Mica Surfaces In Aqueous Solutions Research Articles

The mechanism of restructuring of surfactant monolayer on mica surface in aqueous solution: molecular dynamics simulation

Molecular dynamics simulation was employed to investigate the restructuring process of CTAB monolayer at mica/water interface. The reversing process of CTAB monolayer was exploited by diffusion of water molecules, reversing of CTAB molecules with time evolution and restructuring of the surfactant monolayer. The results showed that bromide ions around surfactant head groups diffused into bulk water readily due to the electrostatic repulsion caused by negatively charged mica surface. Meanwhile, because of the electrostatic attraction between water molecules and mica surface, part of water molecules can penetrate the surfactant monolayer to form water channel which bridges bulk water and mica surface. The monolayer structure was disturbed by diffusion of bromide ions and formation of water channel. Few of the head groups of surfactants tended to reverse and enter into aqueous solution. The number of reversed surfactant molecules increased with time evolution. The monolayer restructured into bilayer structure gradually. Finally, a cylindrical aggregate was obtained.

Micropatterned Charge Heterogeneities via Vapor Deposition of Aminosilanes

Aminosilanes are routinely employed for charge reversal or to create coupling layers on oxide surfaces. We present a chemical vapor deposition method to pattern mica surfaces with regions of high-quality aminosilane (3-aminopropyltriethoxysilane, APTES) monolayers. The approach relies on the vapor deposition of an aminosilane through a patterned array of through-holes in a PDMS (poly(dimethylsiloxane)) membrane that acts as a mask. In aqueous solutions the surfaces have regular patterns of charge heterogeneities with minimal topographical variations over large areas. This versatile dry lift-off deposition method alleviates issues with multilayer formation and can be used to create charge patterns on curved surfaces. We identify the necessary steps to achieve high quality monolayers and charge reversal of the underlying mica surface: (1) hexane extraction to remove unreacted PDMS oligomers from the membrane that would otherwise deposit on and contaminate the substrate, (2) oxygen plasma treatment of the top of the membrane surfaces to generate a barrier layer that blocks APTES transport through the PDMS, and (3) low of the vapor pressure of APTES during deposition to minimize APTES condensation at the mica-membrane-vapor contact lines and to prevent multilayer formation. Under these conditions, AFM imaging shows that the monolayers have a height of 0.9 ± 0.2 nm with an increase in height up to 3 nm at the mica-membrane-vapor contact lines. Fluorescence imaging demonstrates pattern fidelity on both flat and curved surfaces, for feature sizes that vary between 6.5 and 40 μm. We verify charge reversal by measuring the double layer forces between a homogeneous (unpatterned) APTES monolayers and a mica surface in aqueous solution, and we characterize the surface potential of APTES monolayers by measuring the double-layer forces between identical APTES surfaces. We obtain a surface potential of +110 ± 6 mV at pH 4.0.

Two-step sharpening process for silicon probe in quartz-based atomic force microscopy sensor

A two-step sharpening process for a Si pillar in a quartz tuning fork sensor was developed for use in frequency modulation atomic force microscopy (FM-AFM). By combining electrochemical etching in HF solution and anisotropic etching in KOH solution, the tip radius was decreased to 120 nm. The Si-probe-attached sensor showed a higher resonance frequency than a tungsten-probe attached sensor, which would lead to a higher force sensitivity. We demonstrated FM-AFM imaging on a cleaved mica surface in aqueous solution by using the fabricated sensor, and atomic resolution was successfully achieved.

An ultra-low noise optical head for liquid environment atomic force microscopy.

The design considerations and eventual performance of a new, ultra-low noise optical head for dynamic atomic force microscopy (AFM) are presented. The head, designed specifically for the study of hydration layers and ion organization next to solid surfaces and biomolecules, displays an integrated tip-sample distance noise below 3 pm. The sensitivity of the optical beam deflection sensor, operating at frequencies up to 8.6 MHz (3 dB roll-off), is typically below 10 fm/√Hz, enabling utilization of high frequency cantilevers of low thermal noise for fundamental and higher mode imaging. Exceptional signal stability and low optical noise are achieved by replacing the commonly used laser diode with a helium-neon laser. An integral photothermal excitation of the cantilever produces pure harmonic oscillations, minimizing the generation of higher cantilever modes and deleterious sound waves characterizing the commonly used excitation by a piezoelectric crystal. The optical head is designed to fit on top of the widespread Multimode(®) (Bruker) piezo-tube and accommodate its commercial liquid cell. The performance of the new AFM head is demonstrated by atomic resolution imaging of a muscovite mica surface in aqueous solution.

Assembly of Oligoglycine Layers on Mica Surface

Assembly of [Gly7-NHCH2-]4C, [Gly7-NHCH2-]3C CH3 and [Gly4NH(CH2)5-]2 peptides on mica surface in aqueous so-lution was studied. The peptides are capable of forming atomically smooth (2.65-4.3 nm in height) layers assembled as polyglycine II. Monomers in the layers are situated normally to the surface. Formation of analogous flat 2D structures also takes place in solution but much more slowly than on mica surface, i.e. negatively charged surface plays an active role promoting the assembly.

Crystallization in Thin Liquid Films Induced by Shear

It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.

Interactions between Two Polyelectrolyte Multilayers Investigated by the Surface Force Apparatus

The distance-dependent interaction between multilayers of alternating polycation (poly-l-lysine) and polyanion (poly-l-glutamic acid) deposited onto mica surfaces in aqueous solution is investigated with the surface force apparatus. At large separations, a long-range, weak attraction is observed. The interaction turns into repulsion as opposite multilayers overlap when the surface separation is reduced. The initially observed exponential force−distance law is ultimately overcome by a steep steric interaction at full compression of the films. The evolution of the force profiles as a function of time and upon subsequent unload−load sequences is monitored. The origin of the different interaction regimes arises from osmotic pressure and the formation of complexes between the oppositely charged chains.

Electrostatic response of hydrophobic surface measured by atomic force microscopy

The arrangement of water molecules at aqueous interfaces is an important question in material and biological sciences. We have measured the force acting on neutral tips as a function of the distance to hydrophobic silicon surfaces and cetyltrimethylammonium bromide monolayers covering mica surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer) which is generated by the reorientation of water molecular dipoles. The exchange of a volume of this region with a dielectric permittivity (εint) by the tip with a dielectric permittivity (εtip) is responsible for the tip attraction when it is immersed in the polarization (hydration) layer. Variable permittivity profiles starting at ε≈11 at the interface and increasing to ε=80 about 10 nm from hydrophobic silicon surfaces and about 50 nm from cetyltrimethylammonium bromide monolayer covering mica surfaces were measured.

Grafting of Diblock Copolymers onto Adsorbed Surfactant Films

We studied the grafting of diblock copolymers onto mica surfaces in aqueous solutions. Because the copolymers are nonadsorbing on mica, grafting is achieved in the presence of adsorbing cationic surfactants. The surfactant film makes a sublayer onto which the hydrophobic moiety of macromolecules can end anchor. By using both a surface force apparatus (SFA) and an atomic force microscope (AFM), we studied how the interactions between the surfactant-coated surfaces are modified by the presence of copolymer and show how these modifications relate to changes in the morphology of the adsorbed surfactant film.

Elasticity of Mutant Myosin Subfragment-1 Arranged on a Functional Silver Surface

Elasticity of a two-dimensionally arranged myosin subfragment-1 (S1) was measured by using a surface forces apparatus. To prepare a two-dimensionally arranged S1-monolayer on a functionalized silver surface, we used genetically engineered Dictyostelium S1 molecules. A highly reactive cysteine residue was fused to the COOH-terminus using the recombinant DNA method. On the other hand, the maleimide groups was self-assembled onto a silver surface. Then the mutant S1 molecules were chemically bound to the functionalized silver surface at its COOH-terminus. This arrangement technique was necessary in order to create a stable S1-monolayer by chemical bond formation onto the silver surface. The occupied area of the single S1 on the silver surface was about 110 nm2. In the interaction between the S1-monolayer and mica surfaces in aqueous solution, a long-range attractive force was observed. The elastic constants (stiffness and Young's modulus) of myosin S1 were evaluated from force-distance profiles in aqueous solution, using the Hertz theory. We found that the stiffness (or spring constant) and Young's modulus of S1 in the absence of nucleotide are 4.4 ± 1.0 pN/nm and 0.71 ± 0.16 GPa, respectively.

Molecular Mechanisms Controlling the Self-Assembly Process of Polyelectrolyte Multilayers

The distance dependent interaction between polyelectrolyte-covered mica surfaces in aqueous solution was investigated with the surface forces apparatus. We find the following: (i) The surface charge changes sign, when an oppositely charged polyelectrolyte from a concentrated polyelectrolyte solution is adsorbed. (ii) Tails and loops of the adsorbed polyions dangle into the bulk phase, inducing a small steric force. If polycations and poyanions are adsorbed on top of each other, a strong short range attractive force is seen due to ion-pair formation after crossing a large repulsive electrostatic/steric barrier. (iii) Obviously, after polyelectrolyte adsorption, there are still nonoccupied binding places (point charges) on the substrate. We show that these adsorption properties regulate the build-up of polyelectrolyte multilayers: Ion pairs between oppositely charged polyion segments and the substrate are formed, until the surface charge is inversed. The electrostatic barrier limits the adsorbed amount, guarantees the equal thickness of consecutively adsorbed layers, and furthermore causes a strong adsorption hysteresis, which leads to conveniently stable polyelectrolyte multilayers in various environmental conditions.

NEW SURFACES AND TECHNIQUES FOR STUDIES OF INTERPARTICLE FORCES

ABSTRACT Several methods of preparing surfaces suitable for interparticle surface forces measurements are presented. Treatment of muscovite mica surfaces with a water vapour plasma introduces surface hydroxyl groups. These groups can react with chlorosilanes to form organo functionalized surfaces. The forces between water vapour plasma treated mica surfaces in aqueous solutions are well described by DLVO theory except for the presence of a short-range (D < 10 Å) additional repulsion. After reaction with a fluorocarbon containing silane the surfaces become hydrophobic and a long-range attraction acts between the surfaces in water. Smooth glass spheres can be obtained by melting a glass rod in a gas burner. These surfaces are ideal substrates for a new type of surface force apparatus that uses a bimorph force sensor for measuring surface forces. The chemistry of the glass surfaces is readily modified

Molecular mechanisms and forces involved in the adhesion and fusion of amphiphilic bilayers.

The surface forces apparatus technique was used for measuring the adhesion, deformation, and fusion of bilayers supported on mica surfaces in aqueous solutions. The most important force leading to the direct fusion of bilayers is the hydrophobic interaction, although the occurrence of fusion is not simply related to the force law between bilayers. Bilayers do not need to "overcome" some repulsive force barrier, such as hydration, before they can fuse. Instead, once bilayer surfaces come within about 1 nanometer of each other, local deformations and molecular rearrangements allow them to "bypass" these forces.

Forces between model polypeptides and proteins adsorbed on mica surfaces

The forces of interaction between proteins and model polypeptides adsorbed onto mica have been measured as a function of the distance of separation between the mica surfaces in aqueous solution. Results were obtained for the basic polypeptide, poly-l-lysine, of molecular weight 4,000 and 75,000, cytochrome c, concanavalin A and myelin basic protein. For cytochrome c no interaction between the adsorbed protein layers was noted until the surfaces were separated by 12.5 nm whereupon an attraction was measured, indicating that the negatively charged mica has been neutralised by adsorption of the positively charged cytochrome c. For all the other proteins, and the poly-l-lysine the force at long range was repulsive and could be explained simply in terms of electrical double layer theory. At short surface separations attractions were noted in all cases (although for concanavalin A calcium and manganese ions needed to be present). These attractions can be accounted for by van der Waals forces for all the systems, with the exception of myelin basic protein for which an attraction ten times larger than that predicted by van der Waals forces was measured. We propose that this additional attraction may be due to hydrophobic interactions between the adsorbed myelin basic protein layers.

Forces between proteins and model polypeptides adsorbed on mica surfaces

The forces of interaction between proteins adsorbed onto mica have been measured as a function of the distance of separation between the two mica surfaces in aqueous solutions. The results for three proteins, myelin basic protein, concanavalin A and cytochrome c, are presented together with the results for a model basic protein, poly-( l-lysine). With the exception of cytochrome c at large separations, the forces of interation are due to charges on the protein surfaces and may be fitted closely to theoretical predictions. For cytochrome c, however, no long-range electrical repulsion is observed, indicating that the negatively charged mica surface has been neutralised by the adsorption of the positively charged protein. At short surface separations, an attracton between the protein surfaces was noted. For concanavalin A, a weak attraction was observed in the presence of calcium and manganese ions only. For poly( l-lysine) and cytochrome c the attraction can be explained simply in terms of van der Waals interactions between the proteis. However, for myelin basic protein the observed attraction was an order of magnitude larger than that predicted by van der Waals theory. We believe that this additional attraction may be due to hydrophobic interactions between the adsorbed myelin basic protein molecules.