Chp 8: Surface Active Agents (Surfactants)

Introduction to Surface Active Agents

  • Definition: Surface active agents, also known as surfactants, are molecules or ions that adsorb at interfaces, lowering the surface tension between liquids or between liquids and solids.
  • Amphiphiles: Surfactants are amphiphilic, possessing both hydrophilic (polar) and hydrophobic (non-polar) regions, allowing them to interact with both polar and non-polar solvents.
  • Applications: Used in pharmaceuticals as:
    • Wetting agents
    • Emulsifiers
    • Solubilizing agents
    • Antifoaming agents

Surfactant Behavior at Interfaces

  • Air-water Interface: Lipophilic chains of surfactants orient upwards into the air.
  • Oil-water Interface: Surfactant chains associate with the oil phase.

Classification of Surfactants

  1. Anionic Surfactants: E.g., Sodium dodecyl sulfate (SDS®)
  2. Cationic Surfactants: E.g., Cetyl trimethyl ammonium bromide (CTAB®)
  3. Ampholytic (Zwitterionic) Surfactants: E.g., Phospholipids
  4. Non-ionic Surfactants: E.g., Polyoxyethylene (Tween®)

Hydrophilic-Lipophilic Balance (HLB)

  • HLB Scale: Measures the hydrophilicity and lipophilicity of emulsifying agents.
    • Higher HLB indicates greater hydrophilicity.
    • Spans (sorbitan esters) have low HLB (lipophilic).
    • Tweens (polyoxyethylene derivatives of Spans) have high HLB (hydrophilic).

Micelles and Critical Micelle Concentration (CMC)

  • Amphiphilic Characteristics: Molecules contain both hydrophilic and hydrophobic parts.
  • Low Concentration: At low surfactant concentrations, amphiphiles exist as individual molecules.
  • Critical Micelle Concentration: As concentration increases, aggregation occurs forming micelles, which can contain 50 or more monomers.
  • Micelle Size: Each micelle has a diameter of about 50 Å, classifying it as colloidal.

Formation and Function of Micelles

  • Micelles: Formed in aqueous solutions from lipid molecules.
    • Structure: Spherical due to the amphipathic nature (both hydrophilic head and hydrophobic tail).
  • Role of Micelles: Increase solubility of materials that are normally insoluble or slightly soluble in the dispersion medium through solubilization.

Shapes of Micelles

  1. Spherical Micelle
  2. Cylindrical Micelle
  3. Bilayer (Laminar Micelle)

Experimental Work

  • Materials Required:
    • Salicylic acid powder, Tween 60, distilled water, phenol red indicator, volumetric and conical flasks, graduated pipettes, burette, filter paper, funnel, balance, NaOH solution (0.05N).
  • Objective: Investigate the effect of Tween concentration on the solubility of salicylic acid.
  • Critical Micelle Concentration (CMC): Concentration of monomers at which micelles form, crucial for understanding surfactant efficiency.

Procedure Overview

  1. Prepare Tween 60 solutions at varying concentrations (0%, 0.05%, 0.5%, 1%, 2%, 3%) using the stock solution (5%).
  2. Combine 25 mL of Tween 60 solutions with 0.25g salicylic acid in conical flasks.
  3. Shake each flask for 10 minutes and allow to settle for another 10 minutes.
  4. Filter if necessary, then titrate 10 mL of each filtrate with 0.05N NaOH using phenol red as an indicator. Record color change from yellow to pink as the endpoint.
  5. Plot salicylic acid solubility (mg/mL or g/100mL) against Tween 60 concentration.

Calculations

  • Molecular Weight: 138.1 g for salicylic acid corresponds to 1L of 1N NaOH.
  • Equivalence Calculations: 0.0069 g of salicylic acid is determined as the chemical factor for calculations.
  • Final Calculation: Chemical factor multiplied by endpoint gives the amount of salicylic acid dissolved per 10 mL medium.