Give support to repulsion, and you'll see attraction. We know why

The results of last year's experiments with mixtures carried out at the University of Stuttgart, Germany, were surprising for many researchers. In one of the systems studied, a repulsive force was acting between the system components. When a second repulsive force was introduced, an unexpected effect was observed: a strong attraction. This unusual result aroused interest of the theoreticians from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw. "Starting from the basics, we have developed a theoretical model of the system studied in Germany and successfully verified its predictions with experimental evidence. That's why we are able to explain, how superposition of two repulsive interactions transforms into attraction," says Prof. Alina Ciach from the IPC PAS.

The system modelled at the IPC PAS was a mixture of water and an oily organic liquid -- lutidine. The mixture included also salt ions. The fluid was placed between two electrically charged walls, one hydrophilic, and another one hydrophobic.

Water is miscible with lutidine only in a certain temperature range. An interesting situation arises close to the critical temperature, where the system cannot "make a decision" if the components should mix or separate. "Under these conditions, the water layer at the hydrophilic wall becomes relatively thick, similarly as the oil layer at the hydrophobic wall. And as water and oil 'dislike' each other, a force emerges to push the walls apart," explains Faezeh Pousaneh from Iran, a PhD student working at the IPC PAS under the International PhD Projects Programme of the Foundation for Polish Science.

The unusual behaviour of the modelled system was revealed after electric charge of the same sign was applied to both walls. A second, electrostatic, repulsion was acting then between the walls, and even so the walls were becoming attractive! "Paper and pencils were set in motion. Using purely analytical calculations, together with Faezeh, we derived specific formulae to describe the course of the phenomenon," says Prof. Ciach.

It turned out that the key element of the model was the assumption that the ions in solution move exclusively in water, while avoiding lutidine. The walls of the system under study were electrically charged, so they attracted ions. "But there is lutidine layer at the hydrophobic wall!," notices Pousaneh. "So an ion faces a dilemma: it wants to get to the wall, but the access is protected by lutidine. And the hurdle can be taken in one way only: by pulling water." As a result of the process described above, the wall surface, earlier hydrophobic, starts to behave like a hydrophilic one, becoming similar in that respect to the other wall. And two hydrophilic walls attract each other.

The team from the IPC PAS intends to continue the research on variants of the modelled systems. "Interactions similar to those described by us occur between charged colloidal particles with selective surfaces. Depending on temperature, the interactions are sometimes repulsive, sometimes attractive," says Prof. Ciach. It turns out that in a narrow temperature range, the potential has a minimum for certain distance between the particles, so it is similar to that one being responsible for arrangement of atoms in nodes of the crystal lattice. "Thus, by controlling temperature we will be able to force a colloid to develop a specific structure. Then it can be preserved and used, for instance in material engineering," stresses Prof. Ciach.