# Kinetic/Optical Properties of Colloids

## Presentation on theme: "Kinetic/Optical Properties of Colloids"— Presentation transcript:

Kinetic/Optical Properties of Colloids
PhD Halina Falfushynska

Motion Thermal motion Techniques for measuring colloidal sizes
Brownian Motion on the microscopic scale Diffusion and translation on the macroscopic scale Techniques for measuring colloidal sizes Sedimentation (under gravitational or applied field) Colligative Properties Scattering techniques

George Gabriel Stokes (1851) Albert Einstein (1905)
Robert Brown (1827) Brownian motion George Gabriel Stokes (1851) Albert Einstein (1905) Brownian motion was discovered by the biologist Robert Brown in He was studying pollen particles floating in water under the microscope, and he observed minute particles within vacuoles in the pollen grains executing the jittery motion that now bears his name. Full explanation by Einstein. Stokes ( ) was an Irish mathematician and physicist. He was a fellow at Pembroke college, but was kicked out for marrying in years later he was allowed back in. The oldest of the trio Lord Kelvin, James Clerk Maxwell and Stokes who made Cambridge famous in the 19th century was appointed Lucasian professor of Mathematics. Peclet Mainz

Sedimentation Potential
Sedimentation Of A Single Particle Generates a Potential

Stokes Law Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers (e.g., very small particles) in a continuous viscous fluid. Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle (in N), μ is the dynamic viscosity (N s/m2), R is the radius of the spherical object (in m), and vs is the particle's settling velocity (in m/s).

Calculate the fall or settling velocity (Vt) for the given details through Stoke's Law formula.
Acceleration of Gravity (g) = 25 m/s2 Particle Diameter (d) = 15 m Density of Medium (ρm) = 5 kg/m4 Particle Density (ρp) = 10 kg/m3 Viscosity of Medium (μ) = 20 kg/m-s Vt = gd2 (ρp - ρm)/18μ

Problem Vt = gd2 (ρp - ρm)/18μ
Find the largest possible diameter for water drops falling in air with a velocity where Stoke’s law can be used In the calculation. Giver: density of air 1.2 kg/m3, μ – 1.8×10-5 kg/(m×s) Vt = gd2 (ρp - ρm)/18μ

A terminal velocity is reached
Where m – mass of particle - specific volume of colloidal particle - solvent density f – particle frictional factor

Stokes Law In the limit of Slow particle motion
Dilute colloidal suspensions Solvent is considered as a continuum of viscosity 

The Frictional Factor The frictional factor in a given medium is obtained from Stokes Law  – solvent viscosity R- particle radius f, is a measure of the resistance to movement of a molecule; this resistance is a function of both the size and the shape of the molecule

The Frictional Factor D – diffusion coefficient
S – sedimentation coefficient

Frictional Factors Frictional factors depend on the particle shape
f increases as Particle asymmetry increases Degree of interaction with solvent increases Define the frictional ratio, f/fo. Ratio of the f value of the particle to that of an unsolvated sphere.

Poiseuille's Law Calculation
In the case of smooth flow (laminar flow), the volume flowrate is given by the pressure difference divided by the viscous resistance. This resistance depends linearly upon the viscosity and the length, but the fourth power dependence upon the radius is dramatically different.

Sedimentation of colloids
buoyant mass The bigger the particles the faster they sediment

Sedimentation Under gravity
Balance method – cumulative mass of settling particles is obtained as a function of time Practical lower limit is about 1 micron Under centrifugal force High Field – up to 4 x 105g is applied. Displacement of boundary is monitored with time Under low field Measure concentration profile in the tube as a function of position.

Erythrocyte sedimentation rate

Erythrocyte sedimentation rate
Sedimentation rate or is the rate at which red blood cells sediment in a period of one hour. To perform the test, anticoagulated blood is placed in an upright tube, known as a Westergren tube. The rate at which the red blood cells fall is measured and reported in mm/h. The ESR is governed by the balance between pro-sedimentation factors, mainly fibrinogen, and those factors resisting sedimentation, namely the negative charge of the erythrocytes (zeta potential).

When an inflammatory process is present, the high proportion of fibrinogen in the blood causes red blood cells to stick to each other. The red cells form stacks called 'rouleaux,' which settle faster. Rouleaux formation can also occur in association with some lymphoproliferative disorders in which one or more immunoglobulin are secreted in high amounts. The ESR is increased by any cause or focus of inflammation. The ESR is increased in pregnancy, inflammation, anemia or rheumatoid arthritis, and decreased in sickle cell anemia and congestive heart failure. The basal ESR is slightly higher in females.

Diffusion Diffusion is one of several transport phenomena that occur in nature. A distinguishing feature of diffusion is that it results in mixing or mass transport without requiring bulk motion.  There are two ways to introduce the notion of diffusion: either a phenomenological approach starting with Fick’s laws and their mathematical consequences, or a physical and atomistic one, by considering the random walk of the diffusing particles

Fick’s Laws of Diffusion
The diffusion flux is proportional to the minus gradient of concentrations. It goes from regions of higher concentration to regions of lower concentration.

Diffusion From the atomistic point of view, diffusion is considered as a result of the random walk of the diffusing particles. In molecular diffusion, the moving molecules are self-propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown.

Diffusion and Frictional Factors
The diffusion coefficient of a suspended particle is related to f via the Einstein Equation For spherical particles where D is the diffusion constant;  μ is the mobility";  T is the absolute temperature kB is Boltzmann's constant;  × 10-23 m2 kg s-2K-1

Diffusion of ions through a membrane
the flux is equal to mobility×concentration×force per gram ion. This is the so-called Teorell formula. The force under isothermal conditions consists of two parts: Diffusion force caused by concentration gradient:  Electrostatic force caused by electric potential gradient:  Here R is the gas constant, T is the absolute temperature, n is the concentration, the equilibrium concentration is marked by a superscript "eq", q is the charge and φ is the electric potential.

Measurement of Diffusion Coefficients
Free boundary methods a boundary between two solutions of different concentrations is formed in a cylindrical cell Determine the evolution of the concentration distribution with time.

Measurement of Diffusion Coefficients
Taylor Dispersion methods NMR Techniques Pulsed gradient spin echo experiments (PGSE) Diffusion oriented spectroscopy (DOSY)

Intensity of transmitted radiation related to solution turbidity ()
Intensity of transmitted radiation related to solution turbidity ()

Nobel Prize for Chemistry for his work on the heterogeneous nature of colloidal solutions.

Light Scattering and Colloidal Sizes
Size and shapes of colloidal systems can be obtained from scattering measurements Advantages of Light Scattering Absolute No perturbations of system Polydispersed systems Fast

Molar Masses from Scattering
Obtain the Rayleigh ratio at 90