OIA1008 SURFACE & INTERFACIAL PHENOMENON I

Surface Tension

Force acting parallel to surface per unit length due to cohesive forces.

Unit: dynes/cm or N/m.

Interfacial Tension

Force per unit length between two immiscible liquids.

Usually lower than surface tension.

Surface Free Energy

Excess potential energy of molecules at the surface vs bulk.

Why Water Forms Droplets

Surface tension minimizes surface area → spherical shape.

Work Required to Increase Surface Area

dW=γ×2L×ds

Capillary Rise Method

γ=12rhρg

Requires clean, vertical capillary and non-viscous liquid.

Du Noüy Ring Method

Measures force to detach ring from surface:

γ=F⋅β2⋅circumference

Wilhelmy Plate Method

Uses rectangular plate to measure force and contact angle.

Influence of Temperature

Surface tension decreases with temperature due to weakened cohesion.

Use of Correction Factor (β)

Adjusts measured surface tension in Du Noüy method for ring geometry.

Spreading Coefficient (S)

S=γS−(γL+γLS)

Positive S

Spreading occurs (e.g., oleic acid on water).

Negative S

No spreading; forms lenses/globules (e.g., mineral oil on water).

Work of Adhesion (Wa)

Energy to separate unlike molecules:

Wa=γL+γS−γLS

Work of Cohesion (Wc)

Energy to separate like molecules:

Wc=2γL

Alternative Spreading Formula

S=Wa−Wc

Effect of Mutual Saturation

Can reverse spreading; excess forms floating lens.

Pharmaceutical Implication

Important for emulsions, suspensions, transdermal drug delivery.

Contact Angle (θ)

Angle between liquid droplet and solid surface.

Good Wetting

θ<90∘ (e.g., water on clean glass)

Poor Wetting

θ>90∘ (e.g., mercury on glass)

Young’s Equation

γS=γSL+γLcos⁡θ

Spreading Coefficient via Contact Angle

S=γL(cos⁡θ−1)

Work of Adhesion via θ

WSL=γL(1+cos⁡θ)

Definition (Wetting Agents)

Surfactants that reduce contact angle → promote wetting.

Examples of Wetting Applications

Powder suspensions, cotton pads, topical lotions, wound cleaning.

HLB Range for Wetting Agents

Optimal HLB = 6–9.

Tablet Wettability

Influences disintegration and dissolution.

Binders Compared

PVP, gelatin, tapioca at 5% w/w.

PVP Results

γ=71.23, cos⁡θ=0.7455, S=−18.13, Disintegration = 17 min.

Gelatin Results

cos⁡θ=0.7230, S = -19.73, Disintegration = 23.5 min.

Tapioca Results

cos⁡θ=0.7570, S=−17.33, Disintegration = 2.0 min (fastest).

Why Control Surface Tension?

Ensures uniform particle dispersion in suspensions/emulsions.

Activated Charcoal

Adsorbs toxins; used in poisoning but risk of vomiting or aspiration.

Emulsifiers & Surfactants

Lower interfacial tension → stabilize emulsions (e.g., Span, Tween).

Examples of Poor Wetting

Sulfur, charcoal—require wetting agents to suspend in liquid.

Spreading of Benzene on Water

γS=72.8, γL=28.9, γLS=35.0

→ S=8.9 dynes/cm (initially spreads)

Final Benzene Spreading

γS′=62.2, γL′=28.8

→ S′=−1.6 dynes/cm (lens forms)

General Rule

Initial S may be positive, but final S often becomes negative.

Clinical Relevance

Proper wetting influences topical formulations, oral disintegrating tablets, and dermal absorption.