Flat Silica Surface Research Articles

Phospholipids and Hyaluronan: From Molecular Interactions to Nano- and Macroscale Friction

Phospholipids and hyaluronan are two key biomolecules that contribute to the excellent lubrication of articular joints. Phospholipids alone and in combination with hyaluronan have also displayed low friction forces on smooth surfaces in micro- and nanosized tribological contacts. In an effort to develop aqueous-based lubrication systems, it is highly relevant to explore if these types of molecules also are able to provide efficient lubrication of macroscopic tribological contacts involving surfaces with roughness larger than the thickness of the lubricating layer. To this end, we investigated the lubrication performance of hyaluronan, the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and mixtures of these two components using glass surfaces in a mini-traction machine. We compared our data with those obtained using flat silica surfaces in previous atomic force microscopy studies, and we also highlighted insights on hyaluronan–phospholipid interactions gained from recent simulations. Our data demonstrate that hyaluronan alone does not provide any lubricating benefit, but DPPC alone and in mixtures with hyaluronan reduces the friction force by an order of magnitude.

PH-Controlled assembly of polyelectrolyte layers on silica nanoparticles in concentrated suspension

HypothesisPreparation of suspensions of nanoparticles (>1 wt%) coated with a polyelectrolyte multilayers is a challenging task because of the risk of flocculation when a polyelectrolyte is added to a suspension of oppositely charged nanoparticles. This situation can be avoided if the charge density of the polymers and particles is controlled during mixing so as to separate mixing and adsorption events. ExperimentsThe cationic polyethylenimine (PEI) and the anionic carboxymethylcellulose (CMC) were used as weak polyelectrolytes. Polyelectrolyte multilayers build-up was conducted by reducing the charge of one of the components during the addition of the next component. Charge density was controlled by tuning pH. Analysis of the suspension of coated nanoparticles was done by means of dynamic light scattering, electrophoresis and small angle x-ray scattering measurements, while quartz crystal microbalance was used to study the build-up process on flat silica surfaces. FindingsCharge density, controlled through pH, can be used as a tool to avoid flocculation during layer-by-layer deposition of polyelectrolytes on 20 nm silica particles at high concentration (∼40 wt%). When added to silica at pH 3, PEI did not induce flocculation. Adsorption was triggered by raising the pH to 11, pH at which CMC could be added. The pH was then lowered to 3. The process was repeated, and up to five polyelectrolyte layers were deposited on concentrated silica nanoparticles while inducing minimal aggregation.

Control of bubble ζ-potentials to improve the kinetics of bubble-particle interactions

Derjaguin and Dukhin (1961) developed a kinetic model for the bubble-particle interactions in flotation using the classical DLVO theory. According to the model, the energy barrier (E1) to flotation should increase with the square of particle ζ-potential due to the increase in the repulsive electrical double-layer (EDL) forces in the thin liquid films (TLFs) of water confined between the two dissimilar surfaces. In the present work, surface force measurements have been conducted by monitoring the changes in local curvatures of the air bubbles approaching flat silica surfaces of varying contact angles, while controlling the bubble ζ-potentials in alkyltrimethylammonium chloride (CnTACl) solutions of varying chain lengths and concentrations. The results show that the kinetics of film thinning and contact angle formation increased with decreasing energy barriers. It has been found, however, that E1 = 0 is not the sufficient condition for maximum kinetics, as the bubble-particle interaction continued to improve with further increases in the negative disjoining pressure in the TLFs. These findings can be attributed to the increase in the hydrodynamic forces in TLFs due to the increases in the attractive EDL and hydrophobic forces. The fundamental information obtained in the present work may be useful for developing novel flotation reagents and designing more efficient flotation machines.

Parametrizing nonbonded interactions between silica and water from first principles

Silica has been used in a vast number of micro/nano-fluidic technologies where interactions of water with silica at the molecular level play a key role. In such small systems, an understanding of mass and heat transport or surface wetting relies on accurate calculations of the water-silica interface coupling through atomic interactions. Molecular dynamics (MD) is a convenient tool for such use, but force field parameters for nonbonded interactions are required as an input, which are very limited in literature. These interaction parameters can be predicted by density functional theory, but dispersion forces are not calculated in standard models for electron correlations that additional correction models have been proposed at different levels of sophistications, and still under development. Accordingly, this work employs state of the art quantum chemistry to compute the binding energies. Force field parameters for silica/water van der Waals interactions were calculated, and later tested in MD simulations of water droplet on silica surface. While the standard dispersion corrections overestimated the binding energy, Becke-Johnson model yielded interactions parameters recovering experimentally measured wetting behavior of silica with a water contact angle of approximately 12.4° on the flat and clean silica surface. Results will be useful for the current molecular modelling attempts by providing transferable parameters for simple silica/water van der Waals interactions as an alternative to existing complex surface interaction models.

Evaluating Water Reactivity at Silica Surfaces Using Reactive Potentials

Understanding the interactions between amorphous silica surfaces and water provides insight into material degradation of silicate glasses and minerals in aqueous environment. Molecular dynamics (MD) simulations of water and nanometer sized silica structures were used in this work to evaluate the reactivity of flat silica surface and surfaces with curvature. We compared two dissociable water/silica potentials, namely the Reactive Force Field (ReaxFF) and the Mahadevan–Garofalini water/silica force field (MGFF) that have been in development over the past decade, to study their performance in simulating bulk water as well as silica–water interactions. Significant differences in the properties of bulk water as well as surface interactions were observed between the two types of potentials, as well in the same potential type with two parametrizations for ReaxFF, suggesting a need for improvement of the existing water/silica ReaxFF potentials. Our simulation results show that a majority of the silanols were form...

Morphological Study of Nascent Growth of α‐Olefin Polymers on Spatially Unconstrained Silica Surfaces

AbstractThe experimental study of polymer growth on spatially unconstrained flat silica surfaces is presented for propylene and ethylene polymerization in liquid and gas phases with high activity metallocene catalyst (rac‐Et(Indenyl)2ZrCl2/ methylaluminoxane at 30–70 °C and 30–50 psig. The study shows a surprisingly rich spectrum of nascent morphologies for each polymer synthesized in two different reaction phases. In propylene polymerization in a liquid phase, polypropylene nanofibrils are first formed at the catalyst sites and quickly develop into microglobular morphology, whereas in ethylene polymerization in a liquid phase, polyethylene nanofibrils formed at the catalyst sites grow to a ribbon‐like morphology. In gas phase polymerization where polymerization rate is much lower due to low monomer concentration, both polymers grow to compact aggregates of globules. Unlike in liquid phase polymerization, no ribbon‐like structure is formed in ethylene polymerization. The sequential polymerization of these olefin monomers has also been carried out and the experimental data show two distinct regions of corresponding polymers as observed in separate ethylene and propylene homopolymerization experiments.

Effect of Approach Velocity on Thin Liquid Film Drainage between an Air Bubble and a Flat Solid Surface

The dynamic drainage process of the liquid film trapped between an air bubble and a flat silica surface over a wide range of hydrodynamic conditions is studied by a newly developed instrument called integrated thin liquid film force apparatus (ITLFFA) under different salt concentrations. The ITLFFA allows the simultaneous measurement of interaction forces and spatiotemporal film thickness with accurate control of bubble approach velocity in a large range of Reynolds number from 0.005 to 135. Our study demonstrates that increasing the bubble approach velocity plays a significant role in the hydrodynamic pressure and fluid flow within the draining film promoting dimple formation and longer drainage time. The drainage time also depends on the competition between the electrical double-layer and van der Waals interactions, which are repulsive in our system, resulting in a flat equilibrium film at the end. The evolution of the draining film is analyzed using the Stokes–Reynolds–Young–Laplace (SRYL) model. Compa...

Size-dependent interactions of silica nanoparticles with a flat silica surface

We have investigated the surface chemistry of SiO2 nanoparticles (NPs) with different sizes and their corresponding interactions with a flat substrate of surface curvature ∼0. As the size of the NPs increases, the SiO2 surface is increasingly covered with H-bonded silanol groups, thereby increasing the ζ-potential and shifting the isoelectric point higher in pH. Interactions between the SiO2 NPs and the flat SiO2 surface were analyzed in situ using quartz crystal microbalance with dissipation (QCM-D) method, and the results were interpreted based on an extended Derjaguin–Landau–Verwey–Overbeek theory. At very low ionic strength (1mM NaCl), there was no particle adsorption onto the surface due to the highly repulsive energy barriers to this interaction. On the other hand, QCM-D results showed that the significant adsorption of SiO2 NPs onto a flat SiO2 surface occurred under conditions of high ionic strength (100mM NaCl). Interestingly, the adsorption behaviors of three different-sized SiO2 NPs on the surface varied considerably with size. SiO2 NPs with small size have high adsorption affinity with the flat SiO2 surface due to an extremely low energy barrier for the interactions, whereas relatively large SiO2 NPs have very weak adsorption affinity with the flat surface due to the repulsive energy barrier formed by the increase in the electrostatic and hydration repulsion energy.

Nitriles at Silica Interfaces Resemble Supported Lipid Bilayers.

Nitriles are important solvents not just for bulk reactions but also for interfacial processes such as separations, heterogeneous catalysis, and electrochemistry. Although nitriles have a polar end and a lipophilic end, the cyano group is not hydrophilic enough for these substances to be thought of as prototypical amphiphiles. This picture is now changing, as research is revealing that at a silica surface nitriles can organize into structures that, in many ways, resemble lipid bilayers. This unexpected organization may be a key component of unique interfacial behavior of nitriles that make them the solvents of choice for so many applications. The first hints of this lipid-bilayer-like (LBL) organization of nitriles at silica interfaces came from optical Kerr effect (OKE) experiments on liquid acetonitrile confined in the pores of sol-gel glasses. The orientational dynamics revealed by OKE spectroscopy suggested that the confined liquid is composed of a relatively immobile sublayer of molecules that accept hydrogen bonds from the surface silanol groups and an interdigitated, antiparallel layer that is capable of exchanging into the centers of the pores. This picture of acetonitrile has been borne out by molecular dynamics simulations and vibrational sum-frequency generation (VSFG) experiments. Remarkably, these simulations further indicate that the LBL organization is repeated with increasing disorder at least 20 Å into the liquid from a flat silica surface. Simulations and VSFG and OKE experiments indicate that extending the alkyl chain to an ethyl group leads to the formation of even more tightly packed LBL organization featuring entangled alkyl tails. When the alkyl portion of the molecule is a bulky t-butyl group, packing constraints prevent well-ordered LBL organization of the liquid. In each case, the surface-induced organization of the liquid is reflected in its interfacial dynamics. Acetonitrile/water mixtures are favored solvent systems for separations technologies such as hydrophilic interaction chromatography. Simulations had suggested that although a monolayer of water partitions to the silica surface in such mixtures, acetonitrile tends to associate with this monolayer. VSFG experiments reveal that, even at high water mole fractions, patches of well-ordered acetonitrile bilayers remain at the silica surface. Due to its ability to donate and accept hydrogen bonds, methanol also partitions to a silica surface in acetonitrile/methanol mixtures and can serve to take the place of acetonitrile in the sublayer closest to the surface. These studies reveal that liquid nitriles can exhibit an unexpected wealth of new organizational and dynamic behaviors at silica surfaces, and presumably at the surfaces of other chemically important materials as well. This behavior cannot be predicted from the bulk organization of these liquids. Our new understanding of the interfacial behavior of these liquids will have important implications for optimizing a wide range of chemical processes in nitrile solvents.

Temperature-Dependent Deicing Properties of Electrostatically Anchored Branched Brush Layers of Poly(ethylene oxide).

The hydration water of hydrophilic polymers freezes at subzero temperatures. The adsorption of such polymers will result in a hydrophilic surface layer that strongly binds water. Provided this interfacial hydration water remains liquidlike at subzero temperatures, its presence could possibly reduce ice adhesion, in particular, if the liquidlike layer is thicker than or comparable to the surface roughness. To explore this idea, a diblock copolymer, having one branched bottle-brush block of poly(ethylene oxide) and one linear cationic block, was electrostatically anchored on flat silica surfaces. The shear ice adhesion strength on such polymer-coated surfaces was investigated down to -25 °C using a homebuilt device. In addition, the temperature dependence of the ice adhesion on surfaces coated with only the cationic block, only the branched bottle-brush block, and with linear poly(ethylene oxide) was investigated. Significant ice adhesion reduction, in particular, at temperatures above -15 °C, was observed on silica surfaces coated with the electrostatically anchored diblock copolymer. Differential scanning calorimetry measurements on bulk polymer solutions demonstrate different thermal transitions of water interacting with branched and linear poly(ethylene oxide) (with hydration water melting points of about -18 and -10 °C, respectively). This difference is consistent with the low shear ice adhesion strength measured on surfaces carrying branched bottle-brush structured poly(ethylene oxide) at -10 °C, whereas no significant adhesion reduction was obtained with linear poly(ethylene oxide) at this temperature. We propose a lubrication effect of the hydration water bound to the branched bottle-brush structured poly(ethylene oxide), which, in the bulk, does not freeze until -18 °C.

Structural Influences on the Fast Dynamics of Alkylsiloxane Monolayers on SiO2 Surfaces Measured with 2D IR Spectroscopy

There is widespread interest in alkyl chain surface monolayers and their applications. In many applications, alkyl monolayers are functionalized with active headgroups. Here we report the impact of major structural variations on the fast dynamics of alkylsiloxane monolayers. The monolayers were deposited with controlled structures on flat amorphous silica surfaces, and the terminal sites were functionalized with a metal carbonyl headgroup. The CO symmetric stretching mode of the headgroup served as a vibrational probe for detecting the fast structural dynamics of the monolayers using two-dimensional infrared vibrational echo spectroscopy (2D IR) to measure spectral diffusion, which is made quantitative by determining the frequency–frequency correlation function (FFCF) from the time-dependent data. Two methods of functionalizing the surface, independent attachment via a single Si–O bond formed with alkylmonochlorosilane precursors and network attachment via siloxane networks (−Si–O–Si–O−) formed with alkyl...

Surface Charge Properties of Nanoparticle: Size Dependency and Boundary Effect

Surface charge properties of nanoparticles play an important role in many applications, such as drug delivery, cellular uptake and their separation. Surface charge properties of silica nanoparticles are investigated as functions of particle’s size, solution properties including pH and background salt concentration, and separation distance from a flat silica surface using a multi-ion charge-regulation model. In contrast to most previous studies utilizing constant surface charges regardless of the local solution properties, surface chemical reactions in the presence of multiple ionic species are considered in this study. The obtained results agree with experimental data available from the literature. Results show that the magnitude of nanoparticle’s surface charge density increases with its size due to the curvature effect and the presence of the nearby boundary induces spatially non-uniform surface charge around the nanoparticle as while decreasing the k h, the degree of nonhomogeneous distribution increases. Due to the surface chemical reactions, both pH and background salt concentration significantly affect the charge properties of silica nanoparticles.

Utilizing Atomistic Simulations To Map Pressure Distributions and Contact Areas in Molecular Adlayers within Nanoscale Surface-Asperity Junctions: A Demonstration with Octadecylsilane-Functionalized Silica Interfaces

To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity-asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.

Modification of silicon oxide surfaces by monolayers of an oligoethylene glycol-terminated perfluoroalkyl silane

Monolayers of a new hybrid compound having two linearly coupled subunits with different functionalities were prepared on the silicon oxide and silicate-like surfaces using trichlorosilane as an anchoring group. The two subunits of the hybrid compound are a perfluorinated alkyl chain and an oligoethylene glycol chain. The synthesis of the new precursor compound, O-methyl hexaethylene glycol-icosafluorododecandiol-trichlorosilane is reported starting from commercially available triethylene glycol and perfluorododecan-1,12-diol as building blocks. The final and intermediate products were characterized by nuclear magnetic resonance (1H NMR, 13C NMR, 19F NMR and 29Si NMR), infrared spectroscopy (IR) and high resolution mass spectrometry with chemical ionization [HRMS (CI)]. The new compound was self-assembled on borosilicate glass, surface-oxidized silicon and silicon oxide surfaces. Properties of the self-assembled films as a function of the adsorption time were studied using contact angle measurements, X-ray photoelectron spectroscopy, polarization modulation infrared reflection absorption spectroscopy and scanning force microscopy. The adsorption of the hybrid silane on an atomically flat silica surface lead to the formation of poorly defined multilayer films. By contrast, the adsorption of the silane onto a less-ordered substrate resulted in the formation of a well-packed monolayer.

QCM-D as a method for monitoring enzyme immobilization in mesoporous silica particles

Enzyme immobilization in mesoporous materials is a field of great interest, with applications in biocatalysis and biosensing. However, the actual immobilization process is not well understood and has mainly been studied by indirect measurements. The present work demonstrates a direct method for real time study of enzyme immobilization in mesoporous silica particles using quartz crystal microbalance with dissipation monitoring (QCM-D). Silica-coated sensors were grafted with amine groups followed by adsorption of small (40nm), spherical mesoporous silica particles, after which the enzyme immobilization into the mesoporous particles could be studied in real time. The influence of pH on the immobilization efficiency was studied using two different enzymes; lipase from Rhizopus oryzae and feruloyl esterase from Fusarium oxysporum. The results showed that the silica particles adsorbed readily to the amine-grafted surface. The QCM-D measurements indicated that considerably more enzyme was immobilized into mesoporous silica particles than to non-porous silica particles and to a flat silica surface. The viscoelastic effect of the immobilized enzymes was visualized by plotting the frequency shift against the corresponding dissipation. It was observed that the immobilization into the porous particles can be divided into two regimes where the first regime is suggested to represent adsorption to the outer surface and pore openings and the second regime represents further adsorption inside the pores. In summary, we demonstrated QCM-D as a novel method for understanding enzyme immobilization in mesoporous particles in real time and the approach may be of general use for studies of entrapment of molecules into porous particles.

Cell interaction study method using novel 3D silica nanoneedle gradient arrays

Understanding cellular interactions with culture substrate features is important to advance cell biology and regenerative medicine. When surface topographical features are considerably larger in vertical dimension and are spaced at least one cell dimension apart, the features act as 3D physical barriers that can guide cell adhesion, thereby altering cell behavior. In the present study, we investigated competitive interactions of cells with neighboring cells and matrix using a novel nanoneedle gradient array. A gradient array of nanoholes was patterned at the surface of fused silica by single-pulse femtosecond laser machining. A negative replica of the pattern was extracted by nanoimprinting with a thin film of polymer. Silica was deposited on top of the polymer replica to form silica nanoneedles. NIH 3T3 fibroblasts were cultured on silica nanoneedles and their behavior was studied and compared with those cultured on a flat silica surface. The presence of silica nanoneedles was found to enhance the adhesion of fibroblasts while maintaining cell viability. The anisotropy in the arrangement of silica nanoneedles was found to affect the morphology and spreading of fibroblasts. Additionally, variations in nanoneedle spacing regulated cell–matrix and cell–cell interactions, effectively preventing cell aggregation in areas of tightly-packed nanoneedles. This proof-of-concept study provides a reproducible means for controlling competitive cell adhesion events and offers a novel system whose properties can be manipulated to intimately control cell behavior.

Self-Assembly of Poly(4-vynil-N-alkylpyridinium) Bromide onto Silica: Effect of Side-Chain Length on Structural Aspects at a Molecular Level

Self-assembly of poly(4-vynil-N-alkyl)pyridinium bromide with alkyl side chains of 2, 5, 7, 10, or 16 carbons from ethanolic solutions onto flat silica surfaces was studied by means of ellipsometry, atomic force microscopy (AFM), contact angle measurements, and sum-frequency generation (SFG) vibrational spectroscopy in the CH3 and CH2 stretch region. Ab initio quantum-chemical calculations on the N-alkylpyridinium side-group with restricted Hartree–Fock (RHF) method and 6-311G (d,p) basis set were done to estimate the charge distribution along the pyridinium ring and the alkyl side-chain. SFG results showed that longer side chains promote the disorientation of the alkyl groups at the surface, corroborating with the contact angle values. AFM images revealed film homogeneity, regardless the alkyl side group. However, after 24 h contact with water, ringlike structures appeared on the film surfaces, when the polycation alkyl side chain had 7 or less carbons, and as the alkyl chain increased to 10 or 16 carbons, the films dewetted because the hydrophobic interactions prevailed over the electrostatic interactions between the pyridinium charged groups and the negatively charged SiO2 surface. Under acid conditions (HCl 0.1 mol·L–1), the film mean thickness values decreased up to 50% of original values when the alkyl side chains were ethyl or pentyl groups due to ion-pair disruption, but for longer groups they remained unchanged. Quantum-chemical optimization and Mulliken electron population showed that (i) from C2 to C15 the positive charge at the headgroup (HG) decreased 0.025, while the charge at combined HG + α-CH2 increased 0.037; and (ii) for C6 or longer, the alkyl side group presents a tilt in the geometry, moving away from the plane. Such effects summed up over the whole polymer chain give support to suggest that when the side chains are longer than 7 carbons, the hydrophobic interaction decreases film stability and increases acid resistance.

Interaction of Silica Nanoparticles with a Flat Silica Surface through Neutron Reflectometry

Neutron reflectometry (NR) was employed to study the interaction of nanosized silica particles with a flat silica surface in aqueous solutions. Unlike other experimental tools that are used to study surface interactions, NR can provide information on the particle density profile in the solution near the interface. Two types of silica particles (25 and 100 nm) were suspended in aqueous solutions of varying ionic strength. Theoretical calculations of the surface interaction potential between a particle and a flat silica surface using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were compared to the experimental data. The theory predicts that the potential energy is highly dependent on the ionic strength. In high ionic strength solutions, NR reveals a high concentration of particles near the flat silica surface. Under the same conditions, theoretical calculations show an attractive force between a particle and a flat surface. For low ionic strength solutions, the particle concentration near the surface obtained from NR is the same as the bulk concentration, while depletion of particles near the surface is expected because of the repulsion predicted by the DLVO theory.

Disentangling protein–silica interactions

We present the results of modelling studies aimed at the understanding of the interaction of a 7 nm sized water droplet containing a negatively charged globular protein with flat silica surfaces. We show how the droplet interaction with the surface depends on the electrostatic surface charge, and that adhesion of the droplet occurs when the surface is negatively charged as well. The key role of water and of the charge-balancing counter ions in mediating the surface-protein adhesion is highlighted. The relevance of the present results with respect to the production of bioinorganic hybrids via encapsulation of proteins inside mesoporous silica materials is discussed.

AFM measurements of forces between silica surfaces

Interaction forces and adhesion between a silica sphere and a flat silica surface in aqueous electrolyte solutions were investigated by atomic force microscopy. The forces were measured as a function of surface separation, pH and NaCl concentration as the surfaces were approaching each other. The adhesion force was determined upon retraction with respect to pH, NaCl concentration and contact time. The magnitude of the long range repulsive force was decreasing with decreasing pH. A short range repulsive force was observed at pH = 2, but no long range repulsive forces were observed at this pH. Force measurements showed that adhesion of silica surfaces in water was obstructed by short and long range repulsive forces. Adhesion was enhanced when both the long and the short range repulsive force was mitigated. A maximum adhesion force of 7.8 mN/m was measured at pH = 12.5 when the short range force vanished and the long range repulsive force was reduced by increasing the NaCl concentration. At pH = 12.5, the work of adhesion was calculated to be 1.2 mJ/m2 according to the Derjaguin–Muller–Toporov (DMT) model. Adhesion energy was much less at pH = 2 (0.3 mJ/m2) due to persistive short range repulsion.