Interfacial Dilational Properties Research Articles

Interactions between Hydrophobically Modified Associating Polyacrylamide and Cationic Surfactant at the Water‐Octane Interface: Interfacial Dilational Viscoelasticity and Relaxation Processes

The interfacial dilational viscoelastic properties of hydrophobically associating block copolymer composed of acrylamide (AM) and a low amount of 2‐ethylhexyl acrylate (EHA) (<1.0 mol%) with a hydrolyzed degree of about 1.5–2.0% at the octane‐water interfaces were investigated by means of two methods: the interfacial tension response to sinusoidal area variations (oscillating barriers method) and the relaxation of an applied stress (interfacial tension relaxation method) respectively. The influence of cationic surfactant cetyl trimethylammonium bromide (CTAB) on the dilational viscoelastic properties was studied. The results obtained by oscillating barriers method showed that dilational modulus decreased moderately with the increase of CTAB concentration. The results obtained by interfacial tension relaxation measurements showed that two main relaxation processes exist in the interface at 7,000 ppm polymer concentration: one is the fast process involving the exchange of hydrophobic blocks between the proximal region and distal region in the interface; the other is the slow relaxation process involving conformational changes of polymer chain in the interface. By adding CTAB, the slow process changed obviously due to the strong electrostatic interaction between oppositely charged surfactant and hydrolyzed part of polymer chain. Only when the CTAB concentration was close to the “equal charge point,” the associations formed mainly by the hydrophobic interaction like that in SDS/polymer system appeared and the characteristic time of fast process decreased obviously. The information of relaxation processes obtained from interfacial tension relaxation measurements can explain the results from dilational viscoelasticity measurements very well.

Stability of thin stagnant film on a solid surface with a viscoelastic air–liquid interface

Linear stability analysis for a film on a solid surface with a viscoelastic air–liquid interface is presented. The interfacial dilatational and shear viscoelastic properties were described by Maxwell models. Dilatational and shear interfacial elasticity and viscosity were shown to improve film stability. When the interfacial rheological properties are extremely large or small, the maximum perturbation growth coefficient is shown to reduce to those for immobile and mobile interfaces respectively. Calculated values of maximum growth coefficient for thin film stabilized by 0.5% β-lactoglobulin approached those of mobile films for thick ( > 2000 nm ) and those for immobile films for thin ( < 100 nm ) films respectively with the values lying between the two limits for intermediate film thicknesses.

Interfacial Dilatational Elasticity and Viscosity of β-Lactoglobulin at Air−Water Interface Using Pulsating Bubble Tensiometry

The ability of proteins to provide stability in foams is greatly influenced by their interfacial dilatational rheological properties. Surface tension response of a pulsatingbubble with an adsorbed layer of beta-lactoglobulin was measured for different frequencies and protein concentrations using a pulsating bubble tensiometer. A methodology, accounting for adsorption/desorption as well as variation of surface concentration due to expansion/contraction, was developed for the evaluation of surface dilatational elasticity and viscosity at different frequencies from these measurements. The adsorption rate constants were inferred from the surface pressure dynamics of protein adsorption using a Langmuir minitrough. The desorption rates were shown to be negligible for beta-lactoglobulin from the surface pressure response of a spread monolayer when subjected to compression in a Langmuir minitrough. The proposed model was employed to infer the interfacial dilatational viscosity and elasticity of an adsorbed beta-lactoglobulin layer at the air-water interface from experimental pulsating bubble data for protein concentrations in the range of 0.01-0.5 wt % at pH 7. As expected, the interfacial dilatational rheological properties were found to be higher at higher protein concentrations, this effect being less pronounced for dilatational elasticity. Heating at 80 degrees C for 30 min was found to result in higher interfacial dilatational viscosity and lower interfacial dilatational elasticity though this difference was within experimental error. The traditional approach for the inference of interfacial dilatational rheological properties is found to overpredict the interfacial dilatational elasticity whereas the viscosity values do not differ significantly from those obtained using the current analysis.

STS-107 OV-102 mission FAST experiment: Slow surface relaxation at the solution-air interface

The experiments are aimed at the study of the interfacial dilational properties, under microgravity conditions. The examined systems are aqueous solutions of the surfactant n-dodecyl-dimethylphosphine oxide, at different concentrations and temperatures. Low frequency harmonic disturbances are imposed to the interfacial area of an air bubble. Also square pulse disturbances are imposed to each studied system. The experimental results grant information about the dynamics of air-liquid single interfaces.

Oscillating drop/bubble tensiometry: effect of viscous forces on the measurement of interfacial tension

The oscillating drop/bubble technique is increasingly popular for measuring the interfacial dilatational properties of surfactant/polymer-laden fluid/fluid interfaces. A caveat of this technique, however, is that viscous forces are important at higher oscillation frequencies or fluid viscosities; these can affect determination of the interfacial tension. Here, we experimentally quantify the effect of viscous forces on the interfacial-tension measurement by oscillating 100 and 200 cSt poly(dimethylsiloxane) (PDMS) droplets in water at small amplitudes and frequencies ranging between 0.01 and 1 Hz. Due to viscous forces, the measured interfacial tension oscillates sinusoidally with the same frequency as the oscillation of the drop volume. The tension oscillation precedes that of the drop volume, and the amplitude varies linearly with Capillary number, Ca = Δ μ ω Δ V / γ a 2 , where Δ μ = μ D − μ is the difference between the bulk Newtonian viscosities of the drop and surrounding continuous fluid, ω is the oscillation frequency of the drop, Δ V is the amplitude of volume oscillation, γ is the equilibrium interfacial tension between the PDMS drop and water, and a is the radius of the capillary. A simplified model of a freely suspended spherical oscillating-drop well explains these observations. Viscous forces distort the drop shape at Ca > 0.002 , although this criterion is apparatus dependent.

The interfacial dilational properties of hydrophobically modified associating polyacrylamide studied by the interfacial tension relaxation method at an oil-water interface.

The interfacial dilational viscoelastic properties of hydrophobically associating block copolymer composed of acrylamide (AM) and a low amount of 2-phenoxylethyl acrylate (POEA) (<1.0 mol%) at the octane-water interfaces were studied by means of the interfacial tension relaxation method. The dependencies of interfacial dilational elasticity and viscous component on the dilational frequency were investigated. The interaction of hydrophobically associating block copolymer [P(AM/POEA)] with sodium dodecyl sulfate (SDS) has been explored. The results show that at lower frequency, the dilational elasticity for different concentration copolymer is close to zero; at higher frequency, the dilational elasticity shows no change with increased frequency; At moderate frequency (10(-3)-1 Hz), the dilational elasticity decreased with a decrease in the dilational frequency. The results show that the hydrophobic groups of [P(AM/POEA)] chains can be associated by inter- or intrachain liaisons in water solution. The dilational viscous component for P(AM/POEA) comes forth a different maximum value at different frequencies when the polymer concentration is different. It is generally believed that the dilational viscous component reflects the summation of the various microscopic relaxation processes at and near the interface and different relaxation processes have different characteristic frequencies. The spectrum of dilational viscous component may appear more than once maximum values at different frequencies. The influence of SDS on the limiting dilational elasticity and viscous component for polymer solution was elucidated. For 5000 ppm polymer solution, the limiting dilational elasticity decreased with an increase in SDS concentration. The dilational viscous component passed through a maximum value with a rise in the dilational frequency, which appeared at different frequency when SDS concentration is different; and the higher is the concentration, the lower is the dilational frequency. It can be explained that macromolecules may be substituted by SDS molecules in the interface and the interaction of molecules decrease, which makes the limiting dilational elasticity decrease. For 200 ppm polymer solution, the limiting dilational elasticity increased firstly and then decreased with SDS concentration increasing. This may be explained that the interfacial polymer concentration is so low that SDS molecules absorbed in the interface dominate dilational properties of the interfacial film even at very low SDS concentration. However, SDS molecules can gradually substitute the polymer molecules in the interface with a rise in SDS concentration, which results in the decrease in the limiting dilational elasticity.

Hydrophobically Modified Associating Polyacrylamide Solutions: Relaxation Processes and Dilational Properties at the Oil−Water Interface

The interfacial dilational viscoelastic properties of hydrophobically associating block copolymer composed of acrylamide (AM) and a low amount of 2-phenoxylethyl acrylate (POEA) (<1.0 mol %) at the octane−water interfaces were investigated by means of two methods: the interfacial tension response to sinusoidal area variations and the relaxation of an applied stress. The dependencies of interfacial dilational modulus and phase angle on the polymer concentration were explored. The influence of sodium dodecyl sulfate (SDS) on the dilational viscoelastic properties of polymer solutions was studied. The results obtained by oscillating barriers method showed that the dilational modulus passed through a maximum value with increasing polymer concentration, while the phase angle decreased with increasing concentration below 200 ppm, then showed very low concentration dependence up to 3000 ppm, and increased dramatically above it. When SDS was added to the aqueous phase, the dilational modulus passed through a maximum with increasing SDS concentration, while the change of phase angle depended on the polymer bulk concentration. The results obtained by the relaxation of an applied stress show that two main relaxation processes exist in the interface at low bulk concentration below the critical aggregation concentration: one is the fast process involving the exchange of hydrophobic microdomains between the proximal region and distal region in the interface with a characteristic time value from several tens of seconds to several seconds at different bulk concentration; the other is the slow relaxation process involving conformational changes of polymer chain in the interface with characteristic time value from 1000 s to several tens of seconds, depending on the bulk concentration. However, there is only one main relaxation process controlling the dilational properties above c*: a fast relaxation process with the characteristic relaxation time of less than 1 s, which is believed to be related to the associations formed by hydrophobic microdomains. Anionic surfactant SDS can influence the dilational properties of polymer solutions by the following ways: first, SDS can absorb onto the interface and bind to the hydrophobic microdomains to change the characteristic times and contributions of the existed relaxation processes of polymer chains; second, SDS can provide a new fast relaxation process involving the exchange of SDS molecules between monomers and mixed micelles in interface. The information on relaxation processes obtained from interfacial tension relaxation measurements can explain the results from dilational viscoelasticity measurements very well. The negative phase angles have been obtained in some case. It is believed that the in-interface slow relaxation process, which sometimes dominates the dilational viscoelasticity of polymer film, is responsible for this phenomenon in our employed experimental method.

A study of interfacial dilational properties of two different structure demulsifiers at oil–water interfaces

The interfacial dilational viscoelastic properties of two demulsifiers with straight chain (SP-169) and branched chain (AE-121) at the oil–water interfaces were investigated by means of the longitudinal waves method and the interfacial tension relaxation method, respectively. The results obtained by the longitudinal waves method showed that the dilational viscous component for AE-121 and SP-169 also passed through a maximum value with increasing concentration. It was found that the maximum value appeared at different demulsifier concentrations during our experiment frequency; and the higher is the dilational frequency, the lower is the concentration. The influences of AE-121 and SP-169 on the dilational viscoelastic properties of the oil–water interface containing surface-active fraction from Iranian crude oil have been measured. The results clearly stated that both demulsifiers could obviously decrease the dilational elasticity of oil–water interface containing surface-active fraction. At low concentration, because of stronger adsorption ability, SP-169 has stronger ability to decreasing the dilational modulus than AE-121. We also found that the dilational modulus of the interface contained surface-active fraction passed through a minimum value with increasing demulsifier concentration for both demulsifiers. This result indicated the dosage of demulsifier had an optimum value. The results obtained by means of interfacial tension relaxation method showed that the slow relaxation processes involve mainly rearrangement in the conformation of the molecules appeared with increasing demulsifier concentration.

Rheological Study of Lysozyme and PEG2000 at the Air−Water and Dichloromethane−Water Interfaces under Ramp Type or Sinusoidal Perturbations

The dilational rheological properties of interfacial films of poly(ethylene glycol) (PEG2000) and hen egg-white lysozyme (HEWL) were studied respectively at the dichloromethane (DCM)−water and air−water interfaces by means of the pendant drop method. In both cases, the observed interfacial behaviors were approached by a model corresponding to a two-dimensional viscoelastic solid. The interfacial layers were characterized by three physical constants: Ee, the equilibrium elasticity, Ene, the nonequilibrium elasticity, and τ, the relaxation time. Because the interfacial dilational properties of the films were studied by using a ramp type perturbation approach or a sinusoidal variations approach, identical rheological physical constants values were obtained for PEG2000 and HEWL. From these studies, the interactions within the interfacial layer and those between the interfacial film and adjacent phases can be indirectly accessed and estimated.

Influence of Demulsifiers of Different Structures on Interfacial Dilational Properties of an Oil–Water Interface Containing Surface-Active Fractions from Crude Oil

The influences of two commercial demulsifiers that have a straight chain and branch chain, respectively, on the dilational viscoelasticity of an oil–water interfacial film containing surface-active fractions from crude oil were investigated. The branch-chain demulsifier AE-121 could efficiently substitute surface-active fractions of different average molecular weights from the oil–water interface, while straight-chain SP-169 could only efficiently substitute those of large average molecular weight. It was apt to form a mix-adsorption layer with surface-active fractions of small average molecular weight. The results showed that the molecular size (or represented by average molecular weights) of the surface-active fractions was an important factor influencing the reciprocity of demulsifiers and surface-active fractions at the oil–water interface. This effect could be well explained by the difference between sizes of surface-active fraction molecules and vacancies between demulsifier molecules at the interface. The results of SDBS also proved this explanation.

Efecto de la temperatura sobre películas de un aislado proteico del suero bovino (WPI) adsorbidas sobre la interfase aceite-agua

Heat-induced interfacial aggregation of a whey protein isolate (WPI) with a high content of β-lactoglobulin, previously adsorbed at the oil-water interface, was studied by interfacial dynamic characteristics (interfacial tension and surface dilational properties) performed in a automatic drop tensiometer coupled with microscopic observation and image analysis of the drop after heat-treatment. The temperature, ranging between 20 and 80 oC, and protein concentration in aqueous bulk phase, ranging between 1.10 -1 and 1.10 -5 % wt/wt, were studied as variables. The pH, and ionic strength were maintained constant at 5 and 0.05 M, respectively. During the heat-treatment, WPI films behave typically as viscoelastic with non-zero phase angle, but with increasing elastic characteristics as the heat-treatment progresses. During isothermal treatment the surface dilational modulus, E, increases and the interfacial tension, σ, and phase angle, φ, decrease with time to a plateau value. The time dependence of E can be quantified by a first-order equation according to two kinetic mechanisms. The rate of thermal changes in WPI adsorbed films increases with protein concentration in solution. Heat-treatment produces irreversible changes in WPI adsorbed films because the interfacial characteristics do not return to original values after cooling back to the initial temperature. Significant changes in interfacial characteristics and drop image associated with interfacial WPI gelation were observed at protein concentration as low as 1.10 -5 % wt/wt, even for heat-treatment at 40 oC.

The effect of temperature on food emulsifiers at fluid–fluid interfaces

Heat-induced interfacial aggregation of a whey protein isolate (WPI) with a high content of β-lactoglobulin (>92%), previously adsorbed at the oil–water interface, was studied by means of interfacial dynamic characteristics performed in an automatic drop tensiometer. Protein concentration in aqueous bulk phase ranging between 1×10 −1 and 1×10 −5 % wt/wt was studied as a variable. The experiments were carried out at temperatures ranging from 20–80°C with different thermal regimes. During the heating period, competition exists between the effect of temperature on the film fluidity and the increase in mechanical properties associated with the interfacial gelation process. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) previously adsorbed at the oil–water interface, was studied by interfacial dynamic characteristics (interfacial tension and surface dilational properties). The temperature, ranging between 40 and 2°C, and the lipid concentration in aqueous oil phase, ranging between 1×10 −2 and 1×10 −4 % wt/wt, were studied as variables. Significant changes in interfacial dynamic characteristics associated with interfacial lipid crystallisation were observed as a function of lipid concentration in the bulk phase. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) at the air–water interface, was studied by π-A isotherms performed in a Langmuir trough coupled with Brewster angle microscopy (BAM). A condensation in monoglyceride monolayers towards lower molecular area was observed as the temperature decreased. This effect was attributed to lipid crystallisation at lower temperatures. BAM images corroborated the effect of temperature on the monolayer structure, as a function of the monoglyceride type.

Investigation of the oscillating bubble technique for the determination of interfacial dilatational properties

A mathematical framework is developed for measuring the dilatational rheological properties of gas-liquid interfaces by analyzing small amplitude radial oscillations of an air bubble blown into a liquid substrate. Contrary to earlier analyses which customarily require the bubble radius change during the oscillation, the present study is based on the readily measurable quantities of bubble pressure change during the oscillation and phase lag between the bubble pressure and the relative displacement of the oscillation generating device. To expose the extensive interrelationships among the various experimental parameters that have a profound effect on bubble dynamics, the analysis is focused on the influence of the variable gas volumes at the sides of the differential pressure transducer, the gas phase compressibility, the dynamic behavior of the transducer membrane and the bubble surface being only a segment of a sphere, as they play a key role in the response of the system. A linear viscoelastic surface behavior is assumed which includes intrinsic surface viscosities and Gibbs elasticities. New data are deduced from previous measurements on the dilatational surface properties of stearic acid monolayers which, being practically insoluble in water, offer an ideal experimental system without the complications due to bulk-interface interactions. For a fixed frequency of oscillation, a virtually constant dilatational surface viscosity is determined throughout the employed concentration range. Yet, surface viscosity is found to slightly increase when the oscillation frequency increases. In addition, for the range of the examined parameters, no firm evidence of a dilatational surface elasticity is observed.

Adsorption of whey protein isolate at the oil-water interface as a function of processing conditions: a rheokinetic study.

In this paper we present surface dynamic properties (interfacial tension and surface dilational properties) of a whey protein isolate with a high content of beta-lactoglobulin (WPI) adsorbed on the oil-water interface as a function of adsorption time. The experiments were performed at constant temperature (20 degrees C), pH (5), and ionic strength (0.05 M). The surface rheological parameters and the interfacial tension were measured as a function of WPI concentration (ranging from 1 x 10(-)(1) to 1 x 10(-)(5)% w/w) and different processing factors (effect of convection and heat treatment). We found that the interfacial pressure, pi, and surface dilational modulus, E, increase and the phase angle, phi, decreases with time, theta, which should be associated with WPI adsorption. These phenomena have been related to diffusion of the protein toward the interface (at short adsorption time) and to the protein unfolding and/or protein-protein interactions (at long-term adsorption) as a function of protein concentration in solution and processing conditions.

Interfacial Dilatational Properties of Milk Proteins Cross-Linked by Transglutaminase

Milk proteins (β-lactoglobulin and sodium caseinate) were adsorbed at air−water (A−W) and oil−water (O−W) interfaces and cross-linked with a microbial Ca2+-independent transglutaminase. Interfacial dilatational moduli were measured by interfacial tension relaxation. Moduli were generally higher at the O−W interface than at the A−W interface for native β-lactoglobulin, but lower at the O−W interface for native sodium caseinate. Cross-linked films generally had higher dilatational moduli than non-cross-linked films at both the A−W and O−W interfaces. Especially for β-lactoglobulin, it was found that 2 h of cross-linking had a more marked effect at the O−W interface than at the A−W interface. This could be explained by greater unfolding of β-lactoglobulin at the O−W interface, as confirmed by measurement of surface pressure−area (π−A) isotherms for β-lactoglobulin spread at both types of interface. For sodium caseinate enhanced unfolding and adsorption at the O−W interface may have inhibited enzyme action at the interface. Keywords: β-Lactoglobulin; caseinate; protein(s); transglutaminase; dilatational rheology

Comparison of the dilational behaviour of adsorbed milk proteins at the air-water and oil-water interfaces

The interfacial dilational properties of two milk proteins, β-casein and β-lactoglobulin, have been compared at the air-water and paraffin oil-water interfaces. The measurements were performed as a function of bulk protein concentration using a modified Langmuir trough technique at a frequency of 0.1 Hz. It appears that the dilational properties of the two proteins are essentially the same whether the upper phase is paraffin oil or air. This is attributed to rather limited penetration of the oil phase by the protein molecules. The interfacial properties have been shown to be dependent upon the bulk concentration of the protein. This, together with the history of the surface, presumably determines the interfacial concentration and conformation of the protein molecules. At low bulk concentrations both the modulus and the loss tangent are similar for the two proteins. It is conjectured that both proteins have a similar, reasonably extended, structure in the interface. At higher bulk concentrations the behaviour of the two proteins diverges. The interfacial dilational modulus for β-lactoglobulin is around 62 mN m −1 for bulk concentrations ⩾ 3× 10 −2 g l −1, and the viscosity of the surface is rather low. It is postulated that this protein forms a cohesive elastic network within the interface. The interfacial dilational modulus for β-casein at similar concentrations is around 13 mN m −1. It is thought that some intermolecular cohesion is present in the interface, but that this is much weaker than for the globular protein. High concentrations of β-casein impart significant dilational viscosity to the interface, due either to diffusional relaxation or to rearrangement of the adsorbed primary layer and multilayers of protein.

Factors Controlling the Stability of Colloid-Stabilized Emulsions: III. Measurement of the Rheological Properties of Colloid-Laden Interfaces

Finely divided insoluble solid particles constitute an important class of emulsifying agents. Colloidal particles that are partially wetted by both the aqueous and the oleic phases are capable of effectively stabilizing emulsions. We have shown in the past that emulsion stability is controlled primarily by the concentration of particles adsorbed at the oil-water interface. At sufficiently high concentrations of particles, colloid-laden oil-water interfaces tend to exhibit non-Newtonian behavior. In Part II of this series, we presented a model to show that colloid-laden oil-water interfaces behave viscoelastically. Here we present an experimental setup that we have designed and built to measure the dilational rheological properties of surfactant and/or colloid-laden oil-water interfaces. Measurements of the interfacial dilational properties of such colloid-laden oil-water interfaces are also presented. The results clearly show that oil-water interfaces containing adsorbed surfactants and/or colloidal particles exhibit viscoelastic behavior. Such viscoelastic interfaces enhance emulsion stability by increasing the magnitude of steric hindrance (i.e., the energy required to displace particles away from the drop-drop contact region) and by decreasing the rate of film thinning between coalescing emulsion droplets.

Measurement of interfacial dilational properties: A software-driven apparatus

The construction of a time-resolved surface viscoelastometer for the study of fluid/fluid interfaces is described in detail. Instrument design and operation are based on the formalism of systems theory. An interfacial-area change (i.e., an external stimulus) is generated according to a given time function; the ensuing dynamic surface tension response is continuously recorded in synchronism with the stimulus. The principal characteristic of the equipment is a computer-controlled mechanical motion of the area-confining device (i.e., of a particular ring barrier, in place of the traditional linear barriers). The software allows a wide variety of oscillatory or transient experiments to be conducted. Instrument performance is illustrated by a typical experimental response, obtained under conditions of small-amplitude area perturbation, for a submicellar aqueous solution of surfactant. The recorded trace is fitted to a theoretical two-parameter model. Results show the effectiveness of the controlled-surface-deformation technique, allowed by the instrument, for determining static and dynamic interfacial properties.

Dilational viscoelastic properties of fluid interfaces-II: Experimental study

A longitudinal wave technique combined with tracer particle measurements offers a promising method for the measurement of dalational,viscoelastic properties of fluid interfaces containing surface active agents and other chemical additives. In this technique, a longitudinal wave disturbance is imparted to an interface by means of a sinusoidafly oscillating barrier, and the resulting response of the interface is measured in terms of amplitude and time tag of the tracer particle motion relative to the original disturbance. An apparatus constructed for this purpose is capable of measuring the dilational elastic properties from 10 −2 to 10 dyne/cm, and the viscosides from 10 −3 to 10 surface poise. Surface viscoelasticity measurements have been conducted on decanoic acid solutions-representing an adsorbed monolayer system; and on stearic acid films-representing a deposited monolayer system. A significant observation from the measurements on stearic acid monolayers in the range of 35–45 Å 2/molecule was that the delational surface viscosity of these monolayers was 15–400 times greater than the corresponding shear viscosity. This experimental evidence, reported for the first time in this study, indicates the extent of the importance of interfacial dilational properties in comparison with interfacial shear properties.