Adsorption Kinetics Of Surfactants Research Articles

High-throughput evaluation of pulmonary surfactant adsorption and surface film formation

The assessment of new therapeutic strategies to cure surfactant-associated lung disorders would greatly benefit from assay systems allowing routine evaluations of surfactant functions. We present a method to measure surfactant adsorption kinetics into interfacial air-liquid interfaces based on fluorescence microplate readers. The principle of measurement is simple, robust, and reproducible: Wells of a microtiter plate contain an aqueous solution of a light-absorbing agent. Fluorescence is excited and collected from the top of the wells so that fluorescently labeled surfactant injected into the bulk can be detected only once adsorbed into the air-liquid interface. Mass transfer from the bulk to the interface is achieved by orbital shaking implemented in the plate reader instrument. The method has been tested and validated by using phospholipids or surfactants of different origins, by using albumin as surfactant inhibitor, and by comparison of results with Wilhelmy balance measurements. The method is suited for implementation in high-throughput screening routines for conditions affecting, or improving, surfactant film formation. In contrast to surface tension measurements, our method gives a direct readout of the amount of surfactant adsorbing into the interface, including the functionally important amount of material firmly associating with the interfacial film.

Experimental Studies on the Adsorption of Two Surfactants on Solid−Aqueous Interfaces: Adsorption Isotherms and Kinetics

A quartz crystal microbalance with dissipation (QCM-D) was used to measure the adsorption from aqueous solutions of CTAB (cationic) and C(12)E(6) (nonionic) surfactants on gold and silica surfaces. QCM-D allows for the determination of adsorption isotherms and also the monitoring of the dynamics of adsorption in real time. By considering the atomic-scale roughness of the solid surfaces and the surface area per head group at the air/water interface, our experiments indicate that at bulk concentrations above the critical micelle concentration adsorbed C(12)E(6) forms a monolayer-like structure on both surfaces and CTAB yields a bilayer-like structure. Although our measurements do not allow us to discriminate between the morphology of the aggregates (i.e., between flat monolayers, hemicylinders, or hemispheres in the case of C(12)E(6) and between flat bilayers, cylinders, or spheres in the case of CTAB), these results are particularly significant when compared to recent QCM-D data reported by Macakova et al. (Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 12436). These authors reported that QCM-D overestimates the amount of CTAB adsorbed on silica by as much as 30-40% as a result of entrapped water. Our analysis suggests that the effect of entrapped solvent is not as important as previously assumed and, in fact, QCM-D may not overestimate the amount of CTAB adsorbed when roughness is considered. Results for the kinetics of adsorption suggest that the aggregate structure as well as whether micelles are present may influence the adsorption mechanism. We discuss our results in the perspective of molecular theories for both the equilibrium and kinetics of surfactant adsorption.

A Theoretical Study on Surfactant Adsorption Kinetics: Effect of Bubble Shape on Dynamic Surface Tension

A planar or spherical fluid-liquid interface was commonly assumed on studying the surfactant adsorption kinetics for a pendant bubble in surfactant solutions. However, the shape of a pendant bubble deviates from a sphere unless the bubble's capillary constant is close to zero. Up to date, the literature has no report about the shape effect on the relaxation of surface tension due to the shape difference between a pendant bubble and a sphere. The dynamic surface tension (DST), based on the actual shape of a pendant bubble with a needle, of the diffusion-controlled process is simulated using a time-dependent finite element method in this work. The shape effect and the existence of a needle on DST are investigated. This numerical simulation resolves also the time-dependent bulk surfactant concentration. The depth of solution needed to satisfy the classical Ward-Tordai infinite-solution assumption was also studied. For a diffusion-controlled adsorption process, bubble shape and needle size are two major factors affecting the DST. The existence of a needle accelerates the bulk diffusion for a small bubble; however, the shape of a large pendant bubble decelerates the bulk diffusion. An example using this method on the DST data of C12E4 is illustrated at the end of this work.

Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: I. Cetyl Pyridinium Chloride on Hydrophilic Silica

A dual-inlet channel flow cell has been developed for the study of the adsorption kinetics of surfactants to solid−liquid interfaces under hydrodynamic control. This cell, with ellipsometric detection of the adsorbed surfactant, has been used to study the adsorption kinetics of cetyl pyridinium chloride (CPC) to hydrophilic silica in 0.1 M KCl and in pure water. The methodology provides detailed insight into the kinetic parameters and casts light on the adsorption mechanisms. The convection−diffusion behavior in the cell was calculated numerically using the backward implicit finite difference (BIFD) method: the CPC monomer and micelle populations were modeled with constant diffusion coefficients and assumed to equilibrate quickly on the time scale of the experiment. A Frumkin model was used to describe the adsorption behavior at the silica surface: the fitting parameters were determined from the equilibrium adsorption isotherm and kinetically limited desorption measurements. The adsorption kinetics in t...

Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: II. Cetyl Pyridinium Chloride on Methyl and Methyl Ether Surfaces

The adsorption of cetyl pyridinium chloride (CPC) in 0.1 M KCl to methyl- and methyl ether-terminated organic monolayers has been studied using a dual-inlet channel flow cell. The surfaces were formed by reaction of H-terminated Si(111) with dodecene and 11-methoxy-1-undecene, respectively. The equilibrium adsorption isotherm, the desorption kinetics, and the adsorption kinetics can be explained self-consistently on the basis of the Frumkin model of adsorption. The equilibrium adsorption isotherms on both surfaces were very close to Langmuirian, indicating that the driving force for adsorption remains roughly constant with coverage. Adsorption was in the mixed diffusion-kinetic regime under the mass transport conditions of the channel flow cell (adsorption times of the order of tens of seconds). Most of the difference in the adsorption energies of the two surfaces appears to be reflected in the adsorption rate constant rather than the desorption rate constant, suggesting a late transition state. Comparing the Frumkin model parameters for the methyl and methyl ether surfaces with those for hydrophilic silica helps clarify and quantify the difference between the surfaces. The nature of the surface has a very-significant effect on the equilibration time for the same initial surfactant distribution because the adsorption rate affects the subsurface concentration and consequently the flux of surfactant to the interface itself. The organic self-assembled monolayers (SAMs) formed by reaction of H-terminated Si(111) with alkenes make robust and versatile substrates for surfactant adsorption experiments. A combination of SAMs with the dual-inlet channel flow cell offers an opportunity for the systematic and quantitative study of the effect of surface chemistry on the adsorption kinetics of surfactants to organic surfaces.

Computer simulation of adsorption kinetics of surfactants on solid surfaces

Adsorption kinetics of surfactants on solid surfaces has been studied by using computer simulation. Both bulk surfactant concentration and diffusion region are explicitly integrated in our model. Depending on the head–surface interaction, our simulation results indicate that there exist two different kinetic modes in adsorption process of surfactants on solid surfaces. One is the four-regime mode and the other is step-wise mode. For the strongly attractive head–surface interaction, four distinct regimes of surfactant adsorption are found: a diffusion-controlled regime, a self-assembly controlled regime, an intermediate coverage regime and a saturated regime. In particular, the adsorption in second regime displays a power-law time dependence with an exponent unrelated to bulk concentrations and diffusion coefficients. While for the weaker adsorption surfaces, the step-wise mode is found. The mode includes a low-coverage nucleation regime and the saturated regime after a sudden aggregation of surfactants on the substrates which occurs stochastically. Besides the head–surface interaction, in this work, the effects of surfactant diffusivity, bulk concentration, the length of diffusion zone and surfactant architecture on the adsorption kinetics are also considered. The simulated adsorption kinetics is compared qualitatively with experimental results.

Adsorption Kinetics of Asymmetric Gemini Surfactants at Air/Water Interface

Asymmetric Gemini surfactants CmH2m+1OCH2CH(OH)CH2N+(CH3)2C8H17Br,referred to as CmOhpNC8(m=10,12 and 14),were synthesized.This type of surfactant molecule has a head region(a hydroxyl and a quaternary ammonium linked by one methylene group)with a small size and two alkyl chains with different lengths.The maximum bubble pressure technique was used to investigate the adsorption kinetics of CmOhpNC8 aqueous solution below their critical micelle concentrations at the air/water interface.The results showed an obvious kinetic process for the adsorption of CmOhpNC8.When CmOhpNC8 were adsorbed on the freshly formed interface,the adsorption process was pure diffusion controlled.When some molecules existed at the interface,the adsorption barrier(Ea)appeared,which reduced with increasing m.This indicated that those molecules with long alkyl chain migrated more easily from the subsurface into the surface,which was attributed to enhanced hydrophobic interaction between the alkyl chains with increasingm.

Adsorption Kinetics in Micellar Solutions of Nonionic Surfactants

Standard models of the adsorption kinetics of surfactants at the air-water surface assume that micelles break down into monomers in the bulk solution and that only monomers adsorb. We show here that micelles of the nonionic surfactant C14E8 adsorb to the surface of a liquid jet at a diffusion-controlled rate. Micellar adsorption can be switched off by incorporation of a small amount of ionic surfactant into the micelle and switched on again by addition of salt. More sophisticated models of adsorption processes in micellar solutions are required that permit a kinetic flux of micelles to the air-water interface.

Short-time behavior of mixed diffusion-barrier controlled adsorption

This paper focuses on the short-time adsorption kinetics of nonionic surfactants onto water/air surfaces, analyzed in the context of the mixed diffusion-barrier controlled adsorption modeling framework. Specifically, we reconcile the apparent contradiction between theoretical prediction and experimental observations on the adsorption kinetics mechanism at short times: while the mixed diffusion-barrier controlled model predicts a barrier-controlled adsorption, as well as the impossibility of a diffusion-controlled adsorption at asymptotic short times, the short-time experimental dynamic surface tension (DST) behavior of many nonionic surfactants has been interpreted to result from diffusion-controlled adsorption at asymptotic short times. This is because the short-time experimental DST of these surfactants displays a t variation, which is considered as a fingerprint for the existence of diffusion-controlled adsorption, based on the short-time asymptotic behavior of the diffusion-controlled adsorption model. As a result of this interpretation, the fundamental physical nature of the energy barrier has been proposed to be associated with high surfactant surface concentrations. In this paper, we derive a new nonasymptotic short-time formalism of the mixed diffusion-barrier controlled model to describe surfactant adsorption onto a spherical pendant-bubble surface, including determining the ranges of time and surfactant surface concentration values where the short-time formalism is applicable. Based on this formalism, we find that one can expect to observe an apparent t variation of the DST at short times even for the mixed diffusion-barrier controlled adsorption model. We analyze the consequence of this finding by re-evaluating the existing notions of the energy barrier. We conclude that the energy barrier is associated with the adsorption of a single surfactant molecule onto a clean surface.

Kinetics of nonionic surfactant adsorption at a fluid-fluid interface from a micellar solution

The kinetics of nonionic surfactant adsorption at a fluid-fluid interface from a micellar solution is considered theoretically. Our model takes into account the effect of micelle relaxation on the diffusion of the free surfactant molecules. It is shown that nonionic surfactants undergo either a diffusion or a kinetically limited adsorption according to the characteristic relaxation time of the micelles. This gives a new interpretation for the observed dynamical surface tension of micellar solutions.

Effect of Humidity on the Adsorption Kinetics of Lung Surfactant at Air−Water Interfaces

The in vitro adsorption kinetics of lung surfactant at air-water interfaces is affected by both the composition of the surfactant preparations and the conditions under which the assessment is conducted. Relevant experimental conditions are surfactant concentration, temperature, subphase pH, electrolyte concentration, humidity, and gas composition of the atmosphere exposed to the interface. The effect of humidity on the adsorption kinetics of a therapeutic lung surfactant preparation, bovine lipid extract surfactant (BLES), was studied by measuring the dynamic surface tension (DST). Axisymmetric drop shape analysis (ADSA) was used in conjunction with three different experimental methodologies, i.e., captive bubble (CB), pendant drop (PD), and constrained sessile drop (CSD), to measure the DST. The experimental results obtained from these three methodologies show that for 100% relative humidity (RH) at 37 degrees C the rate of adsorption of BLES at an air-water interface is substantially slower than for low humidity. It is also found that there is a difference in the rate of surface tension decrease measured from the PD and CB/CSD methods. These experimental results agree well with an adsorption model that considers the combined effects of entropic force, electrostatic interaction, and gravity. These findings have implications for the development and evaluation of new formulations for surfactant replacement therapy.

Effect of surface properties of activated carbons on surfactant adsorption kinetics

This research investigates the adsorption properties of three activated carbons (AC) derived from coconut, coal, and wood origin. A linear relationship exists between the number of water molecules adsorbed onto each AC and the oxygen content determined elemental analysis and XPS. An inverse linear relationship exists between the plateau amount of dodecanoic acid anionic surfactant and the oxygen content on the surface of ACs. The surface charge on each AC’s surface had a linear relationship with the plateau amount of dodecanoic acid. A plug-flow heterogeneous surface diffusion model (PFHSDM) for a fixed-bed adsorption process was developed to describe the adsorption kinetics in a fixed-bed column. The model represents axially dispersed plug-flow, external mass transfer, adsorption equilibrium on the fluid-particle interface, and intraparticle diffusion. The larger molecular dimension of the dodecanoic acid as a more hydrophobic entity than octanoic acid led to a faster external mass transfer rate but a slower surface diffusion rate as estimated from the PFHSDM. The interaction between the organic moiety of surfactant and the AC surface chemistry such as surface oxygen content and surface charge contributes to the adsorption performance in both to the adsorption equilibrium and kinetics.

Kinetics of Adsorption of Anionic, Cationic, and Nonionic Surfactants

The kinetics of adsorption of anionic, nonionic, and cationic surfactants on a cellulose-water interface were investigated. The effects of monovalent, divalent, and a mixture of mono- and divalent salts on the adsorption kinetics of anionic surfactant were also investigated. The cellulose surface is shown to have dual sites of hydrophobic and hydrophilic nature. It is shown that anionic and nonionic surfactant molecules mostly adsorb on the hydrophobic site while cationic surfactant molecules mostly adsorb on the hydrophilic site. A simple model based on two-site adsorption is developed to describe the kinetics of adsorption. The values of rate constants are determined by using the experimental data and are compared with those reported in the literature. The rate constants show that the rates of adsorption of sodium dodecylbenzenesulfonate (NaDBS) and polyethoxylated tert-octylphenol (TX-100) are same and cetyltrimethylammonium bromide is higher than NaDBS and TX-100 at the cellulose-water interface.

Mechanisms of Foam Destruction by Oil-Based Antifoams

Oils and mixtures of oils with hydrophobic particles are widely used in various technologies and consumer products to control foaminess and foam stability. The aim of this review is to summarize our current understanding of the mechanisms of foam destruction by such substances, which are usually called antifoams or defoamers. The experimental results show that two types of antifoam can be distinguished (called for brevity "fast" and "slow") which differ in the modes of their action. Fast antifoams are able to rupture the foam films at the early stages of film thinning. As a result, fast antifoams destroy completely the foam in less than a minute, in a typical foam-stability test. Microscopic observations have shown that the fast antifoams rupture the foam films by the so-called "bridging" mechanisms, which involve the formation of oil bridges between the two surfaces of the foam film. The stability/instability of these oil bridges is explained by using the theory of capillarity. In contrast, the oily globules of the slow antifoams are unable to enter the surfaces of the foam films and are first expelled into the Plateau borders (PBs). Only after being compressed by the narrowing walls of the PBs (due to water drainage from the foam), are the globules of the slow antifoams able to enter the solution surface and to destroy the adjacent foam films. Typically, the process of foam destruction by slow antifoams requires much longer time, minutes or tens of minutes, and a residual foam of well-defined height is observed in the foam tests. The experiments show that there is no direct relation between the magnitudes of the entry, E, spreading, S, and bridging, B, coefficients, on one side, and the antifoam efficiency, on the other side. The only requirement for having active antifoam, with respect to the bridging mechanisms, is that B should be positive. On the other hand, the barrier preventing the emergence of pre-emulsified antifoam globules on the solution surface (so-called "entry barrier") is of crucial importance for the mode of foam destruction and for the antifoam efficiency. Measurements of the entry barrier with recently developed film trapping technique (FTT) showed that antifoams possessing low entry barriers act as fast antifoams, whereas high barriers correspond to slow or inactive antifoams, although E, S, and B coefficients could be strongly positive in the latter case. A good agreement between the magnitude of the entry barrier, measured by FTT, and the height of the residual foam, in the presence of slow antifoams, was experimentally established and theoretically explained. The importance of various factors, such as the size of antifoam globules, oil spreading, kinetics of surfactant adsorption, hydrophobicity of solid particles in mixed oil-solid antifoams, and the presence of amphiphilic additives (foam boosters), is discussed from the viewpoint of the mechanisms of antifoaming. The main experimental methods, used for studying the modes of antifoam action, are briefly described.

Modeling of the Adsorption Kinetics of Surfactants at the Liquid−Fluid Interface of a Pendant Drop

This paper presents a theoretical model for simulating the adsorption kinetics of a surfactant at the liquid-fluid interface of a pendant drop. The diffusion equation is solved numerically by applying the semidiscrete Galerkin finite element method to obtain the time-dependent surfactant concentration distributions inside the pendant drop and inside the syringe needle that is used to form the pendant drop. With the obtained bulk surfactant concentration distributions, the adsorption at the interface is determined by using the conservation law of mass. It should be noted that the theoretical model developed in this study considers the actual geometry of the pendant drop, the depletion process of the surfactant inside the pendant drop, and the mass transfer of the surfactant from the syringe needle to the pendant drop. The present pendant-drop model is applied to study the adsorption kinetics of surfactant C10E8 (octaethylene glycol mono n-decyl ether) at the water-air interface of a pendant drop. The numerical results show that the Ward and Tordai equation, which was derived for adsorption from a semi-infinite surfactant solution to a planar interface, is unsuitable for interpreting the dynamic surface or interfacial tension data measured by using the pendant-drop-shape techniques, especially at low initial surfactant concentrations. The spherical-drop model, which assumes the pendant drop to be a perfectly spherical drop with the same drop volume, can be used to interpret the dynamic surface or interfacial tension data for pendant drops either with high initial surfactant concentrations or with low initial surfactant concentrations in short adsorption durations only. For pendant drops with low initial surfactant concentrations in long adsorption durations, the theoretical model developed in this study is strongly recommended.

Redox-Dependent Surface Tension and Surface Phase Transitions of a Ferrocenyl Surfactant: Equilibrium and Dynamic Analyses with Fluorescence Images

The surface tension data for the surfactant Fc−CH2−N+(CH3)2−(CH2)14CH3−Br- (where Fc represents the ferrocenium cation) display the signatures of a surface gaseous−surface liquid expanded transition with binodals that depend on the oxidization state of the Fc headgroup. The phase transition is imaged using fluorescence microscopy. The equilibrium and dynamic surface tension data (obtained by the pendant bubble method) are analyzed in terms of an equilibrium model that accounts for surface phase transitions and electrostatic interactions and a dynamic model that accounts for the mass transfer kinetics of surfactant adsorption to the interface. The dynamic surface tension of the reduced form of the surfactant is controlled by bulk diffusion to the interface. The dynamic surface tension of the oxidized form of the surfactant is diffusion-controlled at dilute concentrations, but not at more elevated concentrations where adsorption−desorption barriers are important. The kinetic constants are determined from th...

The influence of chain length and electrolyte on the adsorption kinetics of cationic surfactants at the silica–aqueous solution interface

The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica–aqueous solution interface have been investigated using optical reflectometry. The effect of added electrolyte, the length of the hydrocarbon chain, and of the counter- and co-ions has been elucidated. Increasing the length of the surfactant hydrocarbon chain results in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicate that above the cmc micelles are adsorbing directly to the surface and that as the chain length increases the hydrophobicity of the surfactant has a greater influence on the adsoption kinetics. While the addition of 10 mM KBr increases the CTAB maximal surface excess, there is no corresponding increase for the addition of 10 mM KCl to the CTAC system. This is attributed to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) have little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption is increased in the presence of electrolyte. Slow secondary adsorption is seen over a range of concentrations for CTAC in the absence of electrolyte and importantly in the presence of LiCl; the origin of this slow adsorption is attributed to a structural barrier to adsorption.

Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry

Dynamic interfacial tensiometry, gauged by axisymmetric drop shape analysis of static drops or bubbles, provides useful information on surfactant adsorption kinetics. However, the traditional pendant-drop methodology is not readily amenable to the study of desorption kinetics. Thus, the question of sorption reversibility is difficult to assess by this technique. We extend classical pendant/sessile drop dynamic tensiometry by immersing a sessile bubble in a continuously mixed optical cell. Ideal–mixed conditions are established by stirring and by constant flow through the cell. Aqueous surface-active-agent solutions are either supplied to the cell (loading) or removed from the cell by flushing with water (washout), thereby allowing study of both adsorption and desorption kinetics. Well-mixed conditions and elimination of any mass transfer resistance permit direct identification of sorption kinetic barriers to and from the external aqueous phase with time constants longer than the optical-cell residence time. The monodisperse nonionic surfactant ethoxy dodecyl alcohol (C 12E 5), along with cationic cetyltrimethyl ammonium bromide (CTAB) in the presence of added salt, adsorbs and desorbs instantaneously at the air/water interface. In these cases, the experimentally observed dynamic-tension curves follow the local-equilibrium model precisely for both loading and washout. Accordingly, these surfactants below their critical micelle concentrations (CMC) exhibit no detectable sorption-activation barriers on time scales of order a min. However, the sorption dynamics of dilute CTAB in the absence of electrolyte is markedly different from that in the presence of KBr. Here CTAB desorption occurs at local equilibrium, but the adsorption rate is kinetically limited, most likely due to an electrostatic barrier arising as the charged surfactant accumulates at the interface. The commercial, polydisperse nonionic surfactant ethoxy nonylphenol (NP9) loads in good agreement with local-equilibrium theory but shows deviation from the theoretical washout curve, presumably due to slow desorption of solubilized but otherwise water insoluble components. The polymeric nonionic triblock copolymer Pluronic exhibits almost complete irreversible adsorption at the air/water interface over a molecular-weight range from 3 to 14 kDa. Similar irreversible dynamic behavior is observed for adsorption/desorption of the protein bovine serum albumin (BSA) from dilute aqueous solutions at the air/water interface. The new continuous-flow tensiometer (CFT) is a simple, yet powerful, tool to investigate sorption dynamics at fluid/fluid interfaces, especially for larger molecular weight surface-active agents that exhibit significant hindrance to desorption.

Effect of additives on the adsorption of sodium bis(2‐ethyl hexyl sulfosuccinate) at the air‐water interface studied by equilibrium and dynamic surface tension measurements

AbstractUsing equilibrium and dynamic surface tension measurements, we have studied the effect of the addition of poly(sodium 4‐styrenesulfonate) (PSS), sodium chloride, and 1,4,7,10,13,16‐hexaoxyacyclooctadecane (18‐crown‐6) on the surface properties of sodium bis(2‐ethyl hexyl sulfosuccinate) (AOT). The addition of PSS or NaCl weakly increases the maximum packing of AOT, whereas the presence of 18‐crown‐6 slightly decreases the maximum surface coverage. The surfactant adsorption kinetics on the interface is a diffusion‐controlled process. The two asymptotic solutions at long times and at short times to the classic Ward Tordai equation were used to fit dynamic results. At long times there is evidence of the existence of an electrostatic barrier at high surfactant concentration when using pure AOT and AOT mixed with PSS. In binary mixtures of AOT with sodium chloride or 18‐crown‐6, the electrostatic barrier is not observed over the surfactant concentration range studied.

A Study on Surfactant Adsorption Kinetics: The Role of the Data of Equation of State γ(Γ) for C14E8

The adsorption kinetics of C14E8 at the air–water interface was investigated. A new approach to the determination of the adsorption isotherm and the evaluation of model parameters is proposed in this work. Besides the equilibrium surface tension γ(C), a second set of equilibrium data [equation of state γ(Γ)] was measured and used to determine the adsorption isotherm and model parameters. Dynamic surface tension γ(t) for the adsorption of C14E8 onto a freshly created interface was measured and used to determine the controlling mechanism and diffusivity. The data of the equation of state play an important role in this study. No data for C14E8 are available for γ(C)>62 mN/m because an extremely long time is needed for solutions at very dilute concentrations to reach their equilibrium tensions. Equilibrium data, γ(C) and γ(Γ), indicate that the adsorption of C14E8 is anti-cooperative. The adsorption of C14E8 onto a fresh interface is found to be diffusion-controlled. A diffusivity of 7.6×10−6 cm2/s results from the best fit between the entire relaxation data of γ(t) and the model predictions.