Adhesion of single polyelectrolyte molecules on silica, mica, and bitumen surfaces

Atomic Force Microscopy of Hydrolysed Polyacrylamide Adsorption onto Calcium Carbonate

In this work, the interaction of hydrolysed polyacrylamide (HPAM) of two molecular weights (F3330, 11-13 MDa; F3530, 15-17 MDa) with calcium carbonate (CaCO3) was studied via atomic force microscopy (AFM). In the absence of polymers at 1.7 mM and 1 M NaCl, good agreement with DLVO theory was observed. At 1.7 mM NaCl, repulsive interaction during approach at approximately 20 nm and attractive adhesion of approximately 400 pN during retraction was measured, whilst, at 1 M NaCl, no repulsion during approach was found. Still, a significantly larger adhesion of approximately 1400 pN during retraction was observed. In the presence of polymers, results indicated that F3330 displayed higher average adhesion (450-625 pN) and interaction energy (43-145 aJ) with CaCO3 than F3530's average adhesion (85-88 pN) and interaction energy (8.4-11 aJ). On the other hand, F3530 exerted a longer steric repulsion distance (70-100 nm) than F3330 (30-70 nm). This was likely due to the lower molecular weight. F3330 adopted a flatter configuration on the calcite surface, creating more anchor points with the surface in the form of train segments. The adhesion and interaction energy of both HPAM with CaCO3 can be decreased by increasing the salt concentration. At 3% NaCl, the average adhesion and interaction energy of F3330 was 72-120 pN and 5.6-17 aJ, respectively, while the average adhesion and interaction energy of F3530 was 11.4-48 pN and 0.3-2.98 aJ, respectively. The reduction of adhesion and interaction energy was likely due to the screening of the COO- charged group of HPAM by salt cations, leading to a reduction of electrostatic attraction between the negatively charged HPAM and the positively charged CaCO3.

Application of Atomic Force Microscopy as Advanced Asphalt Testing Technology: A Comprehensive Review

In the past three decades, researchers have engaged in the relationship between the composition, macro performance, and microstructure of asphalt. There are many research results in the use of atomic force microscopy (AFM) to study the microstructure and related mechanisms of asphalt. Based on previous studies, the performance of asphalt from its microstructure has been observed and analyzed, and different evaluation indices and modification methods have been proposed, providing guidance toward improving the performance of asphalt materials and benefiting potential applications. This review focuses on the typical application and analysis of AFM in the study of the aging regeneration and modification properties of asphalt. Additionally, this review introduces the history of the rheological and chemical testing of asphalt materials and the history of using AFM to investigate asphalt. Furthermore, this review introduces the basic principles of various modes of application of AFM in the microstructure of asphalt, providing a research direction for the further popularization and application of AFM in asphalt or other materials in the future. This review aims to provide a reference and direction for researchers to further popularize the application of AFM in asphalt and standardize the testing methods of AFM. This paper is also helpful in further exploring the relationship between the microstructure and macro performance of asphalt.

Current Research and Challenges in Bitumen Emulsion Manufacturing and Its Properties

The global increase of road infrastructure and its impact on the environment requires serious attention to develop sustainable and environmentally friendly road materials. One group of those materials is produced by using bitumen emulsion. However, there are still scientific and technical obstacles standing against its regular application. The bitumen emulsion formulation process and compositional optimization are subjected to a high number of degrees of freedom. Consequently, obtaining the desired product is mostly based on a series of random and tedious trials because of the enormous number of tests that are carried out to meet the required properties, such as emulsion stability, viscosity, droplet size (and distribution), and bitumen emulsion chemistry. Several pre-established formulation procedures have been presented in the literature. Some of them have technical limitations to be utilized for practical industrial application, whereas others are still not understood enough to be applied in bitumen emulsion formulation. Therefore, discussing some important issues in this field could be useful to offer a practical guide for bitumen emulsion manufacturers when trying to formulate a well-defined bitumen emulsion to best fit its use in pavement infrastructure rather than to simply to meet standard specifications. This review paper aims to enable the ultimate potential of bitumen emulsion by further reviewing the research progress of bitumen emulsion manufacturing and discussing the literature available up to now on this topic, in the realm of bitumen emulsion manufacturing and emulsion chemistry.

Enhanced Adsorption of Anionic Polymer on Montmorillonite by Divalent Cations and the Effect of Salinity

Adsorption of polymers from an aqueous solution onto clay minerals is of great interest to many applications such as water purification and soil conditioning. Molecular dynamics simulations were performed to study the adsorption of anionic polyacrylamide (APAM) on anionic montmorillonite, in an aqueous solution containing monovalent or divalent salts. Compared with monovalent salts (NaCl), the enhancement of APAM adsorption brought by divalent salts (CaCl₂) was significant, which could not be explained by the Poisson-Boltzmann theory alone. Each solvated Ca²⁺ was coordinated by 4-6 water oxygens in its first coordination shell. One to two of these water molecules were displaced when APAM formed a complex with Ca²⁺. Ca²⁺ ions in the adsorbed Ca²⁺-APAM complexes did not serve as bridges sandwiched between APAM and Mt; instead, the complexes carried a residual positive charge and were subsequently attracted to montmorillonite. The number of adsorbed Ca²⁺-APAM complexes changed with salinity in a nonmonotonic manner, due to the modulation of apparent charges of montmorillonite and APAM by Ca²⁺. Increasing adsorption of Ca²⁺-APAM complexes also promoted APAM adsorption through direct hydrogen bonding with montmorillonite. The findings provided new molecular insights into the long-standing debates on the role of divalent ions in promoting polymer adsorption on like-charged solid surfaces.

The application of atomic force microscopy in mineral flotation

During the past years, atomic force microscopy (AFM) has matured to an indispensable tool to characterize nanomaterials in colloid and interface science. For imaging, a sharp probe mounted near to the end of a cantilever scans over the sample surface providing a high resolution three-dimensional topographic image. In addition, the AFM tip can be used as a force sensor to detect local properties like adhesion, stiffness, charge etc. After the invention of the colloidal probe technique it has also become a major method to measure surface forces. In this review, we highlight the advances in the application of AFM in the field of mineral flotation, such as mineral morphology imaging, water at mineral surface, reagent adsorption, inter-particle force, and bubble-particle interaction. In the coming years, the complementary characterization of chemical composition such as using infrared spectroscopy and Raman spectroscopy for AFM topography imaging and the synchronous measurement of the force and distance involving deformable bubble as a force sensor will further assist the fundamental understanding of flotation mechanism.

Probing Single-Molecule Adhesion of a Stimuli Responsive Oligo(ethylene glycol) Methacrylate Copolymer on a Molecularly Smooth Hydrophobic MoS2 Basal Plane Surface

Molybdenum disulfide (MoS₂) has been receiving increasing attention in scientific research due to its unique properties. Up to now, several techniques have been developed to prepare exfoliated nanosize MoS₂ dispersions to facilitate its applications. To improve its desired performance, as-prepared MoS₂ dispersion needs further appropriate modification by polymers. Thus, understanding polymer-MoS₂ interaction is of great scientific importance and practical interest. Here, we report our results on molecular interactions of a biocompatible stimuli-responsive copolymer with the basal plane surface of MoS₂ determined using single molecule force spectroscopy (SMFS). Under isothermal conditions, the single-molecule adhesion force of oligo(ethylene glycol) methacrylate copolymer was found to increase from 50 to 75 pN with increasing NaCl concentration from 1 mM to 2 M, as a result of increasing hydrophobicity of the polymers. The theoretical analysis demonstrated that single-molecule adhesion force is determined by two contributions: the adhesion energy per monomer and the entropic free energy of the stretched polymer chain. Further data analysis revealed a significant increase in the adhesion energy per monomer with a negligible change in the other contribution with increasing salt concentration. The hydrophobic attraction (HA) was found to be the main contribution for the higher adhesion energy in electrolyte solutions of higher NaCl concentrations where the zero-frequency of van der Waals interaction were effectively screened. The results illustrate that oligo(ethylene glycol) methacrylate copolymer is a promising polymer for functionalizing MoS₂ and that one can simply change the salt concentration to modulate the single-molecule interactions for desired applications.

Molecular Mechanisms of Ultrafiltration Membrane Fouling in Polymer-Flooding Wastewater Treatment: Role of Ions in Polymeric Fouling

Polymer (i.e., anionic polyacrylamide (APAM)) fouling of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes and its relationships to intermolecular interactions were investigated using atomic force microscopy (AFM). Distinct relations were obtained between the AFM force spectroscopy measurements and calculated fouling resistance over the concentration polarization layer (CPL) and gel layer (GL). The measured maximum adhesion forces (Fad,max) were closely correlated with the CPL resistance (Rp), and the proposed molecular packing property (largely based on the shape of AFM force spectroscopy curve) of the APAM chains was related to the GL resistance (Rg). Calcium ions (Ca(2+)) and sodium ions (Na(+)) caused more severe fouling. In the presence of Ca(2+), the large Rp corresponded to high foulant-foulant Fad,max, resulting in high flux loss. In addition, the Rg with Ca(2+) was minor, but the flux recovery rate after chemical cleaning was the lowest, indicating that Ca(2+) created more challenges in GL cleaning. With Na(+), the fouling behavior was complicated and concentration-dependent. The GL structures with Na(+), which might correspond to the proposed molecular packing states among APAM chains, played essential roles in membrane fouling and GL cleaning.

Effects of Water on the Single-Chain Elasticity of Poly(U) RNA

Water, the dominant component under the physiological condition, is a complicated solvent which greatly affects the properties of solute molecules. Here, we utilize atomic force microscope-based single-molecule force spectroscopy to study the influence of water on the single-molecule elasticity of an unstructured single-stranded RNA (poly(U)). In nonpolar solvents, RNA presents its inherent elasticity, which is consistent with the theoretical single-chain elasticity calculated by quantum mechanics calculations. In aqueous buffers, however, an additional energy of 1.88 kJ/mol·base is needed for the stretching of the ssRNA chain. This energy is consumed by the bound water rearrangement (Ew) during chain elongation. Further experimental results indicate that the Ew value is uncorrelated to the salt concentrations and stretching velocity. The results obtained in an 8 M guanidine·HCl solution provide more evidence that the bound water molecules around RNA give rise to the observed deviation between aqueous and nonaqueous environments. Compared to synthetic water-soluble polymers, the value of Ew of RNA is much lower. The weak interference of water is supposed to be the precondition for the RNA secondary structure to exist in aqueous solution.

Inherent stretching elasticity of a single polymer chain with a carbon-carbon backbone

We study the single-chain elasticities of three kinds of neutral polymers with a carbon-carbon (C-C) backbone by atomic force microscopy-based single-molecule force spectroscopy in a nonpolar solvent (octane), aiming at measuring the inherent chain elasticity of this very important class of polymers. The finding that the single-chain elasticities of all three polymers in octane are virtually identical in the entire force region implies that the side chains of the polymers have no detectable effects on the single-chain elasticity. By utilizing the single-chain elasticity from quantum mechanics calculations, the freely rotating chain model can provide the best fitting curve when each C-C bond is set to be the rotating unit. Although there are some exceptions when the side chain is very huge, our work provides a general result for the inherent elasticity of single neutral flexible polymer chains with C-C backbones.

Unexpected temperature-dependent single chain mechanics of poly(N-isopropyl-acrylamide) in water

Poly(N-isopropyl-acrylamide) (PNIPAM) is a paradigm thermally sensitive polymer, which has a lower critical solution temperature (LCST) of ~32 °C in water. Herein by AFM-based single molecule force spectroscopy (SMFS), we measured the single chain elasticity of PNIPAM across the LCST in water. Below LCST, the force curves obtained at different temperatures have no remarkable difference; while above LCST, an unexpected temperature dependent elasticity is observed, mainly in the middle force regime. We found that 35 °C is a turning point of the variation: from 31 to 35 °C, the middle parts of the force curves drop gradually, whereas from 35 to 40 °C, the middle parts rise gradually. A possible mechanism for the unexpected temperature dependent mechanics is proposed. The single chain contraction against external force upon heating from 35 to 40 °C may cast new light on the design of molecular devices that convert thermal energy to mechanical work.

Single molecule desorption studies on immobilized nanoclay particle surfaces

AFM-based single molecule force spectroscopy was performed on sheetlike inorganic particles of Na-montmorillonite to study the pH-dependent adsorption and desorption behavior of polyelectrolytes in aqueous solutions. Polyallylamine macromolecules were covalently attached on gold-coated AFM cantilevers. Heterogeneous surfaces were formed by immobilizing the nanoclay sheets on mica-stripped ultraflat Au(111) surfaces using aminothiol chemistry. Because of the constant surface charge of the particles over a wide pH range, polymer line charge density was the only parameter that affected the adsorption and desorption behavior when the ionic concentration was kept constant. Polarization modulation infrared-reflection absorption spectroscopy (PM-IRRAS) was performed on cast polyallylamine films to study the pH-dependent charge density of polyallylamine molecules. A good correlation was found between the line charge density and the adsorption characteristics of polyallylamine.

Study of Al(OH)3-polyacrylamide-induced pelleting flocculation by single molecule force spectroscopy

A nanocomposite polymer, Al(OH)3-polyacrylamide (Al-PAM) hybrid, was tested as a flocculant. This novel hybrid polymer was found to induce pellet-like floccules, leading to a more effective solid-liquid separation than common polyacrylamide (PAM)-based flocculants. To understand the mechanism of Al-PAM-induced pelleting flocculation, the molecular structure of this hybrid polymer was studied using an atomic force microscope (AFM). The interactions between Al-PAM molecules and a silica surface were measured using single molecule force spectroscopy (SMFS). The Al-PAM molecules were found to have a star-like structure with Al(OH)3 colloidal particles as cores connecting PAM chains. The SMFS results showed a strong attachment of the Al(OH)3 cores to the silica surface with an adhesion force of approximately 1250 pN, in contrast to a weaker adhesion force of only approximately 250 pN for PAM chains on the silica surface. The Al-PAM-induced pelleting flocculation is attributed to its star-like structure.

The interfacial behaviour of single poly(N,N-dimethylacrylamide) chains as a function of pH

We have studied the pH-dependent conformational behaviour of poly(N,N-dimethylacrylamide) (PDMAc) at silicon and gold surfaces using single-molecule force spectroscopy and a quartz crystal microbalance (QCM). Despite the pH dependence, nuclear magnetic resonance and titration experiments demonstrate that PDMAc is not a weak polybase. The interaction between single chains and a silicon surface (with native oxide layer intact) in aqueous solution was investigated using force spectroscopy. Single-molecule force measurements were performed using thiol-functionalized PDMAc and gold-coated AFM cantilevers. The forces of interaction between PDMAc and the native oxide-coated silicon surface vary with the pH. The shape of the retraction curve for low pH solution includes a greater number of 'train' conformations, which suggests a stronger interaction with the surface relative to the surrounding media at low pH. The adsorption behaviour of PDMAc, from liquid onto silicon surfaces, was monitored using a QCM, which shows greater PDMAc adsorption onto silicon at low pH. The force spectroscopy and QCM investigations confirm that the PDMAc chain is more extended and stiffer in low pH solution. We attribute the pH-dependent behaviour to an increased number of hydrogen bonding sites on the silicon surface at low pH.

Imaging techniques applied to characterize bitumen and bituminous emulsions

The purpose of this article is to present some important advances in the imaging techniques currently used in the characterization of bitumen and bituminous emulsions. Bitumen exhibits some properties, such as a black colour and a reflecting surface at rest, which permit the use of optical techniques to study the macroscopic behaviour of asphalt mixes in the cold mix technology based on emulsion use. Imaging techniques allow monitoring in situ the bitumen thermal sensitivity as well as the complex phenomenon of emulsion breaking. Evaporation-driven breaking was evaluated from the shape of evaporating emulsion drops deposited onto non-porous and hydrophobic substrates. To describe the breaking kinetics, top-view images of a drying emulsion drop placed on an aggregate sheet were acquired and processed properly. We can conclude that computer-aided image analysis in road pavement engineering can elucidate the mechanism of breaking and curing of bituminous emulsion.

Adsorption of Polyelectrolyte versus Surface Charge: in Situ Single-Molecule Atomic Force Microscopy Experiments on Similarly, Oppositely, and Heterogeneously Charged Surfaces

We have studied the effect of the pH and surface charge of mica on the adsorption of the positively charged weak polyelectrolyte (PE) poly(2-vinylpyridine) (P2VP) using atomic force microscopy (AFM) single-molecule experiments. These AFM experiments were performed in situ directly under aqueous media. If the mica's surface and the PE are oppositely charged (pH > 3), the PE forms a flat adsorbed layer of two-dimensionally (2D) equilibrated self-avoiding random walk coils. The adsorbed layer's structure remains almost unchanged if the pH is decreased to pH 3 (the mica's surface is weakly charged). At pH 2 (the mica surface is decorated by spots of different electrical charges), the polyelectrolyte chains take the form of a 2D compressed coil. In this pH range, at an increased P2VP concentration in solution, the PE segments preferentially adsorb onto the top of previously adsorbed segments, rather than onto an unoccupied surface. We explain this behavior as being caused by the heterogeneous character of the charged surface and the competitive adsorption of hydronium ions. The further increase of polymer concentration results in a complete coverage of the mica substrate and the charge overcompensation by P2VP chains adsorbed on the similarly charged substrate, due to van der Waals forces.

Charge-transfer complex study by chemical force spectroscopy: a dynamic force spectroscopic approach

Charge-transfer interaction, as a reversible and rapid phenomenon, was evidenced by force microscopy. Pull-off forces were measured between a tip grafted with a trinitrofluorenone derivative and a surface functionalized with an electron-rich aromatic anthracene compound in a dodecane environment. The effect of the sweep time on the measured interaction forces is described, together with an extensive study of a competitive influence of free aromatic molecules in dodecane diluted solutions. These forces depend on the nature of the competitor and its concentration as well as on the velocity of tip/sample separation.