Solutions, Solubility, and Pharmaceutical Applications: Study Notes for Anatomy & Physiology Students

Solutions and Dispersions

Definition and Classification of Solutions

A solution is a chemically and physically homogeneous mixture of two or more substances, typically consisting of a solute dissolved in a solvent. Solutions are classified based on the size of the dispersed particles:

• Molecular Dispersions (True Solutions): Dispersed particles are molecules or ions, with sizes less than 1 nm. Examples: sodium chloride in water.

• Colloidal Dispersions: Particle sizes range from 1 nm to 500 nm. Examples: albumin in water.

• Coarse Dispersions: Particle sizes range from 1 μm to 100 μm. Examples: suspensions and emulsions.

Applications: Solutions are used for drug delivery, as vehicles for administration, and in various pharmaceutical preparations.

Solubility and Saturation

Definitions and Types of Solutions

Solubility is the concentration of solute in a saturated solution under specific conditions. Solutions can be:

• Saturated: Contains the maximum amount of solute at a given temperature and pressure.

• Supersaturated: Contains more dissolved solute than saturation allows, often unstable.

• Unsaturated: Contains less solute than required for saturation.

Expressions of Solubility

• g of solute dissolved in mL of water

• mL of a saturated solution

• molarity of a saturated solution

Pharmacopeial Expressions of Solubility

Extent of Solubility	Parts of Solvent Required for 1 Part of Solute
Very soluble	Less than 1 part
Freely soluble	1 to 10 parts
Soluble	10 to 30 parts
Sparingly soluble	30 to 100 parts
Slightly soluble	100 to 1000 parts
Very slightly soluble	1000 to 10,000 parts
Practically insoluble/insoluble	More than 10,000 parts

Solvent-Solute Interactions

Types of Solvents

• Polar solvents: High dielectric constants, dissolve ionic and polar compounds. Example: water.

• Semi-polar solvents: Intermediate dielectric constants, can induce polarity in non-polar molecules. Examples: alcohols, ketones.

• Non-polar solvents: Low dielectric constants, dissolve non-polar substances. Examples: hydrocarbons, oils.

Mechanisms of Solubility

• Solvation (Hydration): Water molecules surround and stabilize ions or molecules.

• Ionization: Polar solvents can ionize covalent bonds, increasing solubility.

• Hydrogen-bond formation: Solvents with hydrogen-bonding ability can dissolve compounds capable of forming hydrogen bonds.

• Solvent action of non-polar liquids: Non-polar solvents dissolve non-polar compounds via van der Waals forces.

Factors Affecting Solubility and Rate of Solution

Noyes-Whitney Equation

The Noyes-Whitney equation describes the rate of dissolution of a solute particle in a solvent:

$\frac{dC}{dt} = KA (C_s - C)$

where:

• $\frac{dC}{dt}$ = rate of change of concentration of solute in solution

• $K$ = dissolution rate constant

• $A$ = surface area of the solute particle

• $C_s$ = concentration of solute in the diffusion layer (saturated)

• $C$ = concentration of solute in the bulk solution

Key Factors:

• Increasing agitation reduces the thickness of the diffusion layer, increasing dissolution rate.

• Larger surface area of solute increases dissolution rate.

• Higher solubility of the solute increases the rate of dissolution.

• Reducing viscosity of the medium increases dissolution rate.

Temperature Effects

• For most solids, solubility increases with temperature.

• For some salts (e.g., calcium hydroxide), solubility decreases with temperature.

• For non-electrolytes, solubility is not affected by temperature.

Effect of Electrolytes and Complex Formation

• Electrolytes can increase or decrease solubility of non-electrolytes.

• Complex formation (e.g., KI with I2) can enhance solubility.

Example of Complex Formation:

$2I + HgI_2 \rightarrow K_2[HgI_4]$

$AgCl + NH_3 \rightarrow [Ag(NH_3)_2]Cl$

pH Adjustment and Salt Formation

Solubility of weak acids and bases can be affected by pH. The relationship is:

$S = S_0 \left(1 + \frac{K_a}{[H^+]}\right)$

where $S$ is solubility, $S_0$ is intrinsic solubility, $K_a$ is the acid dissociation constant, and $[H^+]$ is hydrogen ion concentration.

• Increasing pH increases solubility of weak acids.

• Decreasing pH increases solubility of weak bases.

Crystalline Versus Amorphous Forms

• Amorphous forms of a compound are generally more soluble than crystalline forms.

• Polymorphic forms can demonstrate different physical properties, including solubility.

Pharmaceutical Solubility Applications

Solubility of Liquids in Liquids

• Completely Miscible Systems: Mix in all proportions. Example: water and alcohol.

• Partially Miscible Systems: Limited solubility, affected by temperature. Example: phenol and water.

Partition of Solutes Between Immiscible Liquids

When a solute is added to a system of two immiscible liquids, it distributes between the two phases according to the partition coefficient:

$P = \frac{C_1}{C_2}$

where $P$ is the partition coefficient, $C_1$ and $C_2$ are the concentrations of the solute in each phase.

• If $P > 1$, the solute is more soluble in the first phase (often organic).

• If $P < 1$, the solute is more soluble in the second phase (often aqueous).

Summary Table: Types of Dispersions

Type	Particle Size	Example
Molecular Dispersion	< 1 nm	Sodium chloride in water
Colloidal Dispersion	1 nm – 500 nm	Albumin in water
Coarse Dispersion	1 μm – 100 μm	Suspensions, emulsions

Key Terms

• Solution: Homogeneous mixture of solute and solvent.

• Solubility: Maximum amount of solute that can dissolve in a solvent at specific conditions.

• Saturated Solution: Contains maximum solute possible at given conditions.

• Partition Coefficient: Ratio of solute concentrations in two immiscible liquids.

• Diffusion Layer: Thin layer surrounding solute particle affecting dissolution rate.

Example Application: Understanding solubility principles is essential for drug formulation, absorption, and delivery in Anatomy & Physiology.