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Dynamic surface tensions and dilational visco-elasticity are easy accessible parameters of liquids. For human body liquids, such as urine, blood serum, amniotic fluid, gastric juice, saliva and others, these parameters are very characteristic for the health status of people. In case of a disease the composition of certain liquids specifically changes and the measured characteristics of dynamic surface tension of the dilational surface elasticity and viscosity reflect these changes in a clear way. Thus, this kind of physico-chemical measurements represent sensitive tools for evaluating the severity of a disease and can serve as control tool for the efficiency of applied therapies. The overview summarises the results of a successful work over about 25 years on this subject and gives specific insight into a number of diseases for which the diagnostics as well as the therapy control have been significantly improved by the application of physico-chemical experimental techniques. [ABSTRACT FROM AUTHOR]

The influence of hexane vapor in the air atmosphere on the surface tension of water and solutions of C 10 EO 8 , C n TAB and proteins are presented. For dry air, a fast and strong decrease of surface tension of water was observed. In humid air, the process is slower and the surface tension higher. There are differences between the results obtained by the maximum bubble pressure, pendant drop and emerging bubble methods, which are discussed in terms of depletion and initial surface load. The surface tension of aqueous solutions of β-сasein (BCS), β-lactoglobulin (BLG) and human serum albumin (HSA) at the interfaces with air and air-saturated hexane vapor were measured. The results indicate that the equilibrium surface tension in the hexane vapor atmosphere is considerably lower (at 13–20 mN/m) as compared to the values at the interface with pure air. A reorientation model is proposed assuming several states of adsorbed molecules with different molar area values. The newly developed theoretical model is used to describe the effect of alkane vapor in the gas phase on the surface tension. This model assumes that the first layer is composed of surfactant (or protein) molecules mixed with alkane, and the second layer is formed by alkane molecules only. The processing of the experimental data for the equilibrium surface tension for the C 10 EO 8 and BCS solutions results in a perfect agreement between the observed and calculated values. The co-adsorption mechanism of dipalmitoyl phosphatidyl choline (DPPC) and the fluorocarbon molecules leads to remarkable differences in the surface pressure term of cohesion Π coh . This in turn leads to a very efficient fluidization of the monolayer. It was found that the adsorption equilibrium constant for dioctanoyl phosphatidyl choline is increased in the presence of perfluorohexane, and the intermolecular interaction of the components is strong. [ABSTRACT FROM AUTHOR]

The drop profile method is used to study the influence of hexane vapor in the air atmosphere on the surface tension of water. It was shown that the results depend essentially on the air humidity. At the interface with dry air a fast and strong decrease of surface tension was observed. In humid air and in a measurement cell filled with water, the process is slower and the resulting surface tension is higher. The experimental conditions are defined at which stable results can be measured. A new theoretical model is developed to describe the influence of alkane vapor in the gas phase on the surface tension of aqueous solutions of the reorientable surfactant C 10 EO 8 . This model assumes that a first interfacial layer is composed by surfactants mixed with alkane, and a second layer which is formed by alkane only. The experimental equilibrium surface tension data for the studied C 10 EO 8 solutions agree very well with those calculated from the given model. [ABSTRACT FROM AUTHOR]

The dynamic surface tension of aqueous solutions of C 10 EO 8 at the interfaces with air and saturated hexane vapor was measured by the maximum bubble pressure tensiometer BPA for adsorption times t ≥ 0.01 s. It can be shown that the dynamic surface pressure of C 10 EO 8 solutions in a saturated hexane atmosphere increases at short adsorption times with increasing surfactant concentration. The theory of diffusion controlled adsorption of two surfactants from the water and gas phases, respectively, was used for data analysis. The obtained experimental results agree with a model based on a reorientation of the C 10 EO 8 molecules, assuming a strong intermolecular interaction between hexane and C 10 EO 8 molecules in the surface layer. Alternatively, an additional enhancement of C 10 EO 8 adsorption activity can be assumed due to the presence of hexane molecules at the interface. [ABSTRACT FROM AUTHOR]

Abstract: Maximum bubble pressure tensiometry is used to measure the dynamic surface tensions of micellar solutions of TX45 and mixtures of SDS/TX45 at the solution/air interface. For measurements of the dilational visco-elasticity of the micellar solutions the buoyant bubble profile tensiometry with harmonic bubble surface area oscillations is applied. The influence of dodecane solubilised in TX45 and mixed SDS/TX45 micelles, respectively, on the dynamic surface tension and dilational characteristics of these micellar solutions is determined. It is shown that the solubilisation of dodecane by micelles leads to a slowdown in the dynamic surface tension change and an increase in the visco-elasticity of the micellar solutions. Theoretical models proposed earlier are used for the description of the influence of solubilised dodecane on the dynamic surface tension and visco-elasticity of micellar TX45 solutions. It is shown that the presence of solubilised dodecane leads to a decrease of the CMC value, and to a decrease of the micelles diffusion coefficient. [Copyright &y& Elsevier]

Abstract: The complementary drop and bubble profile analysis and maximum bubble pressure tensiometry are used to measure the dynamic surface tension of aqueous SDS solutions in the presence of hardness salts (CaCl2 and MgCl2 in the ratio of 2:1 at concentrations of 6 and 40FH). The presence of hardness salts results in an essential increase of the SDS adsorption activity, which indicates the formation of Ca(DS)2 and Mg(DS)2 in the SDS solutions. The surface tension isotherms of SDS in presence of Ca(DS)2 and Mg(DS)2 are described using the generalised Frumkin model. The presence of hardness salts accelerates the ageing of SDS solutions as compared with the addition of 0.01M NaCl due to a faster hydrolysis and hence formation of dodecanol. These results are used to estimate the possible concentration of dodecanol in the studied SDS solutions. The buoyant bubble profile method with harmonic surface oscillations is used to measure the dilational rheology of SDS solutions in presence of hardness salts in the frequency range between 0.005Hz and 0.2Hz. The visco-elasticity modulus in the presence of hardness salts is higher as compared with its values in the presence of 0.01M NaCl additions. The ageing of SDS solutions leads to an essential increase of the visco-elastic modulus. [Copyright &y& Elsevier]

Abstract: The surface tension and dilatational visco-elasticity isotherms for β-casein determined at pH 7 and 9 are essentially the same, but differ remarkably from those measured at the isoelectric point (i.e.p.) of the protein at pH 5. A recently developed thermodynamic model is applied to the experimental data, which were not only obtained at equilibrium, but also under quasi-equilibrium conditions. It turned out that such a model can be adequately applied to data obtained not too far from the equilibrium state of a protein adsorption layer. The change in the model parameters allows to understand slow changes in the structure of the adsorption layer. Even at pH 5, where β-casein is most hydrophobic and in its most compact conformation, the data point to the fact that conformational changes may happen at the interface upon adsorption. [Copyright &y& Elsevier]

Abstract: The adsorption kinetics of mixed non-ionic surfactants can be described by a diffusion controlled adsorption when the correct model is used, i.e. when for each adsorbing species the corresponding thermodynamic model for the adsorption equilibrium is applied. For non-ionic surfactants, such as oxyethylated alcohols C12EO5 and C14EO8 and Tritons X-n, the reorientation model is the appropriate model. The dynamic surface tensions measured by bubble profile tensiometry are described very well by an efficient diffusion coefficient decreasing with the total surfactant bulk concentration. The physical reason for the change in D eff is the effect of convection in the system becoming more pronounced with the increasing in adsorption time. Equimolar mixtures of Triton X-45 and X-165 mimic the adsorption behaviour of Triton X-100. [Copyright &y& Elsevier]

Abstract: The adsorption kinetics, adsorption isotherm and the dilational surface rheology at low oscillation frequencies of sodium dodecyl sulphate (SDS) were studied by drop and bubble profile, and bubble pressure tensiometry. The effect of the inevitably present dodecanol in any SDS solution is taken into account by interpreting the experimental data with theoretical models valid for mixed surfactant solutions. In particular the dilational rheology method appears to be very suitable for demonstrating the continuously on-going hydrolysis reaction in a SDS solution, as the resulting dilational elasticities and viscosities change from a frequency dependencies characteristic to SDS to those expected for dodecanol adsorption layers. For surfactants as active as dodecanol, it turns out that all adsorption and rheological quantities depend significantly on the use of drop or bubble profile tensiometry, i.e. the small reservoir of available surface active molecules in a single droplet has to be considered quantitatively to obtain reliable data. [Copyright &y& Elsevier]

Abstract: The adsorption kinetics of four Tritons (TX45, TX100, TX165 and TX405) was studied by using the complementary drop and bubble profile and maximum bubble pressure tensiometry methods. While TX45 and TX100 can be quantitatively described by a diffusion controlled adsorption mechanism, based on the reorientation model for describing the equilibrium state of adsorption, the results for TX165 and TX405 deviate significantly from this adsorption mechanism. With additional assumptions, such as a finite rate constant for the transition of adsorbed molecules between states of different molar area, and effects of the composition of the Triton samples by many compounds of different ethylene oxide chains, a much better description of the experimental data is possible. [Copyright &y& Elsevier]

Abstract: The presence of micelles in the solution accelerates the adsorption dynamics of Tritons at the solution/air interface, depending on the micelle dissolution rate. Micellar solutions (at CMC and up to about 10× CMC) are studied by maximum bubble pressure tensiometry and oscillating bubble profile tensiometry. The dynamic surface tensions in the time range between 0.1ms up to 50s are analysed by a diffusion controlled mechanism. At very short times the effect of molecular reorientation of the Triton molecules at the surface is observed. The dilational elasticity data at low frequencies (0.005–0.2Hz) provide good agreement with a respective model assuming a very fast micelle dissolution. While for Triton X-45 and X-100 a quantitative agreement with existing theories is observed, the results for Triton X-165 and X-405 agree only qualitatively. This discrepancy is obviously due to the respective composition of the technical surfactants and a quantitative understanding would require an accurate knowledge on the sample composition. [Copyright &y& Elsevier]

Abstract: The adsorption of the non-ionic surfactants Triton X-45, Triton X-100, Triton X-165, and Triton X-405 can be described by a reorientation model, assuming that molar area required by adsorbed molecules depends on the surface coverage. The surface tension isotherms we measured by drop and bubble profile tensiometry, where it was shown that at low surfactant bulk concentrations, significant differences in the measured equilibrium tensions are due to depletion effects from the volume of a single drop. This difference in equilibrium surface tensions, observed between data from drop and bubble experiments, is a good tool to determine the adsorbed amount from the total mass balance. An intersection point of all surface tension isotherms at about 1μmol/l is explained by a respective displacement of the ethylene oxide chain from the interface by the hydrocarbon chain. [Copyright &y& Elsevier]

Abstract: Exact knowledge of the dead time as part of the bubble lifetime in the maximum bubble pressure method is an important prerequisite for accurate dynamic surface tension measurements. The duration of the dead time depends essentially on the capillary geometry and affects significantly the measured surface tensions of concentrated surfactant solutions. Increase of the dead time leads to a significant surface tension decrease of a freshly formed bubble surface due to the significantly higher residual adsorption of the surfactant molecules. It is shown that correct dynamic surface tensions are obtained with the experimental procedure of Sugden''s method only when in addition to the fixed frequency of bubble formation, also the dead time values for the two capillaries are kept constant. [Copyright &y& Elsevier]

Abstract: The analysis of the available bubble life times and dead times for the bubble pressure tensiometer BPA-1S shows that dynamic surface tensions can be measured also for surfactant solutions at concentrations many times higher than the corresponding CMC. For the three nonionic surfactants Triton X-100, Triton X-45, and C14EO8 experiments are performed for solutions with a concentration of up to 200 times the CMC (C14EO8). Comparison of the experimental data with micelle kinetics models yields rate constants for the fast micelle dissolution process, which are in a good agreement with values obtained by other experimental methodologies. [Copyright &y& Elsevier]

The exact description of the adsorption kinetics for surfactant mixtures is possible when the co-adsorbing species follow the same adsorption model. In general cases, the equilibrium state can be understood only by an approximate model so that the simulation of the adsorption kinetics process requires some special assumptions. Based on a diffusional mechanism, we discuss the adsorption of surfactant mixtures and present a procedure which allows a sufficiently accurate determination of the competitive adsorption process of two surfactants via the characteristic parameters of the single components. A correction involved in the simplified model becomes necessary due to differences in the subsurface concentration for each components adsorbing alone or in competition. The results are compared with experimental data for homologous alkyl dimethyl phosphine oxides. [Copyright &y& Elsevier]

At low surfactant concentration the loss of molecules in the bulk phase due to adsorption can be significant if the surface-area-to-bulk-volume ratio is large. This loss of molecules, however, can be used to determine the adsorption of a surfactant if the experiments are performed under well-controlled conditions. Experiments are performed with the model surfactant tridecyldimethylphosphine oxide (C13DMPO) using drop profile analysis and ring tensiometry. Taking into account the different area/volume ratios and the measured equilibrium surface tensions as a measure of the final surfactant bulk concentration, the adsorption of C13DMPO is determined. [Copyright &y& Elsevier]

The maximum bubble pressure tensiometry is the only technique that allows measurements of dynamic surface tensions of surfactant solutions in the short time range down to milliseconds and even below. To reach such extremely short adsorption times, many hydrodynamic and aerodynamic effects have to be taken into consideration in order to measure physical values or to correct for the respective influences. In particular, the design of the instruments, and essentially the geometry of the measuring cell and capillary, are of key importance for accurate experiments. Another important issue is the way of determination of the characteristic bubble times, such as bubble dead time and lifetime. Examples demonstrate that the bubble pressure technique provides a powerful tool for studies of adsorption dynamics even for solutions of higher viscosities. [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 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]

Abstract: An electroneutral two-dimensional solution model has been used to describe the impact of the chain length on complex formation in mixed ionic surfactant solutions, in particular for mixtures of anionic sodium alkanoates (CH3(CH2) n COONa) with various chain lengths and cationic tetradecyl trimethyl ammonium bromide (C14TAB). The equilibrium surface tensions of the homologous series of alkanoates are studied using plate tensiometry. The experimental data of the pure alkanoates, the resulting 1:1 complexes and the isotherms of the mixtures are analyzed on the basis of the Frumkin model. It turns out that the formation of complexes depends on the chain length of the alkanoate as reflected in the adsorption constant b and that the adsorption behavior is dominated by the presence of 1:1 complexes in the mixtures. [Copyright &y& Elsevier]

Abstract: Equilibrium and dynamic surface tension of non-ionic triblock copolymers, Pluronics, varying in the number of poly(ethylene oxide) and poly(propylene oxide) groups have been measured by means of drop profile tensiometry at the water–hexane interface. The experimental data have been explained on the basis of a theory previously developed for protein solutions . This model assumes the coexistence of different adsorbed states at the interface, depending on the surface coverage. It has been shown that the best fit is achieved when a polylayer adsorption is considered. Reasonable values of the adsorption layer thickness and realistic values of the diffusion coefficient are obtained which validate the model proposed. [Copyright &y& Elsevier]

Abstract: The present work is devoted to the study of interactions between a cationic polyelectrolyte and an anionic surfactant in the solution bulk and at the water/air and water/oil interfaces. For this purpose surface and interfacial tensions, zeta potential, ellipsometry and dynamic light scattering (DLS) were used to describe the formation of complexes of polyallylamine hydrochloride (PAH) and sodium dodecyl sulphate (SDS). Surface and interfacial tension results are supported by zeta potential measurements and aggregation size measurements via DLS. Due to electrostatic binding of dodecyl sulphate anions to the positive charges of the polyelectrolyte chains the resulting complexes become more hydrophobic. At the 1:1 ratio of PAH:SDS the complexes precipitate and become recharged due to the additional hydrophobic binding of SDS ions. The complexation leads to a depletion of surfactant and hence to an increase in surface and interfacial tensions. [Copyright &y& Elsevier]

Abstract: The co-adsorption of hexane from the vapor phase and surfactants from an aqueous solution at the surface of drops are studied in dependence of surfactant concentration. For the anionic surfactant SDS as well as the cationic surfactant DoTAB a significant surfactant–hexane co-adsorption and a remarkable decrease in surface tension is observed. This hexane co-adsorption effect increases with surfactant concentration and reaches a maximum value at intermediate surfactant concentrations below the CMC values. Dynamic surface tension measurements by drop profile analysis tensiometry and calculation of the standard deviation from Laplacian shape fitting, demonstrate three different adsorption regions. The kinetics and equilibrium state of adsorption depend significantly on the surfactant concentration. For the initial effects (regular adsorption region) which appear quickly, a thermodynamic model is applied which is based on a mixed adsorption layer approach. [Copyright &y& Elsevier]

Abstract: The adsorption behaviour of alkyl sulphates at the water/oil interface is rather different from that frequently studied at the water/air interface. First of all typical impurities in form of homologous alcohols, immanent for this type of surfactants at the surface of aqueous solutions, are not essential at the water/oil interface due to their high solubility in the oil phase. In the interfacial layers the oil molecules, here hexane, are incorporated into the structure, and have essential impact on the equation of state at low interfacial coverage and in particular at the shorter alkyl chain surfactants. For the longer chain alkyl sulphates (NaC14SO4 and NaC16SO4) and at high interfacial coverage the hexane molecules are squeezed out of the adsorption layer due to the stronger hydrophobic interaction between the surfactants’ alkyl chains. The adsorption characteristics are discussed in the framework of the two well-known thermodynamics Frumkin and reorientation models. [Copyright &y& Elsevier]

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]

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]

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]

Abstract: The dilational rheology of liquid–air interfaces is very sensitive to the contents and composition of surface active molecules in the liquid. Therefore, these rheological quantities along with dynamic surface tension could prove suitable parameters for the diagnosis of certain deseases. The cerebrospinal fluid, expired air condensate and umbilical blood of newborns have been investigated for patients suffering from neurosyphilis and for a reference group for comparison. The visco-elasticity is measured by pendent drop analysis tensiometry with transient and harmonic surface area perturbations. It is shown that the dilational visco-elasticity parameters depend significantly on the disease, i.e. can be used as diagnostic tool. [Copyright &y& Elsevier]

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: 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]

• 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]