Dynamic Surface Tension Measurements Research Articles

Exploring the correlation of dynamic surface tension with antimicrobial activities of quaternary ammonium-based disinfectants.

Quaternary ammonium compound (QAC) disinfectants represent one of our first lines of defense against pathogens. Their killing activities are usually tested through minimum inhibitory concentration (MIC) and time-kill assays, but these assays can become cumbersome when screeningcompounds. We investigated how the dynamic surface tension (DST) measurements of QACs correlate with these antimicrobial activities by testing a panel of potent and structurally varied QACs against the gram-positive Staphylococcus aureus and the gram-negative Pseudomonas aeruginosa. We found that DST values correlated well with bactericidal activity in real-world disinfection conditions but not with MIC values. Moreover, no correlation between these two antimicrobial activities of QACs was observed. We also observed that the bactericidal activity of our QAC panel against the gram-negative P. aeruginosa was severely affected in the presence of hard water. Interestingly, we found that the counterion of the QAC affects the killing of bacteria in these conditions, a phenomenon not observed in most MIC assessments. Moreover, some of our best-in-class QACs show enhanced bactericidal activity when combined with a commercially available QAC. In conclusion, we determined that DSTcan be used as a technique to screen for bactericidal activity of QACs in conditions that mimic real-world disinfection conditions.

Dynamic Interfacial Properties and Foamability of DoTAB/SiO2 Mixtures

The interaction between nanoparticles and cationic surfactants is an exciting and emerging field in interfacial science. This area of research holds significant promise, linking fundamental principles to practical applications in a variety of industries, including chemical processes, biomedical applications and the petroleum industry. This study explores the interaction between dodecyltrimethylammonium bromide (DoTAB) and silica (SiO2) nanoparticles, investigating their influence on dynamic interfacial properties and foam characteristics. Through equilibrium and dynamic surface tension measurements, along with examining the dilational visco-elasticity behavior, this research reveals the complex surface behavior of DoTAB/SiO2 mixtures compared to individual surfactant solutions. The foamability and stability experiments indicate that the addition of SiO2 significantly improves the foam stability. Notably, stable foams are achieved at low SiO2 concentrations, suggesting a cost-effective approach to enhancing the foam stability. This study identifies the optimal stability conditions for 12 mM DoTAB solutions, emphasizing the crucial role of the critical aggregation concentration region. These findings offer valuable insights for designing surfactant-nanoparticle formulations to enhance foam performance in various industrial applications.

Pressure dependence of adsorption isotherm parameters and transport dynamics of a nonionic surfactant, LS-36, at the carbon dioxide (CO[formula omitted]) gas–liquid water interface

Surfactants are important to a wide variety of elevated pressure processes; however, little is understood about the effect of pressure on surfactant interfacial thermodynamics or transport. While some experimental studies exist, concrete conclusions are difficult to infer due to experimental artifacts, e.g., non-equilibrium conditions. We recently showed that few experiments are capable of achieving bulk fluid saturation, which is essential to careful measurements of surfactant thermodynamics and transport dynamics. In other words, the accurate study of surfactant adsorption equilibrium and transport parameters first and foremost requires bulk fluids that are otherwise in equilibrium. This work uses our recently developed high pressure microtensiometer (HPMT), which ensures bulk phase saturation, to measure thermodynamic isotherms for tri(ethylene glycol)-hexa(propylene glycol)-monododecyl ether (LS-36) surfactant (concentrations between 10−7 and 10−3mol/L and pressures between 1 and 57bar) at the CO2 gas–liquid water surface. All measurements were conducted at 22.5 °C. Note that in this study, the surfactant was dissolved in the liquid water phase and is unable to partition into the gaseous CO2 phase. Isotherms at different pressures were determined from surface tension data, analyzed, and compared to determine trends in thermodynamic parameters. Dynamic surface tension measurements were performed to assess transport dynamics in terms of the well-studied diffusion timescale. We find that surface tension depends non-linearly on pressure and surfactant concentration, such that surfactants are less effective at lowering surface tension as pressure increases. Furthermore, surfactant adsorption is a non-monotonic function of pressure and goes through a maximum at intermediate pressures. This result suggests that surfactant adsorption is strongly dependent on the free energy of the CO2-water surface. Overall, this work shows that the thermodynamics of surfactants at equilibrium surfaces under pressure are non-trivial and need additional investigation to quantify the driving forces for surfactant adsorption and to develop models for high pressure processes.

Synergistic Effect of Frequently Found Ions in the Flotation of Pb-Zn Sulfide Ores on Air/Water Interface

The aqueous ions influence the properties of air bubbles and, therefore, the recovery of flotation. This study aims to reveal the synergistic effect of frequently found ions in the flotation of Pb-Zn sulfide ores. In this context, dynamic surface tension measurements, bubble coalescence time, Sauter mean diameter (SMD), bubble size distribution (BSD), and dynamic foam stability (DFS) measurements were carried out using artificial process waters (APWs). APW with the minimum ion concentration is expressed as “APW1” with the ionic strength (I) of 0.03 mol/dm3. The concentration of the ions in APW1 was increased by 3, 5, and 10 times, and thus APW3 (I = 0.08 mol/dm3), APW5 (I = 0.13 mol/dm3), and APW10 (I = 0.26 mol/dm3) were prepared, respectively. The results of this study indicated that the surface tension increased slightly in the presence of APW related to the ion concentration. Potassium ethyl xanthate (KEX) at high concentrations was effective in the reduction of surface tension. As the APW concentration increased, finer bubbles were obtained with a narrower size range. The stability of the foam increased with butyl glycol (BG) and APW concentration. There was no need to use a frother (BG) for the flotation experiments in the presence of APW1 or APWs with higher ionic strength.

Evaluation of surface activity of rhamnolipid biosurfactants produced from rice bran oil through dynamic surface tension

The use of low-cost carbon substrates such as agricultural residues can drastically lower the cost of biosurfactant production. In this study, rice bran oil extracted from agricultural waste was used as a renewable carbon source for biosurfactant production using Pseudomonas aeruginosa PTCC 1340. The biosurfactant was characterized as a glycolipid derivative by thin-layer chromatography and Fourier transform infrared spectroscopy. The yields of biosurfactant from rice bran oil (YRL/S) and biosurfactant to biomass (YRL/X) were 0.246 and 2.81 (g/g), respectively. In addition, the surface activity of the produced biosurfactant was studied using dynamic surface tension measurements and a mono-exponential decay model by estimating the relaxation time of the biosurfactants at the interface. The biosurfactant exhibited acceptable performance in reducing surface tension, as confirmed by examining the dynamic surface tension state and the lowest adsorption time without being affected by the type of salt or concentration. It was found that the adsorption/relaxation of biosurfactants at the interface was considerably affected by the biosurfactant concentration. The produced biosurfactant by the strain considerably reduced the surface tension of water from 70.46 to 25.86 mN/m with a critical micelle concentration (CMC) of 0.09 g/L with rice bran oil as a carbon source. The biosurfactant was also found to be highly effective in suppressing one of the most destructive pathogenic fungi, Macrophomina phaseolina, in terms of its environmental impact. The enhanced physicochemical properties of biosurfactants, such as potential antifungal properties, oil displacement properties, and surface tension-reducing ability, demonstrate the potential of this biosurfactant as a bio-adjuvant and perfect replacement for chemical surfactants in addressing oil spills and environmental decontamination processes.

Effect of air bubbles on the membrane filtration of rhodamine B.

Effect of air bubbles on the membrane filtration of a basic dye, rhodamine B (RB), using a hydrophilic PTFE membrane filter (pore size: 0.20μm) was studied. The air bubbles were generated by vigorously mixing the aqueous solution containing 0.05% (v/v) of 1-butanol with a shaft generator of a homogenizer. RB being far smaller than the pore size of the membrane filter could not be rejected without air bubbles, but it was rejected by the membrane filter in the presence of air bubbles. The rejection ratio increased with increasing the rotation speed of the shaft generator because of the increase in the amount of air bubbles and therefore the increase in the surface area of air bubbles for the adsorption of RB. On the other hand, another basic dye, methylene blue (MB), was negligibly rejected in the same condition. Dynamic surface tension measurement of aqueous solutions containing different amounts of dye indicated that RB strongly adsorbed to the air-water interface, while MB hardly adsorbed. The results obtained in the present study strongly suggest the potential usefulness of air bubbles for the selective microfiltration of dissolved organic molecules or ions.

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.

Methods of Dynamic Tensiometry in Assessing the Biochemical Status of Animals

Introduction . The existing literature explores various aspects of both the method of dynamic tensiometry based on the measurement of dynamic surface tension (DST) of blood, and its use in assessing the biochemical and physiological parameters of animals. Despite the proven usefulness of DPN methods in human medicine, these methods are underestimated in modern physicochemical biology and veterinary medicine. Purpose . In this regard, the aim of the review is to describe the methodological foundations for determining the DPN of animal biological fluids, identifying its features depending on the physiological state of the organism, the biochemical composition of blood serum while improving methods for the early diagnosis of animal diseases. Materials and Methods . The materials are research articles describing blood serum samples of humans, horses and cows, analyzed by two methods of dynamic tensiometry. Results . As a result, a detailed analysis of 84 studies related both to DPN methods and their use in assessing blood parameters was carried out, which made it possible to identify correlations between DPN and biochemical blood parameters in humans, horses and cows. In addition to presenting the measurement features of different methods of dynamic tensiometry, the review of the subject field summarizes their empirically identified prerequisites and consequences in this area. Both for cows and mares, there are a large number of strong correlations between biochemical parameters and blood DPN. Conclusion . Thus, changes in blood biochemistry associated with the structural and functional development of animals in ontogeny are consistent with changes in blood DPN values, which is reflected in the magnitudes of correlations and makes it possible to apply the obtained fundamental results in practice.

A Comprehensive Analysis of Selected Anionic Surfactants Behaviour in Aqueous Systems Containing Metal Ions and Inorganic Acid

The influence of co-ions on the adsorption properties of two surfactants: ABS (dodecylbenzene sulfonic acid) and its sodium salt ABSNa50 in the aqueous systems, has been studied and discussed. On the basis of experimental measurements of equilibrium and dynamic surface tension, a series of parameters were determined, such as critical micelle concentration (CMC), adsorption parameters, diffusion coefficients, and the micellar dissociation constant. It was proved that values of CMC vary according the contents of accompanying metal ions (zinc(II), copper(II), cobalt(II), nickel(II)) and hydrochloric acid in the system under discussion. Moreover, it was found that as the concentration of surfactant in the system increases, the estimated values of the diffusion coefficients decrease. The obtained results indicate the need to analyse the basic surface properties of surfactants in case of their application in the systems containing additional substances.

Surfactant-free air bubble flotation-coagulation for the rapid purification of chloroquine.

A simple and rapid separation method based on surfactant-free air bubble flotation and coagulation was designed for the purification of chloroquine (CQ) from its crude product. An open glass column having a sintered glass filter (for column chromatography) was used as a flotation vessel. The flotation was conducted by pouring the crude CQ into the aqueous solution containing 0.1% (v/v) of 2-propanol followed by feeding air through the glass filter to generate air bubbles. At pH 12, CQ was enriched into the foam temporary generating on the surface of water to form the coagulates within 90s after the start of the air bubble flotation. On the other hand, reactants; 4,7-dichloroquinoline and 4-amino-1-diethylaminopentane, as well as generated impurities remained in the bulk aqueous solution. The result of dynamic surface tension measurement indicated that CQ molecules selectively adsorbed on the air-water interface and the coagulates more strongly adsorbed the interface. Adsorption and coagulation of CQ molecules on the air-water interface were also reproduced in the calculation results of a molecular dynamic simulation. The coagulates were collected from the surface of water by suction and then poured into another flotation vessel for conducting repeated separation. The time required for the respective separation process including air bubble flotation and collection by suction was within 5min. After three-times separation, highly purified (> 99.0%) CQ was obtained with a yield of 72 ± 8%. The amounts of reactants and other impurities reduced into undetectable levels.

Exploring the Protein Stabilizing Capability of Surfactants Against Agitation Stress and the Underlying Mechanisms

The application of surfactants in liquid protein formulation is a common practice to protect proteins from liquid-air interface-induced protein aggregation. Typically, Polysorbate 20 or 80 are used, but degradation of these surfactants can result in particle formation and/or protein degradation. The purpose of the current study was to directly compare three alternative protein stabilizing molecules – Poloxamer 188, hydroxypropyl-cyclodextrin and a trehalose-based surfactant – to Polysorbate 80 for their capacities to reduce agitation-induced protein aggregation and particle formation; and furthermore, investigate their underlying protein stabilizing mechanisms. To this end, a small-volume, rapid agitation stress approach was used to quantify the molecules’ abilities to stabilize two model proteins. This assay was presented to be a powerful tool to screen the protein stabilizing capability of surfactants using minimum of material and time. SEC, turbidity measurements and particle analysis showed an efficient protein stabilization of all tested surfactants as well as cyclodextrin. STD-NMR and dynamic surface tension measurements indicated the competitive surface adsorption to be the main protein stabilizing mechanism of the three surfactants tested. It might also play a role to some extent in the protein stabilization by HPβCD. However, additional mechanisms might also contribute to protein stabilization leaving room for further investigations.

Correlation of performance data of silica particle flotations and foaming properties of cationic and nonionic surfactants for the development of selection criteria for flotation auxiliaries

In this study, the effect of foaming properties of cationic (CnTAB, n = 10, 12, 14, 16) and nonionic (C13EOm, m = 4, 5, 6, 7, 8, 10 and CxEOy, x = 10, 13, 16 and y = 5, 10, 11) surfactants on the efficiency of silica particle removal by foam flotation technology was studied. These two groups of surfactants were selected because they allow for studying the impact of chain length variation of the hydrophobic and hydrophilic chains. To reveal the potential of the surfactants to improve the efficiency of foam flotation for the removal of silica particles their foaming properties were studied by the Bartsch test, the sparing test, and dynamic surface tension measurements. The determined foam parameters such as foamability, foam stability, and foam density were correlated with the most important performance indicators, namely removal and enrichment factor, to identify their impact and significance. The results show a strong correlation between foam flotation performance and foaming properties of the surfactants which are controlled through their molecular structures. The identified correlations will facilitate the selection of suitable surfactants for flotations of real contaminants.

Dynamic surface tension probe for measuring the concentration of extracellular vesicles

The concentration of extracellular vesicles (EVs) is an essential attribute of biofluids and EV preparations. EV concentration in body fluids was correlated with health status. The abundance of EV secreted by cultured cells into growth medium is vital in signaling studies, tissue and disease models, and biomanufacturing of acellular therapeutic secretome. A limited number of physical principles sensitive to EV concertation have been discovered so far. Particle-by-particle counting methods enumerate individual particles scattering light, modulating the Coulter current, or appearing in EM images. The available ensemble techniques in current use rely on the concentration-dependent signal intensity, as in the case of ELISA. In this study, we propose for the first-time the ensemble-based characterization of EV concentration by dynamic surface tension (DST) probe and demonstrate its implementation. We show that DST measurements agree with the widely used NTA measurements of EV concertation. The proposed method is low-cost and requires only basic laboratory equipment for implementation.

Measurements of Static and Dynamic Bubble Surface Tension Using a Deformation-Based Microfluidic Tensiometer.

The surface tension of bubbles is critical for processes involving mixed liquid-gas systems, from sea spray aerosol generation to firefighting foam aspiration. In particular, the size- and surfactant-dependent time scales of dynamic surface tension decay due to adsorption of surface-active chemicals at the curved interface significantly dictate the multiphase system dynamics. While size-dependent surfactant adsorption and interfacial dynamics have been well characterized for liquid-liquid systems using microfluidic platforms, application of microfluidic methods to liquid-gas systems has received less attention. This work uses a high-throughput microfluidic tensiometer to measure the static and dynamic surface tension of microscale bubbles compared with millimeter bubbles characterized by pendant drop. It is shown that the static surface tension measurements for surfactant-free interfaces with microfluidics show good agreement with pendant drop for most systems. At the same time, its accuracy can be affected by bubble pressure, inertia force at high Re, drag force, bubble expansion, and image processing limitation. In the presence of surfactants, the dynamic surface tension measurements show that both smaller bubbles and higher surfactant concentrations can lead to a much shorter time to reach equilibrium compared with pendant drop, similar to the observation for liquid-liquid interfaces. This work shows the potential of a microfluidic tensiometer to capture early time surface tension decay and accurately measure surface tension even in the presence of Marangoni stress tangential to the interface.

Enzymatic Hydrolysis of Triglycerides at the Water-Oil Interface Studied via Interfacial Rheology Analysis of Lipase Adsorption Layers.

The enzymatic hydrolysis of sunflower oil occurs at the water-oil interface. Therefore, the characterization of dynamic interfacial phenomena is essential for understanding the related mechanisms for process optimizations. Most of the available studies for this purpose deal with averaged interfacial properties determined via reaction kinetics and dynamic surface tension measurements. In addition to the classical approach for dynamic surface tension measurements, here, the evolution of the dilational viscoelasticity of the lipase adsorbed layer at the water-oil interface is characterized using profile analysis tensiometry. It is observed that lipase exhibits nonlinear dilational rheology depending on the concentration and age of the adsorbed layer. For reactive water-oil interfaces, the response of the interfacial tension to the sinusoidal area perturbations becomes more asymmetric with time. Surface-active products of the enzymatic hydrolysis of triglycerides render the interface less elastic during compression compared to the expansion path. The lipolysis products can facilitate desorption upon compression while inhibiting adsorption upon expansion of the interface. Lissajous plots provide an insight into how the hysteresis effect leads to different interfacial tensions along the expansion and compression routes. Also, the droplet shape increasingly deviates from a Laplacian shape, demonstrating an irreversible film formation during aging and ongoing hydrolysis reaction, which supports our findings via interfacial elasticity analysis.

Interfacial complexation of a neutral amphiphilic ‘tardigrade’ co-polymer with a cationic surfactant: Transition from synergy to competition

HypothesisNeutral amphiphilic PEG-g-PVAc co-polymer (a “tardigrade” polymer consisting of a hydrophilic polyethylene glycol, PEG, backbone with hydrophobic polyvinyl acetate, PVAc, grafts) can form complexes at the air–water interface with cationic dodecyltrimethylammonium bromide (DTAB) via self-assembly. Compared to anionic SDS, cationic DTAB headgroups are expected to interact strongly with the negatively charged OH– groups from the partial dissociation of the PVAc grafts. We anticipate a transition from synergistic to competitive behaviour, which is expected to be dependent on the surfactant structural characteristics and concentration. ExperimentsDTAB/PEG-g-PVAc mixtures were investigated using a combination of dynamic and equilibrium surface tension measurements, neutron reflectivity (NR) at the air–water interface, and foaming tests. We varied the concentrations of both the DTAB (0.05 to 5 critical micelle concentration, cmc) and that of PEG-g-PVAc (0.2 and 2 critical aggregation concentration, cac). FindingsOur results show that the interfacial interactions between DTAB and PEG-g-PVAc were both synergistic and antagonistic, depending sensitively on the surfactant concentration. At DTAB concentrations below its cmc, a pronounced cooperative adsorption behaviour was likely driven by the hydrophobic interactions between the DTAB tail and the PVAc grafts and the attraction between the DTAB headgroups and the partially dissociated –O- groups in the partially hydrolysed PVAc grafts, forming a mixed layer. This synergistic adsorption behaviour transitioned to a competitive adsorption behaviour at DTAB concentrations above its cmc, leading to polymer-surfactant partition, forming a “hanging” polymer layer underlying a surfactant monolayer at the interface. We postulate that DTAB/PEG-g-PVAc complexation in the bulk contributed to partial depletion of the mixture from the interface. We therefore consider this polymer/surfactant system to be a moderately interacting system at the air–water interface. No discernible differences in the foaming behaviour were observed between the DTAB/PEG-g-PVAc systems and the pure surfactant. Our results suggest that surfactant headgroup characteristics (particularly charges) were crucial in determining the structure and composition of polymer-surfactant complexes at the air–water interface, as well as the foamability and foam stability, whilst the coexistence of the synergistic and competitive adsorption behaviour is attributed to the unique architecture of the tardigrade polymer with amphiphilicity and partial charge, facilitating different surfactant-polymer interactions at different DTAB concentrations.

Water retention mechanism of cellulose ethers in calcium sulfoaluminate cement-based materials

To explore the mechanism to water retention of cellulose ether (CE) in calcium sulfoaluminate (CSA) cement-based materials, three cellulose ethers (CEs) with similar viscosity but different substitution groups were selected in this paper, namely hydroxyethyl cellulose (HEC) and hydroxyethyl methyl cellulose (HEMC) with high and low degree of substitution (DS). Water retention of CE modified CSA cement mortar with different CE dosages was characterized by filter paper method. The water state and content in CE modified CSA cement paste were determined by low field nuclear magnetic resonance (1H NMR). The adsorption and dispersion characteristics of CE in CSA cement paste were analyzed by measuring the organic carbon content in the pore solution. Combined with the measurement of dynamic viscosity and surface tension of pore solution, the mechanism to water retention of CEs in CSA cement-based materials was comprehensively analyzed. The results show that the addition of CE can significantly improve the water retention of CSA cement mortar. At the same dosage, the water retention effect of HEMC is better than HEC, and DS has little effect on the water retention capacity of HEMC. The mechanism to water retention of HEC and HEMC added less than 0.1% dosage in system is governed by water sorption effect of CE. While more than 0.1% dosage, HEC and HEMC in CSA cement-based materials achieve water retention respectively through the adsorption of CE on cement and the formation of three-dimensional polymer network in pore solution.

Anti-staphylococcal activity and mode of action of thioridazine photoproducts

Antibiotic resistance became an increasing risk for population health threatening our ability to fight infectious diseases. The objective of this study was to evaluate the activity of laser irradiated thioridazine (TZ) against clinically-relevant bacteria in view to fight antibiotic resistance. TZ in ultrapure water solutions was irradiated (1–240 min) with 266 nm pulsed laser radiation. Irradiated solutions were characterized by UV–Vis and FTIR absorption spectroscopy, thin layer chromatography, laser-induced fluorescence, and dynamic surface tension measurements. Molecular docking studies were made to evaluate the molecular mechanisms of photoproducts action against Staphylococcus aureus and MRSA. More general, solutions were evaluated for their antimicrobial and efflux inhibitory activity against a panel of bacteria of clinical relevance. We observed an enhanced antimicrobial activity of TZ photoproducts against Gram-positive bacteria. This was higher than ciprofloxacin effects for methicillin- and ciprofloxacin-resistant Staphylococcus aureus. Molecular docking showed the Penicillin-binding proteins PBP3 and PBP2a inhibition by sulforidazine as a possible mechanism of action against Staphylococcus aureus and MRSA strains, respectively. Irradiated TZ reveals possible advantages in the treatment of infectious diseases produced by antibiotic-resistant Gram-positive bacteria. TZ repurposing and its photoproducts, obtained by laser irradiation, show accelerated and low-costs of development if compared to chemical synthesis.

Dynamic Surface Tension of Surfactants in the Presence of High Salt Concentrations

We study the influence of high NaCl concentrations on the equilibrium and dynamic surface tensions of ionic (CTAB) and nonionic (Tween 80) surfactant solutions. Equilibrium surface tension measurements show that NaCl significantly reduces the critical micellar concentration (CMC) of CTAB but has no effect on the CMC of Tween 80. Dynamic surface tension measurements allow comparing the surface tension as a function of time for pure surfactant solutions and in the presence of NaCl. For the ionic surfactant, the dynamics agree with the usual diffusion-limited interfacial adsorption kinetics; however, the kinetics become orders of magnitude slower when NaCl is present. Sum-frequency generation spectroscopy experiments and the equilibrium adsorption measurements show that the presence of NaCl in CTAB solution leads to the formation of ion pairs at the surface, thereby neutralizing the charge of the head group of CTAB. This change, however, is not able to account for the slowing down of adsorption dynamics; we find that it is rather the decreases in the monomer concentration (CMC) in the presence of salt which has the major influence. For the nonionic surfactant, the kinetics of interfacial tension is seen to be already very slow, and the addition of salt does not influence it further. This also correlates very well to the very low CMC of Tween 80.

Influence of thermally induced structure changes in diluted β-lactoglobulin solutions on their surface activity and behavior in foam fractionation

Surface activity is an intrinsic protein feature, leading to the capability of aqueous protein solutions to form foam. This feature provides opportunities for downstream processing, such as usage of foam fractionation for purification. In order to investigate the impact of the surface activity on the performance of the foam fractionation process, protein solutions with different surface activity were produced by different thermal denaturation of aqueous β-lactoglobulin solutions. The effectiveness of the denaturation procedure was verified with circular dichroic spectroscopy, and the impact on surface activity was determined via dynamic surface tension measurement. The increased surface activity resulted in higher foamate flow rates. Furthermore, the effects could be correlated with secondary structure changes and with the dynamic surface pressure. The new result of this study is that the effect of the denaturation of a protein on foam fractionation depends on the protein concentration. At the lower feed concentration, effects became visible, which could not be observed at the higher concentration.