ABSTRACT
II. Aerosol Silica in the Atmosphere 693 A. Particles of natural origin 694 B. Particles of anthropogenic origin 694 C. Particles of extraterrestrial origin 695
IV. Silica-Water Interface 697 A. Similarity between water and liquid silica 698 B. Interaction between water and silica surfaces 699 C. Effect of acids on the water vapor uptake by fumed silica 706
V. Large Surface-to-Volume Ratio Problems 709 A. General description of the problem 710 B. Large surface-to-volume ratio problems in the classical theory
VI. Silica-Aqueous Solution Systems at Low Temperature 716 A. Freezing of pure water adsorbed on silica 717 B. Heterogeneous mechanism of ice formation on partly
silica 730 C. Implication for the stratosphere 736
D.Possibleroleofthesilicasurfaceinthephotodecomposition ofchlorofluorocarbonsinthetroposphere737
VIII.ConcludingRemarks738
References739
Thischapterwillbedevotedtodiscussinghowtheuniquesurfacepropertiesof fumedsilicacanbeusedforobtainingaqueoussystemswithalargesurface-tovolumeratio.Thermodynamicpropertiesandphasetransformationsinsuchsystemsdifferfromthoseofbulkmaterialand,therefore,theirstudyisveryimportant forfundamentalandmanyappliedinterests,inparticular,forcloudphysics,since incloudstheliquidandsolidphasesofwaterarehighlydispersed.Ontheother hand,intheatmosphere,thesilicondioxide(Si02)surfacecanberegardedas representativeofmanynaturaldusts,productsofcombustionprocesses,andparticlesformedduringevaporationofmeteorites.Intheatmosphere,suchaerosol particlescanserveascloudcondensationandice-freezingnuclei.Thespecificinteractionofwaterwiththesurfaceoffumedsilica,whichcanbeconsideredasa laboratorycounterpartofaerosolsilica,allowsonetoobtainthepopulationsof purewaterandsolutionmicrodropletswhichcanserveasamodelforthestudyof freezingandmeltingprocessestakingplaceintheatmosphere.Theknowledgein thisfieldisalsoveryusefulforbetterunderstandingtheformationmechanismof polarstratosphericclouds(PSCs),whicharesupposedtoplayacrucialrolein stratosphericozonedepletion.
