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We studied silica suspensions with chitosan and biodegradable synthetic surfactant lauroyl ethyl arginate (LAE). Hydrophilic and negatively charged silica nanoparticles were neutralised due to the coating with chitosan. That presence of LAE led to the partial hydrophobisation of their surface, which favoured their attachment to the surface of a thin foam film. It was found that the presence of small and medium-sized (6–9 nm) hydrophobic particles in the interfacial layer of lamella foam film inhibited the coalescence and coarsening processes, which prolonged the life of the foam. Furthermore, hydrophobising of 30 nm particles allowed the formation of large aggregates precipitating from the mixture under steady-state conditions. These aggregates, however, under the conditions of the dynamic froth flotation process in the foam column, were floated into the foam layer. As a result, they were trapped in the foam film and Plateau borders, effectively preventing liquid leakage out of the foam. These results demonstrate the efficiency of using chitosan-LAE mixtures to remove silica nanoparticles from aqueous phase by foaming and flotation.

In this work we investigate the surface properties and foamability of saponin and mixed saponin-chitosan solutions. These natural compounds are widely used in various cosmetic, pharmaceutical and food technologies because of their efficiency as bio-active components and their biodegradability. These compounds and their mixture were investigated versus the composition by surface tension and dilational rheology measurements and the respective foams analysed at the formation and during their entire time evolution. The results show that these systems present peculiarities relevant for their utilisation as foam stabilisers, such as strong amphiphilicity of saponin and high values of dilational viscoelasticity. The behaviour of foams has been interpreted on the basis of the adsorption properties at liquid-air interface and the interfacial rheology. Specifically, we found a remarkable effect of the chitosan on the long-time stability of foams. This has been explained considering the changes of the bulk properties induced by chitosan, which influence also the dynamics of the saponin adsorption. This work aims to contribute to the development of new formulations of biodegradable and biocompatible foams for industrial applications, where it is advantageous to reduce the use of synthetic surfactants in commercial products.

The adsorption behaviour of linear poly(oxyethylene) alkyl ether (CnEOm) is best described by a reorientation model. Based on a complete set of experimental data, including the adsorption kinetics, the equilibrium surface tension isotherm and the surface dilational visco-elasticity, the thermodynamic and kinetic adsorption parameters for some CnEOm at the water/air interface were determined. For the study, six CnEOm surfactants were selected (n = 10, 12 and 14 and m = 4, 5 and 8) and were studied by bubble profile analysis and maximum bubble pressure tensiometry. A refined theoretical model based on a reorientation-adsorption model combined with a diffusion-controlled adsorption kinetics and exchange of matter allowed us to calculate the surface layer composition by adsorbing molecules in different orientations. It turns out that at larger surface coverage, the adsorption rate decreases, i.e., the apparent diffusion coefficients are smaller. This deceleration can be explained by the transition of molecules adsorbed in a state of larger molar surface area into a state with smaller molar surface area.

In this review, we highlight and discuss the effects of interfacial properties on the major mechanisms governing the aging of emulsions: flocculation, coalescence and Ostwald ripening. The process of emulsification is also addressed, as it is well recognized that the adsorption properties of emulsifiers play an important role on it. The consolidated background on these phenomena is briefly summarised based on selected literature, reporting relevant findings and results, and discussing some criticalities. The typical experimental approaches adopted to investigate the above effects are also summarised, underlining in particular the role of adsorption at the droplet interface. Attention is paid to different types of surface-active species involved with emulsion production, including solid particles. The latter being of increasing interest in a wide variety of emulsions-related products and technologies in various fields. The possibility to stop the long term aging caused by Ostwald ripening in emulsions is also discussed, quantifying under which conditions it may occur in practice.

The pulmonary surfactant (PS) is a complex mixture of lipids and proteins dispersed in the aqueous lining layer of the alveolar surface. Such a layer plays a key role in maintaining the proper lung functionality. It acts as a barrier against inhaled particles and pathogens, including viruses, and may represent an important entry point for drugs delivered via aerosols. Understanding the physicochemical properties of PS is therefore of importance for the comprehension of pathophysiological mechanisms affecting the respiratory system. That can be of particular relevance for supporting the development of novel therapeutic interventions against COVID-19–induced acute respiratory distress syndrome. Owing to the complexity of the in vivo alveolar lining layer, several in vitro methodologies have been developed to investigate the functional and structural properties of PS films or interfacial films made by major constituents of the natural PS. As breathing is a highly dynamic interfacial process, most applied methodologies for studying PSs need to be capable of dynamic measurements, including the study of interfacial dilational rheology. We provide here a review of the most frequently and successfully applied methodologies that have proven to be excellent tools for understanding the biophysics of the PS and of its role in the respiratory mechanics. This overview also discusses recent findings on the dynamics of PS layers and the impact of inhalable particles or pathogens, such as the novel coronavirus, on its functionality.

The present work is aimed at investigating the chemicophysical properties of the interface between silicone oils (SOs) used in vitreoretinal surgery and aqueous solutions, in the presence of surfactant biomolecules. Such molecules are thought to play an important role in the formation of SO emulsions in vitrectomised eyes, in which the natural vitreous body has been replaced with a SO. In particular, we have measured the interfacial tension (IT) and the interfacial dilational viscoelasticity (DV) of the interface between SO (Siluron 1000) and serum proteins (albumin and γ-globulins) at various concentrations in a Dulbecco alkaline buffer. The equilibrium IT value is relevant for the onset of emulsification, and the DV influences the stability of an emulsion, once formed. The study is complemented by preliminary emulsification tests. The experimental results show that, when proteins are dissolved in the aqueous solution, the rheological properties of the interface change. The IT decreases significantly for physiological protein concentrations, and the DV modulus achieves high values, even for small protein concentrations. The emulsification tests confirm that, in the presence of proteins, emulsions are stable on the time scale of months. We conclude that the measured values of IT in the presence of serum proteins are compatible with the promotion of droplet formation, which, in addition, are expected to be stable against coalescence. Adsorption of biomolecules at the interface with the SO is, therefore, likely to play an important role in the generation of an emulsion in eyes subjected to vitrectomy. These findings are relevant to identify strategies to avoid or control the formation of emulsions in eyes.

Carbonaceous particles are accounted among the most common environmental pollutants. This makes necessary a careful examination of their potential impact on the physico-chemical properties of different biological films, e.g. tear film, skin, or lung surfactant. This work analyzes the interaction of three different types of carbonaceous particles with Langmuir monolayers of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) spread at the water/vapor interface. This type of studies can be exploited as preliminary in vitro tests for evaluating the potential risks and hazards of this type of pollutants for human health, which requires to study the effect of the particles on the ability of lipid layers for reducing the surface tension under static and dynamics conditions. The obtained results have shown that the insertion of particles into lipid films leads to the emergence of excluded area-like effects which modify the interfacial cohesion and packing. This hinders partially the ability of the lipid films for reducing the surface tension, which may present important adverse effects for the normal physiological performance of lipid membranes, and in particular for the performance of lung surfactant layers. Furthermore, the analysis of the response of lipid layers including carbonaceous particles against mechanical stresses also evidenced that particle incorporation worsens the mechanical performance of lipid layers, which may alter both their functional and structural roles.

Lipid layers are considered among the first protective barriers of the human body against pollutants, e.g., skin, lung surfactant, or tear film. This makes it necessary to explore the physico-chemical bases underlying the interaction of pollutants and lipid layers. This work evaluates using a pool of surface-sensitive techniques, the impact of carbon black and fumed silica particles on the behavior of Langmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The results show that the incorporation of particles into the lipid monolayers affects the surface pressure&ndash, area isotherm of the DPPC, modifying both the phase behavior and the collapse conditions. This is explained considering that particles occupy a part of the area available for lipid organization, which affects the lateral organization of the lipid molecules, and consequently the cohesion interactions within the monolayer. Furthermore, particles incorporation worsens the mechanical performance of lipid layers, which may impact negatively in different processes presenting biological relevance. The modification induced by the particles has been found to be dependent on their specific chemical nature. This work tries to shed light on some of the most fundamental physico-chemical bases governing the interaction of pollutants with lipid layers, which plays an essential role on the design of strategies for preventing the potential health hazards associated with pollution.

Particle stabilized liquid foams are used as templates to obtain carbon-based porous materials with open cell structure, high specific area and hierarchical porosity. This route relies on gel casting to strengthen the structure of the liquid foams, followed by high temperature treatment to eliminate organic components and obtain solid foams. The liquid foam stabilizer was a commercial activated carbon powder, characterized by high porosity and irregular morphology of the particles, associated with a cationic surfactant. The micro-structure and the textural properties of the final solid materials have been investigated by Scanning Electronic Microscopy and Nitrogen adsorption methods. The results show that this method can be used to fabricate high specific area porous materials in the form of monoliths, with adequate consistence and mechanical resistance. The materials obtained seem promising for many practical applications such as gas adsorption, filtering and catalysis.

We report here an overview of current trends and a selection of recent results regarding the characterization of emulsions by Diffusing Wave Spectroscopy (DWS). We provide a synopsis of the state of the art of the DWS technique, and a critical discussion of experiments performed on samples in which Brownian and ballistic dynamics coexist. A novel analysis scheme is introduced for DWS experiments on creaming or sedimenting emulsions, allowing to extract not only average values for drop size and drop dynamics - as usual in DWS - but also properties related to the width of the distributions governing these quantities. This analysis scheme starts from a realistic Monte Carlo simulation of light diffusing in the volume of the sample and reaching the detector. This simulation is more accurate than the analytical expressions available for the idealized geometries normally used in DWS interpretation. By disentangling Brownian and ballistic motions we directly access the variance of velocity distribution, σv. In relatively unstable emulsions σv governs the frequency of drop-drop collisions and subsequent coalescence events. Furthermore, when gravity dominates dynamics, as in emulsions subject to sedimentation or creaming, σv is strongly related to the 2nd and 4th moments of drop size distribution. This novel analysis scheme is exemplified investigating freshly formed model emulsions. Results are validated by comparison with microscopy imaging. This analysis is then extended to emulsions with a much broader drop size distribution, resembling those that are planned to be investigated in microgravity by the Soft Matter Dynamics facility onboard the International Space Station (ISS). This review is concluded by sketching some promising directions, and suggesting useful complementarities between DWS and other techniques, for the characterization of transient regimes in emulsions, and of destabilization processes of great practical importance.

This work focuses on the surface properties of binary dispersions of nanoparticles, namely Titania (TiO2) and amorphous silica (SiO2), associated to a short chain ionic surfactant (CTAB) which has the role of modifying their amphiphilicity, inducing the segregation of particle-surfactant complexes at the liquid interface. The foams produced by these dispersions are also investigated. The adsorption of these complexes at water-air interfaces has been investigated by surface tension and dilational rheology measurements. The results obtained, discussed on the basis of the main characteristics of the two types of particles, allow us to understand the role of the dispersion composition and, especially, of the cationic surfactant in the determination of the surface activity of the different complexes and of the properties of the mixed NP layers at the water- air interfaces. In particular, important synergetic effects on the interfacial properties have been observed, when both types of particles are adsorbed. These effects reflect in an enhanced stability of the corresponding foams. In addition, the study evidences the possibility to formulate binary dispersions in order to drive selectively the adsorption of one type of particles at the liquid interface, which can be particularly significant for the formulation of particle-stabilized foams with tailored lamella and cell surface composition/structure.

In this communication, the single element version of the fractional Maxwell model (single-FMM or Scottndash;Blair model) is adopted to quantify the observed behavior of the linear interfacial dilational viscoelasticity. This mathematical tool is applied to the results obtained by capillary pressure experiments under low-gravity conditions aboard the International Space Station, for adsorption layers at the hydrocarbon/water interface. Two specific experimental sets of steady-state harmonic oscillations of interfacial area are reported, respectively: a drop of pure water into a Span-80 surfactant/paraffin-oil matrix and a pure n-hexane drop into a C13DMPO/TTAB mixed surfactants/aqueous-solution matrix. The fractional constitutive single-FMM is demonstrated to embrace the standard Maxwell model (MM) and the Lucassenndash;van-den-Tempel model (Lndash;vdT), as particular cases. The single-FMM adequately fits the Span-80/paraffin-oil observed results, correctly predicting the frequency dependence of the complex viscoelastic modulus and the inherent phase-shift angle. In contrast, the single-FMM appears as a scarcely adequate tool to fit the observed behavior of the mixed-adsorption surfactants for the C13DMPO/TTAB/aqueous solution matrix (despite the single-FMM satisfactorily comparing to the phenomenology of the sole complex viscoelastic modulus). Further speculations are envisaged in order to devise combined FMM as rational guidance to interpret the properties and the interfacial structure of complex mixed surfactant adsorption systems.

A comparative study is reported on the interfacial properties of a set of surfactants and is discussed in terms of the effects on the features of the corresponding oil-water emulsions. The surfactants are saponin, Tween 80 and citronellol glucoside (CG), while the oil is Miglyol 812N&mdash, A Medium Chain Triglyceride (MCT) oil. Due to their high biocompatibility, all these compounds are variously utilized in food, cosmetic or pharmaceutical products. Among the surfactants, which are all soluble in water, CG presents also an important solubility in oil, as shown by the measured partition coefficient. For these systems, dynamic and equilibrium interfacial tensions and dilational viscoelasticity are measured as a function of the surfactant concentration and analyzed according to available adsorption models. In order to compare these results with the time evolution of the corresponding emulsions, the actual surfactant concentration in the matrix phase of the emulsion is accounted for. This may differ significantly from the nominal concentration of the solutions before dispersing them, because of the huge area of droplets available for surfactant adsorption in the emulsion. Using this approach allows the derivation of the correlations between the observed emulsion behavior and the actual surfactant coverage of the droplet interface.

Carbon monolithic adsorbents exhibiting a hierarchical pore structure are produced via a synthesis route based on the stabilization of liquid foams followed by a carbonization step. The macro-microporous structure is achieved by the incorporation of microporous, biomass-derived activated carbon particles in the liquid foam enclosed by a cationic surfactant as stabilizer. This method yields crack-free monoliths (solid foams) with a compressive strength of the order of 20 kPa. The microstructure and the textural properties of the final solid foams have been investigated by means of Scanning Electron Microscopy (SEM) and gas adsorption. The behavior as selective CO2 adsorbents at 25 °C has been evaluated using breakthrough experiments under simulated post-combustion conditions (16% V/V CO2/N2), resulting in a selectivity factor of 13 over N2. The hierarchical pore structure of the monoliths allows a rapid transport of the gas mixture through the macropores with no appreciable pressure drop, retaining more than 90 % of the adsorption capacity (∼ 0.868 mmol/g) after several adsorption/desorption cycles. Moreover, the monolith has shown a CO2 uptake capacity of 2.62 mmol/g under static condition at 1 bar and 25 °C. This study provides guidelines for the design of carbon-based foams decorated with carbon particles, which have morphological and textural properties that can be carefully selected for any gas-selective capture application. The authors would like to acknowledge the financial support received from Generalitat Valenciana (PROMETEO II/2014/004-FEDER and PhD grant Vali+d), the University of Alicante (Ayudas para estancias de personal investigador invitado), the Italian Space Agency, ASI, within the Project PASTA “Particle Stabilized Emulsion and Foams” (n. 2013-028-R.O), and the financial support from MINECO: Project MAT2017-86992-R.

The control of the behavior of oil in water emulsions requires deeper investigations on the adsorption properties of the emulsion stabilizers at the interfaces, which are fundamental to explain the (de)stabilization mechanisms. In this work, we present an extensive study on oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) below its critical micellar concentration. Dynamic tensiometry, dilational rheology, and electrical conductivity measurements are used to investigate the adsorption properties at the droplet interface, whereas the aging of the respective emulsions was investigated by monitoring the macroscopic thickness of the emulsion layer, by microimaging and dynamic light scattering (DLS) analysis, to get information on the drop size distribution. In addition, the droplet coalescence is investigated by a microscopy setup. The results of this multitechnique study allow deriving a coherent scenario where the adsorption properties of this ionic surfactant relate to those of the emulsion, such as, for example, the prevention of droplet coalescence and the presence of other mechanisms, such as Ostwald ripening, responsible for the emulsion aging.

The incorporation of silica nanoparticles (NPs) from the subphase into Langmuir lipid monolayers formed by three components, 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) and Cholesterol (Chol), modifies the thermodynamic and rheological behavior, as well as the structure of the pristine lipid film. Thus, the combination of structural characterization techniques, such as Brewster Angle Microscopy (BAM) and Atomic Force Microscopy (AFM), with interfacial thermodynamic and dilational rheology studies has allowed us to deepen on the physico-chemical bases governing the interaction between lipid molecules and NPs. The penetration of NPs driven by the interaction (electrostatic or hydrogen bonds) with the polar groups of the lipid molecules affects the phase behaviour (surface pressure-area, ? - A, isotherm) of the monolayer. This can be easily rationalized considering the modification of the packing and cohesion of the molecules at the interface as revealed BAM and AFM images. Furthermore, oscillatory barrier experiments have allowed obtaining information related to the effect of NPs on the monolayer response under dynamic conditions that presents a critical impact on the characterization of biological relevant systems because most of the processes of interest for these systems present a dynamic character.

The interaction of carbon black particles (CB) with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) at the water/air interface has been investigated by means of a pool of surface sensitive techniques, in order to analyze the thermodynamic and rheological aspects of these mixed systems. The incorporation of carbonaceous particles to the lipid monolayers induces changes on the surface pressure-area isotherm, as revealed by the shifting to higher surface area of the maximum packing degree of the monolayer, and the decrease of the collapse pressure. These changes are strongly dependent on the DPPC:CB weight ratio at the interface and can be explained by the disruption of the monolayer structure due to the particle incorporation that provokes the modification of the cohesive interactions along the monolayer. Measurements of dilational viscoelasticity against frequency, at different degrees of monolayer compression, have been performed by means of the Oscillatory Barrier method. The rheological response of the monolayer is only slightly affected by the presence of CB, even if a modification of the quasi-equilibrium dilational elasticity, as well as of the frequency dependence of the viscoelastic modulus, is appreciable increasing the particle concentration. Being DPPC the major component of many systems with biological interest (cell membranes, lung surfactant), the results obtained here are expected to contribute to the understanding of the carbon particle interaction with biological relevant systems.

In this work, a method is proposed for the production of carbon porous materials based on the foaming of a carbon colloidal dispersion in the presence of a short chain ionic surfactant (CTAB) and Poly(vinyl alcohol) followed by the in situ polymerization by cross-linking with 2,5-dimethoxy-2,5-dihydrofuran (DHF). The particle stabilized wet foams are used as templates for the gel-casting providing porous materials with carbon particles mainly distributed at the surface of the cells. This sample so obtained is then submitted to a high temperature treatment to remove all organic components from the matrix and get a compact carbon solid foam. The morphology of the samples obtained according to this procedure have been analyzed via Scanning Electron Microscopy (SEM) and different aspects regarding both the procedure and the composition of the initial dispersion have been investigated such as the role of the surfactant, the dependence on the particle concentration and the temperature adopted during the thermal treatment.

The formation and stability of emulsions in vitrectomized eyes is linked to the properties of the silicone oil-aqueous humor interface, in particular the surface tension. In the presence of natural surfactants, such as serum and plasma, the value of the surface tension is likely to change, but little quantitative information is presently available.To this end, we perform accurate experiments measuring the interfacialproperties of the Siluron 1000 (Fluoron GmbH, Ulm, Germany) silicone oil with an aqueous solution in the presence of endogenous-like proteins. It is found that the surface tension is significantly reduced when physiologically realistic concentrations are used. Moreover, the values obtained for the dilational viscoelastic modulus are compatible with the formation of stable emulsions.

In Zambia, every year some parts of the maize fields are abandoned post-planting. Reasons for this are not clearly known. In this paper, we examine the influence of soil and climatic factors on crop abandonment using a six-year (2007–2012) panel data by modeling the planted-to-harvested ratio (a good indicator of crop abandonment) using a fractional and linear approach. Therefore, for the first time, our study appropriately (as supported by the model specification tests that favour fractional probit over linear) models the fractional nature of crop abandonment. Regression results, which are not very different between the two specifications, indicate that, more than anything, high rainfall immediately after planting and inadequate fertilizer are the leading determinants of crop abandonment. In the agro-ecological region where dry planting takes place, low temperature during planting months negatively affects the harvested area. The results have implications on the sustainability of farming systems in the face of a changing climate.

The effect of a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), on the interfacial properties of seawater has been studied by dynamic and equilibrium surface tension and by dilational rheology essays. Important modifications of the surface tension and dilational rheology response have been observed already at the very low CTAB concentrations, where the effects due to the high ionic strength are negligible. The comparison with the effects of CTAB in different seawater models, or in natural seawater fractions, points out the establishment of strong interactions between the surfactant molecules and the lipophilic fraction of organic material dispersed/dissolved in seawater, affecting the interfacial activity of the molecules. Considering the biochemical richness of seawater, these results can be explained assuming interaction mechanisms and adsorption schemes similar to those speculated for protein and other macromolecules in the presence of surfactants, which in fact show similar features. Thus already at the low concentrations the surfactant molecules form highly surface-active complexes with part of the organic fraction of seawater. At the larger surfactant concentrations these complexes compete for adsorption with an excess of free CTAB molecules which, according to the thermodynamic conditions, are most favoured to occupy the liquid interface. The results of this study underline the important role of the sea organic content in enhancing the surface-activity of surfactants, which is relevant for a deeper understand of the direct and indirect effects of these types of pollutants on the physico-chemical environment in the sea coastal areas and develop mitigation strategies.

In the second part of this communication we screen a few tea polyphenols for their ability to interact with pre-adsorbed salivary proteins at the air-liquid interface. Using interfacial dilational rheology characterisation, we measure the variation in surface elasticity of a pre-adsorbed salivary film after exposure to tea polyphenols solutions in order to probe the degree of mutual interaction. We observe that this interaction with salivary proteins varies according to the chemical structure and degree of polyphenols complexity (number of phenols rings). The evolution of the salivary film after exposure to polyphenol solutions is characterised through surface dilational response to appropriate frequency modulation and the results are analysed using a general mixed kinetic model, which reproduces correctly the experimental data except for the galloylated theaflavins. © 2013 Elsevier B.V.

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. © 2013 Elsevier B.V.

The effect of the incorporation of hydrophilic titanium dioxide (TiO2) nanoparticles on the interfacial properties of Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) has been evaluated combining interfacial thermodynamic studies, dilatational rheology, and Brewster angle microscopy (BAM). The results show that the TiO2 nanoparticles are able to penetrate DPPC layers, modifying the organization of the molecules and, consequently, the phase behavior and viscoelastic properties of the systems. Measurements of dilational viscoelasticity against the frequency have been performed, using the oscillatory barrier method, at different values of the surface pressure corresponding to different degrees of compression of the monolayer. The presence of TiO2 nanoparticles also affects the dynamic response of the monolayer modifying both the quasi-equilibrium dilatational elasticity and the high frequency limit of the viscoelastic modulus. The principal aim of this work is to understand the fundamental physicochemical bases related to the incorporation of specific nanoparticles of technological interest into the interfacial layer with biological relevance such as phospholipid layers. This can provide information on potential adverse effects of nanoparticles for health and the environment.

The dilational rheology for solutions of surface active compounds and their mixtures can be measured with various methods of different frequency ranges. The most frequently used techniques are based on oscillating drops and bubbles. Besides these techniques, also capillary wave damping is a siutable method, in particular at higher frequencies. The analysis of the resulting data is mainly based on a diffusion model derived by Lucassen and van den Tempel many years ago. For mixtures of surfactants or proteins with surfactants only recently a functioning theoretical apparatus was represented and applied to different mixed adsorption layers. The present chapter summarizes the experimental tools and corresponding theories, and provides well selected examples of classical surfactants and mixtures of milk proteins mixed with a number of surfactants.

Silica nanoparticles (SiNP) can be incorporated in phospholipid layers to form hybrid organic-inorganic bidimensional mesostructures. Controlling the dynamics in these mesostructures paves the way to high-performance drug-delivery systems. Depending on the different hydrophobicity/hydrophilicity of SiNP, recent X-ray reflectivity experiments have demonstrated opposite structural effects. While these are reasonably well understood, less is known about the effects on the dynamics, which in turn determine molecular diffusivity and the possibility of drug release. In this work we characterize the dynamics of a mixed Langmuir layer made of phospholipid and hydrophobic SiNP. We combine X-ray photon correlation spectroscopy and epifluorescence discrete Fourier microscopy to cover more than 2 decades of Q-range (0.3-80 μm(-1)). We obtain evidence for the onset of an arrested state characterized by intermittent stress-relaxation rearrangement events, corresponding to a gel dominated by attractive interactions. We compare this with our previous results from phospholipid/hydrophilic SiNP films, which show an arrested glassy phase of repulsive disks.

The effect of silica nanoparticles on the thermodynamic and structural properties of monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), palmitic acid (PA) and a mixture of them, has been investigated using a combination of Langmuir trough technique and Brewster Angle Microscopy (BAM). This study shows that the presence of nanoparticles in the aqueous sub-phase affects the surface pressure-area (Pi-A) isotherm, leading to a different phase behavior of the monolayer. The observed effects are explained assuming the incorporation of nanoparticles into the monolayer driven by the adsorption of lipid molecules onto the silica which makes the nanoparticles more hydrophobic. This process hinders the ordering and affects the composition of the investigated lipid monolayers significantly modifying also their quasi-equilibrium dilational elasticity. The reported results are useful in the framework of a wider study aimed at elucidating the impact of nanoparticles on the physico-chemical properties of pulmonary surfactant, being DPPC its major component and PA utilized as an important component in the formulation of therapeutic substitutes. (C) 2011 Elsevier B.V. All rights reserved.

The hydrophobicity of a particle surface can be tuned by the addition of surfactants that change the surface free energy for their attachment to a liquid interface. In this work, we report an experimental study where the wettability properties of silica nanoparticles are modified by the adsorption of alkyltrimethylammonium surfactants (CnTAB, n = 12, 16) on the surface of the particles. We have pointed out that the wettability of the complexes is controlled by an intricate balance of electrostatic and hydrophobic interactions between the particle surface and the surfactant. These interactions play an important role in the structure of the surfactant-particle nanocomposite interfacial layer.

Aqueous dispersions of carbon black and carbon soot containing the cationic surfactant CTAB have been investigated from the point of view of dynamic interfacial tension and surface dilational rheology. In addition the features and stability of emulsions and foams formed from these dispersions have been studied and correlated with the interfacial properties. The results show an interaction of the carbonaceous particles with the CTAB in determining the interfacial properties of dispersions. The investigation on emulsion and foam stability shows a synergetic effect of particles and surfactant in providing water-in-hexane emulsions stable over several months. This synergetic effect, even evident for emulsions, is instead less effective for foams. Results for emulsions and foams stability can be partially correlated with the observations on interfacial properties. Since the utilised particles are representative of particulate pollutants produced by combustion processes. The study evidences qualitatively potential adverse effects on waters which may arise from the association of these particles with common surfactants.

Despite their potential application, the wetting properties of aqueous surfactant solutions on superhydrophobic (SH) solid surfaces have been rarely investigated. In this work single surfactant solutions and binary mixtures of non-ionic and ionic surfactants have been studied in contact with textured and non-textured surface coatings obtained utilizing nanoparticles and a fluorinated polymer. The role of the molecular structure and the concentration in the modification of the wetting properties has been observed both for individual species and mixtures. At supramicellar concentrations a significant decrease in the effectiveness of the mixed solution in wetting the SH surface in comparison to single ionic species has been found. A comparison with the same polymer coating, but in hydrophobic conditions, shows, on the contrary, that the wetting is slightly promoted by the mixture. The molecular size seems to play a positive role in wetting enhancement with the formation of more packed adsorption layer due to molecular penetration. The results of this work should be useful to design surfactant systems for a more effective wetting control.

In this study the effect of particles of nanometric dimensions on the interfacial properties of liquid-liquid systems is addressed. The nanoparticle transfer and the consequent attachment to the fluid interface are mainly governed by their hydrophobic/lipophilic character. For this reason a model particle-surfactant system has been investigated, namely a nanometric colloidal silica dispersion plus CTAB, where the role of the cationic surfactant is varying the particle surface properties adsorbing on them. The interfacial properties of the dispersion against hexane were determined as a function of the surfactant concentration, measuring the interfacial tension and the dilational viscoelasticity versus frequency. These results were then crossed with a study on the behaviour of micrometric oil droplets inside the same dispersions, as well as on the morphology of the respective nanoparticle stabilised emulsions. By this multiple investigation it was possible to find out that diffusion transport of particles from the bulk to the interface plays an important role, as well as the reorganisation of the mixed parlicle-surfactant layer. Moreover from these results the irreversible aspect of the nanoparticle attachment to the fluid interface was also evidenced which, under particular conditions, can provide the formation of a solid-like layer. (c) 2007 Elsevier B.V. All rights reserved.

The adsorption kinetics of ionic surfactant decanoic acid was examined. Dynamic and equilibrium surface tensions (ST) were measured using a video-enhanced pendant bubble tensiometer. The equilibrium ST data and the complete ST relaxation profiles were compared with theoretical profiles predicted by both non-ionic and ionic models. The quasi-equilibrium approach was used in the ionic model to describe the electric field in the electrical double layer. Both non-ionic and ionic generalized-Frumkin models predict the equilibrium and dynamic ST data very well. Using the ionic model, a comparison between the dynamic ST data and the theoretical ST profiles gave a diffusivity of 5.5 × 10−6 cm2/s. The adsorption of decanoic acid onto the air–water interface was determined to be diffusion controlled. Dynamic surface properties (Γ and ψs), the electrical potential ψ(r, t) in the double layer, and the double layer thickness λ(C, t) were also evaluated. It was found that the surface charge density increases very quickly at the early stages (∼1 s) of the surfactant adsorption process. Additionally, the electrical surface potential data implied that the surface charge density becomes significantly stronger at very dilute surfactant concentrations, where π is less than 1 mN/m.

Highly hydro and oleophobic materials applied to needs of the marine environment represent a relatively young field achieving growing interest as innovative solution, where technological and ecological aspects allow to be merged, taking into account the limitations imposed by international laws in terms of environmental protection. In this work a superhydrophobic (SH) coating for seawater applications has been characterized and tested in both laboratory and field conditions, since investigations in real seawater are crucial to evaluate the behaviour of SH surfaces because of a complexity not reproducible in laboratory. Taking into account the real conditions where the surface can operate oleophobicity can be investigated in presence of organic and inorganic pollution. The amphiphobic surface has been also studied to simulate those applications where the surface undergoes to thermal stress like pipelines and desalination plants affected by biofouling. The preliminary wearing test shows the effective resistance to a continuous low velocity impact of micrometric particles of the coating investigated here. Finally, time durability in field tests has been studied as a function of immersion angle in real seawater environment to investigate the role of the coating on early stages of biofilm growth.

The correlation between the interfacial properties of the surfactant adsorbed layer at the water–oil interface and the emulsification and stability of the corresponding emulsion has been investigated. To this aim an oil soluble surfactant, sorbitane monooleate (Span 80), has been studied at the paraffin oil–water interface, together with the water in oil emulsions stabilized by the same surfactant at the same bulk concentrations, above the cmc. The equilibrium and dynamic interfacial tension and the interfacial rheological properties have been measured by coupling two different tensiometers: a capillary pressure tensiometer (CPT) and a drop shape tensiometer. By this way the dilational viscoelasticity has been determined in a frequency range from 0.01 to 20 Hz. To investigate the emulsion properties, besides the observation by optical microscope, a differential scanning calorimeter (DSC) has been used which allows for the evaluation of the drop size distribution. The results show a correlation between the surfactant concentration and the emulsion properties, which can be explained on the basis of the dilational properties. For very concentrated solutions however the stabilizing effect of inverse micelles must be accounted for. Moreover, dilational properties have been found to be also important to determine emulsification conditions.

In this study the interfacial as well as the surface tensions, the partition coefficients Kp, the interfacial elasticities, and the stabilities of o/w-emulsions of the same systems were measured and compared to find a quantitative correlation which can help to control the emulsion stability. The systems in question contain water, paraffin oil and one of four non-ionic technical surfactants commonly used as emulsifiers. While a clear correlation between emulsion stability and equilibrium interfacial tension was found, the different emulsifying capacities cannot be explained by the partition coefficients Kp or the surface tension. However, combining the Kp results with the surface tension data allows us to estimate which emulsifier is more effective in solubilising a given amount of dispersed phase. Another method to classify emulsifiers is to carry out interfacial rheology measurements over a frequency range as broad as possible. Again a clear correlation between interfacial rheology and emulsifying capacity was found. This correlation, however, was restricted to surfactants of the same class. No correlation between interfacial rheology and emulsifying capacity could be found for emulsifiers of different molecular structure.

The dilational elasticities epsilon and viscosities eta of the two non-ionic surfactants n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) and tetraethylene-glycolmonodecylether (C10E4) were studied as a function of the frequency nu and the surfactant concentration c. To have access to a large frequency range two different oscillating drop tensiometers were used. The first is based on evaluating the drop profile and allows to study dilational rheology in a frequency range from 0.005 to 0.2 Hz, while the second is based on measuring the capillary pressure, thus extending the frequency range up to 100 Hz. The concentrations investigated have been chosen similar, or even equal, to those for which the disjoining pressure Pi versus thickness h curves of the respective foam films were known. The results are discussed in the light of the stability of the respective foam films focussing on the observation that the stability of foam films cannot be explained solely by the magnitude of the surface forces. Indeed, the correlation between the surface visco-elasticity and the stability of the respective foam films, which was suggested on the basis of low frequency data in a previous study, is here confirmed and extended to higher frequency. (c) 2006 Elsevier B.V. All rights reserved.

We investigate the interfacial dynamics of a 2D self-organized mixed layer made of silica nanoparticles interacting with phospholipid (DPPC) monolayers at the air/water interface. This system has biological relevance, allowing investigation of toxicological effects of nanoparticles on model membranes and lung surfactants. It might also provide bio-inspired technological solutions, exploiting the self-organization of DPPC to produce a non-trivial 2D structuration of nanoparticles. The characterization of interfacial dynamics yields information on the effects of NPs on the mechanical properties, important to improve performances of systems such as colloidosomes, foams, creams. For this, we combine micro-tracking in real-space with measurement in momentum-space via x-ray photon-correlation spectroscopy and Digital Fourier Microscopy. Using these complementary techniques, we extend the spatial range of investigation beyond the limits of each one. We find a dynamical transition from Brownian diffusion to an arrested state driven by compression, characterized by intermittent rearrangements, compatible with a repulsive glass phase. The rearrangement and relaxation of the monolayer structure results dramatically hindered by the presence of NPs, which is relevant to explain some the mechanical features observed for the dynamic surface pressure response of these systems and which can be relevant for the respiratory physiology and for future drug-delivery composite systems.