Maximum Bubble Pressure Method - Background

Background

When the liquid forms an interface with a gas phase, a molecule on the border has quite different physical properties due to the unbalance of attracting forces by the neighboring molecules. At the equilibrium state of the liquid, interior molecules are under the balanced forces with uniformly distributed adjacent molecules.

However, relatively fewer number of molecules in the gas phase above the interface than condensed liquid phase makes overall sum of forces applied to the surface molecule direct inside of the liquid and thus surface molecules tend to minimize their own surface area.

Such an inequality of molecular forces induces continuous movement of molecules from the inside to the surface, which means the surface molecules has extra energy, which is called surface free energy or potential energy, and such an energy acting on reduced unit area is defined as surface tension.

This is a frame work to interpret relevant phenomena which occurs surface or interface of materials and many methods to measure the surface tension has been developed.

Among the various ways to determine surface tension, Du Noüy ring method and Wilhelmy slide method are based on the separation of a solid object from the liquid surface, and Pendent drop method and Sessile drop or bubble method depend on the deformation of the spherical shape of a liquid drop.

Even though these methods are relatively simple and commonly used to determine the static surface tension, in case that the impurities are added to the liquid, measurement of surface tension based on the dynamic equilibrium should be applied since it takes more time to obtain a completely formed surface and this means that it is difficult to achieve the static equilibrium as a pure liquid does.

The most typical impurity to induce dynamic surface tension measurement is a surfactant molecule which has both of hydrophilic segment, generally called “head group” and hydrophobic segment, generally called “tail group” in a same molecule. Due to the characteristic molecular structure, surfactants migrate to the liquid surface bordering gas phase until an external force disperse the accumulated molecules from the interface or surface is fully occupied and thus cannot accommodate extra molecules. During this process, surface tension decrease as function of time and finally approach the equilibrium surface tension (σ_equilibrium). Such a process is illustrated in figure 1. (Image was reproduced from reference)

Figure 1 – Migration of surfactant molecules and change of surface tension (σ_t1 > σ_t2 > σ_equilibrium)