Disperse systems 6 Flashcards
Thixotropy
A behaviour seen in some non-Newtonian fluids- reversible, time-dependent change in viscosity
What happens with thixotropy?
The particles form a loose 3-dimensional gel structure in the water
Shear forces disrupt the bonds and therefore the structure, allowing particles to line up and the liquid to flow
The longer the shear is applied, the lower the resultant viscosity
Over time, when there is no shear present, they reverse to their original high viscosity structure
Dynamics definition
Studies the relationship between the motion of objects and the cause of the motion
Kinematics definition
Studies the motion of objects without consideration for the forces causing the motion
Kinematic viscosity
Relates a liquid’s viscosity to its inertial force
Measurement of viscosity
Test how fast the sample flows- capillary methods
Test how hard it is to stir- rotating methods
Test how long it takes something to fall through the sample- falling methods
Capillary viscometers
Temperature of 20 +/- 0.1c unless otherwise prescribed
The time required for the level of the liquid to drop from one mark to the other is measured with a stop watch to the nearest one-fifth of a second
The result is only valid if two consecutive readings do not differ by more than 1%
The minimum flow time should be 350s for size no. 1 and 200s for all other sizes
Electrical properties
Dispersed particles have a large surface area and, when
they meet, they tend to associate to decrease their
surface area in contact with the liquid.
If particles become permanently attached to each other,
the clumps will increase in size and sediment and the
system may become impossible to re-disperse
Whether particles rebound and stay separate after
collisions or get permanently attached when they meet
depends on the attractive and repulsive forces, so the charges on particles is an important factor in the
stability of these systems
Most surfaces acquire an electrical charge when
they come into contact with water, in one of
three ways:
Ion dissolution
Ionisation
Ion adsorption
Ion dissolution
Different ions have different solubilities
e.g. silver ions (Ag+) are more soluble in water than iodide ions (I-)
If you placed particles of silver iodide in
water, the more soluble ions (Ag+) would
dissolve into the bulk of the water, leaving
behind the I- ions
The surface would therefore have an
overall negative charge
Ionisation
If a particle is made of molecules that can be
ionised, the molecules at the surface may
become ionised
If this happens, the particle will acquire a surface
charge
This is influenced by the pKa of the ionisable
groups and the pH of the dispersant
– at pH above the pKa of the group, cations are lost or
anions are gained, leaving it negatively charged
– at pH below the pKa, the opposite happens, leaving
the molecule positively charged
– at the isoelectric point, there is no overall net charge
Ion adsorption
A surface charge can be acquired by the
unequal adsorption of ions onto the surface i.e. if
more anions adsorb to a surface, the surface will
develop a net negative charge.
Surfaces in water are more often negatively
charged than positively because cations are
generally more hydrated than anions so tend to
stay in the water phase, leaving the anions free
to adsorb to the surface.
