Liquid Dosage Forms: Pharmaceutical Solutions

LDFs are pourable pharmaceutical formulations containing active drug components and non-drug components (excipients) dissolved or suspended in a suitable solvent or solvent mixtures.
Purpose: Designed to provide maximum therapeutic response for populations with difficulty swallowing solid dosage forms (e.g., pediatric, geriatric patients) and/or to produce rapid therapeutic effects.
Preparation Methods:
- By dissolving the active drug substance in an aqueous or non-aqueous solvent (e.g., alcohol, ether, glycerin).
- By suspending the drug in an appropriate medium.
- By incorporating the drug substance into an oil or water phase.
Classification:
- Monophasic LDFs
- Biphasic LDFs

Definition: Liquid preparations containing one or more soluble chemical substances dissolved in a suitable solvent or mixture of mutually miscible solvents.
Physicochemical terms: Solutions can be prepared from any combination of solid, liquid, or gas.
Characteristics: Homogenous mixture where solute molecules are dispersed in a solvent.

Solid Solution (solvent is solid)
- Solid in solid: Example: Copper in gold = 12 karat yellow gold.
- Liquid in solid: Example: Mercury dissolved in silver = amalgam.
- Gas in solid: Example: Air dissolved in soap = floating soap.
Liquid Solution (solvent is liquid)
- Solid in liquid: Example: Salt in water.
- Liquid in liquid: Example: Alcohol and water.
- Gases in liquid: Example: Carbonated beverages.
Gaseous Solutions (solvent is gas)
- Solid in gas: Example: Sulfur in air.
- Liquid in gas: Example: Water vapor in air.
- Gas in gas: Example: Oxygenated air.

Expression: Grams of solute dissolving in milliliters of solvent.
USP Descriptive Terms of Solubility:
- Very soluble: Less than 1 part of solvent required for 1 part of solute.
- Freely soluble: 1 - 10 parts of solvent required for 1 part of solute.
- Soluble: 10 - 30 parts of solvent required for 1 part of solute.
- Sparingly soluble: 30 - 100 parts of solvent required for 1 part of solute.
- Slightly soluble: 100 - 1,000 parts of solvent required for 1 part of solute.
- Very slightly soluble: 1,000 - 10,000 parts of solvent required for 1 part of solute.
- Practically insoluble or insoluble: More than 10,000 parts of solvent required for 1 part of solute.

Particle size: Reducing particle size (comminution) increases surface area, thus increasing the rate of solution.
Agitation: Increases the rate of solution by removing the concentrated solution from the solute surface, allowing less concentrated solvent to interact.
Temperature: Heating a liquid increases molecular collision frequency between solvent and solute, leading to a more rapid solution.

Temperature:
- Endothermic reaction: Heat is absorbed; an increase in temperature increases solute solubility.
- Exothermic reaction: Heat is given off; an increase in temperature decreases solute solubility. Example: Methyl cellulose and calcium salts like ext{Ca(OH)}_2 are more soluble in cold water.
Molecular Structure: