H2O in nanotubes
Confined in this tiny structure at -69ºC, water has two different densities simultaneously
Normally water solidifies at zero degrees Celsius (ºC), turning to ice. But under certain specific conditions, it remains in the liquid state, even at temperatures well below the freezing point, with unique thermodynamic properties. A group of researchers at the Federal University of the ABC (UFABC) believes that they have uncovered a unique characteristic of H2O molecules under extreme conditions. Contained in micro- and nanotubes made of the organic compound diphenylalanine, and cooled to 204 Kelvin (K), equivalent to -69.15ºC, the water not only remains in the liquid state, which was already known, but simultaneously maintains two distinct phases, a theoretical possibility never before confirmed in a laboratory. “When we store water under these controlled conditions, high and low densities coexist,” states physicist Herculano Martinho, of UFABC, one of the designers of the experiment that provided proof of the occurrence of this dual rearrangement of H2O molecules under the conditions described above.
The results of the study, done without altering the atmospheric pressure acting on the tiny, water-containing structures, were published in the June 1, 2015 issue of the journal Physical Chemistry Chemical Physics (PCCP). The liquid H2O samples were analyzed using Raman spectroscopic techniques, which are able to determine the structural details of a material or substance with high resolution, and x-ray diffraction. The researchers found evidence of two different “physical-chemical signatures” for the water trapped in the micro/nanotubes at -69°C. “We used the most simple nanotubes available to study the properties of trapped water,” explains chemist Wendel Alves, of UFABC, another author of the study. The work of the group coordinated by the two researchers is funded by FAPESP and the National Council for Scientific and Technological Development (CNPq).
Trapping water in micro/nanotubes made of a biological molecule is easier than a layperson might think. The process occurs spontaneously, naturally, with no need to control temperature and pressure, following a well-known recipe. The first step is to purchase diphenylalanine or synthesize it in a laboratory. This peptide, present during the formation of beta-amyloid fibers in Alzheimer’s disease, can be formed into micro/nanotubes through a crystallization process. “We mix amino acids with butyl alcohol and water,” says Martinho. After one or two hours, the tiny diphenylalanine structures form with water molecules inside their channels. “The nanotubes act like sponges and retain the water,” says Alves.
To the naked eye, the structures studied appear to be nothing but a mound of white powder. At the molecular level, the internal arrangement of the material is more complex, although it is easy to decipher. The diphenylalanine molecules connect to each other in groups of six and form a circular structure. An average of 24 H2O molecules are trapped inside this hexamer, in a space with a diameter of 1 nanometer. These circular structures, containing water in their center, connect to each other on the vertical plane and result in nanotubes. A bundle of several nanotubes results in a microtube of diphenylalanine (see figure).
The nanotubes act like porous bricks that join together to form the microtubes’ water-permeable walls. The ease and low cost of producing these structures make studies with peptide nanotubes a frequent topic in current scientific literature. There are groups in Brazil and abroad analyzing the possibility of using them in different fields, as biosensors to carry drugs inside the human body, or even as miniature energy generators (see Pesquisa FAPESP Issue nº 174).
Second critical point
Since diphenylalanine micro/nanotubes are known to contain water, they are good structures to use when analyzing possible changes in H2O molecule properties at low temperatures. “Understanding the behavior of confined water is important for biology and even for the space industry,” says Martinho. “Most of the water in the human body is stored in nano- and microchannels. Equipment corrosion in space is also partially due to the accumulation of water in nanopores.” In their experiment, Martinho, Alves and their colleagues analyzed the behavior of confined water in the range of temperatures from 10 K (-263.15ºC) to 290K (16,85ºC). At 204K, they noted anomalous behavior in the water, indicating the coexistence of different arrangements of H2O molecules.
The possibility of supercooled liquid water being in two different phases at the same time is called a second critical point in physical chemistry jargon. By definition, the critical point of a material is represented by a certain temperature and pressure at which two distinct states of this substance, almost always liquid and gas, coexist. In other words, it is an extremely specific situation in which one cannot say if the material is liquid or gas, since it is both at the same time.
Water’s (first) critical point is when it is heated to 647K (374ºC) and maintained at a pressure 218 times greater than atmospheric pressure. In this situation, liquid water cannot be differentiated from its vapor. In the last two decades, several authors have suggested that at very low temperatures, water could have a second critical point, in which it would continue to be liquid, but would have two different densities, one more viscous and the other less so. Since almost everything crystallizes under these conditions, this theoretical hypothesis is difficult to prove. “Confining the water prevents it from forming a crystal and, thus, the second critical point could become visible,” says theoretical physicist Márcia Barbosa, of the Federal University of Rio Grande do Sul (UFRGS), who studies anomalous H2O molecule behavior. “The articles by the researchers at UFABC provide evidence of a second critical point for water. It’s not the final word on the matter, but it’s a breakthrough.”
Hierarchical self-organization of peptide amphiphiles: fundamental mechanisms and potential applications (nº 2013/12997-0); Grant Mechanism Regular research project; Principal Investigator Wendel Alves (UFABC); Investment R$374,155.00.
FERREIRA, P. M. et al. Relaxation dynamics of deeply supercooled confined water in L,L-diphenylalanine micro/nanotubes. Physical Chemistry Chemical Physics. June 1, 2015.