Adsorbed Surface Films Research Articles

A study of the spread and adsorbed films of milk proteins

1. 1. The spread and adsorbed surface films of some caseins and whey proteins have been examined at the air-water interface. 2. 2. The pressure-area isotherms of spread films have been compared with pressure-concentration isotherms, which were obtained by adding an increasing amount of protein to a constant surface area. 3. 3. It is shown that the pressure-area isotherms are generally unaffected by changes in the structure of the spread molecule, while pressure-concentration isotherms are sensitive to changes in protein structure. 4. 4. The adsorption studies suggest that disordered proteins are more surface active than globular proteins. 5. 5. The surface pressure characteristics of the adsorbed films were compared with the pressure-area and pressure-concentration isotherms. It was concluded that pressure-concentration spread films are similar to adsorbed films. 6. 6. Both pressure concentration isotherms and adsorption behaviour reflect the magnitude of the forces preventing the molecule unfolding at the surface and can be used to provide information about protein structure.

The theory of absolute surface shear viscosity. I

The steady-state velocity field generated at a fluid interface by a slowly rotating disk of radius a thrust into that interface is solved in laminar flow approximation. The theory takes full account of the hydrodynamic coupling between the internal viscosity μ of the substrate with the surface viscosity η of an adsorbed surface film. Results are presented in graphical form of the interfacial velocity field near the rim of the disk for various values of the dimensional ratio η/μα .

The friction and lubrication of polymers

This paper describes a study of the friction of a typical polar polymer, nylon, and a typical non-polar polymer, polyethylene. The friction of unlubricated surfaces varies with load and geometry in a manner that can be explained in terms of the multi-asperity model of Howell & Lodge (1954), modified to allow for a deformation mode that is intermediate between elastic and plastic deformation. Nylon is markedly plasticized by water and its effect as it diffuses into the bulk of the polymer has been studied using hardness measurements. Initially the water produces a softening restricted to the surface layers and this leads to a modest reduction in friction. After a protracted period the bulk softening of the polymer causes an increase in the area of contact (cf. Rubenstein 1961) and the friction again rises. If one of the surfaces is a hydrophilic substance such as glass a more marked reduction in friction occurs when water is introduced to the surface. There is strong evidence that this due to the formation of a weakly held water film, a few molecules thick, attached to the hydrophilic surface. The film survives because the local pressures are not too severe. For this reason water provides effective lubrication for glass sliding on polyethylene or rubber but not for glass on glass where the contact pressures are much higher. Long chain acids produce negligible plasticization of the polymers but lubricate by forming adsorbed surface films. They are not as effective on polymers as on metals and this work supports the view of Fox & Zisman (1952) that this is because the films are not fully condensed. Polyethylene surfaces are lubricated by surface films of oleamide or stearamide. These materials may also be incorporated, in small quantities, within the bulk of the polymer and can diffuse to the surface to provide an effective lubricating layer. Non-polar liquids and some silicones produce a small reduction in the friction of polymers; the effect is more marked if one of the surfaces is of glass (or metal). It is suggested that these liquids form weakly adsorbed films which survive the relatively gentle stresses at the contact regions and so reduce the amount of polymer-polymer or polymer-glass interaction.

An apparatus for measurement of mechanical hysteresis and stress relaxation in adsorbed surface films

A simple device has been constructed which permits measurement of mechanical hysteresis in vertically suspended surface films of adsorbed proteins or synthetic detergents. In addition, the apparatus can be employed in stress relaxation experiments which provide estimates of the viscoelastic properties of the films.

Detection of Electrochemically Adsorbed Surface Films by Reflectance Spectra

THE rate of electrochemical reaction can be strongly influenced by adsorbed species on the electrode surface, and a knowledge of the degree of surface coverage and the heat of adsorption of the species is in many cases essential for a correct interpretation of the reaction mechanism. The surface coverage by chemisorbed oxygen and hydrogen on many metals has been determined electrochemically by pulse1 or rapid potential sweep methods2, where the quantity of electricity required to either reduce or oxidize the surface film has been determined. In general, these methods can be simply applied only if the current flowing during the time of measurement can be attributed to a single reaction. In many cases, however, it is desirable to know the surface coverage with an intermediate during the progress of a reaction or with the products of a simultaneous reaction, and under these conditions only a small fraction of the total current can be attributed to the formation of an adsorbed species. In some cases it may be possible to relate the experimentally measured pseudocapacitance to surface adsorption but here, also, assumptions have to be made on the relative rates of reaction3.

Mechanism of Lubrication for Solid Carbon Materials in Vacuum to 10−9Millimeter of Mercury

The friction and wear characteristics of various carbon materials sliding on metals and aluminum oxide were determined in vacuum at ambient pressures from 760 to 10−9 mm Hg. The friction and wear experiments were conducted with a hemispherically tipped carbon rider, under a load of 1000 gm, sliding on various disks rotating at a speed of 390 ft per minute. The results of this investigation show that additional research on carbon in vacuum is warranted. Adsorbed surface films present on both carbons and metal, as well as the presence of oxide on metals, appreciably influenced the friction and wear obtained with carbons in vacuum. Some impregnants were beneficial in reducing friction and wear of carbon in vacuum while others were not.

Surface films of nonionic detergents—I. Surface tension study

The surface tension vs. concentration curves of aqueous solutions of polyoxyethylene tridecanols, n-dodecanols, n-octadecanols, nonylphenols, and t-octylphenols have been determined as a function of ethylene oxide chain length. From these plots the surface pressure vs. area curves of the adsorbed surface films have been evaluated by the Gibbs' equation. For comparison the surface pressure vs. area curves of the insoluble surface films of polyoxyethylene tridecanols, n-dodecanols, and n-octadecanols have been determined on a 95% saturated sodium nitrate substrate. It is concluded that the amphipathic molecules orient at the air/water interface with the hydrophobic groups above the interface and the ethylene oxide chains forming coils in the aqueous phase. The size of the coils increases with increasing number of chain segments. In addition the effect of electrolytes on the concentration dependence of surface tension of polyoxyethylene nonylphenols has been determined as a function of electrolyte concentration and as a function of the lyotropic number of the ions. A mechanism is postulated for the observed salting out effects.

The rate of spread of liquid pools over horizontal solid surfaces and between approaching parallel flat plates

Some measurements have been made of the rate of radial spread of liquid pools on horizontal surfaces and between approaching parallel flat plates. Graphs giving the dimensions of the pools after different intervals of time and with different surface finishes have been prepared for several liquids on different kinds of surfaces; also derived graphs relating the rate of radial flow with film thickness, etc., have been included. Movement is complex and specific to the surface and chemical character of the liquid but it is possible to follow the effect of adsorbed surface films and to correlate movement with the periphery resistance (Jamin effect). With the use of binary mixtures of liquids, correlations have been found for rate of radial spread with several properties, such as the coefficient of friction, adhesion, contact angles, and swelling. Any increase in surface activity consequent upon molecular disaggregation is accompanied by a higher rate of movement, increased adhesion, etc. Some liquids are more sensitive to the roughening of surfaces than others. The effect of viscosity usually appears to be masked by surface effects, etc., but with liquids of low Jamin resistance, apart from the initial and later stages of spreading, an approximate relation between time and the radius of the pool can be obtained both for a liquid spreading on a horizontal surface and between approaching parallel plates.

Transfer of air through adsorbed surface films as a factor in foam stability

A simple floating bubble method for determining the permeability of adsorbed surface films to air is described. Data are presented for a selected group of pure detergents and detergent mixtures, and factors affecting the interpretation of the data in terms of foam stability are discussed. High foam stability depends on the adsorbed surface film having low permeability to air and high surface viscosity, and both of these effects may be the result of special solute pairs. Measurements of permeability and of surface viscosity provide valuable information regarding interreactions in adsorbed monolayers and in turn indicate the possibility of relating foam stability in a fundamental way to the chemical structure and composition of the detergent.

The influence of temperature on boundary lubrication

The frictional behaviour between mild steel surfaces lubricated with excess of pure hydrocarbons, ketones, alcohols, amides, acids and esters has been investigated at low speeds and under high loads. In all cases a transition from smooth sliding to irregular stick and slip motion takes place at a temperature characteristic of the lubricant employed. Experiments in which lubricant films one or more molecules thick were built up by the Langmuir-Blodgett technique have shown that the transition from smooth sliding to stickslips occurs when the adsorbed surface film of lubricant breaks down and becomes disoriented. Acids and esters are shown to be strongly adsorbed, while hydrocarbons, ketones, alcohols and amides are not appreciably adsorbed. It is shown that adsorption of acids and esters occurs by the interaction of the dipoles in their polar group with the metal atoms in the surface. The results also suggest that molecules of long-chain compounds are oriented on a metal surface in the same way as they have been shown to be arranged on an aqueous surface. Measurement of the coefficient of friction between surfaces lubricated with films one and many molecules thick has shown that under conditions of ‘ boundary lubrication ’ prevailing at high loads and low speeds, excess of lubricant is squeezed out, and lubrication is effected by a unimolecular film adsorbed on each surface. The variation of the coefficient of friction with load in the case of oleic acid shows that orientation with this lubricant extends beyond the primary adsorbed layer. This result accounts for the low values of the coefficient of friction obtained by previous workers, and explains the good lubricating properties of oleic acid. These experiments show that a study of the frictional behaviour provides a m ethod of investigating the properties of surface films on metals.