Surface Force Measurements Research Articles

Interaction and Adsorption of Hydrophobically Modified Polyacrylamide on Silica and Asphaltene Surfaces

The adsorption of polymers affects the cost and oil recovery in oil reservoir exploitation and the flocculation effect in the treatment of oil sand tailings. The adhesion and adsorption of a hydrophobically modified polyacrylamide (HMPAM), i.e., P(AM-NaAA-C16DMAAC), on silica and asphaltene were investigated using surface force measurements, thermodynamic analysis and quartz crystal microbalance with dissipation (QCM-D) measurement. Our study indicates that HMPAM polymer has strong interaction with both silica and asphaltene. The adhesion force of HMPAM on silica was stronger than that on asphaltene surface. Consistently, the adsorption of HMPAM was also greater on silica surface, with a more rigid layer formed on the surface. For HMPAM/silica system, the attractive interaction and the strong adhesion are mainly driven by the hydrogen bonding and electrostatic interaction. For HMPAM/asphaltene system, it is mainly due to hydrophobic interaction between the long hydrocarbon chains of HMPAM and asphaltene. Furthermore, continuous adsorption of HMPAM was detected and multiple layers formed on both silica and asphaltene surfaces, which can be attributed to the hydrophobic chains of HMPAM polymers. This work has illustrated the interaction mechanism of HMPAM polymer on hydrophilic silica and hydrophobic asphaltene surfaces, which provide insight into the industrial applications of hydrophobically modified polymer.

Understanding Dynamics of Electrochemical Double Layers via a Modified Concentrated Solution Theory

Understanding the dynamics of an electrochemical double layer (EDL) is of fundamental importance to a wide array of electrochemical energy devices. We develop in this study a modified concentrated solution theory to simulate EDL dynamics in the concentrated regime, in which the driving force of ion transport in EDL is derived from a free energy functional that considers ion size effect, short-range correlations, and solvent polarization. The model features a mobility matrix with non-zero off-diagonal elements due to short-range correlations. Model results are compared with molecular dynamic simulations and surface force measurement for an EDL in ionic liquids. The model gives out insights into how the ion size, various short-range correlations, and the solvent polarization affect the charging dynamics. The model is instrumental to design, optimization and control of high-power electrochemical devices.

Screening length for finite-size ions in concentrated electrolytes.

The classical Debye-Hückel (DH) theory clearly accounts for the origin of screening in electrolyte solutions and works rather well for dilute electrolyte solutions. While the Debye screening length decreases with the ion concentration and is independent of ion size, recent surface-force measurements imply that for concentrated solutions, the screening length exhibits an opposite trend; it increases with ion concentration and depends on the ionic size. The screening length is usually defined by the response of the electrolyte solution to a test charge but can equivalently be derived from the charge-charge correlation function. By going beyond DH theory, we predict the effects of ion size on the charge-charge correlation function. A simple modification of the Coulomb interaction kernel to account for the excluded volume of neighboring ions yields a nonmonotonic dependence of the screening length (correlation length) on the ionic concentration, as well as damped charge oscillations for high concentrations.

Analysis of interactions between cinnamycin and biomimetic membranes

The interaction between the cinnamycin and the biomimetic membranes was studied using the atomic force microscope(AFM). The bilayer was composed of the monolayer tethered on the gold surface and the outer layer fused with the vesicles on the monolayer. The vesicles were prepared at the desired ratio of dioleoylphosphatidylethanolamine(DOPE) to dioleoylphosphatidylcholine(DOPC). On the bilayer, the surface force measurement was performed with the cinnamycin immobilized covalently on the tip surface. The immobilization led to the presence of the adhesion, which was found while the tip was retracted from the bilayer. In addition, the magnitude of the adhesive force was changed with respect to the composition of DOPE in the outer layer. The difference in the adhesion may be attributed to the mean-molecular-area of DOPE and the specific-binding density on the outer layer. Furthermore, the analysis of the rupture force with respect to the loading rate indicated that the rupture length was around 0.1∼0.13 nm, which was similar to that of a van der Waals bond.

Wetting Transition on Liquid-Repellent Surfaces Probed by Surface Force Measurements and Confocal Imaging.

Superhydrophobic surfaces in the Cassie-Baxter wetting state retain an air layer at the surface which prevents liquid water from reaching into the porous surface structure. In this work we explore how addition of ethanol, which reduces the surface tension, influences the wetting properties of superhydrophobic and smooth hydrophobic surfaces. Wetting properties are measured by dynamic contact angles, and the air layer at the superhydrophobic surface is visualized by laser scanning confocal microscopy. Colloidal probe atomic force microscopy measurements between a hydrophobic microsphere and the macroscopic surfaces showed that the presence of ethanol strongly affects the interaction forces. When the macroscopic surface is superhydrophobic, attractive forces extending up to a few micrometers are observed on retraction in water and in 20 vol % ethanol, signifying the presence of a large and growing gas capillary. Submicrometer attractive forces are observed between the probe particle and a smooth hydrophobic surface, and in this case a smaller gas capillary is formed. Addition of ethanol results in markedly different effects between superhydrophobic and hydrophobic surfaces. In particular, we show that the receding contact angle on the superhydrophobic surface is of paramount importance for describing the interaction forces.

Surface Forces of Asymmetrically Grown Polyelectrolyte Multilayers: Searching for the Charges.

Surface forces are used to investigate the polymer conformation and the surface charge of polyelectrolyte multilayers. Films are prepared from strong polyelectrolytes with low and high linear charge density at 0.1 M NaCl, namely poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS). The multilayer has two growth regimes: in the beginning, the film can contain as many positive as negative monomers. After about 15 deposited layer pairs, a linear growth regime characterized by an excess of cationic PDADMA monomers occurs. Independent of the film composition, at preparation conditions, the film surface is flat, uncharged and partially hydrophobic. Surface force measurements at decreased ionic strength provide insight. For PSS-terminated films electrostatic forces are found. At the beginning of multilayer formation, the surface charge density is negative. However, in the linear growth regime it is positive and low (one charge per 200-400 nm2). This reversal of surface charge density of PSS-terminated films is attributed to excess PDADMA-monomers within the film. PDADMA terminated films show steric forces, chains protrude into the solution and form a pseudobrush, which scales as a polyelectrolyte brush with a low grafting density (1900 nm2 per chain). We suggest a model of polyelectrolyte multilayer formation: PDADMA with its low linear charge density adsorbs with weakly bound chains. Monovalent anions within the film compensate PDADMA monomer charges. When PSS adsorbs onto a PDADMA-terminated multilayer, PSS monomers replace monovalent anions. While electrostatic bonds are formed and dissolved within the polyelectrolyte multilayer, the surface charge density remains zero.

Interfacial Structure and Boundary Lubrication of a Dicationic Ionic Liquid

We report measurements of the normal surface forces and friction forces between two mica surfaces separated by a nanofilm of dicationic ionic liquid using a Surface Force Balance. The dicationic ionic liquid 1,10-bis(3-methylimidazolium)decane di[bis(trifluoromethylsulfonyl)imide] forms a layered structure in nanoconfinement, revealed by oscillatory structural forces. Friction measurements performed at different film thicknesses display quantized friction, i.e., discontinuities in friction as layers are squeezed out and friction coefficients dependent on the number of liquid layers confined between the surfaces. The details of the friction traces indicate a liquidlike film, and, surprisingly, decreasing friction with increasing water content; we discuss possible mechanisms underlying these observations. This latter trait may be helpful in applications where ionic liquid lubricants cannot be insulated against humid environments.

Investigation of the Antifouling Mechanism of Electroless Nickel–Phosphorus Coating against Sand and Bitumen

Antifouling coatings have attracted much attention in applications of industrial equipment and systems. The accumulation of foulants on downhole equipment used in the steam-assisted gravity drainage process for oil sand extraction reduces the recovery efficiency of bitumen and causes failures of the equipment. Applying suitable coatings is expected to endow antifouling property to the related substrate surface. Understanding the interaction mechanism between foulants and antifouling coating is critical for the evaluation and prediction of antifouling performance. In this work, antifouling coating consisting of Ni and P was prepared on the L80 carbon steel substrate, which is a commonly used material in downhole equipment. The surface properties (e.g., morphology and surface energy) of the Ni–P coating and pristine L80 substrates were characterized. The intermolecular and surface forces between typical foulants (e.g., silica and bitumen-coated silica) and Ni–P coated and uncoated L80 substrates were directly measured using the atomic force microscope colloidal probe technique to investigate the fouling mechanisms and predict the fouling behaviors of the foulants at the nanoscale. It was found that the Ni–P coating possessed lower surface energy, weaker attractive van der Waals interaction, and smaller adhesion to both silica and bitumen as compared to the uncoated L80 substrate. Bulk soaking experiments were also conducted accordingly which further demonstrated the antifouling performance of Ni–P coating against silica and bitumen, agreeing well with the surface force measurements. This work has improved the fundamental understanding of fouling behaviors of silica and bitumen and the antifouling mechanism of Ni–P coating at the nanoscale, with potential applications in many related processes in chemical and petroleum industries. The methodologies employed in this work can be readily extended for investigating fouling and antifouling mechanisms of different materials in various engineering fields.

Density functional theory of confined ionic liquids: A survey of the effects of ion type, molecular charge distribution, and surface adsorption.

Ionic liquids confined between two planar charged walls are explored using density functional theory. The effort represents a study of the effects of the molecular structure, molecular charge distribution, and degree of surface adsorption on forces between the surfaces and on the inhomogeneous atom density profiles. Surface adsorption was found to significantly affect both the magnitude and sign of the surface forces, while differences in the distribution of molecular charge did not. On the other hand, different bulk densities were found to produce dramatically different surface forces indicating a difference in the degree of molecular packing at and near surfaces. No long-range forces were found in any of the cases considered. We conclude that in the absence of any specific cation-anion pairing, surface charges are effectively screened, and the surface forces are dominated by short ranged steric and dispersion interactions between adsorbed molecular layers. In many cases, very similar surface forces correspond to very different molecular arrangements, suggesting that unambiguous interpretation of measured surface forces in ionic liquids, in terms of molecular behavior, may be difficult to guarantee.

Surface forces measurement for materials science

Abstract This article reviews the surface forces measurement as a novel tool for materials science. The history of the measurement is briefly described in the Introduction. The general overview covers specific features of the surface forces measurement as a tool for studying the solid-liquid interface, confined liquids and soft matter. This measurement is a powerful way for understanding interaction forces, and for characterizing (sometime unknown) phenomena at solid-liquid interfaces and soft complex matters. The surface force apparatus (SFA) we developed for opaque samples can study not only opaque samples in various media, but also electrochemical processes under various electrochemical conditions. Electrochemical SFA enables us to determine the distribution of counterions between strongly bound ones in the Stern layer and those diffused in the Gouy-Chapman layer. The shear measurement is another active area of the SFA research. We introduced a resonance method, i.e. the resonance shear measurement (RSM), that is used to study the effective viscosity and lubricity of confined liquids in their thickness from μm to contact. Advantages of these measurements are discussed by describing examples of each measurement. These studies demonstrate how the forces measurement is used for characterizing solid-liquid interfaces, confined liquids and reveal unknown phenomena. The readers will be introduced to the broad applications of the forces measurement in the materials science field.

Electrostatic Screening Length in Concentrated Salt Solutions

Thin films (0-30 nm) of very concentrated aqueous monovalent salt solutions (2-10 M of LiCl, NaCl, and CsCl) were examined to determine how ionic strength affects the screening length of the electrostatic potential. Measurements were consistent with a screening length in the range of 3-12 nm. The screening length increased monotonically as a function of salt concentration, and the rate of increase was a function of the monovalent salt type. The results were incompatible with the Debye length of Poisson-Boltzmann theory but consistent with previous measurements of surface forces. The screening length was determined from the surface excess of fluorescein, a dianion under basic conditions, which was present in trace amounts in the thin film and detected via its fluorescence emission. That is, we directly observed that the ion concentration in very concentrated solutions is perturbed far from an interface.

Effect of ζ-Potentials on Bubble-Particle Interactions

Derjaguin and Dukhin (1961) were the first to develop a model for bubble-particle interaction in flotation from first principles by considering the surface forces in wetting (or flotation) films. The model predicted that the energy barriers to bubble-particle attachment arise from the ζ-potentials of fine particles, which corroborates well with an earlier work of Fuerstenau (1957) reported for the flotation of quartz using dodecyl ammonium acetate (DAA) as collector. In the present work, a series of surface force measurements have been conducted by accurately monitoring the changes in bubble curvature during bubble-silica surface interactions. The curvature changes are then used to determine the capillary force, which is the sum of the hydrodynamic and surface forces. By subtracting the former that can also be determined from the curvature changes, one obtains the surface forces (or disjoining pressure). The results obtained as functions of pH and collector concentrations show that control of the ζ-potentials of bubbles relative to those of the silica surfaces is critical for maximizing the negative disjoining pressure, which is conducive to promoting bubble-particle attachment in flotation.

Fundamental study on surface force measurements using QCM

Fundamental study on surface force measurements using QCM

Forces between zinc sulphide surfaces; amplification of the hydrophobic attraction by surface charge.

Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).

A 3-mirror surface force balance for the investigation of fluids confined to nanoscale films between two ultra-smooth polarizable electrodes.

We present a new technique, based on the Surface Force Balance (SFB), for the direct measurement of surface forces between two ultra-smooth and polarizable gold electrode surfaces across thin fluid films. Combining the direct interferometric measurement of surface separation and contact geometry with smooth electrode surfaces has proved challenging in the past, and for this reason, previous measurements with the SFB typically involved two insulating mica surfaces, or one mica surface and one electrode surface, or an alternative less direct measure of the surface separation. Here, we demonstrate that a 3-mirror interferometer can overcome these difficulties: the setup involves two ultra-smooth electrode/mirror surfaces between which the fluid is confined and a third mirror to allow for interferometric detection of the liquid thickness with nanometer resolution and at thicknesses much smaller than the diffraction limit of the light. We conclude with a proof-of-concept measurement across dry nitrogen gas. The technique should prove useful for studying the properties of fluids confined at the nanoscale inside a slit-pore of controlled electrical potential or subject to applied electric fields.

Anomalous Interfacial Structuring of a Non-Halogenated Ionic Liquid: Effect of Substrate and Temperature

We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 °C causes negligible changes in the interaction. At 80 °C and 120 °C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact.

Probing the Interaction Mechanism between Oil-in-Water Emulsions and Electroless Nickel–Phosphorus Coating with Implications for Antifouling in Oil Production

Fouling issues are highly undesirable in oil industries, and stable water–oil emulsion is one of the major causes of fouling on pipelines, upgrading equipment, and other surfaces in oil production. Studying the interfacial interactions between emulsion drops and various metal substrates is of significant importance in the fundamental understanding of fouling mechanisms. In this work, surface force measurements using a drop probe atomic force microscope technique and fouling tests were applied to investigate the fouling and antifouling mechanisms of electron-beam-deposited iron substrates with and without electroless nickel–phosphorus (EN) coatings. The effects of oil or aqueous solution conditions have been systematically investigated, including the asphaltene concentration, salinity, pH, and presence of divalent ions. A theoretical model based on the Reynolds lubrication equation and augmented Young–Laplace equation has been applied to analyze the measured force profiles. Our results indicate that the at...

Novel Fe3O4 based superhydrophilic core-shell microspheres for breaking asphaltenes-stabilized water-in-oil emulsion

Stabilization and destabilization of emulsions play an important role in a wide range of engineering and environmental processes (e.g., oil-water separation, waste water treatment). The destabilization of water-in-oil (W/O) emulsions in the presence of asphaltenes, which form a protective interfacial layer to prevent the coalescence of emulsions, is a well-known challenging issue in oil production. In this study, a novel type of core-shell microspheres with magnetic core functionalized with a superhydrophilic zwitterionic polyelectrolyte shell has been developed and characterized, which shows excellent capability in breaking asphaltene-stabilized W/O emulsions and releasing considerable amount of water with the assistance of an external magnetic field. Surface force measurements using drop probe atomic force microscope (AFM) demonstrated the strong attraction between zwitterionic polyelectrolytes and emulsion droplets. The water-oil interfacial tension was found to increase with the addition of the microspheres, contributing to the coalescence of emulsions. Quartz crystal microbalance with dissipation monitoring (QCM-D) tests demonstrated that asphaltenes could be adsorbed on the polyelectrolyte surface, suggesting that the polyelectrolyte could interact and disrupt the protective asphaltene layer at oil-water interfaces to facilitate the demulsification. The adsorbed asphaltenes on the polyelectrolyte shell can be released when the microsphere was treated in water, showing a reversable asphaltene adsorption/desorption behavior, which has great potential for cyclic reuse. Our results provide useful insights into the fundamental interactions between emulsions in the presence of polyelectrolytes, with implications for the development of new demulsification methods in water-oil separation and other environment related processes.

Surface Forces between Highly Charged Cationic Polyelectrolytes Adsorbed to Silica: How Control of pH and the Adsorbed Amount Determines the Net Surface Charge.

Atomic force microscopy (AFM) and quartz crystal microbalance with dissipation (QCM-D) were employed to investigate the pH-dependent adsorption of poly(diallyldimethylammonium chloride) (polyDADMAC) to silica surfaces as well as the surface forces between these layers. It was found that polyDADMAC adopted a relatively flat conformation when adsorbed to a silica surface and that the adsorbed amount increased with increasing pH. From the surface force measurements it is evident that the surface undergoes a charge reversal upon saturation with polyDADMAC, at the three different investigated pH values, and that some degree of charge regulation of the silica surface takes place during the adsorption process. Finally, the overcharging phenomenon is discussed in terms of a geometrical mismatch due to the different average spacing between the surface charges on the silica surface and the size of the DADMAC monomer.

Reliable and accurate extraction of Hamaker constants from surface force measurements

A simple and accurate closed-form expression for the Hamaker constant that best represents experimental surface force data is presented. Numerical comparisons are made with the current standard least squares approach, which falsely assumes error-free separation measurements, and a nonlinear version assuming independent measurements of force and separation are subject to error. The comparisons demonstrate that not only is the proposed formula easily implemented it is also considerably more accurate. This option is appropriate for any value of Hamaker constant, high or low, and certainly for any interacting system exhibiting an inverse square distance dependent van der Waals force.