto stabilizing all typesof fluidsuspensions, lotions,andmakeup.

INTRODUCTION

The refinedsmectitemineral,magnesium aluminumsilicate(MAS), hasfor decades beenpopularlyusedasa thickener,suspending agent,emulsionstabilizer, andgeneral texture modifier in a number of industries.

The observedrheologicalpropertiesof MAS that accountfor its utility are well described by the popular'*house of cards"modelfor the aqueous colloidalstructure of smectites. Oncethe MAS hasbee•n well dispersed andexistsas discrete platelets, accordingto this model, the weaklypositiveplateletedgesare attractedto the negatively chargedfaces.This attraction,coupledwith face-faceelectrostatic repulsion, is sufficient to establish a cohesive cubic network. This network determines

dispersion properties. In additionto impartingviscosity, it is•responsible for suspending and segregating the internalphaseof emulsions and suspensions. The amountof forcerequiredto disruptthisnetworkdetermines the yieldvalue.Shearexceeding the yield valueproducesreduceddispersion viscosity.Increasingsheargivesdecreasing viscosity, aswithpseudoplastic materials. Cessation of shearallowsdisplaced facesand 275

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edgesto againbemutuallyattractedandreformthe cubicnetwork.Sincethisstructure recovery takesa measurable amountof time,the dispersion is considered thixotropic. The proposedmechanisms and arguments favoringthis view of colloidalsmectite structurehavebeendetailedby van Olphen(1). Specificapplications of MAS to cosmetic andpharmaceutical products havebeendescribed byCarlson (2,3)andCiullo (4).

To date,the mostwidelyusedorganicin combinationwith MAS hasbeensodium carboxymethylcellulose (CMC). Thisanionicgum providessynergistic viscosities with MAS, evenwhenincludedat aslittle as10%of the mineral'sweight.Greaterelectrolyte toleranceand reducedrisein dispersion viscosityovertime are alsoobtained.The most commonuseof the MAS/CMC combinationis in liquid makeupswherethe MAS providesemulsionand suspension stabilization whilethe CMC contributes to smooth flow.

The polyanionic polyheterosaccharide, xanthangum(XG) is a particularly functional materialwhichoffersadditionalpotentialin modifyingthe properties andextending the usefulness of MAS. XG is a high viscositythickenerwith yield valueand high electrolytetolerance.As a pseudoplastic thickener,solutionviscosityis reducedin proportionto the amountof shear.Whenshearis removed,viscosity recovery occurs almostinstantaneously. The colloidalstructurecorresponding to this behavioris not well characterized for this material.The commercial literature(5) citesthe work by Rees(6,7)suggesting that doublehelicesareformedby the polymerchains,whichin turnform flexibleaggregates. Yield valuewouldbe a measure of the forcerequiredto begindissociation of aggregates, with shearthinningresultingfrom furtherdissociation. In the absenceof shear,reaggregation would immediatelytake place.The properties andusesof XG havebeenreviewed byJeanes (8). While XG is widelyusedin industrialand food products,it is not as popularasthe variouscellulose derivatives for cosmeticandpharmaceutical applications. It hasbeen found,however, that useof smallamountsof XG with MAS providesa synergism in bothviscosity andyieldvalue.Thishasfor a numberof yearsbeenusedto advantage in household and agricultural products.This likewisesuggests considerable potential in thepersonal productsfieldfor stableflowableemulsions andsuspensions. The presentinvestigation wasundertaken to demonstrate the effectson dispersion rheologyof combiningMAS and XG in variousratios.The propertiesof each combinationwere comparedto the propertiesof separatepreparations of each component. The synergism in viscosity andyieldvalueandeffecton MAS thixotropy weretherebydemonstrated. All dispersions and solutionsweresuitablypreserved due to the susceptibility of XG to microbialdegradation on aging. EXPERIMENTAL MATERIALS

The magnesium aluminumsilicate(VEEGUM, R. T. VanderbiltCo.), food grade xanthangum(Keltrol,KelcoDiv. of Merck),andparaformaldehyde, 95%(Matheson, Coleman & Bell) were usedas received.Distilled, deionizedwater was usedfor all

preparations. All dispersions and solutionswere made in a Waring Commercial

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Blender(Model 31 L 41). All viscositydeterminations weremadewith a Brookfield Viscometer (ModelLVT) usingtheappropriate singlepointor helipathspindle. PROCEDURE

Dispersion Preparation

The MAS, XG, or MAS/XG dry blendwasaddedto 23 -+ 2øCwaterwith 3 min of highspeedmixingin the blender.The paraformaldehyde wasthenaddedat 0.2%on dispersion weightwith brief slow speedmixing.In eachcase,the amountof water usedwasadjustedto accountfor the moisturecontentof the MAS and/or XG. Three percentdispersions weremadewith MAS:XG ratiosof 29:1,19:1,9:1,4:1,2:1,and 1:1. SeparateMAS dispersions and XG solutionswere madeat solidscorresponding to thosein eachcombination. Separate 3%MAS andXG preparations werealsomade.In all cases,a 750gquantitywaspreparedwhichwaspouredoff immediatelyinto one 16-oz.andtwo 4-oz.glassscrewcapjars. Viscosity Determinations

After eachdispersion waspouredoff, the 16-oz.samplewasallowedto standfor five minutes.A singlepoint viscosity wasthenrun with the appropriate spindleusingthe viscometer at 60 rpm.A readingfor viscosity calculation wastakenafter6 minto allow equilibration of structurebreakdownand buildupin thixotropicsystems. Following the singlepointdetermination, a helipathviscosity wasobtainedusingthe appropriate attachment and a one minuterun at 6 rpm.The 6-minsingle-point run followedby 1-minhelipathrun wasrepeatedafter one, seven,and thirty daysof agingfor each 16-oz.sample.

A simplecomparative measure of thedegreeof viscosity synergism for eachMAS/XG blendwasdetermined by calculating a viscosity synergism factor(VSF)for bothsingle pointandhelipathviscosities of each3%dispersion according to: VSF =

VB•

V,•Z%+

3%

V,,(• (3 -- Z)% '

(1)

where V• is blend viscosity,V,• is MAS viscosity,Z is the level of the MAS componentin the blend dispersion, and V,, is XG viscosity.A VSF of 1 indicates merelyan additiveviscosity is obtainedfrom the blend.Any figuregreaterthan 1 is a measureof the synergismoccurringin the blend. VSF (SinglePoint) and VSF (Helipath)werecalculatedfrom viscosity figuresafterone,seven,andthirty daysof aging. Shear Stress vs. Shear Rate Determination

After each16-oz.samplehad agedfor 11 days,the shearstressvs. shearrate was determined usingtheappropriate singlepointspindle.The shearstress, represented by the numericalviscometer dial reading,wasrecordedat shearratesof 0.6, 1.5,3, 6, 12, 30,60, 30, 12,6, 3, 1.5,and0.6 rpm in succession. The dial readingwasrecordedafter oneminuteof shearat eachrate.Theshearstress (dialreading) vs.shearrate(rpm)was plottedto determine thixotropy, pseudoplasticity, andpresence of yieldvalue.

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Yield Value Determination

After7 daysof aging,yieldvaluemeasurements weremadewiththetwo 4-oz.samples of eachdispersion usinga modification of theprocedure of Bowles,Davie,andTodd (9). Sevendays of aging were allowedto insurethat the colloidalstructurein thixotropicsystems waswellestablished. A singlepointviscosity at 6 rpmaftera 6-min runwasmadefor onesampleof eachdispersion, withviscosity at 12rpmaftera 6-min runfor theother.By usingseparate samples notpreviously sheared, and6-rainrunsto allowstructure equilibration withtherotatingspindle,thixotropy-induced errorsin the original computationsshould be minimized.Better reproducability is therefore ascribed to the yieldvaluecomputation: (W -

YieldValue(dynes/cm 2)=

100

,

(2)

WhereV6'istheviscosity at 6 rpmandV•2'theviscosity at 12rpm.Dueto thenature of the viscometer usedand the non-Newtonian rheologyof the aqueoussystems evaluated, yieldvaluefiguresareconsidered relativeratherthanabsolute. Analagous to the VSF figures,yieldsynergism factors(YSF) werecalculated according to:

YSF =

YB (• 3%

YMz+

Y,,

(3 - z)%'

(3)

whereYBis the blendyieldvalue,YMisthe MAS yieldvalue,Z is the levelof the MAS

in the blenddispersion, and Yx istheXG yieldvalue.A YSF of 1 indicates anadditive yieldvalueis obtainedfrom the blend.Any figuregreaterthan 1 is a measure of the synergism occurringin the blend.

RESULTS

AND

DISCUSSION

Apparentsynergism in viscosityandyieldvaluehavebeenobservedwhenformulating with magnesiumaluminumsilicate/xanthan gum combinations. This hasoccurred with evensmalladditionsof the gum.Relativelylow levelsof the gumhavealsobeen usedto modifythe thixotropicnatureof the MAS. Smoothflow on extendedstorage hasin thiswaybeenmaintainedby a reducedincrease in viscosity on aging. In order to better characterize this MAS-XG

interaction, a series of blends was

preparedandevaluated at variousconcentrations. The 3%levelis presented asa matter of convenience for subsequent evaluation of the separate MAS andXG dispersions. Unlessotherwiseindicated,resultsat 3%are representative of the generalbehavior observed evento thelowestpracticalusagelevels. Figure1 showsthe agingcurvesfor the MAS/XG blendscompared to MAS aloneat 3% solids.Thesedemonstrate the flatteningor stabilizingeffectson MAS viscosity overtime of progressively higherlevelsof XG. This corresponds to the rheological profilederivedfrom plottingshearstressvs. shearrate as in Figure2. A blendwas considered to providea thixotropicdispersion if producinga characteristic hysteresis loopasin CurveA of thisfigure.It wasconsidered to providepseudoplasticity without thixotropyif producinga curvesimilarto CurveB. At the 29:1and 19:1MAS:XG

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Single Point Viscosity (cps) 2:1 MAS/XG 1:1 MAS/XG O--

o

4:1 MAS/XG



9:1 MAS/XG

19:1 MAS/XG

.

29:1MAS/XG MAS

•o-

'- o,1

'

I

1

Week

'

lO

I

1do

Month

Time (days)

Figure 1. Threepercentmagnesium aluminumsilicate/xanthan gum dispersions showingprogressive flatteningof the MAS agingcurvewithincreasing levelsof XG.

ratios,thixotropicdispersions are still obtained.With 10%additionof the gum (9:1 ratio),the dispersion is rendered pseudoplastic and nonthixotropic. This is likewise true for the 4:1, 2:1, and 1:1 blends.

Due to the inherentlyhighviscosity andyieldvalueof XG alone,it isnot obviousthat a synergism trulyexists.To clarifythis,separate MAS andXG dispersions weremade at solidscorresponding to thoseusedin eachblend.Theseresultsare presented in TablesI throughIV. The calculated viscosity synergism factors(VSF) by Equation1 andyieldsynergism factor(YSF)byEquation3 providea simple,illustrative indication of actual,synergistic valuesasopposed to expected additivevalues. In thesetables,a generalincrease in viscosity synergism is notedasthe dispersions age, indicatinga colloidalstructurebecomingprogressively more complex.There is, however,an interestingdifferencein VSF valuesdependingon the type of viscosity determination used.Singlepointvaluesarederivedfrom a stationary rotatingspindle allowedto turn for 6 min. This allowsequilibrationof colloidalstructurebreakdown and rebuilding.On a practicalbasis,the singlepoint valuesrelateto the working viscosity of the system, i.e.,whilebeingspread,poured,sprayed, etc.Helipathvalues are obtainedfrom an inverted-Tspindlecuttinga slow helicalpath.The crosspiece constantlyencountersnew materialand leavesthat which its previousmotion has

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Shear

Rate

(Viscometer Spindle RPM) 60-

20

40

gO

8'0

1(•0

Shear Stress (ViscometerDial Reading)

Figure2. Typicalrheograms denoting (A) a thixotropic system, (B)a pseudoplastic system.

disrupted. The combination of slowspeedandhelicalpathminimizenon-Newtonian effectsandprovidea relativemeasure of the apparentviscosity underlow shearor staticconditions.In usingtheseblendsin actualformuladevelopment, singlepoint derivedvaluesgenerallybetterreflectobserved performance. Possiblemeansof interactionof smectites with polysaccharides in aqueoussystems hasbeensummarized byTheng(10).Althoughconsiderations of possible platelet-gum molecule colloidalconfigurations to explaintheobserved effectsarebeyondthescope of thispaper,it is assumed that interaction takesplacemainlythroughattractionof anioniccarboxylate groupsof thegumto the slightlypositivesmectiteplateletedges. Somehydrogenbondingto silicateoxygenson plateletfacesmayalsooccur,but this isprobablyminordueto electrostatic repulsion of carboxylate groupsandthenegative faces.The strongest interactionbetweenMAS andXG is seento occurat ratiosfrom 9:1 to 2:1,with the 4:1 ratio especially pronouncedby both VSF and YSF figures.In this range,the associationof polymer chainswith MAS plateletsprogressively modifiesthe MAS cubicnetworkinto a morecomplexand flexiblestructure. This becomeslesspronounced at the 1:1ratio. In the areasof cosmeticsand pharmaceuticals, the modificationof magnesium aluminumsilicaterheologycausedby inclusionof xanthangum will normallybe associated morewith the derivedyield valuesynergism than viscositysynergism. The lattercanlikelybe obtainedaseasilyandat lowercostwith similarMAS/CMC blends. The observedyield valuegain, however,has not been readilyobtainablewith the additionof organicthickenersother than XG. In effect,this yield valuesynergism allows greaterlattitude in balancingviscositywith stabilityin suspensions and emulsions. For example,this may enablesuspension stabilityat lower viscosity than wouldotherwisebe possible.Smoothflow properties withoutneedof agitation,even

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after extendedstorage,havealsobeenobservedas an addedbenefitrelatedto the non-thixotropic natureof the MAS-XG blendsat the preferred9:1to 2:1ratios.The balanceof viscosityand yield value will enable such blendsto be used in many applications at levelsbelow 1%.Theseproperties suggestparticularutility for the MAS-XG combinationin stabilizingfree flowinglotions,suspensions, and liquid makeups. CONCLUSION

Usefulandinteresting properties areobtainedwhenmagnesium aluminumsilicateand xanthangumareusedin combination. Synergism in bothviscosity andyieldvaluecan be obtained.The thixotropyimpartedto aqueous systems by the mineralcanalsobe minimized.Blendswith magnesium aluminumsilicate:xanthangumratiosin the range of 9:1to 2:1arerecommended for the bestbalanceof viscosity, yieldvalue,and flow. Such blends will be effectivein stabilizingflowable lotions, suspensions, and makeups. REFERENCES

(1) H. vanOlphen, An Introduction ToClayColloid Chemistry, (WileyInterscience, NewYork,1977)pp 92-110.

(2) B.C. Carlson, VEEGUM in eyemakeup, Am.Perf Cosmet., 86, 39-44(March1971). (3) B.C. Carlson, VEEGUM in cosmetic gelsandsticks,Cosmet. Toiletries, 92, 81-86(July1977). (4) P. A. Ciullo,Magnesium aluminumsilicatein water-in-oilemulsions, Drug Cosmet. Ind., 126, 50-56 (May 1980). (5) Xanthan Gum/Kdtrol/Kdzana natural biopoOsaccharide for scientific water control,SecondEd., Kelco Div. of Merck Co., Inc.

(6) D. A. Rees,Shapely polysaccharides, Biochem. J., 126,257-273(1972). (7) D. A. Rees,Biophysical Society WinterMeeting,London(1973). (8) A.Jeanes, Applications of extracellular microbial polysaccharide-polyelectrolytes: reviewof literature, including patents,J. Polym. Sci.Part C,Symp.No. 45,209-227(1974). (9) R. L. Bowles,R. ?. DavieandW. D. Todd,Interpretation of Brookfieldviscosities, Mod.Plastics, 33, 140-148(November1955).

(10)B. K. G. Theng,Formation andSProperties ofClay-Polymer Complexes, (Elsevier, NewYork,1979)pp 243-261.