Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 53

Soap cmc (mol/l)25°c

C7COOK 0.4

C9COOK 0.1

C11OOK 0.025

C7COOCs 0.4

These data show that CMC decreases by a factor of 4 for each increase in chain
length by -CH 2 CH 2 -.

3.3 GIbbS adSorPtIon equatIon In SolutIonS

A pure liquid (such as water), when shaken, does not produce any foam. This merely
indicates that the surface layer consists of pure liquid (and there is absence of any
minor surface-active impurities). However, if a very small amount of surface-active
agent is added (soap or detergent of about millimolar concentration or about parts per
million by weight is added) and the solution shaken, foam is produced at the surface
of the solution. This indicates that the surface-active agent has accumulated at the
surface (meaning that the concentration of the surface-active agent is much higher
at the surface than in the bulk phase; in some cases, many thousand times) and thus
forms a thin-liquid film (TLF) that constitutes the bubble. In fact, one can use bubble
or foam formation as a useful criterion for the purity of the system. It can be generally
observed at the shores of lakes or ocean that foam bubbles are formed under different
conditions. If water in these sites is polluted, then very stable foams are observed.
If one adds an inorganic salt, such as NaCl, instead of detergent, then no foam is
formed. Foam formation indicates that the surface-active agent adsorbs at the sur-
face, and forms a TLF (consisting of two layers of amphiphile molecules and some
water). This has led to many theoretical analyses of surfactant concentration (in the
bulk phase) and surface tension (consequent on the presence of surfactant molecules
at the surface). The thermodynamics of surface adsorption has been extensively
described by the Gibbs adsorption theory (Chattoraj and Birdi, 1984).
Further, the Gibbs adsorption theory has also been used for systems other than
solutions (such as solid-liquid or liquid 1 -liquid 2 , adsorption of solute on polymers,
etc.). In fact, the Gibbs adsorption theory will be applicable to any system in which
adsorption takes place at an interface.

3.3.1 gIbbS a d S o r p T I o n T h e o r y a T l I q u I d I n T e r f a c e S
The surface tension γ of water changes with the addition of organic or inorganic
solutes at constant temperature and pressure (Defay et al., 1966; Chattoraj and Birdi,
1984; Birdi, 1989, 1997). The extent of surface tension change and the sign of the
change are determined by the molecules involved (see Figure. 3.1).
The γ values of aqueous solutions generally increase with different electrolyte
concentrations. The magnitude of γ of aqueous solutions containing organic solutes
invariably decreases. As mentioned earlier, the surface of a liquid is where the den-
sity of the liquid changes to that of a gas by a factor 1000 (Chapter 1, Figure 1.1).