Flashcards in Lecture 9 Deck (48)
What is salting out?
when you add something which is soluble in one component,
or its solubility is markedly different in the TWO components
This causes the mutual solubility of the liquid pair to decrease
The UCT will increase (e.g. 0.1M napthalene to phenol water)
and LCT will decrease (e.g. 0.1M KCl added to phenol water mixture)
What is blending?
where the mutual solubility of the pair is increased by adding an additive which is soluble in water components in the same proportion to the same extent
This cases UCT to decrease (e.g. succinict acid or sodium oleate added to phenol water mixture)
and LCT to increase
What happens when you add an additive to any 2 pairs?
it becomes 3 components.
how can you determine the solubility on each component?
using a ternary phase diagram
e.g. 1 phase in which all components are all soluble
2 phases in which one of the components is not soluble
what factors influence the thermodynamic solubility of a drug?
What is lattice energy?
interactions among solute molecules in the crystal
what is cavitation energy?
interactions among the solvent molecules in the space required to accomodate the solute
what is solvation energy?
stablising interactions between solute and solvent
what is entropy?
a measure of randomness or disorder within a system
what is enthalpy?
amount of heat absorbed or evolved during the dissolution process
What are the steps of solvation?
1) initiall solvent and solute are segregated from each other.
The primary interacts are between other molecules of the same type
2) lattice energy and cavitation energy interactions are broken. Entropy increaases slightly due to disruption of hydrgoen bonds among the solvent molecules. Solute starts to move into solution
3) when solute is surrounded by the solvent, solvation energy interactions are formed. Entropy decreases due to mixing of solute and solvent as well as the new short range order due to the presence of the solute
how can energy changes which take place within solute solvent interactions be exlained?
using gibbs energy equation
what is free energy?
the energy available for any system to do work
What must G be for the process to be spontaneous?
What does a -G value mean?
we dont require extra energy to break bonds
What is enthalpy?
the amount of heat absorbed or evolved during the dissolution process
What is an exothermic reaction?
if heat evolves when molecules of solute are dispersed into the solvent.
Indicated by a negative ΔH value
What is an endothermic reaction?
if heat is abosrbed when molecules of ions or solute are separated. Indicated by a positive ΔH balue
What is ΔS?
the change in entropy
POSITIVE for any process such as dissolution due to mingling of solute and solvent and the disruption of the ordered network of bonds among solvent molecules
What factors affect the solubility of solids?
molecular structure of solute
nature of solvents
acid base properties of drug
how does temperature influence the solubility of solids?
a change in temeprature either increases or decreases solubility of a substance depending on the nature of the reaction
if dissolution is endothermic, heat is absorbed naturally so increasing temperature will increase solubility
if dissolution is exothermic, heat is naturally released, so an increased temperature will decrease solubility
What is partiuclar about the temperature solubility relationship of sodium sulphate
it has a temperature where solubility increases, and another temperature where it decreases
How is solubility phenomena used in first aid?
the cold pack.
there is salt, ammonium nitrate and water, when mixted the endothermic reaction takes place so takes heat from its surroundings
how does molecular structure affect solubility of solids?
- structure of solutes affects the solubility of each solvent
-straight compunds are less soluble than branched compounds
- small change in molecular structre can have a marked effect
Why is testosterone more soluble?
it has a higher solubility due to the OH functional group
an introduction of which functional group can decrease solubility?
How can molecular structure be changed?
What can result from the conversion of a weak acid to its sodium salt?
it can lead to a much greater degree of ionic dissociation of the compound when it dissociates in water
e.g. the weak acid is partially soluble due to partial ionisation however the solubility of salicylic acid is practically insoluble
how can we use solubility phenomena for other principles?
higher solubility of a drug can result in a bitter taste, we can mask this by adding a palmitate to the compound so you dont get the taste as soon as you take the medicine
can reduce excessive degadation of certain drugs in the gut
can enhance or reduce the rate of absorption of drugs
What is a cosolvent?
a substance added to obtain an aqueous system that contains solute in their excess of their solubility in water e.g. ethanol and propylene glycol
What do surfactants have?
both hydrophilc and hydrophobic portions
What can surfactants form?
What does solubility of a surfactant depend on?
their HLB value,
High HLB is hydrophilic, low HLB is hydrophobic
Why are crystals hard to solubilise?
organised structures like crystals require more energy to break the bonds
the change in enthalpy is determined by the strength of interactions between adjacent molecules in crystal lattice
amorphous substances are easier to solubilise
What are solvates?
the product formed by solvation
What are hydrates?
the product formed where water is the solvation molecule
Why do hydrated crystals typically exhibit lower solubility in water?
Water can form a H bond between the 2 drug molecules and tie the lattice together creating a mch stronger and stable lattice
Which form of drug always as a higher solubility?
the ionised form
In the dissolution of acetic acid, what happens in acidic conditions? what happens in basic conditions?
in acidic conditions solubility decreases
in basic conditions solubility increases
What can the change in solubility in different pH be determined by?
the pHP equation where pHp is the pH below which an acid precipitates
What happens to the solute in a mixture of 2 immiscible solvents?
It will try to distribute between the 2 phases
what is partitioning?
how well the drug will distribute between 2 systems
How can the partition coefficient be predicted?
using octanol and water.
We obtain the ratio of concentration of a substnace in the octanol phase, to its concentration in the aqueous phase at equillibrium at a constant temperature
Then we obtain a LogP
What does a higher LogP mean?
the solute is more lipophilic i.e. less polar
what does a lower LogP mean?
the solute is more hydrophilic i.e. more polar
What is partition coefficient useful to?
release of drug from dosage forms
What is an old but widely used method to determine the parition coefficient?
shake flask method
Where the flask with the 2 immiscible liquids is shaken and partition is achieved when the 2 phases are separated.
the concentration of the solute in each phase is measured
the 2 phases are mutually saturated with each other