92168.pdf

80 Liquid-gas and liquid-liquid interfaces

Table 4.2 Surface-active agents

Anionic

Sodium stearate

Sodium oleate

Sodium dodecyl sulphate

Sodium dodecyl benzene sulphonate

Cationic

Dodecylamine hydrochloride

Hexadecyltrimethyl ammonium bromide

Non-ionic

Polyethylene oxides

Spans (sorbitan esters)

Tweens (polyoxyethylene sorbitan esters)

Ampholytic

Dodecyl betaine

CH 3 (CH 2 ) 16 COCTNa+

CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COCr Na f

CH 3 (CH 2 )i i .QH 4 .SO 3 NaH

CH 3 (CH 2 ),,NH$a

CH 3 (CH 2 ) 15 N(CH 3 KBr

e.g. CH 3 (CH 2 ),,(O.CH 2 .CH 2 ) 6 OH*

• (CH 3 ) 2

CH 2 COO

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

the oscillating jet method approach that of pure water but fall rapidly

as the surfaces are allowed to age^46 '^150.

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-