This work involved the use of AFM and AFM-IR to acquire surface topography, hydrophobicity and chemical distribution at nanoscale on chemically modified polymeric membranes. Firstly, AFM was used to obtain the topography images, showing the pore size, porosity and also surface roughness of the polymeric membranes. Secondly, AFM chemical force mode was applied to probe nanoscale hydrophobicity on modified membranes. I established a linear correlation (R2=0.994) between the measured adhesion force and water contact angles. Moreover, I tested this technique for nanoscale hydrophobicity measurements on engineered nanoparticles and carbon-based nanomaterials successfully. Thirdly, the hybrid AFM-IR offers accurate chemical identifications and distribution of additives on modified membranes based on their characteristic IR peaks. The IR mapping images showed the inhomogeneous modifiers’ distribution on membrane surface, which could not be resolved by regular FTIR due to the diffraction limit. The hybrid AFM techniques are believed to be critical for revolutionizing the nanoscale characterization for material properties in a wide spectrum of applications ranging from surface contamination or fouling process, material weathering process, capsules coating in pharmaceutical engineering, to antifouling surface design (e.g., eye contact lenses and sanitary coating).
Determination of surface hydrophobicity or wettability of nanomaterials and nanoparticles (NPs) is often challenged by heterogeneous properties of NPs that vary with particle size, shape, surface charge, aggregation states, and surface sorption or coating. This study first summarized inherent limitations of water contact angle, octanol-water partition coefficient (Kow) and surface adsorption of probe molecules in probing nanomaterial hydrophobicity. Then, we demonstrated the principle of a scanning probe method based on atomic force microscopy (AFM) for the local surface hydrophobicity measurement. Specifically, we measured the adhesion forces between functionalized AFM tips and self-assembly monolayers (SAMs) to establish a linear relationship between adhesion force and water contact angles based on the continuum thermodynamic approach (CTA). This relationship was used to determine local surface hydrophobicity of seven different NPs (i.e., TiO2, ZnO, SiO2, CuO, CeO2, α-Fe2O3, and Ag), which agreed well with bulk contact angles of these NPs. Some discrepancies were observed for Fe2O3, CeO2 and SiO2 NPs, probably because of surface hydration and roughness effects. Moreover, the solution pH and ionic strength had negligible effects on the adhesion forces between the AFM tip and MWCNT or C60, indicating that hydrophobicity of carbonaceous nanomaterials is not influenced by pH or ionic strength (IS). By contrast, natural organic matters (NOM) appreciably decreased the hydrophobicity of MWCNT and C60 due to surface coating of hydrophilic NOM. This scanning probe method has proved to be reliable and robust toward the accurate measurement of nanoscale hydrophobicity of individual NPs or nanomaterials in liquid environments.
Wen's Research Group
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May. 2019:Congratulations to Xiaonan and Kaiqin for winning Poster Competition of the NJWEA Conference!