OIA1008 SURFACE & INTERFACIAL PHENOMENON

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40 Terms

1

Surface Phenomena

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

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2

Interface

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

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3

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).

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4

Interfacial Tension

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

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5

Work of Cohesion (Wc)

The energy required to separate identical molecules in a liquid.

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6

Work of Adhesion (Wa)

The energy required to separate two different molecules.

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7

Spreading Coefficient (S)

Determines whether a liquid will spread or form droplets.

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

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8

Gas-Gas Interface

No real interface exists.

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9

Liquid-Gas Interface

Example: Water surface exposed to air.

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10

Liquid-Liquid Interface

Example: Oil and water in an emulsion.

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11

Solid-Liquid Interface

Example: Suspensions, drug adsorption on excipients.

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12

Solid-Gas Interface

Example: Aerosols, adsorption on solid surfaces.

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13

Solid-Solid Interface

Example: Powder particle interactions.

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14

Capillary Rise Method

Measures liquid rise in a capillary tube.

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

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15

Du Noüy Ring Method

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

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16

Wilhelmy Plate Method

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

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17

Bubble Pressure Method

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

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18

Drop Weight Method

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

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19

Pendant Drop Method

Uses drop shape analysis to calculate surface tension.

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20

Sessile Drop Method

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

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21

Temperature Increase

Decreases surface tension due to increased molecular motion.

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22

Addition of Electrolytes

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

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23

Surfactants

Lower surface tension by adsorbing at the interface.

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24

Impurities

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

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25

Surfactants (Surface-Active Agents)

Molecules with hydrophilic and hydrophobic regions that reduce surface tension.

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26

Critical Micelle Concentration (CMC)

The concentration above which surfactants form micelles.

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27

Types of Surfactants

Anionic (e.g., sodium lauryl sulfate)

Cationic (e.g., benzalkonium chloride)

Nonionic (e.g., Tween, Span)

Amphoteric (e.g., lecithin)

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28

Micelle Formation

Aggregation of surfactant molecules to reduce free energy.

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29

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

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30

Wetting Agent

Reduces contact angle, improves liquid spreading on solids.

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31

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

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32

Detergency

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

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33

Hydrophilic-Lipophilic Balance (HLB)

Scale from 0–20 indicating surfactant balance.

Low HLB (< 6) → W/O emulsifier

High HLB (> 8) → O/W emulsifier

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34

Emulsifiers

Reduce interfacial tension to stabilize emulsions.

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35

Foaming Agents

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

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36

Antifoaming Agents

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

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37

Surface Films

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

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38

Drug Absorption & Adsorption

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

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39

Emulsion Stability

Surfactants stabilize emulsions for drug formulations.

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40

Suspension Formulation

Wetting agents prevent particle aggregation in suspensions.

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