Page 90 - Introduction to Colloid and Surface Chemistry

80 Liquid-gas and liquid-liquid interfaces

Table 4.2 Surface-active agents
Anionic
Sodium stearate CH 3(CH 2) 16COCTNa +
Sodium oleate CH 3(CH 2) 7CH=CH(CH 2) 7COCr Na f
Sodium dodecyl sulphate
Sodium dodecyl benzene sulphonate CH 3(CH 2)i i .QH 4.SO 3 Na H

Cationic
Dodecylamine hydrochloride CH 3(CH 2),,NH$a
Hexadecyltrimethyl ammonium bromide CH 3(CH 2) 15N(CH 3KBr

Non-ionic
Polyethylene oxides e.g. CH 3(CH 2),,(O.CH 2.CH 2) 6OH*
Spans (sorbitan esters)
Tweens (polyoxyethylene sorbitan esters)
Ampholytic
Dodecyl betaine - (CH 3) 2

CH 2COO
*Abbreviated C, 2E 6 to denote hydrocarbon and ethylene oxide chain lengths.

the oscillating jet method approach that of pure water but fall rapidly
46 150
as the surfaces are allowed to age ' .

Thermodynamics of adsorption - Gibbs adsorption equation

The Gibbs adsorption equation enables the extent of adsorption at a
liquid surface to be estimated from surface tension data.
The quantitative treatment of surface phenomena involves an
important uncertainty. It is convenient to regard the interface
between two phases as a mathematical plane, such as SS in Figure
4.12. This approach, however, is unrealistic, especially if an adsorbed
film is present. Not only will such a film itself have a certain thickness,
but also its presence may influence nearby structure (for example, by
dipole-dipole orientation, especially in an aqueous phase) and result
in an interfacial region of varying composition with an appreciable
thickness in terms of molecular dimensions.
If a mathematical plane is, nevertheless, taken to represent the
interface between two phases, adsorption can be described conveni-