Physical and chemical properties of solutions I-III

solution

homogenous molecular dispersion of solute dissolved in solvent. The solute is typically the substance that undergoes a phase transition

characteristics of solutes in solvents

solutes interact with solvents of different extents depending on their physical and chemical properties

ideal solution

complete uniformity of intermolecular forces in a solution. Thus, solvent/solvent, drug/drug and drug/solvent interactions are equal

• thermoneutral dissolution (no heat is taken or evolved in dissolution)

• no shrinkage

• structurally similar systems exhibit ideal behavior

• surface tension, refractive index, viscosity, vapor pressure and weighted averages of each pure substance

• complete uniformity

real or non-ideal solution

forces of interaction between solvent/solvent, drug/drug, solvent/drug interactions are unequal

most pharmaceutical solutions

colligative property

alteration of the properties of pure solvent due to number of solute pieces within the solution. These properties predominantly relate to the absolute number of species as opposed to the nature of the solute

effect of solute on one colligative property

• lowering of vapor pressure

• boiling point elevation

• freezing point depression

• increased osmotic pressure

vapor pressure lowering

dissolution of a non-volatile solute in a solvent will reduce the vapor pressure of a liquid as the mole fraction increases

when 2 volatile solvents are mixed the vapor pressure will be the sum of the weighted mean of each solvent

Raoult’s law

ideal substances obey Raoult’s law (states the vapor pressure of a solution is proportional to the mole fraction of solvent)

boiling point elevation

Just a solute lowers the vapour pressure (reduces evaporation), it will also elevate the boiling point of the solutions (anti-boil)

ΔT_b = Elevated boiling point

K_b = Molal elevation constant

m = Molal concentration

Freezing point depression

Just as a solute lowers the vapor pressure (reduces evaporation), it will also depress the freezing point of the solutions (anti-freeze)

ΔT_f = Freezing point depression

K_f = Molal freezing point constant

m = Molal concentration

osmotic pressure

The pressure that must be applied to a solution to prevent the movement of solvent into it when solution and solvent are separated by a semi-permeable membrane (pressure due to the tendency of the solvent to move across a semi-permeable membrane from an area of low solute concentration to one of high concentration)

osmotic pressure

p  = Osmotic pressure (atm)

M = Molar concentration of solution

R = Ideal gas constant

T =  absolute temperature (K)

Tonicity: osmotic pressure in cells

isotonic: equal concentration of dissolved species inside the cell vs. outside the cell

Hypertonic: higher number of dissolved species outside the cell vs. inside the cell

Hypotonic: lower number of dissolved species outside the cell vs. inside the cell

dissociation influences colligative property

1m solution of a non-ionic depresses the freezing point of water by 0.512 degrees, but a ionizable substance will dissociate which influences the overall number of species in water