interfacial phenomena

а, б) – inside the volume of liquid

в) – in the surface layer

gas

liquid

The surface tension of most liquids decreases with increasing temperature

Measurement of surface tension by Stalagmometerdrop-weight methods

n0, ρ0, σ0 – number of droplets, density and surface tension of water,

n, ρ, σ – …… of investigated liquid

0

00

n

n

grh2

1

Where g is gravity = 9,8 m/s2

For zero contact angle,

For the rise of a liquid up a narrow capillary

N.B. In practice, the capillary rise method is only used when the contact angle is zero, owing to the uncertainty in measuring contact angles correctly

Involuntary surface phenomena

Cohesion is the interaction between moleculars inside one phase (homogeneous system).

Adhesion is the interaction between moleculars inside of the different phases

Heterogeneous formation of a new phase

Spreading of the liqid on the surface of other liquid

Formation of spherical drops

Absorption is the process of arbitrary absorption of the substance by volume

Chemisorption - chemical interaction adsorbent with adsorbate

Difference between Adsorption and AbsorptionDifference between Adsorption and Absorption

POSITIVE AND NEGATIVE ADSORPTION

In certain cases - solutions of electrolytes, sugars, etc. - smallincreases in surface tension due to negative adsorption are noted. Here, because the solute-solvent attractive forces are greater than the solvent-solvent attractive forces, the solute molecules tend to migrate away from the surface into the bulk of the liquid.

SpreadingAdhesion and cohesionThe work of adhesion between two immiscible liquids is equal to the work required to separate unit area of the liquid-liquid interface and form two separate liquid-air interfaces (Figure: Work of adhesion (a) and of cohesion (b), and is given by the Dupre equation

Spreading of one liquid on another

When a drop of an insoluble oil is placed on a clean water surface, it may behave in one of three ways:1. Remain as a lens, as in Figure 4.16 (non-spreading).2. Spread as a thin film, which may show interference colours, until it is uniformly distributed over the surface as a 'duplex' film. (A duplex film is a film which is thick enough for the two interfaces - i.e. liquid-film and film-air - to be independent and possess characteristic surface tensions.)3. Spread as a monolayer, leaving excess oil as lenses in equilibrium, as in Figure 4.17.

Harkins defined the term initial spreading coefficient (for the case of oil on water) as

Substituting in the Dupre equation, the spreading coefficient can be related to the work of adhesion and cohesion

Yung’s equation Cos θ = γs-g - γl-g / γl-g

σ = rhgd/2cos θ

Adhesional wettingIn adhesional wetting, a liquid which is not originally in contact with the solid substrate makes contact and adheres to it. In contrast to spreading wetting, the area of liquid-gas interface decreases.

Immersionai wettingIn immersional wetting, the solid, which is not originally in contact with the liquid, is immersed completely in the liquid. The area of liquid-gas interface, therefore, remains unchanged

Surface tension of aqueous sodium chloride solutions at 20°C

Surface activityMaterials such as short-chain fatty acids and alcohols are soluble in both water and oil (e.g. paraffin hydrocarbon) solvents. The hydrocarbon part of the molecule is responsible for its solubility in oil, while the polar —COOH or -OH group has sufficient affinity to water to drag a short-length non-polar hydrocarbon chain into aqueous solution with it. If these molecules become located at an air-wateror an oil-water interface, they are able to locate theirhydrophilic head groups in the aqueous phase and allow the lipophilic hydrocarbon chains to escape into the vapour or oil phase

Figure shows the effect of lower members of the homologousseries of normal fatty alcohols on the surface tension of water. The longer the hydrocarbon chain, the greater is the tendency for the alcohol molecules to adsorb at the air-water surface and, hence, lower the surface tension. A rough generalisation, known as Traube's rule, is that for a particular homologous series of surfactants theconcentration required for an equal lowering of surface tension in dilute solution decreases by a factor of about 3 for each additional CH2 group.If the interfacial tension between two liquids is reduced to asufficiently low value on addition of a surfactant, emulsification will readily take place, because only a relatively small increase in the surface free energy of the system is involved.

SURFACE ACTIVITY OF DRUGSEven small drug molecules are frequently amphiphilic,and therefore also generally surface active. This meansthat the drug tends to accumulate at or close to aninterface, be it a gas/liquid, solid/liquid or liquid/liquidinterface. This surface activity frequently depends on thebalance between electrostatic, hydrophobic and van derWaals forces, as well as on the drug solubility. Sincethe former balance depends on the degree of chargingand screening, the surface activity, and frequently alsothe solubility of the drug, it often depends on thepH and the excess electrolyte concentration.

the surface activities of drugs may contribute to their biologicalaction, although the relationship between surface activity and biological effect is less straightforward.