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surface and interfacial tension Cheat Sheet (DRAFT) by

surface and interfacial tension

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Surface and Interface

surface
boundary between 2 phases, one of them is in the gaseous state
interface
boundary between 2 immiscible phases has context menu

Types

gas-liquid
surface tension
gas-solid
adsorption
e.g., antifl­atu­lents
liquid­-liquid
interf­acial tension
e.g., emulsions
solid-­liquid
wetting
e.g., tablets

Surface tension definition and units

Definition
Force per unit length that must be applied to counte­rba­lance the net inward pull.
 
the work done to increase the surface area of a mass of liquid by 1 cm2. (Units: erg/cm2)
Units
dyne/cm
 
Nm-1
 
mNm-1
 
erg/cm2

surface tension (γ)

Cohesive
interm­ole­cular forces between like molecules
Adhesive
interm­ole­cular forces between unlike molecules
inward force
Molecules of the liquid at the surface exhibit an inward force toward the bulk, therefore contract the surface & pull it towards the inside.

Net force

Surface molecule
somewhat down because of the inward force
Bulk molecule
zero (cancel each others)

Examples on ST

1. Water film on hand Water
2. Water dipping from a tap
3. Water beading on a leaf
4. Soap bubbles
5. Paper clip on water surface
6. Coin on water surface
 

ST of water

Unit
73 dyne/cm at 25C
Why are water droplets spherical?
High stability, low energy, low SA
 
A sphere has the smallest possible surface area to volume ratio
Surface free energy
Surface > Bulk
 
The surface layer of a liquid possesses additional energy as compared to the bulk liquid. This energy increases when the surface of the same mass of liquid increases.

Cleansing effect between hot and cold water

There is an inverse relation between surface tension & temper­ature
1. high temp.
2. more kinetic energy
3. weakens cohesion
4. lower ST, which is defined as specific temp.
Hot water is better cleaning agent than cold water, because it has lower ST, so can get better into pores and fissures, while cold wate can form only bridges between them

IT (Inter­facial Tension)

Definition
the force per unit length existing at the interface between two immiscible liquid phases
 
the work required to separate the 2 sections of liquids.
Unit
dyne/cm
 
erg/cm2
The interf­acial tension reflects the intera­ction between the 2 phases:
1. high IT
 
2. low molecular intera­ctions
 
3. low adhesive
 
4. high cohesive
 
5. immiscible liquids
completely miscible
high intera­ctuins
immiscible
low intera­ctions
 

Measur­ements of ST and IT

1. Capillary rise method
2. Du Nouy ring method
3. Wilhelmy plate method
4. Pendent drop method
5. Spinning drop method
6. Bubble pressure method

1. Capillary rise method

Capillary placed in a liquid contained in a beaker, the liquid generally rises up the tube a certain distance
liquid rises due to the ST, at some point it will stop due to its weght. so the upwaed movement will be balanced by the downward force of gravity
ST determined by measuring the rise in the capillary
ST= 0.5 rhpg
r = radius, h = hight, p = density, g = gravity

2. DuNouy ring method

slowly lifting a platin­um-­irr­idium ring from the surface of a liquid.
The force required to detach the ring from the liquid surface is recorded in dynes on a calibrated dial.
This force is equivalent to the maximum pull exerted on the ring by the surface)

3. Wilhelmy plate method

a thin plate usually made from glass or platin­um-­ori­ented perpen­dicular to the interface and attached to a scale or balance via a thin metal wire
The force on the plate due to wetting is measured and used to calculate the surface tension.

4. Pendant drop method (Drop weight)

A liquid drop hanging on the bottom of a capillary tube starts to fall when the weight of the drop is in an equili­brium state with the surface tension.
The drop is falling when the weight (mg) is equal to the circum­ference (2πr) multiplied by the surface tension (σ).
mg = 2 π r σ
we can use a reference liquid of known surface tension (mostly using water as a reference) to compare with the liquid which we are interested in

Methods used to measure IT between 2 liquids

 

Solid-­liquid interface (Contact angle θ)

If a drop of liquid is placed on a flat, smooth, horizontal solid surface, it forms a drop.
contact angle (θ): drop will exhibit a definite angle against the solid. It’s contact angle (θ): the angle the tangent to the liquid surface makes with the solid surface over which it spreads.
θ is determined by the intera­ctions across the three interf­aces; solid/­liq­uid­/gas.
θ depends on the nature of both the solid surface and the liquid drop.
θ (range of 0 = 180)
θ for pure water-pure silver is 90°
Contact angle differs depending on surface type "even same types of drops”
θ reflects wetting of the surface
On contact of a liquid droplet with a surface, the behavior of the liquid drop will depend on the balance between the cohesive forces of the liquid and the adhesive forces with the solid surface.
If the attractive forces between the liquid & solid exceed the cohesive forces in the liquid drop > low contact angle (α < 90°) > good wetting
e.g., water drop on glass surface
High θ = bad wetting