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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.

In this work studies of growing water droplets in air are presented, with the aim to assess the applicability limit of the Droplet Profile Analysis Tensiometry (PAT). High inflow rates are applied for systems containing surfactants with high adsorption rates. However, under dynamic formation conditions, the measured surface tension values deviate from the theoretical values even for pure systems. Therefore, Computational Fluid Dynamics (CFD) is applied to gain detailed insight into the hydrodynamics of the growing drop. Since the flow is dominated by surface tension forces, an interface tracking approach is applied, which is able to capture the flow in a physically correct way. At high flow rates the inflow jet is not fully dissipated before approaching the free surface. Therefore, the pressure profile inside the drop is not uniform as is required in the derivation of the Gauss Laplace equation. The shape of the drop no longer represents the Gauss Laplace profile corresponding to the theoretical surface tension coefficient. As result we can confirm that the evaluation of surface tension by fitting to the Gauss Laplace Equation is not valid for dynamic droplet formations. Indications for future improvements of the evaluation procedure are provided.

The displacement of mucin from the droplet simultaneously and sequentially was studied using drop profile analysis tensiometry (PAT-1) instrument equipped with special designed instrument to allow the bulk exchange of the droplet. The dynamic and equilibrium surface tension profiles were used to discuss the stability of the formed mixed surfactant–protein layer at the interface. The viscoelastic data also were used to determine the extent of protein at the interface. The results showed that the simultaneously formed surfactant–protein layer at the complex is more surface active than that formed sequentially. The results were discussed according different mechanisms describing the interaction between the protein molecules and the cationic surfactant. The study explained the antimicrobial activity of the studied surfactant in the biological field.

Profile analysis tensiometry (PAT) is presently the most frequently used technique for measuring surface tensions of liquids. The basis of this methodology is however an equilibrium force balance as given by the Gauss–Laplace equation. Therefore, its application under dynamic conditions, i.e. for growing drops or bubbles, is questionable. We discuss the limits of the applicability of PAT under dynamic conditions by using a growing drop configuration equipped with a high speed video camera. The systems studied are the water/air and water/hexane interface. The obtained “dynamic” drop profiles are analyzed by fitting the classical Gauss–Laplace equation. The results are additionally compared with experimental data obtained from capillary pressure tensiometry. The analysis allows defining three different regions related to respective drop expansion rates.

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

Protein corona adsorption layers on nanoparticle surfaces, dispersed in biological fluids, can significantly change the interfacial interactions, reactivity, and mobility of the original nanoparticles designed as a drug carrier or existing as bioaerosols, bacteria, or viruses. The evaluation of the level of interactions (hard/soft corona), dispersion stability, and the ratio of the attached proteins per nanoparticle are essential parameters for the characterization of the protein corona formation on nanoparticle (PCN). In spite of development of several experimental techniques for this purpose, still more powerful, economical and fast measuring techniques are needed to work in-situ at original solution samples (near the physiological conditions), without requirement of additional sample preparation which can change the quality/quantity of the original interactions. In this study, a novel experimental protocol based on the analysis of dynamic interfacial properties (ADIP) is developed for in-situ evaluation of the protein–nanoparticle interactions under original conditions. For this purpose, dynamic surface tension and interfacial elasticity values of the bovine serum albumin (BSA) solutions alone and in mixed solutions with silica nanofluids are measured using drop profile analysis tensiometry. A considerable difference between the dynamic surface tension of BSA and protein corona solutions (BSA + SiO2 NPs complexes) demonstrates significant adsorption of the protein molecules at nanoparticles, confirmed by Fourier Transform Infrared Spectroscopy (FTIR) analysis. PCN complexes illustrate much slower kinetics of adsorption due to a smaller diffusion coefficient according to their larger size. The free proteins available in the solution were estimated considering early-time values of the dynamic surface tension, used for estimation of the adsorbed proteins per unit area of the silica surface (mol/cm2), considering the initial protein concentration in the bulk. The results show very good agreement with others’ results provided by AFM, DLS, UV–vis Spectroscopy, Multi-Parametric Surface Plasmon Resonance (MP-SPR), and Quartz Crystal Microbalance (QCM). Our novel experimental protocols and data analysis, demonstrates promising results for differentiation of the hard and soft corona layers, and unique for soft corona layer recognition, which is difficult by other available techniques, due to quick disturbances of weak protein-protein interaction in soft corona. The measured interfacial elasticity values confirm PCN formation and provides additional information for better differentiations of the corona layers.

The adsorption dynamics of silica nanoparticles (NP) and cetyltrimethylammonium bromide (CTAB) complexes is studied via dynamic surface properties characterization by the drop Profile Analysis Tensiometry (PAT). Considering the hypothesis that a nanoparticle with a certain number of attached surfactant molecules can be considered as a unified surface-active complex, the equilibrium surface tension for fixed CTAB/NP mixing ratios were considered to construct respective adsorption isotherms. The results can be well described by the Frumkin adsorption model. The fitting parameters of the Frumkin model for different mixing ratios demonstrate that complexes with higher mixing ratios occupy less space at the interface and show weaker interaction with each other. Also, the Ward and Tordai equation was fitted to the dynamic surface tension data for each ratio. The results show that by increasing the mixing ratio the adsorption of complexes tends toward a diffusion-controlled mechanism. On the other hand, for lower mixing ratios, the adsorption is affected by adsorption/desorption barriers so that a mixed diffusion-kinetic-controlled mechanism is recognized and discussed.

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 aim of this experimental study is to get insight into the combined interfacial and bulk properties of silica nanoparticles and CTAB cationic surfactant mixture in water/oil systems. The study is focused on low surfactant/nanoparticle ratios to avoid extra free surfactants in the bulk. Surfactant concentration was fixed at 9.0 × 10 −2 mM, (0.1 CMC) and the nanoparticle concentration varied between 0 and 5 wt.%. Drop profile analysis tensiometry (PAT) was utilized to measure the equilibrium and dynamic interfacial tension at water/heptane interface. To comprehend the layer structure of nanoparticle-surfactant complex at the interface, the interfacial rheology was also studied using drop oscillation experiments. The formation of the surfactant-nanoparticle complex layer at the drop surface and its different structures were explained using surface pressure vs. drop surface area curves and the standard deviations of experimental drop profiles from Young-Laplace equation during large compression/expansion cycles. The results demonstrate significant changes in the standard deviation at the onset of closed-packed state of surfactant-nanoparticle complex collapse. This illustrates the formation of irregular adsorbed layer (multilayer formation). The number of surfactant-nanoparticle complexes at drop surface was calculated by accurately assessing the surface coverage at the maximum packing of particles at the interface. For the studied concentration range, surfactant-nanoparticle complexes demonstrate surface activity higher than similar surfactant concentration, however with slower dynamics of adsorption. Elasticity measurements during large compression/expansion path can also provide more details about the structure of mono-/multi-layer formation.

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.

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 inflow pattern of liquid into a droplet is studied experimentally using a surface active dye and compared with results of CFD simulations. The results show visual agreement between experiments and simulations. The CFD simulations show also good agreement with the surface tension measured by drop profile analysis tensiometry (PAT). The inflow of the surfactant induces a Marangoni instability caused by the local arrival of the surfactant at the drop surface. The onset of this Marangoni instability observed experimentally has a delay of about 10 s when compared with the simulation results. Different scenarios are discussed, including a boundary layer barrier, a kinetic-controlled adsorption mechanism, the way of renewing a mobile interface, and a critical Marangoni number required for the onset of such instability. It turns out that besides the commonly defined critical Marangoni number, the main reason for delay of Marangoni instability is possibly the mobility of the drop surface which can prevent the establishment of a quasi static adsorbed layer required for the appearance of such surface effect.

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.

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.

The coaxial double capillary for controlling pendant drops is an interesting experimental technique for investigating adsorption kinetics and transport properties of soluble components in the bulk which can exchange with the interface. The key process of this technique is the exchange of the entire bulk of a droplet without any direct perturbations of the interface due to forced convection. Therefore efficient exchange conditions should be determined in order to provide results during the exchange protocol that can be used for a reliable data interpretation of the adsorption process. In this paper we report on the flow into a drop formed with a double capillary set-up, as visualized by using a dye and simulated by computational fluid dynamics (CFD) of the flow field and mass transfer during the drop bulk exchange. Regarding the experimental protocols, for better monitoring and control of the droplet size a pulse-like flow with different injection volumes (dV) and pulse intervals (dt) combined with drop profile analysis tensiometry (PAT1) was used. The visualized experimental concentration fields are confirmed by the concentration contours obtained from CFD simulations. Both approaches give access to estimate the moment when the dye arrives at the drop surface. The drop surface exchange is also monitored via the measured surface tension. The computational results for average drop interface concentration (DIC) and drop outlet concentration (DOC) are discussed for different conditions.

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.

Dynamic surface tension of non-ionic surfactants pluronic F-168 and poly(dimethylsiloxan) bis(hydroxyalkyl) terminated (PDS) were studied using the drop profile analysis tensiometry. The comparison of these data with those from styrene heterophase polymerization studies show that only in the PDS:pluronic mixing ratio range between 1:0.2 and 1:0.8 the contribution of each surfactant component manifests its effect and provides appropriate conditions for the synthesis of polymer particles with required properties.