Marta Krasowska

Influence of adsorbed gas at liquid/solid interfaces on heterogeneous cavitation

The presence of dissolved gas at the solid/liquid interface can play a crucial role in heterogeneous cavitation. Here we focus our attention on the relationship between gas conditions and cavitation nucleation at planar solid surfaces with alternating hydrophobic/hydrophilic properties. Tapping mode atomic force microscopy and optical microscopy were used to monitor the gas adsorption on the patterns before sonication. Scanning electron microscopy revealed the effects of collapsing cavitation bubbles on the irradiated surfaces. High intensity ultrasonic irradiation (20 kHz) induces the formation of an interfacial gas layer at the solid surface immersed in different liquid media (water saturated with different gases, such as argon, nitrogen or carbon dioxide) by accelerating the adsorption of dissolved gas. Subsequently, the gas rearranges in diverse nano- or microstructures which take further part in the cavitation process. A solvent-exchange method was also applied to induce the formation of artificial gaseous domains accumulated at the solid surface in order to facilitate the cavitation process. By varying the gas adsorption time it is possible to accelerate or to slow down heterogeneous cavitation. The experimental findings on heterogeneous cavitation are discussed in terms of interfacial bubble nucleation and bubble attraction and growth on patterned solid surfaces in liquid media.

Carboxymethylcellulose Adsorption on Molybdenite: The Effect of Electrolyte Composition on Adsorption, Bubble–Surface Collisions, and Flotation

The adsorption of carboxymethylcellulose polymers on molybdenite was studied using spectroscopic ellipsometry and atomic force microscopy imaging with two polymers of differing degrees of carboxyl group substitution and at three different electrolyte conditions: 1 × 10(-2) M KCl, 2.76 × 10(-2) M KCl, and simulated flotation process water of multicomponent electrolyte content, with an ionic strength close to 2.76 × 10(-2) M. A higher degree of carboxyl substitution in the adsorbing polymer resulted in adsorbed layers that were thinner and with more patchy coverage; increasing the ionic strength of the electrolyte resulted in increased polymer layer thickness and coverage. The use of simulated process water resulted in the largest layer thickness and coverage for both polymers. The effect of the adsorbed polymer layer on bubble-particle attachment was studied with single bubble-surface collision experiments recorded with high-speed video capture and image processing and also with single mineral molybdenite flotation tests. The carboxymethylcellulose polymer with a lower degree of substitution resulted in almost complete prevention of wetting film rupture at the molybdenite surface under all electrolyte conditions. The polymer with a higher degree of substitution prevented rupture only when adsorbed from simulated process water. Molecular kinetic theory was used to quantify the effect of the polymer on the dewetting dynamics for collisions that resulted in wetting film rupture. Flotation experiments confirmed that adsorbed polymer layer properties, through their effect on the dynamics of bubble-particle attachment, are critical to predicting the effectiveness of polymers used to prevent mineral recovery in flotation.

In Situ ATR FTIR Spectroscopic Study of the Formation and Hydration of a Fucoidan/Chitosan Polyelectrolyte Multilayer

The formation of fucoidan/chitosan-based polyelectrolyte multilayers (PEMs) has been studied with in situ Fourier transform infrared (FTIR) spectroscopy. Attenuated total reflectance (ATR) FTIR spectroscopy has been used to follow the sequential build-up of the multilayer, with peaks characteristic of each polymer being seen to increase in intensity with each respective adsorption stage. In addition, spectral processing has allowed for the extraction of spectra from individual adsorbed layers, which have been used to provide unambiguous determination of the adsorbed mass of the PEM at each stage of formation. The PEM was seen to undergo a transition in growth regimes during build-up: from supra-linear to linear. In addition, the wettability of the PEM has been probed at each stage of the build-up, using the captive bubble contact angle technique. The contact angles were uniformly low, but showed variation in value depending on the nature of the outer polymer layer, and this variation correlated with the overall percentage hydration of the PEM (determined from FTIR and quartz crystal microbalance data). The nature of the hydration water within the polyelectrolyte multilayer has also been studied with FTIR spectroscopy, specifically in situ synchrotron ATR FTIR microscopy of the multilayer confined between two solid surfaces. The acquired spectra have enabled the hydrogen bonding environment of the PEM hydration water to be determined. The PEM hydration water is seen to have an environment in which it is subject to fewer hydrogen bonding interactions than in bulk electrolyte solution.

Probing of polyelectrolyte monolayers by zeta potential and wettability measurements

Detection of the very first step of polyelectrolyte adsorption onto a solid support is of great importance for understanding mechanisms of solid surface modification. It was shown that streaming potential and contact angle measurements can be successfully used for polyelectrolyte (PE) adsorption characterization in a broad range of surface coverage. Cationic polyallylamine hydrochloride (PAH) was used for the formation of the layer. The electrokinetic characteristics of the substrate covered by the PAH layer were compared with contact angles measured under wet (captive air bubble/substrate in water) and dry (sessile water droplet/dried substrate) conditions. It has been demonstrated that contact angle values determined under both conditions are in good agreement. The observed rapid increase in the contact angle from zero for the bare mica surface to the value close to one characteristic of the PAH monolayer appears in the same PAH coverage range as zeta potential value changes due to adsorption. These results show that wettability can be as sensitive to the presence of small amounts of adsorbed species as electrokinetic measurements.

Organic energy devices from ionic liquids and conducting polymers

The use of smart technologies in our daily lives, from smartphones to auto-dimming windows to touch sensors, has become pervasive. With growing desire for these devices to be conformable and flexible, traditional materials are being replaced to create a class of products known as active organic electronic devices (OEDs). These new devices owe their ability to switch electrical and/or optical function to the intimate interaction between an inherently conducting polymer and electrolyte, typically an ionic liquid. Herein, we provide the first observations that specific ionic liquids can reduce or oxidise conducting polymers upon intimate contact in the absence of any electrical stimuli. The ability to reduce or oxidise the inherently conducting polymer depends on the cation and anion pair within the ionic liquid. Extending the utility of this phenomenon is made by fabricating OEDs such as prototype fuel cells, supercapacitors and smart windows.

Influence of n-hexanol and n-octanol on wetting properties and air entrapment at superhydrophobic surfaces.

Superhydrophobic surfaces have recently attracted a lot of attention due to their self-cleaning properties. The superhydrophobic surfaces used in our studies were prepared using a mixed inorganic-organic coating. In order to check how short chain surface active agents affect the surface energy of such surfaces, their wettability (sessile drop technique) and the kinetics of the three phase contact formation were studied. It was found that with increasing concentrations of n-hexanol and n-octanol the surface energy of these surfaces was only slightly changed, i.e. a small decrease in contact angle values with increasing solution concentration was detected. Even for the most concentrated n-hexanol and n-octanol solutions, the contact angles were in the range 145-155° and the drop rolled off, indicating that the studied surfaces stayed superhydrophobic. Air bubbles, upon collision with such superhydrophobic surfaces, spread over the superhydrophobic surface within milliseconds in the studied solutions.

Adhesion between silica particles in an alcohol medium

The destruction times of the sediment column structures of hydrophobic and hydrophilic silica particles were measured using a simple device. Experiments were carried out for different fractions of both silica particles in alcohols ranging from ethanol to decanol. On the basis of linear relationships between the reciprocal of the destruction times of the silica sediment column structures and average diameters of the silica fractions, the density of alcohols and the work of cohesion of alcohols, and the critical values of these parameters were determined. For the silica particle/alcohol/silica particle system having a critical value of the average diameter of the particle (or the critical density of alcohol, or the critical work of cohesion of alcohol), the detachment force of one silica particle from another was found to be equal to the attachment force between them. The values of the detachment force were found to decrease with increasing length of the hydrocarbon chain of the alcohols. Using these force values and the critical work of cohesion of alcohol, the radii of the contact planes between two silica particles and then the attachment forces were calculated. The attachment force values increased with increasing length of the hydrocarbon chain of the alcohols. It has also been found that the critical parameters (diameter of silica particles, density and work of cohesion of alcohols) depend on the surface properties of the solid. Hydrophilization of the silica surface caused an increase of the average critical diameter and the critical work of cohesion and a decrease of the critical density and, as a consequence, an increase of the attachment force. The increase of the destruction time with increasing length of the hydrocarbon chain of alcohols was caused, on one hand, by the decrease of the detachment forces and the perimeter of the contact planes, and, on the other, by the increase of the attachment force.

Adsorption of a PEO Surfactant from aqueous solution to silica nanoparticle films studied with in situ ATR-IR spectroscopy and colloid probe AFM

Polyethoxylated (PEO) surfactant adsorption to silica under aqueous conditions is an important physical process in a multitude of industries. Consequently, a considerable number of spectroscopic and other studies have been carried out to ascertain the molecular/structural details of the adsorbed surfactant and the kinetics of PEO surfactant adsorption. However, the use of infrared spectroscopy to probe surfactant adsorption at the silica/aqueous solution interface has been limited because of the instability of silica particle films under aqueous conditions and the opacity of silicon prisms below 1300 cm-1 typically employed for these studies. The work presented here provides infrared spectroscopic measurements of silica particle films formed from differing suspension pH on a diamond internal reflection prism to probe silica particle film stability as a function of pH. The films formed from a suspension pH of 2.5 were found to be the most stable owing to a sol-gel transition of the colloidal suspension upon drying and the reduction in electrostatic repulsion between silica nanoparticles, creating a tightly packed nanoparticle film. Colloid probe atomic force microscopy (CP-AFM) was used to confirm the alteration of surface forces between silica nanoparticles as a function of pH. Particle films from silica suspensions of pH 2.5 were formed in situ on an attenuated total reflection infrared diamond prism and used to probe Triton X-100 adsorption from an aqueous solution. The obtained infrared spectra revealed a critical surface aggregation concentration at a solution concentration of 0.14 mmol L-1, Triton X-100 forms discrete micelles at the silica surface, and the PEO head group preferentially adopts a helical conformation. Most intriguingly, a breakup of the silica particle film was observed at the critical micelle concentration of the surfactant. This is due to the repulsive steric forces arising from the interactions between the PEO corona of the surfactant micelles formed at the silica surface, as confirmed by the CP-AFM measurements.

Recent Advances in Macro ATR-FTIR Microspectroscopic Technique for High Resolution Surface Characterisation at Australian Synchrotron IR Beamline

Highly collimated synchrotron-IR beam offers 100-1000 times higher brightness than that of internal IR source used in laboratory-based FTIR instruments, enabling acquisition of high-quality FTIR spectra at diffraction-limited spatial resolution. Such properties make synchrotron-IR an excellent analytical platform for acquiring spatially resolved chemical “mapping” of materials at lateral resolution between 3-10mm. Attenuated total reflection (ATR) FTIR technique is widely used for probing surface-specific molecular information of materials. Coupling synchrotron-IR beam to an ATR element further enhances the lateral resolution greater than those in transmission/reflectance, by a factor of refractive index (n) of the ATR element. For mapping measurements using Ge element (n=4), this has the effect of not only reducing the beam focus size (improving the lateral resolution) bya factor of 4, but also reducing the mapping step size by 4 times relative to the stage step motion. As a result, ATR-FTIR measurement at Australian Synchrotron IR Beamline can be performed at minimum beam size of 1.9mm/1.2mm (with 20x/32x objective), and at minimum mapping step size of 250 nm. Unlike microscopic-ATR (micro-ATR) technique, macro-ATR approach requires only a single contact between the ATR element and the sample throughout the measurement minimising potential of sample damage and also providing a faster scanning speed. This work presents recent advances in macro-ATR devices developed at Australian Synchrotron. Two macro ATR devices have been made available for the users since February 2016. The first model, “hybrid macro-ATR”, was developed by modifying the cantilever arm of the standard macro-ATR unit to accept Ge ATR elements with different facet sizes (1mm, 250μm and 100μm in diameter) normally used with micro-ATR objective. While the larger tip works well with softer materials that do not require high pressure, the small tips can provide higher pressure and allow measurements inside smaller regions with limited access suitable for hard/rough surfaces. The other macro-ATR device, “soft-contact piezo-controlled macro-ATR”, was designed specifically for analysis of delicate and soft materials, by using a unique combination of piezo-controlled linear translation stages to achieve precise positioning and gentle approach of the sample towards the ATR facet. The capabilities of the technique have been demonstrated through a diverse range of research from material and food science to biology and single fibres.

Formation and enzymatic degradation of poly-l-arginine/fucoidan multilayer films

A polyelectrolyte multilayer (PEM) system based on biopolymers has been constructed and studied in its formation and enzymatic breakdown. The multilayer is composed of fucoidan (a proven antimicrobial/anti-inflammatory seaweed-based polysaccharide) and poly-l-arginine (a polypeptide that can be readily degraded with trypsin to yield arginine, a known NO donor), thus making the multilayer a potential dual action surface treatment for wound dressings. Studies on the formation of the multilayer revealed that the film built-up in the expected stepwise manner with consistent reversal of the zeta potential upon the adsorption of each subsequent polyion. The completed film (8 bilayers) was seen to have low hydration (30% water), as determined by H2O/D2O solvent replacement studies using the quartz crystal microbalance, with an adsorbed mass (without hydration water) of approx. 4.8μgcm-2, as determined by quantitative attenuated total reflectance Fourier transform infrared (ATR FTIR) spectroscopy. The enzymatic breakdown of the film in response to exposure to trypsin was also investigated, and the film was seen to release both polymers over time, with a projected complete film removal period of approximately 24h. Critically, this information was determined using ATR FTIR spectroscopy experiments, which allowed unambiguous deconvolution of the removal rates of the two polyions, which is information that cannot be obtained from other methodologies used to study enzymatic breakdown of surface films.