Introduction to Surface and Interfacial Phenomenon
Syllabus & Topics
- 1Liquid Interface: Definition; two types of interfaces (liquid-liquid, liquid-air).
- 2Surface Tension (γ): Inward pull on surface molecules. Units: mN/m or dynes/cm. Water: 72 mN/m at 25°C.
- 3Interfacial Tension: Tension at the interface of two immiscible liquids (lower than either surface tension).
- 4Surface Free Energy: Work done per unit area to extend surface. γ = dG/dA.
- 5Measurement of Surface Tension: Capillary rise method, Drop weight/stalagmometer method, Ring method (du Noüy tensiometer), Wilhelmy plate method.
- 6Measurement of Interfacial Tension: du Noüy ring method.
- 7Spreading Coefficient (S): S = γ(liquid-air) – [γ(oil-air) + γ(oil-water)]. If S > 0, spontaneous spreading occurs.
- 8Adsorption at Liquid Interfaces: Gibbs adsorption isotherm. Positive adsorption = surfactant enrichment at surface (lowers surface tension).
- 9Surface Active Agents (Surfactants): Definition, structure (amphiphilic – hydrophilic head + hydrophobic tail).
- 10Classification of Surfactants: Anionic (SDS, Sodium lauryl sulfate), Cationic (Benzalkonium chloride), Non-ionic (Tweens, Spans), Amphoteric (Lecithin).
- 11Critical Micelle Concentration (CMC): Concentration above which surfactants form micelles (aggregates).
- 12HLB Scale (Griffin’s method): Numerical 0-20 scale quantifying surfactant hydrophilicity. HLB 3-6: W/O emulsifiers (Spans). HLB 8-18: O/W emulsifiers (Tweens). HLB 7-9: Wetting agents. HLB 15-18: Solubilizers. HLB calculation: HLB = 20 × (1 – S/A).
- 13Solubilisation: Incorporation of a water-insoluble drug into micelles using surfactants above CMC. Applications: Vitamins A, D, E, K in aqueous formulations.
- 14Detergency: Mechanism – wetting, oil removal, emulsification, dispersion, and preventing re-deposition.
- 15Adsorption at Solid Interface: Adsorption from solution on solid surfaces.
- 16Freundlich Adsorption Isotherm: x/m = K × C^(1/n). Log (x/m) = log K + (1/n) log C. Empirical, applicable at moderate concentrations.
- 17Langmuir Adsorption Isotherm: Monolayer adsorption model. x/m = (a × C)/(b + C). Linear form: C/(x/m) = 1/ab + C/a. Assumptions: uniform surface, no interaction between adsorbed molecules.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is the HLB Scale and how is it used?
The HLB (Hydrophilic–Lipophilic Balance) scale ranges from 0 to 20 and classifies surfactants based on the balance between their hydrophilic and hydrophobic portions. Surfactants with low HLB values (1–6) are more lipophilic and are typically used as water-in-oil (W/O) emulsifiers, such as Sorbitan monostearate (HLB = 4.7). Surfactants with high HLB values (8–18) are more hydrophilic and are used as oil-in-water (O/W) emulsifiers, such as Polysorbate 80 (HLB = 15). In formulation practice, surfactants are often blended to match the Required HLB of the oil phase to obtain a stable emulsion.
Q2. What is the Critical Micelle Concentration (CMC)?
Critical Micelle Concentration (CMC) is the minimum concentration of a surfactant above which surfactant molecules aggregate to form micelles. Below the CMC, surfactant molecules exist as individual monomers in solution, while above the CMC, micelles form and significantly enhance solubilization capacity. Surface tension decreases sharply as the concentration approaches the CMC and then becomes nearly constant once micelles begin forming.
Q3. What is the Spreading Coefficient?
The spreading coefficient (S) determines whether a liquid will spread over a solid surface and is given by the equation S = γ_L – (γ_S + γ_LS), where γ_L is the surface tension of the liquid, γ_S is the surface tension of the solid, and γ_LS is the interfacial tension between the liquid and solid. If S > 0, the liquid spreads spontaneously over the surface, whereas if S < 0, the liquid forms droplets and does not spread. This concept is particularly important in pharmaceutical coating, wetting, and adhesion processes.
Q4. What is the difference between Freundlich and Langmuir isotherms?
The Freundlich isotherm is an empirical model that applies over a range of concentrations and allows multilayer adsorption, expressed as x/m = KC^(1/n). In contrast, the Langmuir isotherm is a theoretical model that assumes monolayer adsorption on uniform surfaces with no interaction between adsorbed molecules and a definite saturation point, expressed as x/m = aC/(b + C). Thus, Freundlich describes heterogeneous surface adsorption, while Langmuir describes ideal monolayer adsorption on homogeneous surfaces.
Q5. What is Solubilisation in Pharmacy?
Solubilisation is the process of increasing the apparent solubility of a poorly water-soluble drug by incorporating it into the hydrophobic core of micelles formed by surfactants above their CMC. For example, Polysorbate 80 is widely used to solubilize fat-soluble vitamins such as A, D, E, and K in aqueous solutions and injectable formulations, thereby improving their stability and bioavailability.