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Two different approaches have been used in the literature to describe the effects of ionisation of surfactants on the surface pressure, Π. One approach is based on a molecular model for a charged monolayer, in which the mutual repulsion of the long-chain surfactant ions results in an additional surface pressure, Πel, calculated with the Gouy–Chapman theory for the formation of a diffuse electric double layer, and with counterion binding in the Stern–Helmholtz layer adjacent to the surfactant monolayer. The other approach regards the surface as a two-dimensional solution defined as a Gibbs dividing surface, which is electroneutral by definition. In this approach, the adsorption of any ion is the sum of its excesses in the monolayer and the electrical double layer; no assumptions are made about the spatial distribution of charges. It has been shown that both models can produce a reasonable description of experimental results obtained for solutions of a single ionic surfactant (RX) with or without inorganic electrolyte (XY). In many cases, measurements of Π vs. mean ionic activity at different salt concentrations (cXY) are found to coincide on a single curve, implying that at given mean ionic activity both adsorption and Π are independent of cXY, i.e. that double-layer contributions to the surface pressure are negligible. In addition, the electroneutral 2-D solution approach has resulted in a simple explanation of several typical features of mixed ionic surfactant solutions, in particular for mixtures of anionic and cationic surfactants. In mixed solutions too, double-layer effects appear to be negligible. We present arguments for such negligibility. One reason is a significant degree of binding between adsorbed surface active ions (R) and counterions (X); another is that for 1:1 electrolytes, the contribution of the diffuse double layer to the adsorption of the combination (RX) vanishes. As a result, it is possible to interpret the same experimental data in terms of both models. [Copyright &y& Elsevier]

Bubble profile analysis tensiometry was used to study the surface tension and dilational visco-elasticity of individual and mixed solutions of β-casein (BCS) and β-lactoglobulin (BLG) at various mixing ratios of the components. The results for the individual protein solutions were analysed based on a new theory, which is applicable in the pre-critical and post-critical concentration range of the surface tension isotherm. It is shown that these proteins are capable of displacing each other from the adsorption layer, which affects the surface tension and visco-elasticity modulus of the adsorbed layers. The model developed for protein mixtures agrees well with the experimental data and the calculated dependencies of the surface properties on the solution composition confirm the mutual influence of added proteins. Image 1 • A model for mixed adsorption layers is applied successfully to protein mixtures. • The theory predicts the equations of state for mixed BLG/BCS adsorption layers. • For the mixed protein layers the dilational visco-elasticity is correctly obtained. • At short times the mixed adsorption layers are dominated by the random coil BCS. • At longer adsorption times the globular protein BLG replaces the random coil BCS. [ABSTRACT FROM AUTHOR]

The theoretical description of the adsorption of surfactants at interfaces between aqueous solutions and oil was based over a very long time on models derived for the solution/air interface. Thus, most of the experimentally observed peculiarities could not be specifically considered but were merely interpreted in terms of a penetration of oil molecules into the alkyl chain layer of the adsorbed surfactant molecules. These penetrating oil molecules enhance the surfactant adsorption as compared to the water/air interface. Later on, for the special situations at water/oil interfaces a competitive adsorption of surfactant and oil molecules was postulated, allowing a much better description of experimental data. This picture, however, was unable to explain why the interfacial tension of the water/oil interface decreases very quickly when extremely small amounts of surfactants are added to the water. This effect cannot be of competitive nature, but a cooperativity of surfactant and oil molecules forming a mixed adsorption layer is required instead. This cooperative effect means that already few surfactant molecules adsorbed at the interface can induce a significant ordering of oil molecules in the interfacial layer. This new interfacial structure, in turn, attracts further surfactant molecules to adsorb. Improving the theoretical description of experimental data was finally achieved by applying suitable adsorption models for the two adsorbing compounds, i.e. a Frumkin adsorption model for the oil molecules and a Langmuir, Frumkin, or reorientation model for the adsorbing surfactant molecules. Here, the progress in modelling surfactant adsorption at water/oil interfaces is discussed mainly for the homologous series of the cationic surfactants C n TAB, of the anionic surfactant SDS, and members of the homologous series of the non-ionic surfactants C n DMPO at water/alkane interfaces. Unlabelled Image • Surfactants adsorb at the water/oil interface at much lower bulk concentrations as compared to the water/air interface. • At very low concentrations surfactant and oil molecules adsorb in a cooperative way. • With increasing concentration surfactant and oil molecules compete at the interface. • Depending on the packing structure of surfactants, oil molecules are squeezed out from the interfacial layer. • A thermodynamic model allows describing all states of the mixed adsorption layer. [ABSTRACT FROM AUTHOR]

The thermodynamic and structural parameters of monolayers of N -alkanoyl-substituted alanine with 8–17 methylene fragments in the alkyl chain are calculated on the basis of the quantum chemical semiempirical PM3 method. Four optimized structures of monomers are obtained. The energetically most preferred conformer is used for construction of dimers with “sequential” and “parallel” orientation of the hydrophilic parts of the monomers and tetramers with square structure. The hydrophobic chains of N -alkanoyl-substituted alanine are found to be inclined in the dimers to the normal of the spread monolayer: δ = 10° and φ = 43°. Comparison of clusterization Gibbs’ energy for small clusters suggests that 2D film formation takes place by formation of linear associates comprised of dimers with “parallel” orientation of the hydrophilic groups. The temperature increase causes growth of linear 1D clusters leading to the dendritic structure of the monolayer. The parameters of the unit cell of 2D film are calculated: a = 4.65 Ǻ, b = 6.20 Ǻ, the angle between the sides of the cell θ = 105˚. The tilt angle of amphiphilic molecules to the normal of the air/water interface is t = 47°. The calculated parameters of the unit cell are in good agreement with the existing experimental data. The spontaneous clusterization threshold for N -alkanoyl-substituted alanine at the air/water interface is calculated to be 15 carbon atoms in the chain at 288 K. This is only 2 carbon atoms more than the experimental value and within the margin of error of the model used for the calculation. [ABSTRACT FROM AUTHOR]

The influence of temperature on the dynamic surface tension of water in heptane vapour is studied using drop profile analysis tensiometry. The water drops are formed in air saturated by heptane and water vapours. For long life times a new phenomenon is found: a sharp decrease of surface tension from about 60 mN/m down to 30 mN/m. The time until this sharp surface tension sets in decreases with increasing temperature. This phenomenon is attributed to the formation of heptane adsorption layers with a significant thickness. To ensure that the sharp surface tension decrease is not an artefact, the experimental error (deviation of drop profiles from the Young-Gauss-Laplace equation) was determined using harmonic oscillations imposed to the surface of pure heptane drops. It was shown that fitting errors below 10 μm in the determination of the drop radius do not affect the calculated surface tension value. The sharp surface tension decrease was observed with fitting errors below 5 μm, so that this phenomenon was explained to be caused by the formation of multilayers. The surface tensions and adsorbed amounts are described by a model developed earlier. The experimental results depend essentially on the experimental method used. In another experiment the atmosphere in the measuring cell was pre-saturated only by water vapour, and heptane (pentane) was added onto the cell bottom just immediately before the water drop was formed. The increase of temperature results in a slower adsorption process which is opposite to the case where the composition of the mixed atmosphere inside the cell was established prior to the experiments. [ABSTRACT FROM AUTHOR]

The dynamics of surfactant interfacial layers was first discussed more than a century ago. In 1946 the most important work by Ward and Tordai was published which is still the theoretical basis of all new models to describe the time dependence of interfacial properties. In addition to the diffusion controlled adsorption mechanism, many other models have been postulated in literature, however, well performed experiments with well defined surfactant systems have shown that the diffusional transport is the main process governing the entire formation of surfactant adsorption layers. The main prerequisite, in addition to the diffusional transport, is the consideration of the right boundary condition at the interface, given by a respective equation of state. In addition to the classical models of Langmuir and Frumkin, also the so-called reorientation or interfacial aggregation models are to be assumed to reach a quantitative description of respective experimental data. Moreover, the adsorption of surfactants at the interface between water and a gas phase different from air can be strongly influenced by the type of molecules within the gas phase, such as alkane vapours. These oil molecules co-adsorb from the gas phase and change the adsorption kinetics strongly. Besides the discussion of how to apply theoretical adsorption kinetics models correctly, a large number of experimental data are presented and the way of a quantitative analysis of the adsorption mechanism and the main characteristic parameters is presented. This includes micellar solutions as well as mixtures of surfactants of ionic and non-ionic nature. [ABSTRACT FROM AUTHOR]

Abstract: This report aims at (i) presenting a quantitative interpretation of interfacial dilational moduli (|E|) for four proteins at three different interfaces and (ii) identifying the main parameters responsible. The proteins were adsorbed from aqueous solution against air, n-tetradecane and sunflower seed oil, as a function of protein concentration and adsorption time. Experimentally, a dynamic drop tensiometer is a convenient instrument to generate the required sinusoidal oscillations for compression/expansion of interfaces (Benjamins et al., 1996 [1]). Theoretically, a simple two-dimensional solution model with a constant molecular area of the protein described the data only at fairly low pressures. Much better agreement over the entire elastic range was found with a recent extension of the model. This extension accounted for adsorbed proteins adopting smaller molecular areas with increasing surface pressure. Three factors dominated the values of the dilational modulus: (i) rigidity of protein molecules, (ii) degree of interfacial non-ideality and (iii) tension of the clean interface (Benjamins et al., 2006 [2]). The last factor is clearly of great relevance to food emulsions. For each protein at different interfaces, the elasticity increased with the enthalpy parameter (Η S) of the equation of state. Elasticity and Η S both increased with the clean-interface tension, γ 0, i.e., with decreasing polarity of the interface (Benjamins et al., 2006 [2]; Fainerman et al., 2003 [3]). The elasticity of the different proteins also increased with increasing rigidity of the molecules, indicating a lower compressibility of the molecular area at the interface. Pure viscosities were never observed in our experience. However, viscoelastic behaviour was found at high pressures, i.e., in densely packed surfaces. The measured viscous phase angles strongly decreased at still higher pressures, indicating that the active relaxation mechanism slowed down with increasing molecular packing density. Specific kinetic models are yet to be developed for such mechanisms. [Copyright &y& Elsevier]

Abstract: The adsorption of mixtures of β-casein and the anionic surfactant sodium dodecyl sulphate and the cationic dodecyl trimethyl ammonium bromide were studied at the water/hexane interface by drop profile analysis tensiometry and compared with those obtained for the water/air interface. The obtained interfacial tension isotherms suggest a gradual replacement of the protein molecules from both liquid interfaces with increasing surfactant concentration and a fixed amount of protein. The mechanical properties of the interfacial layers were probed by dilational rheological investigations. The results show, that although the equilibrium state is described by the existing theoretical models adequately, the quantitative analysis of the dilational visco-elasticity data requires further refinement of the corresponding model. [Copyright &y& Elsevier]

Abstract: The dilational rheology for mixed solutions of surface active compounds is easy to measure with existing methods, however, there are very few attempts only to analyse the resulting data on the basis of a theoretical model. Models by Jiang et al., Joos, and by Lucassen and van den Tempel (for single compounds) are compared here and applied to experimental data. The mentioned models converge to each other after some small modifications and represent a very good scientific background for studies of the mechanical behaviour of mixed adsorption layers. As example, the experimental results for mixed solutions containing a globular protein (β-lactoglobulin) and a non-ionic surfactant (alkyl dimethyl phosphine oxide) are analysed by the obtained dependencies, using some new proposed numerical procedures. [Copyright &y& Elsevier]

Abstract: The frequency and concentration dependencies of the dilational elasticity and viscosity of adsorbed surfactant layers is yet not quantitatively understood. Especially at a surface coverage above 50% the measured high frequency limits of the elasticity pass typically a maximum or reach a plateau while classical models expect an exponential increase to very high values. The consideration of an intrinsic two-dimensional compressibility allows a much better description of experimental data. There are various surfactant systems discussed in literature following this new model. Although the impact of these interfacial characteristics on the dilational rheology is significant, the adsorption isotherm and equation of state change only slightly. [Copyright &y& Elsevier]

In the framework of quantum chemical approximations, namely MINDO/3, MNDO, AM1 and PM3, the thermodynamic functions characteristic of the formation of dimers of alkanols (n=8–16) are calculated both in vacuum and in water. These values are compared with those calculated from experimental Π versus A isotherms of n-dodecanol, n-tetradecanol and n-hexadecanol using a thermodynamic model that assumes equilibrium between oligomers and highly aggregated condensed phase domains in the monolayer. The results indicate consistency between experimental data, the molecular-statistical theory developed to describe surfactant aggregation (formation of small oligomers in the fluid (gaseous) state) in the monolayer, and the values obtained by the quantum chemical method PM3. In particular, it was shown by PM3 calculations that, in perfect agreement with the experiments, the aggregation degree of an oligomer increases with the alkyl chain length. The increased chain length leads also to increasing absolute values of the free energy per monomer for the formation of highly aggregated domains; and the enthalpy and entropy constituents of the free energy. [Copyright &y& Elsevier]

Graphical abstract Abstract The dilational visco-elasticity of surfactant adsorption layers was measured at low frequencies by the drop profile analysis tensiometry using oscillating drops. As the studied non-ionic surfactant C 13 DMPO (tridecyl dimethyl phosphine oxide) is soluble in water and in hexane, the partitioning of the surfactant between the two solvents had to be taken into consideration. The diffusion controlled exchange of matter theory was generalized in order to take into consideration the curvature of the interface, the diffusional transport in both adjacent bulk phases as well as the transfer across the liquid interface. Using two configurations, i.e. water drop in hexane and hexane drop in water, it is shown that the frequency dependence of the visco-elasticity modulus and the phase angle can be well described when the correct partition coefficient is applied. The surface activity of the selected surfactant C 13 DMPO is optimum to demonstrate the impact of matter transfer across the interface on the dilational visco-elasticity of interfacial adsorption layers of non-ionic surfactants. [ABSTRACT FROM AUTHOR]

The surface tension isotherms and dilational visco-elasticity for three nonionic surfactants (C 10 EO 8 , C 12 EO 5 and C 14 EO 8 ) were comparatively studied using the bubble and drop profile analysis tensiometry. The experiments based on drop profiles were analysed assuming the depletion of surfactant molecules from the bulk of the drop due to adsorption. To process the experimental results, two theoretical adsorption layer models (Frumkin and reorientation) were applied, while the reorientation model provides a better description of the experimental results. In addition, the visco-elasticity moduli obtained from drop and bubble profile experiments were compared. It was shown that at higher surfactant concentrations the drop profile method provides visco-elasticities systematically larger than those obtained from bubble profile experiments, even though the adsorption-related depletion has been correctly considered. The proposed correction protocol provides the option for direct comparison between data obtained by the drop profile analysis method with those from other methods. [ABSTRACT FROM AUTHOR]

On the basis of surface tension and surface visco-elasticity measurements performed using profile analysis tensiometry with single pendent drops the surface properties of the alkyl phospholipid Inositol-C2-PAF were determined. Because of the very low bulk concentrations of Inositol-C2-PAF used in the studies, the obtained adsorption data had to be analysed including the depletion of the bulk concentration due to adsorption at the surface of the measured drops. The determined critical micellar concentration (CMC) of aqueous Inositol-C2-PAF solutions was found to be 1.1 μM, similar to other anti-tumour and anti-inflammatory substances. [ABSTRACT FROM AUTHOR]

Drop profile analysis tensiometry is applied to measure the dilational visco-elasticity of BLG at the buffered aqueous solution/tetradecane (W/TD) interface using oscillating drops of TD immersed in W at frequencies between 0.01–0.2 Hz. The buffered solutions were investigated at pH 3, pH 5 (isoelectric point) and pH 7 at different buffer concentrations (1 mM, 10 mM and 100 mM). The real part of the complex visco-elasticity shows a maximum when plotted as a function of the interfacial pressure Π. In contrast to the water/air surface (W/A) where we observe maximum elasticity values between 15 and 20 mN/m, at the W/TD interface these maximum values are up to 65–70 mM/m, which is in parallel with the much higher interfacial pressure values at the W/TD interface when compared to the W/A surface. [ABSTRACT FROM AUTHOR]

Using drop profile analysis tensiometry the adsorption dynamics and the equilibrium equation of state of β-lactoglobulin (BLG) at the water/tetradecane (W/TD) interface are studied at pH 3, 5 and 7. The data are well described by a thermodynamic adsorption model using almost identical model parameters for all three pH values except for the surface activity coefficient. The surface pressure isotherms at the water/air (W/A) surface exhibit much steeper run than interfacial pressure at the W/TD interface for any of the studied cases at pH 3, 5 and 7, and the calculated adsorption isotherm data point at smaller adsorbed amounts for these pH. This seems to be in contrast to the much larger interfacial pressure changes reached at high BLG concentrations at the W/TD interface, which are almost three times higher than those at the W/A surface. The observations can be explained by a strong interaction between BLG and the oil molecules at the interface. The dynamic interfacial tensions can be adequately described by a mixed adsorption model, assuming a diffusional transport of the protein molecules in the aqueous bulk phase and an adsorption mechanism which assumes a change of the adsorption activity parameter in dependence of the interfacial coverage. [ABSTRACT FROM AUTHOR]

The adsorption kinetics of β-lactoglobulin (BLG) at the water/tetradecane (W/TD) interface as studied by drop profile analysis tensiometry is significantly controlled by the diffusional transport in the aqueous solution bulk. However, due to the contact with the hydrophobic oil phase the protein molecules change their conformation in order to adapt to the interfacial environment. This conformational change can be expressed via the adsorption activity constant. The analysis of the dynamic interfacial tensions leads to much lower activities at short adsorption times and low surface coverages. This allows to conclude that in the early stage of the adsorption layer formation the structure of the BLG adsorption layer at the W/TD interface is similar to that at the water/air (W/A) interface. [ABSTRACT FROM AUTHOR]

The surface tension and dilational surface visco-elasticity of the individual solutions of the biopolymer DNA and the azobenzene-containing cationic surfactant AzoTAB, as well as their mixtures were measured using the drop profile analysis tensiometry. The negatively charged DNA molecules form complexes with the cationic surfactant AzoTAB. Mixed DNA + AzoTAB solutions exhibit high surface activity and surface layer elasticity. Extremes in the dependence of these characteristics on the AzoTAB concentration exist within the concentration range of 3 × 10 −6 –5 × 10 −5 M. The surface tension of the mixture shows a minimum with a subsequent maximum. In the same concentration range the elasticity shows first a maximum and then a subsequent minimum. A recently developed thermodynamic model was modified to account for the dependence of the adsorption equilibrium constant of the adsorbed complex on the cationic surfactant concentration. This modified theory shows good agreement with the experimental data both for the surface tension and the elasticity values over the entire range of studied AzoTAB concentrations. [ABSTRACT FROM AUTHOR]

In the framework of the model for surfactant clusterization description at the air/liquid and liquid/liquid interfaces proposed earlier (Vysotsky et al. J. Phys. Chem. B, 2015, 119 (7), 3281–3296) and based on the quantum chemical semi-empirical PM3 method the structural and thermodynamic parameters of the formation of 1-( N -alkyl)-imidazole monolayers with alkyl chain lengths of 6–16 carbon atoms at the air/water interface are calculated. The model allows the calculation of the thermodynamic parameters (enthalpy, entropy and the Gibbs’ energy) of monomers, clusters and monolayers at the interface between two liquids or between the air and liquid phases. To obtain such results one needs to calculate the corresponding thermodynamic parameters for both adjacent phases within the semi-empirical PM3 method (in the frameworks of the COSMО procedure), then to calculate the increments from the different types of the pairwise interactions and, with the help of the additive scheme, to calculate the required parameters. It has been shown that 1-( n -alkyl)-imidazole forms monolayers with hexagonal and oblique unit cells normal conditions. The parameters of the unit cell are calculated for each of these monolayers: 4.6 Ǻ and 7.2 Ǻ (the angle γ between the sides of the unit cell is equal to 62°) for the hexagonal unit cells and 4.6 Ǻ and 4.6 Ǻ ( γ = 80°) for the oblique unit cells. Enthalpy, entropy and Gibbs’ energy for small clusters (dimers–hexamers), representing the structural elements of the investigated monolayers, are calculated. The probable way of the clusterization process is proposed on the basis of these parameters. In the framework of the model the thermodynamic functions of the monolayer formation are calculated. These results indicate that for both types of the monolayers (with oblique and hexagonal unit cells) spontaneous clusterization is possible when the alkyl chain length is ≥14–15 carbon atoms. [ABSTRACT FROM AUTHOR]

The adsorption of surfactants from aqueous solution at the water/air interface is changed when the air phase contains hexane vapor. This co-adsorption of surfactant and hexane depends on the hexane vapor pressure. A thermodynamic model developed for the adsorption of surfactant mixtures can be adapted to the present situation. The surfactants studied were SDS, C 12 TAB and C 12 DMPO, and the dependence of their adsorption characteristics on the partial hexane vapor pressure was determined. The co-adsorption of hexane from the vapor phase increases the surface activity of the adsorbing surfactants. [ABSTRACT FROM AUTHOR]

The equations of state, adsorption isotherms and functions of the distribution of protein molecules in liquid interfacial layers with respect to molar area and the equations for their viscoelastic behavior are presented. This theory was used to determine the adsorption characteristics of β-casein and β-lactoglobulin at water/oil interfaces. The experimental results are shown to be describable quite adequately by the proposed theory with consistent model parameters. The data analysis demonstrated that the β-casein molecule adsorbed at equilibrium conditions is more unfolded as compared with dynamic conditions, and this fact causes the significant increase of the adsorption equilibrium constant. The theory assumes the adsorption of protein molecules from the aqueous solution and a competitive adsorption of alkane molecules from the alkane phase. The comparison of the experimental equilibrium interfacial tension isotherms for β-lactoglobulin at the solution/hexane interface with data calculated using the proposed theoretical model demonstrates that the assumption of a competitive adsorption is essential, and the influence of the hexane molecules on the shape of the adsorption isotherm does in fact exist. [ABSTRACT FROM AUTHOR]

The adsorption of proteins at liquid interfaces happens at rather low bulk concentrations due to their rather high surface activity. In contrast typical surfactants start to decrease the surface tension at bulk concentration in the range of mmol/l and reach a minimum value at about two or three orders of magnitude higher concentration. The two proteins studied here, β-lactoglobulin and β-casein, adsorb already remarkably at much lower concentrations, i.e. less than 1 μmol/l. When smallest amounts of a non-ionic surfactant are added to low concentrated protein solutions, changes in the surface tension are observed which cannot be explained by the existing theoretical models. An agreement with the experimental data can be achieved only when an increased surface activity of the protein due to the interaction with the surfactants is assumed. [ABSTRACT FROM AUTHOR]

Dynamic interfacial parameters are the key properties of interfaces in many modern technologies and can be studied in various ways. For applications like foams and emulsions, the dynamics of adsorption and the dilational and shear rheology of liquid–fluid interfaces are investigated most frequently. This work gives an insight into recently developed new experimental approaches, such as fast capillary pressure tensiometry for growing and oscillating drops. These experiments are presented in comparison to more classical techniques like drop profile tensiometry and capillary wave damping. Progress in these experiments based on generated interfacial perturbations can be expected only by a close link to respective CFD simulations. We also present the state of the art of CFD simulations, which have reached a high level during the last decade and provide a substantial basis for dynamic interfacial experiments. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hexane molecules adsorb from the vapour phase at the surfactant solution surface. [•] The presence of hexane enhances the C n TAB adsorption. [•] High C n TAB concentration, hexane molecules are removed from the adsorption layer. [•] Good theoretical fitting with a model for competitive adsorption of two components. [Copyright &y& Elsevier]

Highlights: [•] C n TAB molecules adsorb from the water phase at the water/oil interface. [•] The presence of hexane phase enhances the C n TAB adsorption. [•] Lengths of the oil and surfactant molecules define the adsorption layer structure. [•] Good theoretical fitting is obtained with the Frumkin Ionic Compressibility model. [ABSTRACT FROM AUTHOR]

Abstract: The superposition-additive approach developed previously was shown to be applicable for the calculations of the thermodynamic parameters of formation and atomization of conjugate systems, their dipole polarizability, molecular diamagnetic susceptibility, π-electronic ring currents, etc. In the present work, the applicability of this approach for the calculation of the thermodynamic parameters of formation and clusterization at the water/air interface of alkanes, fatty alcohols, thioalcohols, amines, nitriles, fatty acids (C n H2 n +1X, X is the functional group) and cis-unsaturated carboxylic acids (C n H2 n −1COOH) is studied. Using the proposed approach the thermodynamic quantities determined agree well with the available data, either calculated using the semiempirical (PM3) quantum chemical method, or obtained in experiments. In particular, for enthalpy and Gibbs’ energy of the formation of substituted alkane monomers from the elementary substances, and their absolute entropy, the standard deviations of the values calculated according to the superposition-additive scheme with the mutual superimposition domain C n −2H2 n −4 (n is the number of carbon atoms in the alkyl chain) from the results of PM3 calculations for alkanes, alcohols, thioalcohols, amines, fatty acids, nitriles and cis-unsaturated carboxylic acids are respectively: 0.05, 0.004, 2.87, 0.02, 0.01, 0.77, and 0.01kJ/mol for enthalpy; 2.32, 5.26, 4.49, 0.53, 1.22, 1.02, 5.30J/(molK) for absolute entropy; 0.69, 1.56, 3.82, 0.15, 0.37, 0.69, 1.58kJ/mol for Gibbs’ energy, whereas the deviations from the experimental data are: 0.52, 5.75, 1.40, 1.00, 4.86kJ/mol; 0.52, 0.63, 1.40, 6.11, 2.21J/(molK); 2.52, 5.76, 1.58, 1.78, 4.86kJ/mol, respectively (for nitriles and cis-unsaturated carboxylic acids experimental data are not available). The proposed approach provides also quite accurate estimates of enthalpy, entropy and Gibbs’ energy of boiling and melting, critical temperatures and standard heat capacities for several classes of substituted alkanes. For the calculation of thermodynamic functions of clusterization of dimers, trimers and tetramers of fatty alcohols, thioalcohols, amines, carboxylic acids and cis-unsaturated carboxylic acids two superposition-additive schemes are proposed which ensure the correct superimposition of the molecular graphs, including intermolecular hydrogen–hydrogen interactions in the clusters. The calculations involve the thermodynamic parameters of clusterization obtained earlier by the PM3 method. It is shown that the proposed approach reproduces quite accurately the values calculated earlier and is applicable for the prediction of the thermodynamic parameters of the formation of surfactant monolayers. [Copyright &y& Elsevier]

Abstract: Even though sodium dodecyl sulphate (SDS) is the most frequently studied surfactant, its properties at liquid interfaces are not easily accessible. This is mainly caused by the fact that in aqueous solution SDS is subject to hydrolysis, by which the homologous dodecanol (C12OH) is formed. Due to its enormously high surface activity it competes with SDS at the interface. We demonstrate here that this “natural” impurity C12OH does not remarkably affect the adsorption dynamics of SDS at the water/hexane interface, due to its high solubility in hexane. Therefore, the dynamic adsorption properties can be determined independent of disturbing dodecanol effects. The surfactant adsorbs diffusion controlled and the interfacial tension isotherm at the water/hexane interface is well described by a Frumkin model. However complementary experiments via direct admixture of dodecanol in hexane indicate a significant decrease in interfacial tension of the water–hexane interface at concentrations higher than 10−3 mol/l in hexane. This condition may happen when the oil phase is distributed as small droplets in a high concentrated solution of SDS. The distribution coefficient of C12OH between water and hexane is estimated from adsorption experiments to be Kp = co /cw =6.7×103. [ABSTRACT FROM AUTHOR]

Abstract: Maximum bubble pressure and pendent drop profile analysis tensiometry are used to study the dynamic surface tension and rheological characteristics of saliva samples taken from volunteers of various sexes and age, and children with caries. The tensiometric characteristics of the secretions released by parotid, submandibular and sublingual glands are studied, and age-related and sex-related changes of some tensiometric characteristics of saliva are found. The differences are most essential for the tensiometric characteristics of saliva samples taken from caries-free children as compared with those taken from children with caries. Thus, the dynamic interfacial tensiometry is very sensitive and capable of providing information additional to the biochemical data about the properties of saliva, and could be recommended for the development of new diagnostic methods in dentistry. [Copyright &y& Elsevier]

Abstract: Depending on the bulk composition, adsorption layers formed from mixed protein/surfactant solutions contain different amounts of protein. Clearly, increasing amounts of surfactant should decrease the amount of adsorbed proteins successively. However, due to the much larger adsorption energy, proteins are rather strongly bound to the interface and via competitive adsorption surfactants cannot easily displace proteins. A thermodynamic theory was developed recently which describes the composition of mixed protein/surfactant adsorption layers. This theory is based on models for the single compounds and allows a prognosis of the resulting mixed layers by using the characteristic parameters of the involved components. This thermodynamic theory serves also as the respective boundary condition for the dynamics of adsorption layers formed from mixed solutions and their dilational rheological behaviour. Based on experimental studies with milk proteins (β-casein and β-lactoglobulin) mixed with non-ionic (decyl and dodecyl dimethyl phosphine oxide) and ionic (sodium dodecyl sulphate and dodecyl trimethyl ammonium bromide) surfactants at the water/air and water/hexane interfaces, the potential of the theoretical tools is demonstrated. The displacement of pre-adsorbed proteins by subsequently added surfactant can be successfully studied by a special experimental technique based on a drop volume exchange. In this way the drop profile analysis can provide tensiometry and dilational rheology data (via drop oscillation experiments) for two adsorption routes — sequential adsorption of the single compounds in addition to the traditional simultaneous adsorption from a mixed solution. Complementary measurements of the surface shear rheology and the adsorption layer thickness via ellipsometry are added in order to support the proposed mechanisms drawn from tensiometry and dilational rheology, i.e. to show that the formation of mixed adsorption layer is based on a modification of the protein molecules via electrostatic (ionic) and/or hydrophobic interactions by the surfactant molecules and a competitive adsorption of the resulting complexes with the free, unbound surfactant. Under certain conditions, the properties of the sequentially formed layers differ from those formed simultaneously, which can be explained by the different locations of complex formation. [Copyright &y& Elsevier]

Abstract: The adsorption dynamics and equilibrium properties of hen egg-white lysozyme at the air/water interface is studied using bubble and drop profile analysis technique, ellipsometry and infrared reflection absorption spectroscopy (IRRAS). The set of equilibrium data can be described by a recently developed thermodynamic model. Using the equilibrium adsorption parameters the adsorption dynamics can be well described by a diffusion controlled mechanism at the lower concentrations and a mixed model at higher lysozyme concentrations where conformational changes in the surface layer become time determining. [Copyright &y& Elsevier]

Abstract: The standard technique for measuring surface or interfacial tension by drop profile analysis requires two main steps: (i) acquisition of drop/bubble images and determination of the profile coordinates via edge detection techniques; (ii) fitting of the theoretical drop/bubble profile to the experimental coordinates using the interfacial tension γ as adjustable parameter. As to edge detection technique, usually the position of the maximum grey level gradient is assumed to be the drop edge. In order to increase the accuracy of edge detection the procedure of fitting a normal distribution function to the experimental grey level gradients yields accurate results when the drop edge is located into the distribution centre. Conventional algorithms use the arc length along the drop profile as independent variable and fit in a vertical, horizontal or normal direction to the experimental shape coordinates, requiring that the coordinates have to be interpolated between experimental points. For small drops having an almost spherical shape, this technique leads to rather large errors. To avoid this interpolation problem a transformation of the Gauss–Laplace equation into polar coordinates can provide higher accuracy. For this, the angle of rotation is used as independent variable and the origin of the coordinate system is located exactly between the drop apex and the capillary tip at which the drop or bubble is formed. [Copyright &y& Elsevier]

Abstract: The composition of the pulmonary surfactant and the border conditions of normal human breathing are relevant to characterize the interfacial behavior of pulmonary layers. Based on experimental data methods are reviewed to investigate interfacial properties of artificial pulmonary layers and to explain the behavior and interfacial structures of the main components during compression and expansion of the layers observed by epifluorescence and scanning force microscopy. Terms like over-compression, collapse, and formation of the surfactant reservoir are discussed. Consequences for the viscoelastic surface rheological behavior of such layers are elucidated by surface pressure relaxation and harmonic oscillation experiments. Based on a generalized Volmer isotherm the interfacial phase transition is discussed for the hydrophobic surfactant proteins, SP-B and SP-C, as well as for the mixtures of dipalmitoylphosphatidylcholine (DPPC) with these proteins. The behavior of the layers depends on both the oligomerisation state and the secondary structure of the hydrophobic surfactant proteins, which are controlled by the preparation of the proteins. An example for the surface properties of bronchoalveolar porcine lung washings of uninjured, injured, and Curosurf treated lavage is discussed in the light of surface behavior. An outlook summarizes the present knowledge and the main future development in this field of surface science. [Copyright &y& Elsevier]

Abstract: The desorption of proteins from liquid interfaces depends on the conditions under which they have been adsorbed. At low concentrations, the adsorption process takes a comparatively long time and the molecules arriving at the interface have enough space and time to adsorb and unfold. In contrast, adsorption from higher concentrated solutions is faster and adsorbing molecules strongly compete from the beginning of the process. The rate of desorption is studied as a function of the adsorption layer coverage in order to understand to what extend protein adsorption is reversible. The experimental findings cannot give a clear answer on the reversibility, however, the theoretical analysis shows that desorption rates for proteins are many orders of magnitude lower than those for usual surfactants. [Copyright &y& Elsevier]

The experimental data on the surface rheological characteristics of dodecyl dimethyl phosphine oxide solutions obtained in a fully automatic oscillating bubble device under microgravity conditions in the frequency range 0.01–100 Hz are presented. The complex surface elasticity modulus is obtained form the amplitude- and phase-frequency characteristics of established pressure oscillations in a closed cell without calibration experiments by direct calculation of the necessary coefficients. The characteristics of the adsorption layers obtained from the elasticity modulus are in good agreement with adsorption isotherms and equations of state accounting for the intrinsic (2D) monolayer compressibility. [Copyright &y& Elsevier]

The transition state of dimerization at the interface and between the initial and final clusterization products is analyzed. The calculations were performed using the PM3 molecular orbital approximation, implemented in the MOPAC 2000 software. It is shown that the account of the Gibbs energy ΔG leads to a saddle point at the potential energy surface for the two-dimensional cluster formation and re-clusterization in monolayers of normal fatty alcohols (n=8, 12, and 16). The re-organization of a linear trimer and monomer into two dimers was considered. The maximum value ΔG=110 kJ/mol for n-tetradecanol is observed for a distance of 10.5 Å which corresponds to the transition state of re-clusterization reaction. If the distance between the n-tetradecanol trimer and monomer is lower than 10.5 Å, then tetrameric clusters are formed again. For higher distances the monomer is detached from the trimer. The dependencies of the parameters of the activated complex corresponding to the alcohol dimerization reaction on the chain length are analyzed. The position of the maximum on the Gibbs energy curve for dimerization at 298 K corresponds to an intermolecular distances for the dimers in the transition state of 6.31–6.69 Å. [Copyright &y& Elsevier]

The influence of micellar dissolution processes on the dynamic surface tensions of micellar solutions of SDS and SDS–DTAB mixtures was investigated by bubble pressure tensiometry. Based on a kinetic model the micelle dissociation constants are determined from dynamic surface tension data. The calculated micelle dissociation constants of SDS were in the range between 260 and 1600 s-1, i.e., the slow relaxation time for the SDS micelle dissociation process was in the range between 0.004 and 0.0006 s. These results are in good agreement with the experimental data for SDS (slow relaxation process), obtained by others using different methods. The calculated micelle dissociation constants of the SDS–DTAB complex were in the range between 0.3 and 30 s-1. [Copyright &y& Elsevier]

• Cooperative effects on the adsorption of alkane and surfactant molecules at very low concentrations are studied. • Quantum chemical PM3 simulations support the thermodynamic results for the alkane : surfactant ratios at the interface. • There are several principles of alkane and surfactant co-adsorption at the aqueous solution/alkane interface. • Results give a completely new picture of adsorption layers at the water/oil interface. A thermodynamic model is used to analyse the adsorbed layers of C n DMPO (n = 12, 13), C n TAB (n = 10, 12, 14, 16) and SDS at the aqueous solution/hexane and aqueous solution/dodecane interfaces. The combined model assumes that the adsorption of alkane from the oil phase is governed by the Frumkin model, and the adsorption of the surfactant molecules from the solution obeys the reorientation model. We are able to show that at low surfactant concentrations a remarkable decrease of interfacial tension occurs due to a cooperative formation of a mixed adsorption layers by surfactant and alkane molecules. By a quantum chemical semi-empiric (PM3) simulation we analyse the orientation of oil and surfactant molecules in the interfacial layer, and calculate the relative amounts of adsorbed surfactant and alkane. The values obtained from the thermodynamic models agree well with those results calculated via the PM3 simulation. The results show, that at the interface the short-chain hexane molecules can form bi-layers with the long-chain surfactants, while the dodecane and surfactant molecules having commensurable chain lengths are mainly forming mixed monolayer. [ABSTRACT FROM AUTHOR]