Adsorption and surface tension of ionic surfactants at the air–water interface: review and evaluation of equilibrium models (original) (raw)
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2005
In this work we describe a simple thermodynamic method for determination of the adsorption (amount per unit area) of ionic surfactant. The latter is obtained from the interfacial tension isotherm measured in the presence of arbitrary (and fixed) concentration of inorganic electrolyte. Polynomial fit of the interfacial tension versus the logarithm of the mean ionic activity is combined with the Gibbs adsorption equation written in a form suitable for arbitrary content of salt. This procedure is an extension of the approach of Rehfeld [J. Phys. Chem. 71 (1967) 738]. We have performed measurements with sodium dodecyl sulfate (SDS) on water/air and water/oil (n-hexadecane and Soybean Oil) boundaries, at different salt concentrations (10 and 150 mM NaCl). Wilhelmy plate method was used for the water/air measurements; for water/oil we applied drop shape analysis with pendant drops. The obtained isotherms, together with literature data, are processed and the adsorption is determined. The results are compared and discussed in view of the role of the salt and the type of the hydrophobic phase. On oil/water boundaries the adsorption is always lower than that on air/water surface; addition of inert electrolyte increases the adsorption. We analyze theoretically the asymptotic behavior of the adsorption as a function of the solute concentrations in the limit of high surface coverage. The treatment is based on models existing in the literature (Langmuir isotherm with account for the counterion binding, as formulated by Kalinin and Radke [Colloids Surf. A 114 (1996) 337]; the activity coefficients were taken into consideration in the frames of the Debye-Hückel theory). The obtained asymptotic functional dependence of the adsorption is used for fitting of data. The agreement is always good, in the concentration region below and near the critical micellization concentration (CMC). From the fits we determine the limiting adsorption at maximum coverage (i.e., at saturation); therefrom, the degree of coverage of the interface with surfactant is estimated. It turns out that at the CMC the coverage is lower than about 90%. Thus, we confirm literature results for absence of saturation with ionic surfactants at the CMC. The dependence of the surface coverage upon the mean ionic activity is rather insensitive toward the type of the fluid interface (air/water, oil/water with different hydrocarbons), and the salt concentration.
Adsorption of Ionic Surfactants at the Air−Solution Interface
Langmuir, 2000
Neutron reflection (NR) and surface tension methods were compared for accessing the equilibrium adsorption isotherm of various ionic surfactants at the air-water interface. Four custom-synthesized anionics were investigated in detail: sodium dihexyl sulfosuccinate (di-C6SS), bis(1H,1H-perfluoro-npentyl)sodium sulfosuccinate (di-CF4), bis(1H,1H,5H-octafluoro-n-pentyl)sodium sulfosuccinate (di-HCF4), and bis(1H,1H,5H-octafluoropentyl)-2-sulfoglutaconate (di-HCF4GLU). Commercial n-alkyltrimethylammonium bromide (Cn-TAB) cationic surfactants with C12, C14, and C16 chain lengths were also studied. The experiments examined the validity of the Gibbs equation, and the prefactor 2, for these seven compounds. Effects of contaminants, trace levels of polyvalent metal ions, and hydrophobic impurities were assessed for the anionics. When added at low levels, tetrasodium ethylenediaminetetraacetate (EDTA) was effective for eliminating effects of metallic impurities, and foam fractionation was used to remove hydrophobic contaminants. The effects of these treatments on the apparent surface excess and procedures for obtaining agreement between the neutronic and tensiometric isotherms are described. Finally, it was confirmed that the Gibbs prefactor of 2 applies for all these 1:1 ionic surfactants.
Physical chemistry chemical physics : PCCP, 2014
In the present paper, we aim to investigate the dependence of surface tension on the surface properties and reveal the counter-ion effects on the adsorption of ionic surfactants on the solution surface. The surface tension, surface excess and surface concentration (defined as the amount of surfactant adsorbed in the surface phase divided by the surface area) of two anionic surfactants, namely dodecyl sulfate sodium and dodecyl sulfate caesium, dissolved in non-aqueous polar solvent formamide have been separately measured at 6 °C through independent experiments. Then, the correlation of surface tension with surface concentration and that of surface tension with surface excess is inspected in detail. It was found that there is a linear relationship between the surface tension and the surface concentration for the pure solutions of each surfactant, but their surface tension and surface excess cannot be correlated linearly. It is striking that the same surface tension-surface concentrat...
Adsorption and structure of the adsorbed layer of ionic surfactants
Advances in Colloid and Interface Science, 2006
Our goal in this study was to investigate theoretically and experimentally the adsorption of ionic surfactants and the role of different factors in the mechanism of adsorption, the adsorption parameters and the structure of the adsorbed layer. We used available literature data for the interfacial tension, σ, vs. concentration, C s , for sodium dodecyl sulfate (SDS) in three representative systems with Air/Water (A/W), Oil/Water (O/W) and Oil/Water + 0.1 M NaCl (O/WE) interfaces. We derived 6 new adsorption isotherms and 6 new equations of state (EOS) based on the adsorption isotherms for non-ionic surfactants of Langmuir, Volmer and Helfand-Frisch-Lebowitz (HFL) with interaction term βθ 2 / 2 in the EOS, θ = αΓ being the degree of coverage, with Γadsorption and αminimum area per molecule. We applied Gouy equation for high surface potentials and modified it to account for partial penetration of the counterions in the adsorbed layer. The equations were written in terms of the effective concentration C = [C s (C s + C el)] 1 / 2 , where C s and C el are, respectively concentrations of the surfactant and the electrolyte. We showed that the adsorption constant K was model independent and derived an equation for the effective thickness of the adsorbed layer, δ s. We found also that the minimum area per molecule, α, is larger than the true area, α 0 , which depends on the adsorption model and is a function of the adsorption Γ. The interaction term βθ 2 / 2 in the Langmuir EOS was found to be exact for small β ≪ 1, but for the Volmer EOS it turned out to be only a crude approximation. Semi-quantitative considerations about the interaction between adsorbed discrete charges revealed that at A/W interface part of the adsorbed surfactant molecules are partially immersed in water, which leads to decreased repulsion and increased adsorption Γ. At O/W the larger adsorption energy keeps the surfactant molecules on the surface, so that the electrostatic repulsion is stronger, which translates into negative β's, larger α's and smaller adsorption. The addition of electrolyte partly screens the repulsion at O/W, leading to decreased α and increased adsorption. We determined K, α and β by a three-parameter fit. The constant K was found to be model independent and smaller for A/W than for O/W, because of the smaller adsorption energy. The values of α were larger for O/W than for A/W and decreased for O/W upon addition of electrolyte in agreement with the theory. For the Volmer model α was smaller than for Langmuir's model and both were found to increase with decreasing Γagain in agreement with the theoretical predictions. It turned out that θ never exceeds 0.5 i.e. the adsorbed layer is never saturated. We tried to determine which adsorption model gave better results by calculating theoretically the Gibbs elasticity, but it turned out that when the results were plotted vs. an experimental variable, say C, all curves collapsed in a single one, which coincided with the respective experimental curve. This means that it is impossible to determine the adsorption model by using only interfacial tension data.
Adsorption Kinetics of Ionic Surfactants with Detailed Account for the Electrostatic Interactions
Journal of Colloid and Interface Science, 1997
perturbation: the larger the deviation from equilibrium ad-The problem of diffusion-controlled adsorption from a nonsorption, the larger the diffusion flux tending to eliminate micellar solution of an ionic surfactant in the absence of added the perturbation. electrolyte is solved analytically for the case of small deviations While the adsorption kinetics from non-ionic surfactant from equilibrium. For that purpose the electro-diffusion equations solutions is comprehensively studied (1-10), the respective of the transport of surfactant ions and counterions are combined theory of adsorption from ionic surfactant solutions is somewith the Poisson-Boltzmann equation for the electrical field. The what behind. Indeed, there are theoretical studies describing resulting set of equations is linearized and Laplace transform the equilibrium state, i.e., the equation of state of the ionic is applied. Analytical expression for the Laplace image of the adsorption is obtained in terms of elementary functions. Simple surfactant adsorption monolayer (11-14). However, the formulae for the short-time and long-time asymptotics of adsorptheoretical interpretation of data for adsorption kinetics of tion and surface tension relaxation are derived. To illustrate the ionic surfactants (15-22) meets some difficulties. The latter effect of the electrostatic interactions we calculated the theoretical originate from the non-linearity of the set of equations dedependence of the characteristic relaxation time on the bulk surscribing the electro-diffusion process. Three types of apfactant concentration and surface potential for aqueous surfactant proaches to this problem can be found in the literature: (i) solutions in contact with various non-aqueous phases (air, hepnumerical methods (23, 24), (ii) approximate analytical extane, decane, petroleum ether ) and two surfactants: SDS and pression derived by using an assumption for quasi-equilib-DTAB. The general trend is that the electrostatic effects decelerrium regime of surfactant adsorption (23, 25-28), and (iii) ate the process of adsorption, as it could be expected. The derived exact asymptotic expressions for the case of small deviations exact analytical expressions quantifying these effects can be directly applied for the interpretation of experimental data for the from equilibrium. The third is the approach we follow in kinetics of ionic surfactant adsorption. The reliability of our apthe present study. proach is verified through a comparison with other available theo-The characteristic extent of the electrical double layer is ries. ᭧ 1997 Academic Press determined by the Debye length, k 01 (see Eq. 9 below). Key Words: ionic surfactants; adsorption kinetics; dynamic sur-Dukhin et al. (25-28) presented a quasi-equilibrium model face tension.
Langmuir, 1994
The adsorption isotherms of cationic surfactants adsorbed from nearly neutral solutions on polar surfaces show, as a rule, two steps: one at small surface coverages and the other one terminating at a plateau corresponding to the crictical micelle concentration. The second step is generally believed to be due to the formation of bilayered surface aggregates (admicelles). The first step corresponds to the formation of a monolayer-like hydrophobic phase which may well be composed of single noncondensed monomers or to the creation of monolayered aggregates (hemimicelles). In order to study such two-step isotherms and the corresponding heats of adsorption, we have generalized our new theoretical approach for the case when the two kinds of surface aggregates coexist on a solid surface. The extended theoretical treatment can be applied also to monomer-admicelle surface equilibria by putting equal to one the aggregation number of hemimicelles in appropriate theoretical expressions. The obtained theoretical expressions for adsorption isotherms and heats of adsorption were next fitted to the experimental data obtained at CNRS Laboratory in Montpellier. Our computer exercises showed that when agreement with experiment was achieved, the calculated aggregation number ofhemimicelles was unity. This suggests that the first step on the isotherms of cationic surfactants is due to single monomer adsorption, in accordance with such an assumption often expressed in literature, but not sufficiently documented. The aggregation numbers determined by computer are smaller than those found previously by us for zwitterionic surfactants and are much smaller than the surface aggregation numbers found experimentally for anionic surfactants adsorbed at the waterfoxide interface.
Journal of Molecular Liquids, 2015
Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, namely pentanol, octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium dodecyl sulphate have been performed at the free surface of water at two different surface densities, namely 1 mol/m 2 (corresponding to unsaturated adsorption layer), and 4 mol/m 2 (corresponding to saturated adsorption layer), on the canonical ensemble at room temperature. The surfactants have been chosen in such a way that the effect of their headgroup charge as well as alkyl tail length on the properties of the adsorption layer can be separately investigated. The results are analysed in terms of the molecular level structure of the adsorption layer; organisation of the different groups and molecules along the macroscopic surface normal axis as well as conformation and orientation of the apolar tail is investigated in detail. In addition, the roughness of the surface of the aqueous phase is also analysed, using the ITIM method for accurately locating the real, capillary wave corrugated surface of the aqueous phase.
Langmuir, 2003
The main target of this study is to develop a theoretical method for determining small contents of dodecanol in samples of sodium dodecyl sulfate (SDS) by a detailed analysis of surface-tension isotherms. As a tool for our analysis, we employ the van der Waals model. Its application to data for alkanols and anionic surfactants gives an excluded area per adsorbed molecule equal to the geometrical area of the molecular cross section and adsorption energies consonant with Traube's rule. Because the dodecanol and SDS have different excluded areas, we extended the van der Waals model for the case of a two-component adsorption layer, with account for the counterion binding in the Stern layer. General expressions for the surface free energy, two-dimensional equation of state, surface chemical potentials, adsorption isotherms, and surface dilatational elasticity are derived. The experimental surface-tension isotherms are fitted by varying only one adjustable parameter. The model was successfully tested against data for solutions of SDS with a known content of dodecanol. Knowing the parameters of the model, we computed various properties of the surfactant adsorption layer. The results show that the presence of a small amount of dodecanol leads to a considerable increase of the total adsorption and surface elasticity. Even a relatively small (0.2 mol %) fraction of dodecanol in SDS may lead to a predominant content (up to 86 mol %) of dodecanol in the mixed adsorption layer. We applied the model for determining unknown contents of dodecanol in SDS samples at different stages of purification. The addition of NaCl may lead to a significant reduction in the mole fraction of dodecanol in the adsorption layer. The developed theoretical model and computational procedure are also appropriate for a quantitative analysis and computer modeling of the adsorption from other mixed ionic-nonionic surfactant solutions, at both air-water and oil-water interfaces.
Langmuir, 1993
The adsorption isotherms of cationic surfactants adsorbed from nearly neutral solutions on polar surfaces show, as a rule, two steps: one at small surface coverages and the other one terminating at a plateau corresponding to the crictical micelle concentration. The second step is generally believed to be due to the formation of bilayered surface aggregates (admicelles). The first step corresponds to the formation of a monolayer-like hydrophobic phase which may well be composed of single noncondensed monomers or to the creation of monolayered aggregates (hemimicelles). In order to study such two-step isotherms and the corresponding heats of adsorption, we have generalized our new theoretical approach for the case when the two kinds of surface aggregates coexist on a solid surface. The extended theoretical treatment can be applied also to monomer-admicelle surface equilibria by putting equal to one the aggregation number of hemimicelles in appropriate theoretical expressions. The obtained theoretical expressions for adsorption isotherms and heats of adsorption were next fitted to the experimental data obtained at CNRS Laboratory in Montpellier. Our computer exercises showed that when agreement with experiment was achieved, the calculated aggregation number ofhemimicelles was unity. This suggests that the first step on the isotherms of cationic surfactants is due to single monomer adsorption, in accordance with such an assumption often expressed in literature, but not sufficiently documented. The aggregation numbers determined by computer are smaller than those found previously by us for zwitterionic surfactants and are much smaller than the surface aggregation numbers found experimentally for anionic surfactants adsorbed at the waterfoxide interface.
Effect of electrolytes on surface tension of ionic surfactant solutions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
An improved model of ionic surfactant adsorption was developed by considering penetration of counterions into the interfacial Stern layer. Accordingly, the surface layer was treated as quasi two-dimensional electrolyte in which the electroneutrality condition was not fulfilled. Moreover, in the model the finite size of surfactant head-groups and counterions were taken into account as well as lateral electrostatic interactions. The model was confronted with experimental data obtained for cetyltrimethylammonium bromide in different electrolytes. It was found, that the model correctly reflected the variation in surface tension isotherms with the ionic strength of the solution. Contrary to previous models of ionic surfactant adsorption, our model allowed to account for the strong dependence of adsorption on type of counterion present in solution. Using the model a dependence of Stern layer potential and diffuse double layer potential on the concentration of surfactant in solution also was predicted.