OIA1008 SURFACE & INTERFACIAL PHENOMENON

Surface Phenomena

Study of interactions at interfaces (solid-liquid, liquid-gas, liquid-liquid).

Interface

The boundary between two phases (e.g., solid-liquid, gas-liquid).

Surface Tension (γ)

The force per unit length acting at the surface of a liquid to counterbalance inward molecular pull (unit: dynes/cm or N/m).

Interfacial Tension

The force per unit length at the interface between two immiscible liquids.

Work of Cohesion (Wc)

The energy required to separate identical molecules in a liquid.

Work of Adhesion (Wa)

The energy required to separate two different molecules.

Spreading Coefficient (S)

Determines whether a liquid will spread or form droplets.

S = Wa - Wc (If S > 0, spreading occurs).

Gas-Gas Interface

No real interface exists.

Liquid-Gas Interface

Example: Water surface exposed to air.

Liquid-Liquid Interface

Example: Oil and water in an emulsion.

Solid-Liquid Interface

Example: Suspensions, drug adsorption on excipients.

Solid-Gas Interface

Example: Aerosols, adsorption on solid surfaces.

Solid-Solid Interface

Example: Powder particle interactions.

Capillary Rise Method

Measures liquid rise in a capillary tube.

Formula:γ=12rhρgγ = \frac{1}{2} r h ρ g.

Du Noüy Ring Method

Measures force needed to detach a platinum-iridium ring from the surface.

Wilhelmy Plate Method

Measures force required to detach a plate from the liquid surface.

Bubble Pressure Method

Determines surface tension by observing the pressure of a forming bubble.

Drop Weight Method

Measures the weight of a single drop falling from a capillary.

Pendant Drop Method

Uses drop shape analysis to calculate surface tension.

Sessile Drop Method

Analyzes the angle of a liquid drop on a solid surface.

Temperature Increase

Decreases surface tension due to increased molecular motion.

Addition of Electrolytes

Can either increase or decrease surface tension, depending on ion interactions.

Surfactants

Lower surface tension by adsorbing at the interface.

Impurities

Hydrophilic impurities increase surface tension, while hydrophobic ones reduce it.

Surfactants (Surface-Active Agents)

Molecules with hydrophilic and hydrophobic regions that reduce surface tension.

Critical Micelle Concentration (CMC)

The concentration above which surfactants form micelles.

Types of Surfactants

Anionic (e.g., sodium lauryl sulfate)

Cationic (e.g., benzalkonium chloride)

Nonionic (e.g., Tween, Span)

Amphoteric (e.g., lecithin)

Micelle Formation

Aggregation of surfactant molecules to reduce free energy.

Spreading

Occurs when a liquid distributes over another surface.

S = γS - (γL + γLS)

γS = surface tension of sublayer

γL = surface tension of liquid

γLS = interfacial tension

Wetting Agent

Reduces contact angle, improves liquid spreading on solids.

Contact Angle (θ)

The angle between a liquid droplet and solid surface.

θ = 0° → Complete wetting

θ < 90° → Good wetting

θ > 90° → Poor wetting

θ = 180° → No wetting

Detergency

The ability of surfactants to remove dirt or oil from surfaces.

Hydrophilic-Lipophilic Balance (HLB)

Scale from 0–20 indicating surfactant balance.

Low HLB (< 6) → W/O emulsifier

High HLB (> 8) → O/W emulsifier

Emulsifiers

Reduce interfacial tension to stabilize emulsions.

Foaming Agents

Trap gas in liquid, forming a stable foam (e.g., Sodium lauryl sulfate).

Antifoaming Agents

Reduce foam formation (e.g., silicone oils).

Surface Films

Thin molecular layers at an interface (e.g., monolayers in pulmonary surfactants).

Drug Absorption & Adsorption

Surface-active agents enhance or reduce drug absorption on excipients.

Emulsion Stability

Surfactants stabilize emulsions for drug formulations.

Suspension Formulation

Wetting agents prevent particle aggregation in suspensions.