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.