Interaction forces and molecular adhesion between pre-adsorbed poly(ethylene imine) layers

Synthesis and Study of a New Type of Fluorinated Polyether Demulsifier for Heavy Oil Emulsion Demulsification

To solve the problem of heavy oil demulsification difficulties in Liaohe Oilfield, phenolamine resin initiator was synthesized from p-trifluoromethyl phenol, and then FB series fluorinated polyether demulsifiers were synthesized by block polymerization using ethylene oxide (EO) and propylene oxide (PO) as raw materials. The demulsifiers were characterized by infrared spectroscopy, cloud point, hydrophilic-lipophilic balance (HLB) value, and surface tension. The demulsifying and dehydrating properties were tested by demulsifying and dehydrating experiments, the demulsification mechanism was analyzed by the microscopic demulsification process test, and the influence of demulsifier addition and demulsifying temperature on demulsifying performance was also studied. The results showed that under the condition of the optimum demulsification temperature of 60 °C and the optimum demulsifier dosage of 100 mg/L, the water removal (%) of fluorinated polyether demulsifier of FB 4 was the highest, and the overall water removal (%) of 50 mL crude oil emulsion in Liaohe Oilfield reached 90.33% within 2 h, which was better than the current demulsifier used in Liaohe crude oil.

Electrokinetics in Micro-channeled Cantilevers: Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics

The combination of atomic force microscopy (AFM) with nanofluidics, also referred to as FluidFM, has facilitated new applications in scanning ion conductance microscopy, direct force measurements, lithography, or controlled nanoparticle deposition. An essential element of this new type of AFMs is its cantilever, which bears an internal micro-channel with a defined aperture at the end. Here, we present a new approach for in-situ characterization of the internal micro-channels, which is non-destructive and based on electrochemical methods. It allows for probing the internal environment of a micro-channeled cantilever and the corresponding aperture, respectively. Acquiring the streaming current in the micro-channel allows to determine not only the state of the aperture over a wide range of ionic strengths but also the surface chemistry of the cantilever’s internal channel. The high practical applicability of this method is demonstrated by detecting the aspiration of polymeric, inorganic and hydrogel particles with diameters ranging from several µm down to 300 nm. By verifying in-situ the state of the aperture, i.e. open versus closed, electrophysiological or nano-deposition experiments will be significantly facilitated. Moreover, our approach is of high significance for direct force measurements by the FluidFM-technique and sub-micron colloidal probes.

Chemical unfolding of protein domains induces shape change in programmed protein hydrogels

Programmable behavior combined with tailored stiffness and tunable biomechanical response are key requirements for developing successful materials. However, these properties are still an elusive goal for protein-based biomaterials. Here, we use protein-polymer interactions to manipulate the stiffness of protein-based hydrogels made from bovine serum albumin (BSA) by using polyelectrolytes such as polyethyleneimine (PEI) and poly-L-lysine (PLL) at various concentrations. This approach confers protein-hydrogels with tunable wide-range stiffness, from ~10-64 kPa, without affecting the protein mechanics and nanostructure. We use the 6-fold increase in stiffness induced by PEI to program BSA hydrogels in various shapes. By utilizing the characteristic protein unfolding we can induce reversible shape-memory behavior of these composite materials using chemical denaturing solutions. The approach demonstrated here, based on protein engineering and polymer reinforcing, may enable the development and investigation of smart biomaterials and extend protein hydrogel capabilities beyond their conventional applications.

Interactions of Silver Nanoparticles Formed in Situ on AFM Tips with Supported Lipid Bilayers

A facile approach for functionalizing atomic force microscopy (AFM) tips with nanoparticles (NPs) will provide exciting opportunities in the field of tip-enhanced vibrational spectroscopy and in probing the interactions between NPs and biological systems. In this study, through successive exposure to polydopamine and AgNO₃ solutions, the apex of AFM tips was functionalized with silver nanoparticles (AgNPs). The AgNP-modified AFM tips were used to measure the interaction forces between AgNPs and supported lipid bilayers (SLBs) formed on mica, as well as to probe the penetration of SLBs by AgNPs, with an emphasis on the effect of human serum albumin (HSA) proteins. AgNPs experienced predominantly repulsive forces when approaching SLBs. The presence of HSA resulted in an enhancement in the repulsive interactions between AgNPs and SLBs, likely through steric repulsion. Finally, the forces required for AgNPs to penetrate SLBs were higher in the presence of HSA probably due to the increase in the effective size of the nanoscale protuberances on the AFM tip stemming from the formation of protein coronas around the AgNPs.

Evaluation of Adhesion Forces for the Manipulation of Micro-Objects in Submerged Environment through Deposition of pH Responsive Polyelectrolyte Layers

Optimization of surface treatment for reversible adhesion of micro-objects in liquid environment for the need in microassembly processes is presented. A spherical borosilicate probe and planar oxidized silicon wafer substrates were modified by deposition of pH sensitive polyelectrolyte films through layer-by-layer technique. Branched polyethylenimine (b-PEI) and poly(sodium styrenesulfonate) (PSS) were deposited in alternating manner on surfaces, and the influence of polyelectrolyte concentration, pH of deposition, and number of layers on the adhesion were successively examined. The multilayer buildup was followed by optical reflectometry (OR) and dissipative quartz crystal microbalance (QCM-D). The adhesion forces were monitored in aqueous environment at variable pH values by colloidal probe AFM microscopy. The thermodynamic work of adhesion was derived from the pull-off forces by using the Johnson-Kendall-Roberts (JKR) model and compared to the work of adhesion determined from contact angle measurements. It was found out that they correlate well, however, the values accessed from JKR model were underestimated, which was attributed mainly to the effect of surface roughness. Obtained results have demonstrated that it is possible to achieve repeatable reversible adhesion with the change of pH of submerged environment by appropriately tailoring the surface properties and therefore the prevailing surface forces.

Multiscaling Approach for Non-Destructive Adhesion Studies of Metal/Polymer Composites

The adhesion of polyamide 6 (PA6) and polyethylene (PE) toward an aluminum alloy (Al-A) and a dual phase steel (DPS) is studied by contact angle (CA) measurements and atomic force microscopy (AFM). With the combination of the two methods the adhesion properties on a macro- and (sub)microscopic scale can be determined in a nondestructive way. The work of adhesion per area (Wad) of the studied metal/polymer hybrids qualitatively scales the same on both length scales, that is, Al-A/PA6 > DPS/PA6 > Al-A/PE, DPS/PE. The polymer dominates the adhesion. The lower adhesion for PE toward the metal surfaces is explained by dominating van der Waals attraction forces, whereas adhesion for PA6 can also be attributed to attractive polar forces such as hydrogen bonding. For metal/PA6, Wad on a macro- and microscopic length scale is similar. For metal/PE, a discrepancy is measured with lower adhesion values on the microscopic scale than on the macroscopic scale.

Chain Length and Grafting Density Dependent Enhancement in the Hydrolysis of Ester-Linked Polymer Brushes

Poly(N,N-dimethylacrylamide) (PDMA) brushes with different grafting density and chain length were grown from an ester group-containing initiator using surface-initiated polymerization. Hydrolysis of the PDMA chains from the surface was monitored by measuring thickness of the polymer layer by ellipsometry and extension length by atomic force microscopy. It was found that the initial rate of cleavage of one end-tethered PDMA chains was dependent on the grafting density and chain length; the hydrolysis rate was faster for high grafting density brushes and brushes with higher molecular weights. Additionally, the rate of cleavage of polymer chains during a given experiment changed by up to 1 order of magnitude as the reaction progressed, with a distinct transition to a lower rate as the grafting density decreased. Also, polymer chains undergo selective cleavage, with longer chains in a polydisperse brush being preferentially cleaved at one stage of the hydrolysis reaction. We suggest that the enhanced initial hydrolysis rates seen at high grafting densities and high chain lengths are due to mechanical activation of the ester bond connecting the polymer chains to the surface in association with high lateral pressure within the brush. These results have implications for the preparation of polymers brushes, their stability under harsh conditions, and the analysis of polymer brushes from partial hydrolysates.

Accurate predictions of forces in the presence of multivalent ions by Poisson-Boltzmann theory

Forces between positively and negatively charged colloidal particles across aqueous salt solutions containing multivalent ions are measured directly with the atomic force microscope (AFM). The measurements are interpreted quantitatively with Poisson-Boltzmann (PB) theory. Thereby, the surface potentials and regulation properties of the particle surfaces are extracted from symmetric measurements between the same types of particles. This information is used to predict force profiles in the asymmetric situations involving different types of particles without any adjustable parameters. These predictions turn out to be very accurate, which demonstrates that the mean-field PB theory is reliable down to distances of about 5 nm. While various reports in the literature indicate that this theory should fail due to neglect of ion correlations, such effects seem important only at higher concentrations and smaller distances.

Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation

This review summarizes the current understanding of adsorption of polyelectrolytes to oppositely charged solid substrates, the resulting interaction forces between such substrates, and consequences for colloidal particle aggregation. The following conclusions can be reached based on experimental findings. Polyelectrolytes adsorb to oppositely charged solid substrates irreversibly up to saturation, whereby loose and thin monolayers are formed. The adsorbed polyelectrolytes normally carry a substantial amount of charge, which leads to a charge reversal. Frequently, the adsorbed films are laterally heterogeneous. With increasing salt levels, the adsorbed mass increases leading to thicker and more homogeneous films. Interaction forces between surfaces coated with saturated polyelectrolyte layers are governed at low salt levels by repulsive electric double layer interactions, and particle suspensions are stable under these conditions. At appropriately high salt levels, the forces become attractive, principally due to van der Waals interactions, but eventually also through other forces, and suspensions become unstable. This situation can be rationalized with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Due to the irreversible nature of the adsorption process, stable unsaturated layers form in colloidal particle suspensions at lower polyelectrolyte doses. An unsaturated polyelectrolyte layer can neutralize the overall particle surface charge. Away from the charge reversal point, electric double layer forces are dominant and particle suspensions are stable. As the charge reversal point is approached, attractive van der Waals forces become important, and particle suspensions become unstable. This behaviour is again in line with the DLVO theory, which may even apply quantitatively, provided the polyelectrolyte films are sufficiently laterally homogeneous. For heterogeneous films, additional attractive patch-charge interactions may become important. Depletion interactions may also lead to attractive forces and suspension destabilization, but such interactions become important only at high polyelectrolyte concentrations.

Coating of AFM probes with aquatic humic and non-humic NOM to study their adhesion properties

Atomic force microscopy (AFM) was used to study interaction forces between four Natural Organic Matter (NOM) samples of different physicochemical characteristics and origins and mica surface at a wide range of ionic strength. All NOM samples were strongly adsorbed on positively charged iron oxide-coated silica colloidal probe. Cross-sectioning by focused ion beam milling technique and elemental mapping by energy-filtered transmission electron microscopy indicated coating completeness of the NOM-coated colloidal probes. AFM-generated force-distance curves were analyzed to elucidate the nature and mechanisms of these interacting forces. Electrostatics and steric interactions were important contributors to repulsive forces during approach, although the latter became more influential with increasing ionic strength. Retracting force profiles showed a NOM adhesion behavior on mica consistent with its physicochemical characteristics. Humic-like substances, referred as the least hydrophilic NOM fraction, i.e., so called hydrophobic NOM, poorly adsorbed on hydrophilic mica due to their high content of ionized carboxyl groups and aromatic/hydrophobic character. However, adhesion force increased with increasing ionic strength, suggesting double layer compression. Conversely, polysaccharide-like substances showed high adhesion to mica. Hydrogen-bonding between hydroxyl groups on polysaccharide-like substances and highly electronegative elements on mica was suggested as the main adsorption mechanism, where the adhesion force decreased with increasing ionic strength. Results from this investigation indicated that all NOM samples retained their characteristics after the coating procedure. The experimental approach followed in this study can potentially be extended to investigate interactions between NOM and clean or fouled membranes as a function of NOM physicochemical characteristics and solution chemistry.

Adhesion of colloidal particles on modified electrodes

The adhesion between colloidal silica particles and modified electrodes has been studied by direct force measurements with the colloidal probe technique based on the atomic force microscope (AFM). The combination of potentiostatic control of gold electrodes and chemical modification of their surface with self-assembled monolayers (SAMs) allows for the decoupling of forces due to the electrical double layers and functional groups at the solid/liquid interface. Adhesion on such electrodes can be tuned over a large range using the externally applied potential and the aqueous solution's ionic strength. By utilizing cantilevers with a high force constant, it is possible to separate the various contributions to adhesion in an unambiguous manner. These contributions comprise diffuse-layer overlap, van der Waals forces, solvent exclusion, and electrocapillarity. A quantitative description of the observed adhesion forces is obtained by taking into account the surface roughness of the silica particle. The main component of the adhesion forces originates from the overlap of the electrical double layers, which is tuned by the external potential. By contrast, effects due to electrocapillarity are of only minor importance. Based on our quantitative analysis, a new approach is proposed that allows tuning of the adhesion force as a function of the externally applied potential. We expect this approach to have important applications for the design of microelectromechanical systems (MEMS), the development of electrochemical sensors, and the application of micro- and nanomanipulation.

Investigating forces between charged particles in the presence of oppositely charged polyelectrolytes with the multi-particle colloidal probe technique

Direct force measurements are used to obtain a comprehensive picture of interaction forces acting between charged colloidal particles in the presence of oppositely charged polyelectrolytes. These measurements are achieved by the multi-particle colloidal probe technique based on the atomic force microscope (AFM). This novel extension of the classical colloidal probe technique offers three main advantages. First, the technique works in a colloidal suspension with a huge internal surface area of several square meters, which simplifies the precise dosing of the small amounts of the polyelectrolytes needed and makes this approach less sensitive to impurities. Second, the particles are attached in-situ within the fluid cell, which avoids the formation of nanobubbles on the latex particles used. Third, forces between two similar particles from the same batch are being measured, which allows an unambiguous determination of the surface potential due to the symmetry of the system. Based on such direct force measurements involving positively and negatively charged latex particles and different polyelectrolytes, we find the following forces to be relevant. Repulsive electrostatic double-layer forces and attractive van der Waals forces as described by the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) are both important in these systems, whereby the electrostatic forces dominate away from the isoelectric point (IEP), while at this point they vanish. Additional non-DLVO attractive forces are operational, and they have been identified to originate from the electrostatic interactions between the patch-charge heterogeneities of the adsorbed polyelectrolyte films. Highly charged polyelectrolytes induce strong patch-charge attractions, which become especially important at low ionic strengths and high molecular mass. More weakly charged polyelectrolytes seem to form more homogeneous films, whereby patch-charge attractions may become negligible. Individual bridging events could be only rarely identified from the retraction part of the force profiles, and therefore we conclude that bridging forces are unimportant in these systems.

Adsorption of Spherical Polyelectrolyte Brushes: from Interactions to Surface Patterning

Adsorption of colloidal particles constitutes an attractive route to tailor the properties of surfaces. However, for efficient material design full control over the particle-substrate interactions is required. We investigate the interaction of spherical polyelectrolyte brushes (SPB) with charged substrates based on adsorption studies and atomic force spectroscopy. The brush layer grafted from the colloidal particles allows a precise adjustment of their adsorption behavior by varying the concentration of added salt. We find a pronounced selectivity between oppositely and like-charged surfaces for ionic strengths up to 10 mM. Near the transition from the osmotic to the salted brush regime at approximately 100 mM attractive secondary interactions become dominant. In this regime SPB adsorb even to like-charged surfaces. To determine the adhesion energy of SPB on charged surfaces directly, we synthesize micrometer-sized SPB. These particles are used in colloidal probe AFM studies. Measurements on oppositely charged surfaces show high forces of adhesion for low ionic strengths that can be attributed to an entropy gain by counterion release. Transferring our observations to charge patterned substrates, we are able to direct the deposition of SPB into two-dimensional arrays. Considering that numerous chemical modifications have been reported for SPB, our studies could open exiting avenues for the production of functional materials with a hierarchical internal organization.

Torsional noise of a colloidal probe in contact with surface-grafted PEG layers

Surface coatings modify the interaction between microparticles and surfaces. To analyze the effect of a PLL-g-PEG surface coating on the sticktion of a microparticle, we analyzed the torsional Brownian fluctuations of a colloidal probe attached to an atomic force microscope (AFM) cantilever. Results obtained with uncoated surfaces indicate that the torsional oscillation due to noise was only weakly affected by hydrodynamic effects. Upon mechanical contact, however, the uncoated probe stuck to the uncoated surface. Coating probe or surface with a PLL-g-PEG brush polymer reduced the lateral interaction (sticktion and friction) of the colloidal probe. For such a combination, the torsional fluctuations persisted during mechanical compression of the brush layer. The results demonstrate that a PLL-g-PEG coating can effectively reduce the lateral interaction of a microparticle with a surface and prevent sticktion.

Conformation of poly(styrene sulfonate) layers physisorbed from high salt solution studied by force measurements on two different length scales

The conformation of poly(styrene sulfonate) (PSS) layers physisorbed from 1 M NaCl is determined by force measurements and imaging on two length scales. With colloidal probe technique steric forces as predicted for neutral grafted brushes are observed. On decrease and increase of the NaCl concentration, the grafting density remains constant, yet the brush thickness swells and shrinks reversibly with the salt concentration with an exponent of -0.3. At low salt conditions, the brush length amounts to 30% of the contour length, a behavior known for polyelectrolyte brushes and attributed to the entropy of the counterions trapped in the brush. Between a PSS layer and a pure colloidal silica sphere, the same steric forces are observed, and additionally at large separations (beyond the range of the steric repulsion) an electrostatic force is found. A negatively charged AFM tip penetrates the brush--a repulsive electrostatic force between the tip and surface is found, and single chains can be imaged. Thus, with the nanometer-sized AFM tip, the flatly adsorbed fraction of the PSS chains is seen, whereas the micrometer-sized colloidal probe interacts with the fraction of the chains penetrating into solution.

Diffuse layer properties of thiol-modified gold electrodes probed by direct force measurements

The diffuse layer properties of modified gold electrodes under potentiostatic control have been determined by direct force measurements. These measurements have been performed with a colloidal probe consisting of a silica particle attached to the cantilever of an atomic force microscope. The gold electrodes were modified by self-assembled monolayers (SAMs) of different thickness. Additionally, the terminating functional groups of the monolayer have been varied. The interaction force profiles have been fit to the full solutions of the nonlinear Poisson-Boltzmann equation. An accurate quantitative description of the force profiles has been obtained by taking charge regulation between the surfaces into account. The diffuse layer potentials obtained from these fits were studied in dependence of the potential applied to the gold electrode. The capacitance of the SAM and the potential of zero charge (pzc) have been determined for various SAMs of different thickness and surface termination. The values obtained by our direct force measurements are in agreement with the ones reported by classical electrochemical techniques. The capacitance of the SAM depends primarily on the thickness of the monolayer and its crystalline structure. Pronounced differences in the pzc for the different functional groups have been found. These changes are related to the dipole moment of the functional group terminating the SAM. Our data are in agreement with ion adsorption, but this effect seems to be less pronounced than for bare gold electrodes.

Enhanced aggregation of alginate-coated iron oxide (hematite) nanoparticles in the presence of calcium, strontium, and barium cations

Early-stage aggregation kinetics studies of alginate-coated hematite nanoparticles in solutions containing alkaline-earth metal cations revealed enhanced aggregation rates in the presence of Ca2+, Sr2+, and Ba2+, but not with Mg2+. Transmission electron microscopy (TEM) imaging of the aggregates provided evidence that alginate gel formation was essential for enhanced aggregation to occur. Dynamic light scattering (DLS) aggregation results clearly indicated that a much lower concentration of Ba2+ compared to Ca2+ and Sr2+ was required to achieve a similar degree of enhanced aggregation in each system. To elucidate the relationship between the alginate's affinities for divalent cations and the enhanced aggregation of the alginate-coated hematite nanoparticles, atomic force microscopy (AFM) was employed to probe the interaction forces between alginate-coated hematite surfaces under the solution chemistries used for the aggregation study. Maximum adhesion forces, maximum pull-off distances, and the work of adhesion were used as indicators to gauge the alginate's affinity for the divalent cations and the resulting attractive interactions between alginate-coated hematite nanoparticles. The results showed that alginate had higher affinity for Ba2+ than either Sr2+ or Ca2+. This same trend was consistent with the cation concentrations required for comparable enhanced aggregation kinetics, suggesting that the rate of alginate gel formation controls the enhanced aggregation kinetics. An aggregation mechanism incorporating the gelation of alginate is proposed to explain the accelerated aggregate growth in the presence of Ca2+, Sr2+, and Ba2+.

Functionalizable Nanomorphology Gradients via Colloidal Self-Assembly

We present a novel approach for the fabrication of tailored nanomorphology gradients on metal oxide surfaces. We first show the direct formation of a nanocolloidal density gradient by a dip-coating process. The obtained silica nanoparticle gradients are then subjected to a heat treatment. Control of this sintering step allows the precise tailoring of the particle morphology on the surface. Both these processes together provide a new tool to form precise, tunable, and material-independent nanomorphology gradients.