DISPERSE SYSTEMS PART 1 ✅

Surface tension

Force acting along the surface of a liquid that makes the surface behave like a stretched elastic membrane

Physical explanation of surface tension

Molecules at the surface experience unequal attraction and pull inward, creating surface contraction

Association between surface tension and lungs

Surface tension in alveoli must be lowered by surfactant to prevent collapse during exhalation

Condition caused by lack of lung surfactant

Neonatal respiratory distress syndrome due to insufficient pulmonary surfactant

What is missing in neonatal RDS

Pulmonary surfactant that lowers alveolar surface tension

Surface tension calculation

γ = F / (2L)

Methods to measure surface tension

Du Noüy ring method, capillary rise method, Wilhelmy plate method

Use of spreading coefficient

Determines whether one liquid will spread over another surface

What the spreading coefficient measures

Tendency of a liquid to spread on another phase

Spreading coefficient formula

S = γs – γl – γsl

Interpretation of spreading coefficient

If S > 0, spreading occurs; if S < 0, no spreading

Pharmaceutical products where spreading coefficient matters

Creams, ointments, lotions, skin oils, sunscreens, topical emulsions

Why measuring wetting tendency matters

Poorly wetting powders resist dispersion and form clumps in liquid formulations

Parameter used to measure wetting tendency

Contact angle

What the contact angle measures

The angle between solid surface and liquid drop reflecting degree of wetting

Is a slight contact angle good

A slight contact angle indicates good wetting

Meaning of large contact angle

Powder will float and resist wetting; poor dispersion expected

How wetting of powders is measured

Measure contact angle between powder surface and liquid

Why some powders cannot disperse in liquid

They are hydrophobic with very high contact angles

How to facilitate wetting

Add wetting agents (surfactants) to decrease contact angle and improve powder dispersion

Define zeta potential

Potential difference between the tightly bound ion layer around a particle and the bulk liquid

Grandma version of zeta potential

The amount of “electrical charge” particles carry that keeps them from sticking together

Difference between Nernst potential and zeta potential

Nernst is the surface potential; zeta is the potential at the slipping plane farther from particle surface

Why zeta potential is important

It predicts particle repulsion, aggregation, and suspension stability

How zeta potential affects suspension stability

High zeta potential → particle repulsion and stable suspension; low zeta potential → aggregation

Types of pharmaceutical suspensions

Oral suspensions, topical suspensions, injectable suspensions, otic/ophthalmic suspensions

Attributes of a good suspension

Redisperses easily, no caking, uniform particle size, appropriate viscosity, slow sedimentation

Define deflocculated suspension

Particles remain separate, settle slowly, form hard cake

Define flocculated suspension

Particles form loose aggregates, settle quickly but redisperse easily

Define structured and flocculated suspension

Suspension stabilized with polymer structure and loose floccules; best for stability

Best suspension type for stability

Structured flocculated suspension

Three steps of suspension formulation

Wetting of powder → controlled flocculation → adding structured vehicle

Role of wetting in suspension formulation

Removes air from particles and allows the liquid to fully contact powder surfaces

Agents used for wetting

Sodium lauryl sulfate, polysorbates (Tween), alcohol, glycerin

Structured/suspending polymers examples

Methylcellulose, carboxymethylcellulose, carbomer, xanthan gum

Agents used as structured vehicles

CMC, HPMC, carbomer, veegum, xanthan gum

Define floccule

Loose, fluffy cluster of aggregated particles

Why flocculation is desirable

Prevents formation of hard cake and allows easy redispersion

How to achieve flocculation

Add flocculating agents to partially neutralize particle charges

Good zeta potential range for flocculation

Between –10 mV and +10 mV

Agents used to form floccules

Electrolytes, surfactants, polymers

Which forms hard cakes

Deflocculated suspensions

Repulsive and attractive forces between particles

Van der Waals attraction and electrostatic repulsion determine aggregation behavior

Optimum distance between particles

Several angstroms; at this distance attraction is minimal and particles do not cake

What happens when particles get too close

Attractive forces dominate, causing aggregation and caking

How to distinguish flocculated vs deflocculated suspension

Flocculated: clear supernatant + fluffy sediment; Deflocculated: cloudy supernatant + hard cake