Interferometric Surface Force Apparatus Research Articles

Forces between Hydrophilic Surfaces Adsorbed with Apolipoprotein AII Alpha Helices

To provide better understanding of how a protein secondary structure affects protein-protein and protein-surface interactions, forces between amphiphilic alpha-helical proteins (human apolipoprotein AII) adsorbed on a hydrophilic surface (mica) were measured using an interferometric surface force apparatus (SFA). Forces between surfaces with adsorbed layers of this protein are mainly composed of electrostatic double layer forces at large surface distances and of steric repulsive forces at small distances. We suggest that the amphiphilicity of the alpha-helix structure facilitates the formation of protein multilayers next to the mica surfaces. We found that protein-surface interaction is stronger than protein-protein interaction, probably due to the high negative charge density of the mica surface and the high positive charge of the protein at our experimental conditions. Ellipsometry was used to follow the adsorption kinetics of this protein on hydrophilic silica, and we observed that the adsorption rate is not only controlled by diffusion, but rather by the protein-surface interaction. Our results for dimeric apolipoprotein AII are similar to those we have reported for the monomeric apolipoprotein CI, which has a similar secondary structure but a different peptide sequence and net charge. Therefore, the observed force curves seem to be a consequence of the particular features of the amphiphilic alpha-helices.

Interactions between bottle-brush polyelectrolyte layers: Effects of ionic strength and oppositely charged surfactant

Interactions between cationic bottle-brush polyelectrolyte layers adsorbed on mica across salt and oppositely charged surfactant solutions were investigated with the interferometric surface force apparatus, and the results were compared with what is known for similarly charged linear polyelectrolytes. Ellipsometric measurements demonstrated that the bottle-brush polyelectrolytes, which contain 45 units long poly(ethylene oxide) side chains, are more readily desorbed than linear equivalents when the ionic strength of the solution is increased. It is argued that this is due to the steric repulsion between the poly(ethylene oxide) side chains that reduces the surface affinity. The preadsorbed bottle-brush polyelectrolyte layers were also exposed to sodium dodecyl sulfate (SDS) solutions. It was found that the presence of SDS affected the force profiles less than observed for similarly charged linear polyelectrolytes. This observation was attributed to excluded volume constraints imposed by the poly(ethylene oxide) side chains that reduces the accessibility of the charged polyelectrolyte segments and counteracts formation of large aggregates within the layer.

Surface Interactions during Polyelectrolyte Multilayer Build-Up. 2. The Effect of Ionic Strength on the Structure of Preformed Multilayers

Interactions between surfaces bearing multilayer films of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) were investigated across a range of aqueous KBr solutions. Three layer films (PAH/PSS/PAH) were preassembled on mica surfaces, and the resulting interactions were measured with the interferometric surface force apparatus (SFA). Increasing the ionic strength of the medium resulted in a progressive swelling of the multilayer films. Interactions in solutions containing more than 10(-3) M KBr were dominated by a long-ranged steric repulsion originating from compression of polyelectrolyte segments extending into solution. In 10(-1) M KBr, repeated measurements at the same contact position showed a considerable reduction of the range and the strength of the steric force, indicating a flattening of the film during initial approach. Furthermore, this flattening was irreversible on the time scale of the experiments, and measurements performed up to 72 h after the initial compression showed no signs of relaxation. These studies aid in understanding the dominant interactions between polyelectrolyte multilayers, including polyelectrolyte films deposited on colloidal particles, which is important for the preparation of colloidally stable nanoengineered particles.

Interactions and Conformations of α-Helical Human Apolipoprotein CI on Hydrophilic and on Hydrophobic Substrates

Interactions between amphiphilic alpha-helical human apolipoprotein CI (APO CI) adsorbed on hydrophilic and on hydrophobic surfaces were studied using an interferometric surface force apparatus in an effort to understand the surface conformation and the binding activity of this protein. We used mica as the hydrophilic substrate and polymerized octadecyltriethoxysilane (OTE)-covered mica as the hydrophobic substrate. The OTE monolayer and the OTE Langmuir-Blodgett film were studied using Brewster angle microscopy and atomic force microscopy, respectively. We found that interaction forces between layers of APO CI adsorbed on hydrophilic and on hydrophobic surfaces are mainly due to electrostatic double-layer forces at large surface distances and to steric repulsive forces at small distances. In some cases, no force was measured prior to finding a steric wall, suggesting that a complete neutralization of the surface charge was achieved by the protein adsorption. Protein layer thickness values allow us to give an image of the organization and conformation of the APO CI protein on surfaces. The adhesion obtained in both kinds of surfaces indicates that the interaction between the hydrophobic sides of the APO CI proteins is stronger than that between the hydrophilic sides of the protein. (Less)

Polymer Bilayer Formation Due to Specific Interactions between β-Cyclodextrin and Adamantane: A Surface Force Study

The purposes of this study are to utilize the interactions between an adamantane end-capped poly(ethylene oxide) (PEO) and a cationic polymer of beta-cyclodextrin to build polymer bilayers on negatively charged surfaces, and to investigate the interactions between such layers. The association of this system in solution has been studied by rheology, light scattering, and fluorescence measurements. It was found that the adamantane-terminated PEO (PEO-Ad) mixed with the beta-cyclodextrin polymer gives complexes where the interpolymer links are formed by specific inclusion of the adamantane groups in the beta-cyclodextrin cavities. This results in a higher viscosity of the solution and growth of intermolecular clusters. The interactions between surfaces coated with a cationized beta-cyclodextrin polymer across a water solution containing PEO-Ad polymers were studied by employing the interferometric surface force apparatus (SFA). In the first step, the interaction between mica surfaces coated with the cationized beta-cyclodextrin polymer in pure water was investigated. It was found that the beta-cyclodextrin polymer adsorbs onto mica and almost neutralizes the surface charge. The adsorbed layers of the beta-cyclodextrin polymer are rather compact, with a layer thickness of about 60 A (30 A per surface). Upon separation, a very weak attractive force is observed. The beta-cyclodextrin solution was then diluted by pure water by a factor of 3000 and a PEO-Ad polymer was introduced into the solution. Two different architectures of the PEO-Ad polymer were investigated: a four-arm structure and a linear structure. After the adsorption of the PEO polymer onto the beta-cyclodextrin layer reached equilibrium, the forces were measured again. It was found that the weak repulsive long-range force had disappeared and an attractive force caused the surfaces to jump into contact, and that the compressed layer thickness had increased. The attractive force is interpreted as being due to a specific recognition between the hydrophobic adamantane groups on the PEO-Ad polymer and the hydrophobic cavity in the beta-cyclodextrin molecules. Furthermore, the attractive force observed on separation has increased significantly, which is a further indication of a specific interaction between the beta-cyclodextrin polymer and the adamantane groups.

DNA and cationic surfactant complexes at hydrophilic surfaces. An ellipsometry and surface force study.

The adsorption and formation of DNA and cationic surfactant complexes at the silica-aqueous interface have been studied by ellipsometry. The interaction between the DNA-surfactant complexes at the mica-aqueous interface has been determined by the interferometric surface force apparatus. Adsorption was as expected not observed on negatively charged hydrophilic surfaces for DNA and when DNA-cationic surfactant complexes were negatively charged. However, adsorption was observed when there is an excess of cationic surfactant, just below the point of phase separation. The adsorption process requires hours to reach steady state. The adsorbed layer thickness is large at low surface coverage but becomes more compact and thinner at high coverage. A long-range repulsive force was observed between adsorbed layers of DNA-cationic surfactant complexes, which was suggested to be of both electrostatic and steric origin. The forces were found to be dependent on the equilibration time and the experimental pathway.

Polyelectrolytes as adhesion modifiers

Adsorbed layers of polyelectrolytes have been studied with atomic force microscopy (AFM) and the interferometric surface force apparatus (SFA). Particular emphasis was put on determining the effect of the polyelectrolyte charge density on surface topography, and the effect of the polyelectrolyte coating on the adhesive properties. The AFM was employed to image individual polymer chains at low adsorption densities and to characterize the layer topography and coverage at higher adsorption densities. The adhesive properties between two polyelectrolyte-coated surfaces in air were determined as a function of the number of contacts made at any given spot. The data provide evidence for formation of electrostatic bridges, particularly when highly charged polyelectrolytes are used. Further, material transport between the surfaces is observed when the polyelectrolyte is either highly charged or have a very low charge density. For intermediate charge densities we could not observe any indication of material transfer. The adhesion between one polyelectrolyte-coated surface and one bare surface was initially higher than that between two polyelectrolyte-coated surfaces. However, due to material transfer between the two surfaces the adhesion decreased significantly with the number of times that the surfaces were driven into contact. For the polyelectrolytes of the lowest charge density the results suggest that entanglement effects contribute to the adhesive interaction. The modification of the adhesion by polyelectrolytes in practical systems such as in the case of dry-strength additives to improve paper resistance is also considered.

Forces between Xylan-Coated Surfaces: Effect of Polymer Charge Density and Background Electrolyte

The forces between xylan-coated mica surfaces at pH 10 have been studied using the interferometric surface force apparatus. Xylan is a polysaccharide that is composed of (1,4)-linked β-D-xylopyranose units, which are partially substituted at C2 with 4-O-methyl-α-D-glucuronic acid units and at C3 with α-L-arabinosyl units. Two different xylans were studied, one with 0.5% and one with 9% of the segments containing a carboxylic acid group. It was found that both xylans adsorbed onto the negatively charged mica surface despite their negative charge. The interactions between the xylan-coated surfaces were dependent on the charge density of the polymer. The xylan with higher charge density adsorbed in a more extended conformation leading to more long-range repulsion. The shapes of the force curves were consistent with the forces expected for anchored polyelectrolyte brushes. After compression the length of the protruding chains decreased. An increase in background electrolyte concentration led to a decrease in the range of the repulsion. This effect was, as expected, more pronounced for the high charge density xylan. In the presence of CaCl2 not only the long-range forces but also the layer thickness at high compressive load decreased.

Interfacial Interaction between Sodium Dodecyl Sulfate and Hydrophobically Modified Ethyl(hydroxyethyl)cellulose. A Surface Force Study

The effect of addition of sodium dodecyl sulfate (SDS) to a layer of hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC) preadsorbed on hydrophobized mica has been studied with an interferometric surface force apparatus (SFA). The force between the surfaces was always repulsive on both compression and separation. The range of this force, which is 1500 Å for the HM-EHEC layers, undergoes a non-monotonic change on addition of SDS. Initially, at low SDS concentrations the layer expand, while at higher surfactant concentrations a contraction of the adsorbed layer is observed. At 1.5 mM SDS the range of the force is at maximum, while at 6 mM SDS the layer is significantly thinner than before addition of SDS. The results are discussed in terms of polymer−surfactant interfacial association and competitive adsorption. The data obtained by surface force measurements agree qualitatively with ellipsometry findings on the same system.

The Effect of a Cationic Polyelectrolyte on the Forces between Two Cellulose Surfaces and between One Cellulose and One Mineral Surface

The effect of a cationic polyelectrolyte, PCMA, on the forces between two cellulose surfaces and between one cellulose surface and one mica surface has been studied using the interferometric surface force apparatus (SFA). The cellulose surfaces were prepared by Langmuir–Blodgett deposition of trimethylsilyl cellulose onto hydrophobized mica. Prior to measurements the surfaces were desilylated to obtain pure cellulose. Introduction of a cationic polyelectrolyte into the solution drastically changed the interactions between the cellulose layers. It was found that the cationic polyelectrolyte does adsorb onto the cellulose surface, although the adsorbed amount is low. The adsorbed layer is very thin, as expected at a low electrolyte concentration. Before the adsorption has reached equilibrium, when only some polyelectrolyte had adsorbed, the adhesion between the surfaces was high, and it was noted that the cellulose layer was damaged on separation. After a longer adsorption time an electrostatic repulsion and no adhesion were observed between the polyelectrolyte-coated cellulose surfaces. An electrostatic repulsion was observed between cellulose and mica. When cationic polyelectrolyte was introduced to the system it overcompensated the charges on both surfaces, and the range and magnitude of the double-layer force was higher than without polyelectrolyte. The relevance of the results to flocculation mechanism and efficiency in cellulose systems is discussed.

Interactions between cellulose surfaces: effect of solution pH

The forces acting between cellulose surfaces have been studied using the interferometric surface force apparatus. The cellulose surfaces were prepared by Langmuir-Blodgett deposition of trimethylsilyl cellulose (TMSC) onto hydrophobized mica. Prior to measurements, the surfaces were desilylated to obtain pure cellulose. The degree of silylation and the molecular weight of the TMSC both affect the structure of the deposited layer. This was observed from the surface pressure-area isotherm, force versus distance curves, and atomic force microscopy images. The forces between the cellulose surfaces were found to depend on the pH of the solution. In dilute electrolyte solutions, the cellulose film was uncharged and rather compact when the pH of the solution was 6.0. However, when the pH was increased to 7.3, the cellulose film swelled considerably and a long-range steric force was measured. The swelling of the film is interpreted as being due to the dissociation of a few carboxylic acid groups present along the cellulose chain. The forces measured were, however, dominated by steric interactions. The repulsion does not increase substantially when the pH is increased from 7.3 to 9.5. Our results suggest that the pKa of the acid groups present within the cellulose film is larger than it would be in the bulk aqueous solution.

Surface interactions in emulsions and liposome solutions

We have used two surface force techniques to investigate the interactions between hydrophilic and negatively charged mica and glass surfaces across concentrated and dilute oil-in-water emulsions, as well as across aqueous liposome solutions. It was observed with both the interferometric surface force apparatus, using mica surfaces and with the non-interferometric MASIF technique, employing glass surfaces, that emulsion droplets adsorbed onto the surfaces. Under a high compressive force the adsorbed emulsion droplets were coalescing and this resulted in a phase separation in the gap between the surfaces. The forces measured in the presence of the capillary condensate compared favourably with theoretical expectations. In contrast, no adsorption of large aggregates could be detected in the liposome solution. Instead it appears that upon adsorption the liposomes break down and the surfaces become covered by a disordered layer of phospholipids. Measurements with the interferometric surface force apparatus, that allows absolute distances to be determined, allow us to draw the conclusion that some bilayer aggregates are adsorbed and that the inner layer consists of an intercalated monolayer.

Evaluation of the structure of adsorbed layers of -casein from ellipsometry and surface force measurements

Studies of the adsorption of ϐ-casein to hydrophobic and hydrophilic silica surfaces by time-resolved ellipsometry and direct measurements of the forces between adsorbed ϐ-casein layers using an interferometric surface force apparatus are reviewed. Adsorption to hydrophobic surfaces was rapid and plateau surface excess and mean thickness values of 2.8 mg m−2 and 66 Å were measured. Coating the surfaces with protein caused the long-range attractive force to be replaced by a long-range repulsive force and as the surfaces were brought into closer contact, this was overcome by an attractive force. Adsorption of ϐ-casein to hydrophilic surfaces was much slower and the plateau surface excess was higher. Accessibility of the adsorbed protein to endoproteinase Asp-N was also higher on this surface. Interactions between the coated hydrophilic surfaces were entirely repulsive. Possible structures for the adsorbed protein layers are discussed.

Investigation of a 31 Charged Cationic Polyelectrolyte Interacting with Sodium Dodecyl Sulfate in Bulk Solution and as a Preadsorbed Layer on Mica. Low Ionic Strength

The behavior of mixtures of a cationic polyelectrolyte of intermediate charge density (31%) and anionic surfactant both in bulk and at a negatively charged solid surface has been investigated. The polyelectrolyte employed was a random copolymer of the neutral acrylamide and cationic [3-(2-methylpropionamido)propyl]trimethylammonium chloride, and the surfactant was sodium dodecyl sulfate (SDS). Measurements of flocculation and electrophoretic mobility in bulk solution clearly showed a phase separation with a maximum in turbidity corresponding roughly to charge neutralization of the polyelectrolyte/surfactant complex. Conductivity measured at low SDS concentrations showed a strong uptake of ions by the polyelectrolyte at a critical SDS concentration. The interferometric surface force apparatus was used to measure forces between muscovite mica surfaces onto which the polyelectrolyte was adsorbed from a 20 ppm, 10-4 M KBr solution. After the polymer solution was replaced with polymer-free 10-4 M KBr solution, a weak long range double-layer repulsion was observed between approaching surfaces, with a bridging attraction dominating at smaller separations. A weak adhesion was observed upon separating the surfaces. Addition of 4.2 × 10-4 M SDS caused a large swelling of the preadsorbed polymer layer due to formation of an associative complex between the polymer and surfactant. Further increasing the SDS concentration decreased the force and layer thickness due to screening of the electrostatic repulsion within the layers and some desorption of polymer molecules. That a slight desorption did occur was supported by electron spectroscopy for chemical analysis measurements. The results broaden the understanding of the effect of polyelectrolyte charge density on the interaction of preadsorbed polyelectrolyte layers with oppositely charged surfactants.

Forces between Adsorbed Layers of β-Casein

The forces between β-casein layers adsorbed onto mica surfaces have been measured as a function of surface separation by using the interferometric surface force apparatus. Both hydrophilic pure mica and a mica surface, which has been made hydrophobic by Langmuir−Blodgett deposition of dimethyldioctadecylammonium bromide, were used. A long-range repulsive force, most probably of electrostatic origin, was observed between β-casein layers adsorbed on hydrobized mica. The results suggest that β-casein forms a monolayer on this surface, where the outer part is less densely packed and protrudes far out into the solution. This open brushlike structure can readily be compressed on which an attractive force arises. The portion of the monolayer closest to the hydrophobized surface is much more compact and has low compressibility. On the pure hydrophilic surface a bilayer structure is more likely, with the a compact inner layer and an outer layer which has a similar structure to the monolayer formed on a hydrophobic...

Surface Force Studies of Langmuir–Blodgett Cellulose Films

The interactions between cellulose surfaces, between chitosan-coated surfaces, and between one cellulose surface and one chitosan-coated surface have been studied using the interferometric surface force apparatus (SFA). The cellulose surfaces were prepared by depositing trimethylsilyl cellulose on hydrophobized mica using the Langmuir–Blodgett technique. The surfaces were desilylated in a humid HCl atmosphere to obtain regenerated cellulose. ESCA measurements and wetting studies demonstrated that the desilylation process was effective. Scanning force microscopy studies showed the cellulose surfaces to be smooth with a root-mean-square roughness of 0.16 nm. Surface force and ellipsometry measurements illustrate that the cellulose film swells considerably in humid air and in water, suggesting that it is mostly amorphous. The interaction between two cellulose surfaces is dominated by a steric repulsion caused by a few dangling tails. On separation an attractive force was present both between two cellulose surfaces and between one cellulose surface and one chitosan surface. The long-range interaction between cellulose and chitosan was shown to be attractive.

Alkyl Glucosides on Hydrophobic Surfaces Studied by Surface Force and Wetting Measurements

The wetting properties of hydrophobized surfaces in the presence of alkyl glucosides have been studied using a Wilhelmy balance technique, and the interparticle forces acting between such surfaces have been investigated using the interferometric surface force apparatus. The adsorption at concentrations well below the cmc is rather limited both for the anomerically pure octyl β-glucoside and for the technical product based on the branched octyl glucoside, 2-ethylhexyl glucoside. However, the adsorption results in a decreasing surface hydrophobicity, and consequently the force needed to separate two such surfaces from each other is reduced. At the cmc for octyl β-glucoside, a surfactant monolayer is adsorbed on each surface. The interaction between the glucose units gives rise to a short-range repulsive force. Beyond the range of this repulsive force a weak attraction is experienced. Addition of a small amount of octyl α-glucoside does not change the short-range interaction significantly. The technical 2-ethylhexyl glucoside surfactant behaves similarly to the anomerically pure surfactant at low concentrations. The solution with the technical glucoside phase-separates at high surfactant concentration. It is observed that under such conditions a thick layer is deposited on each surface.

Techniques for measuring surface forces

The forces acting between colloidal particles and between surfaces are of utmost importance for determining the behaviour of dispersed systems and adhesion phenomena. Several techniques are now available for direct measurement of these surface forces. In this review we focus on some of these methods. Two techniques for measuring forces between macroscopic solid surfaces; the interferometric surface force apparatus, known as the SFA, and a novel instrument which is based on a bimorph force sensor, the so-called MASIF, are described in some detail. Forces between a macroscopic surface and a particle can be measured with the atomic force microscope (AFM) using a colloidal probe, or by employing total internal reflection microscipy (TIRM) to monitor the position of a colloidal particle trapped by a laser beam. We also describe two different techniques that can be used for measuring forces between “soft” interfaces, the thin film balance (TFB) for single foam, emulsion and solid/fluid/fluid films, and osmotic stress methods, commonly used for studying interactions in liquid crystalline surfactant phases or in concentrated dispersions. The advantages and limitations of each of these techniques are discussed and typical results are presented.

FOAM FILMS AND SURFACE FORCE STUDIES OF AQUEOUS SOLUTONS OF OCTYL-β-GLUCOSIDE

ABSTRACT The interactions at the air/liquid and the liquid/solid interface have been studied for octyl-β-glucoside. With the thin film balance the thickness of the equilibrium liquid films is determined as a function of surfactant concentration, inert salt concentration and pH. At high surfactant concentration and/or at low pH black films, about 4.6 nm thick, are formed. The stabilizing forces in the black films are related to the interactions within and between the layers. The forces acting between octyl-β-glucoside surfactant layers adsorbed on hydrophobized mica were investigated with the interferometric type surface force apparatus. The forces measured between the sugar head-groups were similar to those acting between other small nonionic groups like dimethylamine oxide and monoglycerides. However, considerably more long-range repulsions are observed between surfactants with oligoethylene oxide head-groups.