STRATIFICATION IN EMULSION FILMS

Formation of films possessing a layered or stratified structure has been observed with foam films from liquid crystals, from concentrated surfactant solutions and in liquid layers on the surface of water. The stratifying films have a structure similar to that of the smectic phase which soaps are known to form in the bulk solution at high concentrations. The repeating units of which such films are built are the so-called black films: each unit consists of two surfactant layers interleaved by a thin aqueous core. In the study presented here we have observed that stratification can also take place in emulsion films from concentrated aqueous surfactant solutions. We have compared these results with those obtained for foam films using the same surfactant, i.e. sodium dodecylsulfate.     

Disjoining pressure and stratification in asymmetric thin-liquid films

We directly measure, for the first time, disjoining pressure isotherms for asymmetric oil/aqueous surfactant/gas (i.e., pseudoemulsion) films using a modified version of the porous-plate technique first developed by Mysels in conjunction with thin-film interferometry. Dynamic film-thinning experiments are also performed on individual foam and pseudoemulsion films. At SDS surfactant concentrations above the critical micelle concentration (CMC) (0.1 M SDS), the pesudoemulsion films exhibit the same step-wise layer thinning observed in foam films under similar conditions. Further, we conduct dynamic thinning experiments on solid/liquid/gas systems and show that aqueous 0.2 M CTAB films sandwiched between glass and air also display discrete thinning transitions. All of these stratification transitions arise from oscillations in the disjoining pressure isotherm, generated by amphiphilic structuring within the film.For 0.1 M SDS dedecane/air pesudoemuslion films, the slope and peak height of the disjoining-pressure oscillations increase with each subsequent amphiphilic layer as film thickness decreases. Magnitudes of the structural forces are low (<100 Pa) but the length scale of the oscillations is large (10 nm) and rathe far reaching (50 nm). Moreover, for 0.1 M SDS solutions, the capillary pressures associated with film rupture are significantly lower for pseudoemulsion films (0.1 kPa) when compared to foam films (15 kPa) at equivalent conditions. Taken together, our dynamic thinning and equilibrium disjoining pressure measurements indicate that stratification in 0.1 M SDS films has little effect on both kinetic and thermodynamic films stability.     

Stepwise thickening in aqueous foam films stabilized by sodium naphthenates

During drainage of a foam film formed from an aqueous sodium naphthenate solution, a transient, local, stepwise thickening process was observed. Film stratification is related to the stepwise thinning drainage process where individual layers of material are drained from a film. The process typically involves the appearance of a sequence of small, uniformly thick spots that eventually expand to the size of the film. The appearance and growth of each spot represents a discrete decrease in the thickness of the film. The size of each decrease or step typically corresponds to the size of one or more lamella layers of the stratified film. Stepwise thinning was observed in a foam film formed from an aqueous sodium naphthenate solution, as frequently reported for a variety of systems. However, during the drainage process, a transient stepwise thickening process was also observed. Bright spots began to appear and grow, indicate a discrete increase in the thickness of a portion of the film. This local, stepwise thickening process appeared to be an alternate and temporary drainage process directly related to the expansion of the stepwise thinning spots.     

Thinning of foam films of micellar surfactant solutions

Drainage in microscopic circular foam films depends significantly on the radial (tangential) mobility of the film surfaces and is accelerated as compared to the limiting case of tangentially immobile surfaces, where velocity of thinning is described by the classical Reynolds’ equation (outflow of viscous fluid from a cylindrical gap between two solid plates). The structure and composition of the adsorption layer and the interfacial mass transfer determine the tangential mobility of the film surfaces and, hence, the measured velocity of film thinning. Experiments with soluble surfactants below the critical micelle concentrations (CMC) have exhibited the effect of dynamic interfacial elasticity. At relatively low bulk concentrations, the interfacial mass transfer is governed by surface diffusion; close to CMC (saturated adsorption layer), the limiting case of tangentially immobile surfaces can be reached and at concentrations above the CMC the film thinning is accelerated again. Here, we report freshly established data on the kinetic behavior of foam films from micellar solutions of soluble nonionic surfactants (decyl-octaoxyethylene alcohol and dodecyl-octaoxyethylene alcohol) in a wide range of concentrations above the CMC aiming to investigate the effect of partially disintegrated micelles acting as sources of surfactant molecules at the surface.     

The metastable states of foam films containing electrically charged micelles or particles: experiment and quantitative interpretation

The stepwise thinning (stratification) of liquid films containing electrically charged colloidal particles (in our case - surfactant micelles) is investigated. Most of the results are applicable also to films from nanoparticle suspensions. The aim is to achieve agreement between theory and experiment, and to better understand the physical reasons for this phenomenon. To test different theoretical approaches, we obtained experimental data for free foam films from micellar solutions of three ionic surfactants. The theoretical problem is reduced to the interpretation of the experimental concentration dependencies of the step height and of the final film thickness. The surface charges of films and micelles are calculated by means of the charge-regulation model, with a counterion-binding (Stern) constant determined from the fit of surface tension isotherms. The applicability of three models was tested: the Poisson-Boltzmann (PB) model; the jellium-approximation (JA), and the cell model (CM). The best agreement theory/experiment was obtained with the JA model without using any adjustable parameters. Two theoretical approaches are considered. First, in the energy approach the step height is identified with the effective diameter of the charged micelles, which represents an integral of the electrostatic-repulsion energy calculated by the JA model. Second, in the osmotic approach the step height is equal to the inverse cubic root of micelle number density in the bulk of solution. Both approaches are in good agreement with the experiment if the suspension of charged particles (micelles) represents a jellium, i.e. if the particle concentration is uniform despite the field of the electric double layers. The results lead to a convenient method for determining the aggregation number of ionic surfactant micelles from the experimental heights of the steps.     

Thin liquid films containing micelles or nanoparticles

The stratification/microstructure formation inside suspended thin liquid films containing micelles or other nanoparticles is reviewed with the aim of clarifying some key issues and suggesting unresolved questions requiring further investigation. New experiments reveal that the micellar layer thinning phenomenon is a reversible process in films formed from non-ionic micellar solutions. The important factors contributing to the correlation between the ordered microstructure formation that leads to a high structural stabilization force in a single free suspended film and the stability of macrodispersions (such as foams and emulsions and particle suspensions) are also discussed.     

Micellar solutions of ionic surfactants and their mixtures with nonionic surfactants: Theoretical modeling vs. Experiment

Here, we review two recent theoretical models in the field of ionic surfactant micelles and discuss the comparison of their predictions with experimental data. The first approach is based on the analysis of the stepwise thinning (stratification) of liquid films formed from micellar solutions. From the experimental step-wise dependence of the film thickness on time, it is possible to determine the micelle aggregation number and charge. The second approach is based on a complete system of equations (a generalized phase separation model), which describes the chemical and mechanical equilibrium of ionic micelles, including the effects of electrostatic and non-electrostatic interactions, and counterion binding. The parameters of this model can be determined by fitting a given set of experimental data, for example, the dependence of the critical micellization concentration on the salt concentration. The model is generalized to mixed solutions of ionic and nonionic surfactants. It quantitatively describes the dependencies of the critical micellization concentration on the composition of the surfactant mixture and on the electrolyte concentration, and predicts the concentrations of the monomers that are in equilibrium with the micelles, as well as the solution’s electrolytic conductivity; the micelle composition, aggregation number, ionization degree and surface electric potential. These predictions are in very good agreement with experimental data, including data from stratifying films. The model can find applications for the analysis and quantitative interpretation of the properties of various micellar solutions of ionic surfactants and mixed solutions of ionic and nonionic surfactants.     

Foaming,foam films,antifoaming and defoaming

A general introduction to foams, the initial stages in the production of foams in aqueous solution, foam structures and the classification of bulk foams according to their lifetimes and stability are presented. Fundamental studies on horizontal and vertical isolated foam lamellae with emphasis on drainage and stability are reviewed. For freshly prepared foams containing fairly thick lamellae, the mechanical-dynamical properties of the surface adsorbed layers (surface tension gradients) are decisive for retaining stability. Important parameters to be taken into consideration are the surface elasticity, viscosity (bulk and surface), gravity drainage and capillary suction. Also the film should exhibit low permeability to gases. Providing the stability of a foam film (containing dilute surfactant) is retained during the initial dynamic drainage process, then eventually a static (equilibrium) situation will be reached at film thicknesses < 100 nm. In this region, interfacial interactions dominate and the stability of the film must be discussed in terms of the intermolecular forces (electrostatic double layer repulsion, dispersion force attraction and steric forces). This may lead to the formation of common black and Newton black films and these structures have been shown to be resilient to rupture and have low gas transfer characteristics. At high surfactant concentrations (>c.m.c.) stabilization of films and foams can occur by a micellar laying mechanism (stratification). Antifoaming and defoaming theories are presented, together with the mechanisms of heterogeneous antifoaming agents (non-polar oil, hydrophobic solid particles or mixtures of both) including recent theories describing the role of the emulsion and pseudo-emulsion film in the stability of foams containing oil droplets. Finally, defoaming by ultrasonic waves is briefly reviewed.     

Disruption of viscoelastic beta-lactoglobulin surface layers at the air-water interface by nonionic polymeric surfactants

Nonequilibrium interfacial layers formed by competitive adsorption of beta-lactoglobulin and the nonionic triblock copolymer PEO99-PPO65-PEO99 (F127) to the air-water interface were investigated in order to explain the influence of polymeric surfactants on protein film surface rheology and foam stability. Surface dilatational and shear rheological methods, surface tension measurements, dynamic thin-film measurements, diffusion measurements (from fluorescence recovery after photo bleaching), and determinations of foam stability were used as methods. The high surface viscoelasticity, both the shear and dilatational, of the protein films was significantly reduced by coadsorption of polymeric surfactant. The drainage rate of single thin films, in the presence of beta-lactoglobulin, increased with the amount of added F127, but equilibrium F127 films were found to be thicker than beta-lactoglobulin films, even at low concentration of the polymeric surfactant. It is concluded that the effect of the nonionic triblock copolymer on the interfacial rheology of beta-lactoglobulin layers is similar to that of low molecular weight surfactants. They differ however in that F127 increases the thickness of thin liquid films. In addition, the significant destabilizing effect of low molecular weight surfactants on protein foams is not found in the investigated system. This is explained as due to long-range steric forces starting to stabilize the foam films at low concentrations of F127.     

Investigation of thin films formed from liposome suspensions on quartz substrate

The stability of thin wetting films formed from 0.15 M NaCl solutions containing small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles of different concentrations on quartz surface has been investigated by the microinterferometric method. The intensity of monochromatic light reflected from both film surfaces has been recorded as a function of the time of film thinning. Two temperatures were used in the experiments (20 and 35 degrees C). Films containing 10(-3), 5x10(-3) and 10(-2) mg/ml DMPC were unstable and ruptured, while films with 10(-1) and 1 mg/ml DMPC were stable. Film stability was explained on the basis of hydrophobic interactions. Film thickness dependence on time was calculated. The kinetics of film thinning did not obey Reynolds equation and a linearization was observed in co-ordinates ln(h) as a function of time. This phenomenon was explained by a non-homogeneous thinning process, which might be due to the existence of some areas of different structure of the DMPC adsorption layers.     

Amphiphilic nanostructures in foam films

The revised articles outline the potential of microscopic foam film instrumentation as an investigation tool in studying the amphiphilic nanostructures in aqueous surfactant solutions. The impact of amphiphilic nanostructures on the drainage behaviour and stability of foam films is traced for surfactant solutions of concentrations orders of magnitude above CMC (micellar solutions) to about two orders of magnitude lower than CMC (premicellar solutions). It is found that in the high-concentration domain the micellar entities affect mainly the stability of the films. In the low-concentration domain, the presence of smaller crumbly aggregates (premicelles), plays a significant role for the kinetic stability of the films. Through the mechanism of Marangoni effect, an enhanced coupling of the specific film hydrodynamics and the mass transfer of the surfactant is obtained. The result is a sharp rise in the kinetic stability of the foam films. The importance of this trend of research is related to providing better insight into the self-assembling phenomena and into the factors that determine the drainage and the stability of thin liquid films. The results have potential and actual applications in food, cosmetic and pharmaceutical industries, as well as in biology and medicine.     

Hydrodynamics of thin liquid films effect of the surfactant on the rate of thinning

Summary A theory of the effect of the surfactant on the rate of thinning of foam films is presented. The formulae obtained for the separately treated cases of low and high concentrations cover the whole concentration range. The effect of both bulk and surface diffusion is taken into consideration and it is demonstrated that the relative importance of the latter increases with the decrease of the film thickness. The role of the surface diffusion for the stability of foam films is discussed. It is shown that films stabilized with soluble surfactants never strictly obey Reynolds'eq. [19] so that the actual velocity of thinning can be substantially higher than that calculated by the quoted equation.     

Competitive adsorption of -α-lysophosphatidylcholine/β-lactoglobulin mixtures at the interfaces of foams and foam lamellae

The stability of foams formed with the protein β-lactoglobulin as a function of increasing concentration of the lipid analogue -α-lysophosphatidylcholine were investigated using a microconductivity technique. The drainage, surface diffusion and thickness properties of thin liquid films (foam lamallae) were also studied using optical microscopy including epi-illumination, fluorescence recovery after photobleaching and film interferometry techniques. In addition, the surfactant binding properties of the protein were examined. The addition of small quantities of -α-lysophosphatidylcholine to β-lactoglobulin (molar ratio, R < 7:1) increased the foam stability, whereas a slightly higher concentration of surfactant in the mixture (R = 10) caused foam destabilisation. The explanation of these observations is based on changes in the composition and structure of the adsorbed interfacial layers of the thin films caused by competitive displacement of the protein by the surfactant.     

Fluorescence drainage profiles of thin liquid films

Fluorescence drainage-profiles of thin liquid films formed from Rhodamine solutions containing anionic, cationic and non-ionic surfactants have been investigated. It is found that the influence of system variables such as electrolyte concentration, dye concentration, film environment and solution viscosity can be evaluated by means of the profiles. The addition of sodium chloride leads to the expansion (non-ionic surfactant), or contraction (anionic surfactant) of the profiles. In the case of the cationic surfactant, it reduces the number of fringes in the profiles. The height of selected fringes changes linearly with dye concentration, and no fringes are observed when the films are submerged in non-polar solvents. The influence of solution viscosity on film thickness and drainage rate is demonstrated by the number and frequency of fringes in the profile. The formation of first and second "black films" from solutions containing varying concentrations of sodium chloride can be shown.     

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.     

Change of Surface Structure upon Foam Film Formation

The charge distribution and coverage with surfactant molecules at foam film surfaces plays an important role in determining foam film structure and stability. This work uses the concentration depth profiling technique neutral impact collision ion scattering spectroscopy to experimentally observe the charge distribution in a foam film for the first time. The charge distribution at the surface of a foam film and the surface of the corresponding bulk liquid were measured for a cationic surfactant solution and the surface excess as well as the electric potential were determined. Describing the internal pressure of foam films by using the electrochemical potential is introduced as a new concept. The foam film can be seen to have a more negative surface charge compared to the bulk liquid surface due to re‐arranging of the surfactant molecules. It is discussed how the change in surface excess and electric potential change the electrochemical potential and the stability of the foam film.     

Charge reversal at the air/water interface as inferred from the thickness of foam films

Experiments are reported with foam films from aqueous solutions with increasing concentration of a cationic surfactant. A correlation is established between the foam film thickness and the possible variation of diffuse electric layer potential at the air/water interface from a negative value in absence of surfactant to positive values at higher surfactant concentrations. It is concluded that a charge reversal at the air/water interface is expected to occur under increasing concentration of cationic surfactants in aqueous solutions.     

Decay of standing foams: drainage,coalescence and collapse

A summary of recent theoretical work on the decay of foams is presented. In a series of papers, we have proposed models for the drainage, coalescence and collapse of foams with time. Each of our papers dealt with a different aspect of foam decay and involved several assumptions. The fundamental equations, the assumptions involved and the results obtained are discussed in detail and presented within a unified framework.Film drainage is modeled using the Reynolds equation for flow between parallel circular disks and film rupture is assumed to occur when the film thickness falls below a certain critical thickness which corresponds to the maximum disjoining pressure. Fluid flow in the Plateau border channels is modeled using a Hagen-Poiseuille type flow in ducts with triangular cross-section.The foam is assumed to be composed of pentagonal dodecahedral bubbles and global conservation equations for the liquid, the gas and the surfactant are solved to obtain information about the state of the decaying foam as a function of time. Homogeneous foams produced by mixing and foams produced by bubbling (pneumatic foams) are considered. It is shown that a draining foam eventually arrives at a mechanical equilibrium when the opposing forces due to gravity and the Plateau-border suction gradient balance each other. The properties of the foam in this equilibrium state can be predicted from the surfactant and salt concentration in the foaming solution, the density of the liquid and the bubble radius.For homogeneous foams, it is possible to have conditions under which there is no drainage of liquid from the foam. There are three possible scenarios at equilibrium: separation of a single phase (separation of the continuous phase liquid by drainage or separation of the dispersed phase gas via collapse), separation of both phases (drainage and collapse occurs) or no phase separation (neither drainage nor collapse occurs). It is shown that the phase behavior depends on a single dimensionless group which is a measure of the relative magnitudes of the gravitational and capillary forces. A generalized phase diagram is presented which can be used to determine the phase behavior.For pneumatic foams, the effects of various system parameters such as the superficial gas velocity, the bubble size and the surfactant and salt concentrations on the rate of foam collapse and the evolution of liquid fraction profile are discussed. The steady state height attained by pneumatic foams when collapse occurs during generation is also evaluated.Bubble coalescence is assumed to occur due to the non-uniformity in the sizes of the films which constitute the faces of the polyhedral bubbles. This leads to a non-uniformity of film-drainage rates and hence of film thicknesses within any volume element in the foam. Smaller films drain faster and rupture earlier, causing the bubbles containing them to coalesce. This leads to a bubble size distribution in the foam, with the bubbles being larger in regions where greater coalescence has occurred.The formation of very stable Newton black films at high salt and surfactant concentrations is also explained.     

Structure of liquid films of an ordered foam confined in a narrow channel

A bamboo foam is the simplest case of an ordered foam confined in a narrow channel. It is made of a regular film distribution, arranged perpendicularly to the channel. Our work consists of studying the structural properties of several films taken in a drained foam. X-ray experiments highlighted the equality of the equilibrium thickness for each film within a foam. The same thickness was found as by measurements of disjoining pressure isotherms, proving as well that films of a bamboo foam behave like isolated ones. The refinement of X-ray data by a simple model of specular reflectivity showed a significant variation of the electronic distribution of the surfactant layer for a common black film forwarding from one equilibrium state to another. A discussion on the organization of the surfactant molecules to the gas/liquid interface and film is proposed.     

Scanning probe microscopy of adsorption layers of sodium polystyrenesulfonate/dodecyltrimethylammonium bromide complexes

Scanning probe microscopy is used to study adsorption films of sodium polystyrenesulfonate/dodecyltrimethylammonium bromide complexes, which are transferred from aqueous solution surfaces to the mica surface by the Langmuir-Blodgett method. It is established that the results obtained at surfactant concentrations below critical micelle concentration agree with the conclusions on the structure of the films dased on the data of the dilatational surface rheology (Noskov, B.A., Loglio, G., Miller, R., J. Phys. Chem. B, 2004, vol. 108, p. 18615). At low concentrations of the low-molecular-mass surfactant, a thin adsorption film containing a large number of holes is formed on the surface of an aqueous solution. As the surfactant concentration is increased to reach the region of a dramatic reduction in the dynamic surface elasticity, the morphology of the film drastically changes; i.e., dense three-dimensional aggregates are formed on the solution surface.