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colloids A colloid is a dispersion of small particles of one material in another, where the size of the particles is smaller than 500 nm (the range of the wavelength of visible light, therefore they are not visible with an ordinary optical microscope). dispersed particles (e.g. oil droplets, clay particles, etc.) continuous phase (e.g. water) colloidal system (e.g. milk, muddy water, etc.) classification of colloids dispersed particles solid continuous phase fluid solid fluid (solid) (solid) (solid) sol foam foam sol, (suspen- emulsion sion), gel aerosol, gaseous smoke, (dust) Aerosol, aerosol, Nebel fog gaseous Schaum foam dispersion of a material into fine particles is connected to a drastic increase in surface: original surface of the complete cube: A = 6 a² after cube is split into 10x10x10 subunits: A = 1000x0.01x6 a² = 60 a² a surface tension γ F γ = dw/dσ γ = F/(2l) l fluid film liquid surfaceinduced pressure P P = 2γ /r surface-induced pressure ∆P = 2γ/r ∆P = 2γ/r ∆P = 4γ/r r droplet (gas-)bubble foam bubble distribution of hydrophobic material at an oilwater interface: oil β water σ α distribution of hydrophilic material at an oilwater interface: oil β water σ α distribution of amphiphilic material (surfactant) at an oil-water interface: oil β surfactant, e.g. soap water σ α spreading of surfactant on an oil-water interface: σ ∗ γ α π = F/l γ water movable barrier oil β an amphiphilic component in pure water may form micelles: surfactant, e.g. soap water surfactants stabilize colloidal dispersions of particles: continuous phase particle 10 Nanometer surfactant examples for colloidal systems: - food (milk, yoghurt, mayonnaise, raw juices etc.) - paint (liquid dispersions, spray, electrophoresis etc.) - glues (white wood glue, glue stick etc.) - lubricants (e.g. oil-water mixture for cooling) - technical aerosols (e.g. air-gasoline mixture) - pollutants (smoke, foam on natural water surfaces) - pharmaceutical applications (e.g. Aspirine® in water) - natural occurance of colloids: clay in water, clouds, fog, dust, planctonic microorganisms