Dynamics of interfacial layers-experimental feasibilities of adsorption kinetics and dilational rheology

Doxorubicin Interaction with Lipid Monolayers Leads to Decreased Membrane Stiffness when Experiencing Compression-Expansion Dynamics

Physical membrane models permit to study and quantify the interactions of many external molecules with monitored and simplified systems. In this work, we have constructed artificial Langmuir single-lipid monolayers with dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidylserine (DPPS), or sphingomyelin to resemble the main lipid components of the mammalian cell membranes. We determined the collapse pressure, minimum area per molecule, and maximum compression modulus (C_s^-1) from surface pressure measurements in a Langmuir trough. Also, from compression/expansion isotherms, we estimated the viscoelastic properties of the monolayers. With this model, we explored the membrane molecular mechanism of toxicity of the well-known anticancer drug doxorubicin, with particular emphasis in cardiotoxicity. The results showed that doxorubicin intercalates mainly between DPPS and sphingomyelin, and less between DPPE, inducing a change in the C_s^-1 of up to 34% for DPPS. The isotherm experiments suggested that doxorubicin had little effect on DPPC, partially solubilized DPPS lipids toward the bulk of the subphase, and caused a slight or large expansion in the DPPE and sphingomyelin monolayers, respectively. Furthermore, the dynamic viscoelasticity of the DPPE and DPPS membranes was greatly reduced (by 43 and 23%, respectively), while the reduction amounted only to 12% for sphingomyelin and DPPC models. In conclusion, doxorubicin intercalates into the DPPS, DPPE, and sphingomyelin, but not into the DPPC, membrane lipids, inducing a structural distortion that leads to decreased membrane stiffness and reduced compressibility modulus. These alterations may constitute a novel, early step in explaining the doxorubicin mechanism of action in mammalian cancer cells or its toxicity in non-cancer cells, with relevance to explain its cardiotoxicity.

Dynamic interfacial properties and foam behavior of licorice root extract solutions

Licorice (Glycyrrhiza glabra) is a useful plant of the family Fabaceae, with sweet-tasting roots. The root extract of this plant is rich in saponins, so it can be considered a source of natural surfactants. This research provides some applicable information about the dynamic surface tension and foam behavior of aqueous solutions of licorice root extract (LRE). The pendant drop shape analysis was utilized to study the surface tension and dilational surface rheology of LRE at the water/air interface. The Bikerman type experiment was used to measure foamability and foam stability of aqueous LRE solutions. The equilibrium surface tensions reveal that the LRE contains surface-active components and is capable of reducing the surface tension by 25 mN/m at the critical aggregation concentration (CAC). The surface dilational visco-elasticity measurements proved that the adsorption layers are predominantly of elastic nature. Also the foamability and foam stability show a meaningful correlation with the dynamic surface properties. This study aims to contribute to the development of appropriate utilization of the benefits provided by a biosurfactant source in foam-related commercial applications.

Review of the role of surfactant dynamics in drop microfluidics

Surfactants are employed in microfluidic systems not just for drop stabilisation, but also to study local phenomena in industrial processes. On the scale of a single drop, these include foaming, emulsification and stability of foams and emulsions using statistically significant ensembles of bubbles or drops respectively. In addition, surfactants are often a part of a formulation in microfluidic drop reactors. In all these applications, surfactant dynamics play a crucial role and need to be accounted for. In this review, the effect of surfactant dynamics is considered on the level of standard microfluidic operations: drop formation, movement in channels and coalescence, but also on a more general level, considering the mechanisms controlling surfactant adsorption on time- and length-scales characteristic of microfluidics. Some examples of relevant calculations are provided. The advantages and challenges of the use of microfluidics to measure dynamic interfacial tension at short time-scales are discussed.

Microparticle Brownian motion near an air-water interface governed by direction-dependent boundary conditions

HYPOTHESIS

Although the dynamics of colloids in the vicinity of a solid interface has been widely characterized in the past, experimental studies of Brownian diffusion close to an air-water interface are rare and limited to particle-interface gap distances larger than the particle size. At the still unexplored lower distances, the dynamics is expected to be extremely sensitive to boundary conditions at the air-water interface. There, ad hoc experiments would provide a quantitative validation of predictions.

EXPERIMENTS

Using a specially designed dual wave interferometric setup, the 3D dynamics of 9 μm diameter particles at a few hundreds of nanometers from an air-water interface is here measured in thermal equilibrium.

FINDINGS

Intriguingly, while the measured dynamics parallel to the interface approaches expected predictions for slip boundary conditions, the Brownian motion normal to the interface is very close to the predictions for no-slip boundary conditions. These puzzling results are rationalized considering current models of incompressible interfacial flow and deepened developing an ad hoc model which considers the contribution of tiny concentrations of surface active particles at the interface. We argue that such condition governs the particle dynamics in a large spectrum of systems ranging from biofilm formation to flotation process.

Effect of Divalent and Monovalent Salts on Interfacial Dilational Rheology of Sodium Dodecylbenzene Sulfonate Solutions

This study presents the equilibrium surface tension (ST), critical micelle concentration (CMC) and the dilational viscoelasticity of sodium dodecylbenzene sulfonate (SDBS)-adsorbed layers in the presence of NaCl, KCl, LiCl, CaCl2 and MgCl2 at 0.001–0.1 M salt concentration. The ST and surface dilational viscoelasticity were determined using bubble-shape analysis technique. To capture the complete profile of dilational viscoelastic properties of SDBS-adsorbed layers, experiments were conducted within a wide range of SDBS concentrations at a fixed oscillating frequency of 0.01 Hz. Salts were found to lower the ST and induce micellar formation at all concentrations. However, the addition of salts increased dilational viscoelastic modulus only at a certain range of SDBS concentration (below 0.01–0.02 mM SDBS). Above this concentration range, salts decreased dilational viscoelasticity due to the domination of the induced molecular exchange dampening the ST gradient. The dilational viscoelasticity of the salts of interest were in the order CaCl2 > MgCl2 > KCl > NaCl > LiCl. The charge density of ions was found as the corresponding factor for the higher impact of divalent ions compared to monovalent ions, while the impact of monovalent ions was assigned to the degree of matching in water affinities, and thereby the tendency for ion-pairing between SDBS head groups and monovalent ions.

Effect of Surfactant Dynamics on Flow Patterns Inside Drops Moving in Rectangular Microfluidic Channels

Drops contained in an immiscible liquid phase are attractive as microreactors, enabling sound statistical analysis of reactions performed on ensembles of samples in a microfluidic device. Many applications have specific requirements for the values of local shear stress inside the drops and, thus, knowledge of the flow field is required. This is complicated in commonly used rectangular channels by the flow of the continuous phase in the corners, which also affects the flow inside the drops. In addition, a number of chemical species are present inside the drops, of which some may be surface-active. This work presents a novel experimental study of the flow fields of drops moving in a rectangular microfluidic channel when a surfactant is added to the dispersed phase. Four surfactants with different surface activities are used. Flow fields are measured using Ghost Particle Velocimetry, carried out at different channel depths to account for the 3-D flow structure. It is shown that the effect of the surfactant depends on the characteristic adsorption time. For fast-equilibrating surfactants with a characteristic time scale of adsorption that is much smaller than the characteristic time of surface deformation, this effect is related only to the decrease in interfacial tension, and can be accounted for by the change in capillary number. For slowly equilibrating surfactants, Marangoni stresses accelerate the corner flow, which changes the flow patterns inside the drop considerably.

Comparison of surfactant mass transfer with drop formation times from dynamic interfacial tension measurements in microchannels

Dynamic interfacial tension was studied experimentally during drop formation in a flow-focusing microchannel. A low viscosity silicone oil (4.6 mPa s) was the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. An anionic (sodium dodecylsulfate, SDS), a cationic (dodecyltrimethylammonium bromide, DTAB) and a non-ionic (Triton™ X-100, TX100) surfactant were added in the dispersed phase, at concentrations below and above the critical micelle concentration (CMC). For SDS and DTAB the drop size against continuous phase flowrate curves initially decreased with surfactant concentration and then collapsed to a single curve at concentrations above CMC. For TX100 the curves only collapsed at surfactant concentrations 8.6 times the CMC. From the collapsed curves a correlation of drop size with capillary number was derived, which was used to calculate the dynamic interfacial tension at times as low as 3 ms. The comparison of the surfactant mass transport and adsorption times to the interface against the drop formation times indicated that surfactant adsorption also contributes to the time required to reach equilibrium interfacial tension. Criteria were proposed for drop formation times to ensure that equilibrium interfacial tension has been reached and does not affect the drop formation.

Dilatational rheology of water-in-diesel fuel interfaces: effect of surfactant concentration and bulk-to-interface exchange

Micrometer-sized water droplets dispersed in diesel fuel are stabilized by the fuel's surface-active additives, such as mono-olein and poly(isobutylene)succinimide (PIBSI), making the droplets challenging for coalescing filters to separate. Dynamic material properties found from interfacial rheology are known to influence the behavior of microscale droplets in coalescing filters. In this work, we study the interfacial dilatational properties of water-in-fuel interfaces laden with mono-olein and PIBSI, with a fuel phase of clay-treated ultra-low sulphur diesel (CT ULSD). First, the dynamic interfacial tension (IFT) is measured using pendant drop tensiometry, and a curvature-dependent form of the Ward and Tordai diffusion equation is applied for extracting the diffusivity of the surfactants. Additionally, Langmuir kinetics are applied to the dynamic IFT results to obtain the maximum surface concentration (Γ∞) and ratio of adsorption to desorption rate constants (κ). We then use a capillary pressure microtensiometer to measure the interfacial dilatational modulus, and further extract the characteristic frequency of surfactant exchange (ω0) by fitting a model assuming diffusive exchange between the interface and bulk. In this measurement, 50-100 μm diameter water droplets are pinned at the tip of a glass capillary in contact with the surfactant-containing fuel phase, and small amplitude capillary pressure oscillations over a range of frequencies from 0.45-20 rad s-1 are applied to the interface, inducing changes in interfacial tension and area to yield the dilatational modulus, E*(ω). Over the range of concentrations studied, the dilatational modulus of CT ULSD with either mono-olein or PIBSI increases with a decrease in bulk concentration and plateaus at the lowest concentrations of mono-olein. Characteristic frequency (ω0) values extracted from the fit are compared with those calculated using equilibrium surfactant parameters (κ and Γ∞) derived from pendant drop tensiometry, and good agreement is found between these values. Importantly, the results imply that diffusive exchange models based on the equilibrium relationships between surfactant concentration and interfacial tension can be used to infer the dynamic dilatational behavior of complex surfactant systems, such as the water-in-diesel fuel interfaces in this study.

Adsorption of Equimolar Mixtures of Cationic and Anionic Surfactants at the Water/Hexane Interface

In mixed solutions of anionic and cationic surfactants, called catanionics, ion pairs are formed which behave like non-ionic surfactants with a much higher surface activity than the single components. In equimolar mixtures of NaCnSO4 and CmTAB, all surface-active ions are paired. For mixtures with n + m = const, the interfacial properties are rather similar. Catanionics containing one long-chain surfactant and one surfactant with medium chain length exhibit a strong increase in surface activity as compared with the single compounds. In contrast, catanionics of one medium- and one short chain surfactant have a surface activity similar to that of the medium-chain surfactant alone. Both the Frumkin model and the reorientation model describe the experimental equilibrium data equally well, while the adsorption kinetics of the mixed medium- and short-chain surfactants can be well described only with the reorientation model.

Phase-Dependent Surfactant Transport on the Microscale: Interfacial Tension and Droplet Coalescence

Liquid-liquid emulsion systems are usually stabilized by additives, known as surfactants, which can be observed in various environments and applications such as oily bilgewater, water-entrained diesel fuel, oil production, food processing, cosmetics, and pharmaceuticals. One important factor that stabilizes emulsions is the lowered interfacial tension (IFT) between the fluid phases due to surfactants, inhibiting the coalescence. Many studies have investigated the surfactant transport behavior that leads to corresponding time-dependent lowering of the IFT. For example, the rate of IFT decay depends on the phase in which the surfactant is added (dispersed vs continuous) due in part to differences in the near-surface depletion depth. Other key factors, such as the viscosity ratio between the dispersed and continuous phases and Marangoni stress, will also have an impact on surfactant transport and therefore the coalescence and emulsion stability. In this feature article, the measurement techniques for dynamic IFT are first reviewed due to their importance in characterizing surfactant transport, with a specific focus on macroscale versus microscale techniques. Next, equilibrium isotherm models as well as dynamic diffusion and kinetic equations are discussed to characterize the surfactant and the time scale of the surfactant transport. Furthermore, recent studies are highlighted showing the different IFT decay rates and its long-time equilibrium value depending on the phase into which the surfactant is added, particularly on the microscale. Finally, recent experiments using a hydrodynamic Stokes trap to investigate the impact of interfacial surfactant transport, or "mobility", and the phase containing the surfactant on film drainage and droplet coalescence will be presented.

Motion of micro- and nano- particles interacting with a fluid interface

In this article, we review both theoretical models and experimental results on the motion of micro- and nano- particles that are close to a fluid interface or move in between two fluids. Viscous drags together with dissipations due to fluctuations of the fluid interface and its physicochemical properties affect strongly the translational and rotational drags of colloidal particles, which are subjected to Brownian motion in thermal equilibrium. Even if many theoretical and experimental investigations have been carried out, additional scientific efforts in hydrodynamics, statistical physics, wetting and colloid science are still needed to explain unexpected experimental results and to measure particle motion in time and space scales, which are not accessible so far.

A Multi-Scale Approach to Modeling the Interfacial Reaction Kinetics of Lipases with Emphasis on Enzyme Adsorption at Water-Oil Interfaces

The enzymatic hydrolysis of triglycerides with lipases (EC 3.1.1.3.) involves substrates from both water and oil phases, with the enzyme molecules adsorbed at the water-oil (w/o) interface. The reaction rate depends on lipase concentration at the interface and the available interfacial area in the emulsion. In emulsions with large drops, the reaction rate is limited by the surface area. This effect must be taken into account while modelling the reaction. However, determination of the interfacial saturation is not a trivial matter, as enzyme molecules have the tendency to unfold on the interface, and form multi-layer, rendering many enzyme molecules unavailable for the reaction. A multi-scale approach is needed to determine the saturation concentration with specific interfacial area so that it can be extrapolated to droplet swarms. This work explicitly highlights the correlation between interfacial adsorption and reaction kinetics, by integration of the adsorption kinetics into the enzymatic reaction. The rate constants were fitted globally against data from both single droplet and drop swarm experiments. The amount of adsorbed enzymes on the interface was measured in a single drop with a certain surface area, and the enzyme interfacial loading was estimated by Langmuir adsorption isotherm.

Structure and Dynamics of Confined Liquids: Challenges and Perspectives for the X-ray Surface Forces Apparatus

The molecular-scale structure and dynamics of confined liquids has increasingly gained relevance for applications in nanotechnology. Thus, a detailed knowledge of the structure of confined liquids on molecular length scales is of great interest for fundamental and applied sciences. To study confined structures under dynamic conditions, we constructed an in situ X-ray surface forces apparatus (X-SFA). This novel device can create a precisely controlled slit-pore confinement down to dimensions on the 10 nm scale by using a cylinder-on-flat geometry for the first time. Complementary structural information can be obtained by simultaneous force measurements and X-ray scattering experiments. The in-plane structure of liquids parallel to the slit pore and density profiles perpendicular to the confining interfaces are studied by X-ray scattering and reflectivity. The normal load between the opposing interfaces can be modulated to study the structural dynamics of confined liquids. The confinement gap distance is tracked simultaneously with nanometer precision by analyzing optical interference fringes of equal chromatic order. Relaxation processes can be studied by driving the system out of equilibrium by shear stress or compression/decompression cycles of the slit pore. The capability of the new device is demonstrated on the liquid crystal 4'-octyl-4-cyano-biphenyl (8CB) in its smectic A (SmA) mesophase. Its molecular-scale structure and orientation confined in 100 nm to 1.7 μm slit pores was studied under static and dynamic nonequilibrium conditions.

Mass Transfer Accompanying Coalescence of Surfactant-Laden and Surfactant-Free Drop in a Microfluidic Channel

The coalescence of two different drops, one surfactant-laden and the other surfactant-free, was studied under the condition of confined flow in a microchannel. The coalescence was accompanied by penetration of the surfactant-free drop into the surfactant-laden drop because of the difference in the capillary pressure and Marangoni flows causing a film of surfactant-laden liquid to spread over the surfactant-free drop. The penetration rate was dependent on the drop order, with considerably better penetration observed for the case when the surfactant-laden drop goes first. The penetration rate was found to increase with an increase of interfacial tension difference between the two drops, an increase of flow rate and drop confinement in the channel (for the case of the surfactant-laden drop going first), an increase of viscosity of the continuous phase, and a decrease of viscosity of the dispersed phase. Analysis of flow patterns inside the coalescing drops has shown that, unlike coalescence of identical drops, only two vortices are formed by asymmetrical coalescence, centered inside the surfactant-free drop. The vortices were accelerated by the flow of the continuous phase if the surfactant-laden drop preceded the surfactant-free one, increasing the rate of penetration; the opposite was observed if the drop order was reversed. The mixing patterns on a longer time scale were also dependent on the drop order, with better mixing being observed for the case when the surfactant-laden drop goes first.

Registration of high-frequency waves on the surface by the interference methods

Capillary waves are frequently used to measure the surface tension of liquids. However, this approach has not found wide application in the manufacture of modern commercial tensiometers because of the limitations imposed by capillary wave excitation techniques and the labor input associated with its practical implementation. In this paper we introduce a modified version of the capillary wave method which allows one to avoid the existing limitations and disadvantages. The distinguishing features of the proposed technique are as follows: acoustic wave generation and application of an interferometry technique for 3D surface profile reconstruction. A dynamic speaker with controlled vibration frequency and amplitude is used to produce acoustic vibrations. Application of a conventional Fizeau interferometer and the spatial phase shifting method makes it possible to perform surface form measurements with a high accuracy. For calculating wavelengths and the damping co-efficient, the surface profile is fitted with a decaying cylindrical wave equation. The accuracy of surface tension measurement by the modified capillary wave technique is 0.3 %. Owing to the non-contact way of wave generation and the small amounts of the examined fluid, the proposed method can be used in different studies.

The Role of Electrostatic Repulsion on Increasing Surface Activity of Anionic Surfactants in the Presence of Hydrophilic Silica Nanoparticles

Hydrophilic silica nanoparticles alone are not surface active. They, however, develop a strong electrostatic interaction with ionic surfactants and consequently affect their surface behavior. We report the interfacial behavior of n-heptane/anionic-surfactant-solutions in the presence of hydrophilic silica nanoparticles. The surfactants are sodium dodecyl sulfate (SDS) and dodecyl benzene sulfonic acid (DBSA), and the diameters of the used particles are 9 and 30 nm. Using experimental tensiometry, we show that nanoparticles retain their non-surface-active nature in the presence of surfactants and the surface activity of surfactant directly increases with the concentration of nanoparticles. This fact was attributed to the electrostatic repulsive interaction between the negatively charged nanoparticles and the anionic surfactant molecules. The role of electrostatic repulsion on increasing surface activity of the surfactant has been discussed. Further investigations have been performed for screening the double layer charge of the nanoparticles in the presence of salt. Moreover, the hydrolysis of SDS molecules in the presence of silica nanoparticles and the interaction of nanoparticles with SDS inherent impurities have been studied. According to our experimental observations, silica nanoparticles alleviate the effects of dodecanol, formed by SDS hydrolysis, on the interfacial properties of SDS solution.

Dynamic interfacial tension of surfactant solutions

The dynamics of surfactant interfacial layers was first discussed more than a century ago. In 1946 the most important work by Ward and Tordai was published which is still the theoretical basis of all new models to describe the time dependence of interfacial properties. In addition to the diffusion controlled adsorption mechanism, many other models have been postulated in literature, however, well performed experiments with well defined surfactant systems have shown that the diffusional transport is the main process governing the entire formation of surfactant adsorption layers. The main prerequisite, in addition to the diffusional transport, is the consideration of the right boundary condition at the interface, given by a respective equation of state. In addition to the classical models of Langmuir and Frumkin, also the so-called reorientation or interfacial aggregation models are to be assumed to reach a quantitative description of respective experimental data. Moreover, the adsorption of surfactants at the interface between water and a gas phase different from air can be strongly influenced by the type of molecules within the gas phase, such as alkane vapours. These oil molecules co-adsorb from the gas phase and change the adsorption kinetics strongly. Besides the discussion of how to apply theoretical adsorption kinetics models correctly, a large number of experimental data are presented and the way of a quantitative analysis of the adsorption mechanism and the main characteristic parameters is presented. This includes micellar solutions as well as mixtures of surfactants of ionic and non-ionic nature.

Phase inversion emulsification: Current understanding and applications

This review is addressed to the phase inversion process, which is not only a common, low-energy route to make stable emulsions for a variety of industrial products spanning from food to pharmaceuticals, but can also be an undesired effect in some applications, such as crude oil transportation in pipelines. Two main ways to induce phase inversion are described in the literature, i.e., phase inversion composition (PIC or catastrophic) and phase inversion temperature (PIT or transitional). In the former, starting from one phase (oil or water) with surfactants, the other phase is more or less gradually added until it reverts to the continuous phase. In PIT, phase inversion is driven by a temperature change without varying system composition. Given its industrial relevance and scientific challenge, phase inversion has been the subject of a number of papers in the literature, including extensive reviews. Due to the variety of applications and the complexity of the problem, most of the publications have been focused either on the phase behavior or the interfacial properties or the mixing process of the two phases. Although all these aspects are quite important in studying phase inversion and much progress has been done on this topic, a comprehensive picture is still lacking. In particular, the general mechanisms governing the inversion phenomenon have not been completely elucidated and quantitative predictions of the phase inversion point are limited to specific systems and experimental conditions. Here, we review the different approaches on phase inversion and highlight some related applications, including future and emerging perspectives.

Adsorption of polyelectrolytes and polyelectrolytes-surfactant mixtures at surfaces: a physico-chemical approach to a cosmetic challenge

The use of polymer and polymer - surfactant mixtures for designing and developing textile and personal care cosmetic formulations is associated with various physico-chemical aspects, e.g. detergency and conditioning in the case of hair or wool, that determine their correct performances in preserving and improving the appearance and properties of the surface where they are applied. In this work, special attention is paid to the systems combining polycations and negatively charged surfactants. The paper introduces the hair surface and presents a comprehensive review of the adsorption properties of these systems at solid-water interfaces mimicking the negative charge and surface energy of hair. These model surfaces include mixtures of thiols that confer various charge densities to the surface. The kinetics and factors that govern the adsorption are discussed from the angle of those used in shampoos and conditioners developed by the cosmetic industry. Finally, systems able to adsorb onto negatively charged surfaces regardless of the anionic character are presented, opening new ways of depositing conditioning polymers onto keratin substrates such as hair.

Polymeric Janus nanoparticles from triblock terpolymer micellar dimers

Well-defined polymeric Janus nanoparticles have been synthesized by a novel method of combining self-assembly of simple ABC linear triblock terpolymers into nanostructured dimers and crosslinking of the conjunction between the opposite hemispheres.