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1、1ColloidChemistryofSilicaAnOverviewHoracioE.Bergna,DuPont,Wilmington,DE19880-0228Silicondioxideisthemaincomponentofthecrustoftheearth.Combinedwiththeoxidesofmagnesium,aluminum,calcium,andiron,itformsthesilicatemineralsinourrocksandsoil.Overmillionsofyearssilicondioxide,ors
2、ilica,hasbeenseparatedfromtheoriginalsilicaterocksbytheactionofwatertoappearasquartz.Inafewplacesitwasdepositedintheamorphousformasopal.OurEnglishwordsilicahasaverybroadconnotation:itincludessilicondioxideinallitscrystalline,amorphous,soluble,orchemicallycombinedformsinwhi
3、chthesiliconatomissurroundedbyfourorsixoxygenatoms.Thisdefinitelyexcludesalltheorganosiliconcompoundsmadebymaninwhichcarbonatomshavebeenlinkeddirectlytosiliconatoms—commonlyreferredtoas"silicones",whichdonotoccurinnature.Silicaissolubleenoughinwatertoplayimportantrolesinma
4、nyformsoflife.Itformstheskeletonsofdiatoms,theearliestformoflifethatabsorbedsunlightandbegantoreleaseoxygenintotheatmosphere.Manyplantsusesilicatostiffenstemsandformneedlesonthesurfaceforprotection.Asanimalsdeveloped,theroleofsilicabecamelessobvious.Buteachoneofuscontainsa
5、bouthalfagramofsilica,withoutwhichourbonescouldnothavebeenformed,andprobablyalsonotourbrains.Silicahasplayedakeyrolesincethebeginningofcivilization,firstinflintfortoolsandweaponsandinclayandsandforpottery.ThehighstrengthanddurabilityofRomancement2000yearsagoisnowknowntobed
6、uetotheuseofaspecialvolcanicashthatisanalmostpureformofamorphouscolloidalsilica.Todaythereisactiveresearchontheuseofthesomewhatsimilarsilicafumefromelectricfurnacestomakeasuper-strongPortlandcement.Ourpresenttechnologywouldbeverydifferentwithoutthesilicaforthecatalystsofou
7、roilrefineries,forthemoldsforcastingthesuperalloysinourjetengines,formodernglassandceramics,electronicmicrocircuits,quartzcrystals,andfiberoptics.0065-2393/94/0234-0001$12.86/0©1994AmericanChemicalSociety2THECOLLOIDCHEMISTRYOFSILICA—RalphK.Iler,AlexanderMemorialLecture,Aus
8、tralia,1989.ReprintedwithpermissionfromChemistryinAustralia(October1986,p355).SILICONDIOX
