Surface and Interfacial Phenomenon notes

Surface Tension (ST)

Force per unit length acting at surface at right angle (N/ meter) or ( Dyne/cm)
Indicate strength of Cohesive force (force between like molecules)
Examples:

‡ Formation of spherical globules in emulsion

‡ Shape of falling water drop

‡ Shape of mercury drop on flat surface

‡ Rise in capillary tube

Interfacial Tension (IT)

Force per unit length acting at interface at right angle (N/m)
Indicates strength of adhesive force (force between unlike molecules)

Factors affecting ST and IT

Temperature–T increases → Kinetic Energy increases → weakening of cohesive forces, hence ST decreases.
Electrolytes
Surface Active agents

Determination of ST and IT

1. Capillary rise method–Only ST can be determined.

2. DuNouy Tensiometer–Both ST and IT can be determined.

3. Bubble pressure

4. Drop weight or Drop count method–Both ST and IT can be determined.

Capillary rise method–Based on Young–Laplace Equation

P = 2Ƴ/r

P = hρg

So Ƴ = h ρ g × r/2

Where Ƴ is surface tension, ρ is density of liquid, h is height occupied by liquid, r is inside radius of capillary tube.

Drop weight method–Instrument used: Stalagmometer

Mg = 2 π r × Ƴ

Surface Free energy–It is the work required to increase the area of liquid by 1 cm2 .

Work done = Ƴ × 2L × d

Surface Free energy ΔG = Ƴ × Δ A

Spreading coefficient

Spreading coefficient (S) = Work of adhesion (Wa ) –Work of cohesion (Wb )

Work of adhesion Work required breaking the attraction between unlike molecules

(Wa ) = ƳL + ƳS – ƳLS

Work of cohesion Work required to separate the molecule of spreading liquid (Wb ) = 2 ƳL

S = Wa – Wb

S = (ƳL + ƳS – ƳLS) – 2 ƳL

S = ƳS – (ƳL + ƳLS)

Spreading occurs when S is positive, that is, Surface tension of sub-layer liquid is greater than sum of surface tension of spreading liquid and IT between sub-layer liquid and spreading liquid.
Initially, spreading coefficient may be positive or negative, but finally, it is always negative.

Adsorption

Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase.

Adsorbate: material being adsorbed.
Adsorbent: material doing the adsorbing. Examples are activated carbon or ion-exchange resin.
Surface excess can be defined as:

Where Volume is the volume of the solution from which the adsorption is occurring onto the surface with total surface area = surface area.

Surface excess is defined as the mass adsorbed per surface area. A more fundamental definition is given by the Gibbs relationship.

Where: µi = the molar free energy of solute i. Ci is the bulk concentration of this solute.

The Gibb’s expression simply uses Ƭ as a proportionality constant to relate the change in solute molar free energy to surface tension (y) during adsorption.

The underlying principle here is that for the adsorption process, changes in the sum of all solute free energy must be accounted for in changes in the surface tension during the adsorption process.

For a single solute:

Therefore,

Results in increases in Ƭ (surface concentration) dγ /dC < 0

Results in decrease in Ƭ dγ/ dC > 0

Types of adsorption

Exchange adsorption (ion exchange)–Electrostatic due to charged sites on the surface. Adsorption goes up as ionic charge goes up and as hydrated radius goes down.
Physical adsorption: Van der Waals attraction between adsorbate and adsorbent. The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface. This is relatively weak, reversible adsorption, capable of multilayer adsorption.
Chemical adsorption: Some degree of chemical bonding between adsorbate and adsorbent characterized by strong attractiveness. Adsorbed molecules are not free to move on the surface. There is a high degree of specificity and typically, a monolayer is formed. The process is seldom reversible

Generally, some combination of physical and chemical adsorption is responsible for activated carbon adsorption in water and waste water.

Adsorption Equilibria

If the adsorbent and adsorbate are contacted long enough, an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms.

qe = mass of material adsorbed (at equilibrium) per mass of adsorbent.

Ce = equilibrium concentration in solution when amount adsorbed equals qe.

qe /Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption, etc

Isotherm models

1. Langmuir isotherm

This model assumes monolayer coverage and constant binding energy between the surface and adsorbate. The model is:

Q0 a represents the maximum adsorption capacity (monolayer coverage) (g solute/g adsorbent).

Ce has units of mg/L.

K has units of L/mg

For the Langmuir model, linearization gives:

A plot of Ce /qe versus Ce should give a straight line with intercept:

2. BET (Brunauer, Emmett and Teller) isotherm

This is a more general, multi-layer model. It assumes that a Langmuir isotherm applies to each layer and that no transmigration occurs between layers. It also assumes that there is equal energy of adsorption for each layer except for the first layer.

CS = saturation (solubility limit) concentration of the solute. (mg/litre)

KB = a parameter related to the binding intensity for all layers.

3. Freundlich isotherm

For the special case of heterogeneous surface energies (particularly good for mixed wastes) in which the energy term, KF , varies as a function of surface coverage, we use the Freundlich model.

n and KF are system specific constants.

For the Freundlich isotherm, use the log-log version:

A log-log plot should yield an intercept of log KF and a slope of 1/n.

Factors which affect adsorption extent (and therefore affect isotherm) are:

1. Adsorbate:

In general, as solubility of solute increases the extent of adsorption decreases. This is known as the “Lundelius’ Rule”. Solute-solid surface binding competes with solutesolvent attraction.

Factors which affect solubility include molecular size (high MW-low solubility), ionization (solubility is minimum when compounds are uncharged), polarity (as polarity increases get higher solubility because water is a polar solvent).

2. pH:

pH often affects the surface charge on the adsorbent as well as the charge on the solute. Generally, for organic material, as the pH goes down, adsorption goes up.

3. Temperature:

Adsorption reactions are typically exothermic i.e., H rxn is generally negative. Here heat is given off by the reaction therefore as T increases extent of adsorption decreases.

4. Presence of other solutes:

In general, get competition for a limited number of sites therefore get reduced extent of adsorption or a specific material.

Wetting

Intimate contact between solid and liquid or liquid and liquid

Application

Initial step in preparation of emulsion and suspension
In granulation process
Film coating requires wetting and spreading of liquid over tablet surface

Surface and Interfacial Phenomenon notes / Types of adsorption physical pharmacy notes / Surface tension Notes / PDFS Notes