Drop Profile Analysis Tensiometry Research Articles

Evaluation of Interfacial Structure of Self-Assembled Nanoparticle Layers: Use of Standard Deviation between Calculated and Experimental Drop Profiles as a Novel Method.

The self-assembly of nanoparticles (NPs) at interfaces is currently a topic of increasing interest due to numerous applications in food technology, pharmaceuticals, cosmetology, and oil recovery. It is possible to create tunable interfacial structures with desired characteristics using tailored nanoparticles that can be precisely controlled with respect to shape, size, and surface chemistry. To address these functionalities, it is essential to develop techniques to study the properties of the underlying structure. In this work, we propose an experimental approach utilizing the standard deviation of drop profiles calculated by the Laplace equation from experimental drop profiles (STD), as an alternative to the Langmuir trough or precise microscopic methods, to detect the initiation of closely packed conditions and the collapse of the adsorbed layers of CTAB-nanosilica complexes. The experiments consist of dynamic surface/interfacial tension measurements using drop profile analysis tensiometry (PAT) and large-amplitude drop surface area compression/expansion cycles. The results demonstrate significant changes in STD values at the onset of the closely packed state of nanoparticle-surfactant complexes and the monolayer collapse. The STD trend was explained in detail and shown to be a powerful tool for analyzing the adsorption and interfacial structuring of nanoparticles. Different collapse mechanisms were reported for NP monolayers at the liquid/liquid and air/liquid interfaces. We show that the interfacial tension (IFT) is solely dependent on the extent of interfacial coverage by nanoparticles, while the surfactants regulate only the hydrophobicity of the self-assembled complexes. Also, the irreversible adsorption of nanoparticles and the increasing number of adsorbed complexes after the collapse were observed by performing consecutive drop surface compression/expansion cycles. In addition to a qualitative characterization of adsorption layers, the potential of a quantitative calculation of the parameter STD such as the number of adsorbed nanoparticles at the interface and the distance between them at different states of the interfacial layer was discussed.

Interaction of catalyst nanoparticles and pollutant molecules in photocatalytic wastewater treatment: Novel characterization via dynamic surface properties

In photocatalytic processes for wastewater treatment, the degradation reactions mainly occurs at the surface of the catalyst and nearby regions. Therefore, understanding of the interactions between the catalyst and pollutant in a photocatalytic degradation system is of utmost importance, determining the affinity of the pollutant molecules to be attracted or repulsed from the photocatalyst particles. The aim of this experimental study is to get a better insight about the interactions between the catalyst nanoparticles and pollutant molecules via a novel characterization method based on dynamic surface phenomena analysis. Drop profile analysis tensiometry (PAT) is used for this purpose for dynamic surface tension and elasticity measurements of dye solutions (as pollutants), and in mixed solutions with catalyst particles at different pH conditions. The results demonstrate that the cationic dye Malachit Green (MG) has a low surface activity at pH 6.5, with an equilibrium surface tension about 69 mN/m. While it becomes much more surface active at pH 9 and surface tension is decreased to 59 mN/m. Adding TiO2 nanoparticles to MG solution at pH 9 causes a significant increase in surface tension, with much slower dynamics of adsorption at air/water interface. These results indicate a significant attraction and attachment of MG molecules to the surface of TiO2 nanoparticles, creating MG-TiO2 complexes, which have mobility much slower than the free MG molecules. This observation is in a good correlation with observed higher efficiency of MG degradation around pH 9. The capability of this novel experimental protocol is also examined for the anionic dye Eriochrom Black-T (EBT). According to the results, a significant interaction between TiO2 nanoparticles and EBT exist at pH 3, which is also well in accordance with reported higher efficiency of EBT degradation around pH 3. The elasticity measurements performed in this work, also support these results as an important characterization parameter for estimation of the molecular interactions in mixed adsorbed layers.

Can Enzymatic Treatment of Sugar Beet Pectins Reduce Coalescence Effects in High-Pressure Processes?

While sugar beet pectins (SBPs) are well known for effectively stabilizing fine oil droplets in low-fat food and beverages, e.g., low-fat dressings and soft drinks, it often fails in products of higher oil contents. The aim of this study was to improve the emulsifying properties of SBPs and, consequently, their ability to reduce coalescence during high pressure homogenization of products with increased oil content. Therefore, the molecular size of SBPs was reduced by partial cleavage of the homogalacturonan backbone using the enzymes exo- and endo-polygalacturonanase and varying incubation times. The sizes of SBPs were compared based on the molecular size distribution and hydrodynamic diameter. In addition, to obtain information on the interfacial activity and adsorption rate of SBPs, the dynamic interfacial tension was measured by drop profile analysis tensiometry. The (non)modified SBPs were used as emulsifying agents in 30 wt% mct oil–water emulsions stabilized with 0.5 wt% SBP at pH 3, prepared by high-pressure homogenization (400–1000 bar). By analyzing the droplet size distributions, conclusions could be drawn about the coalescence that occurred after droplet breakup. It could be shown that SBPs modified by exo-polygalacturonanase stabilized the oil–water interface more rapidly, resulting in less coalescence and the smallest oil droplets. In contrast, SBPs modified with endo-polygalacturonanase resulted in poorer emulsification properties, and thus larger oil droplets with increasing incubation time. The differences could be attributed to the different cleavage pattern of the enzymes used. The results suggest that a minimum molecular size is required for the stabilization of fine oil droplets with SBPs as emulsifiers.

A Multistate Adsorption Model for the Characterization of C13DMPO Adsorption Layers at the Aqueous Solution/Air Interface.

Experimental data for tridecyl dimethyl phosphine oxide (C13DMPO) adsorption layers at the water/air interface, including equilibrium surface tension and surface dilational viscoelasticity, are measured by bubble and drop profile analysis tensiometry at different solution concentrations and surface area oscillation frequencies. The results are used to assess the applicability of a multistate model with more than two possible adsorption states. For the experiments with single drops, the depletion of surfactant molecules due to adsorption at the drop surface is taken into account. For the assessment, the same set of model parameters is used for the description of all obtained experimental dependencies. The agreement between the proposed model and the experimental data shows that for the nonionic surfactant C13DMPO, the description of the adsorption layer behavior by three adsorption states is superior to that with only two adsorption states.

Characterization of protein corona formation on nanoparticles via the analysis of dynamic interfacial properties: Bovine serum albumin - silica particle interaction

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.

Dynamics of adsorption of CTAB-Silica nanoparticle complexes: New experiments and modeling approach

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.

Can optical fiber compete with profile analysis tensiometry in critical micelle concentration measurement?

Abstract Critical micelle concentration (CMC) is one of the important nominal characteristics of the surfactants which can be measured using various methods. In this study, to detect the CMC of two ionic surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), two methods were utilized: (a) optical fiber and (b) drop profile analysis tensiometry (PAT) techniques. The spectrum width center and surface tension of the solutions at different concentrations of the surfactant were measured. The preliminary outcomes showed a compliance between optical fiber method and PAT technique. However, there were differences in the behavior of two surfactants in optical fiber measurement. In this method, when the solid surface of fiber is put in the system, the interactions between surfactant molecules and the fiber surface must be carefully considered.

Drop Size Dependence of the Apparent Surface Tension of Aqueous Solutions in Hexane Vapor as Studied by Drop Profile Analysis Tensiometry

Surface tension experiments were performed using the drop profile analysis tensiometry method. The hexane was injected into the measuring cell at certain times before the formation of the solution drop. The influence of the capillary diameter and solution drop size on the measured apparent dynamic surface tension was studied. The amount of hexane transferred from the vapor phase to the drop was estimated. For large pure water drops, it was shown that the ageing of the drop in the hexane vapor during a long time resulted in the formation of a liquid hexane phase covering the drop, but the volume of this phase did not exceed 0.5 mm3. On the contrary, for surfactant solution drops the volume of the hexane phase covering the drop was essentially larger. Experiments with solution drops were performed to measure the surface tension within a wide range of surfactant concentration. It was found that the dependencies of dynamic surface tension on the C13DMPO and C14EO8 solutions concentration exhibit maxima at concentrations of about 1–2 μmol/L for C14EO8 and 2–5 μmol/L for C13DMPO at ageing times of 100 to 1000 s; these maxima were shown to exist also at equilibrium. This phenomenon is presumably ascribed to the competitive character of simultaneous adsorption of hexane and surfactant.

Adsorption process for phospholipids of different chain lengths at a fluorocarbon/water interface studied by Du No\xfcy ring and spinning drop

Fluorocarbons are novel systems in the fast-growing fields of diverse biomedical applications and fluorocarbon-water emulsions. However, characterization of these systems with modern measuring techniques such as drop profile analysis tensiometry is almost impossible because of practically identical refractive indexes and high-density differences. Due to the material properties of the fluorocarbon-water system, the invasive Du Noüy ring is the most appropriate method to measure interfacial tensions over long times. However, the influence of the ring on a fluorocarbon/water interface packed with phospholipids needs careful analysis. For the proof of methodology, the spinning drop tensiometry was used for comparison as a non-invasive technique to measure interfacial tension between water and perfluoroperhydrophenanthrene (PFPH) covered by 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) proving almost identical results. This demonstrates the validity of the invasive measurement technique for the studied system. The Du Noüy ring method was applied for further measurements of phospholipids with different chain lengths (1,2-dmyristoyl-sn-glycero-3-phostphatidylcholine, DMPC; 1,2-distearoyl-sn-glycero-3-phosphatidylcholine, DSPC) which revealed a difference in interfacial adsorption kinetics and equilibrium tensions. The Du Noüy ring tensiometry is appropriate to examine the slow adsorption kinetics of phospholipids emulsifying fluorocarbons. The results enable functional optimization of fluorocarbon emulsions regarding physical emulsification parameters and the selection of lipids.

Influence of hydrophilic silica nanoparticles on the adsorption layer properties of non-ionic surfactants at water/heptane interface

There is a notable paucity of studies investigating the impact of charged nanoparticles on the interfacial behavior of nonionic surfactants, assuming that the interactions are negligible in the absence of electrostatic forces. Here, we argue about our observations and the existence of a complex interfacial behavior in such systems depending on the type and chemical structure of surfactant. This study set out to investigate the effects of interactions between hydrophilic silica nanoparticles (NP) and non-ionic surfactants on water/heptane dynamic interfacial properties using drop profile analysis tensiometry (PAT). Three surfactants were studied, namely Triton X-100 (significantly soluble in water phase), C12DMPO (well soluble in both phases) and SPAN 80 (oil-soluble). The different chemical structures and partition coefficients of the surfactants enabled us to cover possible interactions and differentiate between bulk and interfacial interactions. We observed that hydrophilic silica NPs had a negligible effect on the interfacial behavior of Triton X-100, that they increased the surface activity of C12DMPO when both compounds are initially in the aqueous phase. Most interestingly is that the added NPs generated unstable interfacial NP-surfactant complexes and reduced the pseudo-equilibrium interfacial tension of oil-soluble surfactant, Span 80, even though NPs and surfactants were in different bulk phases.

Dilational interfacial rheology of tridecyl dimethyl phosphine oxide adsorption layers at the water/hexane interface

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 C13DMPO (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 C13DMPO 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.

Direct Determination of the Distribution Coefficient of Tridecyl Dimethyl Phosphine Oxide between Water and Hexane

Drop profile analysis tensiometry is applied to determine the distribution coefficient of a nonionic surfactant for a water/hexane system. The basic idea is to measure the interfacial tension isotherm in two configurations: a hexane drop immersed in the surfactant aqueous solutions at different bulk concentrations, and a water drop immersed into a hexane solution of the same surfactant. Both types of experiments lead to an isotherm for the equilibrium interfacial tensions with the same slope but with a concentration shift between them. This shift refers exactly to the value of the distribution coefficient.

Dilational Viscoelasticity of Proteins Solutions in Dynamic Conditions.

Drop profile analysis tensiometry used in the oscillating drop mode provides the dilational viscoelasticity of adsorption layers at liquid interfaces. Applied during the progress of adsorption the dynamic surface rheology can be monitored. For β-casein solutions at the same surface pressure values, the larger the dynamic dilational viscoelasticity the longer the adsorption time, i.e., the smaller the studied protein concentration is. For β-lactoglobulin and human serum albumin, the differences in the viscoelasticity values are less or not dependent on the adsorption time at identical surface pressures. The observed effects are caused by the flexibility of BCS, while the globular proteins BLG and HSA do not change their conformation significantly within the adsorption layer.

Adsorption and surface dilational visco-elasticity of CnEOm solutions as studied by drop profile analysis tensiometry

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.

Surface Tension Measurements with the Drop Profile Analysis Tensiometry—Consideration of the Surfactant Mass Balance in a Single Drop

In drop profile analysis tensiometry, the ratio of drop surfaces area S to volume V is large, i.e., S/V >> 1. In such a case, the concentration of a surfactant within the drop bulk decreases due to adsorption at the drop surface. In contrast, in bubble profile analysis tensiometry, we have S/V << 1 so that depletion due to adsorption is negligible. A protocol is presented to determine the correct adsorption parameters of surfactants from surface tension data measured by bubble and drop profile analysis tensiometry. The procedure is applied to experimental data measured for selected surfactants of different adsorption activities: C10OH, CTAB, Tween 20, and the equimolar mixture SDS + DoTAB. The results show that for surfactants with higher surface activity, the differences between the surface tensions measured with the drops and bubbles profile analysis tensiometry, respectively, are larger, while for less surface-active surfactants, such as SDS, the results obtained from drop and bubble profile experiments are very close. The correction procedure is based on the same set of adsorption parameters used to fit both the experimental data obtained from drop-based measurements (which involve the depletion effects) and those data measured in a way that depletion effects are negligible.

Adsorption of alkane vapor at water drop surfaces

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 60mN/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 10μm 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 5μm, 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.

Competition of Thermomyces lanuginosus lipase with its hydrolysis products at the oil–water interface

Lipase–catalyzed hydrolysis of triglycerides yields glycerol and free fatty-acids, provided that the enzyme is non-regioselective. For an Sn-1,3 regioselective enzyme, such as lipase from Thermomyces lanuginosus, the final product is no longer glycerol but Sn-2 monoglyceride instead. However, surface active molecules generated by lipolysis may have a detrimental effect on the interfacial biocatalysis since it is known that low molecular weight surfactants can displace proteins from interfaces. By using drop profile analysis tensiometry, we evaluated the interfacial properties of the lipase–generated molecules and their competitive effect on the adsorption behavior of the lipase and on the proceeding lipolysis. Our results show that even at concentration ratios of 8.64×10−4M (Sn-2 monoglyceride) to 2.5×10−7M (lipase), the final interfacial pressure values are very similar as for the system containing the lipase alone (i.e. ∼26 mN/m). This is a strong indication that monoglycerides, as the most interfacially active products generated during regioselective lipolysis, are expelled from the oil–water interface by the lipase. We attribute this effect to intermolecular lipase–lipase interactions, resulting in a low desorption probability of the lipase. For low oleic acid concentrations, the interfacial tension is solely determined by the lipase, while for higher concentrations, lipase and oleic acid both contribute to the tension values. We propose a hypothesis based on the preferential interaction of oleic acid molecules with hydrophobic sites on the lipase. The pH dependence of the adsorption rate and the interfacial activity of the lipase were also investigated.

Silica nanoparticles cationic surfactants interaction in water-oil system

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 9.0×10−2mM, (0.1 CMC) and the nanoparticle concentration varied between 0 and 5wt.%. 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.

Mixed Protein/Hexane Adsorption Layers Formed at the Surface of Protein Solution Drops Surrounded by Hexane Vapor

The surface tension of aqueous solution drops of ß‐casein, ß‐lactoglobulin, and human serum albumin at the interface to air and hexane saturated air are measured by drop profile analysis tensiometry. The results indicate that the dynamic and equilibrium surface tension for a hexane vapor atmosphere are considerably lower as compared to the values at the interface with pure air. The experiments are performed with the initial adsorption of protein followed by hexane adsorption, and with subsequent removal of hexane from the measuring cell after equilibrium is attained. Two models are proposed, both of which assume the diffusion of protein in the solution and either diffusion‐governed or barrier‐governed hexane coadsorption. The new developed theoretical equilibrium model assumes a double layer adsorption in these systems, with the first layer composed of protein mixed with hexane and the second layer formed by hexane molecules only. The experimental and calculated equilibrium surface tension data are in a perfect agreement. Note the model parameters are exactly the same as those found for the individual compounds.

Dilational visco-elasticity of BLG adsorption layers at the solution/tetradecane interface – Effect of pH and ionic strength

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 0.01–0.2Hz. The buffered solutions were investigated at pH 3, pH 5 (isoelectric point) and pH 7 at different buffer concentrations (1mM, 10mM and 100mM). 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 20mN/m, at the W/TD interface these maximum values are up to 65–70mM/m, which is in parallel with the much higher interfacial pressure values at the W/TD interface when compared to the W/A surface.