In short: pharmaceutics is the physical chemistry behind medicines, governing how a drug dissolves, flows, stays dispersed and degrades. FLN Tutors (Freelancer Network) offers one-to-one help with solubility, rheology, dispersed systems and stability kinetics, explained step by step. Matched to a verified specialist who knows your course, your university's marking criteria, and your deadline.
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Pharmaceutics is the physical and chemical science underlying dosage forms. It asks why a drug behaves as it does: what controls its solubility, how fast it dissolves, whether a suspension will stay dispersed, how a cream flows, and how quickly the active degrades.
It is the theory that formulation applies. For the design and manufacture of dosage forms see pharmaceutical formulation.
This page supports coursework. It is not a guide to preparing or using any medicine.
Most drugs are weak acids or weak bases, so their solubility depends on how ionised they are, and that depends on pH relative to the pKa.
The Henderson-Hasselbalch relationship gives the ratio of ionised to unionised species.
For a weak acid: pH − pKa = log([ionised] ÷ [unionised])
Worked example. A weak acid of pKa 4.0 in a solution at pH 6.0:
log(ratio) = 6.0 − 4.0 = 2, so the ratio is 10² = 100 : 1 ionised to unionised.
So 100 parts in 101 are ionised, which is 99.0% ionised and 1.0% unionised.
The ionised form is far more water-soluble; the unionised form crosses membranes more readily. This is the central tension in oral drug delivery: the conditions that dissolve a drug are often not the conditions that let it be absorbed. A weak acid is largely unionised in the stomach and ionised in the intestine, which is why absorption cannot be predicted from solubility alone.
Dissolution rate depends on surface area, the diffusion coefficient, the thickness of the diffusion layer, and the difference between saturation solubility and the concentration already in solution.
Practical consequences examined regularly:
Note the distinction examiners press on: solubility is how much dissolves at equilibrium, dissolution rate is how fast it gets there. A drug can be reasonably soluble yet dissolve far too slowly to be absorbed in the time available.
The partition coefficient describes how a compound distributes between an organic phase and water, and it is expressed as log P.
Worked example. A drug is shaken between equal volumes of octanol and water. At equilibrium, the octanol layer contains 80 mg and the aqueous layer 20 mg.
P = 80 ÷ 20 = 4, so log P = log 4 = 0.60
For ionisable compounds, log D is used instead, since it accounts for ionisation at a stated pH. Very low log P means poor membrane permeability; very high log P means poor aqueous solubility and often poor dissolution. Useful oral drugs generally fall in an intermediate range, which is the reasoning behind the familiar rule-of-thumb property filters.
| Behaviour | Description | Example type |
|---|---|---|
| Newtonian | Viscosity independent of shear rate | Water, simple syrups |
| Pseudoplastic (shear-thinning) | Viscosity falls as shear increases | Many gels and polymer solutions |
| Dilatant (shear-thickening) | Viscosity rises as shear increases | Concentrated dense suspensions |
| Plastic | Flows only above a yield value | Some ointments and pastes |
| Thixotropic | Thins with time under shear, recovers at rest | Well-formulated suspensions |
Thixotropy is the property a suspension formulator wants: thick at rest so particles do not settle, thin while being shaken and poured, then thick again. Explaining why that combination is desirable is worth more than naming the behaviour.
Suspensions. Settling follows Stokes' law: rate increases with the square of particle radius and with the density difference between particle and medium, and decreases with viscosity. So settling is slowed by reducing particle size, by raising viscosity, and by narrowing the density difference.
A flocculated suspension forms loose aggregates that settle quickly to a large, easily redispersed sediment. A deflocculated suspension settles slowly but forms a dense, hard cake that will not redisperse. Controlled flocculation is usually preferred, because a suspension that cannot be shaken back into uniformity cannot be dosed accurately. This trade-off is a standard exam question.
Emulsions. Two immiscible liquids, one dispersed in the other, stabilised by an emulsifier. Instability takes distinct forms: creaming, where the dispersed phase rises or sinks but remains as droplets and can be redispersed; flocculation of droplets; coalescence, where droplets merge irreversibly; and phase inversion. Creaming is reversible and coalescence is not, and confusing the two is a frequent error.
The HLB system classifies emulsifiers by their balance of hydrophilic and lipophilic character, with lower values suiting water-in-oil emulsions and higher values oil-in-water.
Most drug degradation in solution is treated as first order, so the rate depends on the concentration remaining.
For first-order decay: k = 0.693 ÷ t½
Example. A drug has a half-life in solution of 2 years at its storage temperature.
k = 0.693 ÷ 2 = 0.3465 per year
Shelf life is often quoted as t₉₀, the time to fall to 90% of the labelled content:
t₉₀ = 0.105 ÷ k = 0.105 ÷ 0.3465 = 0.303 years, about 3.6 months
Note how much shorter t₉₀ is than the half-life. Pharmaceutical shelf life is governed by the small permitted loss, not by half-life, and quoting a half-life as a shelf life is a serious misunderstanding.
For zero-order degradation the rate is constant and independent of concentration, which applies to suspensions of poorly soluble drugs where the saturated solution is continually replenished from solid.
| Route | Typical control |
|---|---|
| Hydrolysis | Reduce water content, control pH, use a dry dosage form |
| Oxidation | Antioxidant, inert headspace, chelate trace metals |
| Photolysis | Amber or opaque container, protective carton |
| Microbial growth | Preservative, low water activity, sterile manufacture |
| Physical change | Control temperature and humidity, avoid freeze-thaw |
Temperature dependence follows the Arrhenius relationship, which is what allows accelerated stability studies at elevated temperature to support a predicted shelf life. The prediction is only valid where the degradation mechanism does not change with temperature, and that assumption has to be stated rather than assumed silently.
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Pharmaceutics is the physical and chemical science underlying medicines, covering solubility, dissolution, partitioning, rheology, dispersed systems such as suspensions and emulsions, and the kinetics of degradation. It supplies the theory that formulation and dosage form design then apply in practice.
Pharmaceutics is the underlying science explaining why materials behave as they do. Formulation is the applied activity of turning an active ingredient into a dosage form using that science. Courses often teach them together, with pharmaceutics first because formulation decisions depend on it.
Most drugs are weak acids or bases, and their degree of ionisation depends on pH relative to their pKa. The ionised form is much more water-soluble, so solubility changes markedly along the gastrointestinal tract, which is why absorption varies with site as well as with the drug.
It relates pH, pKa and the ratio of ionised to unionised drug, so you can calculate what fraction of a weak acid or base is ionised at a given pH. That fraction determines both aqueous solubility and the ability to cross membranes.
Solubility is how much dissolves at equilibrium, a thermodynamic quantity. Dissolution rate is how quickly it gets into solution, a kinetic one. A drug can be adequately soluble yet dissolve far too slowly to be absorbed during its transit through the gut.
Because dissolution rate is proportional to the surface area exposed to the dissolution medium, and dividing a given mass into smaller particles greatly increases that area. Micronisation is therefore a standard approach for poorly soluble drugs, though it can worsen powder flow.
Sink conditions exist when the dissolved concentration stays well below saturation solubility, so the concentration gradient driving dissolution remains high. Dissolution tests use a large enough volume to maintain them, otherwise the measured rate would fall simply because the medium approaches saturation.
The partition coefficient is the ratio of a compound's concentration in an organic phase to that in water at equilibrium, usually reported as log P using octanol and water. It indicates lipophilicity and so predicts membrane permeability and likely solubility behaviour.
Log P describes the partitioning of the unionised species only. Log D is the distribution coefficient at a stated pH, accounting for all species present including ionised forms. For ionisable drugs, log D at physiological pH is the more informative value.
A thixotropic system thins when sheared and rebuilds its structure at rest. In a suspension this is ideal: it is thick enough standing on a shelf to stop particles settling, thin enough to shake and pour accurately, then thickens again afterwards.
Flocculated suspensions form loose aggregates that settle fast into a bulky sediment which redisperses easily on shaking. Deflocculated suspensions settle slowly but form a hard, dense cake that will not redisperse, making accurate dosing impossible, so controlled flocculation is normally preferred.
Stokes' law describes the settling rate of particles in a suspension, which increases with the square of particle radius and with the density difference between particle and medium, and decreases with viscosity. It identifies the three ways a formulator can slow settling.
Creaming is the separation of an emulsion's dispersed phase to the top or bottom while the droplets remain intact, so it is reversible on shaking. Coalescence is the merging of droplets into larger ones, which is irreversible and signals genuine breakdown of the emulsion.
The hydrophilic-lipophilic balance system assigns surfactants a number describing their relative affinity for water and oil. Lower values suit water-in-oil emulsions and higher values oil-in-water, so it guides emulsifier selection. It is a starting point used alongside experimental screening rather than a substitute for it.
Because pharmaceutical shelf life is usually defined as the time for content to fall to ninety per cent of the label claim, not fifty per cent. For first-order degradation that point arrives far sooner, so quoting a half-life as a shelf life badly overstates how long a product lasts.
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