colloids 2

Question 1

what is wrong with lyophobic sols?

Answer 1

they are thermodynamically unstable

Question 2

how to stabilize lyophobic sols

Answer 2

• by the presence of electric charges on their surfaces aka electrical double layer

Question 3

effect of adding small amounts of electrolytes to lyophobic colloids?

Answer 3

It stabilizes them by providing a surface charge

Question 4

effect of adding large amounts of electrolytes to lyophobic colloids?

Answer 4

coagulation of colloidal particles due to decreased zeta potential (decrease in repulsion)

Question 5

which systems are thermodynamically stable?

Answer 5

Lyophilic and association colloids

Question 6

what is the effect of adding small amount of electrolytes to lyophilic

Answer 6

does not result in coagulation
decrease zeta potential
solvent sheath remains

Question 7

stability of lyophilic colloids

Answer 7

1- electric double layer ( repulsion)

2) Solvation ( formation of solvent sheath surrounding the particles)

Question 8

what happens if large amounts of electrolytes are added to lyophilic?

Answer 8

salting out occurs which is coagulation and sediment ion

Question 9

why does salting out happen

Answer 9

due to the reduced zeta potential and dehydration of colloidal particles

Question 10

DLVO theory

Answer 10

theory suggests that we can predict the stability of colloids by calculating the sum of their electrostatic repulsive forces and their van der walls forces

Question 11

dlvo calculation used

Answer 11

the total potential energy (Vt) = repulsive forces (Vr) + attraction forces (Va)

Question 12

EXPLAIN DLVO plot

Answer 12

primary minimum: closer to the y-axis therefore distance. between particles is small, the van der waal forces predominate
coagulation occurs and its irreversible

primary max: distance increases, repulsive forces predominate, this region is stable , particles remain in dispersion

secondary minimum- as distance further increases , repulsive forces decreases, there are weak attractive forces between particles

• flocculation occurs

Question 13

stability of lyophilic colloids is affected by

Answer 13

-addition of electrolytes
-addition of solvents
-coacervation
- sensization and protective colloid action
-steric stabilization

Question 14

how can we change lyophilic to lyophobic in order to get a precipitate?

Answer 14

adding an anti solvent, in order to change the interactions between particles and solvent

this results in dehydration which is the removal of the solvent sheath

plus coagulation which is basically the precipitate

Question 15

Coacervation

Answer 15

the separation of a colloid-rich layer and a diluted phase layer from a lyophilic sol as the result of adding another substance

Question 16

the colloid-rich layer is present as

Answer 16

form of an amorphous liquid called coacervate

Question 17

types of coacervation

Answer 17

simple- separation of a colloid rich layers occurs by salting out by adding electrolytes or anti solvent

complex- occurs by mixing two lyophilic colloids of opposite charges , thus attraction occurs

ex NH3+ AND COO-

Question 18

WHAT is coacervation useful for

Answer 18

micro-encapsulation - where the the coacervate is formed in an insoluble suspension of a drug

the macromolecular material will coat the suspended solid particles

Question 19

sensitization

Answer 19

adding small amount of lyophilic colloid to lyophobic colloid of opposite charges

reduce zeta potential
reduce repulsion
aggregates = coagulation

as a result the lyophobic colloid becomes more sensitive to added colloids

Question 20

Lyophobic dispersions are unstable in the prescence of even small concentration of electrolytes. why?

Answer 20

Effect is due to neutralisation of the charge on the particles.

Question 21

PROTECTIVE colloid action

Answer 21

adding large amount of lyophilic colloid to a lyophobic colloid of opposite charge

thus lyophilic forms a solvent sheath protecting lyophobic colloid against coagulation

thus lyophobic acts as lyophilic

Question 22

Gold number

Answer 22

it is a way to measure the strength of the lyophilic colloids protective power

• minimum weight in milligrams of a protective colloid needed to protect lyophobic upon the addicting of electrolytes which prevents coagulation

Question 23

what is the general idea of gold number strength

Answer 23

the lower the gold number , the stronger/ greater protective action

Question 24

steric stabilization

Answer 24

stabilizing colloid particles with non ionic macromolecules (long chains) placed on the surface of colloid like a brush

when the length of layers increases the van der wal forces are MASKED thus no attraction thus stability