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.

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.

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.

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.

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.

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.

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.