Sheet mica

A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

The sample preparation of these mica sheets is a delicate process that requires some experience and often takes 1-2 days prior to an SFA experiment. Tln-ough successive cleaving, one has to prepare 1-5 p.m thick and imifonn sheets of mica. Mica is a natural material that is available m different qualities [44]. 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

It was the idea of Winterton [2] to glue the otherwise fragile mica sheets onto polished silica discs to give them better mechanical stability, especially for friction experiments. The glue layer detemiines the final... 

The earliest SFA experiments consisted of bringing the two mica sheets into contact m a controlled atmosphere (figure Bl.20.61 or (confined) liquid medium [14, 27, 73, 74 and 75]. Later, a variety of surfactant layers [76, 77], polymer surfaces [5, 9, fO, L3, 78], poly electrolytes [79], novel materials [ ] or... 

Protems can be physisorbed or covalently attached to mica. Another method is to innnobilise and orient them by specific binding to receptor-fiinctionalized planar lipid bilayers supported on the mica sheets [15]. These surfaces are then brought into contact in an aqueous electrolyte solution, while the pH and the ionic strength are varied. Corresponding variations in the force-versus-distance curve allow conclusions about protein confomiation and interaction to be drawn [99]. The local electrostatic potential of protein-covered surfaces can hence be detemiined with an accuracy of 5 mV. 

Ohnishi S ef a/1999 Presence of particies on meit-cut mica sheets Langmuir 15 3312-6... 

Israelachvili and his colleagues have used the SEA to study the interactions between surface layers of surfactant and of other molecules representing functionalised polymer chains, adhesion promoters or additives. Typically a monolayer of the molecule concerned is deposited onto cleaved mica sheets. The values of surface energies obtained from the JKR equation (Eq. 18) throw some interesting light on the nature and roughness of surface layers in contact. 

In a typical experiment, Israelachvili deposited monolayers of surfactants onto cleaved mica sheets, and evaluated the surface energies using the JKR equation. Fig. 11 contrasts results for mica coated with monolayers of (a) L-a-dipalmitoyl-phosphatidylethanolamine (DMPE) where j/a = = 27 mJ/m and (b) hexa-decyltrimethylammonium bromide (CTAB) where = 20 mJ/m and = 50 mJ/m. ... 

GUmmer-blatt, n. mica sheet, -blattchen. n. mica scale or lamina, -erde. /. micaceoiis earth-... 

Before being glued to the glass sample, one face of each mica sheet has to be coated or spread with a thin layer of silver in 50 — 60 nm thick (reflectivity 96 % —98 %) for facilitating the optical interference. As shown in Fig. 16(a), when a beam of white light goes up vertically through the lower sample and reaches the silver film, the beam is partly re-... 

Fig. 16—A sketch of samples and fringes of FECO. (a) Mica sheets, silver films, and the light path, (b) Fringes before and after shift.

If there is a small increase in the distance between the two silver films, corresponding to a change in the gap between two mica sheets, the fringes will shift to the longer wavelengths (Fig. 16(b)), by a small interval of... 

Sample surfaces are atomically smooth surfaces of cleaved mica sheets for SFA, and various colloidal spheres and plates for a colloidal probe AFM. These surfaces can be modified using various chemical modification techniques, such as Langmuir-Blodged (LB) deposition [12,19] and silanization reactions [20,21]. For more detailed information, see the original papers and references texts. 

Sample preparation for AFM analysis is relatively simple. Generally, a desired amount of sample is absorbed onto a smooth and clean substrate surface, for example, a freshly cleaved mica surface. For example, to prepare a food macromolecule sample for AFM imaging in air, the diluted macromolecule solution is disrupted by vortexing. Then, a small aliquot (tens of microliters) of vortexed solution is deposited onto a surface of freshly cleaved mica sheet by pipette. The mica surface is air dried before the AFM scan. A clean surrounding is required to avoid the interference of dust in the air. Molecular combing or fluid fixation may be applied to manipulate the molecule to get more information. 

In 1963, Armin Weiss (then at the University of Heidelberg, Germany) reported the intercalation of amino acids and proteins in mica sheet silicates (Weiss, 1963). Some years later, U. Hoffmann, also from Heidelberg, published an article titled Die Chemie der Tonmineralien (The Chemistry of Clay Minerals), in which he mentioned possible catalytic activity of clays in processes which could have led to the emergence of life (Hoffmann, 1968). 

Many of the various techniques associated with metal film preparation have recently been reviewed by Klemperer (76). Much of the catalytic work with thick continuous films has used a cylindrical reaction vessel (Fig. 7a). This cylindrical geometry permits a cylindrical sleeve of mica sheet to be inserved and used as the film substrate for epitaxial film growth... 

The force-distance profiles presented in the following sections are generally plotted as F(D)/Rv.D, i.e. the force axis is normalized by dividing F(D) by the mean radius R of the mica sheets. In the Derjaguin approximation (11),... 

Figure 8.11 Illustration of Mauguin twisted nematic cell, reported in 1911. Substrates are thin mica plates, which are uniaxial with their optic axis parallel to plane of plates. Apparently, uniaxial crystal stmcture of mica produces strong azimuthal anchoring of nematic LCs of Lehmann, such that director is parallel (or perpendicular) to optic axis of mica sheets at both surfaces. Mauguin showed that method of Poincard could be used to explain optics of system if it was assumed that LC sample created layer of material with uniformly rotating optic axis in twisted cells.

A hot solution (0.1—1 %) is added drop-wise onto a pre-heated substrate such as a glass slide or a piece of mica sheet (the substrate may have been coated with vapor-deposited C). After being dried and solidified, the resulting thin film specimen is mounted onto a specimen grid with the aid of water surface if the specimen is insoluble in water. When a glass slide is used as a substrate, a dilute aqueous solution (<0.02M) of hydrofluoric acid (HF) might be used for easy detachment. The resulting thin film can be... 

The structure, the thickness, and the interactions generated by the presence of the adsorbed polymer layers have been extensively studied [17], In particular, the force between two polymer-covered mica sheets in various solvency conditions has been probed via the SEA technique [27,28], The force is purely repulsive in a good solvent and becomes attractive as the solvent gets poorer. However, these studies concern only a regime of large interaction compared to the thermal energy and are restricted to interactions between solid surfaces. 

Dipolar Absorption. When the dielectric behavior of hydrated Na+-zeolites is investigated between two electrodes blocked from the sample with thin mica sheets, the conduction process is eliminated and two absorp-... 

The surface forces technique measures the force between molecules (eg. surfactants, polymers) adsorbed on mica sheets. In the case of large molecules such as polymers, the measurement is most sensitive to the regions closest to the solution and provides little direct information about the region adjacent to the surface. As it is a measurement between macroscopic surfaces, it is unable to provide information on microscopic chemical differences at the interface. Infrared spectroscopy could provide additional information about the quantity of adsorbed material on the mica surface, the identity and orientation of the adsorbed species, and possibly the nature of the surface linkage. 

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