Certain molecules have two distinct components with differing affinity for solutes. The part of the molecule that has an affinity for polar solutes, such as water, is said to be hydrophilic. The part of the molecule that has an affinity for non-polar solutes, such as hydrocarbons, is said to be hydrophobic. Molecules with hydrophilic and hydrophobic components are said to be amphiphilic (Figure A).
Amphiphilic molecules display distinct behavior when interacting with water. The polar part of the molecule seeks to interact with water while the non-polar part shuns interaction with water. There are two ways in which such molecules achieve both these states. An amphiphilic molecule can arrange itself on the surface of water so that the polar part interacts with the water and the non-polar part is held above the surface (either in the air or in a non-polar liquid) as shown in Figure B. The presence of these molecules on the surface disrupts the cohesive energy at the surface and thus lowers the surface tension. Such molecules are called ‘surface active’ molecules or surfactants.
Another arrangement for the molecules allows each component to interact with its favored environment. Molecules can form aggregates or micelles in which the hydrophobic portions are oriented within the cluster and the hydrophilic portions are exposed to the solvent. An example of a spherical micelle is shown in Figure C.
The proportion of molecules present at the surface of a liquid or as micelles in the bulk of a liquid depends on their concentration. At low concentrations, surfactants in low concentrations occupy the surface of the liquid. As the surface becomes crowded with surfactant, additional molecules arrange into micelles. This concentration is called the Critical Micelle Concentration (CMC) and can be found by measuring surface tension. The graph below shows surface tension versus log of concentration of surfactant added:
The graph shows the three phases: 1, at low concentrations of surfactant only slight change in surface tensionis detected; 2, additional surfactant decreases surface tension; and 3, the surface becomes fully loaded, with no further change in surface tension.
CMC is found as the point where the baseline of minimal surface tension and the slope where surface tension shows linear decline intersect. Surface tension versus log concentration may be plotted by measuring a series of manually mixed solutions, or automatically using an Attension® Sigma 700 or 701 with optional dispenser. Manual mixing and testing surface tensions for a series of solutions is both time consuming and may present real problems. By using the automated features of the Sigma range the stability of surface tension measurements can be easily handled. The Sigma 700 and 701 programs also automate the mixing of concentrations for even spacing across the log scale and assist with graphics to help find CMC.
CMC measurements with Sigma 700/701 can be performed by either adding concentrated surfactants to a surfactant free solution or by diluting a concentrated surfactant solution with pure solvent. Measurement of CMC starts with preparing the software settings and choosing a method (Wilhelmy plate or Du Noüy ring). Thereafter, a probe is chosen accordingly and the measuring vessel is filled with the initial solution and Sigma 700/701 will automatically complete the experiment.
OneAttension software has an AutoSearch option that divides the experiment into two phases, and thus speeds up your experiments with unknown liquids. The Search phase allows for larger additions to be made during the first portion of the experiment where non-critical data calculations occur. When ST values drop significantly, the instrument switches to the Measurement phase and follows the normal experimental protocol. When all requested additions have been made the experiment terminates and the data is automatically stored for analysis.