Physical + Chemical Solution Properties Flashcards
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Solution Outline
A molecular dispersion. Clear, no deflecting of life. Solute binds with solvent on the molecular level (1 phase)
Vapour Outline
Substance that is liquid at room temperature. Liquid phase in a gas carrier
Non-electrolyte solutes characteristics
non-ionisable (doesn’t dissociate to form ions), don’t conduct electricity and it’s colligative properties depends on the no. molecules in solution
Electrolyte Solute Characteristics
Ionisable (does dissociate to form ions), conduct electricity and it’s colligative properties depend on no. of ions in solution
Ideal Solution Outline
Complete uniformity of molecular forces in solution. Drug and solvent molecules want to interact with each other as much as they want to interact with themselves.
Conditions foe Ideal Solutions
Thermoneutral (neither exo/endothermic dissolution), no change in net system volume, drug and solvent have similar structures. The interactions (vapour pressure, surface tension, refractive index and viscosity) are weighted averages of each pure substance (equal influence)
Real/ Non-ideal solution Outline
Molecular forces in solution are non-uniform.
Positive Deviation of Raoult’s Law Outline
Drug-drug and solvent-solvent interactions are stronger then drug-solvent interactions
Negative Deviation of Raoult’s Law Outline
Drug-solvent interactions are stronger then drug-drug and solvent-solvent interactions
Colligative Outline
Collection of properties that change together as solute dissolves in solvent.
4 Main colligative properties Outline
Vapour pressure lowering, boiling point elevation, freezing point depression and osmotic pressure increase
Relationship between vapour pressure and amount evapourated
High vapour pressure = large amount of water evaporated. Lowering of vapour pressure = decreasing amount of solvent evaporated from surface
Positive deviation and vapour pressure relationship
Positive deviation = stronger drug-drug, solvent-solvent interactions = higher vapour pressure = more solvent lost
Negative Deviation and vapour pressure relationship
negative deviation = stronger drug-solvent interaction = lower vapour pressure = less solvent lose (drug molecules hold solvent in place)
Result of adding non-volatile solute to solvent
Increased conc = reduced vapour pressure (negative deviation)
Relationship between negative deviation = vapour pressure = boiling point
negative deviation = stronger drug-solvent interactions = decreased vapour pressure = increased boiling point
Molal Elevation and Molal Freezing Constant Outline
Constant is different from solvent to solvent. Use molality instead of molarity as weight is used instead of volume due to temperature dependency. Molal elevation and freezing constants for the same solvents are also different
Relationship between molal concentration and elevated boiling point
Increased molal concentration = bigger increase in elevated boiling point
Relationship between vapour pressure and freezing point
The lower the vapour pressure = the lower the freezing point
Relationship between molal concentration and freezing point depression
Higher molal concentration = lower freezing point depression
Osmotic Pressure Def
Force solution exerts on solvent preventing solvent from passing through semipermeable membrane and entering solution. Opposes osmosis
Relationship between osmatic pressure and molar conc of solution
increased molar conc = increased osmatic pressure
Relationship between osmatic pressure and absolute temp (K)
increase in temp = increase in osmatic pressure
Van’t Hoff Factor (i) Outline
Numerical value that accounts for electrolytes ability to dissociate when calculating osmotic pressure. i = the number of ions an electrolyte produces when dissociated
Relationship between Van’t Hoff Factor and osmotic pressure
The larger the Van’t Hoff Factor = the higher the osmatic pressure
Why might Van’t Hoff Factor deviate from expected when measured
Tendency for some dissociated ions to rejoin meaning that they no longer act like independent ions
Pharmaceutical Relevance of colligative properties
Understand interactions between solvent and drug. Examples: maintenance of physiological tonicity when admining drugs (prevent irritation and drug precipitation), humectants (excipients that lower vapour pressure = decreasing evaporation), propellants (excipients that increase vapour pressure for aerosols)
How drugs alter solution pH
Acidic drugs lower solution pH (release H+ into solution when dissolving) and alkaline drugs increase solution pH (take up H+ ions from solution when dissolving). Changes in pH of drug can effect chemical (ionisation), microbial (optimal growth) and physical (precipitation) stability
Buffer Outline
Substance in solution that prevents sharp increase/decrease of pH. Consists of weak acid/base and it’s conjugate base/acid in a 1:1 ratio (needs to be at pKa). Acid components donate protons to base added, base accepts prtons from acid added. Net pH remains the same
Buffering Capacity Outline
Defined by conc of strong acid/base required to change the pH of 1L buffer by 1 pH unit. Depends on conc of buffer components
Buffer excipient considerations
shouldn’t alter physiological pH of admin site (irritation), toxicity, pKa and drug + excipient solubility and permiability, chemical stability and patient acceptability
Example of a buffer
phosphoric acid (acid) and monosodium phosphate (conjugate base)
Precipitation Outline
Solute no longer has affinity for solvent and falls out of solution forming a suspension. Eventually solute settles at bottom as precipitate and solvent = supernate
Precipitation Causes
Solid state change (polymorphism), temp change (eg refrigeration), cosolvent dilution, pH change, adding of a new species (eg one of opposite charge to solute), degradation (eg loss of hydrophilic components in molecule) and adding of solute past the point of saturation (always make up subsaturated solutions)
Chelation Outline
Binding of an ion to a substance that donates an electron pair. Lowers bioavailability and can act intercation possibilities are changed
Chelators as Excipients
Used as antioxidants
Adsorption Def
Binding of a substance to a media’s interface. 2 types: physisorption and chemosorption
Absorption Def
Binding of a substance throughout a phase
Leaching Outline
Movement of a subtance from a container into phase it holds. Eg plasticisers from soft pastics (eg bags) into a lipophilic liquid.
Physiosorption Examples
Weak Van der Waals, ion-dipole and dipole-dipole interactions
Adsorbate def
Substance binding to interface
Adsorbent Def
Interface that substance is binding to
