This record concerns an open question from Zhang, Luo, Ling, Huang, and Xue, Membranes 14 (2024) 233 (DOI 10.3390/membranes14110233), who prepared carbon-doped TiO2 nanofilms ('GLTO') on porous alpha-Al2O3 by a vapor-liquid interfacial reaction of glycerol with TiCl4 vapor followed by calcination at 250 C in air, achieving 90.9 L/m2/h/bar methanol permeability, 93.2% Congo Red rejection, and a molecular weight cut-off near 575 Da. The paper's explicit open question (Section 3.1) is whether the calcination-induced decrease of the water contact angle — 98.895 to 68.782 degrees for the 5-minute-reaction sample and 50.856 to 34.641 degrees for the 1-minute sample — should be attributed to increased surface porosity or to reduced residual carbon content.

Background

Organic solvent nanofiltration (OSN) is pressure-driven membrane separation of solutes roughly in the 200-1000 Da range from organic solvents; polymer membranes often swell or degrade in aggressive solvents, so ceramic (metal-oxide) membranes on porous alumina supports are valued for chemical and thermal stability. Molecular layer deposition (MLD) denotes gas-phase, self-limiting surface reactions between a metal precursor such as TiCl4 and an organic diol, which build hybrid organic-inorganic ('organometallic') films one reaction cycle at a time; calcination burns out the organic fraction, leaving a doped metal-oxide layer with nanometer-scale pores. A foundational result is Sengupta et al., Science 381 (2023) 1098-1104 (DOI 10.1126/science.adh2404), which made carbon-doped metal oxide (CDTO) interfacial nanofilms giving ultrafast, precise molecular separation. Zhang et al. (Membranes 14 (2024) 233, DOI 10.3390/membranes14110233) extend this to glycerol, a triol containing both 1,2-diol and 1,3-diol groups: glycerol wetted into the porous alpha-Al2O3 support reacts with TiCl4 vapor at the vapor-liquid interface, and calcination at 250 C in air yields a carbon-doped TiO2 nanofilm the authors call GLTO, with methanol permeability up to 90.9 L/m2/h/bar, 93.2% rejection of Congo Red from methanol, a molecular weight cut-off (MWCO, the solute molecular weight rejected at 90%) of about 575 Da, and a mean pore size of about 0.92 nm estimated via Ferry's equation (a classical membrane-transport relation linking solute rejection to pore size) combined with a log-normal pore-size distribution model. Two measured surface properties matter here. First, wettability, quantified by the water contact angle (the angle a 3 microliter water drop makes on the surface; smaller means more hydrophilic). Second, composition: X-ray photoelectron spectroscopy (XPS) shows the carbon fraction falls from 63.80% after 250 C calcination to 30.84% after 300 C calcination, the higher temperature also nucleating anatase (the tetragonal crystalline polymorph of TiO2) crystallites. The authors observe that the organometallic hybrid films are hydrophobic (water contact angles of 98.895 degrees at 5 minutes of interfacial reaction and 50.856 degrees at 1 minute), while the corresponding calcined GLTO films are more hydrophilic (68.782 degrees at 5 minutes and 34.641 degrees at 1 minute), and they note that both higher surface porosity and lower carbon content plausibly contribute — but these two factors are not independently varied or quantified. Related precursor chemistry is understood separately: mechanistic MLD studies of titanicone (the hybrid Ti-organic film grown from TiCl4 and ethylene glycol) films (Kim et al., Chemistry of Materials 36 (2024) 247-255, DOI 10.1021/acs.chemmater.3c02018) address film-growth instability, not membrane wettability.

Problem Statement

For carbon-doped TiO2 (GLTO) nanofilms prepared by vapor-liquid interfacial reaction of glycerol with TiCl4 vapor on porous alpha-Al2O3 supports followed by calcination in air, determine quantitatively whether the calcination-induced increase in hydrophilicity — the water contact angle falling, for films paired by interfacial reaction time, from 98.895 degrees (organometallic hybrid film, 5-minute reaction) to 68.782 degrees (calcined GLTO, 5-minute reaction), and from 50.856 degrees (1-minute hybrid) to 34.641 degrees (1-minute GLTO) — is caused by the increase in nanoscale surface porosity, by the reduction in residual carbon content, by both, or by their interaction; that is, disentangle and quantify the separate contributions of surface porosity and residual carbon fraction to the water contact angle. This is the source paper's explicit open question ('Further research is needed in due time'). Because calcination couples carbon burn-out to pore generation, the reported contact angles alone do not separate the two factors; the requested decomposition is therefore made identifiable by an explicit model and its assumptions, or by a concrete experiment that independently varies calcination temperature and time while measuring both surface porosity and residual carbon content. A qualitative attribution to both factors does not by itself quantify their separate contributions.

References

  1. Wenjing Zhang, Jiangzhou Luo, Honglei Ling, Lei Huang, Song Xue, "Carbon-Doped TiO2 Nanofiltration Membranes Prepared by Interfacial Reaction of Glycerol with TiCl4 Vapor", Membranes 14 (2024) 233. DOI: 10.3390/membranes14110233, https://doi.org/10.3390/membranes14110233
  2. Bratin Sengupta, Qiaobei Dong, Rajan Khadka, Dinesh Kumar Behera, Ruizhe Yang, Jun Liu, Ji Jiang, Pawel Keblinski, Georges Belfort, Miao Yu, "Carbon-doped metal oxide interfacial nanofilms for ultrafast and precise separation of molecules", Science 381 (2023) 1098-1104. DOI: 10.1126/science.adh2404, https://doi.org/10.1126/science.adh2404
  3. Hyeongjin Kim, Jieun Hyun, Gaeun Kim, Eunsang Lee, Yo-Sep Min, "Origin of Instability of Titanicone Grown by Molecular Layer Deposition Using TiCl4 and Ethylene Glycol", Chemistry of Materials 36 (2024) 247-255. DOI: 10.1021/acs.chemmater.3c02018, https://doi.org/10.1021/acs.chemmater.3c02018