Physical chemistry of interfaces: liquids, electrostatics, dynamics

My interest is physical chemistry of interfaces: boundaries between two phases. Near the interface, the translational symmetry is broken, and then the physical property is different from that in bulk. We can see interfaces, namely bubbles and droplets in our daily life. Chemical products such as soap and foods such as milk are well-known colloidal solutions which have many interfaces. Porous media such as sponge, polymeric gels, and even soil also have many interfaces. Although interfaces are familiar to us, theoretical treetment of interfaces is more difficult than that of bulk due to translational asymmetry. Since this diffuculty has attracted many physicists, a famous physisit Wolfgang Pauli used to say "God made the bulk; surfaces were invented by the devil.".
Specially, I am interested in (aquous) electrolyte solutions because it is the most familiar to us. Any purified water has trace of ions dissociated from water molecules themselves. Seawater inlcudes many kinds of electrolytes, and it circulates in atomosphere through cloud, rain, and finally it permeates into the soil. Moreover, our blood and cells include ions which play a significant role to control nerves and physiological functions. Furthermore, the interface between electrodes and electrolytes must be important in battery scicence. Study of electrolyte solutions is a bit classical in physical chemistry, but it will definitely give a large impact on geophysics, biology, and engineering.

Surface tension of electrolyte solutions: Jones-Ray effect

Surface tension of water increases when we add electrolytes, namely NaCl or KCl. Over 80 years ago, in 1934 Onsager and Samaras theoretically explained that image repulsion force between ion and interface increases the surface tension. However, the next year, in 1935 Jones and Ray reported that in experiments the surface tension decreases up to added salt concentration 1mM, and after that the surface tension goes up.This clearly contradicted with the Onsager-Samaras theory, and it caused a widespread controversy with involving Langmuir and so on. After that, many different laboratories repoted the surface tension minimum, but still the convincing theoretical explanation has been lacking. I study this effect by focusing on impurity effect.

Electrokinetics

Interfaces between electrolyte solutions and another phase is usually spontaneously charged by physico-chemical mechanism. Amount of surface chages at the interface is measured in experiments, and the most common method is zeta potential measurement. The amount of surface charges is very important because it has a relation with stability of colloidal solutions. The physico-chemical mechanism of negative zeta potential is still not clear for air-water interface, which is a famous unsolved problem. Another problem is the differences between the surface charge estimated from zeta potentail and from titration measurement. Analysis of the theoretical model with taking inhomogeneous profiles of water dielectric constant and viscosity into account reveals a power-law behavior between the two surface charges. The exponent of this power law allow us to estimate interfacial viscosity near a surface, which differs from bulk viscosity and depends on hydrophilicity or hydrophobicity.

Polymer and electrolyte solutions

Since polymers have larger molecular weight than normal molecular liquids, their characteristic length and time scale is longer than those of molecular liquids as shown in softmatter physics. Moreover, polymers must be useful material because they change their property by changing their chemical structure and they are easy to deform into any shape. I study on zeta potential of polymer solutions, specially the effect of their adsorption layer on the surface. It is experimentally known that non-ionic polymers often show non-zero zeta potential. I also study origin of the non-zero zeta potential of neutral surface.