Physiochemical properties 1 Flashcards
What is a SOLUTION?
A mixture of 2 or more components that form a homogeneous molecular dispersion
What is a SINGLE PHASE SYSTEM?
Not another phase dispersed in another
What is a SOLVENT?
The component present in greater amount (usually liquid)
What is a SOLUTE?
The component present in smaller amount (usually solid or liquid)
What is SATURATED SOLUTION?
Where the solute is at the limit of its solubility at a given temperature and pressure
What are the 2 types of solution?
- Liquid in Liquid
2. Solid in Liquid
Describe liquid in liquid
- solutions of this type can be divided into 2 categories:
- Completely miscible, e.g. ethanol + water- one phase
- Partially miscible, e.g. water + ether- two phases
- Miscibility: Mutual solubility of components in liquid in liquid system
- Partial miscibility is affected by temperature
- Partially miscible binary liquid mixtures are used with a separating funnel to determine partition coefficients.
Describe solid in liquid
- solutions of non-electrolytes (non ionic) e.g. organic molecules phenol, sucrose
- solutions of electrolytes (forms ions in solution)
- strong electrolytes (fully ionised over a wide ph range_
- weak electrolytes (ionisation dependent on ph)
How do drugs enter solution?
Dissolution of a solid solute
- solute molecule leaves particle
- creation of a cavity in the solvent
- solute molecule enters the cavity
What is solvation and hydration?
Solvation: process of binding of solvent to solute molecules.
Hydration: solvation when the solvent is water.
What is the structure of water?
- tetrahedral structure, angle 104.5
- Flickering cluster model: involves clusters of H-bonded ‘ice like’ water within pools of unbound water
What is an IDEAL SOLUTION?
There are no interactions between dissolved solutes and obey Raoult’s law
What is a REAL SOLUTION?
Interaction occur between dissolved solutes which reduce the effective concentration
Activity coefficient
= activity/concentration
Describe polar solvents
- water, methanol, acetic acid have a high relative permittivity (dielectric constant)
- Dissolve ionic solutes by reducing the force of attraction between oppositely charged ions in crystalline solids
- They can break covalent bonds
- They can solvate molecules and ions through dipole-dipole forces.
Describe Semi-polar solvents
- Acetone, methanol, ethanol
- Can dissolve polar and non-polar substances
- Solvent molecules dipole movement or H-bonding groups enable POLAR solutes to be dissolved
- SEMI-POLAR solvents can induce a degree of polarity in NON-POLAR solvent molecules.
Describe Non-polar solvents
- Hexane, pentane, oils&fats
- Low dielectric constant, lack H-bonding groups prevent these solvents from dissolving ionic or polar solutes
- Unable to break covalent bonds
- Dissolve NON-POLAR compounds only
- Non-polar solutes are held in solution through Van Der Waals interactions with the solvent.
Solubility of weak electrolytes
ph and ionisation:
- The solubility of weak acids and bases is influenced by ph
- By modifying the ph we can alter the solubility of a drug
- ionised species is more soluble than the unionised species
Weakly Acidic Drugs
- name examples
- solubility Equation
- ph at which drug will start to precipitate
- Warfarin , NSAIDs
- ionised species of these will be hydrated and more soluble
- The solubility can be determined by Henderson-Hasselbatch equation:
Pka= ph + log [HA]/[A-] - ph= pka + log s-so/so
- s= saturation solubility at a given ph
- so= solubility of undissociated drug (FREE BASE)
Weakly Basic Drugs
- name examples
- solubility equation
- ph at which drug will start to precipitate
- Ranitidine, Cimetidine
- pka= ph + log [BH+]/[B]
- ph= pka + log so/s-so
What is an Amphoteric drug?
- isoelectric point
- Oxytetracycline, nitrazapam
- Contain both ACIDIC and BASIC groups so will have 2 or more pka values
- ISOELECTRIC POINT- is the ph at which the drug carries no net electric charge
ph = (pka1 + pka2)/2
Define partitioning
- Partition coefficient equation
The distribution of a substance between 2 immiscible phases
- p= Co/Cw
How is partition coefficient calculated?
- Drug added to a separating funnel containing 2 immisicible liquids
- The flask is shaken for up to 1 hr and then left so that the 2 phases separate out
- Conc in the aqueous phase is determined by spectroscopy or
- Conc in the organic phase would be determined by subtraction
Apparent partition coefficient
- for weak acids and bases
WEAK ACID:
Log [p/papp - 1] = ph - pka
WEAK BASE:
Log [p/papp - 1] = pka - ph
What are the limitations of the ph- partition theory?
- The small intestine are designed for absorption
- Surface area of up to 200m2 as a result of microvilli
- long residence time and excellent blood flow
Active transport
- Molecules may be particularly vital to the body that active transport mechanisms exist
- mechanisms exist for substances such as: Ions, glucose, vitamins, amino acids
- Active transport means that molecules may be absorbed even if they are ionised or highly hydrophillic
