Suspensions of colloidal particles in water

Question 1

What happens to most oxides suspended in water?

Answer 1

~ they will hydrate

Question 2

What happens to oxides in water under acidic conditions?

Answer 2

~ particle surface develops a positive charge
~ MOH + H3O+ = MOH2+ + H2O

Question 3

What happens to oxides in water under basic conditions?

Answer 3

~ particle surface develops a negative charge
~ MOH + H2O = MO- + H3O+

Question 4

What happens to particles in neutral water? What happens with acid or base additions?

Answer 4

~ they may naturally form a charged surface

~ acid or base additions can bring the surfaces back to an uncharged state

Question 5

What is the point of zero charge?

Answer 5

~ the pH at which particle surfaces have no charge
~ close to the isoelectric point determined from zeta potential measurements

Question 6

What’s an example of an acidic and basic charge?

Answer 6

~ acidic: silica
~ basic: MgO

Question 7

What is the PZC related to?

Answer 7

~ it’s related to the ionic charge Z and the radius r
~ PZC = A - B(Z/r)

Question 8

What are high charge small cations?

Answer 8

~ acidic
~ their PZC requires highly acidic conditions
~ under neutral conditions, they develop a negative surface charge

Question 9

What happens to ions in solution of charge opposite to that of the particle surface?

Answer 9

~ the ions are attracted to the surface, surrounded and partially electrically shielded by water molecules

Question 10

Explain the electrical double layer.

Answer 10

~ an electrical DOUBLE LAYER forms with an interior layer of immobile ions (STERN LAYER) that’s surrounded by a DIFFUSE LAYER of thermally agitated mobile ions, that adopt a stead ystate conc. gradient extending to a distance at which the surface charge is fully compensated

~ double layer thickness is ~3nm

Question 11

What happens under an external electric field?

Answer 11

~ the particle and the stern layer (and possibly a short distance into the diffuse layer) are induced to move (HYDRONAMIC PLANE OF SLIPPAGE)

Question 12

What is the potential at the slippage plane?

Answer 12

~ the zeta potential: a measure of particle surface charge minus stern layer charge

~ can be calculated from the known applied field and from particle velocity measurements

Question 13

How is particle velocity measured?

Answer 13

~ aka electrophoretic velocity

~ determined by direct microscopic measurement, or doppler shift measurements on scattered laser light

Question 14

Why is an acid or base added in regards to the zeta potential?

Answer 14

~ to bring the particle surface to the PZC, the zeta potential goes to zero, the pH at this point is the isoelectric point

Question 15

How can particles be induced to mutually repel?

Answer 15

~ if the surface charge is adequately high

~ correspondingly the concentration of counterions in the diffuse layers is relatively high
~ as particles approach, the conc. of counterions between the particles exceeds conc. on the opposite sides of the particles
~ these counterions apply a net force pushing the particles apart

Question 16

What happens if you add acid or base to shift the pH away from the isoelectric point (PZC)?

Answer 16

~ this deflocculates the suspension
~ sharp drop in viscosity

~ adding acid or base provides counterions
~ large monovalent counterions allow the diffuse layer to extend greater distances into the liquid, favoring mutual repulson

Question 17

What are some coagulant ions?

Answer 17

~ Li+, Mg2+, SO42-

Question 18

What are some deflocculant ions?

Answer 18

~ NH4+, Ba2+, NO3-

Question 19

What is the effect of large additions of counterions?

Answer 19

~ can facilitate charge compensation over a short distance, compressing the diffuse layer
~ diminishing returns for acid/base additions

Question 20

What zeta potential is required for electrostatic repulsion?

Answer 20

~ around 25 mV