Pyrene fluorescence at air/sodium dodecyl sulfate solution interface

The dependence of pyrene fluorescence spectra on the concentration of sodium dodecyl sulfate (SDS) was observed, where the solution was prepared from water saturated with pyrene. The values of the I(1)/I(3) ratio from the bulk solution and from the upper meniscus region in an optical cell were similar but decreased rapidly around the critical micelle concentration (cmc) of SDS, indicating that pyrene molecules preferred to be solubilized in the micelles having a lower dielectric constant. The fluorescence intensity of the excimer indicated the concentration of pyrene molecules at the air/solution interface or the surface activity of pyrene molecules. In addition, the intensity from the meniscus region is much larger than that from the bulk at the concentrations below the cmc, whereas there was no difference in the intensity between the bulk and the meniscus above 8 mmol dm(-3) of SDS. The analysis of the fluorescence intensity from the excimer strongly suggests the presence of molecular aggregates that are favorable to the pyrene molecules just like the micelles in the bulk, making them less movable.     

Adsorption at the air/water interface in dodecylammonium chloride/sodium dodecyl sulfate mixtures

The composition and properties of the adsorption films of dodecylammonium chloride/sodium dodecyl sulfate at the air/water interface depend on interactions between the film molecules and equilibria in the bulk phase (monomer-micelle and/or monomerprecipitate equilibria).The negative value of surface molecular interaction parameter ^mon calculated using the regular solution theory indicates strong attractive interactions between adsorbed molecules. Electrostatic interactions between oppositely charged ionic head groups enhance the adsorption of surfactants and decrease the minimum molar area of surfactant molecules at the air/water interface. The addition of an oppositely charged surfactant enhances packing at the air/water interface and transition from a liquid expanded to a liquid condensed state. Surface potential measurements reveal positive values for the mixtures investigated, implying the cationic surfactant ions are closer to the surface than the anionic ones.     

Adsorption and precipitation processes at cadmium oxide/sodium sulfate aqueous solution interface

The results of experimental studies of the adsorption of ions at the cadmium oxide/electrolyte solution interface are presented. On the basis of kinetic changes in the concentration of cadmium, hydroxide, and sulfate ions in the solution, the processes occurring in this system are discussed. It was found that cadmium oxide is transformed into the hexagonal form of cadmium hydroxide. The surface charge data for cadmium oxide/aqueous Na₂SO₄ are presented.     

Adsorption of alkyl trimethylammonium bromides at the air/water interface

A number of features of the adsorption of alkyl trimethylammonium bromides with nc=10,12,14, and 16 at the air/water interface were studied. First, the adsorption isotherms were calculated from experimental surface tension vs concentration curves by means of the Gibbs equation. Second, a novel method was used to estimate the adsorption free energy change. From the analysis of these data it was concluded that the hydrophobic driving force for the adsorption first increases with increasing adsorbed amount and then levels off in a plateau, which holds true for all four homologues. This peculiar behavior was interpreted by the formation of a thin liquid-like alkane film at the air/water interface once a certain adsorbed amount is exceeded. The hydrophobic contribution to the standard free energy change of adsorption was compared with those values previously determined for alkyl sulfate homologues. This comparison suggests that the alkyl trimethylammonium type surfactants behave as if their alkyl chain was approximately one methylene group shorter than those of the corresponding alkyl sulfates.     

Adsorption of sodium octylbenzene-sulfonate at the liquid/air interface

The surface tensions of aqueous solutions containing sodium octylbenzenesulfonate were measured by means of a processor tensiometer at 20°, 25°, 30° and 35°C. A test for the layer model and thickness of the adsorbed phase is proposed. By means of a linear regression the surfactant area at liquid/air interface was calculated. Finally, the molar-and surface-related thermodynamic functions were discussed.     

Adsorption of multiple ammonium salts at the air/solution interface

The interfacial behavior of aqueous solutions of newly synthesized bis- and tris-ammonium salts (i.e., bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]alkylamine dichlorides and bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]dialkylammmonium trichlorides, respectively) was analyzed, both experimentally and theoretically. The dynamic and equilibrium surface tension of multiple ammonium salt solutions was measured by using a pendant drop shape analysis method. The determined surface tension isotherms indicated the lack of significant differences in surface activity between bis- and tris-ammonium salts, contrary to the expectations for divalent and trivalent surfactant ions. That effect was explained by assuming the formation of multiple surfactant ion-counterion associates. Taking into account the association process, a good correlation between experimental data and theoretical predictions was obtained by means of the "surface quasi two-dimensional electrolyte" (STDE) model of ionic surfactant adsorption. The degree of association necessary to explain the lack of difference in surface activity between bis- and tris-ammonium salts was in quantitative agreement with the results of measurements of the concentration of free chloride anions in the surfactant solution.     

Examination of surface adsorption of sodium chloride and sodium dodecyl sulfate by surface potential measurement at the air/solution interface

The surface potential (DeltaV) of the air/sodium chloride solution interface was measured by using an ionizing (241)Am electrode method at 298.2 K. The surface potential steeply increased from 0 up to 15 mV with increasing concentration, then gradually increased up to 20 mV between 1 and 10 mmol dm(-3), and finally stayed almost constant at 20 mV up to the concentration of 20 mmol dm(-3). This result means that sodium ions concentrate more just near the air/solution interface, whereas chloride ions concentrate more far below the interface above the bulk region of electroneutrality. The dipole moment was derived from the surface potential value, from which the width of the interfacial layer was estimated as a function of the magnitude of electric charge. As for the sodium dodecyl sulfate solution, on the other hand, the surface potential steeply decreased from 0 down to -80 mV with increasing concentration from 0 to 0.01 mmol dm(-3), then rapidly increased up to -50 mV between 0.1 and 3 mmol dm(-3), then linearly increased up to 0 mV with increasing concentration from 3 mmol dm(-3) up to the CMC, 8 mmol dm(-3), then quite rapidly decreased again down to -82 mV from the CMC to 10 mmol dm(-3), and finally stayed almost constant at -82 mV up to the concentration of 20 mmol dm(-3). The above variations of the surface potential cannot be elucidated by the conventional surface excess, and therefore, the new concept of surface adsorption was presented for a simple salt and a typical anionic surfactant.     

Mixtures of sodium dodecyl sulfate/dodecanol at the air/water interface by computer simulations

Molecular dynamics simulations of monolayers of surfactant mixtures at the air/water interface were performed where the binary mixture was composed of sodium dodecyl sulfate (SDS) and dodecanol molecules. At the same ratio of SDS and dodecanol molecules, two monolayer mixtures were prepared. In the first monolayer, all the dodecanol molecules were placed together in the center of the simulation box, whereas in the second monolayer, those molecules were uniformly distributed in the surface area in such a way that they were far from each other. Simulations of both systems indicate that the dodecanol tails in the first monolayer are straighter and more ordered than those in the second monolayer. From the present results, we observed new insights of how the different molecules should array or distribute at the interface in real systems. Finally, studies of the interfacial water around the different surfactants were also analyzed, showing that they are closer to the polar headgroups of dodecanol than to the SDS headgroups.     

Adsorption behavior and dilational rheology of the cationic alkyl trimethylammonium bromides at the water/air interface

The dynamic and equilibrium surface tensions of C(n)TAB solutions for n = 12, 14, and 16 are studied using ring and bubble pressure tensiometry. Together with respective literature values, including neutron reflectivity and dilational surface rheology measurements, the experimental data are analyzed on the basis of two theoretical models, the Frumkin model and a modified reorientation model that takes into account an intrinsic compressibility of adsorbed surfactant molecules. It turns out that this new reorientation model, earlier applied to nonionic surfactant adsorption layers, is also applicable to ionic surfactants and superior to the Frumkin isotherm. All adsorption properties of one particular surfactant can be described by a single set of model parameters.     

Competitive adsorption of sodium dodecyl sulfate and polyethylene oxide at the air/water interface

Geoinspired synthetic chrysotile, which represents an ideal asbestos reference standard, has been utilized to investigate homomolecular exchange of bovine serum albumin (BSA), the major plasma protein, between the adsorbed and dissolved state at the interface between asbestos fibers and biological medium. FTIR spectroscopy has been used to quantify BSA structural modifications due to surface adhesion on chrysotile fibers as a function of the surface coating extent. Circular dichroism spectroscopy has been used to investigate the adsorption/desorption equilibrium through analysis of the BSA structural perturbations after protein desorption from chrysotile surface. Data results show clearly that in the solid state BSA modifications are driven by surface interaction with the substrate, following a bimodal adsorption evidenced by two different binding constants. On the other hand, BSA desorbed in solution is able to rearrange, in the lack of substrate, although keeping irreversible modifications with respect to the native species. The lack of regaining its native structure certainly affects albumin interaction with biological environment. The present investigation on the stoichiometric synthetic geoinspired chrysotile nanocrystals is the first approach toward a deeper attempt to use standard synthetic chrysotile reference samples in mimicking the behavior of asbestos fibers and allows to better understand their interaction with a biological environment.     

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.     

Adsorption and direct probing of fibrinogen and sodium myristate at the air/water interface

Fibrinogen (FB), a serum protein, is considered a major inhibitor of lung surfactant function at the lining layer of the alveoli. In this study, the adsorption of aqueous bovine FB at the air/water interface was investigated with tensiometry and directly probed for the first time with ellipsometry and infrared reflection adsorption spectroscopy (IRRAS). The tension results show that FB has moderate surface activity. The surface densities of FB were calculated by using two different ellipsometry models to range from 3±0.2 to 17±2 mg/m², for 7.5 to 750 ppm of FB in water at 25°C. Although FB at concentrations from 75 to 750 ppm reached about the same steady surface tension value, the surface densities at 750 ppm FB were substantially larger. The same techniques were used for studying aqueous mixtures of 7.5 to 750 ppm FB with 2 mM of sodium myristate (SM) to investigate a possible interaction of the SM with the protein. The behavior of the FB/SM mixtures was found to be close to that of SM alone. The surface tension of the FB/SM mixtures reached values less than 10 mN/m under surface area oscillation at 20 or 80 rpm. These results and the ellipsometry and the IRRAS results indicate that at a concentration of 2 mM SM, FB, up to 750 ppm, does not inhibit the surfactant surface-tension-lowering function. In certain cases the results demonstrate that FB and SM may act cooperatively in lowering the surface tension.     

Competitive adsorption of neutral comb polymers and sodium dodecyl sulfate at the air/water interface

The interfacial behavior of aqueous solutions of four different neutral polymers in the presence of sodium dodecyl sulfate (SDS) has been investigated by surface tension measurements and ellipsometry. The polymers comprised linear poly(ethylene oxide) with low and high molecular masses (10(3) and 10(6) Dalton (Da), respectively), and two high molecular mass methacrylate-based comb polymers containing poly(ethylene oxide) side chains. The adsorption isotherms of SDS, determined by Gibbs analysis of surface tension data, are nearly the same in the presence of the high molecular mass linear polymer and the comb polymers. Analysis of the ellipsometric data reveals that while a single surface layer model is appropriate for films of polymer alone, a more sophisticated interfacial layer model is necessary for films of SDS alone. For the polymer/surfactant mixtures, a novel semiempirical approach is proposed to determine the surface excess of polymer, and hence quantify the interfacial composition, through analysis of data from the two techniques. The replacement of the polymer due to surfactant adsorption is much less pronounced for the high molecular mass linear polymer and for the comb polymers than for the low molecular mass linear polymer. This finding is rationalized by the significantly higher adsorption driving force of the larger polymer molecules as well as by their more amphiphilic structure in the case of the comb polymers.     

Adsorption of polyprotic acid at the water/air interface

A rigorous thermodynamic treatment appropriate for surface adsorption from mixed aqueous solution of alkali and polyprotic acid was derived. Those equations were applied to mixed aqueous solution/air systems of alkali metal hydroxide and Fe^III complex with ethylenediamine- N, N, N′,N′-tetraacetate (Fe-EDTA). Surface density of each species arising from Fe-EDTA was separately evaluated, and thus, surface activity of Fe-EDTA was studied, especially its dependence on pH and how it is influenced by the counter cations. Fe-EDTA was positively adsorbed at the water/air interface at very low pHs and negatively at high pHs. The pH range of positive adsorption of Fe-EDTA with potassium ion, as a counter ion, was wider than that with sodium ion. Thus, potassium ion, a structure breaker, tended to smooth surface adsorption of Fe-EDTA at the water/air interface, whereas sodium ion, a structure maker, tended to withdraw Fe-EDTA from the interfacial region.     

Adsorption of sugar surfactants at the air/water interface

The adsorption isotherms of n-decyl-β-D-glucoside (β-C(10)G(1)) as well as various n-alkyl-β-D-maltosides (β-C(n)G(2)) with n=8, 10, 12 and 14 were determined from surface tension measurements. Based on the analysis of the adsorption isotherms, the total free energy change of adsorption was determined and a novel method was proposed to determine the maximum adsorbed amount of surfactant. It can be concluded that the driving force for adsorption first increases with increasing adsorbed amount of the sugar surfactants and then levels off in a plateau. This peculiar behaviour is interpreted as formation of a thin liquid-like alkane film of overlapping alkyl chains at the air/water interface once a certain adsorbed amount is exceeded. The driving force of adsorption depends on the alkyl chain length only and is not affected by the type of the head group. The hydrophobic contribution to the standard free energy change of adsorption was compared with the values of sodium alkylsulfate and alkyltrimethylammonium bromide surfactants. This comparison reveals that the hydrophobic driving force of adsorption is the largest for the sodium alkylsulfates, whereas it is the same for the sugar surfactants and the alkyltrimethylammonium bromides.     

Adsorption of procaine at the mercury/electrolyte solution interface

The adsorption of the local anaesthetic procaine hydrochloride at the mercury/electrolyte interface solution is followed using capacitance measurements. The adsorption is studied at various procaine concentrations, in potassium chloride, potassium bromide or potassium fluoride used as supporting electrolytes, and at various pH values and temperatures. Procaine has basic properties with two acidity constants K. The results indicate the way the procaine molecules orientate at the interface. In all cases studied no hemimicelles or condensed film are observed.     

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     

Adsorption of methylmethacrylate-vinylferrocene copolymers at the solution/solid interface

A series of methylmethacrylate–vinylferrocene random copolymers was synthesized and characterized. Their adsorption from toluene and chloroform was measured onto pyrogenic silicas. The level of adsorption depended on the solvent, the surface area of the adsorbent, and the copolymer composition. Thus, an inverse adsorption-solubility relationship for toluene and chloroform was observed. However, in solvents such as tetrahydrofuran, 2-butanone, and cyclobutanone, which have strong interaction with silica, this trend was not evident. The compositional dependence of the adsorption of these copolymers in toluene and chloroform is similar. Initially, adsorption tends to increase with the vinylferrocene content in the polymer, and at equimolar copolymer compositions the adsorption reaches a maximum which is followed by a decrease in the adsorption values at high vinylferrocene contents. Gel permeation chromatography(GPC) measurements allowed us to conclude that high molecular weight polymer was preferentially adsorbed.     

Adsorption of protein/surfactant complexes at the air/aqueous interface

We use optical reflectometry and surface pressure techniques to measure co-adsorption of the anionic surfactant sodium dodecyl sulfate (SDS) and the protein lysozyme at the air-aqueous interface. We observe lysozyme/SDS co-adsorption behavior in two different buffers for which solution-phase binding data are available in the literature. The co-adsorption of lysozyme/SDS complexes is controlled by the mode of protein/surfactant binding that occurs in solution. In a pH 5.0 acetate buffer, the extent of co-adsorption is weakly dependent on SDS concentration throughout the specific and transitional binding regimes. In a pH 6.9 phosphate buffer, the extent of co-adsorption is weakly dependent on SDS concentration in the specific binding regime, but it increases dramatically, giving rise to multilayer co-adsorption, in the transitional binding regime. In both buffers, the extent of co-adsorption dramatically decreases in the cooperative binding regime. Lysozyme/SDS co-adsorption is strongly influenced by kinetically trapped non-equilibrium adsorbed layer states, such that adsorbed amounts are markedly path-dependent. Surface pressure measurements by themselves do not capture the variations in adsorption in the different binding regimes, nor do they capture the path-dependency of co-adsorption.     

Adsorption of polyelectrolyte/surfactant mixtures at the air-solution interface: poly(ethyleneimine)/sodium dodecyl sulfate

Neutron reflectivity and surface tension have been used to characterize the adsorption of the polyelectrolyte/ionic surfactant mixture of poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) at the air-water interface. The surface tension behavior and adsorption patterns show a strong dependence upon the solution pH. However, the SDS adsorption at the interface is unexpectedly most pronounced when the pH is high (when the polymer is essentially a neutral polymer) and when the polymer architecture is branched rather than linear. For both the branched and the linear PEI polymer/surfactant complex formation results in a significant enhancement of the amount of SDS at the interface, down to surfactant concentrations approximately 10(-6) M. For the branched PEI a transition from a monolayer to a multilayer adsorption is observed, which depends on surfactant concentration and pH. In contrast, for the linear polymer, only monolayer adsorption is observed. This substantial increase in the surface activity of SDS by complexation with PEI results in spontaneous emulsification of hexadecane in water and the efficient wetting of hydrophobic substrates such as Teflon. In regions close to charge neutralization the multilayer adsorption is accentuated, and more extensively ordered structures, giving rise to Bragg peaks in the reflectivity data, are evident.