9. Measurement of surface tension by
Stalagmometer
drop-weight methods
n0, ρ0, σ0 – number of
droplets, density and
surface tension of
water,
n, ρ, σ – …… of
investigated liquid
0
0
0
ρ
ρ
σσ
n
n
=
11. 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
12.
13.
14.
15. 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
16. Absorption is the process of arbitrary absorption of
the substance by volume
Chemisorption - chemical interaction adsorbent
with adsorbate
20. In certain cases - solutions of electrolytes, sugars, etc. - small
increases 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.
21.
22.
23.
24.
25. Spreading
Adhesion and cohesion
The 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
26. 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.
27.
28. 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
31. Adhesional wetting
In 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 wetting
In 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
35. Surface activity
Materials 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-water
or an oil-water interface, they are able to locate their
hydrophilic head groups in the aqueous phase and
allow the lipophilic hydrocarbon chains to escape
into the vapour or oil phase
36.
37.
38.
39. Figure shows the effect of lower members of the homologous
series 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 the
concentration 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 a
sufficiently 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.
40. SURFACE ACTIVITY OF DRUGS
Even small drug molecules are frequently amphiphilic,
and therefore also generally surface active. This means
that the drug tends to accumulate at or close to an
interface, be it a gas/liquid, solid/liquid or liquid/liquid
interface. This surface activity frequently depends on the
balance between electrostatic, hydrophobic and van der
Waals forces, as well as on the drug solubility. Since
the former balance depends on the degree of charging
and screening, the surface activity, and frequently also
the solubility of the drug, it often depends on the
pH and the excess electrolyte concentration.
the surface activities of drugs may contribute to their
biological
action, although the relationship between surface activity and
biological effect is less straightforward.
