Solutions - physical properties

Question 1

What is a solution?

Answer 1

homogenous mixture of 2 or more substances

the solute is dispersed uniformly throughout the solvent

Question 2

How does a solution form?

- what are the steps?

Answer 2

1 - solvent molecules are attracted to surface ions (solute)
2 - each solute ion is surrounded by solvent molecules = solvated a.k.a dissolution
3 - enthalpy changes when each interaction is broken or formed - the intermolecular force is ion-dipole

Question 3

How do you determine the enthalpy changes in a solution?

Answer 3

1 - separation of solute particles
2 - separation of solute particles creates ‘holes’ = gaps
3 - formation of new interactions between solute and solvent

if the end reaction is at a lower enthalpy than the start reaction = exothermic reaction - more energy given out than taken in

if the end reaction is at a higher enthalpy than the start reaction = endothermic reaction - more energy taken in than given out

Question 4

What are the degrees of saturation for a solution?

Answer 4

Unsaturated solution

• less than the maximum amount of solute for that temperature is dissolve
• no solid left in the flask = solvent is still able to react as not enough solute was added

Saturated solution

• solvent holds as much solute as it can at that temperature
• undissolved solid remains in the flask = all of the solvent has reacted with solute so excess solute cannot react/dissolve
• dissolved solute is in dynamic equilibrium with solid solute particles = they can switch with each other, if more dissolves then some will switch to solid particles

Question 5

What is solubility?

Answer 5

equilibrium term
between solid and a saturated solution
it is dynamic - able to switch between dissolved solute and solid solute particles

Question 6

What is dissolution?

Answer 6

rate of dissolving
‘like dissolves like’
- polar substances dissolve in polar solvents
- non-polar substances dissolve in non-polar solvents

attractions between solute particles must be overcome by attractions of the solvent for the solute particles

Question 7

What is the solubility constant?

Answer 7

the equilibrium constant is called the solubility constant when dissolving = Ks

concentration of the products to the power of their stoichiometry

Question 8

What are the factors affecting the solubility constant?

Answer 8

temperature

liquid
- solubility of solid solutes in liquid solvents increases as temperature increases
gases
- solubility of gases in liquid solvents decrease as temperature increases
- gases will try to escape the solution at higher temperatures

Question 9

What is an acid and base in the Lowry-Bronsted Theory?

Answer 9

acid - can donate a proton

base - accepts a proton

Question 10

What is a buffer solution?

Answer 10

a solution formed by a weak acid ( or weak base) and a salt of the conjugate base ( or conjugate acid)

• wake acid and conjugate base
• weak base and conjugate acid
  = conjugate means it is the same but with or without an additional proton

is able to withstand small additions of acid or base without much change in the pH of the solution

Question 11

What is bioavailability?

Answer 11

rate and extent of absorption

amount of drug available to exert a pharmacological effect = active effect

Question 12

What are the factors affecting bioavailability of the drug?

Answer 12

dissolution
- depends on the solubility of the drug
permeation/absorption
- depends on particle size 
- depends on the ionisation and hydrophilic/hydrophobic nature of the drug

both are limiting factors
example - non-water soluble drugs are dissolution rate limited

Question 13

What is the partition coefficient?

Answer 13

the partition coefficient, P, shows the ability of a
molecule to partition (move) between 2 phases = lipid and water-soluble

P = solubility in octanol / solubility in water (oil/water)

• usually logP is tabulated
• the bigger logP is the more hydrophobic the molecule is
• the smaller logP is the more hydrophilic the molecule is

Question 14

What is a co-solvent?

Answer 14

a substance added to a mixture of 2 or more substances that are poorly soluble or immiscible with each other

increases the solubility of poorly soluble substances by increasing the solvent power

• the drug must be more soluble in the co-solvent than the solvent
• the co-solvent must be completely miscible with water

Question 15

What are examples of co-solvents?

Answer 15

water 
glycerine - glycerol
propylene glycol
ethanol 
polyethyl glycol (PEG)

Question 16

What is the formula to calculate solubility in mixed solvents?

Answer 16

logS = mx + c

S - solubility
x - volume fraction of co-solvent

x = co-solvent / (co-solvent + solvent)

Question 17

What are some excipients used in solutions?

Answer 17

co-solvents
chelators
anti-oxidants

Question 18

What is the purpose of chelators?

Answer 18

group of substances used to stop metal ions catalysing the oxidation of the drug

remove metal ions from the body/blood

example of metal ions - copper, iron = most effective
example of chelators - edataate (EDTA)

Question 19

What is the purpose of anti-oxidants?

Answer 19

prevent oxidation of the drug
- they are oxidised preferentially

example - BHT = butylated hydroxy toluene