Adsorption layer properties and foam film drainage of aqueous solutions of tetraethyleneglycol monododecyl ether

The aim of the present study is to clarify how the surfactant adsorption layer properties are related to the course of the drainage parameters of microscopic foam films in the special case of aqueous solutions of the non-ionic amphiphile tetraethyleneglycol monododecyl ether (C₁₂E₄), containing premicellar nanostructures. The scope of the research covers adsorption dynamics, construction of equilibrium adsorption isotherms, studies on surface rheology of the interfacial layers and microscopic foam film drainage kinetics. It is established that in the premicellar concentration domain considerable irregularities of the adsorption layer properties are observed: two plateau regions are registered in the experimental surface tension isotherm along with unusual changes of the surface rheological characteristics. The systematic investigation of the drainage of microscopic foam films obtained from these solutions show that the dependencies of basic kinetic parameters of the films on the amphiphile concentration run in synchrony with the changes in the adsorption layer properties. This fact is related to the presence of smaller surfactant aggregates (premicelles). They are presumed to be organized as Platonic bodies. The premicelles play also a significant role in the kinetic stability of the films. The importance of this research is in providing better insight into the initial stages of self-assembling phenomena and into the factors determining the adsorption layer properties and the drainage behaviour of thin liquid films.     

Effect of ionic surfactants on drainage and equilibrium thickness of emulsion films

This paper presents new theoretical and experimental results that quantify the role of surfactant adsorption and the related interfacial tension changes and interfacial forces in the emulsion film drainage and equilibrium. The experimental results were obtained with plane-parallel microscopic films from aqueous sodium dodecyl sulphate solutions formed between two toluene droplets using an improved micro-interferometric technique. The comparison between the theory and the experimental data show that the emulsion film drainage and equilibrium are controlled by the DLVO interfacial forces. The effect of interfacial viscosity and interfacial tension gradient (the Marangoni number) on the film drainage is also significant.     

Effect of double-layer repulsion on foam film drainage

This paper presents an experimental validation of new theoretical development for foam film drainage, which focuses on the role of surface forces. The drainage of microscopic foam films (with radii smaller than 100 μm) from aqueous solutions of 10⁻⁶ to 10⁻⁴ mol/L sodium dodecyl sulphate (SDS) was studied by means of an improved Scheludko micro-interferometric technique which consisted of a conventional Scheludko cell, a high-speed camera system, and the software for digital analysis Optimas used for the digitisation of the interferometric images to obtain the monochromatic light intensity. The experimental technique allowed fast processing of the interferometric data for determining the transient film thickness with high accuracy. The zeta-potential of the air–water interface was determined from the electrophoretic mobility of micro-bubbles in SDS solutions of the same concentrations. Advanced predictions for the electrical double-layer repulsion at either constant surface potential or constant surface charge were employed. Significant discrepancy between the theoretical prediction and the experimental data was obtained. The analysis showed that the adsorption layer, which is located on the film surfaces, is far away from equilibrium, while the theory assumes condition close to equilibrium. In this term the interaction between the film surfaces is affected by the dynamics of the adsorption layers during the film drainage.     

Investigation of the isotherms of the disjoining pressure of wetting films of binary nonionic solutions by the ellipsometric method

The paper deals with an experimental investigation into the influence of the second component on the thicknesses of the wetting films of a nonionic solvent. A technique has been developed for the production of pure, smooth, thin glass substrates for wetting liquid films.

The use of these glass substrates enabled us to exclude the influence on the experimental results of such noncontrollable factors as roughness and pollution of the substrate surface. The isotherms of the disjoining pressure of wetting films of a number of two-component mixtures of nonionic liquids on glass substrates were experimentally determined. The film thicknesses were measured by an ellipsometric method; the disjoining pressure for the film was preset by adjusting the pressure of solvent vapours. The results obtained demonstrate a qualitative agreement with the theory of the adsorption component of disjoining pressure developed by Derjaguin and Churaev.

It is also shown that even very small additions of a polar substance to a nonpolar solvent may cause a marked change in the thickness of films. In addition to adopting the theory of the adsorption component of disjoining pressure, certain assumptions are made about the formation of the structural component resulting from the addition of a polar component to quantitatively describe the results obtained. The contribution of the adsorption and structural components of disjoining pressure to the stability of films of solution is estimated.     

An experimental test of the fractal model for drainage of foam films

Drainage of foam films with different radii (50-150 μm), stabilized by hexathylene glycol dodecyl ether C(12)E(6) and in a presence of 0.024 M NaCl, were analyzed in the light of a recent dynamic fractal classification of [1]. The latter accounts for the effect of film surface corrugations developed during the film drainage. For simplicity, the film surface mobility is neglected since the presence of surfactants reduces dramatically the film surface velocity. The magnitude of surface non-homogeneities, caused by the film drainage, is accounted via a dynamic fractal dimension parameter α being spanned between zero and two. Depending on the α-value the film drains by different kinetic laws. For example, if the thin film is planar α=2 and it drains according to the Reynolds law; if α=1 the film contains an axisymmetric dimple causing faster drainage; if α=1/2 the film exhibits number of asymmetric dimples and the film drains even faster; finally if α=0 the film contains spatially uncorrelated domains causing the fastest possible drainage. The present analysis of experimental data suggests that the parameter α is inversely proportional to the film radius R and it is independent of the type and concentration of surfactants. A semi-empirical model for α is proposed, thus completing the generic dynamic fractal classification.     

Determination of Bulk and Surface Diffusion Coefficients from Experimental Data for Thin Liquid Film Drainage

This note presents a method for the determination of the surface diffusion coefficient and surface diffusion flux. The theoretical considerations are based on the Onsager linear theory for the definition of the surface diffusion flux and on the Einstein theorem for the definition of the surface diffusion parameter. In this interpretation the surface diffusion coefficient differs from the one commonly defined in the literature. It does not depend on the surfactant concentration and it is a function only of the type of surfactant and the liquid/liquid interface. The theoretical calculations indicate that the effect of the surface diffusion on the film drainage is stronger than that predicted by previous theoretical studies. The experimental data for thin liquid film drainage in the case of low surfactant concentration in the continuous phase could be used for the calculation of the bulk and surface diffusion coefficients. In the present study we utilized the experimental data for the drainage of nitrobenzene films stabilized by different concentrations of dodecanol. Copyright 2000 Academic Press.     

Film drainage and interfacial instabilities in polymeric systems with diffuse interfaces

We report an experimental investigation on the effect of mutual diffusion in polymeric systems on film drainage between two captive drops. The main objective is to study the influence of diffuse interfaces on film drainage. This is done by using material combinations with different interfacial properties and interferometric visualization of the film between two interacting drops. For highly diffusive systems film drainage is observed to be, in contrast to immiscible systems, non-axisymmetric and unstable immediately after the film formation (at a few micrometers film thickness). Depending on whether the total thickness of the diffusion layers in the film is smaller or larger than the thickness of the film, Marangoni convection is found to enhance or delay film drainage. Enhanced film drainage is determined to be in order of 100 times faster than predicted by the current models, while delayed film drainage is observed after a drainage period where experimental and predicted results (assuming, a partially mobile interface) are in close agreement.     

Drainage of the air-water-quartz film: experiments and theory

Experimental results of the kinetics of drainage of the trapped water film between an approaching air bubble and a quartz plate have been analysed using recent theoretical advances in formulating and solving the flow problem in deformable films. Excellent agreement is obtained between experimental data and a model that assumes the bubble-water interface is tangentially immobile in its hydrodynamic response. The coupling between hydrodynamic pressure, disjoining pressure and film deformation is critical in determining the dynamic behaviour of the drainage process. The Reynolds parallel film model that omits the effects of film deformation predicts results that are qualitatively incorrect.     

The adsorption of O2 on Pb films and the effect of quantum modulation: a first-principles prediction

Using first-principles calculations based on density-functional theory, we systematically study the adsorption of O(2) molecules on ultrathin Pb(111) films ranging from 3 to 11 monolayers (MLs). It is found that no matter how thick the film is, the O(2) molecule prefers to adsorb at the threefold hcp hollow site where it lies parallel to the surface. The adsorption mechanism is discussed from the hybridization of p orbitals of O(2) and Pb. The adsorption energy of O(2) on the Pb(111) film, about several hundred meV, shows a 2 ML oscillation with the thickness. This study well confirms the modulation of the surface reactivity of Pb films induced by the quantum well states, which is compatible with the previous experimental observation.     

Measurement of adsorption of a single component from the liquid phase: modelling investigation and sensitivity analysis

In this work, we consider an alternative approach for the measurement of adsorption from the liquid phase. Consider a mixture consisting of a non-adsorbed component (B) and an adsorbed component (A) present at some low concentration. Initially, a feed of component B only flows through a column packed with an adsorbent. Then, the feed is switched to the mixture of A and B. As soon as the mixture enters the column, there will be a reduction in the outlet flow rate as component A leaves the liquid phase and passes into the adsorbed phase. There are three stages to this work. The first is to develop overall and component balances to show how the amount adsorbed of component A can be determined from the variation in the column outlet flow rate. The second is to determine the actual variation in the column outlet flow rate for both plug flow and axial-dispersed plug flow. The final stage is to consider the suitability of a gravity-fed system to deliver the feed to the column. An analysis of the results shows that the experimental arrangement should be able to accurately monitor adsorption from the liquid phase where the mass fraction of the solute is of the order of 1%: the limiting experimental factor is how constant the volumetric flow rate of the liquid feed can be maintained.     

Binary Adsorption of Phenol and m-Cresol Mixtures onto a Polymeric Adsorbent

Adsorption processes are gaining interest as methods of purifying industrial effluents. Most industries discharge effluents containing several components. The adsorption of phenol and m-cresol mixtures from aqueous solutions onto a macroporous polymeric adsorbent, Duolite ES-861, was investigated experimentally in a fixed-bed adsorber for different flowrates, feed concentrations and bed initial conditions (clean or pre-saturated).The experimental results are presented in this work, where the major objective is placed on the modelling of these fixed bed adsorption experiments using an extended Langmuir isotherm equation for two components, based on single component equilibrium data obtained for phenol and m-cresol.The model presented in this paper takes into account axial dispersion of the liquid phase, film diffusion and intraparticle mass transfer and successfully simulates the adsorption behaviour of the phenol and m-cresol mixtures.     

Influence of electrostatic and chemical heterogeneity on the electric-field-induced destabilization of thin liquid films

A numerical model for thin liquid film (<100 nm) drainage in the presence of an external electric field is developed. Long-wave theory is applied to approximate and simplify the governing equations. A spatiotemporal film morphology evolution equation thus obtained is then solved using a combination of finite difference to resolve the spatial dimensions and an adaptive time step ODE solver for the temporal propagation. The effect of fluid properties, namely, viscosity and surface tension, on the film drainage time is observed for a homogeneous electric field, which leads to random dewetting spots. Electrically heterogeneous fields, achieved by modeling electrodes with various periodic patterns, are explored to identify their effect on the drainage time and behavior. Finally, the chemical heterogeneity of the substrate is coupled with the periodic electric heterogeneity to understand the implications of combined heterogeneity. It is observed that the introduction of any heterogeneity results in faster drainage of the film when compared to that of the homogeneous field. In all cases, the thin film is drained, leaving submicrometer-scale structures at the interface. Well-controlled surface patterns are found on the application of periodic heterogeneity. This study effectively demonstrates the immense potential of electrically induced thin film drainage as a means for faster de-emulsification and for the creation of ordered submicrometer-scale surface patterns on soft materials.     

Modeling the sorption kinetics of divalent metal ions onto mineral adsorbent using integral method

A mathematical model has been developed that could predict kinetic parameters for the adsorption of divalent cations (lead, copper and zinc) onto low-grade rock phosphate using experimental data. The experiments were conducted with the initial concentrations of metal ions ranging from 10 to 100 mg/L. The mathematical model is based on application of Freundlich isotherm to mass transfer across the film surrounding the adsorbent. A code in C programming is used to numerically integrate the model equation, and to obtain the best simulated values of Freundlich constants K, N, order of reaction n, and film transfer coefficient, alpha. It is observed that the adsorption of metal ions on rock phosphate is more sensitive to N,n, and alpha in comparison to K, and lead is adsorbed more favorably than copper and zinc.     

Application of the chromatographic step profile method for determination of water film pressure and surface free energy of quartz

Summary Investigation of water adsorption by the step profile method (Glueckauf method) was carried out with the help of a modified gas chromatography equipped with thermal-conductivity detector. On the basis of the adsorption isotherm obtained, the water film pressure and the polar component of the surface free energy of quartz were calculated. The calculated value of the polar component of the surface free energy of quartz agrees with analogous values obtained by other methods.     

Correlation of Adsorption Equilibrium Data for Water Vapor on F-200 Activated Alumina

Single component adsorption equilibrium data for water vapor on commercially available activated alumina F-200 measured in a previous study (Serbezov, 2003) is correlated by two adsorption isotherm equations, both of which are based on the adsorption potential theory. The first equation is the well known Dubinin-Astakhov (D-A) equation. The second equation is obtained from a methodology proposed by Kotoh et al. (1993). It is referred to as a dual mechanism adsorption potential (DMAP) equation because it is a linear combination of two D-A terms with n = 1 where each term accounts for a specific mechanism of water retention. The D-A equation has two fitting parameters; the DMAP equation has three fitting parameters. The DMAP model provides a better fit for the adsorption data than the D-A model, while neither model describes the desorption data well. Analysis of the DMAP equation parameters shows that most of the water is retained by virtue of capillary condensation. In addition to fitting the experimental data, the heat of adsorption was calculated as function of the relative humidity and adsorbent loading. When capillary condensation is present, the heat of adsorption is only slightly higher than the latent heat of vaporization.     

Modeling the kinetics of gas adsorption in multilayer porphyrin films

A kinetic model is proposed to describe the diffusion and adsorption behavior of gas in multilayer films. Numerical solutions are attained on time scales of seconds using a finite differencing approximation to the kinetic equations. Predictions of this model are compared to experimental data for the case of NO2 diffusing through a porphyrin film. The model predicts a binding energy for the NO2 porphyrin interaction of 0.72 eV. It also predicts that for this system diffusion is the limiting factor for the adsorption response time of the film, although the recovery time is determined by both the diffusion coefficient and NO2 binding energy. Comparison with experiment gives a predicted diffusion coefficient of approximately 10(-14) m2.s-1.     

On the Theory of the Stability of Dipole Molecule Solution Interlayers in Apolar Solvents: 4. A Localized Adsorption at the Dipole Orientation Parallel to the Interface

Image forces and their contribution to the disjoining pressure of solution films at the orientation of dipole molecules of adsorbed component parallel to the film interfaces are analyzed. Expressions for the calculation of the correlation attraction of adsorbed monolayers under various conditions of adsorption on opposite film boundaries are obtained. Relation for calculating the contribution of interaction between the adsorbed monolayers and their images to the film disjoining pressure is derived. Numerical estimates of the disjoining pressure arising due to aforementioned interactions are made.     

Adsorption behavior of glucose oxidase on a dipalmitoylphosphatic acid monolayer and the characteristics of the mixed monolayer at air/liquid interfaces

A dipalmitoylphosphatic acid (DPPA) monolayer at the air/liquid interface is used as a binding layer to incorporate glucose oxidase (GOx) from the subphase. The effects of the adsorption time of GOx on the behavior of the mixed DPPA/GOx monolayer and the relevant structure of the mixed LB film were studied using the characteristics of the pressure-area (pi-A) isotherm, Brewster angle microscopy (BAM), and atomic force microscopy (AFM). The experimental results show that two equilibrium states of GOx adsorption exist in the presence of a DPPA monolayer. The first equilibrium stage occurs at tens of minutes after spreading of DPPA, and a surface pressure of ca. 7.5 mN/m is obtained. The second equilibrium stage approaches slowly, and a higher equilibrium surface pressure (ca. 16 mN/m) was obtained at ca. 8 h after the first stage. The BAM and AFM images show that, after the second equilibrium stage is reached, a more condensed phase and rough morphology are obtained on the mixed DPPA/GOx monolayer, indicating a higher amount of GOx incorporated into the mixed film. For the first equilibrium stage of GOx adsorption, DPPA molecules can still pack regularly and closely under compression, suggesting that GOx molecules are mainly located beneath the DPPA monolayer at the compressed state. A more uniform phase was detected on a film prepared after the first equilibrium stage was reached. The present result indicates that distinct structures and properties of mixed DPPA/GOx films can be prepared from the various stages of GOx adsorption.     

Dynamics of interactions involving deformable drops: hydrodynamic dimpling under attractive and repulsive electrical double layer interactions

A model developed previously to analyze force measurements between two deformable droplets in the atomic force microscope [Langmuir 2005, 21, 2912-2922] is used to model the drainage of an aqueous film between a mica plate and a deformable mercury drop for both repulsive and attractive electrical double-layer interactions between the mica and the mercury. The predictions of the model are compared with previously published data [Faraday Discuss. 2003, 123, 193-206] on the evolution of the aqueous film whose thickness has been measured with subnanometer precision. Excellent agreement is found between theoretical results and experimental data. This supports the assumptions made in the model which include no-slip boundary conditions at both interfaces. Furthermore, the successful fit attests to the utility of the model as a tool to explore details of the drainage mechanisms of nanometer-thick films in which fluid flow, surface deformations, and colloidal forces are all involved. One interesting result is that the model can predict the time at which the aqueous film collapses when attractive mica-mercury forces are present without the need to invoke capillary waves or other local instabilities of the mercury/electrolyte interface.     

Interaction of hexadecylbetainate chloride with biological relevant lipids

The present work investigates the interaction of hexadecylbetainate chloride (C(16)BC), a glycine betaine-based ester with palmitoyl-oleoyl-phosphatidylcholine (POPC), sphingomyelin (SM), and cholesterol (CHOL), three biological relevant lipids present in the outer leaflet of the mammalian plasma membrane. The binding affinity and the mixing behavior between the lipids and C(16)BC are discussed based on experimental (isothermal titration calorimetry (ITC) and Langmuir film balance) and molecular modeling studies. The results show that the interaction between C(16)BC and each lipid is thermodynamically favorable and does not affect the integrity of the lipid vesicles. The primary adsorption of C(16)BC into the lipid film is mainly governed by a hydrophobic effect. Once C(16)BC is inserted in the lipid film, the polar component of the interaction energy between C(16)BC and the lipid becomes predominant. Presence of CHOL increases the affinity of C(16)BC for membrane. This result can be explained by the optimal matching between C(16)BC and CHOL within the film rather by a change of membrane fluidity due to the presence of CHOL. The interaction between C(16)BC and SM is also favorable and gives rise to highly stable monolayers probably due to hydrogen bonds between their hydrophilic groups. The interaction of C(16)BC with POPC is less favorable but does not destabilize the mixed monolayer from a thermodynamic point of view. Interestingly, for all the monolayers investigated, the exclusion surface pressures are above the presumed lateral pressure of the plasma membranes suggesting that C(16)BC would be able to penetrate into mammalian plasma membranes in vivo. These results may serve as a useful basis in understanding the interaction of C(16)BC with real membranes.