Surfactant Spreading Research Articles

On the modulus of continuity of weakly differentiable functions

We prove global existence of a nonnegative weak solution to a degenerate parabolic system, which models the spreading of insoluble surfactant on a thin liquid film.

Fluorescent visualization of a spreading surfactant

The spreading of surfactants on thin films is an industrially and medically important phenomenon, but the dynamics are highly nonlinear and visualization of the surfactant dynamics has been a long-standing experimental challenge. We perform the first quantitative, spatiotemporally resolved measurements of the spreading of an insoluble surfactant on a thin fluid layer. During the spreading process, we directly observe both the radial height profile of the spreading droplet and the spatial distribution of the fluorescently tagged surfactant. We find that the leading edge of a spreading circular layer of surfactant forms a Marangoni ridge in the underlying fluid, with a trough trailing the ridge as expected. However, several novel features are observed using the fluorescence technique, including a peak in the surfactant concentration that trails the leading edge, and a flat, monolayer-scale spreading film that differs from concentration profiles predicted by current models. Both the Marangoni ridge and the surfactant leading edge can be described to spread as R∝tδ. We find spreading exponents δH≈0.30 and δΓ≈0.22 for the ridge peak and surfactant leading edge, respectively, which are in good agreement with theoretical predictions of δ=1/4. In addition, we observe that the surfactant leading edge initially leads the peak of the Marangoni ridge, with the peak later catching up to the leading edge.

An experimental method for measuring the spreading velocity of surface active substances on thin films of liquid substrate

This paper reports the development of a new experimental method suitable for the determination of the spreading velocity of surface active substances on a thin film of liquid substrate over a solid surface. Previous research showed that when oil and surfactants spread on the surface of a deep body of water, the spreading velocity is usually quite high, being in the range of several tens of centimeters per second. However, the spreading velocities of these substances on a thin water film are much slower. This is because that the spreading of an oil or a surfactant on water surface is not just a two dimension phenomenon, but a phenomenon involving the movement of water in a small depth below the surface. An additional process impeding oil or surfactant spreading in this situation is the interaction of the spreading molecules with the liquid/soild interface underneath water surface. The experimental method developed in this work uses a Wilhelmy balance to monitor the average spreading velocity of surface active substances on a thin water film over a porous solid substrate. The spreading of cellulose hydrophobization agents over wet paper web during the papermaking process is used as an example to demonstrate the capability of the method.

On the effect of pH on spreading of surfactant solutions on hydrophobic surfaces

Surfactants are invaluable in a number of agricultural applications in products such as pesticides and herbicides. In these products, surfactants are very often used in conjunction with acidifiers in order to improve their half-life. In this paper, we investigate how the change in pH affects surfactant wetting and spreading. We compare the performance of a conventional surfactant, Triton ® X-100, with that of a trisiloxane superspreader, Silwet ® L-77, on a number of polymer coated surfaces exhibiting various degrees of hydrophobicity. Silwet ® L-77 in water based solutions showed very good wetting capability on all surfaces. However, its wetting ability was drastically reduced with the addition of acetic acid. On the other hand, Triton ® X-100 was not affected by the addition of acid and exhibited the same spreading behaviour as in water-based solutions.

Influence of Liquid-Layer Thickness on Pulmonary Surfactant Spreading and Collapse

Pulmonary surfactant spreads on the thin (∼0.1 μm) liquid layer that lines the alveoli, forming a film that reduces surface tension and allows normal respiration. Pulmonary surfactant deposited in vitro on liquid layers that are several orders of magnitude thicker, however, does not reach the low surface tensions (∼0.001 N/m) achieved in the lungs during exhalation when the surfactant film compresses. This is due to collapse, a surface phase transition during which the surfactant film, rather than decreasing surface tension by increasing its surface density, becomes thicker at constant surface tension (∼0.024 N/m). Formation of the collapse phase requires transport of surfactant to collapse sites, and this transport can be hindered in thinner liquid layers by viscous resistance to motion. Our objective is to determine the effect of the liquid-layer thickness on surfactant transport, which might affect surfactant collapse. To this end, we developed a mathematical model that accounts for the effect of the liquid-layer thickness on surfactant transport, and focused on surfactant spreading and collapse. Model simulations showed a marked decrease in collapse rates for thinner liquid layers, but this decrease was not enough to completely explain differences in surfactant film behavior between in vitro and in situ experiments.

Certain peculiarities of the solutocapillary convection

This paper presents experimental results on solutocapillary Marangoni convection, an effect that occurs in a thin horizontal layer of the inhomogeneous solution of a surface-tension-active agent (surfactant) either near the free upper boundary of the layer or near the surface of an air bubble injected into the fluid. A procedure using interferometry is developed for simultaneously visualizing convective flow structures and concentration fields. A number of new phenomena are observed, including the deformation and rupture of the liquid layer due to a surfactant droplet spread over its surface; bubble self-motion (migration) toward higher surfactant concentrations; self-sustained convective flow oscillations around stationary bubbles in a fluid vertically stratified in concentration; and the existence of a threshold for a solutal Marangoni flow in thin layers. A comparison of solutocapillary and thermo-capillary phenomena is made.

A comparative study of mechanisms of surfactant inhibition

Pulmonary surfactant spreads to the hydrated air–lung interface and reduces the surface tension to a very small value. This function fails in acute respiratory distress syndrome (ARDS) and the surface tension stays high. Dysfunction has been attributed to competition for the air–lung interface between plasma proteins and surfactant or, alternatively, to ARDS-specific alterations of the molecular profile of surfactant. Here, we compared the two mechanisms in vitro, to assess their potential role in causing respiratory distress. Albumin and fibrinogen exposure at or above blood level concentrations served as the models for testing competitive adsorption. An elevated level of cholesterol was chosen as a known adverse change in the molecular profile of surfactant in ARDS. Bovine lipid extract surfactant (BLES) was spread from a small bolus of a concentrated suspension (27 mg/ml) to the air–water interface in a captive bubble surfactometer (CBS) and the bubble volume was cyclically reduced and increased to assess surface activity of the spread material. Concentrations of inhibitors and the concentration and spreading method of pulmonary surfactant were chosen in an attempt to reproduce the exposure of surfactant to inhibitors in the lung. Under these conditions, neither serum albumin nor fibrinogen was persistently inhibitory and normal near-zero minimum surface tension values were obtained after a small number of cycles. In contrast, inhibition by an increased level of cholesterol persisted even after extensive cycling. These results suggest that in ARDS, competitive adsorption may not sufficiently explain high surface tension, and that disruption of the surfactant film needs to be given causal consideration.

Thinning of drying latex films due to surfactant

Lateral non-uniformities in surfactant distribution in drying latex films induce surface tension gradients at the film surface and lead to film thinning through surfactant spreading. Here we investigate the influence of the surfactant driven to the air–water interface, during the early stages of latex film drying, on the film thinning process which could possibly lead to film rupture. A film height evolution equation is coupled with conservation equations for particles and surfactant, within the lubrication approximation, and solved numerically, to obtain the film height, particle volume fraction, and surfactant concentration profiles. Parametric analysis identifies the effect of drying rate, dispersion viscosity and initial particle volume fraction on film thinning and reveals the conditions under which films could rupture. The results from surface profilometry conform qualitatively to the model predictions.

Local film thinning due to concentration gradient of an insoluble surfactant at an interface

The local thinning of a viscous liquid film on a substrate driven by a surface (or interfacial) tension gradient due to a concentration gradient of a monolayer of an insoluble surfactant initially non-uniformly distributed at a liquid interface relevant to chemical engineering, biomedical and other applications is investigated. A simple model is presented for the temporal evolution of the profiles of radial variation in the thickness of a thin liquid film, the effects of gravity and capillarity due to deformation of the interface in slowing down the film thinning process being allowed. As time increases, the surfactant spreads and the radius of its front increases inversely with decrease in the two-third power of the film thickness at the center. The model describes well not only the published experimental results but also those obtained by other authors using numerical simulations of a set of coupled partial differential equations.

Surfactant Spreading on Thin Viscous Films: Nonnegative Solutions of A Coupled Degenerate System

We consider the Navier--Stokes system for an incompressible fluid coupled with a convection-diffusion equation for surfactant molecules on the free surface. The lubrication approximation leads to a coupled system of parabolic equations, consisting of a degenerate fourth-order equation for the film height and a second-order equation for the surfactant concentration. A proof based on energy estimates shows the existence of global weak solutions which in addition fulfill an integral inequality (entropy condition) which ensures positivity properties for the solution.

The spreading and superspeading behavior of new glucosamide-based trisiloxane surfactants on hydrophobic foliage

In order to find how the molecular structure and HLB (hydrophilic–lipophilic balance) value of trisiloxane surfactant affect its wetting and spreading behavior on leaf surface, the time-dependent and the concentration-dependent spreading performance of five new glucosamide-based trisiloxane surfactants on wheat and cabbage leaf surface were investigated and compared with that of Silwet L-77 ®. The effect of different epicuticular waxes on surfactants’ spreading behavior was also investigated, and the extent of adsorption on epicuticular waxes was calculated. The results showed that the HLB values of surfactants play a very important role for showing superspreading behavior. Furthermore, the spreading performance was also affected by the molecular structure of surfactant and leaf epicuticular waxes. A new trisiloxane surfactant II show a good superspreading behavior on cabbage and wheat foliage, which is similar with that of Silwet L-77 ®. This indicated that surfactant II have a great potential to being an excellent foliage ‘superspreader’.

Polyoxometalate Monolayers in Langmuir–Blodgett Films

Langmuir and Langmuir-Blodgett (LB) films of a variety of polyoxometalates of different shapes, sizes, and charges were prepared by taking advantage of the adsorption properties of these polyanions on a positively charged monolayer of an organic surfactant spread on water. Three different aspects were investigated. 1) The electrochemical and electrochromic properties of LB films containing the easily reducible polyoxoanion [P2Mo18O62]6-. Absorbance changes of these LB films deposited onto an ITO substrate have been induced by repeated switching of the applied potential. These changes are due to the formation of the colored reduced forms of the polyanion. Coloration and bleaching of the LB film occur very quickly and are reversible. 2) The preparation of LB films based on magnetic polyoxometalates, such as the Keggin anions, [CoW12O40]6- and [SiMn(OH2)W11O39]6-, or containing magnetic clusters of increasing nuclearities such as [Co4(H2O)2(PW9O34)2]10- and [Co4(H2O)2(P2W15O62)2]16- based on a Co4O16 ferromagnetic cluster, and the polyoxometalates [Co9(OH)3(H2O)6(HPO4)2(PW9O34)3]16- and [Ni9(OH)3-(H2O)6(HPO4)2(PW9O34)3]16- based on a nonanuclear M9O36 cluster. 3) The preparation of LB films of the giant heteropolyoxomolybdate, [Na3(NH4)12][Mo57Fe6(NO)6O174(OH)3-(H2O)24]76 H2O.

Synthetic Surfactants: The Search Goes on

When Avery and Mead1 first described surfactant deficiency as being responsible for hyaline-membrane disease, it seemed as if the therapeutic implications would be simple: Because we knew that pulmonary surfactant was made up primarily of phosphatidyl choline, perhaps administering a solution or aerosol of this readily available lipid to the surfactant-deficient lung should solve the problem. Unfortunately, this simplistic approach did not work regardless of whether surfactant was delivered to humans2 or animals.3 Improving the spreading characteristics by adding alcohols or altering the physical preparation improved the function somewhat,4,5 but the first dramatic effect on pulmonary function and survival was shown only with modified natural surfactants that had been extracted from lung minces6,7 and airway washes of animal lungs.8 Various head-to-head trials of natural versus synthetic surfactants have consistently favored the natural preparations.9 Accordingly, although 2 synthetic surfactant preparations (Exosurf and Pumactant) were approved for commercial distribution in the United States and Britain, respectively, Exosurf is no longer readily available in the United States, and Pumactant was recently removed from the market in Britain. The search for a synthetic preparation that is equivalent or superior to the animal-derived surfactants has continued, because there has always been concern about the potential antigenic and infectious complications that might be associated with animal-derived … Address correspondence to John Kattwinkel, MD, Department of Pediatrics, University of Virginia, Charlottesville, VA 22908. E-mail: jk3f{at}virginia.edu

Mechanism of the enhanced spreading of some mixtures of anionic and cationic hydrocarbon chain surfactants on a highly hydrophobic polyethylene surface

AbstractThe objective of this investigation was to measure the interfacial properties and interactions among mixtures of different cationic and anionic surfactants at the hydrophobic solid/aqueous solution interface to explain the different spreading factors or behavior of the mixed surfactants on a highly hydrophobic polyethylene (PE) film. A synergistic effect in the spreading of the mixed surfactant solutions on the PE film was observed when the surfactants were added sequentially. Other interfacial phenomena related to this surfactant spreading, such as interaction parameters at the solid/liquid (S/L) and liquid/air (L/A) interfaces in the mixtures adsorbed at various interfaces and dynamic contact angles of the mixed surfactant solutions during the process of spreading on the PE substrate, were investigated to obtain insight into this enhanced spreading. All the interaction parameters were more negative than −20, indicating very strong interaction between these cationic and anionic surfactants. The interaction parameters at the S/L interface were more negative than at the L/A interface, showing that the attractive interaction at the S/L interface was stronger than at the L/A interface. The spreading was related to the difference in the interaction parameters at the S/L and L/A interfaces and to the dynamic contact angle.

Synergism in the Spreading of Hydrocarbon-Chain Surfactants on Polyethylene FilmAnionic and Cationic Mixtures by a Two-Step Procedure

A study has been made of the adsorption, interaction, and spreading of mixtures of anionic and cationic surfactants at the aqueous solution/polyethylene (PE) interface. When a drop of an aqueous solution of an anionic or cationic hydrocarbon-chain surfactant is placed on a highly hydrophobic PE film (contact angle of water > 90 degrees ), it spreads to an area very little larger than that of a drop of water of the same volume. If the anionic and cationic hydrocarbon-chain surfactant solutions are mixed prior to being applied to PE film, synergism is small, if any, and the reproducibility of the experimental results is poor. However, when the cationic and anionic aqueous solutions are applied on the PE film in a sequential manner, a remarkable synergism in spreading is observed and the results are very reproducible. The area spread by an aqueous solution of the anionic-cationic mixture may be more than 400 times that of aqueous solutions of the same volume and surfactant concentration of the individual surfactant components. Previous work in this laboratory on surfactant systems showing synergism in spreading on PE film, but only weak interaction at the aqueous solution/air interface, showed that the synergy was due to changes at the aqueous solution/PE interface and not to the changes at the aqueous solution/air or PE/air interface. Investigation of the adsorption behavior at the aqueous solution/solid interface of two of the anionic-cationic mixtures studied here indicates the reason for differences in spreading behavior observed with different anionic-cationic mixtures. The more similar the adsorption tendencies at the solid/aqueous solution interface of the anionic and cationic surfactants, and the closer their adsorption to an equimolar monolayer there, the stronger their interaction there and the greater their enhancement of the spreading. A mechanism is proposed for the synergy in spreading observed, based upon the difference between the surface tension in the precursor film at the spreading interface and that at the top of the spreading drop.

Fingering phenomena associated with insoluble surfactant spreading on thin liquid films

We study the linear and nonlinear stability of a thick surfactant deposition spreading on a thin liquid film using transient growth analysis (TGA) and direct numerical simulations (DNS) of the two-dimensional lubrication equations, respectively. Results of the TGA of the one-dimensional spatially and temporally evolving base state reveal disturbance growth and the selection of a perturbation of intermediate wavenumber. This perturbation targets the ‘contact region’ between the deposition and the underlying thin liquid film and grows despite the absence of intermolecular forces. Increasing the initial thickness ratio of the deposition to the thin film and decreasing the relative magnitude of capillarity and surface diffusion further amplify perturbation growth. The DNS results clearly show the formation of fingers in the contact region behind the surfactant leading edge and provide further confirmation of the TGA findings.

Convergence of a finite-element approximation of surfactant spreading on a thin film in the presence of van der Waals forces

We consider a fully practical finite-element approximation of the following system of nonlinear degenerate parabolic equations: ∂u/∂t + 1/2 ⊇.(u 2 ⊇[σ(υ)]) - 1/3 ⊇.(u 3 ⊇ω) = 0, ω = -c Δu+δu -ν + au -3 , ∂υ/∂t + ⊇.(u υ ⊇[σ(υ)]) - ρΔυ - 1/2 ⊇.(u 2 υ⊇ω) = 0. The above models a surfactant-driven thin-film flow in the presence of both attractive, a > 0, and repulsive, δ > 0 with υ > 3, van der Waals forces; where u is the height of the film, υ is the concentration of the insoluble surfactant monolayer and σ(υ):= 1 - υ is the typical surface tension. Here ρ ≥ 0 and c > 0 are the inverses of the surface Peclet number and the modified capillary number. In addition to showing stability bounds for our approximation, we prove convergence, and hence existence of a solution to this nonlinear degenerate parabolic system, (i) in one space dimension when p > 0; and, moreover, (ii) in two space dimensions if in addition υ ≥ 7. Furthermore, iterative schemes for solving the resulting nonlinear discrete system are discussed. Finally, some numerical experiments are presented.

Hybrid Langmuir–Blodgett monolayers containing clay minerals: effect of clay concentration and surface charge density on the film formation

To control the properties of hybrid organo-clay films prepared by the Langmuir–Blodgett (LB) method, the film formation mechanism should be understood. This work aimed to understand what occurs at the air–water interface after the spreading of cationic surfactants (octadecyl rhodamine B, 3,3′-dioctadecyl oxacarbocyanine) on aqueous dispersions of smectite clay minerals (saponite, montmorillonite, hectorite, laponite), resulting in hybrid organo-clay films. Information on the amount of surfactant molecules and clay particles in the hybrid films was obtained with surface pressure versus molecular area isotherms, attenuated total reflection infrared spectroscopy, ultraviolet-visible spectroscopy and atomic force microscopy. X-ray reflectivity measurements indicated that the surfactant molecules had adsorbed on only one side of the clay mineral lamella. With increasing clay concentration of the dispersion, the isotherms shifted to a lower lift-off area, a minimum lift-off area (MLA) was reached and then the lift-off area increased again. Films made at lower than the MLA clay concentration consisted of two phases: an organic phase and a hybrid organo-clay phase. Films made at the MLA clay concentration consisted of dense monolayers of the surfactant molecules and single clay mineral lamellae. The density of the surfactant molecules was highly correlated with the surface charge density of the clay minerals. These films had low water content. Films made at higher than the MLA clay concentration contained less surfactant, aggregates of the clay mineral particles, residual Na+ ions and water. With the clay concentration of the dispersion and the surface charge density of the clay mineral, the properties of hybrid organo-clay LB films can be adjusted.

Biogeochemistry of Aromatic Surfactants in Microtidal Estuaries

This paper summarizes several studies on input, distribution, and fate of two different types of aromatic surfactants, the anionic linear alkylbenzene sulphonates (LAS) and nonionic alkylphenol polyethoxylates (APnEO), in a karstic estuary characterized by sharp salinity gradients. A combination of reversed-phase and normal-phase HPLC analysis followed by spectrofluorimetric detection allowed detailed characterization of homologue and oligomer compositions of aromatic surfactants in wastewater and estuarine waters. The distribution patterns of homologues and/or oligomers in the wastewater plume reveal a significant alteration of the original composition of both surfactants by biotransformation and physico-chemical partitioning. The vertical distribution of surfactant residues in the estuarine water column is characterized by pronounced maxima at the air–brackish water and brackish water–sea water interface. The brackish water–sea water interface is a very efficient barrier, which prevents spreading of surfactants into the deeper layers. In addition, the interface seems to be a site of high biological activity, including biotransformation of surfactant residues. As a consequence, this layer can contain enhanced levels of lipophilic metabolites of APnEO. Biotransformation is the main mechanism for the removal of both surfactant classes from the water column. However, our studies suggest that a significant percentage of surfactant residues discharged into the stratified estuaries reaches the coastal sea due to the incomplete removal in the estuary. This situation is a consequence of a rather fast flushing of the brackish layer and a very slow biotransformation kinetics in the saline layer, especially in the winter period.

Surfactant−Surfactant Molecular Interactions in Mixed Monolayers at a Highly Hydrophobic Solid/Aqueous Solution Interface and Their Relationship to Enhanced Spreading on the Solid Substrate

Spreading of mixed aqueous hydrocarbon-chain surfactant solutions on the solid polyethylene (PE) surface has been studied. Synergistic effect on the spreading of the mixed surfactant solution on the PE film has been observed, and the obtained spreading is comparable to superspreading normally obtainable from trisiloxane-based surfactants. Some other interfacial phenomena related to surfactant spreading, such as surfactant−surfactant molecular interactions in the mixtures adsorbed at various interfaces, dynamic contact angle change of the mixed surfactant solutions during the process of spreading on the hydrophobic PE substrate, and surfactant adsorption at the solid/liquid and air/liquid interfaces have been investigated. It is suggested that stronger surfactant−surfactant attractive interactions and greater adsorption at the PE powder/aqueous solution interface than at the air/aqueous solution interface account for the observed spreading enhancement in the mixed hydrocarbon-chain surfactant systems, which is also accompanied by lower dynamic contact angles, implying greater dynamic spreading coefficients.