Unimolecular Film Research Articles

Intercalation of alkanols and glycols into zirconium(IV) hydrogenphosphate monohydrate

Alkanols and glycols have been intercalculated in the layered structure of Zr(HPO4)2·H2O by regenerating salt forms of the exchanger with the protonated alkanols or glycols. The increase in the interlayer spacing of the obtained compounds with the length of the guest molecule suggests that n-alkanols are intercalated as a bimolecular film of extended molecules while glycols form a unimolecular film. Owing to the weak forces holding the molecules within the layers of zirconium phosphate and their high interlayer spacing, the intercalated compounds have been used successfully to intercalate other polar organic molecules. The use of zirconium phosphate–alkanol compounds as ion exchangers in aqueous and non-aqueous media is also reported and discussed.

Interferometric and x-ray diffraction study of “built-up” molecular films of some long chain compounds

By using the Blodgett and Langmuir technique, monomolecular films of barium palmitate, barium margarate, barium stearate and barium behenate have been built-up to films of 2, 4 and 7 mono-molecular layer thicknesses, on a silver substrate. The metrical thickness has been determined by multiple-beam interferometry. The c-spacings measured by X-rays are found to be twice the molecular chain length. A correlation between the interferometric measurement of the thickness and the lattice spacing normal to the film, is established. The correlation between the unimolecular film thickness and the chain length of the corresponding fatty acid leads to the conclusion that the fatty acids spread on a water surface mono-molecularly, oriented with their hydrocarbon chains almost at right angles to the substrate.

Multiple Beam Interferometric Study of `Built-up' Films of Barium Stearate

A direct determination of the metrical thickness of mono- and multi-molecular films of barium stearate has been carried out with the use of multiple beam interferometry. The thickness of the unimolecular film, as obtained from measurements on a four-layer film, is 25.7 ± 0.3 A which is in excellent agreement with the length of the molecular chain of barium stearate (=25.75 ± 0.25 A) as obtained from Fankuchen's X-ray data. The technique employed eliminates differential phase change effects and does not involve the refractive index of the films. The result confirms the monomolecular spreading of stearic acid on a water surface.

Radioiodine exchange between iodostearic acid monolayers and substrate iodide. II

The kinetics of exchange of radioiodine between α-iodostearic acid spread as a unimolecular film on aqueous solutions of KI have been studied. The possible influence on the exchange rate of the film characteristics as determined by the pressure-area isotherms and their variation with pH have been studied. The effect of insoluble barium soap formation on the exchange velocity at high pH values has also been investigated. Exchange reactions and the relation between surface radioactive count rate and surface concentration of film molecules have been studied in both spread and collapsed films.

Energy Relations in Unimolecular Film Formation: The Spreading of Cetyl Alcohol and Palmitic Acid on Aqueous Surfaces

Energy Relations in Unimolecular Film Formation: The Spreading of Cetyl Alcohol and Palmitic Acid on Aqueous Surfaces

Researches on monolayers. Part 4.—A study of dying processes by the use of the unimolecular film balance

The first page of this article is displayed as the abstract.

Treatment of Glass and Steatite Ceramic with Quaternary Ammonium Compounds for the Improvement of Electrical Insulation Resistance

THE adsorption and condensation of water on the surface of insulators was responsible for the failure of much electrical equipment under service conditions in the tropics. From the literature of flotation1, it was known that the adsorption of a unimolecular film of an amphipathic (polar – non-polar) compound gives to solid surfaces a paraffin-like character, rendering them less readily wetted by water. Some typical flotation reagents were therefore applied to glass and steatite ceramic surfaces in an endeavour to improve their electrical insulation resistances under moist conditions. As might be anticipated from the work of Wark2, the quaternary ammonium compounds were effective. The accompanying table shows the results obtained with various substituent radicals for glass microscope slides and steatite terminal bushings. These were cleaned with chromic acid, immersed for about one second in an aqueous solution of the quaternary ammonium salt, and then washed thoroughly in distilled water.

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.

Problems concerning the Efficiency of Oils as Mosquito Larvicides.—Part I. The Stability of Oil Films on the Surface of Water

Films of oil which have originally covered the whole of the surface of an area of water frequently break up and leave the oil only in patches, and the rest of the surface uncovered. An investigation has revealed that this may be the result of either or both of two circumstances.It has been shown that instability may result from :—I. Certain relationships between the volatility of the aromatic and aliphatic hydrocarbons composing the oil mixture.II. The presence of certain polar substances dissolved in the oil.I. The volatility relationships which determine the stability or instability are as follows :—(a) If the aromatic hydrocarbons are more volatile than the aliphatic, a tendency exists for the oil to become concentrated into one central thick lens with the rest of the surface uncovered by oil.(b) If the whole of the aromatics are more volatile than the whole of the aliphatics, this action is very rapid and occurs whatever the proportion of the mixture.(c) If the difference in the volatility is only small, the phenomenon is only observed when the proportion of aromatics to aliphatics is very near to unity.(d) If the boiling range of the aromatics overlaps the boiling range of the aliphatics (as occurs when two oils containing both types of constituents are mixed), the phenomenon is much less clearly defined.(e) Olefinic oils, e.g., oils obtained by cracking, behave like aliphatic oils in so far as this behaviour is concerned.It is believed that these changes would not occur if the oil was extremely pure, since lenses are only stable if subject to lateral pressure from an invisible film of contaminating material.II.—The effect of polar materials can be summarised:—(a) Solutions of certain organic substances in oil tend to make unstable films on the surface of water; rupture of such a film (by wind or other agitation) allows a unimolecular film of the added substance to spread at the air-water interface. This forces back the oil film. The greater the agitation the less the area of surface eventually covered by oil. In this class of substances are fats, fatty acids, naphthenic acid (a natural constituent of petroleum), some sulphonic acids and cholesterol.(b) Certain other substances act in a contrary manner. They do not promote instability but actually counteract and, if present in sufficient proportion, overcome the instability produced by the first type of substance. The substances falling in this group are all at present of unknown composition, but include “ cracked spirit gum ” (a material derived by polymerisation of olefinic hydrocarbons), certain resins, and some substance present in petroleum and remaining in the residue after distillation. The last of these acts as a natural “ stabiliser ” and masks the instability phenomenon to some extent in commercial oils. It is found in largest quantity in the undistilled residue of aromatic concentrates.(c) The quantitative relationship between the “ stabiliser ” and the substances promoting instability is not linear. Successive increments of “ stabiliser ” equalise larger and larger amounts of the other.(d) Long-chain molecules such as fatty acids are the most difficult to overcome by “ stabiliser.” Naphthenic acid is more easily counteracted, cholesterol still more easily.The following recommendations can therefore be made as to the desirable composition of anti-malarial mixtures, so far as film stability is concerned :—1. The oil should contain either a small aromatic content or a very high one ; it should not contain 50 per cent. aromatics unless they are very high-boiling (e.g., of lubricating base fraction).2. It should consist of a mixture of wide and overlapping cuts of oil.3. It should not contain fats or fatty acids added as spread-aiders. If spread-aiders are desired, resins should be employed.This research was financed in its entirety by a grant contributed equally by the Anglo-Iranian Oil Company, the Asiatic Petroleum Company, and the Burmah Oil Company. To the research chemical staffs of these companies I express my thanks for samples of oil and for much technical information and advice. My thanks are also due to Professor P. A. Buxton and Professor N. K. Adam for advice and help.

Reaction kinetics in films. The hydrolysis of γ-stearolactone

In previous communications it was shown that it was possible from observation of the phase boundary potential to study the kinetics of reactions proceeding at air liquid interfaces. A chemical change occurring in a unimolecular film at an air liquid interface my result in ( a ) a change in the magnitude of the vertical component of the apparent electric moment of the film-forming molecule, as observed in the oxidation of unsaturated acids; ( b ) an aggregation due to greatly reduced adhesion as in the spontaneous oxidation of long chain mercaptans to form disulphides; or ( c ) the diffusion of the product away from the surface as is the case in hydrolysis of γ -stearolactone. A consideration of such heterogeneous systems reveals several factors of interest in the sphere of reaction kinetics. In some instances it is evident that the velocity of the process may be governed by the rate of diffusion of products away from the surface rather than by the rate of chemical reaction. If the chemical process is the slower, then it will determine the velocity of the reaction. In a surface reaction, not complicated by diffusion, the velocity would be determined by the collision frequency, the probability of energy transfer involved in activation, and an orientation factor. This latter factor of molecular orientation, doubtless present in many heterogeneous catalyses, is clearly defined in the oxidation of films of unsaturated acids, and as it will be shown an alteration in the inclination of the polar group result in a significant change in the reaction velocity constant, also in the hydrolysis of the lactone.

The Unimolecular Film in Heterogeneous Reactions

THE formation of unimolecular films on solid surfaces has been demonstrated by Langmuir1 for gases and by Carver2 for toluene, but others employing different materials have obtained results interpreted as indicating an adsorbed layer of multi-molecular character. At the moment the position is such that “definite agreement about the validity of direct experimental evidence is hardly reached”3 and for this reason it may be of value to record the discovery of two unimolecular films made during our most recent work on the combination of hydrogen with sulphur in Pyrex glass. For the first time, so far as we are aware, this reaction has now been studied at low hydrogen pressures of the order of 3 mm. at the temperature employed, namely, 343° C. In the light of previous experience5, the most rigid precautions were taken to ensure the absence of oxygen or moisture from the reactants.

Societies and Academies

LONDON Royal Society, March 9. J. H. AWBERY and E. GRIFFITHS: The heats of combustion of carbon monoxide in oxygen and of nitrous oxide in carbon monoxide at constant pressure. The heat of combustion of carbon monoxide in oxygen is 282,730 joules per mole at 20 ° C., correct to within 3 parts in 1,000. The heat of reaction when nitrous oxide burns in carbon monoxide is 364,340 joules per mole, correct to 2 or 3 parts in 1,000. From these two results, the heat of formation of nitrogen dioxide at constant atmospheric pressure and at a temperature of 20 ° C. is found to be 81,610 joules, that is, 19.50 cal.15 ° per mole. This quantity is determined as the difference between two heats of combustion, which were carried out with the same apparatus used in the same manner for both sets of experiments.—R. W. FENNING and F. T. COTTON: A bomb calori meter determination of the heats of formation of nitrous oxide and carbon dioxide. By means of a bomb calorimeter the heats of reaction of oxygen and nitrous oxide in carbon monoxide and hydrogen respectively were measured. Thus, by difference, two values were obtained for the heat of formation of nitrous oxide. A value for the heat of formation of carbon dioxide from carbon monoxide and oxygen was also obtained. Since the combustion of oxygen in hydrogen formed the calibration process, all the values given are based on the acceptance of 68,320 gm. cal.15 ° as the heat of formation of water (liquid) at constant pressure and at 25 ° C. On this basis, the heats of formation of nitrous oxide and carbon dioxide at a constant pressure of 1 atmosphere and at 20 ° C. were found to be—19.74 ± 0.07 kcal.15 °, or 82,600 ± 290 international joules, and 67.655 ± 0.035kcal.15 °, or 283,090 ± 150 international joules respectively. A. H. HUGHES and E. K. RIDEAL: On the rate of oxidation of monolayers of unsaturated fatty acids. The method of surface potentials has been employed to study chemical reactions occurring in a unimolecular film; the reactions examined being the oxidation by acidified potassium permanganate of long chain unsaturated aliphatic acids such as oleic acid. The reaction velocity depends on the accessibility of the double bond to the oxidising agent, and decreases markedly on compression of the film. Reaction velocity has been studied as influenced by the position of the double bond in relation to the polar carboxyl headgroup. Autoxidation in a unimolecular film has been observed for the elæostearic acids, and the effect of hydroquinone as an antioxygen to this reaction has been examined.

On the surface potentials of unimolecular films. Part IV.—The effect of the underlying solution and transition phenomena in the film

In previous papers of this series (1) it was shown that the method of surface potentials could be developed to give quantitative measurements of the physico-chemical properties of unimolecular films at an air liquid interface. Several problems of fundamental importance arise when a closer investigation is made of the true meaning of the phrase “surface potential of a unimolecular film.”

On the surface potentials of unimolecular films of long chain fatty acids.–Part I. Experimental method

Molecular orientation in a film formed at an air liquid interface was ascribed by Hardy to an inwardly directive attractive force along the normal to the surface, the condition of minimum potential involving two terms, one relating to the variation in density, the other to the orientation of the field with the "polar" groups, which are accordingly attracted inwards towards the bulk phase. That "polar" groups do in fact posses large fields is confirmed from an examination of the electric moments of compounds containing these groups. In previous communications it was shown that the characteristics and phase of films could be affected not only by alterations in the variables of temperature and two dimensional pressure as shown experimentally by Langmuir and by Adam, but also by altering the extent of the adhesion of the asymmetrically situated polar group to the substrate. Alterations in the extent of this adhesion will naturally affect the field and it is the object of this communication to attempt an examination of the behaviour of films from this point of view. The modification of an interfacial potential difference produced by the presence of a unimolecular film was shown by Helmholtz to be given by the expression 4 πn μ = ∆V where the change in potential ∆V is caused by n molecules per square centimetre of the film forming substance, each molecule possessing an average electric moment of effective vertical component μ.

On the surface potentials of unimolecular films of long chain fatty acids. Part II.–The evaluation of the resolved electric moments

In Part I a method was described for the determination of the change in interphase potential at an air liquid interface caused by the presence of a unimolecular film of myristic acid and the values for this change (∆V) were plotted as a function of the molecular area. On application of the Helmholtz equation ∆V = 4 πn μ we obtain a mean value for the vertical component of the molecular electric moment of a molecule of myristic acid when present in a film in the expanded state of μ = 2.29. 10 -19 e. s. u.

On the stability of unimolecular films. Part I.—The conditions of equilibrium

Of the factors which determine the state of equilibrium of a unimolecular film on the surface of water the two which have been subjected to the most exhaustive examination are the effects of changes in temperature and pressure. In previous communications a generalisation of the effects of kinetic agitation on the state of a film, and the conditions of equilibrium imposed by the phase rule on the various states of such films have been described. As pointed out originally by Hardy, in film formation the magnitude and direction of the adhesional forces between the surface and the polar "heads" of the spreading molecules is one of primary importance. The effects of replacement of one type of polar head by another have been examined in detail by numerous investigators; but the effect on the state of a film of one substance of changing the magnitude of this adhesion, a factor clearly as important as the conditions of temperature and pressure, has not hitherto been considered. In these communications the results of an investigation on the effects of such a change are described. Unimolecular films of long chain fatty acids on the surface of water can exist in three well-defined states, the highly dispersed or vaporous, the expanded and the condensed. The expanded state, first noted by Labrouste, was considered originally by Adam to be vaporous in character. Schofield and Rideal ( loc. cit .), however, from an examination of the isothermals of the force area curves for a series of acids concluded that expanded that the true vapour pressure of a film of myristic acid at ordinary temperatures should be about 0∙2 dynes per centimetre. A similar suggestion was made independently by Langmuir and later accepted by Adam, when with a sensitive apparatus he effected the measurement of the vapour pressure of expanded films, which proved to be in good agreement with that predicted by Schofield and Rideal. It seemed probable that expanded films consist of some arrangement of tilted molecules with the chains still enmeshed and the heads free, as suggested by Schofield and Rideal and Langmuir. Adam, however, has concluded that the heads adhere to one another with sufficient tenacity to render this view improbable.

On the stability of unimolecular films. Part II.— Dissolution in alkaline solutions

In the previous communications (Parts I and Parts II) it was shown that the conditions of equilibrium of a unimolecular film palmitic acid were influenced both by the temperature and the attraction of the polar head-group for the underlying solution, which in the case of the carboxyl group could be altered by changing the acidity or alkalinity of the surface layers of the solution. On alkaline surfaces this attraction increases to such an extent that not only does an expanded film tend to contract to form a condensed film, but actual dissolution may occur. Adam noticed that condensed films of palmitic acid showed a small slow decrease in area on a solution of P H 8∙5, while on a solution of P H 10∙0 this decrease in area look place so rapidly that he could not obtain force/area curves. An attempt has been made to examine the mechanism of the-dissolution process and to investigate the relationship between the rate of solution and the alkalinity of the underlying solution. The experiments were carried out on an apparatus designed so that the pressure of the film was maintained constant as the film dissolved. As the molecules left the film for the underlying solution the gaps in the film are closed by the pressure, and the area of the film gradually decreases. The factors influencing this rate of decrease in area at constant pressure have been studied.

On the stability of unimolecular films. Part II.-The mechanism of film expansion

In Part I it was shown that the adhesion of a unimolecular film of a fatty acid to an underlying aqueous solution could be varied by alteration of the Hydrion concentration of the solution. Increasing the alkalinity effected an increase in the adhesional force of the polar beads, and under isothermal con­ditions a film could be converted from the expanded to the liquid condensed and even to the solid condensed state, by causing an increase in these adhesional forces, this process being perfectly reversible. Whilst ionisation of the acid occurs over a bruited range of P H , the alteration in adhesional forces by a change in P H and the effects of such change on the state of the film extend, contrary to the conclusions of Egner and Hägg,* over a much wider range of P H . Since contraction and expansion of the film coincide with an increase decrease respectively in the adhesional forces holding the polar heads to the surface, we may inter that expansion is effected by a gradual tilting of the molecules from the close packed formation existing in the solid condensed state. We have noted that Müller* from X-ray determinations on crystals of fatty acids suggested that it seemed possible that even in a film in the solid condensed state the molecules were already tilted. Objections to this view were raised by Adam, since he found but one characteristic area for long chain molecules in the solid condensed state, which was, with few exceptions, independent of the nature of the head group. He further found that the area in the liquid condensed state*was dependent on the nature of the head group.

The behaviour of crystals and lenses of fats on the surface of water.—Part II. The effect of temperature on the equilibrium pressure

In Part I a study was made of the rate of spread and unimolecular nature of films of fatty substances, containing a long hydrocarbon chain and a polar “head,” over a surface of pure water or of N/100 HC1. It was shown that after a definite interval of time, the surface became saturated with a unimolecular film in equilibrium with either a crystal or a lens of the substance in question, and that the time taken to attain equilibrium as well as the equilibrium surface tension was a function of the temperature. In the present paper the effect of increasing temperature on the two-dimensional equilibrium pressure is considered in detail.

The catalytic action of copper. Part VII.—A study of the effect of pressure on the rate of dehydrogenation of alcohols

The assumption that reaction occurs in a uni-molecular film covering the surface of the copper can be tested by studying the effect of pressure on the change. If all the adsorbed molecules can react, then increasing the pressure should increase the thickness of the film, and therefore the reaction velocity. If the molecules only react in the uni-molecular film next to the catalyst surface, then varying the pressure only effects the change in so far as affects the life of a molecule in the surface layer. This effect is known to be small. Hence the reaction velocity should be practically independent of the pressure, provided the catalyst is completely covered with the adsorbed alcohol film. At low pressures the velocity would be expected to fall with pressure, since the surface is not completely covered. However, since the aldehydes are known to poison the bare copper surface, it is desirable to avoid very low pressures in the experimental work.