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Harmonic surface oscillations of drops are used to study the dilational surface rheology of solutions of the oxyethylated alcohols C10ЕO8, C12ЕO5 and C14ЕO8 at the fixed oscillation frequency of 0.1 Hz for adsorption times between 200 and 5000 s. It was found that at shorter adsorption times the visco-elasticity modulus is lower than its value at longer adsorption times or in the equilibrium state of the adsorption layer at the same surface pressure. This mechanism is explained by the fact that an actual surfactant, characterized by a certain number of EO groups, is in fact a mixture of compounds with a quite wide range of these groups. As molecules with a smaller number of EO groups exhibit a higher visco-elasticity modulus, the changes in the composition of the adsorbed layer during the adsorption lead to the given situation. These considerations are supported by theoretical calculations of the adsorption for mixed surfactants solution.

The surface tension isotherms and dilational visco-elasticity for three nonionic surfactants (C10EO8, C12EO5 and C14EO8) 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.

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.

The adsorption of proteins (β-lactoglobulin, β-casein) and the oxyethylated non-ionic surfactants C10EO8 and C14EO8 at the aqueous solution/air interface is strongly enhanced and accelerated by the presence of hexane vapour in the air phase caused by the co-adsorption of hexane molecules. Due to this hexane co-adsorption, the dependence of dilational visco-elasticity modulus on surface pressure is shifted towards larger surface pressure values. The adsorption kinetics of the studied non-ionic surfactants shows the same picture as that observed for the two proteins. In contrast, the desorption process of the hexane molecules from a pre-adsorbed mixed adsorption layer is very slow. This decelerated desorption is explained by a rather large desorption energy required by the hexane molecules. The experimental data are compared with several theoretical models developed earlier. The results allow estimating the activation energy for the hexane desorption from adsorption layers of the two studied non-ionic surfactants.

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. Refereed/Peer-reviewed

The adsorption kinetics of beta-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. Refereed/Peer-reviewed

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.

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.

A thermodynamic model is used to analyse the adsorbed layers of CnDMPO (n = 12, 13), CnTAB (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.

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, C12TAB and C12DMPO, 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.

The adsorption from mixed aqueous solutions of polyallyl amine hydrochloride (PAH)/sodium dodecyl sulphate (SDS), both in absence and presence of the inorganic electrolyte NaCl, at the solution/tetradecane interface is studied experimentally by drop profile analysis tensiometry. The results are discussed in terms of a competitive adsorption of PAH + SDS complexes and unbound SDS molecules. A thermodynamic model, developed originally for the adsorption of protein/surfactant mixtures, is modified such that it takes into account the formation of PAH + SDS complexes and the influence of the SDS concentration on their surface activity. It is shown that at low SDS concentrations the properties of the adsorbed monolayer are determined mostly by the adsorption of complexes, while at high SDS concentrations the properties are determined mostly by the SDS adsorption and the effect of the PAH + SDS complexes is negligibly small. The proposed modified model demonstrates good agreement with the experimental data.

The dynamic interfacial tension and dilational rheology of alkyltrimethylammonium bromides (C n TAB) solutions are studied for n = 10, 12, 14 and 16 using drop profile analysis and capillary pressure tensiometry. The experimental data are analyzed on the basis of two theoretical models, the Frumkin Ionic Compressibility and the Langmuir Compressibility models [1] . We show that even the simplified Langmuir Compressibility model is able to adequately describe the dynamic properties of the investigated ionic surfactants. Dynamic experiments point out the difficulties related to short chain surfactants due to their very fast adsorption kinetics which cannot be studied with conventional tensiometer setups.

The adsorption behaviour of β-Lactoglobulin (BLG) at the air/water surface and the visco-elastic properties of the BLG adsorption layers are discussed in terms of quasi-equilibrium adsorption isotherms and equation of state gained by comparing a model with a set of experimental data obtained by bubble profile analysis tensiometry. The presented simultaneous characterization of the adsorption isotherm and the dilational visco-elasticity under dynamic and equilibrium conditions is based on measurements of the dynamic surface pressure and its response to sinusoidal bubble area variations. The thermodynamic model was applied to the set of experimental data under the assumptions of monolayer compressibility, surface dimerization and development of a secondary layer. The effect of the bubble surface age on the adsorption isotherm and equation of state, and also on the visco-elastic characteristics is discussed and it allows concluding that the existence of a bilayer structure, which is formed beyond a certain protein adsorption, is responsible for the observed relaxation processes.

The superposition-additive approach developed earlier was shown to be applicable for the calculations of the thermodynamic parameters of formation and atomisation of conjugate systems, their dipole polarisability, molecular diamagnetic susceptibility, π-electronic ring currents etc. In this publication, the applicability of this approach for the calculation of the thermodynamic parameters (enthalpy and Gibbs’ energy of formation from elementary substances, and absolute entropy) of alkanes, fatty alcohols, thioalcohols, amines, nitriles, fatty carboxylic acids and nitriles with the general composition CnH2n+1X (X is the functional group) is studied. It is shown that the thermodynamic quantities determined using the proposed approach agree to within the satisfactory precision with the available calculated and experimental data.

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.

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.

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 [1] . 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.

The adsorption of the anionic surfactant SDS in mixtures with non-ionic surfactants of the Triton type (X-45, X-100, X-165, X-405) is studied by bubble profile analysis tensiometry at the solution/air interface. The surface tension isotherms are described by a thermodynamic model for mixed surfactants which allows consideration of different adsorption models for the two compounds: a reorientation model for the Tritons and a Frumkin model for SDS. The experimental data are very well described by the model while for the mixtures the characteristic parameters of the single compounds have been used.

Surface and interfacial tension measurements have been performed at the water/air and water/hexane interface by drop profile analysis tensiometry for a series of alkyl trimethylammonium bromides with different chain lengths (C10, C12, C14 and C16). The effect of neutral phosphate buffer (10 mM, pH 7) has been investigated and the results have been compared with literature data obtained for the same surfactants in the absence of salt. The use of a modified Frumkin isotherm (Ionic Compressibility) taking into account the mean activity of all ions in solution leads to a real improvement of the data interpretation, while the classical Frumkin Compressibility model yields similar results but overestimates the adsorption parameters. At the water/hexane interface, the hexane molecules are incorporated in the surfactant layer and it results a kind of competitive adsorption for the shortest chain surfactant (10 carbons). For the longest chains, the attractive interactions between the hydrophobic chains of the adsorbed surfactants are strong enough to replace the solvent molecules from the interface.

For the characterisation of interfaces between two immiscible liquids, capillary pressure tensiometry is the most versatile method. To measure short time interfacial tensions it has to be applied under very dynamic conditions for which the following protocols have been developed: 1—continuously growing drop (CGD); 2—pre-aged growing drop (PGD); 3—stopped growing drop (SGD). The CGD is a procedure analogous to the well-known maximum bubble pressure tensiometry for liquid–gas interfaces, however, the processes of drop detachment and formation of the subsequent drop require a rather complex data analysis. For the PGD protocol based on a growing drop with an initially pre-established equilibrium adsorption layer, the complexities are less, but for higher surfactant concentrations it is not optimum. The SGD protocol provides a drop with a fresh surface and the dynamic interfacial tension can be monitored in the absence of hydrodynamic effects. All three protocols complement each other and provide a good set of data for dynamic interfacial tensions at short adsorption times.

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 C 12 EO 5 and C 14 EO 8 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.

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.

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.

The dilational rheology of adsorption layers of four Tritons (TX45, TX100, TX165 and TX405) was studied by using the oscillating drop and bubble profile methods at frequencies between 0.005 Hz and 0.2 Hz. It is shown that the visco-elasticity modulus |E| for TX45 solutions can be excellently described by a rigorous reorientation model. For the description of the dilational rheology of TX100 adsorption layers an additional compressibility of the adsorbed molecules at high surface pressure has to be considered. Also the phase angle ϕ as a function of concentration can be described quantitatively for the two Tritons. For TX165 and TX405, having a much larger number of ethylene oxide units, the models is not in good agreement with the experimental data. For these molecules, obviously the presence of homologues in the surfactant samples play a larger role and the dependencies of |E|(c) and ϕ(c) show features as common for surfactant mixtures.

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.

The mixed adsorption layers of hen egg-white lysozyme/SDS for fixed protein concentration and varying surfactant concentrations at the air/water interface have been studied using bubble and drop profile analysis tensiometry and ellipsometry. The set of equilibrium and kinetic data are compared with thermodynamic and adsorption kinetic models developed recently. The parameters used for the mixed system are essentially those parameters obtained for the individual components. For a lower SDS concentrations the agreement with the model is good while in the transition region to a mainly surfactant controlled surface layer is poor.

A novel procedure of analysis of protein adsorption at liquid interfaces is proposed in terms of the steady state (equilibrium) surface pressure–concentration curves and also the dynamic interfacial tension–time curves obtained for each individual protein concentration. The analysis is performed on the basis of a thermodynamic model that has been recently extended to the study of adsorption dynamics. The procedure presented is based on the modification of the area values given to the protein at the oil–water interface with respect to those used at the air–water interface. This is done in order to account for the differences inferred from the experimental data. In this manner, and in view of the excellent correlation found between theory and experiment, the model substantially deepens the understanding of the adsorption process in two steps. Firstly, the fitting of the equilibrium isotherm provides a set of characteristic parameters that quantifies the interfacial structure adopted by the protein at the oil–water interface in terms of the partial molar area. Subsequently, the optimal set of parameters obtained by this procedure is also used in the fitting of the dynamic curves and similarly the area parameters changed according to experimental findings. Such practice yields as a conclusion a direct relationship between the degree of interfacial unfolding attained by the protein at the tetradecane–water interface and the bulk concentration, representing an innovative quantification of the kinetics of protein adsorption.

The adsorption–calorimetric method is used to study the adsorption values and adsorption heats of benzene at the modified kaolinite and silica samples. The entropy diagrams of the benzene adsorbed at these samples are calculated. The results are compared with the entropy losses calculated from the ideal models of localised and non-localised adsorption. It is shown that the nature of the inorganic matrix, the modifier type and the modification degree determine the character of the benzene sorption on the studied sorbents, which can be described either by the supermobile model, non-localised model or non-localised model with partial loss of rotational degrees of freedom. For an infrequent case of supermobile adsorption of benzene at the modified silica, the frequencies of the vibrations of the molecule perpendicular to the surface are calculated.

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.

Adsorption of water, methanol and benzene at carboxyl groups at the edge of the graphene sheets at the side face of graphite microcrystallites is studied. The adsorption enthalpies of the studied substances are determined using quantum chemical calculations. The adsorption of water molecules is shown to result in the formation of dimers at the adsorption centre, while only single methanol or benzene molecules are adsorbed. The formation of water molecule clusters on a hydrophilic centre at the surface of graphite and other adsorbents is shown to be a characteristic feature of water adsorption.

A model for the adsorption kinetics of globular proteins at a liquid/fluid interface is described which takes into consideration the transport of molecules in the bulk by diffusion and a process accounting for the transition between the adsorption states of the protein molecules. The model is based on a recently published thermodynamic model for the adsorption isotherm of proteins at a liquid/fluid interface, which considers various adsorption states with different molar interfacial area. The model calculations show the particular features common to protein adsorption kinetics: induction period for low bulk concentrations, steep interfacial tension changes, significant effect of the transition kinetics on the overall adsorption process. A comparison with dynamic surface tension data for β-lactoglobulin at the water/air interface shows good agreement. In full agreement with the generally excepted physical picture, it turns out that the adsorption kinetics is significantly influenced by the transfer kinetics between the different adsorption states of the protein molecule. The estimation of the transfer rate constant yields values of the order of k =10 −3 s −1 .

Calculations of energetic and structural characteristics of clusters are performed to model certain states of the systems involved in the process of Mg 2+ →Ca 2+ exchange in water solution — montmorillonite system. The energy of this ion, exchange of Δ F ≈−215 kJ mol −1 , was calculated. While the sign of Δ F corresponds to the substitution Ca 2+ →Mg 2+ , which actually takes place the absolute values of the energetic yield of the exchange reaction exceed those found in the experiments by two orders of magnitude. This discrepancy is shown to be mainly due to the energetic effects caused by the expansion of the montmorillonite interlayer gap Δ d =0.14 nm during the substitution of [Mg(H 2 O) 12 ] 2+ aquacations by larger [Ca(H 2 O) 16 ] 2+ ones. It follows from the estimations based on the dilatometric data for the interlayer swelling of layer silicates and related variations in the thermodynamic characteristics that this increase in Δ d corresponds to the increase in the internal energy of the adsorbent by Δ U ∼200 kJ kg −1 , or ∼200 kJ per one gram-equivalent of exchange cations, which agrees satisfactorily with the calculated values Δ F .

The dynamic surface tension of aqueous solutions of alkyl dimethyl phosphine oxides with different alkyl chain length is studied. The experimental results obtained for the lower homologues (C8–C12) were found to correspond rather satisfactorily with the values predicted by the classical diffusion model. For the higher homologues (C13–C15), however, the measured dynamic surface tensions are higher than the theoretically predicted ones. Fitting would lead to a theoretical diffusion coefficient for, say, C14DMPO, which was 3 to 4 times larger than the physical values. A diffusion-controlled adsorption kinetics theory is proposed which considers two states of CnDMPO at the surface, each with its characteristic different partial molar area value. This theory agrees with the experimental data with typical diffusion coefficients.