Diffusion-controlled adsorption kinetics at the air/solution interface

 To describe diffusion-controlled adsorption, the diffusion equation is solved under different initial and boundary conditions by means of a Laplace transformation. By solving this equation, it has been found that the solution, which Ward and Tordai used, is only applicable for x>0; therefore, it is incorrect if the derivation is made at x = 0. Ward and Tordai did not notice this and the first derivation was made at x = 0 in order to get the dynamic surface adsorption, Γ(t). In this paper, an accurate solution, which is applicable for x≥ 0, is given and the expression for Γ(t) is obtained. Furthermore the relationship between the dynamic surface tension and Γ(t) is derived. As an example, the dynamic surface tensions of an aqueous octyl-β-d-glucopyranosid solution were measured by means of the maximum bubble pressure method. By using the derived theory it has been proved that the controlling mechanism of the adsorption process of this surfactant at the long-time-adsorption limits changes as a function of the bulk concentration; only at dilute concentration is it controlled by diffusion. Received: 26 July 1999/Accepted in revised form: 16 September 1999     

Ellipsometric study of surfactant adsorption at the aqueous solution/air interface

Summary Ellipsometry has been applied to study the adsorption of sodium dodecylsulfate (NaDS) at the air/solution interface of the surfactant in water and aqueous sodium chloride. Results are expressed by the ellipticitye and the anglea which the major axis of the ellipse forms with the plane of incidence of the light. The ellipticity is found to change its sign at low NaDS concentrations and to pass a maximum somewhat below the cmc. Below the maximum the increment in ellipticity Aee is a linear function of the surface excess concentration dodecylsulfate. The slope e/gd this linear relation is found to decrease when inert electrolyte (NaCl) is added. The azimuth anglea increases slightly with NaDS concentration near and above the cmc. The results are discussed in terms of the Drude theory.

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Zusammenfassung Die Adsorption von Natrium Dodecylsulfat (NaDS) an der Oberfläche wäßßriger Lösungen wurde mit Hilfe eines Ellipsometers untersucht. Die Meßergebnisse werden durch die Elliptizität e und den Winkel a zwischen der Hauptachse der Ellipse und der Einfallsebene des Lichtstrahls ausgedrü ckt. Die Elliptizität wechselt ihr Vorzeichen im Bereich geringer Konzentrationen von NaDS und läuft durch ein Maximum etwas unterhalb der cmc des Tensids. Unterhalb des Maximums wird eine lineare Beziehung zwischen dem Inkrement der Elliptizität e und der Oberflächen-Überschußkonzentration von Dodecylsulfat gefunden. Durch Zugabe eines inerten Elektrolyten (NaC1) wird die Steigung e/ stark verringert. Der Azimuth-Winkel a nimmt im Bereich der cmc des Tensids schwach zu. Die Ergebnisse werden im Rahmen der Drude-Theorie diskutiert.

     

Hofmeister anion effect on aqueous phase behavior of heptaethylene glycol dodecyl ether

The aqueous phase behavior of heptaethylene glycol dodecyl ether (C12E7) was investigated in the presence of sodium salts of Cl-, I-, and ClO4-. Pseudo binary T-X phase diagrams were constructed for these mixtures by means of differential scanning calorimetry. The salting-out electrolyte NaCl expanded the Lalpha region toward higher temperatures and shrank the H1 region toward lower temperatures compared with the salt-free system. On the contrary, the salting-in electrolytes NaI and NaClO4 induced shrinkage of the Lalpha region and an expansion of the H1 phase. The influence of these salts on the mesophase regions was more pronounced for the Lalpha phase than for the H1 phase, and area of the Lalpha phase region decreased in the sequence of NaCl > none > NaI > NaClO4, consist with the Hofmeister series of the anions. This salt effect on the mesophase stability in aqueous nonionic surfactant mixture would be qualitatively interpreted in terms of the salt effect on the hydration of the polyoxyethylene chain in the surfactant molecules.     

Dynamic surface tension and adsorption kinetics of beta-casein at the solution/air interface

A diffusion model is proposed to describe the adsorption kinetics of proteins at a liquid interface. The model is based on the simultaneous solution of the Ward-Tordai equation and a set of recently developed equations describing the equilibrium state of the adsorption layer: the adsorption isotherm, the surface layer equation of state, and the function of adsorption distribution over the states with different molar areas. The new kinetics model is compared with dynamic surface tensions of beta-casein solutions measured with the drop/bubble profile and maximum bubble pressure methods. The adsorption process for low concentrations is governed by the diffusion mechanism, while at large protein concentrations this is only the case in the initial stage. The effective diffusion coefficients agree fairly well with literature data. The adsorption values calculated from the dynamic surface tension data agree very well with the used equilibrium adsorption model.     

Competitive adsorption of fibrinogen and dipalmitoylphosphatidylcholine at the air/aqueous interface

The competitive adsorption of fibrinogen (FB) and DPPC at the air/aqueous interface, in phosphate buffer saline at 25 degrees C, was studied with tensiometry, infrared reflection absorption spectroscopy (IRRAS), and ellipsometry. For FB/DPPC mixtures with 750 ppm (0.075 wt%) FB and 1000 ppm (0.10 wt%) DPPC, the tension behavior was found to be similar to that of FB when alone, even with DPPC and FB being at the interface. Thus, FB interferes with adsorption of DPPC and inhibits its surface tension lowering ability. When FB protein is introduced in the solution after a DPPC monolayer has formed, the adsorption of FB is inhibited by the DPPC monolayer. When a DPPC monolayer is spread onto a solution with a preadsorbed FB layer, the DPPC monolayer excludes FB from the surface and controls the tension behavior with little inhibition by FB. When a DPPC dispersion is introduced with the Trurnit method, or sprayed dropwise, onto an aqueous FB/DPPC surfaces, the DPPC layer formed on the surface prevents the adsorption of FB and dominates the surface tension behavior. These results have implications in controlling the inhibition of lung surfactant tension behavior by serum proteins, when they leak at the alveolar lining layer, and in developing surfactant replacement therapies for alveolar respiratory diseases.     

Surface mixed adsorption films of 2,4,6- trimethylphenol–2,4,6-trichlorophenol at aqueous solution/air interface

 The results of experimental studies of the adsorption at the solution/air interface from an aqueous mixture: 2,4,6-trimethylphenol–2,4,6-trichlorophenol are presented. The surface properties of the above-mentioned mixture were studied by surface potential and surface tension measurements. These measurements were carried out as a function of the concentration of 2,4,6-trimethylphenol aqueous solution at a constant concentration of 2,4,6-trichlorophenol. Using the results obtained and based on the Gibbs equation, Helmholtz formula and Motomura’s method the relative surface excesses of adsorbed substances, effective dipole moments, surface molar fractions of solutes and miscibility of adsorbed films were determined. Received: 7 November 1997 Accepted: 26 February 1998     

Interaction between anionic and cationic gemini surfactants at air/water interface and in aqueous bulk solution

The mixture of the anionic O,O′-bis(sodium 2-lauricate)-p-benzenediol (C₁₁pPHCNa) and cationic (oligoona)alkanediyl-α, ω-bis(dimethyldodecylammonium bromide) (C₁₂-2-E_x-C₁₂·2Br) gemini surfactants has been investigated by surface tension and pyrene fluorescence. The results show that the surface tension γ drops faster with total surfactant concentration C_T for α₁ = 0.1 or 0.3 than for α₁ = 0.7 or 0.9, where α₁ is the mole fraction of C₁₁pPHCNa in the bulk solution on a surfactant-only basis. The fast drop in γ for α₁ < 0.5 indicates strong adsorption at the air/water interface owing to the interaction between oppositely charged components, resulting in the formation of the adsorption double layers in the subsurface. The slow descent in γ for α₁ > 0.5 is attributed to the pre-aggregation in the solution before the critical micelle concentration cmc. A possible mechanism is proposed.     

Influence of the polyelectrolyte poly(ethyleneimine) on the adsorption of surfactant mixtures of sodium dodecyl sulfate and monododecyl hexaethylene glycol at the air-solution interface

The polyelectrolyte poly(ethylenenimine), PEI, is shown to strongly influence the adsorption of the anionic-nonionic surfactant mixture of sodium dodecyl sulfate, SDS, and monododecyl hexaethylene glycol, C(12)E(6), at the air-solution interface. In the presence of PEI, the partitioning of the mixed surfactants to the interface is highly pH-dependent. The adsorption is more strongly biased to the SDS as the pH increases, as the PEI becomes a weaker polyelectrolyte. At surfactant concentrations >10(-4) M, the strong interaction and adsorption result in multilayer formation at the interface, and this covers a more extensive range of surfactant concentrations at higher pH values. The results are consistent with a strong interaction between SDS and PEI at the surface that is not predominantly electrostatic in origin. It provides an attractive route to selectively manipulate the adsorption and composition of surfactant mixtures at interfaces.     

On the adsorption kinetics of surface-chemically pure n-dodecanoic acid at the air/water interface

The dynamic surface tension data for n-dodecanoic acid in 0.005 M hydrochloric acid, for as-received as well as for surface-chemically pure solutions, show the presence of a prolonged induction period, clearly indicating that the adsorption of this nonionic surfactant is not simply diffusion-controlled. A kinetic model for the reversible formation of monolayer islands, long known in the field of electrochemistry, is shown to also apply to the adsorption of n-dodecanoic acid at the air/water interface. The rate constant increases linearly with increasing bulk concentration, while the induction time decreases exponentially. The phenomenon of nucleation at the air/water interface is consistent with the direct experimental observation of the formation of solid-like patches as the interfacial region is drastically compressed.     

Phase diagram of the liquid crystal system of water-decanol-lanthanum nitrate-decaethylene glycol monododecyl ether

The phase behavior of a self-organizing system based on the nonionic surfactant of decaethylene glycol monodecyl ether-lanthanum nitrate hexahydrate-water and decanol is presented. A system phase diagram was constructed. The concentration, temperature ranges of existence and types of liquid crystal phases were established. Analysis of structural changes during the phase transitions hexagonal phase-lamellar phase-isotropic liquid was made on the basis of data obtained by viscosimetry and self-diffusion NMR.     

The influence of pH on phosphatidylcholine monolayer at the air/aqueous solution interface

The measurements of the interfacial tension at the air/aqueous subphase interface as the function of pH were performed. The interfacial tension of the air–aqueous subphase interface was divided into contributions of individuals. A simple model of the influence of pH on the phosphatidylcholine monolayer at the air/hydrophobic chains of phosphatidylcholine is presented. The contributions of additive phosphatidylcholine forms (both interfacial tension values and molecular area values) depend on pH. The interfacial tension values and the molecular areas values for LH⁺, LOH⁻ forms of phosphatidylcholine were calculated. The assumed model was verified experimentally.     

Beta-casein bilayer adsorption at the solution/air interface: experimental evidences and theoretical description

Ellipsometric and surface pressure studies of beta-casein adsorption layers at the water/air interface support the idea of a model that assumes the formation of a second layer adjacent to the primary adsorption layer. A thermodynamic model describes the concentration behavior of the surface pressure and the adsorbed amount with one and the same set of model parameters over the entire concentration range studied.     

Adsorption of TX100 and TX165 at the aqueous solution–air interface: free enthalpy of adsorption and equation of state

Surface tension measurements of aqueous solutions of TX100 and TX165 were made at different temperatures. The thermodynamic parameters of the adsorption calculated from the surface tension measurements, using several methods, have given slightly different values. The equation of state for the TX100 and TX165 monolayers at the solution–air interface was analysed. There is excellent agreement between the modified Volmer equation of state and the experimental π–A isotherms. Received: 13 July 1999/Accepted in revised form: 20 October 1999     

Kinetics of phase separation and coarsening in dilute surfactant pentaethylene glycol monododecyl ether solutions

We investigated the phase separation phenomena in dilute surfactant pentaethylene glycol monodedecyl ether (C(12)E(5)) solutions focusing on the growth law of separated domains. The solutions confined between two glass plates were found to exhibit the phase inversion, characteristic of the viscoelastic phase separation; the majority phase (water-rich phase) nucleated as droplets and the minority phase (micelle-rich phase) formed a network temporarily, then they collapsed into an usual sea-island pattern where minority phase formed islands. We found from the real-space microscopic imaging that the dynamic scaling hypothesis did not hold throughout the coarsening process. The power law growth of the domains with the exponent close to 1/3 was observed even though the coarsening was induced mainly by hydrodynamic flow, which was explained by Darcy's law of laminar flow.     

Adsorption kinetics at air/solution interface studied by maximum bubble pressure method

A general dynamic surface adsorption equation (t) for maximum bubble pressure method was derived by solving Ficks diffusion equation for the bubbles under different initial and boundary conditions. Different from the planar surface adsorption(Ward-Tordai equation), the derived dynamic surface adsorption (t) for the short time consists of two terms, one of them reflects the geometric effect caused by the spherical bubble surface. This kind of effect was discussed.The equilibrium surface tension _eq and the dynamic surface tension (t) of aqueous C₁₀E₈ (CH₃(CH₂)₉(OCH₂CH₂)₈OH) solution at temperature 25 °C were measured by means of Wilhelmy plate method and maximal bubble pressure method respectively. In the region of t0 (short time limits) a good agreement of experimental results with the theory was reached and the adsorption was controlled by diffusion. However, for the long time limits, a mixed diffusion-kinetics controlled process was proved.     

Kinetics of the desorption of surfactants and proteins from adsorption layers at the solution/air interface

It is shown by experiments that the DeSNa desorption kinetics is governed by a pure diffusion mechanism, while the desorption of more surface active surfactants such as C13DMPO and Triton X-100 obeys a mixed mechanism. The BLG desorption kinetics, as shown by experiments, is determined by a barrier mechanism. From the analysis of the temperature dependence of the BLG desorption kinetics it is possible to calculate the activation energy of this process, which is quite close to the free energy of BLG adsorption. The theoretical model of desorption kinetics predicts that these two energetic parameters are approximately equal to each other if the adsorption activation energy is low. This can explain the fact that the higher the adsorption activity of a substance is, the lower is its desorption rate.     

Effect of sucrose on the properties of caffeine adsorption layers at the air/solution interface

Sweet and bitter tastes are known to be mediated by G-protein-coupled receptors. The relationship between the chemical structure of gustable molecules and their molecular organization in saliva (aqueous solution) near the surface of the tongue provides a useful tool for elucidating the mechanism of chemoreception. The interactions between stimulus and membrane receptors occur in an anisotropic system. They might be influenced by the molecular packing of gustable molecules within an aqueous solvent (saliva) close to the receptor protein. To investigate the molecular organization of a sweet molecule (sucrose), a bitter molecule (caffeine), and their mixture in an aqueous phase near a "wall", a hydrophobic phase, we modeled this using an air/liquid interface as an anisotropic system. The experimental (tensiometry and ellipsometry) data unambiguously show that caffeine molecules form an adsorption layer, whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine molecules at the air/water interface gradually increases with the volume concentration and is delayed when sucrose is added to the solution. Spectroscopic ellipsometry data show that caffeine in the adsorption layer has optical properties practically identical to those of the molecule in solution. The results are interpreted in terms of molecular association of caffeine with itself at the interface with and without sucrose in the subphase, using the theory of ideal gases.     

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.