“Interlocking holes” in graphene oxide membranes help separate water from alcohol
Computational simulations carried out by a group of physicists from the State Universities of Campinas (Unicamp) and Federal Universities of ABC (UFABC) provided an explanation for a unique property of membranes made of graphene oxide, sheets with a hexagonal structure formed mainly by carbon atoms and a small number of oxygen and hydrogen atoms. As thin as an atom, when stacked on top of each other, these membranes form a structure capable of acting as a filter to separate the constituents of a homogeneous solution – a mixture of two or more liquids whose components are no longer distinguishable with the naked eye.
The mixture studied by the researchers included 50% water and 50% alcohol (ethanol or methanol). The graphene oxide membranes let water through and retained alcohol. According to the theoretical model proposed by physicists, this particularity is due to the formation of “interconnected holes” between the membranes through which the water molecules all pass together, leaving the alcohol molecules behind. The functioning of this molecular labyrinth that attracts only water was described in a work published in February in the scientific journal carbon.
The filtration properties of graphene oxide membranes are well known experimentally. “But now we are able to describe in a more comprehensive and detailed way what happens in this process”, explains physicist Douglas Galvão, from Unicamp, one of the authors of the work. It is not a mechanical filtration in which molecules of a certain size pass through the graphene mesh while others, larger ones, are retained. It is a mechanism that involves chemical attraction. “Graphene oxide membranes swell and have greater spacing between their planes. Thus, two-dimensional channels are formed that induce the passage of water”, says Pedro Autreto, from UFABC, another author 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 bridges or hydrogen bonds. By comparison, it is as if only the water molecules joined hands to get out of 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 pure, formed only by carbon atoms, but without oxygen, there is an inversion in the selectivity of the membrane”, comments physicist Daiane Damasceno Borges, who is doing a postdoctoral internship at Unicamp and participated in the work. “In this case, alcohol molecules pass through the filter instead of water molecules”, adds Cristiano Woellner, also a postdoctoral fellow at Unicamp.
Despite its theoretical nature, the work can be useful for creating new types of filters, which are more efficient and cheaper. “Simulations like these allow great control of variables and can guide the development of biofuels, in which it is necessary to separate ethanol from water”, comments physicist Leandro Seixas, from the Center for Advanced Research in Graphene, Nanomaterials and Nanotechnologies (MackGraphe), from Universidade Presbiteriana Mackenzie, who did not participate in the study.
Project
Center for Computational Engineering and Sciences – Cecc (nº 13/08293-); Modality Research, Innovation and Dissemination Centers Program (Cepid); Responsible researcher Munir Salomão Skaf (Unicamp); Investment R$ 18.478.546,78 (for the entire project).
Scientific article
BORGES, DD et al. Insights into the mechanism of water-alcohol separation in multilayer graphene oxide membranes: Entropic versus enthalpic factors. Carbon. v. 127, p. 280-6. Feb. 2018.
