interfacial Phenomena

chemistry

cohesive forces

like molecules - within the bulk - hold a phase together

yLS :

Interfacial tension between the sublayer and the spreading liquid

yL:

Surface tension of the spreading liquid

yS

Surface tension of the sublayer liquid

adhesive forces

unlike molecules - occur across a boundary of contact

molecules at the surface

cohesive and adhesive forces

Liquid-gas interface

the adhesive forces are small

the surface free energy

the work required to increase the surface area by 1m2

surface tension

surface of separation between a liquid and air saturated with vapour

interfacial free energy

work required to increase the interface by 1m2

Interfacial tension is < surface tension

Antonoffs rule

the interfacial tension between 2 mutually saturated liquids (A and B) is equal to the difference the surface tensions of each of the two saturated liquids measured against air

YAB = YA - YB

Measurements of surface and interfacial tensions

Capillary rise method
DuNouy Tensiometer
Wihemy plate method
Maximum bubble pressure method
Drop weight method
The stalagmometer

The Capillalry Rise Method

if the capillary tube has a radius r: y = rhpg
where
p = density of liquid
g = acceleration due to gravity
h = height

It can be used to measure surface tension but not interfacial

Du Nouy tensiometer

y = dial reading in dynes / (2x the ring circumference) x correction factor

Wilhemy Plate Method

This method also makes use of a tensiometer but with a plate attached. The plate (glass or platinum) is oriented perpendicular to the surface or interface. The plate makes contact with the surface of the liquid. The liquid will wet the Wilhelmy plate upwards and in this case the surface tension acts along the perimeter of the plate

the surface tension of water

decreases as temperature rises

Spreading of one liquid over another

when a drop of one liquid (e.g. oleic acid) is placed on the surface of another immiscible liquid (e.g. water), it may remain as a lens of it may spread over the surface to form a thin "duplex film".

S = ys - (yL +yLs)

YS = surface tension of the sublayer liquid
YL = surface tension of the spreading liquid
YLS = interfacial tension between the sublayer and the spreading layer
S = spreading coefficient

if S is positive spreading occurs

Solid - liquid interfaces

wetting of solids

if liquid is poured onto a solid surface it may spread over the surface spontaneously in which case it is said to "wet" the solid layer or alternatively it may remain as cohesive globule and not form a spreading wetting layer

The term "wetting" refers to the displacement from a surface of one fluid by another.

The term is most commonly applied to the displacement of air from a liquid or solid surface by water or anaqueous solution.

The term "wetting agent" is applied to any substance that increases the ability of water or an aqueous solution to displace air from a liquid or solid surface.

Contact Angles and the Wetting of Solids

A drop of liquid when placed on a solid surface will come to equilibrium and forms an angle of contact (q)between the liquid and the solid.

The angle is measured through the liquid.

If θ is

If θ is \>90 degrees wetting does not take place

The critical surface tension (γc)

the surface tension of a liquid that would completely wet the solid of interest

Zisman plot to determine yc

Young's equation

Want to maximise Cosθ -A wetting agent or surface active agent (surfactant) can decrease the contact angle by altering gGL or gSL

y GS = y SL + y GL Cos (angle)

Surfactant Molecular Structure:

Surfactants are compounds that:
- are amphiphilic

Hydrophilic head group and hydrophobic tail group.

their water-insoluble hydrophobic group may extend out of the bulk water phase, into the air or into the oil phase, while the water-soluble head group remains in the water phase;

- lower the surface tension between liquid and air by adsorbing at the liquid-gas interface;

- lower the interfacial tension between two liquids by adsorbing at the liquid-liquid interface, (or between a liquid and a solid);

- may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants

A wetting agent or surface active agent (surfactant) may be added to the liquid phase to promote the lowering of γGL, also adsorbed onto the surface of the powder can reduce γSL.

Surfactants almost always cause a reduction in γGL-surfactants molecules will insert themselves in the air-liquid surface and interfere with liquid-liquid cohesive interactions, reducing the surface tension.

The effect on the interfacial tension depends on the nature of the interactions

If the surfactant molecules orient at the solid-liquid interface with their polar ends towards the substrate and hydrophobic ends towards the liquid, the wettability of an aqueous solution will be reduced.

However, at higher concentration of surfactant, the surfactant ions adsorb by hydrophobic interaction with the already adsorbed layer, thus exposing their hydrophilic ends to the solution such that the surface becomes more readily wetted.

Thus the contact angle may increase and subsequently decrease following the addition of increasing concentrations of surfactant to a solution.

In contrast, if the surfactant molecules are oriented with their hydrophilic groups towards the liquid, the hydrophilicity of the substrate is increased and it becomes more wettable.

The Importance of Contact Angles

Wettability of powders for suspension formulations

Influence on dissolution

slow release tablets

Transdermal drug delivery, topical lotions etc.

Displacement of dirt and debris by using detergents (surfactants)

Adsorption ≠ Absorption

Adsorption is a surface or interfacial phenomenon only.

Adsorbent: solid on which adsorption takes place

Adsorbate: substance which is adsorbed

Adsoprtion at L-G Surfaces and L-L Interfaces:

Substances which are adsorbed at solid-liquid, liquid-liquid and liquid-gas interfaces are called surface active agents or surfactants (sometimes also called tensides).

amphiphiles

hydrophilic-lipophilic balance (HLB).

e.g. sodium stearate, myricyl alcohol

Adsorbed substances may be said to form either "soluble monolayers" or "insoluble films" depending on the solubility of the adsorbate in the liquid subphase.

Uses ofsurfactants: as wetting agents, emulsifying agents, solubilising agents, detergents

Gibbs Adsorption Equation in Relation to Soluble Monolayers:

Γis the surface excess concentration of surfactant:
i.e., the amount of surfactant per unit area of surface in excess of that in the bulk of liquid at a particular bulk concentration.

For non-ionic surfactants e.g. cetomacrogol:

Γ = (-c÷RT) x ( dγ÷dc)

C = the concentration in the bulk liquid.
R = the gas constant
T = the absolute temperature
dγ÷dc=the change in surface tension of the solution with change of bulk concentration.

For ionic surfactants e.g. sodium lauryl sulphate, cetrimide:
Γ=-(1 ÷xRT1) (dγ ÷ dlnc)
X=1 in diluted solution
X= 2 in concentrated solution

Adsorption at Solid-Liquid and Solid-Gas Interfaces:

Adsorption at S-L interfaces may be considered as and attempt to reduce the surface free energy of the solid.

A solid on which adsorption takes place is called the adsorbent.

The substance which is adsorbed is the adsorbate.

Adorption phenomena may be the result of a physical or chemical process.

Physical adsorption (or van der Waals' adsorption):

relatively weak interaction between adsorbent and adsorbate.

-Tends to multi layered adsorption

Chemical adsorption (chemisorption):

-Involves a degree of specific chemical interaction between adsorbent and adsorbate.

-New chemical bonds are generated

-Mono layer adsorption only

-ie. Rust (oxidation), catalysis(hydrogenation)

Physical Adsorption:

1.Weak physical forces and low heat of adsorption
2.Exothermic
3.Nonspecific
4.Tends to multi layered adsorption
5.Dynamic equilibrium
6.Spontaneou

Chemical Adsorption:

1.Strong chemical bonds and higher heat of adsorption
2.Endothermic
3.Specific
4.Monolayers only usually formed
5.No dynamic equilibrium
6.Not spontaneous

Applications of Gas Adsorption

removal of odours

surface area and particle size measurement of powders

operation of gas masks

Applications of Liquid Adsorption:

decolorizing solutions

adsorption chromatography: a separation technique

filtration

wetting

detergency

adsorption of noxious substances from the alimentary tract

Factors Affecting Adsorption:
Solubility of adsorbate:
Lundelius' rule :

the extent of adsorption of a solute is inversely proportional to its solubility in the solvent from which adsorption occurs.

In order for adsorption to occur solute-solvent bonds must first be broken.

pH of the solution:

Adsorption is generally increased as ionization of the adsorbate is suppressed. The unionized form of most drugshas a lower solubility in aqueous solution.

Pressure of gas or concentration of a solution:

Freundlich adsorption isotherm x/m = Kp1/n

x/m = Kc1/n

X/m= amount adsorbed per unit mass of adsorbent
C= equilibrium concentration
P= equilibrium pressure K and n are constants

Temperature:

Physical adsorption is generally an exothermic process and so the amount adsorbed usually decreases as temperature increases whereas chemical adsorption increases as temperature increases (endothermic process).

Nature of the adsorbent:
Surface area:

the amount of adsorbate adsorbed is proportional to the surface area of the solid

Affinity between adsorbent and adsorbate:

large molecules and highly surface active substances are more readily adsorbed

if the surfactant molecules orient at the solid-liquid interface with their polar ends towards the substrate and hydrophobic ends towards the liquid, the wettability of an aqueous solution will be reduced.

- This orientation of surfactants molecules at the surface occurs if they are adsorbing to ionic or polar substrates.

at higher concentration of surfactant, the surfactant ions adsorb by hydrophobic interaction with the already adsorbed layer, thus exposing their hydrophilic ends to the solution such that the surface becomes more readily wetted.

- Thus the contact angle may increase and subsequently decrease following the addition of increasing concentrations of surfactant to a solution