A Multistate Adsorption Model for the Characterization of C13DMPO Adsorption Layers at the Aqueous Solution/Air Interface.

Experimental data for tridecyl dimethyl phosphine oxide (C13DMPO) adsorption layers at the water/air interface, including equilibrium surface tension and surface dilational viscoelasticity, are measured by bubble and drop profile analysis tensiometry at different solution concentrations and surface area oscillation frequencies. The results are used to assess the applicability of a multistate model with more than two possible adsorption states. For the experiments with single drops, the depletion of surfactant molecules due to adsorption at the drop surface is taken into account. For the assessment, the same set of model parameters is used for the description of all obtained experimental dependencies. The agreement between the proposed model and the experimental data shows that for the nonionic surfactant C13DMPO, the description of the adsorption layer behavior by three adsorption states is superior to that with only two adsorption states.

Evaluating the Impact of Hydrophobic Silicon Dioxide in the Interfacial Properties of Lung Surfactant Films.

The interaction of hydrophobic silicon dioxide particles (fumed silicon dioxide), as model air pollutants, and Langmuir monolayers of a porcine lung surfactant extract has been studied in order to try to shed light on the physicochemical bases underlying the potential adverse effects associated with pollutant inhalation. The surface pressure-area isotherms of lung surfactant (LS) films including increasing amounts of particles revealed that particle incorporation into LS monolayers modifies the organization of the molecules at the water/vapor interface, which alters the mechanical resistance of the interfacial films, hindering the ability of LS layers for reducing the surface tension, and reestablishing the interface upon compression. This influences the normal physiological function of LS as is inferred from the analysis of the response of the Langmuir films upon the incorporation of particles against harmonic changes of the interfacial area (successive compression-expansion cycles). These experiments evidenced that particles alter the relaxation mechanisms of LS films, which may be correlated to a modification of the transport of material within the interface and between the interface and the adjacent fluid during the respiratory cycle.

Surface Properties of Saponin-Chitosan Mixtures.

The surface properties of saponin and saponin-chitosan mixtures were analysed as a function of their bulk mixing ratio using vibrational sum-frequency generation (SFG), surface tensiometry and dilational rheology measurements. Our experiments show that saponin-chitosan mixtures present some remarkable properties, such as a strong amphiphilicity of the saponin and high dilational viscoelasticity. We believe this points to the presence of chitosan in the adsorption layer, despite its complete lack of surface activity. We explain this phenomenon by electrostatic interactions between the saponin as an anionic surfactant and chitosan as a polycation, leading to surface-active saponin-chitosan complexes and aggregates. Analysing the SFG intensity of the O-H stretching bands from interfacial water molecules, we found that in the case of pH 3.4 for a mixture consisting of 0.1 g/L saponin and 0.001 g/L chitosan, the adsorption layer was electrically neutral. This conclusion from SFG spectra is corroborated by results from surface tensiometry showing a significant reduction in surface tension and effects on the dilational surface elasticity strictly at saponin/chitosan ratios, where SFG spectra indicate zero net charge at the air-water interface.

Monitoring stem cell differentiation using Raman microspectroscopy: chondrogenic differentiation, towards cartilage formation.

Mesenchymal Stem Cells (MSCs) have the ability to differentiate into chondrocytes, the only cellular components of cartilage and are therefore ideal candidates for cartilage and tissue repair technologies. Chondrocytes are surrounded by cartilage-like extracellular matrix (ECM), a complex network rich in glycosaminoglycans, proteoglycans, and collagen, which, together with a multitude of intracellular signalling molecules, trigger the chondrogenesis and allow the chondroprogenitor to acquire the spherical morphology of the chondrocytes. However, although the mechanisms of the differentiation of MSCs have been extensively explored, it has been difficult to provide a holistic picture of the process, in situ. Raman Micro Spectroscopy (RMS) has been demonstrated to be a powerful analytical tool, which provides detailed label free biochemical fingerprint information in a non-invasive way, for analysis of cells, tissues and body fluids. In this work, RMS is explored to monitor the process of Mesenchymal Stem Cell (MSC) differentiation into chondrocytes in vitro, providing a holistic molecular picture of cellular events governing the differentiation. Spectral signatures of the subcellular compartments, nucleolus, nucleus and cytoplasm were initially probed and characteristic molecular changes between differentiated and undifferentiated were identified. Moreover, high density cell micromasses were cultured over a period of three weeks, and a systematic monitoring of cellular molecular components and the progress of the ECM formation, associated with the chondrogenic differentiation, was performed. This study shows the potential applicability of RMS as a powerful tool to monitor and better understand the differentiation pathways and process.

Vibrational Spectroscopy for In Vitro Monitoring Stem Cell Differentiation.

Stem cell technology has attracted considerable attention over recent decades due to its enormous potential in regenerative medicine and disease therapeutics. Studying the underlying mechanisms of stem cell differentiation and tissue generation is critical, and robust methodologies and different technologies are required. Towards establishing improved understanding and optimised triggering and control of differentiation processes, analytical techniques such as flow cytometry, immunohistochemistry, reverse transcription polymerase chain reaction, RNA in situ hybridisation analysis, and fluorescence-activated cell sorting have contributed much. However, progress in the field remains limited because such techniques provide only limited information, as they are only able to address specific, selected aspects of the process, and/or cannot visualise the process at the subcellular level. Additionally, many current analytical techniques involve the disruption of the investigation process (tissue sectioning, immunostaining) and cannot monitor the cellular differentiation process in situ, in real-time. Vibrational spectroscopy, as a label-free, non-invasive and non-destructive analytical technique, appears to be a promising candidate to potentially overcome many of these limitations as it can provide detailed biochemical fingerprint information for analysis of cells, tissues, and body fluids. The technique has been widely used in disease diagnosis and increasingly in stem cell technology. In this work, the efforts regarding the use of vibrational spectroscopy to identify mechanisms of stem cell differentiation at a single cell and tissue level are summarised. Both infrared absorption and Raman spectroscopic investigations are explored, and the relative merits, and future perspectives of the techniques are discussed.

Adsorption of Sodium Dodecyl Sulfate at Water-Dodecane Interface in Relation to the Oil in Water Emulsion Properties.

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.

Effect of the Incorporation of Nanosized Titanium Dioxide on the Interfacial Properties of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine Langmuir Monolayers.

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.

Hydrophobic Silica Nanoparticles Induce Gel Phases in Phospholipid Monolayers.

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.

Two-dimensional DPPC based emulsion-like structures stabilized by silica nanoparticles.

We studied the mechanical and structural properties of mixed surface layers composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and silica nanoparticles (NPs). These layers are obtained by spreading a DPPC Langmuir monolayer on a colloidal silica dispersion. The transfer/incorporation of NPs into the DPPC monolayer, driven by electrostatic interactions, alters the molecular orientation, the mechanisms of domain formation, and consequently the phase behavior of the surface layer during compression. The investigation of these systems by means of complementary techniques (Langmuir trough, fluorescence microscopy, ellipsometry, and scanning electron microscopy (SEM)) shows that the incorporated NPs preferentially distribute along the liquid expanded phase of DPPC. The layer assumes the stable and homogeneous bidimensional structure of a two-dimensional (2D) analogue of a Pickering emulsion. In fact, the presence of particles provides a circular shape to the DPPC domains and stabilizes them against growth and coalescence during the monolayer compression.

Timing of transition between capillaroscopic patterns in systemic sclerosis.

OBJECTIVE: To investigate the timing of transition through different patterns of nailfold microvascular damage in patients with systemic sclerosis (SSc). METHODS: In this medium-term longitudinal study, 38 SSc patients (median disease duration 12 months) with the early scleroderma pattern of microangiopathy seen on baseline nailfold videocapillaroscopy (NVC) were followed up by NVC for a median of 84 months. The evolution of the NVC pattern over time was monitored and recorded. RESULTS: At the end of followup, the NVC pattern was still that of early scleroderma in 47% of the patients. The active scleroderma pattern was seen in 34%, the late scleroderma pattern in 13%, and a normal pattern in 5%. The mean± SD time of progression from the early to the active pattern and from the early to the late pattern was of 28 ± 20 months and 36± 29 months, respectively. In the subgroup of patients whose microangiopathy progressed from the early to the late NVC pattern, the time of progression from the early to the active pattern was only 8± 1 months (P = 0.01), demonstrating that there is a subset of patients with rapid progression of microangiopathy. Clinical symptoms progressed in accordance with the nailfold morphologic changes in 60% of the SSc patients. CONCLUSION: The results of this longitudinal study demonstrate dynamic transition of microvascular damage through different NVC patterns of microangiopathy in ∼50% of SSc patients. It is recommended that patients exhibiting rapid progression from the early to the active NVC pattern (<1 year) should be monitored closely, since the evidence suggests that they are at risk of rapid progression to the advanced (late) NVC pattern of microangiopathy that is associated with further clinical manifestations of SSc.

Properties and structure of interfacial layers formed by hydrophilic silica dispersions and palmitic acid.

The properties and structure of different types of interfacial layers obtained from aqueous dispersions of nanometric silica and palmitic acid (PA) have been studied and characterized by different diagnostics and measurements. The investigations concern PA monolayers spread on the silica dispersions, dispersions in contact with PA solutions in oil and silica dispersions containing PA, aiming at elucidating the role of the PA interaction with the particles and investigating the surface-activity of the originated silica-PA complexes. Drop shape tensiometry was utilized to measure the dynamic surface and interfacial tension while a Langmuir trough apparatus was used to obtain compression isotherms of the spread PA layers and to measure the dilational viscoelasticity according to the oscillating barrier method. Brewster angle microscopy and ellipsometry were utilized to investigate the lateral and vertical structure of the interfacial layers. From this multifold approach emerges a complex picture of the features of these interfacial layers that can be rationalized on the basis of the adsorption of PA on the particle surface. The results evidence a threshold in PA adsorption above which particles change from hydrophilic to partially hydrophobic, promoting their incorporation into the interfacial layer.

Hydrophobisation of Silica Nanoparticles Using Lauroyl Ethyl Arginate and Chitosan Mixtures to Induce the Foaming Process.

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.

Emulsification and emulsion stability: The role of the interfacial properties.

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.

Recent developments in emulsion characterization: Diffusing Wave Spectroscopy beyond average values.

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.

The Role of Endogenous Proteins on the Emulsification of Silicone Oils Used in Vitreoretinal Surgery.

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.

Optical observation of high-frequency drop oscillations by a spectrum compression technique applied to the capillary pressure tensiometry.

Image acquisition and subsampling of periodic high-frequency drop oscillations is presented as an advantageous metrological procedure in capillary pressure tensiometry (CPT). The observation of a finite sequence of single tone or of multiharmonic cycles, subsampled in an expanded time-scale interval, allows the characteristics of the real oscillations to be well-reconstructed in a frequency-compressed spectrum, where each component is translated toward lower frequencies. The introduced technique is applied to nanoliter-sized water drops, oscillating in a hydrocarbon matrix up to 150 Hz frequency, by using a standard PAL CCD camera provided with an electronic shutter. Application examples show the important role of this technique in data analysis and interpretation of typical high-frequency oscillating drop/bubble experiments. In particular, this technique is effective to check the onset of critical hydrodynamic effects and allows for the determination of the intrinsic elasticity of the liquid/cell system as a function of frequency by comparison of the liquid volume, as displaced by a piezo-actuator, and the actually observed drop volume-amplitude oscillation. The knowledge of this quantity is fundamental for the calculation of the dilational viscoelasticity from the acquired pressure data in the CPT.

Surface properties and foamability of saponin and saponin-chitosan systems.

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.

Effect of nanoparticles on the interfacial properties of liquid/liquid and liquid/air surface layers.

An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant. These properties are the result of different phenomena such as the particle attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid interfacial environment, all those phenomena are closely correlated. The equilibrium and dynamic interfacial tensions of the liquid/air and liquid/oil interfaces have been measured as a function of the surfactant and particle concentration. The interfacial rheology of the same systems has been also investigated by measuring the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particle-surfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface and by the surfactant redistribution between the particle and fluid interface.

Influence of surface processes on the dilational visco-elasticity of surfactant solutions.

The mechanical properties of liquid-fluid systems, like the dynamic interfacial tension and interfacial rheology are closely related to the kinetic processes involved and to the behaviour of the adsorbed molecules. Therefore, provided suitable models and experimental methods are set, investigating these properties allows qualitative and quantitative information on these processes to be drawn. This paper presents recent developments in dilational rheology of liquid-fluid adsorption layers, including experimental methods, models and experimental data concerned with surfactants undergoing transformations in the adsorption layer. Models account both for relaxation due to surfactant diffusion and to processes internal to the adsorption layer. In particular surfactant reorientation, aggregation phase transitions and interfacial chemical reactions have been considered as possible reorganisation processes. The presented approach, allows the dilational viscoelasticity to be derived as a function of the perturbation frequency and of the equilibrium and kinetic parameters of the system. The results can also be easily specified for insoluble monolayer. The principal experimental techniques are reviewed and the recent progresses in the implementation of an Oscillation Bubble/Drop method for Capillary Pressure Tensiometer are discussed in detail. Two experimental studies of surfactants characterised by re-orientation and aggregation phase transition are presented. Beside providing a wider comprehension of these mechanisms, the interpretation of the dilational visco-elasticity data, according to the developed models, allows the effective estimation of the equilibrium and kinetic parameters.