The ascorbyl palmitate - water system: phase diagram and state of water

ERICA P. SCHULZ; PAULA V. MESSINA; PALMA S.D.; ALLEMANDI D. A.; PABLO C. SCHULZ

COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS

ELSEVIER SCIENCE BV

Año: 2011 vol. 375 p. 178 - 185

0927-7757

This workaddressestheascorbylpalmitate(Asc16)aqueoussystems.Ithasbeenstudiedthroughdif- ferential scanningcalorimetry(DSC)andpolarizingmicroscopy(PM)atdifferenttemperaturesovera wide rangeofconcentrations(weightfractions(w/w)between0.05and1.0),allowingthedetermina- tion ofacompletephasediagram.Whenpossible,thefindingswerecorroboratedwithliteraturedata. Systems below C¡Ö0.48 wtfractionshowhydratedcrystalsinanisotropicliquid,whichleadtoalamel- lar liquidcrystalwhenheatedtoabout60 ◦C. Abovethisconcentration,thephasetransitionoccursat about 80 ◦C, givingacubicliquidcrystalwhichinturnbecomeslamellarliquidcrystalatabout90 ◦C. The textureofthislamellarmesophaseisdifferenttothatproducedatlowerconcentrations.Ithasbeen determined, throughDSCanalysisofthemeltingpeaksofwater,computersimulationofthehydration of Asc16moleculesandamodelofthehydratedaggregates(hydratedcrystalsandlamellarmesophase), that lowconcentrationliquidcrystalsexistprovidedbulkwaterispresentinthesystem.Twoother kinds ofwaterbutbulkweredetected:afirsthydrationshell,withwatermoleculesstronglyattachedby hydrogen bondstotheoxygenandhydrogenatomsofthepolarheadgroup,whichdoesnotmeltandis then undetectablebyDSC(11.47¡À0.95 watermoleculespersurfactantmolecule)andasecondhydration layer containing59¡À17 watermoleculespersurfactantone(apartfromthoseinthefirsthydrationlayer). The numberofwatermoleculesinthesecondhydrationlayerdecreasesasthesurfactantconcentration increases, andvanishesat C = 0.62¡À0.01 wtfraction(computersimulationandstructuremodeldata)or at C = 0.667¡À0.007 (DSC).Atveryhighconcentrationstheformationofmesophasesisprecededbythe melting ofthechains,givingawaxytexturewhenviewedatthepolarizingmicroscope,maintainingthe structure ofcrystalsbecausetheanhydrouspolarbilayerremains"solid". C¡Ö0.48 wtfractionshowhydratedcrystalsinanisotropicliquid,whichleadtoalamel- lar liquidcrystalwhenheatedtoabout60 ◦C. Abovethisconcentration,thephasetransitionoccursat about 80 ◦C, givingacubicliquidcrystalwhichinturnbecomeslamellarliquidcrystalatabout90 ◦C. The textureofthislamellarmesophaseisdifferenttothatproducedatlowerconcentrations.Ithasbeen determined, throughDSCanalysisofthemeltingpeaksofwater,computersimulationofthehydration of Asc16moleculesandamodelofthehydratedaggregates(hydratedcrystalsandlamellarmesophase), that lowconcentrationliquidcrystalsexistprovidedbulkwaterispresentinthesystem.Twoother kinds ofwaterbutbulkweredetected:afirsthydrationshell,withwatermoleculesstronglyattachedby hydrogen bondstotheoxygenandhydrogenatomsofthepolarheadgroup,whichdoesnotmeltandis then undetectablebyDSC(11.47¡À0.95 watermoleculespersurfactantmolecule)andasecondhydration layer containing59¡À17 watermoleculespersurfactantone(apartfromthoseinthefirsthydrationlayer). The numberofwatermoleculesinthesecondhydrationlayerdecreasesasthesurfactantconcentration increases, andvanishesat C = 0.62¡À0.01 wtfraction(computersimulationandstructuremodeldata)or at C = 0.667¡À0.007 (DSC).Atveryhighconcentrationstheformationofmesophasesisprecededbythe melting ofthechains,givingawaxytexturewhenviewedatthepolarizingmicroscope,maintainingthe structure ofcrystalsbecausetheanhydrouspolarbilayerremains"solid".