Elias I. Franses

Adsorption dynamics of surfactants at the air/water interface: a critical review of mathematical models, data, and mechanisms

Abstract This review covers equilibrium and dynamic aspects of surface tension and adsorption, primarily of single nonmicellar or premicellar surfactants at the air/water interface. Equilibrium tension data have been related to the Langmuir adsorption equilibrium isotherm and the Gibbs isotherm. A set of Langmuir isotherm parameters has been compiled. Dynamic adsorption models with the diffusion-controlled and mixed-kinetic mechanisms are discussed in some detail and applied to several sets of data. Apparent diffusion coefficients, inferred from dynamic tension data, and kinetic adsorption parameters, derived from application of the models to the data, are presented and critically evaluated. The adsorption of most of the surfactants examined is slower than predicted by diffusion-controlled models. Exceptions are mostly non-ionic surfactants and certain alcohols.

Adsorption and surface tension of ionic surfactants at the air–water interface: review and evaluation of equilibrium models

Abstract A series of old and new equilibrium tension models are reviewed and evaluated for single premicellar ionic surfactants at the air–water interface with or without added salt with a common ion. Several experimental methods used to measure surface tensions, adsorbed densities, and surface potentials are also reviewed. The models are based on the Gibbs adsorption isotherm, and classified as ‘pseudo-nonionic’ when the surface charge is ignored or ‘ionic’ when the surface charge and its electric double layer are accounted for. The former models fit and represent well tension and adsorption density data but are not predictive, primarily because the underlying adsorption isotherms, the Langmuir or the Frumkin, are independent of salinity. Ionic models are to an extent predictive, based on the Davies or a combined Frumkin–Davies isotherm, and provide estimates of the adsorbed density and surface potential. Counterion binding is incorporated in the new models using a fractional binding parameter analogous to that used in micellar models. Certain advanced binding models proposed by Kralchevsky et al., Kalinin and Radke, and Warszyski et al. are also examined. The models are tested with tension data at 25°C for sodium dodecylsulfate (SDS) in the presence of several sodium chloride (NaCl) concentrations. Both the model predictions and the fitted parameter values are evaluated with respect to physical plausibility and overall goodness of fit to the available data. Although the pseudo-nonionic models can fit the data well, the fitted parameters depend strongly on salinity. The more advanced ionic models can fit the data nearly as well as the pseudo-nonionic models, and provide a plausible description of the surface electrostatics. More detailed electrostatic models, and reliable data on both adsorbed densities and surface potentials at the surfactant–water interface are needed for developing more definitive and less empirical models, and for improving further our fundamental understanding of the adsorption and tension behavior of ionic surfactants.

Contact angle temperature dependence for water droplets on practical aluminum surfaces

Abstract This paper presents an experimental investigation of the temperature dependence of the quasistatic advancing contact angle of water on an aluminum surface polished in accordance with surface preparation techniques commonly employed in boiling heat transfer studies. The surface, speculated to contain aluminum oxide and organic residue left behind from the polishing process, was characterized with scanning electron microscopy, surface contact profilometry, and ellipsometry. By utilizing a pressure vessel to raise the liquid saturation temperature, contact angles were measured with the sessile drop technique for surface temperatures ranging from 25 to 170°C and pressures from 101.3 to 827.4 kPa. Two distinct temperature-dependent regimes were observed. In the lower temperature regime, below 120°C, a relatively constant contact angle of 90° was observed. In the high temperature regime, above 120°C, the contact angle decreased in a fairly linear manner. Empirical correlations were developed to describe this behavior which emulated previous experimental data for nonmetallic surfaces as well as theoretical trends.

Ultrathin PMMA films spin-coated from toluene solutions

Abstract Films of atactic poly(methyl methacrylate) (PMMA) were produced by spin-coating from toluene solutions, and their properties were compared to similar PMMA films produced by spin-coating from chloroform [Walsh and Franses, Thin Solid Films 347 (1999) 167]. Two-angle ellipsometry at λ=6328 A was used to probe the film thicknesses, refractive indices, and their overall quality and uniformity. Ellipsometry was also used with microspot optics at one angle to determine the film thickness uniformity. The films from toluene were approximately 4-fold thinner, but more uniform and of higher quality, than the films from chloroform, evidently because of the lower volatility and slower evaporation, of toluene. Films with thicknesses d1=0.003 to 1 μm were produced for initial PMMA concentrations c=0.1–10 wt.%, and spinning speeds of 1000–3000 rpm. The thicknesses fit the equation d1 (μm)=0.92 c1.56 ω−0.51. The ω-dependence agrees with predictions of simple theoretical models. The results may find use in production of high-quality polymer films for resists or other applications.

Preparation and characterization of monodisperse polymer microspheroids

Abstract An original scheme for producing monodisperse polymer microspheroids of precise size and shape was developed. Graft-copolymer-stabilized poly(methyl methacrylate) (PMMA) microspheres with diameters of 0.5 to 1.4 μm were prepared by dispersion polymerization. The microspheres were embedded in a matrix of poly(dimethylsiloxane) (PDMS) normally containing 1 to 10% PMMA particles. The matrix was subsequently crosslinked. The composite elastic material was deformed under uniaxial extension, at a temperature well above the glass transition temperature Tg of the particles, to produce prolate microspheroids with aspect ratios up to 8. The microspheroids were physically set by cooling below Tg and were then recovered after selective chemical degradation of the PDMS matrix. The particles were characterized by scanning electron microscopy and by a previously developed method of transmission electron microscopy with metallic double shadow casting (Keville et al., J. Microsc. 142, 327 (1986)). Deviations in the dimensions and aspect ratios were typically less than 5%. Uniaxial deformation of properly synthesized materials produced a smooth prolate shape when the particles were less stiff than the matrix, even when they were uncrosslinked. Stable dispersions of spheroids were prepared in alkanes and PDMS. These novel dispersions of model particles are well suited for elucidating the effects of shape and orientation on various properties of colloidal dispersions.

Modified Langmuir—Hinselwood kinetics for dynamic adsorption of surfactants at the air/water interface

Abstract The Langmuir—Hinselwood (L-H) equation is the simplest kinetic equation which is consistent with Langmuirs equilibrium isotherm. This kinetic equation cannot describe well the dynamic surface tension data for octanol, sodium dodecyl sulfate (SDS), and other surfactants. A new kinetic equation for the rate of adsorption from the subsurface (d l /d t = k a L c (θ, t )(1−θ) exp(− B θ)− k d L Γ exp(− B θ). where θ is the fractional surface coverage Γ/Γ m , c (θ, t ) is the subsurface concentration, and k a L , k d L , and B are constants) includes modification of the kinetics but not of the equilibrium isotherm. The new equation describes better the capture efficiency of the interfacial monolayer for additional surfactant, and can describe activation barriers for adsorption and desorption, or cooperative adsorption caused by primarily attractive interactions between the monolayer and the dissolved surfactant. This equation was used in a new model of mixed kinetics for one-dimensional diffusion/adsorption/desorption. For octanol and heptanol, the initial adsorption rate is controlled by intrinsic adsorption/desorption kinetics (slow adsorption/desorption). With increasing surface coverage, dynamic adsorption gets closer to the diffusion-controlled limit (fast adsorption/desorption relative to diffusion). This indicates attractive and cooperative interactions of alcohol molecules in the monolayer. For sodium di-2-ethylhexylsulfosuccinate (DESS or AOT) and SDS, adsorption is much slower than predicted by diffusion-controlled models. The modified L-H equation in a mixed-kinetics model can fit the data well. The capture efficiency factor, k a L exp(− b θ), increases with increasing SDS concentration c SDS or NaCl concentration c s , indicating that adsorption is strongly affected by electrostatic barriers. For c s = 0 and c SDS = 1.7 to 5.9 m M (for θ c c > 0.4 and a high salt concentration, the parameter B may involve substantial steric or other interactions.

Dynamic tension behavior of aqueous octanol solutions under constant-area and pulsating-area conditions

Dynamic tension data for 0.3 and 0.6 mM aqueous octanol solutions at 25°C and constant-area conditions were obtained and compared with previous diffusion-controlled and mixed kinetics adsorption models. The results show that tension drops more slowly than predicted by the diffusion-controlled model. The modified Langmuir-Hinshelwood equation in a mixed kinetics model describes the data quite well. Moreover, the same solutions were examined with a pulsating bubble surfactometer at 10–80 cycles min−1, with the bubble radius oscillating between 0.40 and 0.55 mm. The dynamic tension oscillates between a tension maximum γmax > γe and a minimum γmin < γe, where γe is the equilibrium tension. The tension amplitude (γmax - γmin) increases with frequency, because the adsorption process is too slow to follow exactly the area changes. The amplitude decreases with increasing concentration from 0.3 to 3 mM. Phase lags between γ(t) and A(t) and low γmin can be predicted by convective-diffusion mass transfer models at spherical coordinates, or even by simple planar mass transfer models, without considering intrinsic surface rheology effects. Comparisons of pulsating-area tension data with the models indicate that intrinsic adsorption-desorption rates must be considered in the overall rate of adsorption. Certain discrepancies between the model and the data are attributed in part to measurement errors and in part to the use of the Frumkin equation of state for nonequilibrium surface densities. The results are relevant to foam generation and lung surfactants.

Techniques to measure dynamic surface tension

Recent developments of techniques used to measure dynamic surface tension aim at incorporating measurements during area changes, and improving speed of measurements, automation, and range of time scales (≤ 1 ms). New techniques incorporate better definition of surface age and of the flow field, before or during measurements. They point to truly dynamic measurements, where the transient interface shape is rigorously analyzed in terms of the flow, pressure, and concentration fields.

Adsorption dynamics of single and binary surfactants at the air/water interface

Abstract Models of diffusion-controlled adsorption of mixed surfactants at the air/water interface reveal a rich variety of interesting behavior. The models predict the following. A maximum in the dynamic adsorption concentration for the less surface-active surfactant can occur if this component has a higher initial bulk concentration. The maximum is even more pronounced for small diffusion lengths, at which the dynamic subsurface concentration of the fast-adsorbing component can temporarily exceed that of the bulk. If an impurity is more surface-active than the main component, then small amounts of the impurity can drastically alter the adsorption behavior of the latter, as shown in some examples. These phenomena can be partly explained in terms of the timescales of adsorption for single surfactants described by the Henrys law or Langmuir isotherms, and partly by the competition of the components for surface sites. Since dynamic surface concentration data are not yet available for binary surfactants, the model predictions are used to calculate dynamic surface tensions and compare them with dynamic tension data. The results have implications for the dynamic and equilibrium selectivities of foam fractionation processes and for the dynamics of free-surface flows.

Deformation and breakup of a stretching liquid bridge covered with an insoluble surfactant monolayer

The breakup of surfactant-laden drops and jets is of technological interest and fundamental scientific importance. Surfactants are routinely used to control the breakup of drops and jets in applications ranging from inkjet printing to crop spraying. Accurate computation of breakup of surfactant-laden drops and jets is often the key to the development of new applications and to providing a rational fundamental understanding of both existing and emerging applications. While highly accurate algorithms for studying the breakup of surfactant-free drops and jets are well documented and much is now known about the dynamics in such situations, little is known by contrast about the closely related problem of interface rupture when surfactant effects cannot be neglected. The deformation and breakup of a stretching liquid bridge of an incompressible Newtonian fluid whose surface is covered with an insoluble surfactant monolayer are analyzed here experimentally and computationally. In the experiments, high-speed visu...