skip to main content

Title: Measurement of the surface hydrophobicity of engineered nanoparticles using an atomic force microscope
Determination of the surface hydrophobicity or wettability of nanomaterials and nanoparticles (NPs) is often challenged by the 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 the water contact angle, octanol–water partition coefficient ( K ow ) 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-assembled monolayers (SAMs) to establish a linear relationship between the adhesion forces and water contact angles based on the continuum thermodynamic approach (CTA). This relationship was used to determine the local surface hydrophobicity of seven different NPs ( i.e. , TiO 2 , ZnO, SiO 2 , CuO, CeO 2 , α-Fe 2 O 3 , and Ag), which agreed well with bulk contact angles of these NPs. Some discrepancies were observed for Fe 2 O 3 , CeO 2 and SiO 2 NPs, probably because of surface hydration and roughness effects. Moreover, the solution pH and ionic strength had negligible effects on more » the adhesion forces between the AFM tip and MWCNTs or C 60 , indicating that the hydrophobicity of carbonaceous nanomaterials is not influenced by pH or ionic strength (IS). By contrast, natural organic matter (NOM) appreciably decreased the hydrophobicity of MWCNTs and C 60 due to surface coating of hydrophilic NOM. This scanning probe method has been proved to be reliable and robust toward the accurate measurement of the nanoscale hydrophobicity of individual NPs or nanomaterials in liquid environments. « less
Authors:
;
Award ID(s):
1756444 1235166
Publication Date:
NSF-PAR ID:
10100822
Journal Name:
Physical Chemistry Chemical Physics
Volume:
20
Issue:
37
Page Range or eLocation-ID:
24434 to 24443
ISSN:
1463-9076
Sponsoring Org:
National Science Foundation
More Like this
  1. Based on tunable properties, engineered nanoparticles (NPs) hold significant promise for water treatment technologies. Motivated by concerns regarding toxicity and non-biodegradability of some nanoparticles, we explored engineered magnetite (Fe 3 O 4 ) nanoparticles with a biocompatible coating. These were prepared with a coating of rhamnolipid, a biosurfactant primarily obtained from Pseudomonas aeruginosa . By optimizing synthesis and phase transfer conditions, particles were observed to be monodispersed and stable in water under environmentally relevant pH and ionic strength values. These materials were evaluated for U( vi ) removal from water at varying dissolved inorganic carbon and pH conditions. The rhamnolipid-coated iron oxide nanoparticles (IONPs) showed high sorption capacities at pH 6 and pH 8 in both carbonate-free systems and systems in equilibrium with atmospheric CO 2 . Equilibrium sorption behavior was interpreted using surface complexation modeling (SCM). Two models (diffuse double layer and non-electrostatic) were evaluated for their ability to account for U( vi ) binding to the carboxyl groups of the rhamnolipid coating as a function of the pH, total U( vi ) loading, and dissolved inorganic carbon concentration. The diffuse double layer model provided the best simulation of the adsorption data and was sensitive to U( vi )more »loadings as it accounted for the change in the surface charge associated with U( vi ) adsorption.« less
  2. Abstract

    Most small asteroids are defined as “rubble piles” or bodies with zero tensile strength and large bulk porosity. The cohesive forces that hold them together act at the grain scale, and their magnitude is often estimated from similar materials when used in simulations. Improving the accuracy of predictions of asteroid strengths requires suitable laboratory measurements of relevant materials, as well as increasing the availability of materials from sample return. Atomic force microscopy (AFM) is well suited for force measurements relative to particle–particle interactions. In this work, we use AFM force measurements to evaluate the cohesive forces that act between micron-sized grains. We investigate the effect of the sizes of the interacting grains of JSC-1 lunar simulant using three sample sizes (<45, 75–125, and 125–250μm) and three spherical AFM tip diameters (2μm, 15μm, and 45μm). In all cases, adhesion forces were larger at ambient relative humidity (RH), where the water layer on the surface of the grains is more prominent, creating a larger meniscus between the tip and the grain upon contact. We observed weaker adhesion with larger grain/tip size, which can be attributed to the changing contact area between the samples and the tips. We expect that our approachmore »will pave the way to a better understanding of regolith surface properties such as adhesion and cohesion and provide suitable input for models that can be used to predict the evolution of asteroids and their particle behaviors.

    « less
  3. To unravel fouling and defouling mechanisms of protein, saccharides and natural organic matters (NOM) on polymeric membrane during filtration, this study investigated filtration characteristics on polyvinyl chloride (PVC) ultrafiltration membranes with bovine serum albumin, dextran, humic acid as model foulants. Membrane fouling and defouling performances were analyzed through monitoring the flux decline during filtration and flux recovery during physical backwash. Physico-chemical properties (e.g., hydrophobicity and surface charge) of PVC membrane and foulants were characterized, which were used in the extended Derjaguin–Landau–Verwey–Overbeek (EDLVO) theory to calculate the interaction energies between membrane foulant and foulant-foulant. The results showed that at the later filtration stages the fouling rate was strongly correlated with the deposition rate, which was determined by the interaction energy profile calculated by EDLVO. Moreover, the adhesion forces of membrane–foulant and foulant–foulant were further measured by atomic force microscopy (AFM) with modified colloidal probes. A positive correlation (R2 =0.845) between particle detachment rate (determined by adhesion force) and defouling rate was developed for BSA and HA foulants that led to cake layer formation. By contrast, dextran defouling rate was off this correlation as dextran partially clogged membrane pores due to its smaller size.
  4. Protein adsorption onto nanomaterials is a process of vital significance and it is commonly controlled by functionalizing their surface with polymers. The efficiency of this strategy depends on the design parameters of the nanoconstruct. Although significant amount of work has been carried out on planar surfaces modified with different types of polymers, studies investigating the role of surface curvature are not as abundant. Here, we present a comprehensive and systematic study of the protein adsorption process, analyzing the effect of curvature and morphology, the grafting of polymer mixtures, the type of monomer (neutral, acidic, basic), the proteins in solution, and the conditions of the solution. The theoretical approach we employed is based on a molecular theory that allows to explicitly consider the acid–base reactions of the amino acids in the proteins and the monomers on the surface. The calculations showed that surface curvature modulates the molecular organization in space, but key variables are the bulk pH and salt concentration (in the millimolar range). When grafting the NP with acidic or basic polymers, the surface coating could disfavor or promote adsorption, depending on the solution’s conditions. When NPs are in contact with protein mixtures in solution, a nontrivial competitive adsorption processmore »is observed. The calculations reflect the balance between molecular organization and chemical state of polymers and proteins, and how it is modulated by the curvature of the underlying surface.« less
  5. Surface adsorption of two commonly detected emerging contaminants, amlodipine (AMP) and carbamazepine (CBZ), onto model colloidal microplastics, natural organic matter (NOM), and fullerene nanomaterials have been investigated. It is found that AMP accumulation at these colloidal–aqueous interfaces is markedly higher than that of CBZ. Measurements of surface excess and particle zeta potential, along with pH-dependent adsorption studies, reveal a distinct influence of colloidal functional group on the adsorption properties of these pharmaceuticals. AMP shows a clear preference for a surface containing carboxylic group compared to an amine modified surface. CBZ, in contrast, exhibit a pH-dependent surface proclivity for both of these microparticles. The type of interactions and molecular differences with respect to structural rigidity and charge properties explain these observed behaviors. In this work, we also demonstrate a facile approach in fabricating uniform microspheres coated with NOM and C 60 nanoclusters. Subsequent binding studies on these surfaces show considerable adsorption on the NOM surface but a minimal uptake of CBZ by C 60 . Adsorption induced colloidal aggregation was not observed. These findings map out the extent of contaminant removal by colloids of different surface properties available in the aquatic environment. The methodology developed for the adsorption study also opensmore »up the possibility for further investigations into colloidal–contaminant interactions.« less