Mechanical and optical properties of colloidal solutions

1. Mechanical and optical properties of colloidal solutions Kinetic properties of the dispersed systems Investigation methods of the dispersed systems according to their kinetic properties. 3. Optical properties 4. Opticalinvestigation methods of the dispersed systems Assistant Kozachok S.S. prepared

2. PROPERTIES OF COLLOIDAL SOLUTIONS The main characteristic properties of colloidal solutions are:

6. The motion of colloidal particle in dispersed medium Brownian motion Direction of the particle Average Brownian displacement

7. 1. Fick's first law of diffusion (analogous with the equation of heatconduction) states that the mass of substance dm diffusing in the xdirection in a time dt across an area Sis proportional to theconcentration gradient dc/dx at the plane in question: • (The minus sign denotes that diffusion takes place in the direction ofdecreasing concentration.)

8. The proportionality factor D is called the diffusion coefficient. If – dc/dx = 1, S = 1 and dt = 1 D = dm, diffusion coefficient equals to the mass of substancediffusingin a time dt across an area S at the concentration gradient equals to 1. [D] = m2/s Einstein equation for spherical particles of the dispersed systems D = kT/6πηr diffusion coefficient is inversely proportsiynyry to particle radius

9. 2. The rate of change of concentration at any given point is given byan exactly equivalent expression, Fick's second law:

10. Osmotic pressureof colloid solutions: 1. Osmotic pressure is very low: 2. Osmotic pressure is inversely proportional to the cube of radius of particles and is directly proportional to raise to the cube (third) power of its dispersion in the same dispersed medium:

11. Hepp-Skatchard’s osmometr Colloidal solution Capillary with toluol where 2.53·105 – constant at 25ºС π= [cm of water shaft] manometer solvent membrane

12. Sedimentation equilibrium Sedimentation rate (Stock’s equation):

13. Sedimentation analysis It consists of the obtaining sedimentation curve, that shows the dependence of the sediment mass m of the dispersed phases, which is settled down till certain time t. For monodispersed systems (with the same particles size) this dependence is line: m = Qυt/H where Q – general mass of the dispersed phases; H – initial height of column of the dispersed system. But all real dispersed systems are polydispersed and that’s why the sedimentation rate for different fraction is different: large particlessettle down faster, smaller – slowly. Therefore sedimentation curve is bent to the axis of ordinates.

14. Tangent of the slope in specific point adjacents to the sedimentationdetermines the sedimentationvelocity for the corresponding particles. Knowing the sedimentation rate of the corresponding particles of separated fractions can be determined the particle’s size (radius)

15. Sedimentation curves mono- and poly-disperced systems Content of separated fraction

16. Distribution curve of the particles of the dispersed phase according to the size Q r

17. Sedimentometers:а) Phygorovski’ b) Vagner’ h , wherem –mass settled down fraction, Q– general mass of powders

18. The scheme, which explains the color of the atmosphere Sky blue Sun Sunset (Sunrise)red Observation point Earth Rayleigh equation:

19. The intensity of the transmitted light beam is defined according to Rayleigh equation: where I0 is the intensity of the incident light beam, It is the intensity of the transmitted light beam, n1 and n0 – the refractive indices of the particles and the dispersion medium. λ - wavelength

20. Optical properties Optical microscopy Colloidal particles are often too small to permit direct microscopicobservation. The resolving power of an optical microscope (i.e. Thesmallest distance by which two objects may be separated and yetremain distinguishable from each other) is limited mainly by thewavelength λ of the light used for illumination. The numerical aperture of an optical microscope is generally less than unity. With oil-immersion objectives numerical apertures up to about 1.5 are attainable, so that, for light of wavelength 600 nm, this would permit a resolution limit of about 200 nm (0.2 μ.m). Since the human eye can readily distinguish objects some 0.2 mm (200 μm) apart, there is little advantage in using an optical microscope, however well constructed, which magnifies more than about 1000 times. Further magnification increases the size but not the definition of the image.

21. Particle sizes as measured by optical microscopy are likely to be inserious error for diameters less than c. 200 nm. • Two techniques for overcoming the limitations of optical microscopyare of particular value in the study of colloidal systems. Theyare electron microscopyand dark-field microscopy – the ultramicroscope

22. The transmission electron microscope Toincrease the resolving power of a microscope so that matter ofcolloidal (and smaller) dimensions may be observed directly, thewavelength of the radiation used must be reduced considerably belowthat of visible light. Electron beams can be produced with wavelengthsof the order of 0.01 nm and focused by electric or magnetic fields,which act as the equivalent of lenses. The resolution of an electronmicroscope is limited not so much by wavelength as by the technical difficulties of stabilising high-tension supplies and correcting lensaberrations. The useful range of the transmission electron microscope for particle size measurement is c. 1 nm-5 μm diameter.

23. The use of the electron microscope for studying colloidal systems is limited by the fact that electrons can only travel unhindered in high vacuum, so that any system having a significant vapour pressure must be thoroughly dried before it can be observed. A small amount of the material under investigation is deposited on an electron-transparent plastic or carbon film (10-20 nm thick) supported on a fine copper mesh grid. The sample scatters electrons out of the field of view, and the final image can be made visible on a fluorescent screen.

25. Dark-field microscopy-the ultramicroscope Dark-field illumination is a particularly useful technique for detectingthe presence of, counting and investigating the motion of suspendedcolloidal particles. It is obtained by arranging the illumination systemof an ordinary microscope so that light does not enter the objectiveunless scattered by the sample under investigation. Lyophobic particles as small as 5-10 nm can bemade indirectly visible in this way. The two principal techniques of dark-field illumination are the slitand the cardioid methods. 1)In the slit ultramicroscope of Siedentopfand Zsigmondy (1903) the sample is illuminated from the side by anintense narrow beam of light from a carbon-arc source

27. Scheme ultramicroscope Diaphragm Light source Lens Lens

28. 2)The cardioid condenser (a standard microscope accessory) is anoptical device for producing a hollow cone of illuminating light; thesample is located at the apex of the cone, where the light intensity ishigh (Figure 3.4).

29. Dark-field microscopy is, nevertheless, anextremely useful technique for studying colloidal dispersions andobtaining information concerning: 1. Brownian motion. 2. Sedimentation equilibrium. 3. Electrophoretic mobility. 4. The progress of particle aggregation. 5. Number-average particle size (from counting experiments and aknowledge of the concentration of dispersed phase). 6. Polydispersity (the larger particles scatter more light and thereforeappear to be brighter). 7. Asymmetry (asymmetric particles give a flashing effect, owing todifferent scattering intensities for different orientations).

30. Light scattering When a beam of light is directed at a colloidal solution or dispersion,some of the light may be absorbed (colour is produced when light ofcertain wavelengths is selectively absorbed), some is scattered andthe remainder is transmitted undisturbed through the sample. The Tyndall effect-turbidity All materials are capable of scattering light (Tyndall effect) to someextent. The noticeable turbidity associated with many colloidaldispersions is a consequence of intense light scattering. A beam ofsunlight is often visible from the side because of light scattered bydust particles. Solutions of certain macromolecular materials mayappear to be clear, but in fact they are slightly turbid because of weaklight scattering. Only a perfectly homogeneous system would notscatter light; therefore, even pure liquids and dust-free gases are veryslightly turbid.

31. The turbidity of a material is defined by the expression where I0 is the intensity of the incident light beam, It is the intensity of the transmitted light beam, l is the length of the sample and τ is the turbidity. This expression is used in Turbidimetry. It is based on the measuring of the intensity of the transmitted light beam. Measurement of scattered light As we shall see, the intensity, polarisation and angular distribution of the light scattered from a colloidal systemdepend onthe size and shape of the scattering particles, the interactions between them, and the difference between the refractive indices of the particles and the dispersion medium.

32. Light-scattering measurements are, therefore, ofgreat value for estimating particle size, shape and interactions, andhave found wide application in the study of colloidal dispersions,association colloids, and solutions of natural and synthetic macromolecules. The intensity of the light scattered by colloidal solutions ordispersions of low turbidity is measured directly. A detectingphotocell is mounted on a rotating arm to permit measurement of thelight scattered at several angles, and fitted with a polaroid forobserving the polarisation of the scattered light (see Figure 3.5).

34. Doty nephelometr photometer Limb Limb Plate Source of light Flasks containing colloidal solution

35. Nephelometry is based on the measuring of the the intensity of the scattered light beam by the dispersed system. It,1/It,2 = c1/c2; It,1/It,2 = V1/V2; It = k νV2I0=kCVI0 where, k – constant, C = νV – volume concentration of the dispersed phase

36. Thanks for attention