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A comparative study of transport characteristics of composite membranes based on graphene oxide prepared by Hummers' (H-GO) and Brodie (B-GO) methods is presented. By using Raman and XPS spectroscopy combined with gas and vapor measurements at non-zero pressure drop, it is shown that the difference in preparation methods results not only in different composition and microstructure of the membranes, but also in different water vapor permeability and resistance towards pressure drops during membrane performance. The H-GO samples are found to be more defective and stronger oxidized with C/O ratio of 1.8, whereas B-GO revealed a total C/O ratio of 2.6 with more perfect microstructure. The higher oxidation degree of H-GO membranes allows one to achieve higher water vapor permeability (up to ∼170 Barrer at 100 % humidity) but dramatically lower stability towards pressure revealing the irreversible loss in permeability up to 46 % during the application of pressure drop of 1 bar. In contrast, B-GO membranes show slightly lower permeability (∼140 Barrer at 100 % humidity) but enhanced pressure stability revealing the irreversible permeability loss of only 4 % at pressure drop of 1 bar which is about 10-fold smaller compared to H-GO stability. This could be explained by the difference in microstructural features of the H-GO and B-GO. Graphene oxide prepared by Hummer's method has more flexible and defective nanosheets, whereas Brodie's method gives rise to more rigid nanosheets with more perfect microstructure. The obtained results suggest that it is possible to prepare graphene oxide membranes with high resistance towards pressure using only the composition-microstructure interplay without additional modification with pressure-stabilizing agents. KEYWORDS graphene oxide membranes, Hummers' method, Brodie method, oxidation degree, pressure stability, water vapor permeability ACKNOWLEDGEMENTS The work is supported by the state program of world-class scientific and educational centers (assignment number FEWG-2021-0014) for the youth laboratory on the research direction "Studying gas permeability and physicochemical properties of sealing composite and carbon materials". The authors are grateful to Dr. Andrei Eliseev and his laboratory (Lomonosov Moscow State University) for help with experimental studies. The authors acknowledge shared services center "Surface and novel materials" of UdmFRC of UB RAS for XPS data acquisition.

Composite membranes are formed based on ultrathin 20 nm-thick selective layers of graphene oxide nanoflakes deposited on porous anodic alumina substrates. The long-term dynamics of permanent gases transport and water vapor permeability across the composite membranes is measured during 240 days (8 months). It is revealed that the permeability towards permanent gases remains nearly constant during a prolonged period of time. Contrary, water vapor flux decreases rapidly within the first 30 days from the membrane preparation moment and reaches about 80% of permeability loss during 8 months. The rapid decrease of membrane permeability during the first month could be attributed to a gradual packing of graphene oxide nanoflakes, particularly, locating in the surface sublayers, into more tight microstructure due to the evaporation of remaining solvent (membrane drying) under ambient conditions. Further decrease in permeability during more prolonged time could be caused additionally by deoxygenation of surface GO nanoflakes preventing water vapors diffusion into the GO film. This phenomenon, the so called "ageing" accompanies graphene oxide thin films similarly to some types of highly-permeable polymers. Holding the aged membrane under saturated water vapors, and even liquid water, didn't allow one to revitalize completely its permeability. The obtained results should be taken into account when designing membranes and other devices based on graphene oxide and its derivatives. KEYWORDS graphene oxide nanoflakes, anodic alumina, membrane ageing, dehumidification, water vapor transport. ACKNOWLEDGEMENTS The work is supported by the Russian Science Foundation (project 22-23-00662 "Membranes for molecular filtration and membrane electrocatalysis based on reduced graphene oxide").