Surface Properties of Protein-Polyelectrolyte Solutions. Impact of Polyelectrolyte Hydrophobicity.

The strong influence of an amphiphilic polyelectrolyte, poly(N,N-diallyl-N-hexyl-N-methylammonium chloride), on the surface properties of solutions of globular proteins (lysozyme, ß-lactoglobulin, bovine serum albumin, and green fluorescent protein) depends on the protein structure and allows elucidation of the contribution of hydrophobic interactions in the protein-polyelectrolyte complex formation at the liquid-gas interface. At the beginning of adsorption, the surface properties are determined by the unbound amphiphilic component, but the influence of the protein-polyelectrolyte complexes of high surface activity increases at the approach to equilibrium. The kinetic dependencies of the dilational dynamic surface elasticity with one or two local maxima give a possibility to distinguish clearly between different steps of the adsorption process and to trace the formation of the distal region of the adsorption layer. The conclusions from the surface rheological data are corroborated by ellipsometric and tensiometric results.

DNA Penetration into a Lysozyme Layer at the Surface of Aqueous Solutions.

The interactions of DNA with lysozyme in the surface layer were studied by performing infrared reflection-absorption spectroscopy (IRRAS), ellipsometry, surface tensiometry, surface dilational rheology, and atomic force microscopy (AFM). A concentrated DNA solution was injected into an aqueous subphase underneath a spread lysozyme layer. While the optical properties of the surface layer changed fast after DNA injection, the dynamic dilational surface elasticity almost did not change, thereby indicating no continuous network formation of DNA/lysozyme complexes, unlike the case of DNA interactions with a monolayer of a cationic synthetic polyelectrolyte. A relatively fast increase in optical signals after a DNA injection under a lysozyme layer indicates that DNA penetration is controlled by diffusion. At low surface pressures, the AFM images show the formation of long strands in the surface layer. Increased surface compression does not lead to the formation of a network of DNA/lysozyme aggregates as in the case of a mixed layer of DNA and synthetic polyelectrolytes, but to the appearance of some folds and ridges in the layer. The formation of more disordered aggregates is presumably a consequence of weaker interactions of lysozyme with duplex DNA and the stabilization, at the same time, of loops of unpaired nucleotides at high local lysozyme concentrations in the surface layer.

Dynamic Surface Properties of Fullerenol Solutions.

Application of dilational surface rheology, surface tensiometry, ellipsometry, Brewster angle, and transmission electron and atomic force microscopies allowed the estimation of the structure of the adsorption layer of a fullerenol with a large number of hydroxyl groups, C60(OH) X ( X = 30 ± 2). The surface properties of fullerenol solutions proved to be similar to the properties of dispersions of solid nanoparticles and differ from those of the solutions of conventional surfactants and amphiphilic macromolecules. Although the surface activity of fullerenol is not high, it forms adsorption layers of high surface elasticity up to 170 mN/m. The layer consists of small interconnected surface aggregates with the thickness corresponding to two-three layers of fullerenol molecules. The aggregates are not adsorbed from the bulk phase but formed at the interface. The adsorption kinetics is controlled by an electrostatic adsorption barrier at the interface.

Adsorption of Denaturated Lysozyme at the Air-Water Interface: Structure and Morphology.

The application of protein deuteration and high flux neutron reflectometry has allowed a comparison of the adsorption properties of lysozyme at the air-water interface from dilute solutions in the absence and presence of high concentrations of two strong denaturants: urea and guanidine hydrochloride (GuHCl). The surface excess and adsorption layer thickness were resolved and complemented by images of the mesoscopic lateral morphology from Brewster angle microscopy. It was revealed that the thickness of the adsorption layer in the absence of added denaturants is less than the short axial length of the lysozyme molecule, which indicates deformation of the globules at the interface. Two-dimensional elongated aggregates in the surface layer merge over time to form an extensive network at the approach to steady state. Addition of denaturants in the bulk results in an acceleration of adsorption and an increase of the adsorption layer thickness. These results are attributed to incomplete collapse of the globules in the bulk from the effects of the denaturants as a result of interactions between remote amino acid residues. Both effects may be connected to an increase of the effective total volume of macromolecules due to the changes of their tertiary structure, that is, the formation of molten globules under the influence of urea and the partial unfolding of globules under the influence of GuHCl. In the former case, the increase of globule hydrophobicity leads to cooperative aggregation in the surface layer during adsorption. Unlike in the case of solutions without denaturants, the surface aggregates are short and wormlike, their size does not change with time, and they do not merge to form an extensive network at the approach to steady state. To the best of our knowledge, these are the first observations of cooperative aggregation in lysozyme adsorption layers.

Molybdenum Sulfide for Hydrogen Evolution Reaction: The Importance of Solution Dynamic Wetting Behavior in The Drying Process.

MoSx serving as a hydrogen evolution reaction electrocatalyst is known for its morphology sensitive characteristic. The low temperature thermo-decomposition method provides an easy and energy saving pathway to produce highly active MoSx on carbon paper substrates. However, during the precursor solution drying process, the dynamics of liquid wetting behavior dominates the morphology of the precursor salt and eventually the morphology of MoSx. As a result, here, for the first time, by carefully pairing the substrate hydrophobicity and solvent polarity, the cohesive force between solvent molecules and adhesive force between solvent and carbon substrate can be tuned, and thus the MoSx morphology can be controlled. Pairing hydrophilic carbon paper with DMF + H2O mixing solvent results in a relatively strong adhesive force, as a result, we are able to lower the overpotential required at the benchmark current density, 10 mA/cm2, to as low as 0.160 V and boost the current density to 40 mA/cm2 at -0.2 V vs RHE. This mainly results from the low charge transfer resistance and the well wrapped MoSx on carbon paper fiber structure. Furthermore, this well wrapped MoSx on hydrophilic carbon paper was proved to be comparably stable for constant voltage electrolysis operation.

Phase Transitions in DNA/Surfactant Adsorption Layers.

The adsorption layers of complexes between DNA and oppositely charged surfactants dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) at the solution/air interface were studied with surface tensiometry, dilational surface rheology, atomic force microscopy, Brewster angle microscopy, infrared absorption-reflection spectroscopy, and ellipsometry. Measurements of the kinetic dependencies of the surface properties gave a possibility to discover the time intervals corresponding to the coexistence of two-dimensional phases. One can assume that the observed phase transition is of the first order, unlike the formation of microaggregates in the adsorption layers of mixed solutions of synthetic polyelectrolytes and surfactants. The multitechniques approach together with the calculations of the adsorption kinetics allowed the elucidation of the structure of coexisting surface phases and the distinguishing of four main steps of adsorption layer formation at the surface of DNA/surfactant solutions.

On the uniqueness of the receding contact angle: effects of substrate roughness and humidity on evaporation of water drops.

Could a unique receding contact angle be indicated for describing the wetting properties of a real gas-liquid-solid system? Could a receding contact angle be defined if the triple line of a sessile drop is not moving at all during the whole measurement process? To what extent is the receding contact angle influenced by the intrinsic properties of the system or the measurement procedures? In order to answer these questions, a systematic investigation was conducted in this study on the effects of substrate roughness and relative humidity on the behavior of pure water drops spreading and evaporating on polycarbonate (PC) surfaces characterized by different morphologies. Dynamic, advancing, and receding contact angles were found to be strongly affected by substrate roughness. Specifically, a receding contact angle could not be measured at all for drops evaporating on the more rugged PC surfaces, since the drops were observed strongly pinning to the substrate almost until their complete disappearance. Substrate roughness and system relative humidity were also found responsible for drastic changes in the depinning time (from ∼10 to ∼60 min). Thus, for measurement observations not sufficiently long, no movement of the triple line could be noted, with, again, the failure to find a receding contact angle. Therefore, to keep using concepts such as the receding contact angle as meaningful specifications of a given gas-liquid-solid system, the imperative to carefully investigate and report the inner characteristics of the system (substrate roughness, topography, impurities, defects, chemical properties, etc.) is pointed out in this study. The necessity of establishing methodological standards (drop size, measurement method, system history, observation interval, relative humidity, etc.) is also suggested.

A Study on the Dilational Modulus Measurement of Polyacrylic Acid Films at Air-Water Interface by Pendant Bubble Tensiometry.

The dilational modulus (E) of polymer films has been commonly measured using the oscillating ring/bubble/drop methods with an external force, and often without specifying the state of the adsorbed film. This study explores an approach where E was determined from the relaxations of surface tension (ST) and surface area (SA) of natural perturbations, in which ST and SA were monitored using a pendant bubble tensiometer. The E of the adsorbed film of PAA (polyacrylic acid) was evaluated for aqueous solutions at CPAA = 5 × 10-4 g/cm3, [MW = 5, 25, and 250 (kDa)]. The E (=dγ/dlnA) was estimated from the surface dilational rate (dlnA/dt) and the rate of ST change (dγ/dt) of the bubble surface from the natural perturbation caused by minute variations in ambient temperature. The data revealed that (i) a considerable time is required to reach the equilibrium-ST (γeq) and to attain the saturated dilational modulus (Esat) of the adsorbed PAA film, (ii) both γeq and Esat of PAA solutions increase with MW of PAA, (iii) a lower MW solution requires a longer time to reach its γeq and Esat, and (iv) this approach is workable for evaluating the E of adsorbed polymer films.

Spread Layers of Lysozyme Microgel at Liquid Surface.

The spread layers of lysozyme (LYS) microgel particles were studied by surface dilational rheology, infrared reflection-absorption spectra, Brewster angle microscopy, atomic force microscopy, and scanning electron microscopy. It is shown that the properties of LYS microgel layers differ significantly from those of ß-lactoglobulin (BLG) microgel layers. In the latter case, the spread protein layer is mainly a monolayer, and the interactions between particles lead to the increase in the dynamic surface elasticity by up to 140 mN/m. In contrast, the dynamic elasticity of the LYS microgel layer does not exceed the values for pure protein layers. The compression isotherms also do not exhibit specific features of the layer collapse that are characteristic for the layers of BLG aggregates. LYS aggregates form trough three-dimensional clusters directly during the spreading process, and protein spherulites do not spread further along the interface. As a result, the liquid surface contains large, almost empty regions and some patches of high local concentration of the microgel particles.

Impact of denaturing agents on surface properties of myoglobin solutions.

The addition of denaturants strongly influences the surface properties of aqueous myoglobin solutions. The effect differs from the results for mixed solutions of the denaturants and other globular proteins, for example, bovine serum albumin (BSA), lysozyme and ß-lactoglobulin (BLG), although the surface properties of the solutions of the pure proteins are similar. The kinetic dependencies of the dynamic surface elasticity of myoglobin solutions with guanidine hydrochloride (GuHCl) reveal at least two adsorption steps at denaturant concentrations higher than 1 M: a very fast increase of the dynamic surface elasticity to approximately 30 mN/m at the beginning of adsorption, and a slower growth to abnormally high values of 250-300 mN/m. At the same time, the surface elasticity of BSA/GuHCl, BLG/GuHCl and lysozyme/GuHCl solutions is a non-monotonic function of the surface age, and does not exceed 50 mN/m close to equilibrium. The high surface elasticity of myoglobin/GuHCl solutions may be associated with protein aggregation in the surface layer. The formation of aggregates is confirmed by ellipsometry and Brewster angle microscopy. The addition of ionic surfactants to protein solutions leads to the formation of myoglobin/surfactant complexes, and the kinetic dependencies of the dynamic surface elasticity display local maxima indicating multistep adsorption kinetics, unlike the corresponding results for solutions of other globular proteins mixed with ionic surfactants. Ellipsometry and infrared reflection-absorption spectroscopy allow tracing the adsorption of the complexes and their displacement from the interface at high surfactant concentrations.

Dynamic Surface Properties of Mixed Dispersions of Silica Nanoparticles and Lysozyme.

The surface properties of mixed aqueous dispersions of lysozyme and silica nanoparticles were studied using surface-sensitive techniques in order to gain insight into the mechanism of the simultaneous adsorption of protein/nanoparticle complexes and free protein as well as the resulting layer morphologies. The properties were first monitored in situ during adsorption at the air/water interface using dilatational surface rheology, ellipsometry, and Brewster angle microscopy. Two main steps in the evolution of the surface properties were identified. First, the adsorption of complexes did not lead to significant deviations in the dynamic surface elasticity and dynamic surface pressure from those for a layer of adsorbed lysozyme globules. Second, through the gradual displacement of protein globules from the interfacial layer as a result of further complex adsorption, the layer became more dense with much higher dynamic surface elasticity (∼280 mN/m compared to ∼80 mN/m for a pure protein layer). These layers were shown to be fragile and could be easily broken into separate islands of irregular shape by a weak mechanical disturbance. The layer properties were then monitored following their transfer to solid substrates using atomic force microscopy and scanning electron microscopy. These layers were shown to consist of nanoparticles surrounded by a rough shell of protein globules, whereas some particles tended to form filamentous aggregates. This comprehensive study provides new mechanistic and morphological insight into the surface properties of a model protein/nanoparticle system, which is of fundamental interest in colloidal science and can be extended to systems of physiological relevance.

Adsorption kinetics of heptadecafluoro-1-nonanol: Phase transition and mixed control.

HYPOTHESIS: The adsorption kinetics of heptadecafluoro-1-nonanol (C9H2F17OH) onto a clean air-water interface at low surfactant concentrations (equilibrium surface tension, γ(C) > 65 mN/m) has been reported, and the controlling mechanism was found to be mixed diffusive-kinetic controlled (Kuo et al., JCIS 402 (2013) 131). However, it remains to be determined what the adsorption kinetics are at higher concentrations. Hence, the dynamic surface tension, γ(t) of C9H2F17OH was measured and compared with the theoretical γ(t) curves predicted from phase transition model. EXPERIMENTS: A video-enhanced pendant bubble tensiometer was used to measure the γ(t) data of aqueous C9H2F17OH solutions at higher concentrations (C > 7.7 × 10-9 mol/cm3). A new generalized Frumkin-Langmuir phase transition model was built up to simulate the γ(C) and γ(t) data. FINDINGS: At higher surfactant concentrations, a constant-γ region at 64.8 mN/m was observed for one hundred to a few thousand seconds during the γ(t) relaxation. This constant-γ region implies the existence of a phase transition of the adsorbed surfactant monolayer at air-water interface. The γ(t) data of C9H2F17OH can be simulated perfectly using this mixed-controlled phase transition model with the adsorption rate constants ß1 = 1.0 ±â€¯0.5 and ß2 = 13 ±â€¯4 (105 cm3/mol·s). It is therefore concluded that the adsorption process of C9H2F17OH onto a clean air-water interface is of mixed-control.

Surface Equation of State of Nonionic CmEn Surfactants.

The surface equation of state (γ vs Γ/Γref), based on the experimental data from rapid expansion of a pendant bubble, has been used on studying the adsorption kinetics for C12E6 (Pan et al. J. Colloid Interface Sci. 1998, 205, 213) and C14E8 (Lin et al. J. Colloid Interface Sci. 2001, 244, 372). In those studies, the relationship between surface tension γ and relative surface concentration Γ/Γref was applied, and the γ(Γ/Γref) data profiles were found very useful for the determination of the adsorption isotherm and its parameters. In this work, the value of Γref was evaluated from the equilibrium surface tension data γ(C) and the Gibbs adsorption equation. It was found that the dependence of γ vs Γ is more sensitive than γ(Γ/Γref) and brings more help to studying the adsorption kinetics. The role of γ(Γ) data was examined using the equilibrium γ(C) and dynamic γ(t) data of six nonionic polyoxyethylene surfactants CmEn and two alcohols. The γ(Γ) data play an important role on the determination of the adsorption isotherm, model parameters, and controlling mechanism, and also on the evaluation of surfactant diffusivity. Applying γ(Γ) data with γ(ln C) data does bring a much better result on studying the adsorption behavior. According to the γ(Γ) data, (i) the Langmuir isotherm clearly fails to describe the above nonionic surfactants, and (ii) the interaction between the adsorbed surfactant molecules is significant.

Surface tension increment due to solute addition.

Addition of solute into solvent may lead to an increase in surface tension, such as salt in water and water in alcohol, due to solute depletion at the interface. The repulsion of the solute from the interface may originate from electrostatic forces or solute-solvent attraction. On the basis of the square-well model for the interface-solute interaction, we derive the surface tension increment Deltagamma by both canonical and grand-canonical routes (Gibbs adsorption isotherm) for a spherical droplet. The surface tension is increased linearly with the bulk concentration of the solute c(b) and the interaction range lambda. The theoretical results are consistent with those obtained by experiments and Monte Carlo simulations up to a few molarity. For weak repulsion, the increment is internal energy driven. When the repulsion is large enough, the surface tension increment is entropy driven and approaches the asymptotic limit, Deltagamma approximately c(b)k(B)Tlambda, due to the nearly complete depletion of the solute at the interface. Our result may shed some light on the surface tension increment for electrolyte solutions with concentration above 0.2M.

Impingement dynamics of water drops onto four graphite morphologies: from triple line recoil to pinning.

Droplet impingement experiments at low Weber number (We=7.61) were conducted by digitizing silhouettes of impacting water drops onto four graphite surfaces with different hydrophobicities. The relaxation of the wetting diameter, the droplet height and the dynamic contact angle were determined for characterizing the peculiar impact and spreading dynamics for the unalike surfaces, typified by four distinctive topographies carefully analyzed by scanning electron microscopy. After the initial inertially dominated phase, during which the drops spreading onto all substrates showed a similar behavior, the expected recoil phase was not observed for droplets impacting onto graphite surfaces characterized by 90° and 120° advancing contact angles. A few milliseconds after the impact, the drops exhibited a pinned configuration due to the peculiar morphology of these graphite substrates: randomly distributed cavities on a generally smooth surface, in fact, stopped the movement of the triple line. This behavior, however, was not detected for water droplets impinging onto graphite surfaces characterized by 160° advancing contact angles, which instead presented the usual retraction phase. Finally, the graphite surface characterized by 140° advancing contact angles showed a mixed behavior due to a transition of the drop configuration from Cassie-Baxter to Wenzel state.

The adsorption kinetics of a fluorinated surfactant--heptadecafluoro-1-nonanol.

The adsorption kinetics of heptadecafluoro-1-nonanol (C8F17CH2OH) onto a clean air-water interface were studied. Video-enhanced pendant (emerging) bubble tensiometry was employed to measure the equation of state and the dynamic/equilibrium surface tensions. Relaxation profiles of the surface tension for heptadecafluoro-1-nonanol molecules absorbing onto a freshly created air-water interface were obtained and simulated from theory. The adsorption of the fluoroalcohol C8F17CH2OH was found to be cooperative from the comparison of the equilibrium surface tension data at γ(C)>65 mN/m to the prediction of the Frumkin model. The comparison was made for the entire relaxation period of the tension data and the model predictions. The controlling mechanism of the adsorption process was found to be mixed diffusive-kinetic control. Values of the adsorption/desorption rate constants of C8F17CH2OH were estimated from these dynamic surface tension profiles with a diffusivity of 5.93×10(-6) cm(2)/s, which was evaluated from the Wilke-Chang equation for C8F17CH2OH.

Accurate surface tension measurement of glass melts by the pendant drop method.

A pendant drop tensiometer, coupled with image digitization technology and a best-fitting algorithm, was built to accurately measure the surface tension of glass melts at high temperatures. More than one thousand edge-coordinate points were obtained for a pendant glass drop. These edge points were fitted with the theoretical drop profiles derived from the Young-Laplace equation to determine the surface tension of glass melt. The uncertainty of the surface tension measurements was investigated. The measurement uncertainty (σ) could be related to a newly defined factor of drop profile completeness (Fc): the larger the Fc is, the smaller σ is. Experimental data showed that the uncertainty of the surface tension measurement when using this pendant drop tensiometer could be ±3 mN∕m for glass melts.

A simple method for measuring the superhydrophobic contact angle with high accuracy.

A modified selected-plane method for contact angle (theta) measurement is proposed in this study that avoids the difficulty of finding the real contact point and image-distortion effects adjacent to the contact point. This method is particularly suitable for superhydrophobic surfaces. The sessile-drop method coupled with the tangent line is the most popular method to find the contact angle in literature, but it entails unavoidable errors in determining the air-solid base line due to the smoothness problem and substrate tilting. In addition, the tangent-line technique requires finding the actual contact point. The measurement error due to the base line problem becomes more profound for superhydrophobic surfaces. A larger theta deviation results from a more superhydrophobic surface with a fixed base line error. The proposed modified selected-plane method requires only four data points (droplet apex, droplet height, and two interfacial loci close to the air-solid interface), avoiding the problem of the sessile-drop-tangent method in finding the contact point and saving the trouble of the sessile-drop-fitting method for best fitting of the numerous edge points with the theoretical profile. A careful error analysis was performed, and a user-friendly program was provided in this work. This method resulted in an accurate theta measurement and a method that was much improved over the classical selected plane and the sessile-drop-tangent methods. The theta difference between this method and the sessile-drop-fitting method was found to be less than three degrees.

Dynamic behaviors of droplet impact and spreading: water on five different substrates.

The dynamic wetting behaviors, especially the droplet morphology, of a water droplet impinging on five substrate surfaces were investigated. A water drop was released from 13.6 mm above a solid surface and impinged on substrates. The images (the silhouette and 45 degrees top view) were sequentially recorded from the moment that the droplet impacted the solid surface until it reached equilibrium. The entire profile of each of the water droplets during spreading was obtained from the digitized recorded images. The digitized droplets were then used to detail the spreading mechanism, including information on the relaxations of the wetting diameter, droplet height, contact angle, and spreading velocity. A comparison of the full droplet profiles allows us to clarify the independent motion of two related but independent components, the central region and rim, of an impinging droplet. An interesting plateau region in the droplet height relaxation curve was observed in the first cycle for all substrate surfaces. For hydrophobic surfaces (paraffin and Teflon), three particular growth modes in the droplet height relaxation curve were detected in every oscillation cycle during the early spreading stages. It only took three and four oscillation cycles for a water droplet on the glass and quartz substrates, respectively, to dissipate its energy and reach its equilibrium state. However, it took 72 and 28 oscillation cycles for a water droplet on the Teflon and paraffin substrates, respectively. Moreover, several other new phenomena were also observed.

A theoretical study on surfactant adsorption kinetics: effect of bubble shape on dynamic surface tension.

A planar or spherical fluid-liquid interface was commonly assumed on studying the surfactant adsorption kinetics for a pendant bubble in surfactant solutions. However, the shape of a pendant bubble deviates from a sphere unless the bubble's capillary constant is close to zero. Up to date, the literature has no report about the shape effect on the relaxation of surface tension due to the shape difference between a pendant bubble and a sphere. The dynamic surface tension (DST), based on the actual shape of a pendant bubble with a needle, of the diffusion-controlled process is simulated using a time-dependent finite element method in this work. The shape effect and the existence of a needle on DST are investigated. This numerical simulation resolves also the time-dependent bulk surfactant concentration. The depth of solution needed to satisfy the classical Ward-Tordai infinite-solution assumption was also studied. For a diffusion-controlled adsorption process, bubble shape and needle size are two major factors affecting the DST. The existence of a needle accelerates the bulk diffusion for a small bubble; however, the shape of a large pendant bubble decelerates the bulk diffusion. An example using this method on the DST data of C12E4 is illustrated at the end of this work.