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Physical Chemistry of Interfaces

  • Hiroki Matsubara, Associate Professor
The adsorbed film of surfactant has three distinct physical phases so called gaseous (G), expanded (L) and condensed (S) phases which is respectively corresponding to two-dimensional gas, liquid and solid states. When surfactant adsorbed films undergo two-dimensional phase transitions, which is normally driven by temperature, pressure, and concentration variations, the coexistence of surface phase domains can be observed. Our laboratory has studied the adsorption of surfactant at the air-water and oil-water interfaces more than 40 years and now we can set the physical state of interfaces as desired. One of the interesting findings of recent studies is the surface phase transition driven stability switching of foam and emulsion. The project on the relation between surface composition and foam film stability in binary surfactant systems is also in progress.

1. 2D phase diagram

Our strong point is a rigor thermodynamic treatment of surface tension () that allows us to evaluate the difference between partial molar quantities in the adsorbed film and those in the bulk solution. The discontinuity of thermodynamic quantities determines boundaries between G, L, and S states (K. Motomura, JCIS, 1978, 64, 348). By applying similar treatment to binary surfactant systems, one can also calculate the composition of mixed adsorbed film () in equilibrium with the surfactant solution of composition () at given (M. Aratono et al., JCIS, 1998, 200, 161). The figure below is the surface tension vs. total surfactant molality () in equilibrium with the surfactant solution of composition () curves for a nonionic–ionic surfactant mixed adsorbed film of different .

Figure 1

To construct 2D phase diagram, values at given surface tension (red line) are plotted against (colored arrows). Applying the following thermodynamic equation

to the obtained vs. curve, can be calculated. In the example shown here, vs. and vs. curves are drawn by solid and chained curves. From this diagram, it is realized that the two compositions are considerably different from each other and the adsorbed film is abound to ionic surfactant (the second component) compared to bulk solution.

Figure 2

Most of existing studies of foam and emulsion systems did not take into account of the composition difference between the bulk and adsorbed film. This is however crucial for the flotation and creaming processes through the change in the surface charge density. We are trying to clarify the relation between 2D phase diagram and foam film stability in a variety of mixed surfactant systems.

2. Surface phase transition driven thickness transition of foam film

A basic unit of foam is a thin aqueous film stabilized by surfactant adsorbed films and, in order to prevent foam film from rapturing, some sort of repulsion across the foam film is required. The metastable foam film stabilized by electrical double layer repulsion between adsorbed films is called a common black film (CBF). One of the unique features of CBFs is the thinning transition to a Newton black film (NBF) in which the electrical repulsion is diminished by added electrolyte, and the overlap of the hydration shells of surfactant head groups stabilized the foam film. We recently reported the first example of the temperature-driven thinning transition between different-thickness CBFs. The phase transition of the surfactant-alkane mixed adsorbed film was used for this purpose. Whether the surface phase is of the liquid or solid type was determined by ellipsometry (red lines in the figure below) and X-ray reflectometry.

Figure 3

For the cetyltrimethylammonium chloride (CTAB)-tetradecane (C14) system, this 2D transition occurs at about 21 °C and the thickness of foam film (filled square) shows clear discontinuity at the temperature very close to the 2D transition (E. Ohtomi et al., Chem. Lett. 2012, 41, 1300). Similar procedure is also applicable to OW emulsions.

3. Dynamics of surfactant adsorption

In reality, some foams are very stable even after the CBF-NBF transition. A plausible idea to explain this phenomena is slow exchange rate of surfactant molecules between the adsorbed film and the bulk. In the final deforming process, bare water surfaces has to be in contact and to create the bare water surface the desorption of surfactants and/or the compression of the adsorbed film were required. We have previously construced the 2D phase diagram for the mixed adsorbed film of dihexanoylphosphatidylcholine (DC6PC) and dioctanoyl-phosphatidylcholine (DC8PC) and found that the excess Gibbs energy of adsorption (dotted curve) was negative which suggests a synergistic attractive intermolecular interaction between two species in the adsorbed film. For the same system, the dilational viscoelasticity measurements based on the elctrocapillary wave (ECW) method suggested that the surface elasticity (red curve) has a maximum at the surface composition of the minimum of the excess Gibbs energy of adsorption. In this case, a staggered arrangement was proposed as a favorable intermolecular arrangement for lipid molecules and the existence of in-plane attraction between different species is considered to resist both the desorption and compression of surfactant molecules in the adsorbed film (Takajo et al., JPC C, 2012, 117, 1097).

Figure 4

Here we used the electrocapillary wave of 200 Hz, however, it is true that other relaxation modes which response different perturbation frequency exist for the adsorbed film. To study the overall picture of adsorption dynamics and deforming, we combined the dynamic surface tension and surface quasi-elastic light-scattering measurements with ECW experiment.