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j. Soc.Cosmet.hem., 9, 383-392 (November/December988)

Method o determinesiliconeson humanhair by atomicabsorption pectroscopyEMMETT G. GOOCH and GRETCHEN S. KOHL, Dow CorningCorporation,200 West alzburg oad,Midland,MI 48686-0994.Receivedune4, 1987.SynopsisThis method onsistsf an enzyme igestion f hair sampleso whichsilicone-containingair productshavebeenapplied.The releasedrganosiloxanes recoveredy liquid-liquid xtractionnto an organicsolvent.The siliconcontentof the solvent xtract s measured y atomicabsorption pectroscopyAAS) todetermine he total massof deposited ilicone n the hair.

INTRODUCTIONHair doesnot containnaturallyoccurring mountsof organosiliconsilicone) pecies(1). Generally,naturallyoccurring iliceousmoietiesare in the form of silica, silicates,or substituted ilicates.Unlike most commonly ound siliceousmaterials,organosi-1oxane olymers, rovided heyarenot crosslinked,resolublen organic olvents. hisorganosiloxaneolubility n organicsolvents orms the basicconcept or a selectivesolventextraction pproach.However,somesiloxanes uchasaminofunctional iloxanesbind so tightly to hair that mere solventextraction s not sufficient or quantitativeremoval.Therefore,he first step n the developmentf this methodwas o find a way to physi-callyreleasehe siloxanesromthe hair. The nextstepwasan organic olvent xtractionof the siloxane,ollowedby atomicabsorption pectroscopynalysis f the extract.EXPERIMENTALMATERIALSTest tresses eremadeof virgin European, aturalbrownhair obtained rom DeMeoBrothers, nc. The siloxanes sedare illustrated n Figure 1 and wereappliedas emul-sions,diluted o 0.25 or 1.0 weight% silicone ctive n deionizedwater, asspecifiedfor eachexperiment.A 200-g treatmentbath of diluted emulsionwas used or each12-g hair tressprepared.Each tresswaspre-washed ith a non-silicone-containingshampoo, nd then dipped nto a treatmentbath for 30 seconds.he tresswasrinsedunder 0C unning apwater or 30 seconds,nd henair driedprior o sample repa-ration.The dimethylpolysiloxaneluids (dimethicones)sed or AAS calibrationwere DowCorning 200 fluids,350 and 1000centistokeiscosity.

383

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384 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

Me OHII o-)-(-siO--Si-- -- 3;-HMe RH(CH2)2NH2

Me Me MeMe. I I /M SI-O--Si--O-)-Si--O--)--Sie I ' [ ' V MeMe Me R MeH(CHz)zNHzLYLAIODnTCOI

Me Me Me

l __O_Si /e'Si-O--S -- '-----MeMe Me MeFigure 1. Siloxanestructures.

Papainenzyme s isolated rom the latex of the Caricapapaya lant. Suitable ources fthis enzymencludedPfaltz& Bauer item #PO1100) and SigmaChemicalcatalog#P-3375). Methyl isobutylketone,concentratedydrochloric cid (36-37%), andsodium ulfitewere eagent-gradevailableromany bulk supplier.Kimble 4.0-dram vials catalog 60957) with polyethylenerotected apswereusedfor the samplepreparation, nd extractionwas completedwith 5-ml plastic ransferpipets Fishercatalog 13-711-9A).

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METHOD FOR SILICONES ON HAIR 385INSTRUMENTATION

The atomic bsorptionssay asperformed n a PerkinElmerModel380 spectrometerwith a siliconhollowcathodeamp and a 5-cm slot nitrousoxide-acetyleneurner.Sampleswere preparedwith the aid of a Burrell wrist-actionshakerand an IEC Clin-ical model 28 centrifuge,quipped itha 6 X 52-mlangle otor nd50-mlshields.Photoswere akenon Polaroid olapan ype-52 ilm, through he objective f a JEOLT300 scanning lectronmicroscope. air samples eremountedon an aluminum abwith double-sidedape and then gold-sputteredor 50 seconds. he tab was thenmountedn the microscopend the samplesiewedwithout ilting the stage.No dif-ferencen imageresolutionwasdetectable y rotating he angleof the stage.Thephotoswere akenat a magnification f 2000 x.SPECIFIC SILICON ASSAY TEST PROCEDURE

[Special ote:As this s a traceelemental nalysisechnique, aremustbe takenduringsamplepreparationo avoid nadvertent ontamination f the samples nd reagents.Latexgloves houldbe worn if the analysthasusedsilicone-containingosmetics,oavoidsamplecontamination.]Sample reparationrocedure.sing solvent-cleanedcissors, bout 50-75 hair fiberswerecut from eachpreviouslyreated,driedhair tress o be evaluated. he fibers romthe tresswerecut just below he (carnauba-waxed)oundendof the tress, lippingoffand discardinghe upper2.5 cm of these airs rootend).The remainingiberswerecut into approximately .3-cm length clippingswith the cleanscissors.Individual amples ereprepared y weighing .10 _+0.02 g of the clippedhair nto ataredvial, usingsolvent-cleanedweezers. he hair sampleweightwas ecordedo usefor calculatingmg/kg Si from the reported AS results.Samples ere hendilutedwith10.0 _+ 0.10 g of enzymesolution,aspreparedn the formulabelow:

Papainenzyme 0.13 -+ 0.02 gSodiumsulfite 2.00 _+ 0.10 gDistilled water 100.00 _+ 0.10 g[Shaken ntil dissolved,hen adjustedo pH 6.8 with HC1 (conc.).This recipe senough or nine samples nd a blank. Freshenzyme olutions houldbe preparedoreachday's esting.]Each vial was then cappedand shaken ightly to wet the hair clippings. A blanksample,of just 10.0 g of enzyme olution,wasalsoprepared.All of the samples erethenplacedn a blockheateror in an ovensetat 63C _+ 3 for threedays,swirlingeachsampleat leastonceduring the heatingcycle o make sure that all of the hairsample s in the solutionand not adhering o the sidesof the vial.To eachsample,sevenmilliliters of methyl isobutylketoneMIBK) and five hundredmicroliters0.5 ml) of concentratedeagent ydrochloriccidwereadded.The sampleswere then placed n a Burrell mechanical hakerand agitated or 30 minutes. Theextraction ialswere hen spun n an IEC Clinical centrifuge t moderate peed or20-30 minutes to permit phaseseparation. he 4.0-dram vials will fit into 50-mlcentrifuge hields. ollowing eparation,he top (solvent)ayerof each amples drawnoff with a transfer ipet and reservedn a newvial for AAS analysis.

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Instrumentalibrationndsamplenalysis. iliconwasdeterminedn the extracts y AAS,usinga nitrousoxide-acetylenelame.The atomicabsorptionpectrometernitializationprocedureollowed he manufacturer'secommendedalues i.e., for gas flows andlamp current). The 251.6-nm silicon wavelength was used. After the instrumentwarm-upperiod, the gas low, nebulizeruptake, and burnerheadalignmentadjust-mentsweremadewhile aspirating Dow Corning 200 fluid-MIBK solution on-taining60 to 70 xgSi/ml. The final optimization djustmentsor bestsensitivity ndsignal-to-noiseatio weremadewhile aspirating he water-saturatedlank solventandcalibratingsolutions.Calibrationn both wet and dry solvent olutions asnecessaryecauseilicon s verysensitive o small changesn flame temperature.The water which saturateshe MIBKduring he solvent xtraction tepalters he temperature nd uel-oxidant toichiometryof the N20-C2H2 flame. The atomicabsorptionesponseor silicon n wet MIBK isperhaps educed o 70% as comparedo the samequantity of silicon n dry MIBK.Dry MIBK siliconcalibration olutions erepreparedrom Dow Corning 200 fluid,which hasa polymerrepeatingunit molecularweight of 74. g/unit of Me2SiO. Itcontains 7.9 percentsilicon.Viscosity radesbetween100 centistokesnd 1000 cen-tistokes re permissibleor useasstandards. or a typicalstandard, 0 mg of DC 200fluid wasweighed nto a 250-ml flaskpartially illed with MIBK. DC 200 fluid tendsto adhere o dry glasssurfaces nd may requirea long contact ime with the solvent oensurecompletesolubility of the entire sample.An exact weight of 50 mg is notessential, ut that weight in 250 ml will result n a siliconconcentrationf 75 }zg ofsiliconper milliliter of solvent,or 75 mg/kg Si on a wt/volume asis.The calibrationplot of Si concentration s. absorbance as inear from the zero inter-cept througha siliconconcentrationf 75 mg/kg provided hat the atomicabsorptionspectrometer asset up underoptimum conditions.The exactsiliconconcentration ascalculated sing he followingequation:

Sampleweight (milligrams+ 0.1 mg) x 28.1 x 100074.1 x 250ml = microgramsf Si per ml

This weight/volumeiliconstandardmay be seriallydilutedwith MIBK to producecalibrating olutionsower n silicon ontent srequired.To make he wet solvent tandardalibrationolutions,n aliquotof the standard asshaken ith an excessf water,whichwascentrifugedut, prior o use.Forexample,to a 25-ml aliquotof calibrating olution, ive ml of waterand0.25 ml of concentratedHCI reagentwere added. Dry standards an be storedand will remain constant,butwet standardseteriorate n standing ndsoweremadeanew or eachday's esting. naddition o the standards,he backgroundolventwhich s aspiratednto the instru-ment between amplesmust alsobe water-saturated. supplyof MIBK storedoverwater wasmaintained o use or background olvent.Sampleswere aspiratednto the AA spectrometer, nd the absorbancealuesat 251.6nm were ecorded longwith the absorbancealues or the wet calibrating olutions.Acalibrationplot of absorbances. siliconconcentration asconstructed. he concentra-tion of silicon n the sample xtracts as ecordedrom the calibration lot, and thenmultipliedby the extractsolvent olumeof 7 ml to obtain he total weight of silicon

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METHOD FOR SILICONES ON HAIR 387

recoveredrom the sample.The total weight of silicon,dividedby the hair sampleweight,gave he concentrationf silicon n the hair. The quantityof siloxane olymertreatmenton the hair wasdetermined y multiplying he siliconvalueby the propergravimetric actor or the specific olymer.A carefully lignedand adjusted tomicabsorptionpectrometerypically ecords nabsorbancef 0.050 absorbancenits or 10 xgof silicon er milliliter (10 ppm) n wetMIBK. For AAS, sensitivitys generally efinedas that quantityof analyteper milli-liter whichwill produce n absorptionignalof onepercent,or an absorbancef 0.0044absorbancenits. The sensitivityor organosiliconn wet MIBK is 0.88 xg of siliconeper milliliter. The instrument houldbe expectedo reproduce n absorbance easure-ment of 0.050 within _+ 0.002 absorbance units. The absolute detection limit forsilicone or these experimentswas 0.002 absorbance nits. An absorbance f 0.002units may not be readilydiscerniblerom the normal background oisewith instru-mentsequippedonly with light-emitting diodearray LED) readouts, ut may be seenon a stripchart ecorder.The detection imit for silicon,based n a total solventvolumeof 7 ml, is about 3 xg, with a precision f -+ 3 }xg. If the spectrometereatures caleexpansionapability,moderate cale xpansionf 2 X or 3 X mayprovidesome ensi-tivity improvement.Scaleexpansions an aid when one s attempting to discernsmalldifferencesn absorbanceignals etween amples.

RESULTS AND DISCUSSION

METHOD SELECTION

Siloxanes,when placed n contactwith porous urfacesuchashair, tend to be adsorbedinto the substratendcanonlybepartially xtracted ith organic olvents. reated airmust be decomposedrior to extraction. everaleagents ere ried for preparingtreatedhair samplesor AAS assay,ncluding austicKOH andNaOH), phosphoricacid,and etramethylguanidine.hile the hairprotein tructureouldbe decomposedwith sodium r potassiumydroxides,hosematerials erenot selected,s heycausedcleavagef the organosiloxaneolymernto cyclics, ilanols, ndsilicates. hose eac-tion productswereeithervolatileor non-extractablen organic olvents. hosphoricacidand etramethylguanidineid not sufficientlyecomposehehairat room emper-ature,even fterseveral eeks, ndwhenheated lso ausediloxaneolymer egrada-tion and generation f non-extractablepecies.As an alternative,an enzymedigestionmethodwasdiscoveredn a literaturesearch2)that employed apain an enzymeound n papaya). his enzymes a proteolytic n-zyme hat is activewith freesulfhydryl roups.An aqueousolution ontaining .13%papainand 2% sodium ulfiteasa catalystwith the pH adjustedo 6.8) washeatedwith a hair sample t 65C or threedays.Samplesrepared y this methodwerefurtherextracted ith methylisobutyletone MIBK) anda smallquantityof hydro-chloric cidbeforeAAS analysis. he HCI servedo precipitate therwise oluble ro-teins ndaided n phase eparationater n theprocedure.heextraction ascompletedby shakinghevial andcontentsna Burrellmechanicalhaker, ndcompletinghaseseparationn a centrifuge.The top (solvent)ayerwasdrawnoff and the siliconcontentwasassayedy atomic bsorptionpectroscopytilizinga nitrous xide-acetylenelame.

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388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTSMETHOD ACCURACY DETERMINATION

The siliconcontentof organosilicon aterialswasdetermined y AAS afterdilution ofthe siloxane f interestn an organic olvent.However, he silicon bsorptionignal,orsignal ntensity,canvary, dependingon the chemical tructureof the siloxane.Calibra-tion error is minimized f a siloxane f known purity and having the samechemicalstructure nd vaporpressure s the material o be assayeds found rom which to pre-pare calibrationstandards.n actualpractice, t was more convenient o compare hesignal intensity of the compounds f interest to calibratingsolutionsprepared romknown purity standardmaterialsand then to decide f the signal response ifferencesbetween he materials ntroduced n acceptableevel of error. If the error wasonly apercentor so relative o the actual siliconcontentof the material, that error s negli-gible when measuring ilicon evelsof a few partsper million. Normal instrumentalsignal-to-noise atios will impart far greater measurement rror, especiallywithreadings lose o the detection imit. Standard eviation f the measurements given nall of the data tables.To test the accuracy f the method or detecting he properratio of Si polymeremul-sion, several ampleswere assayed. able I contains he results rom samples f threedifferentaminofunctionaliloxane mulsions:modimethiconend two trimethylsilyl-amodimethiconeTSA) polymers seeFigure 1] with differentpolymerchain engthsand percentamine functionality. The assayed i valuesare within about 10% of thecalculated values.The calibrating standardused in the previousexamplewas a well-characterized i-methyl fluid with a viscosity f 1000 centistokes. o check he correlation f variousneat organofunctionaliloxane luids with this standard,severalsamples f differentfunctionalitysiloxane luids were analyzed.Thosedata are reported n Table II. The Siassay f thesesiloxane olymersmatchedvery closely o the calculated% Si values lso,except or one polymer (siloxaneE) which contained very long dimethyl siloxanechain, and which because f its high molecularweight is lesssoluble n the organicsolvent.As a resultof these ests, t wasdecided o use he 1000-centistoke imethylfluid as the primarystandard or all of the futuresilicondeterminations.

PRECISION OF METHODThe methodwas tested or reproducibility precision) y analyzingduplicatesamples

Table IQuantitationof % Si in Aminofunctional iloxaneFluids

Calculated Calculated CalculatedSampleemulsion molecularwt % Si % Si in % Si(35% silicone olids) of polymer in polymer emulsion by AAS (+ SD)*

A TSA, x = 96, y = 2 7624 36.7 12.9 12.4 _ .2B Amodimethicone 38038 36.8 12.9 12.1 --- .2C TSA, x = 45.8 3939 35.5 12.8 11.5 --- .2

y= 2.2* Means epresent verages f two determinations,nd standard eviationsSD) represent nly instru-mental variation.

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Table IIAccuracy f UsingDimethylSiloxane tandardsor Quantitation f % Si in VariousOrganofunctionaliloxanes

%SiasSample Calculated W Calculated determined(neat luids) of polymer % Si by AAS ( ___D)*

Siloxane A 8005 35.0 35.6 + .3Siloxane B 7980 35.1 36.2 + .3SiloxaneC 7698 36.4 36.3 + .3Siloxane D 8059 34.7 33.4 -4_-_3Siloxane E 30480 41.3 35.3 + .3Siloxane F 7630 36.7 36.8 -4_-_3* Means epresent veragesf two determinations,nd standard eviationsSD) represent nly instru-mental variation.

preparedby eachof two different esearchers.able III containshe results rom threedifferent treatedhair samples s determinedby two operators. he hair samples n-cluded untreatedhair and two increasingly igher levelsof silicone reatment (withTSA). Variation n the mg/kg Si as determinedby the differentoperatorswas withinexperimentalerror (standarddeviation of +/- 30 mg/kg Si), as was the variationbetween eplicate amples.t shouldbe noted hat an accurate eighingand recordingof the hair sampleweight is important n controlling he accuracy f the calculatedSimg/kg. The Si values or untreated air shouldbe close o the detectionimit or the testshouldbe repeated.With valuesclose o the detection imit, the standarddeviation(SD) of the measurementften approximateshe magnitudeof the reading tself. Theblank shouldmeasure lose o or below he detectionimit, meaning hat there s no Sipresent.Sample eadingshat are n this ow rangeof detection hould lsobe suspectedof havingno Si present.APPLICATIONS OF METHODA series of siloxane emulsions was used to treat hair. The emulsions were all diluted to0.25 wt % solids n deionizedwater, and two TSA fluidsof varyingpolymer ength and% amino unctionalitywere evaluatedrepresentedy x and y, respectively,n Figure

Table IIIPrecision f Method by Two Operators

mg/kg Si by A.A. (+ SD)*Tress # Treatment (1) (2)K13-3 Low conc. TSA 150 + 30 140 --- 30

170 --- 30 150 + 30K13-U Untreated 70 +- 30 60 --- 3070 -+ 30 30 + 30K14-2 High conc.TSA 370 + 30 380 + 30380 + 30* Means epresent verages f two determinations,nd standard eviationsSD) represent nly instru-mental variation.

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390 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTSTable IV

First Series of Aminofunctional Siloxane*-Treated HairTreatment mg/kg Si(. 25% Emulsion) by AA (+ SD)**

A TSA, x = 96, y = 2 1170 + 50B TSA, x = 298, y = 2 4220 + 100C Dimethicone 190 + 20D Blank 30 + 30* TrimethylsilylamodimethiconeTSA).** Means epresent verages f duplicatedeterminations.

1). The depositionevelsof thesepolymerswere compared o that of a dimethiconeemulsion.Table IV containshe results, hat show hat the higher-molecular-weightamine-containingolymerdepositshe most 4220 mg/kg Si), followedby the lower-molecular-weightamine-containingpolymer (1170 mg/kg Si). The dimethiconeshowed nly 190 mg/kg Si, and the blankcontained 30 mg/kg Si.Scanning lectronmicroscopySEM) visuallyconfirmed his progressionf deposition.Figures - 5 showSEM photos akenat a magnification f 2000 x. The untreated air(Figure2) shows ighly definedcuticlescales. o gross urface hanges reapparent orthe dimethicone emulsion-treatedhair (Figure 3). However, the lower-molecular-weight amine-containing olymer reatmentshows full coatingof the cuticle, withlossof cuticledefinition Figure4). The photoof the higher-molecular-weightmine-containing olymer reatment Figure5) shows heavy,"wrinkled"coatingof silicone,obscuringhe cuticledefinition.

Figure 2. SEM photograph f untreatedhair.

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/-

Figure3. SEMphotographf hair reatedwith a dimethiconemulsion.CONCLUSIONSA method s presentedor bulk analysis f the Si contentof hair fibers. The methodconsists f enzymedigestingsilicone-treated air, and extracting he organosiloxaneswith an acidifiedorganicsolvent.The detection imit with current nstrumentations 3

Figure4. SEMphotographf hairtreatedwith low-molecular-weightSA emulsion.

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Figure 5. SEM photograph f hair treatedwith high-molecular-weightSA emulsion.

xgof Si. Reproducibilityf mg/kgdata,betweeneplicate amplesndbetween per-ators during samplepreparation,was within instrumentalmeasurement rror whencareful ttentionwaspaidto accurately eighing he hair sample.Applicationsf thismethod how trendbetween tomic bsorption-determinedi depositionnddeposi-tion asvisuallydeterminedby SEM.REFERENCES(1) C. R. Robbins, Chemicalnd Physical ehavior f HumanHair (Van NostrandReinholdand Co.,1979), p. 39-(2) A. W. Holmes,Degradation f humanhairby papain,Part : The patternof degradation,ext.Res.J., 34, 706-712 (1964).

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