Physical Chemistry of Foods

net interaction energy between surfactant molecules when in the adsorption

layer, which can vary greatly among surfactants.

The adsorption isotherms for surfactants at the A–W and the O–W

interface need not be the same. This is primarily because the net interaction

energy mentioned will often be different for these interfaces. It may even

make a difference whether the oil is a paraffin or a triacylglycerol mixture.

Another condition affecting the isotherm is temperature. By and large, the

surface excess will be smaller for a higher temperature at the same activity of

the surfactant; the difference can be considerable (cf. Figure 8.5b).

Thesurface activityof a solute, i.e., the reciprocal ofc0.5, is higher if

the free energy needed to remove a surfactant molecule from the interface is

larger. For an aqueous solution, this is the case if the hydrophobic part of

the molecule (e.g., an aliphatic chain) is larger relative to the hydrophilic

part (e.g., a carboxyl group). This explains the trend shown by the

surfactants in Figure 10.6; it may also be noted that in such cases

(homologous surfactants) the surface activity is closely related to the

solubility. For the same ratio hydrophobic/hydrophilic, the free energy for

FIGURE10.7 Adsorption isotherms. Surface excessGoverG?(i.e.,Gat surface
saturation) versus concentration in solutioncoverc0.5(i.e.,cforG/G?¼ 0 :5).
Langmuir isotherm and calculated isotherm for Na-stearate at the A–W surface.