Aeroszolok, lioszolok, xeroszolok

Átírás

1 Aeroszolok, lioszolok, xeroszolok Márta Berka és István Bányai, University of Debrecen Dept of Colloid and Environmental Chemistry x. lecture 1

2 Kolloid rendszerek (szerkezet alapján) Kolloid rendszerek inkoherens rendszerek koherens rendszerek gélek diszperziós k. szolok (liofób kolloidok) makromol. asszociációs porodin (pórusos) retikuláris hálós Spongoid szivacsszerő kolloid oldatok liofil kolloidok korpuszkuláris fibrillás lamellás Szol-gél átalakulás: xeroszolok x. lecture 2

3 Diszperziós kolloidok (szolok) Halmazállapot szerint Gázközegő aeroszolok Folyékonyközegő lioszolok Szilárdközegő xeroszolok L/G folyadék aeroszol: köd, permet S/G szilárd aeroszol: füst, kolloid por G/L gázlioszolok (tömény =hab) L/L emulzió S/L liofób kolloid szol, szuszpenzió (aranyszol, fogpaszta) G/S szilárd hab: polisztirol x. lecture 3 hab L/S szilárd emulzió: opál, igazgyöngy S/S szilárd kolloid szuszpenzió: pigmentált polimerek

4 Kolloidok elıállítása (általános) Heterogén rendszerbıl Méretcsökkentés, eloszlatás (porlasztás, ırlés, kolloidmalmok) Homogén közegbıl Kondenzálás (új fázis, méretnövelés) Weimarn szabály Folyadékból (paraffin) Gázfázisból (ködök) Kémiai reakciók útján Monodiszperz kolloidok (kénszol, fémoxid szolok) Makromolekulák polimerizációval Asszociációs kolloidok (konc., hm.) x. lecture 4 Kolloid rendszerbıl: Szol-gél átalakulás oda-vissza

5 Aeroszolok Elıállitás a diszpergálás hatékonysága a felületi feszültségtıl és a viszkozitástól függ, kisebb könnyebb. (porlasztás, ultrahang, -Kondenzálás, köd, permet L/G. -Füst S/G, szmog S/L/G Weimarn szabály: túltelitettség, Q-L, és az oldékonyság, L, és a realtiv túltelitettség, (Q-L/L), szabja meg a diszperzitás fokát Atmospheric aerosols x. lecture 5

6 Atmospheric aerosols S/G, L/G, komplex (S-L)/G (0.001 µ 50 µ), természetes és antropogén eredet: homok, talajok, mezıgazdaság tengervíz permet ipar (korom, por) elılények párolgás/ lecsapódás (felhık, köd, jégkristályok) Vulkánok, sivatagosodás, éghajlati hatások. Anthropogenic Aerosols takes 10 % (1999: NASA Earth Observing System) tüzelı és motorolajok égése, biomassza égése: SO 2 a legfontosabb, savas esık! x. lecture 6

7 Habképzıdés A folyadék a közeg, lehet kolloid mérető, ez a képzıdéskor jól látszik Buborékolás (pórusosos testen) módszere küszöbnyomás, p=2γ/r Eloszor a nagyobb jon ki x. lecture 7

8 Foam Habképzı szükséges nem stabilak Habszerkezet Spherical bubbles <70-75% Foam structure of a wet spherical foam at 400X magnification Polyhedral cells Foam structure of dry hexagonal foam at 400X magnification / x. lecture 8

9 bubble aeration Formation of bubbles P= 2γ r p p r p Két buborék lebeg a folyadek levegı határfelületen. A nyomás a B helyen kisebb mint az A vagy A` helyen Polyhedral cells The arrows show the direction of streaming, hence water will flow to these points, until they become unstable If you add glycerol to a soap solution, the viscosity increases, and the drainage of the foam is slowed down: it takes a longer time before the foam collapses x. lecture 9

10 Stabilization of a foam film Electrostatic stabilization of a foam film Liquid crystals stabilize foams Each interface is electrically charged. As the film thins, the repulsion increases. Good emulsifying are also good foaming agent x. lecture 10

11 Foam Stability, Inhibition and Breaking The stability of a liquid foam is governed by three main processes: Drainage (oldoszer kiaramlas): liquid will drain through the Plateau border channels until an equilibrium state is reached. Coarsening (novekedes): gas diffuses between bubbles - some grow while others shrink and disappear. The net result of this process is that the average bubble size grows in time. Film Rupture (szakadas): if a foam film gets too thin and weak, it will rupture. Eventually the foam will collapse and vanish. Unstable foams are formed from aqueous solutions of short chain acids or alcohols. Metastable foams are typically formed from solution of soaps, synthetic detergents, proteins, saponins, etc. Foam inhibitors - added before foam forms, displace foaming agents, or solubilizing the foaming agents (in micelles) Foam breaking - mechanical, shock waves, compression waves, ultrasonics, rotating discs, heating, an electrical spark. Antifoams - added to existing foams, in the form of small droplets, which spread on the lamellae, thinning and breaking it x. lecture 11

12 Antifoams L With an antifoam on one surface, electrostatic stabilization is lost. (a) Antifoam drop. (b) Entering the surface. (c ) Leading to rupture of the film. Antifoams - added to existing foams, in the form of small droplets, which spread on the lamellae, thinning and breaking it x. lecture 12

13 Examples Marshmellow (mályvacukor)- foam formed from egg white, gelatin, and sugar. Ice cream - refrigerated and aerated at the same time. Ice crystals and fat crystals form the matrix. Dynamic foams: cakes, sponges, bread, meringues, soufflés. Bubbles change at various stages of preparation. Foams on drying, especially in distillation columns. A foam blanket at the surface acts as an insulating layer - causing overheating. Metallic slags (salak) foam probably because of the high viscosity. Cooling stabilizes the foam. Paper making - Caused by lignin, resin, and fatty acids in wood, sulfate soaps from pitch. Also, sizing materials, dyes, fillers, oxidized starch, proteins, etc act as profoamers. Beer - foam should not affect taste, but it remains important. Too little, beer looks "flat". Sources of foam: entrained air in the pouring, in the pressurizing, and from dissolved carbon dioxide. Mostly stabilized by proteins. Protein-polysaccharide complexes are especially x. lecture 13 stabilizing.

14 Examples Firefighting Foams Primarily for fire protection in petroleum storage. Airplane fires. Foam is made in a self-aspirating branchpipe: high pressure pushes the water + foaming agent down a pipe, aspirating air, foaming because of the turbulence ( about 1mm bubbles) and is thrown from about 15 to 75 m. Types: (1) Protein foam liquid - solution of hydrolysed protein, (2) liquid with various perfluorinated surfactants (high performance, non-biodegradable), (3) mixtures of perfluorinated surfactants with proteins x. lecture 14

15 Examples Foams to Immobilize To retard evaporation. Improve insulation. For fumigants (toxic to fungi), insecticides, contraceptives, to keep them in place. Applying thin layers, such as adhesives or etching formulations, dyes or bleaches Capture of aerosols. Aqueous foam is an excellent suspending medium for paper fibers. Pseudo plasticity enables dispersion of long fibers. At low shear stress the fibers are "frozen" in position. Enables the use of long fibers which otherwise orient on coating x. lecture 15

16 Solid foams Solid foams are cellular materials, i.e. materials which are made up from a framework of solid material surrounding gas-filled voids (bubbles). Solid foams can be 100 times lighter than the equivalent solid material. Natural solid foams include wood, bone and sea sponges. The bee's honeycomb is a two-dimensional cellular structure: Honeycomb concrete, habbeton x. lecture 16

17 Emulsion, terminology The emulsion is a dispersed system in which the phases are immiscible or partially miscible. Phase 1 Phase 2 Droplet Dispersed Internal Discontinuous Serum Medium External Continuous Droplet size: µm O/W (oil in water), O/W (water in oil ) emulsions and bicontinuous Polyhedral cells x. lecture 17

18 Emulsion types Identification of emulsion type: 1. Generally, an O/W emulsion has a creamy texture and a W/O emulsion feels greasy 2. The emulsion mixes readily with a liquid which is miscible with the dispersion medium 3. The emulsion is readily coloured by dyes which are soluble in the dispersion medium 4. O/W generally have a much higher electrical conductivity than W/O emulsions The liquid with the greater phase volume need not necessarily be the dispersion medium! Above 74% there is either a phase inversion or the droplets are deformed to polyhedra x. lecture 18

19 Terminology Macroemulsions At least one immiscible liquid dispersed in another as drops whose diameters generally exceed 10 µm. The stability is improved by the addition of surfactants and/or finely divided solids. Considered only kinetically stable. Miniemulsions An emulsion with droplets between 0.1 and 10 µm, reportedly thermodynamically stable. Becher, P. Emulsions, theory and practice, 3rd ed.; Oxford University Press: New York; Microemulsions An emulsion with droplets below 100 nm. A thermodynamically stable, transparent solution of micelles swollen with solubilizate. Microemulsions usually require the presence of both a surfactant and a cosurfactant (e.g. short chain alcohol). Creaming less dense phase rises Inversion internal phase becomes external phase Ostwald ripening small droplets get smaller Flocculation droplets stick together Coalesence droplets combine into larger ones The most important physical properties of an emulsion is its stability x. lecture 19

20 Surface activity in emulsions Emulsions are dispersions of droplets of one liquid in another. Emulsifiers form an adsorbed film around the dispersed droplets. Emulsifiers are soluble, to different degrees, in both phases. Drops flocculate and coalesce spontaneously. In general, emulsions are thermodynamically unstable G= γ A> 0 emulsifiers G= γ A+ work of desorption If the work of desorption of emulsifier is high, the coalescence is prevented x. lecture 20

21 Making emulsions Method of phase inversion High Speed Mixers Condensation methods - solubilize an internal phase in micelles Electric emulsification Intermittent milling szaggatott orles Homogenizer, Mills, Microfluidizer, Sonolator In which fluid streams at high velocities are forced against each other resulting in cavitations, turbulence, and shear. Emulsification proceeds in two steps: -mechanical mixing -stabilization x. lecture 21

22 Emulsion stability Emulsifiers: -surface active materials, -naturally occurring materials, -finely divided solids (Pickering stabilization) Factors favor emulsion stability (see lecture about colloid stability) 1. Low interfacial tension 2. Steric stabilization. Mechanically strong interfacial film (proteins, surfactants, mixed emulsifiers are common. Temperature is important) 3. Electrical double layer repulsions (at lower volume fractions) 4. Relative small volume of dispersed phase 5. Narrow size distribution 6. High viscosity (simple retards the rates of creaming, coalescence, etc.) x. lecture 22

23 Emulsion oil Bicontinous emulsion a sponge-like random network W/O O/W water x. lecture 23

24 Emulsion, emulsifiers self-assembly of binary amphiphilic fluids into bicontinous cubic phases x. lecture 24

25 Emulsifiers The emulsifier stabilizes the emulsion type where the continuous phase is the medium in which it is most soluble. O W O The long tail on the surfactant is to represent the longer range interaction of a hydrophilic molecule through water. A hydrophilic solute in an O/W emulsion. W O A hydrophilic solute in a W/O emulsion x. lecture 25

26 Emulsion Inversion As the concentration increases (A) the droplets get closer until they pinch off into smaller, opposite type of emulsion (B). making milk into butter Milk is a fairly dilute, not very stable O/W emulsion, about 4% fat. Creaming produces a concentrated, not very stable O/W emulsion, about 36% fat. Gentle agitation, particularly when cool, C, inverts it to make a W/O emulsion about 85% fat. Drain, add salt, and mix well. Lo and behold! butter! What remains is buttermilk x. lecture 26

27 Typical food emulsions Food Emulsion type Dispersed phase Continuous phase Stabilization factors, etc Milk, cream O/W Butterfat triglycerides partially crystalline and liquid oils. Droplet size: 1 10 µm Volume fraction: Milk: 3-4% Cream: 10-30% Aqueous solution of milk proteins, salts, minerals, Lipoprotein membrane, phospolipids, and adsorbed casein. Ice cream O/W (aerated to foam) Butterfat (cream) or vegetable, partially crystallized fat. Volume fraction of air phase: 50% Butter W/O Buttermilk: milk proteins, phospholipids, salts. Volume fraction: 16% Imitation cream (to be aerated) O/W Vegetable oils and fats. Droplet size: 1 5 µm. Volume fraction: 10 30% Water and ice crystals, milk proteins, carboxydrates (sucrose, corn syrup) Approx. 85% of the water content is frozen at 20 oc. Butterfat triglycerides, partially crystallized and liquid oils; genuine milk fat globules are also present. Aqueous solution of proteins (casein), sucrose, salts, hydrocolloids. The foam structure is stabilized by agglomerated fat globules forming the surface of air cells. Added surfactants act as destabilizers controlling fat agglomeration. Semisolid frozen phase Water droplets distributed in semisolid, plastic continuous fat phase. Before aeration: adsorbed protein film. After aeration: the foam structure is stabilized by aggregated fat globules, forming a network around air cells; added lipophilic surfactants promote the needed fat globule aggregation x. lecture 27

28 Typical food emulsions Food Emulsion type Dispersed phase Continuous phase Stabilization factors, etc Coffee whiteners O/W Vegetable oils and fats. Droplet size: 1 5 µm. Volume fraction: % Aqueous solution of proteins (sodium caseinate), carbohydrates (maltodextrin, corn syrup, etc.), salts, and hydrocolloids. Blends of nonionic and anionic surfactants together with adsorbed proteins. Margarine and related Products (low calorie spread) W/O Water phase may contain cultured milk, salts, flavors. Droplet size: 1 20 µm Volume fraction: % Edible fats and oils, partially hydrogenated, of animal or vegetable origin. Colors, flavor, vitamins. The dispersed water droplets are fixed in a semisolid matrix of fat crystals; surfactants added to reduce surface tension/promote emulsification during processing. Mayonnaise O/W Vegetable oil. Droplet size: 1 5 µm. Volume fractions: Minimum 65% (U.S. food standard.) Aqueous solution of egg yolk, salt flavors, seasonings, ingredients, etc. ph: Egg yolk proteins and phosphatides. Lecitin (O/W), cholesterine (W/O) Salad dressing O/W Vegetable oil. Droplet size: 1 5 µm. Volume fractions: Minimum 30% (U.S. food standard.) Aqueous solutions of egg yolk, sugar, salt, starch, flavors, seasonings, hydrocolloids, and acidifying ingredients. ph: Egg yolk proteins and phosphatides combined with hydrocolloids and surfactants, where permitted by local food law x. lecture 28

29 HLB (hydrophilic -lipophilic balance) values The amphiphilic nature of many emulsifying agents (particularly non/ionic surfactant) can be expressed in terms of an empirical scale of so-called HLB Applications 3-6 W/O emulsions Nil 7-9 wetting agents 3-6 poor Dispersibility in water 8-15 O/W emulsions 6-8 unstable milky dispersions detergent 8-10 stable milky dispersions solubiliser Translucent dispersion/solution 13- clear solution Solubility in octanol and water x. lecture 29

30 1 20 HLB Scale example x. lecture 30

31 Variation of type and amount of residual emulsion with HLB number of emulsifier. (antagonistic action) Nature of the emulsifying agent determine the type of emulsion x. lecture 31

32 Multiple emulsions Particles as emulsion stabilizers W/O/W double emulsion O/W/O double emulsion Each interface needs a different HLB value. The curvature of each interface is different. Almost all particles are only partially wetted by either phase. When particles are adsorbed at the surface, they are hard to remove the emulsion stability is high. Crude oil is a W/O emulsion and is very old!! (Pickering stabilization) bentonite clays tend to give O/W whereas carbon black tends to give W/O emulsions x. lecture 32

33 Physical properties of emulsions Identification of internal and external phases; W/O or O/W Droplet size and size distributions generally greater than a micron Concentration of dispersed phase often quite high. The viscosity, conductivity, etc, of emulsions are much different than the continuous phase. Rheology complex combinations of viscous (flowing), elastic (when moved a little) and viscoelastic (when moved a lot) properties. Electrical properties useful to characterize structure. Multiple phase emulsions drops in drops in drops in drops, x. lecture 33

34 Multiple phase emulsions drops in drops in drops Drug delivery x. lecture 34

35 Breaking emulsions x. lecture 35

36 Breaking emulsions First, determine type, O/W or W/O. Continuous phase will mix with water or oil. Chemical demulsification, i.e. change the HLB Add an emulsifier of opposite type (antagonistic action). Add agent of opposite charge. Freeze-thaw cycles. Add electrolyte. Change the ph. Ion exchange Raise temperature. Apply electric field. Filter through fritted glass or fibers. Centrifugation x. lecture 36

37 Phase inversion temperature x. lecture 37

38 Emulsion polymerization Meeting place of oil soluble monomer and water soluble initiator Emulsifier surfactant water water soluble Initiator The number of polymer particles finally depend on largely on emulsifier concentration x. lecture 38 Nano-emulsions

39 További példák: szilárd aeroszolok S/G, (füst, por). Gélek (Carbon Black Aggregates) Aktív szén aggregátumok Fumed Silica Aggregates Thermal conductivity: 12 to 16 mw/m K Light transmission: 20 to 80% at 2 cm Particle density: 140 kg/m³ Bulk Density: kg/ m³ Surface area: 700 m2/g Porosity: > 90% Particle size: 5µ - 5 mm Primarily used as reinforcing filler Tire Elastomer composites Plastics, Pipe Printing Inks, Coatings x. lecture Surface Chemistry hydrophobic 39

40 Egyéb fémoxid kolloid porok, füstök (fumed metal oxides) Fumed Silica, Alumina, Titania, and Ceria Láng közegő gyártás (fumed) Felület módosított változatok Elsıdlegesen reológiai adalékok Szilikon/ tömítıszer megerısítés Félvezetık Ragasztók & Bevonatok Festék patronok, gyógyszerek x. lecture 40

41 Aerogel frozen smoke Aerogel is a low-density solid-state material derived from gel in which the liquid component of the gel has been replaced with gas. Extremely low density solid, most notably as an insulator. It is nicknamed frozen smoke, solid smoke or blue smoke due to its semitransparent nature and the way light scatters in the material The first results were silica gels. Aerogel can be made of many different materials based on silica, alumina, chromia, and tin oxide. Carbon aerogels were first developed in the early 1990s x. lecture 41

42 Aerogel A folyadék cseréje gázra State-of-the-Art Manufacturing Technology x. lecture 42

43 Xerogélek: Szilárd, pórusos szilikagél Vízüveg oldatból - szilika szol, flokkulációval - hidrogél nyitott térhálós szerkezet, majd szárítással - xerogel. További térhálósodás a szárítás során. Új típusú kerámiák közel szobahımérsékleten Aerogel x. lecture 43

44 Xerogel bevonatok A szol szintézise, bemerítés, oxidbevonat készítése, nátrium boroszilikát bevonat üvegre alacsony hımérsékleten! Rendezett szerkezető, néhány nanométeres vastagságú oxid bevonatok közel szobahımérsékleten Szárítás, tömörítés x. lecture 44

45 Szol gél technológia és termékei x. lecture 45