Dilational Interfacial Rheology Research Articles

Modifying the interfacial dynamics of oleosome (lipid droplet) membrane using curcumin

Cells store energy in lipid droplets, known as oleosomes, which have a neutral lipid core surrounded by a dilatable membrane of phospholipids and proteins. Oleosomes can be loaded with therapeutic lipophilic cargos through their permeable membrane and used as carriers. However, the cargo can also adsorb between the phospholipids and affect the membrane properties. In the present work, we investigated the effect of adsorbed curcumin on the mechanical properties of oleosome membranes using dilatational interfacial rheology (LAOD). The oleosome membrane had a weak-stretchable behavior, while the adsorption of curcumin led to stronger in-plane interactions, which were dependent on curcumin concentration and indicated a glassy-like structure. Our findings showed that adsorbed curcumin molecules can enhance the molecular interactions on the oleosome membrane. This behavior suggests that oleosomes membranes can be modulated by loaded cargo. Understanding cargo and membrane interactions can help to design oleosome-based formulations with tailored mechanical properties for applications.

Air-water interfacial properties of perfluorosulfonic acid salts with different chain lengths

Perfluorosulfonic acids (PFSA) are a class of per- and polyfluoroalkyl substances (PFAS), which have been extensively used in numerous applications. The toxicity and bioaccumulation of PFAS have become a serious worldwide problem. Foam fractionation has been suggested as a promising remediation method for PFAS removal owing to the amphiphilic nature of most of the currently existing PFAS in the environment, allowing them to migrate to the air-water interface during the foaming process. Nevertheless, the current challenge for widespread utilization of foam fractionation is its low removal efficiency, especially for short-chain PFAS. Foam fractionation performance depends on the air-water interfacial properties as well as the ability for PFAS to adsorb at the air-water interface. In the present study, therefore, we investigate surface tensions of PFSA with long and short chain lengths. Different adsorption models are studied to estimate maximum surface excess concentrations for various PFSA. Our results indicate that long-chain PFSA have lower surface tensions, higher surface activities, and form more rigid air-water interfaces. This study marks the first investigation of dilatational interfacial rheology of PFAS, as well as addressing the gap in understanding PFAS interfacial rheology by focusing on the quantification the storage and loss interfacial moduli, which are known to play a significant role in the foaming process.

Surfactant Adsorption Layers: Experiments and Modeling.

During recent years, great progress has been made in understanding the adsorption of surfactants at liquid interfaces. In addition to tensiometry, new efficient methodologies have been developed, in particular interfacial selective optical methods which allow direct access to the adsorbed amounts and interfacial layer compositions. In addition to these new experimental tools, the thermodynamic description by equations of state now allows one to provide a quantitative picture of surfactant interfacial layers. This is most notable for surfactant layers at water/oil interfaces. Additional knowledge about the structure of interfacial layers was gained through different types of molecular modeling. Improved interrelationships between these three aspects are the challenges for current and future work. Particular attention must be paid to dilational interfacial rheology studies, as these mechanical quantities are much more sensitive to small changes in the interfacial composition and structure.

The Biomimetic System of Oleanolic Acid and Oleic Acid at the Air-Water Interface-Interactions in Terms of Nanotechnology-Based Drug Delivery Systems.

Oleanolic acid (OLA) and oleic acid (OA) are ubiquitous in the plant kingdom, exhibiting a therapeutic effect on human health, and are components of novel pharmaceutical formulations. Since OLA has limited solubility, the utilization of nanotechnology-based drug delivery systems enhancing bioavailability is highly advantageous. We report on the interfacial behavior of the OLA:OA system at various molar ratios, using the Langmuir technique to assess the dependence of the molar composition on miscibility and rheological properties affecting film stability. Specifically, we evaluate the interfacial properties (morphology, thermodynamics, miscibility, and viscoelasticity) of the OLA:OA binary system in various molar ratios, and indicate how the OLA:OA system exhibits the most favorable molecular interactions. We apply the Langmuir monolayer technique along with the complementary techniques of Brewster angle microscopy, dilatational interfacial rheology, and excess free energy calculations. Results demonstrate that the properties of mixed monolayers depend on OLA:OA molar ratio. Most of the systems (OLA:OA 2:1, 1:1, 1:5) are assumed to be immiscible at surface pressures >10 mN/m. Moreover, the OLA:OA 1:2 is immiscible over the entire surface pressure range. However, the existence of miscibility between molecules of OLA and OA in the 5:1 for every surface pressure tested suggests that OA molecules incorporate into the OLA lattice structure, improving the stability of the mixed film. The results are discussed in terms of providing physicochemical insights into the behavior of the OLA:OA systems at the interface, which is of high interest in pharmaceutical design.

Viscoelastic properties of interfacial lignosulfonate films and the effect of added electrolytes

New evidence is presented, which confirmed interfacial gelling of lignosulfonates in presence of di- and trivalent cations. In this article, the viscoelastic properties of lignosulfonate films at the water-xylene interface were studied by dilatational interfacial rheology and interfacial shear rheology. Both techniques showed that increasing lignosulfonate concentration would first increase and then decrease the interfacial modulus. The same trend was observed for increasing salinity. The maximum interfacial modulus corresponded with lignosulfonate aggregation or precipitation and accounted for the best emulsion stability. The film strength increased progressively with the cation charge number. It was argued that multivalent cations provided intermolecular bridging between lignosulfonate molecules, which increased film strength and led to gelling. The decrease of interfacial film strength at high salinity was explained by two mechanisms: (1) For sodium cations, the polyelectrolyte contraction at high ionic strength yielded screening of the functional groups, which are deemed responsible for attractive interactions between lignosulfonate molecules or aggregates. (2) For calcium and aluminum cations, precipitation would reduce the effective bulk concentration, yielding a lower surface coverage. Modelling of the interfacial properties was conducted in addition, which showed that lignosulfonate adsorption was not diffusion-controlled and that lignosulfonate aggregation was affecting the adsorption process. In conclusion, our results revealed a more detailed picture of the mechanisms, which govern the interfacial behavior and properties of lignosulfonates.

How the Influence of Different Salts on Interfacial Properties of Surfactant–Oil–Water Systems at Optimum Formulation Matches the Hofmeister Series Ranking

AbstractIn this work, we present the effects of salts on sodium dodecyl benzene sulfonate micellization and on the interfacial performance of a sodium dodecyl benzene sulfonate–heptane–brine system at optimum formulation, i.e., hydrophilic–lipophilic deviation (HLD) = 0. In order to do that, interfacial tension and dilational interfacial rheology properties of surfactant–heptane–water systems at optimum formulation are measured using an interfacial spinning drop tensiometer with an oscillating velocity, which can accurately measure interfacial rheology properties at both low and ultralow interfacial tensions. The brines used contain one of the following salts: MgCl2, CaCl2, NaCl, NH4Cl, NaNO3, CH3COONa, or Na2SO4. We performed a one‐dimensional salinity scan with each of these salts to achieve an optimum formulation. In relation to the Hofmeister series, we found that, at optimum formulation, systems with chaotropic ions (NH4+, NO3−) present interfaces with ultralow interfacial tensions, very low dilational modulus, and a low phase angle, whereas kosmotropic ions (Mg2+, Ca2+, SO4−2) generate high interfacial tension and high rigidity monolayers. Intermediate ions in the Hofmeister series (Na+, CH3COO−, Cl−) present interfaces with intermediate properties. Furthermore, according to the Hofmeister series, interfaces can be respectively ordered from higher to lower rigidity for surfactant counterions Mg2+ > Ca2+ > Na+ > NH4+ and coions SO42− > CH3COO– > Cl− > NO3−, which correspond to a salting‐out (highest rigidity) and salting‐in (lowest rigidity) effect. We observed that counterions have a more significant effect on surfactant–oil–water system properties than those that act as coions.

Experimental investigation on the dilatational interfacial rheology of dust-suppressing foam and its effect on foam performance

Foam dust suppression is an effective technique for controlling dust at its source. Dilatational interfacial rheology is an important property of aqueous foam. It characterizes the strength and toughness of liquid film and acts on the foaming process and drainage, affecting foaming capacity (FC) and foam stability (FS). The interfacial viscoelastic properties of anionic and nonionic surfactants commonly used were studied by testing the viscoelastic properties with different foaming agent concentrations (FAC) and their effects on foam performance were investigated. The results show that increasing FAC from 0.1 wt. ‰ to 1.0 wt. ‰ generally decreased the viscoelastic modulus, which was negatively correlated with FC: 6501 had a highly linear relationship between FC and viscoelastic modulus. The viscoelastic modulus of K12 and AES were positively correlated with FC in the low-modulus range, then negatively correlated at higher viscoelastic modulus values; the turning point was defined as the optimal viscoelastic modulus. The viscoelastic modulus had a significant positive effect on FS. The surface viscoelasticity contributed greatly to improving FS before the critical micelle concentration (CMC) was reached. After reaching CMC, micelle properties of bulk phase mainly affected the foam life.

New Interfacial Rheology Characteristics Measured Using a Spinning Drop Rheometer at the Optimum Formulation. Part 2. Surfactant–Oil–Water Systems with a High Volume of Middle‐Phase Microemulsion

AbstractThis article is a continuation of our first study on dilational interfacial rheology properties at optimum formulation for surfactant‐oil–water systems at low surfactant concentration just above the cμc. Here, we have investigated a high content of middle‐phase microemulsion with an optimum WIII phase behavior for a system containing sodium dodecyl sulfate, n‐pentanol, and kerosene. A new oscillating spinning drop interfacial rheometer was used to measure the interfacial properties. The very low dilational elasticity moduli and phase angle found at or near hydrophilic–lipophilic deviation (HLD) = 0 are related to the presence of the bicontinuous phase microemulsion and to the fast surfactant exchanges between the bulk and the interface, regardless of the phases involved in the measurement using the spinning drop apparatus, i.e., the two‐phase excess oil and excess water (O‐W) or the bicontinuous microemulsion and excess water (M‐W). We show that at or near optimum formulation, the interfacial tension and the dilational modulus for the M‐W case almost instantly reach equilibrium, because of the high surfactant content in the microemulsion and the fast exchanges between the bulk and the interface. In contrast, when both excess phases (O‐W) are measured, the changes in these properties are slower, due to the scarce presence of surfactants in both phases. The possibility of having almost all the surfactants trapped in the middle‐phase bicontinuous microemulsion could explain the emulsion instability in all the WIII range. This is behaving as if there were no surfactant available in the oil and water phases to stabilize the oil or water droplets thus formed.

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.

Impact of multiple quaternary ammonium salts on dynamic properties of BSA adsorption layer at different pH values.

The interaction mechanism of multiple quaternary ammonium salts (MQAS) with bovine serum albumin (BSA) was examined by the fluorescence quenching method and circular dichroism (CD) spectra. Moreover, the effects of MQAS on the dynamic properties of BSA adsorption layers at different pH values were investigated using dilational interfacial rheology. Results show that the quenching constants increase with an increase in pH values and decrease with an increase in the experiment temperature at pH 5.3. The quenching mechanism is static quenching, and the electrostatic force dominates the interaction between MQAS and BSA at pH 5.3. Due to three positive head groups, MQAS can significantly affect the dynamic interfacial activity of BSA molecules at a relatively low concentration. At pH 4.3, the electrostatic repulsion is unfavorable for the formation of MQAS/BSA complexes. Consequently, MQAS molecules will replace BSA molecules from the interface by competitive adsorption. At the pH value above the isoelectric point of BSA, the electrostatic attraction is better for the formation of MQAS/BSA complexes, which exhibit a rapid adsorption rate and an enhanced interfacial activity. Moreover, the kinetic dependencies of interfacial dilational elasticity for the MQAS/BSA mixtures become nonmonotonous. The appearance of the maximum interfacial elasticity values can be attributed to the formation of tails and loops, which suggests that the addition of MQAS destroys the secondary and tertiary structure of protein in the bulk phase. In addition, the effects of MQAS on the secondary structure of protein were demonstrated by CD spectra.

Protein and lipid oxidation affect the viscoelasticity of whey protein layers at the oil–water interface

Protein and lipid oxidation are prevailing issues that negatively affect the nutritional and sensory quality of food emulsions. It is probable that such oxidative modifications affect the functional properties of proteins, and in particular their ability to form densely packed, interconnected viscoelastic films at the oil–water interface. However, these aspects have hardly been investigated. We induced controlled levels of protein and lipid oxidation using whey protein solution and sunflower oil as substrates, respectively. The adsorption kinetics, surface activity, and dilatational interfacial rheology of whey proteins at the sunflower oil–water interface were investigated using a drop tensiometer. Both protein and lipid oxidation led to a decrease in interfacial elasticity compared to the non‐oxidized samples, though through different pathways: protein oxidation led to a broad range of proteinaceous species, including peptides and aggregates, which did not form an interconnected network. Lipid oxidation induced the formation of surface active compounds, which presumably formed segregated domains at the interface.Practical applications: Oxidative reactions prevent whey proteins to form strong, viscoelastic layers at the oil–water interface. This could, in turn, drastically affect their ability to stabilize food emulsions. These findings suggest that considering the initial oxidative state of ingredients should be an integral part of food emulsion formulation.Protein oxidation leads to the formation of a broad range of proteinasceous material, including peptides and aggregates. Lipid oxidation leads to the formation of surface‐active compounds. Both oxidative reactions decrease the ability of whey proteins to form interconnected, elastic interfacial layers at the oil–water interface.

Interfacial dynamic properties and dilational rheology of mixed anionic and cationic Gemini surfactant systems at air–water interface

The interfacial dynamic properties and dilational rheology of sulfonate Gemini surfactant (SGS-12), two bisquaternary ammonium dibromides with different carbon chain lengths (BQAS-12, 16), and two mixed anionic and cationic Gemini surfactant solutions (SGS-12/BQAS-12 and SGS-12/BQAS-16) air–water interface were investigated using the drop shape analysis. Results indicate that the adsorption mechanism of surfactants is diffusion-controlled at dilute concentrations, whereas the adsorption process gradually shifts to a mixed kinetic-diffusion control with increasing the surfactant concentration. The surface tension of the mixtures reaches the minimum value when the molar ratios of SGS-12/BQAS-12 and SGS-12/BQAS-16 mixtures are 6:5 and 8:5, respectively. The effects of oscillating frequency and concentration on the dilational properties were also examined. As the dilational frequency increases, the dilational modulus of these surfactants increases gradually, but the phase angle decreases. The dilational elasticity peaks at a certain concentration, which is less than the critical micelle concentration. Moreover, the effects of SGS-12 concentration on the dilational interfacial rheology of the BQAS-12/SGS-12 and BQAS-16/SGS-12 mixtures were studied. The surface activities of these mixtures show positive correlation with the phase angle. Moreover, the surface film mainly exhibits viscosity as the surface tension reaches the minimum.

Foams stabilized by mixed cationic gemini/anionic conventional surfactants

The mixed adsorption of a cationic gemini surfactant, ethanediyl-1,2-bis(dodecyldimethylammonium bromide) (abbreviated as 12-2-12), and an anionic conventional surfactant, sodium dodecyl sulfate (SDS), was examined using surface tension measurements. The viscoelastic properties of the mixed films were investigated by dilational interfacial rheology technique. The results showed that the addition of SDS promoted the close packing of adsorbed molecules at the interface, which increased the dilational elasticity of the mixed films. The stability of the foams was determined by the half-life of foam height collapse. The foams generated by 12-2-12/SDS mixtures were more stable than that formed by pure 12-2-12. In the presence of sodium bromide, the foam stability was further enhanced and the surfactant concentration required to attain the maximum effect in stabilizing foams was greatly reduced. The high foam stability could well relate to the high elasticity of the film.

Dilational Interfacial Rheology for Increasingly Deasphalted Bitumens and n-C5 Asphaltenes in Toluene/NaHCO3 Solution

Understanding the properties and behaviors of diluted bitumen/water interfaces under stresses is important for remediating emulsion problems in bitumen production. We studied the dilational interfacial rheology of toluene-diluted mineral-free Athabasca bitumen, its n-pentane (n-C5) asphaltenes, and partially deasphalted Athabasca bitumens in a NaHCO3 solution. n-Heptane (n-C7) added in increasing mass ratios to diluted bitumen produced the partially deasphalted bitumens. The solvent-free deasphalted bitumens and the precipitated n-C7 asphaltenes were recovered for study. The number averaged molecular weight (Mn) of the samples was first measured as a function of the mass concentration in toluene. The slopes of these curves indicated that asphaltenes showed higher aggregation tendencies than the bitumens. This tendency was reduced significantly for the deasphalted bitumens. For all oscillation frequencies studied, the interfacial viscoelastic modulus of Athabasca bitumen-in-toluene reached a maximum at 0.8...

Shear and dilational interfacial rheology of surfactant-stabilized droplets

A new measurement method is suggested that is capable of probing the shear and dilational interfacial rheological responses of small droplets, those of size comparable to real emulsion applications. Freely suspended aqueous droplets containing surfactant and non-surface-active tracer particles are transported through a rectangular microchannel by the plane Poiseuille flow of the continuous oil phase. Optical microscopy and high-speed imaging record the shape and internal circulation dynamics of the droplets. Measured circulation velocities are coupled with theoretical descriptions of the droplet dynamics in order to determine the viscous (Boussinesq) and elastic (Marangoni) interfacial effects. A new Marangoni-induced stagnation point is identified theoretically and observed experimentally. Particle velocimetry at only two points (including gradients) in the droplet is sufficient to determine the amplitudes of the dilational and shear responses. We investigate the sensitivity for measuring interfacial properties and compare results from droplets stabilized by a small-molecule surfactant (butanol) and those stabilized by relatively large block copolymer molecules. Future increased availability of shear and dilational interfacial rheological properties is anticipated to lead to improved rules of thumb for emulsion preparation, stabilization, and general practice.

Effect of alkyl tail length of quaternary ammonium gemini surfactants on foaming properties

This paper reports the investigation about the effect of the alkyl tail length of quaternary gemini surfactants, 2-hydroxyl-propanediyl-α,ω-bis(dimethylalkylammonium bromide), referred to as m-3(OH)- m, where m represents the number of carbon atoms at each alkyl tail, on the foamability and foam stability. To understand the foaming behavior, the adsorption of surfactants at the air/water interface as well as the dilational viscoelasticity of their films was studied using surface tension and dilational interfacial rheology measurements. 14-3(OH)-14 was found to have stronger foamability and stabilize more efficiently the foam than 12-3(OH)-12, however, 16-3(OH)-16 failed to generate foam. The latter was attributed to the self-coiling of the long hexadecyl tails prior to the adsorption of surfactant at the air/water interface, resulting in the ageing effect of the adsorption behavior. For both 12-3(OH)-12 and 14-3(OH)-14, the results revealed a good correlation between the high limit interfacial elasticity and high stability of foam at the level of identical surface excesses.

Electrically stressed water drops in oil

The deformation of water droplets in an electric field has been studied in hydrocarbon liquids. A water drop will elongate in an electric field due to the electrostatic pressure, and becomes unstable when a critical field limit is reached. By applying different voltage waveforms it is possible to measure both transient, time varying and static effects. An experimental method has been developed to study the rheological properties of the water droplet. The experimental results in surfactant-free model oil are in good agreement with classic Taylor theory. In diluted crude oil a correlation was found between the drop behavior in an electric field and the interface elasticity modulus, measured by dilational interfacial rheology. The static drop deformation was reduced with increasing elastic modulus due to adsorption of polar surfactants to the water/oil interface. Drop surface oscillations will be damped by the viscosity of the bulk liquid. The dynamic drop behavior was studied in viscosity standards prepared from different concentrations of polystyrene in toluene. The drop oscillations were modeled as damped oscillator, and the influence of viscosity on the eigen-frequency and damping coefficient was studied for different drop sizes. The oscillations can also explain the premature break-up of water droplets sometimes observed in the experiments.

Interfacial rheology as a tool to study the potential of cyclodextrin polymers to stabilize oil–water interfaces

The aim of this work is to characterize rheologically the interfacial properties of oil/water interfaces stabilized with β-cyclodextrin (CD)-based inclusion complexes. Interfacial experiments were conducted using two different hydrophobic oils: Parsol MCX and Squalene. Dilatational interfacial rheology was performed with a dynamic drop tensiometer, allowing the determination of the interfacial tension from the drop profile, and the calculation of rheological values (phase angle, elastic and viscous moduli) from the correlation between the drop area and the interfacial tension sinusoidal variations. Practically, an oil drop was formed in the aqueous phase containing the CD-derivative. With Parsol MCX, no correlation was observed, indicating that surfactants were not adsorbed at the interface. With Squalene, two β-CD-derivatives were used: monomers and polymers. With monomers, a solid membrane was created, impeding any rheological measurements. This is consistent with the crystallization of the system and the impossibility to formulate an emulsion with monomers. With polymers, an effective correlation of the drop area with the interfacial tension variation demonstrated the adsorption of the CD-complex at the interface. Rheological values clearly indicated the formation of an elastic interface, preventing droplets coalescence: an emulsion formulated with β-CD polymers results in a better stabilization.

Dynamic Asphaltene−Resin Exchange at the Oil/Water Interface: Time-Dependent W/O Emulsion Stability for Asphaltene/Resin Model Oils

The critical electric field (CEF) technique was used to determine the time-dependent stability of water-in-oil emulsions in which asphaltenes stabilize the film. Stabilizing films comprising purely asphaltenes were observed to increase monotonically in stability with time. However, in the presence of resins, particularly in mass ratios of resins to asphaltenes of 0.5−1.0, the stability of the emulsions as probed by CEF were observed to exhibit a very sharp local maximum. Similar behavior was observed in dilatational interfacial rheology experiments using an oscillating drop tensiometer. The dilatational modulus (ε) for the stabilizing film, as obtained from the variation of interfacial tension with interfacial area, of an aging asphaltene/resin model oil droplet in water exhibited a time-dependent local maximum. Values of ε were nominally lower for asphaltene/resin model oil systems than asphaltene model oil systems, qualitatively similar to CEF trends. These observed phenomena are similar to the “Vroman effect”, observed in competitive protein adsorption. One plausible explanation is that resin-solvated asphaltenic aggregates are able to diffuse and adsorb to the interface more quickly than larger pure asphaltenic aggregates, but then a molecular rearrangement occurs in which resins become the primary adsorbent in the monolayer by reptation through the consolidated asphaltene network, displacing the asphaltenes and reducing the stability and the dilatational elasticity.

Sorbitan Tristearate Layers at the Air/Water Interface Studied by Shear and Dilatational Interfacial Rheology

The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.