Surface Forces Apparatus Research Articles

Elucidating the Interfacial Structure at the Ionic Liquid-Solid Interface Using Atomic Force Microscopy and Molecular Dynamics

The structure and properties of ionic liquids (ILs) at the solid-liquid interface will govern their performance in all applications including energy storage and catalysis. Although it is well known that the interfacial structure of ionic liquids is very different to that of classical dilute electrolytes, a comprehensive understanding of the ion structure within the electrical double layer for this class of electrolytes is still lacking. Therefore it is necessary to have experimental methods to elucidate the interfacial ion structure which can be related to electrochemical performance. Atomic force microscopy (AFM) force spectroscopy is capable of probing the layered ion structure formed at the interface, and offers advantages over other techniques (e.g. x-ray and neutron reflectivity, surface force apparatus) due to its high spatial resolution allowing for the ion structure to be examined in a spatially resolved manner. Although there are several studies demonstrating the impressive ability of AFM to probe the interfacial ion structure in ionic liquids, several questions remain regarding the measured AFM response. (e.g. what is the role of the AFM probe?, Do we measure cations or anions? Can we tune ion selectivity?) In this work we examine the AFM force profiles of various ionic liquids at different substrates to examine how the substrate and ionic liquid properties affect the interfacial ion structure. We also use a series of different AFM probes of different geometry and chemistry to determine the effect on the measured force profiles. Our experimental results are compared directly with molecular dynamics simulations, and through a combination of experimental and theoretical approaches we are able to develop a better understanding of the measured force response and factors determining the interfacial structure at the ion-liquid solid interface.

Nanorheology of adsorbed polymer chains immersed in pure solvent.

Long linear chains of polybutadiene are adsorbed on the two surfaces of a surface force apparatus and immersed in pure tetradecane. The hydrodynamic force was measured by drainage experiments and by frequency sweeps at constant distances. We related the hydrodynamic thickness to the chain dimension. The complex modulus encompasses the shear modulus and, at distances lower than the hydrodynamic thickness, a compression modulus. The compression term was related to the static force which appears when the two adsorbed layers are overlapped. The complex shear modulus was interpreted by a two-components hydrodynamic model proposed by P. Sens et al. We first complemented the theoretical model. Then, our experimental data fit the proposed viscoelastic expressions in the entire range of distances. The storage modulus is supposed to be affected by a residue of free chains and by the dispersion of the loop lengths.

Boundary Condition in Liquid Thin Films Revealed through the Thermal Fluctuations of Their Free Surfaces.

We investigate the properties of nanometric liquid films with a new noninvasive technique. We measure the spontaneous thermal fluctuations of the free surfaces of liquids to probe their hydrodynamic boundary condition at a solid wall. The surface fluctuations of a silicon oil film could be described with a no-slip boundary condition for film thicknesses down to 20 nm. Oppositely, a 4 nm negative slip length had to be introduced to describe the behavior of n-hexadecane, consistently with previous surface force apparatus data on the same system. Our results demonstrate that at vanishing flow a nanometric solidlike layer close to the wall may exist according to the nature of the liquid.

Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion.

Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane–membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane–membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force–distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems.

Reversible Adhesion with Polyelectrolyte Brushes Tailored via the Uptake and Release of Trivalent Lanthanum Ions

Applications of end-tethered polyelectrolyte “brushes” to modify solid surfaces have been developed and studied for their colloidal stabilization and high lubrication properties. Current efforts have expanded into biological realms and stimuli-responsive materials. Our work explores responsive and reversible aspects of polyelectrolyte brush behavior when polyelectrolyte chains interact with oppositely charged multivalent ions and complexes, which act as counterions. There is a significant void in the polyelectrolyte literature regarding interactions with multivalent species. This paper demonstrates that interactions between solid surfaces bearing negatively charged polyelectrolyte brushes are highly sensitive to the presence of trivalent lanthanum, La3+. Lanthanum cations have unique interactions with polyelectrolyte chains, in part due to their small size and hydration radius which results in a high local charge density. Using La3+ in conjunction with the surface forces apparatus (SFA), adhesion has been...

Molecular Structure and Equilibrium Forces of Bovine Submaxillary Mucin Adsorbed at a Solid-Liquid Interface.

By combining dynamic light scattering, circular dichroism spectroscopy, atomic force microscopy, and surface force apparatus, the conformation of bovine submaxillary mucin in dilute solution and nanomechanical properties of mucin layers adsorbed on mica have been investigated. The samples were prepared by additional chromatographic purification of commercially available products. The mucin molecule was found to have a z-average hydrodynamic diameter of ca. 35 nm in phosphate buffered solution, without any particular secondary or tertiary structure. The contour length of the mucin is larger than, yet of the same order of magnitude as the diameter, indicating that the molecule can be modeled as a relatively rigid polymeric chain due to the large persistence length of the central glycosylated domain. Mucin molecules adsorbed abundantly onto mica from saline buffer, generating polymer-like, long-ranged, repulsive, and nonhysteretic forces upon compression of the adsorbed layers. Detailed analysis of such forces suggests that adsorbed mucins had an elongated conformation favored by the stiffness of the central domain. Acidification of aqueous media was chosen as means to reduce mucin-mucin and mucin-substrate electrostatic interactions. The hydrodynamic diameter in solution did not significantly change when the pH was lowered, showing that the large persistence length of the mucin molecule is due to steric hindrance between sugar chains, rather than electrostatic interactions. Remarkably, the force generated by an adsorbed layer with a fixed surface coverage also remained unaltered upon acidification. This observation can be linked to the surface-protective, pH-resistant role of bovine submaxillary mucin in the variable environmental conditions of the oral cavity.

Stiffening and unfolding of early deposited-fibronectin increase proangiogenic factor secretion by breast cancer-associated stromal cells

Fibronectin (Fn) forms a fibrillar network that controls cell behavior in both physiological and diseased conditions including cancer. Indeed, breast cancer-associated stromal cells not only increase the quantity of deposited Fn but also modify its conformation. However, (i) the interplay between mechanical and conformational properties of early tumor-associated Fn networks and (ii) its effect on tumor vascularization remain unclear. Here, we first used the Surface Forces Apparatus to reveal that 3T3-L1 preadipocytes exposed to tumor-secreted factors generate a stiffer Fn matrix relative to control cells. We then show that this early matrix stiffening correlates with increased molecular unfolding in Fn fibers, as determined by Förster Resonance Energy Transfer. Finally, we assessed the resulting changes in adhesion and proangiogenic factor (VEGF) secretion of newly seeded 3T3-L1s, and we examined altered integrin specificity as a potential mechanism of modified cell–matrix interactions through integrin blockers. Our data indicate that tumor-conditioned Fn decreases adhesion while enhancing VEGF secretion by preadipocytes, and that an integrin switch is responsible for such changes. Collectively, our findings suggest that simultaneous stiffening and unfolding of initially deposited tumor-conditioned Fn alters both adhesion and proangiogenic behavior of surrounding stromal cells, likely promoting vascularization and growth of the breast tumor. This work enhances our knowledge of cell – Fn matrix interactions that may be exploited for other biomaterials-based applications, including advanced tissue engineering approaches.

Molecular and Surface Interactions between Polymer Flocculant Chitosan-g-polyacrylamide and Kaolinite Particles: Impact of Salinity

Solution salinity plays an important role in the interactions between polymer flocculants and solid colloid particles which determine the flocculation performance. In this work, chitosan-graft-polyacrylamide (chi-g-PAM) was synthesized and characterized. The influence of solution salinity (viz., addition of NaCl and CaCl2) on the flocculation of chi-g-PAM on kaolinite suspension and the interactions between the polymers and solid surfaces was investigated via several complementary measurements and techniques, including settling tests, zeta potential analyzer, quartz crystal microbalance with dissipation (QCM-D), surface forces apparatus (SFA), and atomic force microscope (AFM). Our results show that the initial settling rate (ISR) of kaolinite suspension decreases with increasing the concentration of NaCl and CaCl2 (0.01 to 1 M). The high salinity condition leads to a relatively weak adsorption and more compressed conformations of chi-g-PAM chains on both silica and alumina surfaces (two facets of kaolini...

Simultaneous measurements of molecular forces and electro-optical properties of a confined 5CB liquid crystal film using a surface forces apparatus.

Using a surface forces apparatus (SFA), we studied the forces associated with the reorientation of molecules of a common nematic thermotropic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB), confined between two conducting (silver) surfaces and its optical behavior under the influence of electric fields with varying magnitudes and field directions. A transient attractive force was observed due to partial reorientations of the liquid crystal molecules and the flow of free ions, in addition to a stronger constant capacitance attraction between the silver surfaces. At the same time, the optical properties of the liquid crystals were observed perpendicular to the silver surfaces. Observations of shifts and fluctuations of the extraordinary wave of the (multiple beam) interference fringes measure the refractive index of the director component parallel to the surface, which is sensitive to tilt motion (or reorientation) of the liquid crystal molecules that provided details of the anisotropic orientations of the molecules and domains. Any lateral differential refractive index change is easily observed by optical microscopy. The optical microscope imaging showed that the changes in the optical properties are due to convective flow at domain boundaries of the liquid crystal molecules (and possible free ions) between the two charged surfaces. At low electric fields, propagation of domain boundaries was observed, while at higher electric fields, hexagonal patterns of flowing molecules were observed. The interplay of the force measurements and optical observations reveal a complex dynamic behavior of liquid crystals subjected to varying electric fields in confined spaces.

Adsorption mechanism of myelin basic protein on model substrates and its bridging interaction between the two surfaces.

Myelin basic protein (MBP) is an intrinsically disordered (unstructured) protein known to play an important role in the stability of myelin's multilamellar membrane structure in the central nervous system. The adsorption of MBP and its capacity to interact with and bridge solid substrates has been studied using a surface forces apparatus (SFA) and a quartz crystal microbalance with dissipation (QCM-D). Adsorption experiments show that MBP molecules adsorb to the surfaces in a swollen state before undergoing a conformational change into a more compact structure with a thickness of ∼3 nm. Moreover, this compact structure is able to interact with nearby mica surfaces to form adhesive bridges. The measured adhesion force (energy) between two bridged surfaces is 1.0 ± 0.1 mN/m, (Ead = 0.21 ± 0.02 mJ/m(2)), which is slightly smaller than our previously reported adhesion force of 1.7 mN/m (Ead = 0.36 mJ/m(2)) for MBP adsorbed on two supported lipid bilayers (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775). The saturated surface concentration of compact MBP on a single SiO2 surface reaches a stable value of 310 ± 10 ng/cm(2) regardless of the bulk MBP concentration. A kinetic three-step adsorption model was developed that accurately fits the adsorption data. The developed model is a general model, not limited to intrinsically disordered proteins, that can be extended to the adsorption of various chemical compounds that undergo chemical reactions and/or conformational changes upon adsorbing to surfaces. Taken together with our previously published data (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775), the present results confirm that conformational changes of MBP upon adsorption are a key for strong adhesion, and that such conformational changes are strongly dependent on the nature of the surfaces.

Stick-slip friction of gecko-mimetic flaps on smooth and rough surfaces.

The discovery and understanding of gecko 'frictional-adhesion' adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick-slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick-slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick-slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems.

Scaling from single molecule to macroscopic adhesion at polymer/metal interfaces.

Understanding the evolution of macroscopic adhesion based on fundamental molecular interactions is crucial to designing strong and smart polymer/metal interfaces that play an important role in many industrial and biomedical applications. Here we show how macroscopic adhesion can be predicted on the basis of single molecular interactions. In particular, we carry out dynamic single molecule-force spectroscopy (SM-AFM) in the framework of Bell-Evans' theory to gain information about the energy barrier between the bound and unbound states of an amine/gold junction. Furthermore, we use Jarzynski's equality to obtain the equilibrium ground-state energy difference of the amine/gold bond from these nonequilibrium force measurements. In addition, we perform surface forces apparatus (SFA) experiments to measure macroscopic adhesion forces at contacts where approximately 10(7) amine/gold bonds are formed simultaneously. The SFA approach provides an amine/gold interaction energy (normalized by the number of interacting molecules) of (36 ± 1)k(B)T, which is in excellent agreement with the interaction free energy of (35 ± 3)k(B)T calculated using Jarzynski's equality and single-molecule AFM experiments. Our results validate Jarzynski's equality for the field of polymer/metal interactions by measuring both sides of the equation. Furthermore, the comparison of SFA and AFM shows how macroscopic interaction energies can be predicted on the basis of single molecular interactions, providing a new strategy to potentially predict adhesive properties of novel glues or coatings as well as bio- and wet adhesion.

Characterization of solid-supported dipalmitoylphosphatidylcholine membranes containing cholesterol.

The incorporation of cholesterol into dipalmitoylphosphatidylcholine (DPPC) membranes, even in small amounts, has been shown to significantly alter the properties of the membrane. In this work, force-distance interaction profiles of DPPC membranes containing 8 mol % cholesterol obtained using the surface force apparatus are analyzed in the context of high-resolution structural characterization by atomic force microscopy and neutron reflectometry. The adhesion between the mixed membranes was greater than that for pure DPPC and was variable-depending on the number of defects in the outer membrane leaflets. These defects were only detectable by atomic force microscopy and had an average size of 230 ± 30 nm and 1-5% surface density in the outer leaflet. The adhesion between the membranes monotonically increased as the thickness of the membrane decreased-in direct correlation with the number of defects present (exposed hydrophobic groups) in the membrane contact region. Because of the low diffusion rate of gel-phase membranes, the interaction force profiles were stable and no membrane restructuring was observed.

Mussel adhesive protein provides cohesive matrix for collagen type-1α

Understanding the interactions between collagen and adhesive mussel foot proteins (mfps) can lead to improved medical and dental adhesives, particularly for collagen-rich tissues. Here we investigated interactions between collagen type-1, the most abundant load-bearing animal protein, and mussel foot protein-3 (mfp-3) using a quartz crystal microbalance and surface forces apparatus (SFA). Both hydrophilic and hydrophobic variants of mfp-3 were exploited to probe the nature of the interaction between the protein and collagen. Our chief findings are: 1) mfp-3 is an effective chaperone for tropocollagen adsorption to TiO2 and mica surfaces; 2) at pH 3, collagen addition between two mfp-3 films (Wc = 5.4 ± 0.2 mJ/m2) increased their cohesion by nearly 35%; 3) oxidation of Dopa in mfp-3 by periodate did not abolish the adhesion between collagen and mfp-3 films, and 4) collagen bridging between both hydrophilic and hydrophobic mfp-3 variant films is equally robust, suggesting that hydrophobic interactions play a minor role. Extensive H-bonding, π-cation and electrostatic interactions are more plausible to explain the reversible bridging of mfp-3 films by collagen.

Study of DNA adsorption on mica surfaces using a surface force apparatus.

We report our studies on the adsorption properties of double-stranded DNA molecules on mica surfaces in a confined environment using a surface force apparatus. Specifically, we studied the influence of cation species and concentrations on DNA adsorption properties. Our results indicated that divalent cations (Mg2+ and Co2+) preferred to form uniform and moderately dense DNA layers on a mica substrate. By measuring the interactions between DNA-coated mica and bare mica in an aqueous solution, obvious adhesion was observed in a cobalt chloride solution, possibly due to the ion-correlation attraction between negatively charged DNA and the mica surface. Furthermore, the interaction differences that were observed with MgCl2 and CoCl2 solutions reveal that the specific adsorption behaviors of DNA molecules on a mica substrate were mediated by these two salts. Our results are helpful to elucidate the dynamics of DNA binding on a solid substrate.

Simple transform methods of a force curve obtained by surface force apparatus to the density distribution of a liquid near a surface

We propose two simple methods that transform a force curve obtained by a surface force apparatus (SFA) into a density distribution of a liquid near a surface of the SFA probe. The transform methods are derived based on the statistical mechanics of simple liquids, where the liquid is an ensemble of small spheres. The solvent species is limited to only one component and two-body potential between the solvent spheres is arbitrary. However, two-body potential between the SFA probe and the solvent is restricted to rigid potential (i.e., the transform methods are derived within the restriction of the rigid potential). In addition, Kirkwood and linear superposition approximations are applied in order to derive the transform methods. The transform methods are simply tested in both hard-sphere fluid and Lennard-Jones (LJ) fluid with hard core potential. The tests are computationally practiced using a three-dimensional integral equation theory. It is found that the transform method with Kirkwood superposition approximation (transform method 1) generally reproduces the more precise solvation structure than that with linear superposition approximation (transform method 2). In the test of the hard sphere solvent, it is found that the reproducibility becomes better as the number density of the solvent is lower. Furthermore, it is found in the test of the LJ fluid that the reproducibility becomes better as the two-body potential between the SFA probe and the solvent approaches rigid potential. This is because, the transform methods are derived within the model of the rigid potential. It is verified that the transform methods are useful for obtaining of a rough image of the solvation structure. (However, if evaporation or solidification, a phase transition in a local space sandwiched between the two surfaces, occurs while the experiment, the transform methods should not be used.)

Bridging adhesion of mussel-inspired peptides: role of charge, chain length, and surface type.

The 3,4-dihydroxyphenylalanine (Dopa)-containing proteins of marine mussels provide attractive design paradigms for engineering synthetic polymers that can serve as high performance wet adhesives and coatings. Although the role of Dopa in promoting adhesion between mussels and various substrates has been carefully studied, the context by which Dopa mediates a bridging or nonbridging macromolecular adhesion to surfaces is not understood. The distinction is an important one both for a mechanistic appreciation of bioadhesion and for an intelligent translation of bioadhesive concepts to engineered systems. On the basis of mussel foot protein-5 (Mfp-5; length 75 res), we designed three short, simplified peptides (15–17 res) and one relatively long peptide (30 res) into which Dopa was enzymatically incorporated. Peptide adhesion was tested using a surface forces apparatus. Our results show that the short peptides are capable of weak bridging adhesion between two mica surfaces, but this adhesion contrasts with that of full length Mfp-5, in that (1) while still dependent on Dopa, electrostatic contributions are much more prominent, and (2) whereas Dopa surface density remains similar in both, peptide adhesion is an order of magnitude weaker (adhesion energy Ead ∼ −0.5 mJ/m2) than full length Mfp-5 adhesion. Between two mica surfaces, the magnitude of bridging adhesion was approximately doubled (Ead ∼ −1 mJ/m2) upon doubling the peptide length. Notably, the short peptides mediate much stronger adhesion (Ead ∼ −3.0 mJ/m2) between mica and gold surfaces, indicating that a long chain length is less important when different interactions are involved on each of the two surfaces.

Real-Time Monitoring of Aluminum Crevice Corrosion and Its Inhibition by Vanadates with Multiple Beam Interferometry in a Surface Forces Apparatus

Crevice corrosion (CC) of metals remains a serious concern for structural materials. Yet a real-time in situ visualization of corrosion, and its inhibition within a confined geometry, remains challenging. Here, we present how multiple-beam interferometry in a Surface Forces Apparatus can be utilized to directly visualize corrosion processes in real-time and with Ångstrom resolution within well-defined confinement geometries. We use atomically smooth muscovite mica surfaces to form round-shaped ∼1000 μm2 crevices on aluminum. After exposure to NaCl solutions we can detect and track active sites of aluminum corrosion within this confined geometry. CC of aluminum randomly initiates in the confined crevice mouth, where the distance between apposing surfaces is between 20–300 nm. We can directly track oxide dissolution/formation, and corrosion-rates as well as their retardation due to sodium vanadate inhibitors present in solution. Formation of a compacted oxide layer effectively inhibits CC in 5 mM NaCl solutions with 2.5 mM of added NaVO3, while inhibition rapidly breaks down at chloride concentrations above 50 mM. Breakdown of the inhibition-layers is initiated by rapid dissolution of the protective oxide within the confined zone. Our technique may be adapted for monitoring CC, corrosion inside of crack-tips, and evaluation of inhibitor efficiencies in a variety of metals.

Irreversible structural change of a dry ionic liquid under nanoconfinement.

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Interactions between charged lamellae in aqueous solution.

Interactions between charged surfaces in aqueous solutions, widespread in soft matter and biology, are very complex and, despite many efforts, their full explanation remains challenging. We support the idea that, in contrast to extremely small separations (d≤2 nm), where many effects, prominently those linked to the structure of liquid water, interfere, electrostatics alone rules over larger distances (d≥5 nm) at low ionic strength. We set up specially designed surface force apparatus (SFA) experiments to measure the elastic compressibility modulus of a stack of charged membranes with monovalent counterions, directly and with high precision. We demonstrate that electrostatics alone, if implemented beyond Poisson-Boltzmann theory, fully accounts for the data, nonelectrostatic contributions playing at best a minor role.