Surface tension: looking at the hydrophobic effect in action

Surfactants and amphiphiles are chemical compounds containing both hydrophilic and hydrophobic parts in the same molecule. In classical surfactants such as in SDS, these parts are well separated giving rise to the nomenclature used, hydrophobic tail of hydrocarbon and the hydrophilic and polar sulfate headgroup.

A common way of characterizing amphiphilic compounds is to study them at an air/water interface. Amphiphiles adsorb to the interface and orient so as to have their polar headgroups with water, while projecting the hydrophobic tail into the air, thus forming in the interface a single layer of surfactant molecules. The driving force is the hydrophobic effect.

Upon reaching the solubility limit for the amphiphile monomers, they start to aggregate to form structures known as micelles. Now the non-polar, hyrophobic parts are hidden inside the micelle core while the micelle surface consists of the polar parts. Again, this process is driven by the hydrophobic effect.

Interfaces in numbers - Gibbs isotherm

Adsorption and aggregation of a compound can be monitored through changes in the air/water interfacial tension (surface tension) upon increasing the compound's concentration, thus providing the so-called adsorption isotherm. The data can be described by various adsorption models (we use the Gibbs model) to obtain the following molecular parameters.

• Air-water partitioning coefficient (Kaw)
• True surface area (TSA)
• Critical Micelle Concentration, CMC

In the picture above surface pressure is used instead of surface tension. Surface pressure is related to surface tension through:
π = γ0 - γ
where γ0 is the surface tension for the clean air/water interface and γ is the surface tension measured in the presence of the surface active substance.

The partitioning coefficient, Kaw−1 is determined as the intersection between the extrapolated line and the x-axis, log c. It measures the compound's affinity for the air/water interface, and simplistically put can be understood as the ratio of the species found in the bulk phase and in the interface.

For monolayers the difference between surface concentration and surface excess (slope of the isotherm) becomes negligible. Thus, the surface concentration is inversely proportional to the area available per surfactant molecule in the interfacial region.

The critical micelle concentration (CMC) or solubility limit results in a sharp transition above which the concentration of the free surfactant/amphiphile molecules remains constant, resulting in a plateau in the surface pressure vs. concentration curve.