Mica plate experiments

In a number of experiments,1 the measured repulsive force between two mica plates was greater than that predicted on the basis of the Poisson-Boltzmann equation, when the distance between them was below about 40 A. This higher repulsive force arises only for sufficiently large ionic strengths and has a decay length in the range of 3-10 A. In contrast, the repulsive force between two parallel phospholipid bylayers falls off with a decay length between... 

Earlier experiments on polystyrene adsorbed on mica immersed in cyclohexane showed that below the temperature the force was attractive for large separations between the mica plates, but became repulsive for small separations.27 However, it was found, for the same system, that the attraction persisted even for temperatures higher than . For this reason, its origin was attributed to the formation of polymer bridges between the surfaces.28 Later, another experiment, in which the bridging was essentially eliminated, has shown that at large separations the force was attractive for T < and repulsive for T > ,26... 

Klein (1980) has recently measured directly the forces between two mica plates coated by polystyrene in cyclohexane at a temperature (24 °C) below the 0t/-temperature (35 °C). These measurements showed clearly the existence of a long-range force of attraction that is in part attributable to the mutual attraction between the polystyrene segments under worse than 0-conditions. This experiment will be discussed more fully in Section 13.4.1.2. 

Klein (1980 1982) has performed the most definitive experiments to date on the distance dependence of the steric interactions of polymer coated macroscopic surfaces. Monodisperse polystyrene, of molecular weight 1 x 10 or 6 X 10 , was adsorbed onto the mica plates by incubation of the plates with a very dilute solution (ca 7 x 10 g dm ) of the polymer in cyclohexane at 24 C. The polymer solution (save for ca 1 cm between the plates) was then replaced by pure cyclohexane. 

Rayleigh (i6) succeeded in the technically difficult task of mounting a mica plate for diffusion experiments at temperatures up to 415° C. The principle of his method was to keep an outer annulus of the plate cold and thermally insulated as far... 

Mica is a unique colloid in that it is possible to cleave atomically flat sheets from this clay mineral. These have been exploited in so-called surface forces experiments (Section 1.9.6), where the force between two mica sheets sandwiching a liqnid of interest is measured as a function of distance between them. The method has allowed a direct determination of the Hamaker constant for mica plates separated by different liquids. Recent extensions of the method have enabled the measurement of the forces between polymer layers adsorbed on to the mica, for various polymer architectures at different grafting densities and in the presence of different solvents (or none). 

In order to measure the absorption of the beam of rays coming from the mercury vapor an ionization chamber with a thin mica window in it and containing methyliodide was set up opposite the window, F, and lead plates with holes in them were placed in the line of the beam so that only the radiation coming from the impacts of the electrons against the mercury entered the chamber with sufficient intensity to be detected. That this was the case in the actual experiments is indicated by the fact that no perceptible ionization current could be observed when the mercury pump was not running. A quadrant electrometer measured the ionization current. 

This method has been used to obtain force-separation curves for block copolymers adsorbed on planar substrates and subjected to uniaxial compression. Block copolymers are adsorbed from solution onto atomically flat substrates (e.g. mica, quartz) and the force between the plates measured for separations ranging from —0.1 to contact. Further details of the experiments are given in Section 3.8.2. 

The contents of the trough were then stirred at a constant rate with a magnetic stirrer, and Atr was measured with a roughened mica Wilhelmy plate (13). Adsorption proceeded with the lipid film held at constant area ( 70 cm2) until steady-state values of Atr and AR were attained. Att and AR could be reproduced to 1 dynes/cm and 1 counts/sec, i.e., r 0.1 mg/m2. All experiments were carried out at 22° 2°C. 

Figure 3.4 Examples of the PECO patterns as observed in a real SPA experiment. Top with a gap of 10 nm between the mica surfaces. The curved shape resembles a cross section of the sphere-plate geometry equivalent to the crossed cylinders. Bottom with surfaces in contact. The...

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