Direct Measurement of Molecular Forces

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Second entry by Kelly Miller, AP225 Fall 2011


Derjaguin's review article can be found here: [1]


Keywords

thin films, Lifshitz's Theory of dispersion forces, disjoining pressure

Introduction

Abstract:

The current theory of the stability of colloids and liquid film rests on the direct measurements of the force of interaction between two surfaces separated by a thin film. The first measurements, in the 1950s provided the stimulus of Lifshitz's macroscopic theory of dispersion forces. So far, measurements of disjoining pressure fit well with the theoretical predictions; but there are still difficulties with measuring some of the components of that pressure. [[2]]


This article reviews new ways devised (in 1978) to measure the disjoining pressure in colloid systems and in thin films of liquids

The current theory of the stability of colloids and liquid films (at the time) accounted for the simultaneous effect of the disjoining pressure: 1) resulting from the overlapping of diffuse ionic layers on charged surfaces 2) the forces of molecular attraction 3)resulting from the overlapping of boundary (solvate) layers of liquids

But - there had been trouble figuring out accurate ways of measuring disjoining pressure (especially specific components of disjoining pressure).

The purpose of this paper was to report of ways that had been devised to measure disjoining pressure, in order to further develop the theory and add further credence to Lifshitz's macroscopic theory of dispersion forces.

Context

Solving the problem of being able to measure disjoining pressure was fundamental to the paradigm shift that was occurring at the time that this paper was written.

The paradigm shift was from looking at things as a collection of particles with specific properties (particle-centric) to, instead focussing not on the particles, but on the properties of the thin films between the particles (thin-film centric).

The first measurements made of thin films, conflicted with the existing London-Hamaker theory and "provided the stimulus for the development, by Lifshitz, of the more widely-known macroscopic theory of dispersion forces. The basis of the London-Hamaker microscopic theory was the physical characteristics of separate molecules. The Lifshitz theory, by contrast, used the spectral characteristics of continuous media for calculating the attraction forces. The formulae of Lifshitz's theory relating to the distances at which the retardation effect is fully realized proved to be in good quantitative agreement with those first measurements of the molecular attraction. "[[3]]


B.V.D. and Abrikosova were the first to measure the forces of molecular attraction between solids. They encountered several difficulties:

1) the forces decrease drastically as the distance between the solids increases (at large distances, the forces are extremely small and so the accuracy of the measurements is decreased 2) at small distances, the derivative of the disjoining pressure as a function of distance is large and positive and this makes it necessary to use dynamometers of high rigidity (which have low sensitivity)

Findings and Significance

This paper outlines several ingenious ways of measuring the disjoining pressure and overcoming the problems that had been encountered in the past.

Derjaguin et al. overcame the difficulties experienced by other scientists who had done measurements of the forces of molecular attraction between particles, by using an apparatus (the main part of which was a microbalance with negative feedback) (Depicted below:)

Derjaguin and Abrikosova Apparatus.png


Derjaguin was able to overcome the difficulties by using negative feedback to a) stabilize the distance between the solids and b) the measurement of the attraction force.

(Negative feedback allows the measured force to be automatically counterbalanced by keeping a preset gap width between the solids (even in the range of large values for the derivative of disjoining pressure at a function of distance)

This device was used to measure the molecular attractive forces between two solids in air. The force of attraction between solids 1 & 2 was compensated by the electric current flowing through frame 4 which was rigidly attached to the beam of on a microbalance. The interaction between the electric current and the field of magnet 5 created a balancing torque. The relationship between the current intensity and the attraction force was based on the calibration of the instrument. This was determined by the extent to which two images (at raster 8 and 10) on the image above) were aligned. The electric current was generated by photoelement 11 when it was exposed to light passing through rasters 8 and 10. The distance between the solids could be adjusted by changing one of the rasters.


Another clever way of measuring molecular forces is to use an apparatus such as this one, devised by Sheludko et al.:

Sheludko method.png



This uses thin liquid films bounded on both sides by a liquid. The film is formed in an opening drilled into the porous filter (1) which is pressed by a Teflon cover (2) to the solid substrate (3). The disjoining pressure of the film is found from the differences in the pressure formed by the column of liquid (pgH) where p is the density of the liquid and g is the gravitational acceleration. Polarized light from an ellipsometer passes through a window (7) in the wall of the film. Small fluctuations in the thickness of the film can be measured by observing the change in the angle of the reflected light. By changing the hydrostatic pressure and allowing the system to equilibrate, the disjoining pressure of the thin film can be measured as a function of the thickness of the film.

Conclusion

This is an important review article because it outlines many experiments that were done to measure the disjoining pressure. This was a challenging measurement (at the time) and fundamental to the shift in how systems of particles were thought of. The forces that were measured and reported in this paper were consistant with the theoretical values from Lifshitz's macroscopic theory of dispersion forces. These measurements gave further support for the thin-film centric way of thinking of intermolecular interactions.


References

Derjaguin, B., Rabinovich, Y., Churaev, N.V., Direct Measurement of Molecular Forces, Nature 272, 313 - 318 (23 March 1978)