Gaseous Monolayers Research Articles

Surface Diffusion in Gaseous Monolayers of an Insoluble Surfactant.

Surface diffusion is an important mass transfer mechanism of surfactant molecules within adsorbed layers, which has to be taken into account in many fluid dynamics problems. Although considerable research has been devoted to studying the thermodynamic and rheological properties of surface films, rather less attention has been paid to surface diffusivity measurements. Current measurement methods, which are based on marking part of surfactant molecules in uniform motionless layers with the radiotracer or fluorescence technique, are well suited for use in quite condensed layers, but they do not work in rarefied layers due to increasing contribution of density fluctuations at an interface. In this study, we propose a method for measuring the surface diffusion coefficient in gaseous monolayers of an insoluble surfactant under dynamic conditions, i.e., in the presence of a flow at an interface. Our approach is based on measuring the velocity of thermocapillary flow on the water surface, which contains molecules of an insoluble surfactant. We show that under conditions of the balance between thermo- and solutocapillary tangential stresses the convective motion exists at an interface, which is caused by a blurring of the surface concentration gradient of surfactant molecules due to the surface diffusion mechanism. For calculations of the surface diffusion coefficient, we use the equation proposed earlier in the theoretical study [ Homsy , G. M. ; et al. J. Fluid Mech. 1984 139 , 443 - 459 ]. The surface diffusion coefficient measured by us in gaseous layers is 2-3 orders of magnitude larger than the results for liquid-expanded and liquid-condensed layers obtained by other researchers. Finally, we compare the obtained results with the known measurements of surface diffusion and discuss the limitations of the proposed method.

Relationships between Equilibrium Spreading Pressure and Phase Equilibria of Phospholipid Bilayers and Monolayers at the Air−Water Interface

The intricate interplay between the bilayer and monolayer properties of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE) phospholipids, in relation to their polar headgroup properties, and the effects of chain permutations on those polar headgroup properties have been demonstrated for the first time with a set of time-independent bilayer-monolayer equilibria studies. Bilayer and monolayer phase behavior for PE is quite different than that observed for PC and PG. This difference is attributed to the characteristic biophysical PE polar headgroup property of favorable intermolecular hydrogen-bonding and electrostatic interactions in both the bilayer and monolayer states. This characteristic hydrogen-bonding ability of the PE polar headgroup is reflected in the condensed nature of PE monolayers and a decrease in equilibrium monolayer collapse pressure at temperatures below the monolayer critical temperature, T(c) (whether above or below the monolayer triple point temperature, T(t)). This interesting phenomena is compared to equilibrated PC and PG monolayers which collapse to form bilayers at 45 mN/m at temperatures both above and below monolayer T(c). Additionally, it has been demonstrated by measurements of the equilibrium spreading pressure, pie, that at temperatures above the bilayer main gel-to-liquid-crystalline phase-transition temperature, T(m), all liquid-crystalline phospholipid bilayers spread to form monolayers with pie around 45 mN/m, and spread liquid-expanded equilibrated monolayers collapse at 45 mN/m to form their respective thermodynamically stable liquid-crystalline bilayers. At temperatures below bilayer T(m), PC and PG gel bilayers exhibit a drop in bilayer pi(e) values < or =0.2 mN/m forming gaseous monolayers, whereas the value of pic of spread monolayers remains around 45 mN/m. This suggests that spread equilibrated PC and PG monolayers collapse to a metastable liquid-crystalline bilayer structure at temperatures below bilayer T(m) (where the thermodynamically stable bilayer liquid-crystalline phase does not exist) and with a surface pressure of 45 mN/m, a surface chemical property characteristically observed at temperatures above bilayer T(m) (monolayer T(c)). In contrast, PE gel bilayers, which exist at temperatures below bilayer T(m) but above bilayer T(s) (bilayer crystal-to-gel phase-transition temperature), exhibit gel bilayer spreading to form equilibrated monolayers with intermediate pie values in the range of 30-40 mN/m; however, bilayer pie and monolayer pic values remain equal in value to one another. Contrastingly, at temperatures below bilayer T(s), PE crystalline bilayers exhibit bilayer pie values < or =0.2 mN/m forming equilibrated gaseous monolayers, whereas spread monolayers collapse at a value of pic remaining around 30 mN/m, indicative of metastable gel bilayer formation.

Equilibrium and Dynamics of Soluble Surfactant Penetration into a Langmuir Monolayer of a 2D Aggregating Homologue

The penetration dynamics of a dodecyldimethylphosphine oxid dissolved in the aqueous substrate into a Langmuir monolayer of eicosanyldimethylphosphine oxide and the equilibrium characteristics of the mixed monolayer formed are experimentally and theoretically studied. For the theoretical description of the low-density gaseous monolayers, the generalized Szyszkowski−Langmuir equation of state and the adsorption isotherm equation were applied. After the main phase transition, at which the gaseous phase starts to coexist with a condensed phase as a result of 2D aggregation, the monolayers are considered within the framework of a generalized Volmer equation and a quasichemical model for the aggregational equilibrium. The equations of state for the mixed monolayer are derived corresponding to different assumptions about the composition of 2D aggregates. The adsorption isotherm equations for soluble surfactant within the tightly packed monolayer are derived from the simultaneous solution of the corresponding equations of state and Pethica's equation. For the mixture of insoluble eicosanyldimethylphosphine oxide and soluble dodecyldimethylphosphine oxide, the experimental results were shown to agree well with the theoretical predictions based on the characteristics of these surfactants in the single-component solutions and monolayers. The model based on mixed aggregates is best suited to match the experimental values both for Π−A and Π−t isotherms.

Equations of State for Langmuir Monolayers with Two-Dimensional Phase Transitions

A generalized Volmer's equation is derived for a multicomponent Langmuir monolayer. This general equation is then used to derive the equations of state for the monolayer of a single amphiphile considering the two-dimensional (2D) main phase transition. A theory developed recently (J. Phys. Chem. 1996, 100, 15478) is extended to incorporate the important case that the area per one amphiphile molecule within the aggregate differs from that characteristic for the nonaggregated amphiphile. The general equation derived involves four parameters, two of which refer to the state of the gaseous monolayer whereas the other two parameters express the 2D aggregation constant and the difference between the area per one amphiphile molecule within the aggregate and the area per one nonaggregated molecule. The experimental Π−A isotherms for Langmuir monolayers of various amphiphiles, either measured by the authors or referred to elsewhere, agree well with the proposed model for physically reliable values of model parameters.

Energy and entropy of formation of crystalline 1-pentadecanoic acid from the ideal gaseous monolayer at the air/water interface

Energy and entropy of formation of crystalline 1-pentadecanoic acid from the ideal gaseous monolayer at the air/water interface

Interfacial oxidation of alpha-tocopherol and the surface properties of its oxidation products.

dl-alpha-Tocopherol spread on an acidic subphase as a gaseous monolayer was oxidized slowly to a derivative that was identified by thin-layer chromatography as alpha-tocopherylquinone. The derivative generated the same II-A isotherm as alpha-tocopherylquinone. When the subphase contained gold chloride, alpha-tocopherol was oxidized rapidly and quantitatively to alpha-tocopherylquinone. dl-alpha-Tocopherol spread on a basic subphase as a gaseous monolayer was oxidized slowly to a mixture that contained alpha-tocopherol, a quinone, and a nonpolar derivative. The mixture generated a II-A isotherm with an inflection point below the equilibrium spreading pressure of either alpha-tocopherol or alpha-tocopherylquinone. When potassium ferricyanide was added to the alkaline subphase, alpha-tocopherol was oxidized rapidly to a mixture that contained both the nonpolar derivative (major product) and the quinone (minor product). The nonpolar derivative was isolated by thin-layer chromatography and identified as the spirodienone ether by ultraviolet, infrared, and chemical ionization mass spectra. The spirodienone ether had a low equilibrium spreading pressure that explained the inflection point in the II-A isotherm generated by alpha-tocopherol on an alkaline subphase. Surface area data showed that dl-alpha-tocopherol formed immiscible films with stearyl alcohol and miscible films with oleyl alcohol. II-A isotherms showed that alpha-tocopherol in both immiscible and miscible mixtures was oxidized rapidly on an alkaline potassium ferricyanide subphase to the spirodienone ether. Collapse pressure data showed that the spirodienone ether formed an immiscible film with stearyl alcohol and a miscible film with oleyl alcohol. Interfacial oxidation experiments showed that alpha-tocopherol is oxidized either to tocopherylquinone (acidic subphase) or to the spirodienone ether (alkaline subphase). The natural occurrence of both tocopherylquinone and the spirodienone ether suggests that several types of oxidant stress are found in biological systems. One type of oxidant stress may involve the peroxy radical generating tocopherylquinone; a second type may involve hydroxyl radical-hydroxide ion generating the spirodienone ether.

Energetics of the transition between lecithin monolayers and bilayers

The energetics of the following equilibrium phase transitions for dimyristoyl lecithin in water at 23.5°C were measured by monolayer methods: gel ⇆ gaseous monolayer (11.2 kcal/mole); liquid-crystal ⇆ gaseous monolayer (4.4 kcal/mole); liquid-expanded monolayer ⇆ gaseous monolayer (2.5 kcal/mole). Analysis of the hydrocarbon and polar group contributions to each of these transitions indicates major differences between the internal energies of each of the condensed phases of lecithin. The use of solvent for preparing lecithin monolayers when T < T c ( T c is the gel ⇆ liquid-crystal transition temperature) results in the formation of metastable condensed monolayers.

Adsorption on mercury surfaces. II. Octadecane, octadecyl alcohol, octadecyl amine and stearic acid from solution in cyclohexane

A study was made of the adsorption of octadecane, octadecyl alcohol, octadecyl amine and octadecanoic (stearic) acid from cyclohexane solutions onto mercury surfaces. The amount absorbed was obtained by applying the Gibbs adsorption equation to the observed relationship between the equilibrium interfacial tension and the concentration. Interfacial tensions were measured as functions of time, solution concentration and temperature by the pendant drop technique. Tensions between mercruy and pure cyclohexane could be held constant for periods of several hours which made it possible to study adsorption by this method. The calculated results show that octadecane and octadecyl amine did not adsorb from cyclohexane solution onto mercury. Octadecyl alcohol and stearic acid did adsorb. The amount adsorbed increased with increasing solution concentration up to a maximum (limiting) value. Solution concentration required to obtain limiting adsorption was less for the acid than for the alcohol. The temperature coefficient of adsorption between 30 and 50°C was small. The adsorbed films behaved as gaseous monolayers at low surface pressures and as condensed monolayers at higher surface pressures. The limiting areas per molecule were very close to those normally associated with vertically oriented close packed monolayers at the air-water interface. Thermodynamic calculations indicated physical adsorption of these two polar solutes, and suggested significant structuring of the cyclohexane solvent by mercury.

A study of the effects of soluble surfactants on gas absorption using liquid laminar jets

In recent years the question of the effects of surfactants on gas absorption into liquid systems has arisen from various sources. Previous investigators have concluded that condensed monolayers cause an interfacial resistance to gas absorption while gaseous monolayers do not. In order to further study the effect of gaseous monolayers on gas absorption, a liquid laminar jet was used as a mass transfer contactor. Four surfactants were used: dodecyl trimethylammonium chloride, hexadecyltrimethylammonium chloride, dodecyl sodium sulfate, and hexadecyl sodium sulfate. Carbon dioxide was used as the absorbing gas. It was concluded that the surfactants tested did offer small, but measurable interfacial resistances to gas absorption ranging from 1 to 6 sec/cm. In agreement with previous studies using insoluble surfactants, the interfacial resistance increased with an increasing number of carbon atoms in the hydrophobic portion of the surfactant molecule. In addition, it was found that the interfacial resistance to gas absorption increased with decreasing molecular mass of the hydrophilic portion of the surfactant molecule for a given hydrophobic chain length. It was also concluded that a liquid laminar jet is a convenient mass transfer contactor for experimentally measuring interfacial resistances which are smaller than those which can be accurately detected using quiescent liquid contactors.

Measurement of the thermal effects in the formation of a monolayer from the volume phase on liquid surfaces

1. A calorimeter was designed for the determination of the heats of spreading of insoluble surface-active substances (surfactants) from the volume phase into a monolayer on liquid surfaces. 2. In the case of slow formation of a monolayer on the surface of water, endothermic effects are observed for the “crystal ⇌ gaseous monolayer” process, and the values of the effects, related to the number of molecules leaving the volume phase, are close to the heats of sublimation of the crystalline surfactants studied. 3. An endothermic thermal effect, accompanying the formation of an insoluble monolayer from the volume phase on the surface of water, is characteristic of various classes of surfactants.

Apparent dipole moment of a long-chain surfactant molecule in a gaseous monolayer from surface potential measurements

The Helmholtz model of the simple parallel-plate condenser has often been used to represent the potential across the double layer resulting from spreading a long-chain surfactant at the air/water interface. Guastalla, using a modified Helmholtz model, has suggested a method for determing the apparent dipole moment of the surfactant molecule in the gaseous region. The method involves extrapolation of experimental surface potential and surface concentration data to infinite dilution of surfactant. Surface potentials and surface pressures of gaseous monolayers of myristic and palmitic acids over aqueous substrates at various pit have been investigated. The apparent dipole moment and molecular weight of the spread molecule have been calculated from these data. When the pit of the aqueous substrate is below 4.5, the fatty acid is molecularly dispersed and the low surface potential indicates that the apparent dipole moment of the fatty acid is small; either the fatty acid molecule lies practically flat on the surface of the liquid or interaction between the long-chain fatty acid and the water molecules of the substrate occurs. When the pH of the aqueous substrate is above 4.5, the fatty acid gradually ionizes and the monolayer slowly desorbs. It is concluded that the Guastalla interpretation of the dipole moment is inadequate.

A Survey of Contact Resistance Theory for Nominally Clean Surfaces

While the theory of electrical contact resistance is, for the most part, well known, it is difficult to apply directly to the prediction of experimental results since, in general, the theory involves microscopic parameters beyond the control of the investigator. Recent measurements of contact resistance as a function of the applied contact load, carried out under specified conditions, have yielded results which are in excellent agreement with the general theory. In contrast, however, to a number of previous publications, the results indicate that the contact area is determined completely by the applied load and an effective plastic yield pressure. Under conditions where contact wipe and vibration are held to a practical minimum, the contact area can be specified in terms of a plastic yielding mechanism down to pressures as low as 0.1 gram. In this region the bulk of the contact resistance is seen to be attributable, for nominally clean contacts, to an absorbed gaseous monolayer approximately two angstroms thick.

The influence of electrolytes on gaseous monolayers I. Neutral films

An equimolar mixture of a long-chain cationic and anionic detergent when spread on dilute substrates at the oil/water interface has been shown to obey the theoretical relationship II( A – A 0 ) = kT . However, on concentrated substrates this relationship is no longer obeyed and the increased surface pressure observed has been attributed to a nonspecific penetration of the interface by the substrate ions. Good agreement has been obtained between the number of penetrating ions calculated from the increased surface pressure found on concentrated substrates and the number calculated directly using a modified Stem treatment.

The influence of electrolytes on gaseous monolayers II. Ionized films

An equation of state for a charged monolayer spread on an aqueous solution of a 1:1 electrolyte at 20°C in the absence of cohesive interactions has been derived, namely II = 2 kT / A-A o + 6.10√ c cosh sinh -1 133/ A √ c 6.10√ c . It has been found to apply to soluble charged films at hydrocarbon/water interfaces. In the case of insoluble monolayers of both alkyl quaternary ammonium halides and sodium alkyl sulphates at the hydrocarbon/water interface there is evidence of considerable ( ~50 %) ion-pair association, and this appears to be even further enhanced at the air/water interface at least for sulphate films. Surface potentials, measured by both the radioactive electrode and vibrating plate condenser techniques have been found to be in good agreement. The surface potential of sulphate monolayers in contrast to the ideal behaviour of quaternary ammonium films has been found to be dependent not only on the electrolyte concentration and film area but also on the nature of the interface, the nature of the substrate and at the air/water interface on the chain length. This has been attributed to a reorientation of the film molecules at the interface. Differences in the surface potential behaviour of sulphate and quaternary ammonium films are explained by suggesting that whereas with the sulphate film restricted rotation about the C—O bond can lead to different configurations of the sulphate ion with respect to the alkyl chain, only one configuration is possible for the symmetrical quaternary ammonium ion. Surface potential evidence indicates that contrary to what has been suggested previously, sulphate monolayers are completely dissociated in the acid-base sense.