On the theory of adsorption kinetics of ionic sufactants at fluid interfaces 4. Deceleration of the adsorption rate due to non-equilibrium distribution of adsorbed ions in the diffuse layer

An approximate analytical solution is obtained for the adsorption kinetics equation derived earlier. On the basis of these relations the importance of the consideration of a non-equilibrium diffuse layer has been shown. To describe the retarded adsorption kinetics the distribution of adsorbed ions in the diffuse layer section of multivalent surfactant ions has been taken into account. The rate of adsorption calculated for a non-ionic surfactant is compared with the adsorption rate for monovalent and bivalent ionic surfactants, respectively.     

On the theory of adsorption kinetics of ionic surfactants at fluid interfaces 3. Generalization of the model

It was shown that, in the case of adsorbing ions, the Boltzmann equation cannot be applied in its classical form, but has to be modified by considering the flux of adsorbing ions. From the comparison with the adsorption of nonionic surfactants a ratio results which is the measure of deceleration of adsorption kinetics due to the electric double layerr=K(y _s)/(t). At highr-values the electrostatic deceleration controls the adsorption kinetics process.     

On the application of quantitative models of diffusion-controlled adsorption kinetics at fluid interfaces

The model of diffusion-controlled adsorption kinetics can be solved by different mathematical procedures. For the most important case of a Langmuir isotherm, useful solutions are summarized and extended and the ranges of application are given. New approximations are presented which can be applied to most experiments in practice.Nomenclature a _i coefficients of the collocation solution - a _L Langmuir parameter - a interaction parameter of the Frumkin isotherm - _n coefficients of a power series - c surfactant bulk concentration - c ₀ equilibrium bulk concentration - D diffusion coefficient - surface concentration - ₀ equilibrium surface concentration - _t8 maximum surface concentration - ( _x ) Gamma function - t time - dimensionless time Dedicated to the 60th birthday of G. Kretzschmar.     

Model description of specific adsorption of two anions on electrodes

Simultaneous specific adsorption of two anions on the electrode surface is considered. Calculations involve the use of the Frumkin adsorption isotherm for each anion and a model that describes the electric double layer structure based on one or two Helmholtz planes. Dependences of the ion adsorption on both the electrode potential and the solution composition are calculated for different electrode charges, ion sizes, and capacitance characteristics of the electric double layer.     

Relation between adsorption of some anionic surfactants on barite and solution/air interfaces

The adsorption behaviour of synthesized anionic surfactants with chemical structure RO–Ph–N?N–Ph–X, where R is octyl, dodecyl or cetyl and X is SO₃Na, was analysed. Analysis of the behaviour of the surfactants was made using a modified version of the Frumkin adsorption isotherm. The values of thermodynamic parameters were calculated at the solution/air interface. The relation between adsorption of the surfactants at the solution/air interface and solid/liquid interface was investigated. The surface properties of these synthetic surfactants were studied. The results show that the length of the hydrocarbon chain of these surfactants plays a major role in determining the surface and thermodynamic properties. The results also indicate that there is a good relationship between effectiveness of adsorption of the surfactant and its efficiency as a collector. Copyright © 2003 John Wiley & Sons, Ltd.     

Desorption kinetics of surfactants at fluid interfaces by novel coaxial capillary pendant drop experiments

Surfactants appear in multiphase fluid systems in which the interface and the adjacent bulk phase have been removed from equilibrium. Here, a new method is described for the measurement of rate constants of desorption of surface-active materials from fluid/fluid interfaces and the extent to which adsorption is reversible: the coaxial capillary pendant drop experimental technique.

Kinetic constants are determined by desorption experiments in pendant drops in which the interface adjacent to a surfactant solution is removed from equilibrium by replacing the subphase of the drop with pure water. Further, we demonstrate that although the rate of subphase exchange is comparatively slow with respect to the desorption timescale, it is possible to resolve desorption processes which occur under local equilibrium with the adjacent bulk phase from those that are determined in part by sorption kinetics. Experiments which measure the desorption kinetic coefficient, , using a homologous series of n-alkyl (C₈, C₁₀, C₁₂, C₁₄) dimethyl phosphine oxides are presented.     

Adsorption kinetics of nonionic surfactants using the drop volume method

The kinetic results obtained for the nonionic surfactantsn-octyl,n-decyl, andn-dodecyl dimethyl andn-octyl, andn-decyl diethyl phosphine oxide show purely diffusion controlled adsorption. The drop volume technique applied in a static and dynamic version proves to be useful to measure the adsorption kinetics in the form of surface tensions in function of time. Comparisons of the results obtained from both the static and the dynamic measuring procedure confirm the validity of a theory applied to interpret the kinetic data.Nomenclature a Langmuir parameter - c ₀ surfactant bulk concentration - D diffusion coefficient - surface concentration - ₀ equilibrium surface concentration - ¯ (t)/ ₀ reduced surface concentration - maximum value of - R gas law constant - surface tension - ₀ surface tension of pure water - t time - T absolute temperature     

Structure of adsorption layers of ionic surfactants at the solid/liquid interface

A large number of experimental results of different surfactant adsorption systems (mainly on the silicas) obtained from both equilibrium and kinetic studies under different conditions are interpreted by a model of small individual surface aggregates. The adsorption model is contrasted with the influences of various factors, including electrostatic interaction, hydrophobic interaction, concentrations, types of coions, types of counterions, surfactant structure, alkyl chain length, types of head groups, neutral electrolytes, pH, adsorbent structure, porosity, surface charge density, and surface polarity.Dedicated to Frau Professor Dr. Elsa Ullmann on the occasion of her 80th birthday     

Role played by the head size of some non‐ionic surfactants on their adsorption onto montmorillonite clay

Sorption behavior of polyoxethylene(n)monooleate series [Ol(EO)_n] onto montmorillonite clay was studied at 25°C to investigate the influence of the surfactant's head size on the sorption process. All the tested surfactants exhibited L‐shaped isotherms that means a strong interaction between the adsorbent and the adsorbate. Also, all the obtained isotherms ended with a drastic increase in the isotherm slope at nearly constant equilibrium concentration. This abrupt increase reflected the fairly high affinity of the tested surfactants to the clay surface at high bulk concentration region. The maximum amount adsorbed at the plateau region, Γ_max, was calculated according to the Langmuir adsorption theory and followed the order: Ol(EO)₁₄ > Ol(EO)₂₀ < Ol(EO)₄₀ < Ol(EO)₈₀. In case of short ethylene oxide (EO) chain, Γ_max decreased with the increase in the chain length; but a reverse result was obtained in case of surfactants with longer EO chain length (20 to 80 units). The free energy of adsorption, ΔG°_ad, had negative values indicating the spontaneous adsorption of surfactant molecules onto clay. The values of ΔG°_ad increased with increasing EO units from 14 to 20 units and decreased with further elongation in the EO chain from 20 to 80 units. Copyright © 2004 John Wiley & Sons, Ltd.     

Binding characteristics of ionic surfactants with human hair

Binding isotherms of two types of ionic surfactants, C_mH_{2m +1}SO₄Na (m = 8,10,12) and C_nH_{2n + 1}N⁺(CH₃)₃C1 (n=10, 12), to human hair in aqueous solutions were examined to clarify effects of hydrophobic and electrostatic interaction of ionic surfactants with hair. The binding isotherms of anionic surfactants showed cooperativity with discontinuously increasing shapes, while the binding isotherms of cationic surfactants showed a Langmuir-type, regardless of the difference of a hair condition.The calculated free energy change (— G@#@) for binding, obtained from Klotz' plots, suggests that the binding processes are governed mainly by a hydrophobic interaction, and bound surfactants probably expose their alkyl chains to the aqueous phase, since no-G was observed with the increase of m or n and values of enthalpy change(H) were positive or zero.     

Multilayer adsorption with variable thickness at solid-liquid interfaces

 The multilayer adsorption on the solid/liquid interface in binary mixtures was studied by adsorption space filling with constant and variable layer thickness. Adsorption from benzene/n-heptane mixtures was examined on hydrophilic and hydro-phobic surfaces. The free enthalpy of adsorption, Δ₂₁ G=f (x ₁), was calculated from the adsorption excess isotherm by integration of the Gibbs equation. Supposing that the free enthalpy is mainly due to adsorption in the first layer, the composition of this layer can be calculated from the Δ₂₁ G=f (x ₁) function. It was established that the adsorption layer thickness in benzene/heptane mixtures increases significantly with increasing benzene content. This statement was supported by X-ray diffraction on hydrophobic clay minerals. Received: 2 April 1997 Accepted: 10 June 1997     

On the importance of the purity of surfactant solutions in determining their adsorption kinetics

By means of a theoretical experiment it is demonstrated, that a barrier in the adsorption kinetics of a surfactant can be simulated by the presence of surface-active impurities. Hence, the application of models for the diffusion-kinetic-controlled and/or kinetic controlled adsorption presumes the proof of the surface chemical purity of the surfactant solution under investigation.     

Modeling of specific adsorption of ions at electrodes during the passing from a mixed solution of constant ionic strength to a binary solution

Electrocapillary curves in the mc NaNO₃ + (1 ? m)c NaF mixed solutions are calculated in terms of the Alekseev-Popov-Kolotyrkin model. The analysis of limiting case m = 1 shows that in a NaNO₃ binary solution the model predicts quadratic dependence of the NO ₃ ^? anion adsorption energy on the potential, which contradicts to experimental data. A modification of the model is suggested, which allowed removing this contradiction.     

Synthesis and solution properties of ionic liquid-type polysiloxane bola surfactants

Three novel ionic liquid (IL)-type polysiloxane bola surfactants (ATPS-MA, ATPS-EA, and ATPS-PA) were designed and synthesized using a two-step method. Their chemical structures were characterized by Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (¹H NMR) spectroscopy. Their surface activity and aggregation behavior in aqueous solution were systematically investigated by surface tension measurements, transmission electron microscopy (TEM), and dynamic light scattering (DLS). Surface activity measurement results indicated that the γ_CMC of the three IL-type polysiloxane surfactants are under 25 mN m^?1, and much lower than those of conventional IL hydrocarbon bola surfactants due to the introduction of siloxane group at the end of the hydrophobic chains. TEM and DLS analyses results indicated that the three surfactants can self-assemble into spherical micelles with a range from 50 to 300?nm, indicating potential uses as model systems for biomembranes and vehicles for drug delivery.     

Electric double layer theory for room temperature ionic liquids on charged electrodes: Milestones and prospects

In this review, we shortly summarize the basic theoretical milestones achieved in the mean-field theory of room temperature ionic liquids on charged electrodes since the publication of Kornyshev's seminal article in 2007. We pay special attention to the behavior of the differential capacitance profile and the microscopic parameters of ions that can have a substantial influence on it. Among them are parameters of short-range specific interactions, ionic diameters, static polarizabilities, and permanent dipole moments. We also discuss the recent ‘nonlocal’ mean-field theories that can describe the overscreening behavior of the local ionic concentrations, as well as the crossover from overscreening to crowding.     

On the solution of diffusion controlled adsorption kinetics for any adsorption isotherms

Summary The problem of diffusion controlled adsorption kinetics is solved by application of an implicite difference method. Thereby all forms of adsorption isotherms can be used in the calculations. Computation examples are given concerning Langmuir, Freundlich, Volmer, Frumkin, and van der Waals isotherms.

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Zusammenfassung Es wird das Problem der diffusions-kontrollierten Adsorptionskinetik durch Anwendung eines impliziten Differenzenverfahrens gelöst. Dabei können bei der Berechnung alle Formen von Adsorptionsisothermen zugrunde gelegt werden. Rechenbeispiele für die Langmuir-, Freundlich-, Volmer-, Frumkin- und van-der-Waals-Isotherme werden angegeben.

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Nomenclature c concentration - c _o bulk concentration - D diffusion coefficient - x distance from the surface - t time - surface concentration - ₀ equilibrium surface concentration - saturation surface concentration - k _o/c _o - K ₁,K ₂ parameters of Henry and Freundlich isotherm with the dimension of a length - n parameter of Freundlich isotherm - a ₁,a ₂ interaction parameters - B ₁,B ₂,B ₃,B ₄ parameters of different isotherms with the dimension of a concentration - R gas law constant - T absolute temperature dimensionless parameters C c/c _o - / _o - X x/k - X step size ofX - Dt/k ² - step size of - _e time at - 0.95 Zentralinstitut für Organische Chemie,Bereich Grenzflhenaktive Stoffe, der Akademie der Wissenschaften der DDR, Berlin-AdlershofWith 6 figures and 1 table     

Adsorption kinetics measurements of some nonionic surfactants

Dynamic surface tension values of aqueous surfactant solutions were measured by using the ring and plate method. The mean diffusion coefficients calculated on the basis of the purely diffusion controlled adsorption model vary between 2 · 10^–6 to 7 · 10^–6 cm²/s for all surfactants studied:n-alkanols,n-alkanoic acids, dimethyl and diethyln-alkyl phosphine oxides. That means the surfactants investigated adsorb with a purely diffusion controlled adsorption mechanism and no barriers excist to hinder sorption processes.Nomenclature c ₀ surfactant bulk concentration - D diffusion coefficient - surface concentration - ₀ equilibrium surface concentration - ¯ (t)/ ₀ reduced surface concentration - maximum surface concentration - K ₀/c₀ - surface tension - t time - Dt/K ² reduced time - a _L coefficient of the Langmuir isotherm