Surfactant Film Research Articles

Relationship between Rheological Properties and One-Step W/O/W Multiple Emulsion Formation

Formation of a normal (not temporary) W/O/W multiple emulsion via the one-step method as a result of the simultaneous occurrence of catastrophic and transitional phase inversion processes has been recently reported. Critical features of this process include the emulsification temperature (corresponding to the ultralow surface tension point), the use of a specific nonionic surfactant blend and the surfactant blend/oil phase ratio, and the addition of the surfactant blend to the oil phase. The purpose of this study was to investigate physicochemical properties in an effort to gain a mechanistic understanding of the formation of these emulsions. Bulk, surface, and interfacial rheological properties of adsorbed nonionic surfactant (CremophorRH40 and Span80) films were investigated under conditions known to affect W/O/W emulsion formation. Bulk viscosity results demonstrated that CremophorRH40 has a higher mobility in oil compared than in water, explaining the significance of the solvent phase. In addition, the bulk viscosity profile of aqueous solutions containing CremophorRH40 indicated a phase transition at around 78 ± 2 °C, which is in agreement with cubic phase formation in the Winsor III region. The similarity in the interfacial elasticity values of CremophorRH40 and Span80 indicated that canola oil has a major effect on surface activity, showing the significance of vegetable oil. The highest interfacial shear elasticity and viscosity were observed when both surfactants were added to the oil phase, indicating the importance of the microstructural arrangement. CremophorRH40/Span80 complexes tended to desorb from the solution/solution interface with increasing temperature, indicating surfactant phase formation as is theoretically predicted in the Winsor III region. Together these interfacial and bulk rheology data demonstrate that one-step W/O/W emulsions form as a result of the simultaneous occurrence of phase-transition processes in the Winsor III region and explain the critical formulation and processing parameters necessary to achieve the formation of these normal W/O/W emulsions.

What can we learn from combined SAXS and SANS measurements of the same sample containing surfactants?

We review the information gained when studying the same sample containing surfactants by SANS and SAXS. Specific information can be obtained if intensities I(q) are determined and used on absolute scale as well as over a large q-window, thus extending to the «Porod limit» asymptotic range. Comparing SAXS and SANS peak positions (when present) allows unambiguous identification of structure factors separated from underlying form factors. Absolute scale refers to scattering cross-section while resolution relates to the qmax/qmin ratio. Taking into account "external" knowledge of molecular volumes in constrained fitting from explicit models gives better results on aggregation numbers as well as on interfacial thickness than considering only Patterson functions.We assemble in this review results on adsorption isotherms on surfactant films as well as equations of state, quantifying colloidal interactions. Identification of the topological origin of swelling behaviour are made possible by using combined SAXS and SANS on the largest possible q-range. Best results using this general methodology up to now were obtained by considering data separated from background up to qmax= 0.6 Å−1 − 0.8 Å−1.

Monovalent Cations Trigger Inverted Bilayer Formation of Surfactant Films

We monitored single-layer Langmuir-Blodgett films of behenic acid deposited on silanized glass or silicon substrates by atomic force microscopy (AFM) in liquid. We observed the in situ transformation of the monolayer to a bilayer when the surrounding solution was NaOH or KOH with pH > 8.3. The final state is that of an inverted bilayer, in which both the hydrophobic OTS (octadecyltrichlorosilane) and the alkane chains are exposed to the surrounding solution, defying common intuition based on hydrophobic-hydrophilic energy considerations. Strong sodium-containing carboxylic dimers formed between the headgroups are shown to be responsible for the stabilization of this configuration; calcium ions slow down/inhibit the transformation.

Dynamics within surfactant monolayers

Water-air interfaces are seldom clean, and frequently host monolayers of surface-active (“surfactant”) molecules. In lowering interfacial energy (surface tension), surfactants play key roles in enhanced oil recovery and in mixing, blending, and stabilizing multiphase materials. Surfactant soaps keep us clean, and surfactant monolayers allow us to breathe. Despite their molecularly thin nature, surfactant films can have a profound influence on the behavior of free fluid surfaces. Additionally, molecules within the monolayer can form ordered phases, yet still flow like two-dimensional liquids. Properly visualized,1 such monolayers can reveal extraordinarily beautiful structures. We investigate the flow response of the phospholipid surfactant dipalmitoylphosphatidylcholine (DPPC), which is a significant component of our lung surfactant monolayers, as well as our cells' lipid bilayers. We microfabricate micron-scale ferromagnetic, amphiphilic disks [Fig. ​[Fig.1a],1a], which we place on a water-DPPC-air interface [Fig. ​[Fig.1b]1b] and torque with external electromagnets [Fig. ​[Fig.1c].1c]. Notably, we can visualize the interface directly while we torque the disk. Figure 1 (a) Scanning electron micrograph of an amphiphilic, ferromagnetic microdisk (20μm diameter) with 5μm diameter buttonholes. (b) Schematic of a ferromagnetic microdisk probe at the water-DPPC-air interface and (c) bright-field image of an ... Figure ​Figure1d1d shows a disk within a DPPC monolayer in the liquid-condensed (LC) phase, which consists of a packed layer of individual, 10μm-scale grains, each of which contains a two-dimensional fluid with liquid-crystalline orientational order. The grain structure is visualized with a small fraction (0.1 mol. %) of Texas red-labeled 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DHPE), which cannot pack within the liquid-crystalline domains and is thus forced to the grain boundaries.1 Under steady disk rotation, the monolayer partitions into two distinct regions: a static outer region, where domains show a steady elastic deformation, and an inner “yielded” region, which flows freely [Fig. ​[Fig.1e].1e]. The surface yielding behavior exhibited by the LC-DPPC monolayer is reminiscent of its three-dimensional analog, as when butter is spread.

Physicochemical studies on the interaction of serum albumin with pulmonary surfactant extract in films and bulk bilayer phase

Functionality, structure and composition of the adsorbed films of bovine lipid extract surfactant (BLES), in the absence and presence of bovine serum albumin (BSA), at the air-buffer interface was characterized through surface tension, atomic force microscopy and time of flight secondary ion mass spectrometric methods. Gel and fluid domains of BLES films were found to be altered significantly in the presence of BSA. Differential scanning calorimetric studies on BLES dispersions in presence of BSA revealed that the perturbations of the lipid bilayer structures were significant only at higher amount of BSA. FTIR studies on the BLES dispersions in buffer solution revealed that BSA could affect the lipid head-group hydrations in bilayer as well as the methylene and methyl vibration modes of fatty acyl chains of the phospholipids present in BLES. Serum albumin could perturb the film structure at pathophysiological concentration while higher amount of BSA was required in perturbing the bilayer structures. The studies suggest a connected perturbed bilayer to monolayer transition model for surfactant inactivation at the alveolar-air interface in dysfunctional surfactants.

Interfacial dilational properties of model oil and chemical flooding systems by relaxation measurements

In the present work, the influences of a model organic acid (oleic acid) on the interfacial dilational properties of different types of surfactants (anionic surfactant sodium 4,5-diheptyl-2-propylbenzene sulfonate 377 and asymmetrical anionic Gemini surfactant C 12COONa-p-C 9SO 3Na) and polymers (HPAM and HMPAM) systems were studied at decane–water interface, respectively, by means of interfacial tension relaxation measurements. The decay curves of interfacial tension were fitted by the summation of a number of exponential functions. The dilational elasticity and dilational viscosity component were calculated by Fourier transform and displayed as Cole–Cole plots (plotting ɛ i or ɛ i / ɛ 0 as a function of ɛ r or ɛ r / ɛ 0, respectively). The experimental results show that the interfacial behaviors of both surfactants are part conformance with a diffusion-controlled process and part conformance with a reorientation controlled process. In the case of partly hydrolyzed polyacrylamide Mo-4000, the nature of mixed film is controlled mainly by interaction between polymer chains and surfactant molecules in the interface. As for hydrophobically modified polyacrylamide, the film properties are controlled by the formation of mixed surfactant-hydrophobic block associations in the interface and the reorientation process resulted from exchange of associations between different regions of interface. Under our experimental conditions, the mixed adsorption films formed by oleic acid and surfactant molecules are more compact than pure surfactant films.

Association of Titania with Nonionic Block Copolymers in Ethanol: The Early Stages of Templating and Film Formation

We have prepared self-supporting, free-standing titania-surfactant mesostructured films via spontaneous growth on the surface of ethanolic solutions, as an alternative synthetic route to evaporation induced self-assembly. The initial stages of surfactant templating and interfacial film formation in alcoholic solutions of titania with a polyethylene−poly(ethylene glycol) surfactant have been observed by small angle neutron scattering and Brewster angle microscopy. Variation of parameters including the titania precursor concentration, acid concentration, and surfactant concentration allowed formation times and pathways to be probed. These time-resolved observations of titania development represent a novel achievement in formation studies and have allowed a formation mechanism for titania−surfactant films to be proposed. Micelles in solution undergo an initial slow accumulation of titania, followed by rapid growth of a titania shell, followed again by a slow growth period, and these species accumulate at the...

Lipid environment determines the interfacial behaviour of pulmonary surfactant films in the presence of hydrophobic proteins SP-B and SP-C

Lipid environment determines the interfacial behaviour of pulmonary surfactant films in the presence of hydrophobic proteins SP-B and SP-C

A new cleaning process combining non-ionic surfactant with diamond film electrochemical oxidation for polished silicon wafers

This paper presents a new cleaning process for particle and organic contaminants on polished silicon wafer surfaces. It combines a non-ionic surfactant with boron-doped diamond (BDD) film anode electrochemical oxidation. The non-ionic surfactant is used to remove particles on the polished wafer's surface, because it can form a protective film on the surface, which makes particles easy to remove. The effects of particle removal comparative experiments were observed by metallographic microscopy, which showed that the 1% v/v non-ionic surfactant achieved the best result. However, the surfactant film itself belongs to organic contamination, and it eventually needs to be removed. BDD film anode electrochemical oxidation (BDD-EO) is used to remove organic contaminants, because it can efficiently degrade organic matter. Three organic contaminant removal comparative experiments were carried out: the first one used the non-ionic surfactant in the first step and then used BDD-EO, the second one used BDD-EO only, and the last one used RCA cleaning technique. The XPS measurement result shows that the wafer's surface cleaned by BDD-EO has much less organic residue than that cleaned by RCA cleaning technique, and the non-ionic surfactant can be efficiently removed by BDD-EO.

Towards a comprehensive computational model for the respiratory system

AbstractThis paper is concerned with computational modeling of the respiratory system against the background of acute lung diseases and mechanical ventilation. Conceptually, we divide the lung into two major subsystems, namely the conducting airways and the respiratory zone represented by lung parenchyma. Owing to their respective complexity, both parts are themselves out of range for a direct numerical simulation resolving all relevant length scales. Therefore, we develop detailed individual models for parts of the subsystems as a basis for novel multi‐scale approaches taking into account the unresolved parts appropriately. In the tracheobronchial region, CT‐based geometries up to a maximum of approximately seven generations are employed in fluid–structure interaction simulations, considering not only airway wall deformability but also the influence of surrounding lung tissue. Physiological outflow boundary conditions are derived by considering the impedance of the unresolved parts of the lung in a fully coupled 3D‐1D approach. In the respiratory zone, an ensemble of alveoli representing a single ventilatory unit is modeled considering not only soft tissue behavior but also the influence of the covering surfactant film. Novel nested multi‐scale procedures are then employed to simulate the dynamic behavior of lung parenchyma as a whole and local alveolar ensembles simultaneously without resolving the alveolar micro‐structure completely. Copyright © 2010 John Wiley & Sons, Ltd.

Solitary Structures Sustained by Marangoni Flow

We construct interfacial solitary structures (spots) generated by a bistable chemical reaction or a non-equilibrium phase transition in a surfactant film. The structures are stabilized by Marangoni flow that prevents the spread of a state with a higher surface tension when it is dynamically favorable. In a system without surfactant mass conservation, a unique radius of a solitary spot exists within a certain range of values of the Marangoni number and of the deviation of chemical potential from the Maxvell construction, but multiple spots attract and coalesce. In a conservative system, there is a range of stable spot sizes, but solitary spots may exist only in a limited parametric range, beyond which multiple spots nucleate. Repeated coalescence and nucleation leads to chaotic dynamics of spots observed computationally in Ref. .

ChemInform Abstract: 2H NMR in Liquid Crystals and Membranes

Abstract Deuterium NMR spectroscopy is widely applicable to studies of the structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids. The properties of soft nanomaterials are also accessible on the mesoscopic length scale intermediate between the molecular and bulk dimensions. For membrane lipids in the liquid-crystalline state, rapid axial averaging occurs about the director axis (the membrane normal). One can then relate the profiles of the order parameters | S CD | of the individual C– 2 H labeled segments to average bilayer properties. These include the mean area per molecule and projected acyl chain length, the area compressibility modulus, and the radius of curvature for reverse hexagonal ( H II ) phase nanotubes. In addition, measurements of the relaxation rates for Zeeman order, R 1Z , and quadrupolar order, R 1Q , enable one to investigate the mean-squared amplitudes and time-scales of the fluctuations that underlie the thermodynamic properties. A unified interpretation is provided by a composite membrane deformation model, which fits simultaneously the frequency dependence and the angular anisotropy of the R 1Z and R 1Q relaxation rates. The results suggest the bilayer dynamics in the MHz regime can be modeled in terms of nematic-like deformations of the membrane hydrocarbon interior, together with axial rotations of the lipid acyl chains. A small contribution from internal segmental motions is found, which implies the bilayer microviscosity is comparable to that of a liquid hydrocarbon. Finally, the 2 H-NMR relaxation rates of lipid bilayers containing cholesterol in the liquid-ordered phase suggest a dynamically more rigid bilayer, involving fast axial lipid rotations together with a reduction in collective bilayer deformations. Possible future applications include studies of liquid crystals and thin films of membrane lipids and surfactants, as well as lipid-protein systems.

Nucleation of an Oil Phase in a Nonionic Microemulsion-Containing Chlorinated Oil upon Systematic Temperature Quench

A clear and stable nonionic model microemulsion consisting of pentaoxyethylene glycol dodecyl ether (C(12)E(5)), water, and 1-chlorotetradecane (CLTD) was prepared. This system was subjected to a systematic temperature quench (perturbation out of equilibrium) in steps of 1.0 degrees C from 20.4 to 15.3 degrees C in the unstable region of its phase diagram. The change in turbidity (for droplet volume fractions of 0.02 and 0.08) and hydrodynamic radius (R(h)) (for a droplet volume fraction of 0.02) of the system on its way to its new equilibrium was measured at each quench temperature. For small systematic temperature quenches just below the emulsification failure boundary (EFB) the turbidity decreases and remains constant indicating quick changes in the microstructures. Further lowering of temperature brings the system to the unstable region where the turbidity and light scattering increase sharply as function of time because of expulsion of excess oil from the microemulsion droplets. The newly formed oil-rich droplets grow in size as a function of time. These observations indicate the existence of a narrow but observable metastable region en route to the new equilibrium where both microemulsion droplets and larger oil-rich droplets coexist. The region in which microemulsion droplets are metastable is very narrow and is concentration-dependent. The presence of a metastable region is as for other similar systems attributed to the presence of a free energy barrier for the formation of the larger oil-rich droplets associated with curvature free energy of the surfactant film. The turbidity-time curves were converted to the radius-time curves using a model assuming monodisperse spherical droplets. The obtained results are in good agreement with the results for the hydrodynamic radius. The observed average radius from both type of measurements decreases in the metastable region. By performing calculation of the influence of eccentricity and size polydispersity on the observed radius, we have shown that the distribution of the microemulsion droplets becomes more homogeneous in the metastable region.

Pulmonary Toxicity of Polysorbate-80-coated Inhalable Nanoparticles; In vitro and In vivo Evaluation

Using inhalable nanoparticles (NPs) to treat pulmonary diseases is an emerging field (1). Lungs as part of the mononuclear phagocyte system (MPS) are extensively rich with alveolar macrophages (2). These alveolar macrophages are part of the body’s immune system and responsible for clearing inhaled foreign objects from the alveolar spaces (3), which cause a substantial portion of the inhaled NPs to be cleared after inhalation once NPs are settled in the alveolar spaces (4). Clearance of NPs by macrophages is not exclusive to inhalable NPs; as it is associated with other routes of administrations, such as intravenous i.v. (5). In fact, the failure of early attempts of using NPs for cancer targeting was mainly attributed to the interference of macrophages of the MPS. The early work showed that after i.v. injection, NPs were cleared immediately by phagocytotic cells, specifically by macrophages and Kuppfer cells of the liver (6). Thus, in order to evade macrophages and increase NPs circulation, a new strategy was implemented by changing the NPs surface properties (5). This was accomplished by using hydrophilic surfactants, such as polysorbate 80 to coat the NPs surface (7). Hydrophilic coating of NPs changes the physicochemical characteristics of NPs. This inhibits opsonization, which is the precipitation of antibodies and other proteins on the surface of NPs (8). This strategy was shown to successfully prolong the circulation time of NPs and also enhanced the distribution of NPs to some organs such as the brain (9). Using coated NPs for pulmonary delivery may be associated with increased bioavailability since inhaled NPs may have a higher chance to evade alveolar macrophages. However, the delicate composition of the alveolar space should stay unchanged and kept intact after delivery of any inhalable treatment including NPs. In the alveolar space, a thin film of lung surfactant covers the epithelium. It is composed of phospholipids and proteins (10). The main component of the film is 1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine (DPPC), a phosphatidylcholine with two saturated 16-carbon long fatty acyl chains (11). This phospholipid is a key factor for controlling the surface tension and the gas exchange process in the alveolar sacs (12). Moreover, the ability of the lungs to function properly depends on the integrity of this surfactant layer (13). In vitro, lung-surfactant model systems have been developed to study the impact of NPs on the surfactant layer (14). Changes in the surface pressure-area isotherms induced by NPs were recorded and considered as an indicator for interactions of NPs and components of the surfactant model. Previous studies investigated the effect of non-coated NPs on the integrity of the lung surfactant film in vitro (10,11,14). Azarmi et al. showed in vivo that non-coated NPs are well tolerated by Balb/c nude mice after pulmonary administration (15). However, the impact of coated NPs on the integrity of the surfactant film is yet unknown. Therefore, the aim of this study was to investigate the biophysical interactions of polysorbate-80-coated NPs with DPPC monolayer film in vitro. The results were compared with in vivo observations.

On the intensification of decimeter-range wind waves in film slicks

The physical influence of the intensification of decimeter waves on a water surface covered by a surfactant film is proposed. The damping of a long wave caused by the loss of its energy on the modulation of short (centimeter) wind waves is analyzed. The damping coefficient of waves of decimeter-meter lengths is calculated, and this coefficient is shown to decrease in the presence of a surfactant film. The strong suppression of the spectrum of centimeter wind waves in a film slick is the physical factor responsible for such a decrease. Due to a decrease in the damping coefficient of decimeter-meter waves, their intensity in the slick increases. Within the framework of a model of the spectrally local energy balance of wind waves, the results of calculations satisfactorily agree with the experiment.

Kelvin probe force microscopy in application to biomolecular films: Frequency modulation, amplitude modulation, and lift mode

Kelvin probe force microscopy (KPFM) is a powerful technique to visualize the differences of work function in metals and lateral surface potential distribution in thin organic films. Earlier we have shown that frequency modulated-Kelvin probe force microscopy has significant advantages in both sensitivity and resolution when applied to metal and inorganic interfaces in vacuum. High resolution, high sensitivity, and performance in ambient conditions are required in order to study biologically relevant samples. In this work we compared the resolution of frequency modulation (FM-KPFM), amplitude modulation (AM-KPFM), and lift modes KPFM for imaging the local electrical surface potential of complex biomolecular films and demonstrated that FM-KPFM mode has superior resolution for biological applications. The power of the method was illustrated on pulmonary surfactant films, revealing nm spatial resolution and mV potential sensitivity in ambient air. At this level of resolution this method can provide critical insight into the molecular arrangement and function of complex biosystems in a way that other KPFM modes cannot do. Based on the observed changes in the local surface potential we discovered that excess cholesterol produces nm size electrostatic domains and change the electric fields.

Alkaline/Surfactant/Polymer Processes: Wide Range of Conditions for Good Recovery

Summary Design of an alkaline/surfactant/polymer (ASP) process requires knowledge of the amount of soap formed under alkaline conditions from naphthenic acids in the crude oil. We show here for several crude oils that, when substantial acid is present, the acid number determined by nonaqueous-phase titration is approximately twice that found by hyamine titration of a highly alkaline aqueous phase used to extract soaps from the crude oil. This acid number by soap extraction should provide a better estimate than nonaqueous-phase titration because the extracted soap interacts with the injected surfactant to form surfactant films and microemulsion droplets during an ASP process. In a previous paper (Liu et al. 2008), an unusually wide range of salinities of ultralow oil/water interfacial tensions (IFTs) was found for one alcohol-free crude-oil/anionic-surfactant system under alkaline conditions where naphthenic soaps were present. Solubilization results indicate that this favorable behavior exists with the same surfactant blend and another crude oil. In the same paper, a 1D simulator for the ASP process was presented. Here, this ASP simulator has been used for various acid contents, injected-surfactant concentrations, slug sizes, and salinities to show that high recoveries of waterflood residual oil (> 90%) can be expected for a wide range of near-optimal (Winsor III) and underoptimum (Winsor I) conditions for a constant-salinity process, even with relatively small slug sizes. A key factor leading to this good performance is development of a gradient in soap/surfactant ratio, which ensures that a displacement front with ultralow IFT forms and propagates through the formation. Similar high recoveries can be attained for certain Winsor II conditions but only for much larger slug sizes, owing to the tendency for surfactant to partition into the oil phase and become retarded. Large dispersion, such as might be expected for field conditions, can reduce recovery significantly for small surfactant slugs even for near-optimal and underoptimum conditions. However, this problem can be overcome by injecting the slug or drive at salinities below reservoir salinity, thereby creating a salinity gradient.

Submicron droplet formation in the human lung

The exhaled breath of humans contains droplets originating from the lung lining fluid. An analysis of these droplets for non-volatile proteinaceous biomarkers holds potential as a non-invasive diagnosis of lung diseases. To ease the interpretation of the diagnostic results, the source strength of the particles should be known und therefore an understanding of the particle generation process is required. It is assumed that during reopening of a collapsed terminal airway a liquid bridge of the lung lining fluid ruptures and droplets are generated. The objective of our experimental and theoretical study was to clarify the mechanisms of droplet generation for quiet breathing patterns by investigating in detail the number flux and the particle size distribution in the exhaled breath. The process of liquid film rupture is modelled by computational fluid dynamics analysis from which the droplet size distribution is calculated. In addition the number emission flux and the droplet size distribution are systematically measured in the exhaled breath of healthy volunteers. The strong increase of the particle emission flux with tidal volume and the good agreement between measured and calculated droplet number distribution both showing droplets primarily in the submicron range confirm the present hypothesis that reopening of collapsed airway structures associated with the rupture of a surfactant film is the physical mechanism of droplet generation. This was hypothesized previously in the literature.

Dynamics in water-AOT-n-decane microemulsions with poly(ethylene glycol) probed by dielectric spectroscopy

Water in oil microemulsions, consisting of water, AOT and n-decane, have been used as a model system to investigate the influence of the water soluble polymer PEO on the dynamical behavior of the system. Therefore dielectric relaxation spectroscopy and conductivity, extracted from dielectric spectroscopy, measurements in a wide frequency and temperature range have been applied. The pure microemulsion displays the known phenomenon of percolation that manifests in a steep increase of conductivity at the percolation temperature \(T_\text{P}\). The percolation temperature has been found to be strongly dependent on droplet volume fraction and droplet size. The latter additionally shows that percolation temperature and surfactant film rigidity are proportional. Far from percolation water-AOT-n-decane microemulsions display two dielectric relaxations. The slower one has a relaxation time of \(\tau \approx 3\cdot 10^{-6}~\text{s}\) and can be related to an interfacial polarization at the interface of the water core and the AOT shell (core relaxation). The faster one has a relaxation time of \(\tau \approx 10^{-9}~\text{s}\) and can be related to the ions in the AOT shell(shell or cluster relaxation). While the first is mainly untouched by the percolation phenomenon, the latter undergoes a slowdown and an increase of relaxation strength, both over about two decades, on approaching the percolation transition. Addition of PEO tremendously shifts the percolation transition to higher temperatures, due to adsorption at the AOT layer which leads to an increase in rigidity. Furthermore a lower phase boundary temperature evolves, below which the microemulsion phase separates. The conductivity of the microemulsion is also slightly increased with polymer. The effect on the dielectric properties is only small, where dielectric relaxation times are reduced by the polymer, while only the relaxation strength of the faster relaxation is influenced and also decreases with polymer. The decreased relaxation time of core relaxation can be either due to changes in the core to shell volume ratio or an increased conductivity of the water core. The decrease in relaxation time and strength of the shell relaxation suggest that the ion mobility in the shell increase, while the dipole moment is reduced. Additionally we applied a cluster relaxation model proposed by Cametti et al. (Phys Rev Lett 75(3):569, 1995) and Bordi et al. (J Phys, Condens Matter 8:A19, 1996) to estimate the cluster size evolution.

Encapsulated-polyoxometalates in surfactant/silica gel hybrid films: Electrochemical behavior and main characteristics

Immobilization of various polyoxometalates (POMs) of different sizes and charges in surfactant/silica gel hybrid films is investigated. A change in the electrochemical behavior is noticed between dissolved and immobilized POMs. This change is attributed to the microenvironment effect that POM clusters experience inside the films. The presence of surfactant Triton X-405 in the matrix is essential for these observations. Results from SEM showed rough and porous morphology for the films prepared with surfactant, while smooth and fractured film structure is observed in absence of surfactant. For POMs such as [P 2W 18O 62] 6− and [SiW 12O 40] 4−, the first two one-electron redox waves merged into one two-electron redox wave. However, for other POMs such as [PW 12O 40] 3−, [H 4AsW 18O 62] 7−, [P 2W 15Mo 2V IVO 62] 7−, α 1-[P 2W 17O 61Fe III] 7−, α 2-[P 2W 17VO 62] 8− and [P 8W 48O 184] 40−, the microenvironment effect results only in a shift of the potentials towards the negative values. The magnitude of the shift in potential is shown to decrease with increasing the POM's charge. Results from Particle size analysis showed that Triton X-405 influences the POM's particle size and with increasing the POM's charge, the POM's particle size in presence of Triton X-405 decreases. The latter is attributed to formation of micelles POM-Triton X-405, which consequently create the microenvironment effect for the immobilized POMs. The main characteristics of the surfactant/silica gel immobilized POM films are explored. The influence of the scan rate demonstrates that the redox behavior of the surfactant/silica gel immobilized composites is fast and involves surface-confined electron transfer processes. The study of these films in different proton concentrations showed that they are stable at low pH, and to obtain a better electron transfer, they have to be stored in the reference medium of 0.5 M H 2SO 4.