Michele Ferrari

Drop and bubble shape analysis as tool for dilational rheology studies of interfacial layers.

Drop and bubble shape tensiometry is a modern and very effective tool for measuring dynamic and static interfacial tensions. An automatic instrument with an accurate computer controlled dosing system is discussed in detail. Due to an active control loop experiments under various conditions can be performed: constant drop/bubble volume, surface area, or height, trapezoidal, ramp type, step type and sinusoidal area changes. The theoretical basis of the method, the fitting procedure to the Gauss-Laplace equation and the key procedures for calibration of the instrument are analysed and described.

Adsorption and partitioning of surfactants in liquid–liquid systems

The adsorption at liquid-liquid interfaces is a phenomenon with a remarkable impact on many scientific and technological fields concerning multiphasic systems. Though in principle similar to liquid-vapour, the study and the description of the dynamic aspects of the adsorption processes at liquid-liquid interfaces deserves some specific considerations. In fact, these systems are often characterised by the partitioning of the surfactant between the two liquid phases, which makes much more complex both their modelling and investigation. In some conditions, the partitioning can be the main process controlling the adsorption dynamics. This paper is aimed at reviewing the state-of-the-art of the theoretical modelling and experimental investigation of the adsorption dynamics of surfactants at liquid-liquid interfaces. After a brief introduction to the problem of adsorption dynamics, the principal models utilised to describe the process at liquid-liquid interfaces under different assumptions are critically presented, underlining the influence of the surfactant partitioning, with respect to the relative volumes of the liquid phases and of the initial partitioning conditions. The most important experimental methodologies for the measurement of the dynamic interfacial tension are also critically reviewed by pointing out the specific problems related with the investigation of the adsorption dynamics of surfactants at liquid-liquid interfaces. Moreover, the problem of the measurement of the thermodynamic quantities characterising the partitioning--mainly the partition coefficient--is also addressed, reporting some literature data. Finally, a review of the literature about the experimental work on the subject and an overview of the needs and of the open questions is given.

Mixed DPPC-cholesterol Langmuir monolayers in presence of hydrophilic silica nanoparticles.

Langmuir monolayers of Cholesterol (Chol) and a mixture of Chol with 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), at a ratio of 17:83 in weight, spread on pure water and on silica nanoparticle dispersions, have been investigated measuring the compression isotherms as well as the surface pressure response to harmonic area variation of the monolayer. Aim of this study was to evaluate the effects of the interaction of silica nanoparticles with Chol and the conditions for the incorporation in the monolayer. In previous works on different kind of lipid monolayers, it has been shown that hydrophilic silica nanoparticles dispersed in the sub-phase may transfer into the monolayer, driven by the interaction with the lipid molecules that make them partially hydrophobic. The results here obtained indicate that also for Chol and Chol-DPPC mixtures the presence of silica nanoparticles may have important effects on the phase behaviour and structural properties of the monolayer. As confirmed by complementary structural characterisations, BAM, AFM and ellipsometry, the principal effect of the nanoparticle incorporation is the disruption of the monolayer packing, owing to the alteration of the cohesive interactions of lipid components.

Influence of silica nanoparticles on dilational rheology of DPPC–palmitic acid Langmuir monolayers

The effect of silica nanoparticles on the dynamic behavior of monolayers composed by 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a mixture of DPPC with Palmitic acid (PA), has been investigated by comparing the dilational rheological response of these lipid layers, spread on water and on a silica nanoparticle dispersion. To this aim, the dilational viscoelasticity has been measured against the frequency of the surface area perturbation of the monolayer, according to the Oscillatory Barrier method in a Langmuir trough. These measurements were performed at different values of the surface pressure, corresponding to different degrees of compression of the monolayer. The results show that the incorporation of particles in the layer induces additional surface kinetic processes and, depending on the surface pressure, modifies both the quasi-equilibrium dilational elasticity and the high frequency limit of the viscoelastic modulus. Another important effect concerns the linearity of the dilational rheological response which is appreciably worsened by the presence of nanoparticles. With DPPC being the major component of pulmonary surfactant and PA used as a component in synthetic substitutes of it, the results here obtained are relevant in the framework of wider studies on the effect of nanoparticles on the pulmonary surfactant interfacial properties.

MOLECULAR REORIENTATION IN THE ADSORPTION OF SOME CIEJ AT THE WATER-AIR INTERFACE

Abstract As demonstrated by some recent works the adsorption properties of some n -alkyl poly-oxyethylene glycol ethers can be explained by a model assuming the co-existence of different orientation states for the adsorbed molecules. In the present paper, with the aim of giving complete evidence of the above, the adsorption isotherms and the adsorption kinetics of some n -decyl and dodecyl poly-oxytheylene glycol ethers (C 10 E 8 , C 10 E 5 , C 10 E 4 , C 12 E 5 , C 12 E 8 ) at water/air interface have been studied by using the pendant drop technique. The data have been interpreted with a model considering the possibility of two orientation states for adsorbed molecules. The results of this study show that the model accurately describes both the equilibrium and dynamic adsorption behaviour of these molecules and that adsorption at water/air is controlled by diffusion. Moreover, the analysis of the best fit isotherm parameters of homologue molecules gives interesting information about the configuration of the adsorption layer.

Surface rheology as a tool for the investigation of processes internal to surfactant adsorption layers

The description of adsorption at liquid interfaces has been largely improved after incorporation into models of dynamic processes internal to the adsorption layers, such as surfactant re-orientation, aggregation and chemical reactions. Evidence for most of these processes has been given by qualitative studies utilising direct imaging techniques or by tensiometric investigations. These processes strongly influence the dilational rheological features of the adsorption layer, i.e. the response of interfacial tension to perturbations of the interfacial area. The investigation of the dilational rheology is then very effective to assess the existence of these processes and to characterise their kinetic parameters and equilibrium properties. To this aim specific models and experimental techniques have been recently developed. Here the surface visco-elasticity of adsorbed surfactant layers, epsilon, at liquid-air and liquid-liquid interfaces is measured by means of different oscillating drop/bubble methods, which are based on the interfacial tension response to harmonic perturbations of the interfacial area and have been implemented in capillary pressure and in drop shape tensiometers. The comparison of this data with theoretical predictions allows at the same time the validation of the models and the quantification of the kinetic features of the internal processes.

Surfactant adsorption at superhydrophobic surfaces

Here the wetting of ionic and nonionic surfactant solutions on a superhydrophobic interface, obtained by using a mixed inorganic-organic coating providing a fractal-like structure, has been investigated to define the role of adsorption at these interfaces. The presence of the amphiphilic molecules is effective in lowering the contact angle in a hydrophobic or still superhydrophobic range. Influencing the adsorption properties by acting on the surfactant type and concentration, the spreading of a drop on the surface can be controlled by amphiphiles confining its volume in a restricted space.

Superhydrophobic surfaces for applications in seawater.

OBJECTIVE Technological fields in which seawater is implied are numerorus, working in seawater (shipping, oil industry, marine aquaculture,..), and exploiting seawater in plants (cooling heat-exchange, desalination, power plants,..). All suffer from detrimental effects induced by biofouling mainly enhancing material failures and limiting energetic efficiencies. Among the remediation solutions, technologies coniugating economical, green and efficiency criteria should represent the direction. With the aim to meet these criteria, superhydrophobic (SH) technology attracted many researches for the protection of materials operating in contact with seawater. METHOD In this work, the literature focusing on such technology for the protection of surfaces in contact with seawater has been reviewed, mainly focusing on boat and ship hull protection. RESULTS Despite the growing interest around SH technology in seawater for fouling control and friction drag reduction of hulls, to date literature shows that superhydrophobicity in seawater is still limited if compared with a time window compatible with technological needs (set on years). An evaluation of the causes of early superhydrophobicity loss under operative conditions clearly indicates that, to the best of present knowledge, a SH surface cannot preserve this feature by itself alone (especially in real seawater). Hence, we have considered to highlight the behaviour of SH surfaces in seawater in relation to early stages of biocolonization (conditioning film and pioneering bioslime formation). Considering the annual costs sustained for the biofouling impact control, advantages coming from SH surfaces, in terms of foul control and friction drag reduction, would allow economical savings allowing to cover both the appliance of longevity keeping strategies of the SH surfaces and investments in green technologies of SH coating life cycle (production, storing). In addition a brief outlook is provided on technological fields exploiting seawater in pipelines (power and desalination plants), where the SH surface finishing finds potentially interesting application for fouling and corrosion prevention applications.

Dynamic tensiometric characterization of espresso coffee beverage

Abstract Espresso coffee world-wide success, besides being a phenomenon of fashion, seems to be based on the greater sensory satisfaction it gives to the consumer when compared with coffees prepared with other brewing methods. Conditions normally used in the espresso brewing technique enhance several surface tension-related phenomena such as foam and emulsion formation and stabilisation which strongly affects the organoleptic beverage properties. In spite of the relevant role played by surface tension in several quality characteristics of espresso coffee, little attention has been paid in its determination and its time-dependency has not yet been investigated. In the present work, experimental techniques such as maximum bubble pressure and pendant drop have been used to characterise in a wide time window the dynamic surface tension of air–espresso coffee beverage interface at two different temperatures. The experimental data show a remarkable decrease of the surface tension with time for beverages prepared by using pure arabica as well as pure robusta roasted coffee, with a profile dependent upon the coffee variety. This behaviour is definitely related to the presence of surface active components and is consistent with a system having good wetting properties for oral cavity surfaces. A possible role of some natural surface active chemical components, like lipids, on tensiometric behaviour is discussed.

Preparation of a superhydrophobic surface by mixed inorganic-organic coating

In recent years, superhydrophobic surfaces, with a water-contact angle greater than 150°, have attracted great interest for both fundamental research and practical applications. Due to the small area these surfaces show when in contact with water, interactions with aqueous environment are usually strongly reduced. An original methodology based on a mixed inorganic-organic coating in a multistep procedure is described here allowing a superhydrophobic surface to be prepared starting with a mechanical treatment of a glass surface.