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Colloid: Electrokinetic properties Kausar Ahmad Kulliyyah of Pharmacy http://staff.iiu.edu.my/akausar Physical Pharmacy 2 1 Contents Types of electrokinetic phenomena Measuring zeta potential • Traditional microelectrophoresis • Laser Doppler velocimetry Applications Physical Pharmacy 2 2 Electrokinetic phenomena Electrophoresis • The movement of charged colloidal particles in electric field. • Most practical. Electroosmosis • When the charged solid surface is fixed, the electric field causes a movement of the liquid Streaming potential • Forcing a liquid through a capillary or porous plug induces a difference of electric potentials Sedimentation potential • Forced movement of charged solid particles in a liquid, e.g., due to gravitation induces a difference of electric potentials Physical Pharmacy 2 3 Potentials 0 - potential at charged surface s - potential at Stern layer - potential at plane of shear. • Only zeta potential can be determined experimentally. • Both 0 and s are thermodynamic and theoretical quantities and are calculated from theory only. Physical Pharmacy 2 4 Zeta potential From http://www.silvercolloids.com/Tutorials/Intro/pcs17.html The slipping/ shear plane separates the thin layer of liquid bound to the solid surface (elastic behavior) from the rest of liquid (normal viscous behavior). The electric potential at the shear plane is called zeta potential. Physical Pharmacy 2 5 Determination of zeta potential Measure electrophoretic mobility the electrophoretic mobility is the ratio of the velocity of particles to the field strength Physical Pharmacy 2 6 Traditional Measurement of Electrophoretic Mobility An electrophoresis system consists of a capillary cell with electrodes at either end to which a potential is applied. Observe individual particles using a microscope and time their transit across a graticule. Physical Pharmacy 2 7 Smoluchowski equation From Marian Smoluchowski: applies for m=e/h m electrophoretic mobility e electric permittivity of the liquid h is the viscosity Physical Pharmacy 2 thin double layer when the zeta potential is not too high large colloidal particles and high ionic strengths 8 Huckel Equation Thickness of EDL, 1/κ size of particle is small compared to EDL (or thick EDL) Use Huckel equation: m=e/1.5h Physical Pharmacy 2 9 Complications in zeta potential determination Electrophoretic retardation Relaxation effect Physical Pharmacy 2 10 Laser Doppler Velocimetry Young’s interference fringes formed at stationary level Using Fourier Transform, the correlator produces a frequency spectrum from which the mobility distribution and hence zeta potential are calculated. Physical Pharmacy 2 11 Fluctuation in intensity of scattered light From: http://www.silver-colloids.com/Tutorials/Intro/pcs23.html Physical Pharmacy 2 12 Advantages of laser technique High speed Applicable for nonaqueous, highly conductive, high ionic strength environment Does not differentiate particles i.e. even small ones are detected. Can detect particles < 100 nm Measures over thousands of particles Physical Pharmacy 2 13 Examples of Application To investigate the electrophoretic properties of blood lipid particles in connection to potential heart problems. Control of size and zeta potential of droplets of artificial blood, which is vital for its safe use. The relationship between the zeta potential of certain cells in amniotic fluid, and lung maturity (suggested by a study in Holland). Determination of IEPs are carried out to e.g. confirm the pH for flocculation/coagulation. Determination of CRC can be used to identify the nature of surface groupings e.g. sulfates, carboxylates etc Physical Pharmacy 2 14 Isoelectric point (IEP) • If pH<4 or pH>8 there is sufficient charge to confer stability. • If 4<pH<8 dispersion may be unstable. • Most unstable at around pH 6 (IEP) From: http://www.silver-colloids.com/ Tutorials/Intro/pcs18.html Physical Pharmacy 2 15 Zeta potential of chloramphenicol and glass particles in benzethonium (+) chloride solution. - an adsorbing cation Charge reversal concentration From: Florence & Attwood Physical Pharmacy 2 16 References RJ Hunter, Zeta Potential in Colloid Science, Academic Press (1988) RJ Hunter, Foundations of Colloid Science Volume 2, Clarendon Press Oxford (1989) ID Morrison & S Ross, Colloidal Dispersions, Wiley-Interscience, New York (2002) http://zeta-potential.sourceforge.net/ http://www.silver-colloids.com/Tutorials/Intro/pcs1.html http://www.luminet.net/~wenonah/riddick/chap22.htm Physical Pharmacy 2 17