Contact angle determination of micro- and nanoparticles at fluid/fluid interfaces: the excluded area concept

A novel and simple method for the determination of the contact angle of nano- and microparticles at the liquid/air interface is proposed. The principle is based on the consideration of differences between the pressure/area isotherms of a 2D single-component system of a surfactant and those of binary systems comprised of the same surfactant and the particles to be studied. The theoretical analysis of the contact-angle behavior in this system upon compression allows the prediction of direction of the particles' squeezing out of the surface layer and therefore the distinction between the particles with high contact angle (Theta(p) > 90 degrees) and low (Theta(p) < 90 degrees) hydrophobicity. The application of this method to microparticles of two different hydrophobicities demonstrates good agreement between the obtained contact angles and the corresponding degrees of hydrophobicity given by the particle provider.     

Novel film-calliper method of measuring the contact angle of colloidal particles at liquid interfaces

A simple and reliable film-calliper method of measuring the particle contact angle at the water-air (oil) interface in real time has been developed. Its applicability to submicrometer and micrometer latex and silica particles is demonstrated.     

Determination of surface tension and contact angle from the shapes of axisymmetric fluid interfaces without use of apex coordinates

Drop shape techniques, such as axisymmetric drop shape analysis, are widely used to measure surface properties, as they are accurate and reliable. Nevertheless, they are not applicable in experimental studies dealing with fluid configurations that do not present an apex. A new methodology is presented for measuring interfacial properties of liquids, such as surface tension and contact angles, by analyzing the shape of an axisymmetric liquid-fluid interface without use of apex coordinates. The theoretical shape of the interface is generated numerically as a function of surface tension and some geometrical parameters at the starting point of the interface, e.g., contact angle and radius of the interface. Then, the numerical shape is fitted to the experimental profile, taking the interfacial properties as adjustable parameters. The best fit identifies the true values of surface tension and contact angle. Comparison between the experimental and the theoretical profiles is performed using the theoretical image fitting analysis (TIFA) strategy. The new method, TIFA-axisymmetric interfaces (TIFA-AI), is applicable to any axisymmetric experimental configuration (with or without apex). The versatility and accuracy of TIFA-AI is shown by considering various configurations: liquid bridges, sessile and pendant drops, and liquid lenses.     

Influence of aqueous electrolytes on the wetting behavior of hydrophobic solid polymers-low-rate dynamic liquid/fluid contact angle measurements using axisymmetric drop shape analysis

The interaction of inorganic ions with low-energy hydrophobic surfaces was examined using model systems of solid polymers without ionizable functional surface groups in aqueous electrolyte solutions. Low-rate dynamic contact angle measurements with captive bubbles in conjunction with axisymmetric drop shape analysis (ADSA) were performed to study the influence of electrolyte ions (in the aqueous test solutions) on the wettability of the polymers. When various types of ions were used, no significant change in advancing and receding contact angles was observed. The contact angle hysteresis was small. The zeta potential of the model polymers in aqueous electrolyte solutions was determined from streaming potential measurements. The variation of the zeta potential at different pH levels indicates preferential adsorption of hydroxyl ions at this interface. However, the presence of electrolytes at the interface between water and the different model polymers did not influence the macroscopic contact angle. The results may suggest the absence of any specific interaction between the ions and the solid polymer, as this should result in changes of hydrophobicity. Similar to the air/water interface, the composition and the potential of the polymer/water interface are obviously determined predominantly by the aqueous phase with only slight influence from the solid phase.     

Boundary slip and wetting properties of interfaces: correlation of the contact angle with the slip length

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.     

Phase behavior and morphology of equimolar mixed cationic-anionic surfactant monolayers at the air/water interface: isotherm and Brewster angle microscopy analysis

The phase behavior and morphological characteristics of monolayers composed of equimolar mixed cationic-anionic surfactants at the air/water interface were investigated by measurements of surface pressure-area per alkyl chain (pi-A) and surface potential-area per alkyl chain (DeltaV-A) isotherms with Brewster angle microscope (BAM) observations. Cationic single-alkyl ammonium bromides and anionic sodium single-alkyl sulfates with alkyl chain length ranging from C(12) to C(16) were used to form mixed surfactant monolayers on the water subphase at 21 degrees C by a co-spreading approach. The results demonstrated that when the monolayers were at states with larger areas per alkyl chain during the monolayer compression process, the DeltaV-A isotherms were generally more sensitive than the pi-A isotherms to the molecular orientation variations. For the mixed monolayer components with longer alkyl chains, a close-packed monolayer with condensed monolayer characteristics resulted apparently due to the stronger dispersion interaction between the molecules. BAM images also revealed that with the increase in the alkyl chain length of the surfactants in the mixed monolayers, the condensed/collapse phase formation of the monolayers during the interface compression stage became pronounced. In addition, the variations in the condensed monolayer morphology of the equimolar mixed cationic-anionic surfactants were closely related to the alkyl chain lengths of the components.     

Capillary rise of a non-Newtonian power law liquid: impact of the fluid rheology and dynamic contact angle

The impact of non-Newtonian behavior and the dynamic contact angle on the rise dynamics of a power law liquid in a vertical capillary is studied theoretically and experimentally for quasi-steady-state flow. An analytical solution for the time evolution of the meniscus height is obtained in terms of a Gaussian hypergeometric function, which in the case of a Newtonian liquid reduces to the Lucas-Washburn equation modified by the dynamic contact angle correction. The validity of the solution is checked against experimental data on the rise dynamics of a shear-thinning cmc solution in a glass microcapillary, and excellent agreement is found.     

Fluorescence and ESR studies of the conformational behavior of oppositely charged polyelectrolytes at solid/liquid interfaces

The conformational behavior of oppositely charged polyelectrolytes on alumina in solutions was investigated by means of excimer fluorescence and electron spin resonance spectroscopy using maleic acid-propene copolymer labeled with pyrene or TEMPO. It was found that the ability of the polyanion at the surface for conformational rearrangements is strongly influenced by the constraints of the adsorbed state that restrict its complexation. Polyelectrolyte complexes (PEC) formed by mixing of the oppositely charged polyelectrolytes exhibited extreme coiling due to the screening of the charged groups. The polyelectrolytes undergo spreading during the adsorption process due to the electrostatic attraction. Surface binding can irreversibly limit the flexibility for the reconformation process to a great extent. It is also shown here that a flatter adsorbed state could be reached by sequential adsorption of polyanion and polycation than could be reached by the direct adsorption of the polyelectrolyte complex itself.     

The standard free energy of surfactant adsorption at air/water and oil/water interfaces: Theoretical vs. empirical approaches

The standard free energy of surfactant adsorption represents the work of transfer of a surfactant molecule from the bulk of solution to an infinitely diluted adsorption layer. This quantity can be determined by non-linear fits of surface-tension isotherms with the help of a theoretical model of adsorption. Here, the models of Frumkin, van der Waals and Helfand-Frisch-Lebowitz are applied, and the results are compared. Irrespective of the differences between these models, they give close values for the standard free energy. The results from the theoretical approach are compared with those from the most popular empirical approach. The latter gives values of the standard free energy, which are considerably different from the respective true values, with c.a. 10 kJ/mol for nonionic surfactants, and with c.a. 20 kJ/mol for ionic surfactants. These differences are due to contributions from interactions between the molecules in dense adsorption layers. It is concluded that the true values of the standard free energy can be determined with the help of an appropriate theoretical model. For the processed sets of data, the van der Waals model gives the best results, especially for the determination of the standard adsorption enthalpy and entropy from the temperature dependence of surface tension. The results can be useful for the development of a unified approach to the thermodynamic characterization of surfactants.     

Effect of inorganic salts on the aggregation behavior of branched block polyether at air/water and n-heptane/water interfaces

The effect of inorganic salts (CaCl₂, MgCl₂, NaCl, NaI and NaSCN) on the aggregation behavior of a synthesized polyether with seven poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) (PEO-PPO-PEO) arms attached to a tetraethylenepentamine core (simplified AE73) at air/water and n-heptane/water interfaces has been investigated by interfacial tension and oscillating bubble methods. The additions of NaCl, CaCl₂, and MgCl₂ may facilitate the micellization of AE73 and increase its maximum interfacial excess concentration (Γ_max) due to the “salting out” effect, while NaSCN induces opposite effect and NaI exerts little influence. The adsorption kinetics of AE73 is controlled not only by the diffusion between the bulk solution and the interfacial layer but also by the energetic and steric barriers generated by the already adsorbed molecules. The adsorption relaxation time of AE73 is reduced with the addition of salts and this phenomenon is more prominent at the n-heptane/water interface. The “salting in” ions decrease the dilational modulus of AE73 while the “salting out” ions induce an opposite effect. The mechanisms of the interaction between inorganic ions and the polyether were discussed; the difference in aggregation behavior between linear and branched block polyethers were also compared.     

Interfacial behavior of N-nitrosodiethylamine/bovine serum albumin complexes at the air-water and the chloroform-water interfaces by axisymmetric drop tensiometry

Interfacial properties of N-nitrosodiethylamine/bovine serum albumin (NDA/BSA) complexes were investigated at the air-water interface. The interfacial behavior at the chloroform-water interface of the interaction product of phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), dissolved in the chloroform phase, and NDA/BSA complex, in the aqueous phase, were also analyzed by using a drop tensiometer. The secondary structure changes of BSA with different NDA concentrations were monitored by circular dichroism spectroscopy at different pH and the NDA/BSA interaction was probed by fluorescence spectroscopy. Different NDA/BSA mixtures were prepared from 0, 7.5 x 10(-5), 2.2 x 10(-4), 3.7 x 10(-4), 5 x 10(-4), 1.6 x 10(-3), and 3.1 x 10(-3) M NDA solutions in order to afford 0, 300/1, 900/1, 1 500/1, 2 000/1, 6 000/1, and 12 500/1 NDA/BSA molar ratios, respectively, in the aqueous solutions. Increments of BSA alpha-helix contents were obtained up to the 2 000/1 NDA/BSA molar ratio, but at ratios beyond this value, the alpha-helix content practically disappeared. These BSA structure changes produced an increment of the surface pressure at the air-water interface, as the alpha-helix content increased with the concentration of NDA. On the contrary, when alpha-helix content decreased, the surface pressure also appeared lower than the one obtained with pure BSA solutions. The interaction of DPPC with NDA/BSA molecules at the chloroform-water interface produced also a small, but measurable, pressure increment with the addition of NDA molecules. Dynamic light scattering measurements of the molecular sizes of NDA/BSA complex at pH 4.6, 7.1, and 8.4 indicated that the size of extended BSA molecules at pH 4.6 increased in a greater proportion with the increment in NDA concentration than at the other studied pH values. Diffusion coefficients calculated from dynamic surface tension values, using a short-term solution of the general adsorption model of Ward and Tordai, also showed differences with pH and the NDA concentration. Both, the storage and loss dilatational elastic modulus were obtained at the air-water and at the chloroform-water interfaces. The interaction of NDA/BSA with DPPC at the chloroform-water produced a less rigid monolayer than the one obtained with pure DPPC (1 x 10(-5) M), indicating a significant penetration of NDA/BSA molecules at the interface. At short times and pH 4.6, the values of the storage elastic modulus were larger and more sensible to the NDA addition than the ones at pH 7.1 and 8.4, probably due to a gel-like network formation at the air-water interface.     

Headgroup effects of template monolayers on the adsorption behavior and conformation of glucose oxidase adsorbed at air/liquid interfaces

Stearic acid (SA) and octadecylamine (ODA) monolayers at the air/liquid interface were used as template layers to adsorb glucose oxidase (GOx) from aqueous solution. The effect of the template monolayers on the adsorption behavior of GOx was studied in terms of the variation of surface pressure, the evolution of surface morphology observed by BAM and AFM, and the conformation of adsorbed GOx. The results show that the presence of a template monolayer can enhance the adsorption rate of GOx; furthermore, ODA has a higher ability, compared to SA, to adsorb GOx, which is attributed to the electrostatic attractive interaction between ODA and GOx. For adsorption performed on a bare surface or on an SA monolayer, the surface pressure approaches an equilibrium value (ca. 8 mN/m) after 2 to 3 h of adsorption and remains nearly constant in the following adsorption process. For the adsorption on an ODA monolayer, the surface pressure will increase further 1 to 2 h after approaching the first equilibrium pressure, which is termed the second adsorption stage. The measurement of circular dichroism (CD) spectroscopy indicates that the Langmuir-Blodgett films of adsorbed GOx transferred at the first equilibrium state (π = 8 mN/m) have mainly a β-sheet conformation, which is independent of the type of template monolayers. However, the ODA/GOx LB film transferred at the second adsorption stage has mainly an α-helix conformation. It is concluded that the specific interaction between ODA and GOx not only leads to a higher adsorption rate and adsorbed amount of GOx but also induces a conformation change in adsorbed GOx from β-sheet to α-helix. The present results indicate that is possible to control the conformation of adsorbed protein by selecting the appropriate template monolayer.     

Competitive adsorption of surfactants and hydrophilic silica particles at the oil-water interface: interfacial tension and contact angle studies

The effect of surfactants' type and concentration on the interfacial tension and contact angle in the presence of hydrophilic silica particles was investigated. Silica particles have been shown to have an antagonistic effect on interfacial tension and contact angle in the presence of both W/O and O/W surfactants. Silica particles, combined with W/O surfactant, have no effect on interfacial tension, which is only dictated by the surfactant concentration, while they strongly affect interfacial tension when combined with O/W surfactants. At low O/W surfactant, both particles and surfactant are adsorbed at the interface, modifying the interface structure. At higher concentration, interfacial tension is only dictated by the surfactant. By increasing the surfactant concentration, the contact angle that a drop of aqueous phase assumes on a glass substrate placed in oil media decreases or increases depending on whether the surfactant is of W/O or O/W type, respectively. This is due to the modification of the wettability of the glass by the oil or water induced by the surfactants. Regardless of the surfactant's type, the contact angle profile was dictated by both particles and surfactant at low surfactant concentration, whereas it is dictated by the surfactant only at high concentration.     

Adsorption behavior of glucose oxidase on a dipalmitoylphosphatic acid monolayer and the characteristics of the mixed monolayer at air/liquid interfaces

A dipalmitoylphosphatic acid (DPPA) monolayer at the air/liquid interface is used as a binding layer to incorporate glucose oxidase (GOx) from the subphase. The effects of the adsorption time of GOx on the behavior of the mixed DPPA/GOx monolayer and the relevant structure of the mixed LB film were studied using the characteristics of the pressure-area (pi-A) isotherm, Brewster angle microscopy (BAM), and atomic force microscopy (AFM). The experimental results show that two equilibrium states of GOx adsorption exist in the presence of a DPPA monolayer. The first equilibrium stage occurs at tens of minutes after spreading of DPPA, and a surface pressure of ca. 7.5 mN/m is obtained. The second equilibrium stage approaches slowly, and a higher equilibrium surface pressure (ca. 16 mN/m) was obtained at ca. 8 h after the first stage. The BAM and AFM images show that, after the second equilibrium stage is reached, a more condensed phase and rough morphology are obtained on the mixed DPPA/GOx monolayer, indicating a higher amount of GOx incorporated into the mixed film. For the first equilibrium stage of GOx adsorption, DPPA molecules can still pack regularly and closely under compression, suggesting that GOx molecules are mainly located beneath the DPPA monolayer at the compressed state. A more uniform phase was detected on a film prepared after the first equilibrium stage was reached. The present result indicates that distinct structures and properties of mixed DPPA/GOx films can be prepared from the various stages of GOx adsorption.     

Energy level alignment at organic semiconductor/metal interfaces: effect of polar self-assembled monolayers at the interface

We determined the shifts in the energy levels of approximately 15 nm thick poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] films deposited on various substrates including self-assembled monolayer (SAM) modified Au surfaces using photoelectron spectroscopy. As the unmodified substrates included Au, indium tin oxide, Si (with native oxide), and Al (with native oxide), a systematic shift in the detected energy levels of the organic semiconductor was observed to follow the work function values of the substrates. Furthermore, we used polar SAMs to alter the work function of the Au substrates. This suggests the opportunity to control the energy level positions of the organic semiconductor with respect to the electrode Fermi level. Photoelectron spectroscopy results showed that, by introducing SAMs on the Au surface, we successfully increased and decreased the effective work function of Au surface. We found that in this case, the change in the effective work function of the metal surface was not reflected as a shift in the energy levels of the organic semiconductor, as opposed to the results achieved with different substrate materials. Our study showed that when a substrate is modified by SAMs (or similarly by any adsorbed molecules), a new effective work function value is achieved; however, it does not necessarily imply that the new modified surface will behave similar to a different metal where the work function is equal to the effective work function of the modified surface. Various models and their possible contribution to this result are discussed.     

Tensions at liquid interfaces: a general filter for the separation of micro-/nanoparticles

In this letter, we put forward a new strategy for the separation of micro-/nanoparticles with different sizes and densities by using the tensions at liquid interfaces. The interactions between particles and a liquid-liquid interface have been analyzed. Furthermore, we applied our strategy to the separation of two size-distributed Cu2O particles by using the water and n-pentanol interface, which demonstrates the feasibility of the proposed separation method.     

The boundary element method: the simulation of voltammetry at immiscible liquid/liquid interfaces

The boundary element method is presented as an efficient and powerful method for the analysis of time-dependent electrochemical processes occurring at immiscible liquid/liquid interfaces. This paper outlines the theory and numerical details required for the development and application of two-dimensional transient diffusion models for the simulation of cyclic voltammetry behaviour at a range externally polarised immiscible liquid/liquid interfaces of differing topography. The benefits of the BEM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure and complete elimination of the need for domain discretisation with the time-domain convolution approach. The versatility and efficiency of the numerical procedures are examined with respect to a number of liquid/liquid interface geometries and a series of working curves established to quantify the influence of interface topography on the observed voltammetric behaviour.     

Water behavior revisited at the air/ionic solution and silica/ionic solution interfaces. Extension to coadsorption of ions and organic molecules

This paper reports on investigations about the adsorption at the air-water surface, and for the sake of comparison at the silica-solution interface, of two 1-2 electrolytes, Pb(NO(3))(2) and PbCl(2), at first alone and then from a mixture with carbofuran or with benzene; all of them were at concentrations below 10(-2) M. The limited domain, where the Debye and Hückel formalism for solutions and the Wagner-Onsager-Samaras (WOS) model for surfaces are correct, is then respected. This study was aimed at trying to identify the part played in the surface by the different particles of the system components and in particular the role of water. When aqueous solutions of nonorganic salts are dilute enough, their surface tensions are known to be salt concentration-independent; however, the zero value of the resulting relative adsorption has never been the subject of analysis about water behavior. By combining experimental relative adsorptions and Gibbs excesses calculated from the WOS theory, we will show that, in well-known solutions such as KCl ones, where the negative excess in salt can be very precisely modeled by the WOS theory, the resulting water excess Gamma(W) is negative. The same result can be obtained by taking into account the Ray-Jones effect. This observation drove us to wonder about the results of a similar analysis done on solutions of unsymmetrical electrolytes and on mixtures of salt and organic molecules. Experiments showed that, for all of the systems, Gamma(W) was negative. For a given salt, Gamma(W) was more negative in the presence of organic molecules, and carbofuran was a more efficient water repellent than benzene; water repulsion was greater with nitrates than with chlorides. From these data, it seems that water was repelled toward the solution bulk, whereas ions probably took place between the bulk and a layer of organic molecules. These observations called for a more detailed modeling.     

Kinetics of solute adsorption at solid/solution interfaces: on the special features of the initial adsorption kinetics

The features of the initial adsorption kinetics monitored at short adsorption times are investigated. It is shown that the concave character of the square-root dependence on time may be due to a combined effect of the rate of surface reaction and that of the transport from the bulk to the surface. That effect causes the appearance of a certain subsurface region close to the surface, where the concentration of the sorbate is different from that in the bulk phase. For the purpose of illustration, the initial parts of the kinetic isotherms are analyzed for the RY/F-400 system already studied in our previous paper.     

Molecular theory on dielectric constant at interfaces: a molecular dynamics study of the water/vapor interface

Though the local dielectric constant at interfaces is an important phenomenological parameter in the analysis of surface spectroscopy, its microscopic definition has been uncertain. Here, we present a full molecular theory on the local field at interfaces with the help of molecular dynamics simulation, and thereby provide microscopic basis for the local dielectric constant so as to be consistent to the phenomenological three-layer model of interface systems. To demonstrate its performance, we applied the theory to the water/vapor interface, and obtained the local field properties near the interface where the simple dielectric model breaks down. Some computational issues pertinent to Ewald calculations of the dielectric properties are also discussed.