Computational simulations conducted by a group of physicists from the University of Campinas (UNICAMP) and the Federal University of ABC (UFABC) have provided an explanation for a unique property of graphene oxide membranes, which are sheets formed largely by carbon atoms and a small number of oxygen and hydrogen atoms arranged in a hexagonal lattice structure. As thin as an atom, these membranes are stacked together to form a filter structure capable of separating the constituents of homogeneous solutions—mixtures of two or more liquids whose components cannot be distinguished with the naked eye.
The mixture the researchers studied was 50% water and 50% alcohol (either ethanol or methanol). Graphene oxide membranes allow the water, but not the alcohol, to pass through. According to the theoretical model the physicists proposed, this selectivity is due to the formation of “intercommunicating pores” in the membranes through which the water molecules pass together, leaving the alcohol molecules behind. The mode of operation of this water-permeable molecular labyrinth is described in a paper published in February in Carbon.
The filtration properties of graphene oxide membranes are well known experimentally. “But now we are able to describe the mechanisms at play more completely and in greater detail,” explains physicist Douglas Galvão of UNICAMP, one of the authors of the paper. It is not a mechanical filtering process in which molecules of a certain size pass through the graphene mesh while larger ones are retained. It is a mechanism that involves chemical attraction. “Graphene oxide membranes swell and the spaces between them widen, forming two-dimensional channels that induce the passage of water,” says Pedro Autreto of UFABC, a coauthor of the study.
Within these two-dimensional labyrinths, the presence of oxygen atoms in the graphene sheets causes the hydrogen atoms in the water molecules to form hydrogen bonds. As an approximate analogy, it is as if the water molecules joined hands to make their way through the labyrinth formed by the graphene oxide membranes. The efficiency of the process depends on the degree of oxidation of the membranes. “When graphene is made purely of carbon atoms and has no oxygen, membrane selectivity is reversed,” says physicist Daiane Damasceno Borges, a postdoctoral researcher at UNICAMP who participated in the study. “Alcohol molecules would then move across the filter instead of water,” adds Cristiano Woellner, a fellow postdoctoral researcher at UNICAMP.
While theoretical in nature, the model can be useful in creating new, more efficient and economical filter designs. “Models like these provide a great degree of control over variables and can support the development of biofuels in which ethanol needs to be separated from water,” says physicist Leandro Seixas of the Graphene and Nanomaterials Research Center (MackGraphe) at Mackenzie Presbyterian University, who was not involved in the study.
Center for Computational Science and Engineering (CECC) (no. 13/08293-7); Grant Mechanism Research, Innovation, and Dissemination Centers (RIDC) Program; Principal Investigator Munir Salomão Skaf (UNICAMP); Investment R$18,478,546.78 (overall project).
BORGES, D. D. et al. Insights on the mechanism of water-alcohol separation in multilayer graphene oxide membranes: Entropic versus enthalpic factors. Carbon. Vol. 127, pp. 280–6. Feb. 2018.