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Soft Interfacial Chemistry

  • Takanori Takiue, Professor
  • Yosuke Imai, Assistant Professor
The soft interface such as gas/liquid and liquid/liquid interfaces is regarded as a fundamental structure of soft matters including emulsion, vesicle, biological membrane, and thus the study on the structure and property of soft interface is crucial to understand sophisticated structure-function relation of soft matter. Our group investigates the adsorbed films (Gibbs films) of various surface active substances at soft interfaces by means of macroscopic and microscopic techniques; interfacial tensiometry, synchrotron X-ray reflection (XR), total reflection XAFS, and Brewster angle microscopy (BAM). Our goal is to elucidate the principles of “raft” formation in biological membrane from the viewpoint of “line tension”, which is an excess energy generated at the domain boundary in heterogeneous structure at the interface. Our recent activities are as follows

1. Phase transition and molecular miscibility in the adsorbed film

Figure 1
Figure 1. Self-assembled porphyrin nanotube including linear array of fullerene C60.

The thermodynamic analysis of interfacial tension data provides us the state of adsorbed film in terms of interfacial pressure vs. area per molecule curves which is 2D analogue of pressure–volume isotherm. The adsorbed film of fluorocarbon (FC) alcohol, 1H,1H,2H,2H-perfluorodecanol (F8H2OH), at the hexane/water interface shows two types of phase transitions, gaseous–expanded and expanded–condensed transitions depending on temperature, pressure, concentration of the oil solution. The electron density profile indicated that F8H2OH molecules are densely packed with perpendicular orientation in the condensed state.

In the case of adsorbed FC-diol (1H, 1H, 10H, 10H-perfluorodecane-1,10-diol (FC10diol) film, on the other hand, the molecules form a condensed monolayer with parallel orientation, and then pile spontaneously to form multilayer, which is driven mainly by enhanced interaction between FC chains of FC10diol molecules due to weak interaction between FC10diol and hexane molecules.

Figure 2
Figure 3
J. Phys. Chem. B, 109, 1210–1225 (2005)
J. Phys. Chem. B, 118, 12451–12461 (2014)

2. Domain formation in the adsorbed film and line tension

The structure of adsorbed film is examined mainly by X-ray reflectivity (XR) measurement at BL37XU in SPring-8. According to the electron density profile, it was found that FC alcohol molecules are densely packed like 2D solid, while hydrocarbon (HC) alcohol such as 1-icosanol ones are packed like 2D liquid just above the freezing point even in the condensed state. One of the noticeable findings was that FC alcohol molecules form condensed phase domain with mm size in the expanded state close to the expanded–condensed phase transition point. This heterogeneous film is regarded as a model of “bio-membrane raft”. The size and shape is primarily affected by “line tension”, which is an excess energy at the domain boundary and estimated to be a few to several tens pN (10−12 N).

Our very recent finding is that the expanded – condensed phase transition in the adsorbed F8H2OH film becomes obscure by adding FC-HC hybrid alcohol, 6-perfluorohexyl-hexanol (F6H6OH). According to the domain structure analysis by XR measurement, it was shown that disappearance of the phase transition is well correlated to the increase in domain coverage and the decrease in domain size. This was reasonably explained by the “line adsorption” of F6H6OH at the boundary of the condensed F8H2OH domain.

Figure 4
J. Phys. Chem. B, 119, 12436–12445 (2015)