Neutron and X-ray scattering studies of cholera toxin interactions with lipid monolayers at the air-liquid interface

Using neutron/X-ray reflectivity and X-ray grazing incidence diffraction (GID), we have characterized the structure of mixed DPPE:GM₁ lipid monolayers before and during the binding of cholera toxin (CTAB₅) or its B subunit (CTB₅). Structural parameters such as the density and thickness of the lipid layer, extension of the GM₁ oligosaccharide headgroup, and orientation and position of the protein upon binding are reported. Both CTAB₅ and CTB₅ were measured to have 50% coverage when bound to the lipid monolayer. X-ray GID experiments show that both the lipid monolayer and the cholera toxin layer are crystalline. The effects of X-ray beam damage have been assessed and the monolayer/toxin structure does not change with time after protein binding has saturated.     

Development and rheological investigation of novel alginate/N‐succinylchitosan hydrogels

In the present article alginate hydrogels and novel hydrogels based on blends of alginate/N‐succinylchitosan have been realized in water solution at neutral conditions. The gels have been obtained by crosslinking via the internal setting method using calcium carbonate (CaCO₃) as calcium ions source. A rheological investigation of both the plain alginate and the alginate/N‐succinylchitosan blend hydrogels has been performed by means of oscillatory dynamic measurements. The effect of the inclusion of different amounts of CaCO₃ on the critical deformation (γ_c) characterizing the limit of the linear viscoelastic regime has been studied for the plain alginate gels. The frequency response in small amplitude oscillatory experiments of the plain alginate gels has been investigated in terms of the storage (G′) and loss (G″) modulus behavior. The dynamic data have been interpreted in terms of the Friedrich and Heymann model. The inclusion of the N‐succinylchitosan, in the range 10–50% w/w, had no effect on the γ_c values. On the contrary, when the 10% w/w of the N‐succinylchitosan is added to the plain alginate gels, a significant increase in the storage modulus values is recorded for all the systems analyzed. The gelation kinetics has been investigated and the results indicate that the kinetics process can be accelerated increasing the percentage of Ca⁺² ions and/or including the N‐succinylchitosan in the plain alginate systems. Finally, the morphological analysis of scaffolds obtained from the hydrogels through freeze‐drying revealed an interconnected porous structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1167–1182, 2008     

Tuneable Control of Interfacial Rheology and Emulsion Coalescence

Breaking point: Switchable peptide surfactants are used to demonstrate that the extent of cross‐linking in an interfacial surfactant layer can control the rate of emulsion coalescence. Pictured is the rupture of an aqueous thin film where the peptide layer lacks sufficient strength to prevent hole formation, but nonetheless dramatically slows the rate of hole expansion.

     

Bidimensional mixtures of polymethylmethacrylate with fatty esters at water-air interface

Two-dimensional mixtures were studied between a polymer, polymethylmethacrylate (PMMA) which has at the water-air interface its chains distributed almost parallel to that interface, and two esters, methylstearate (MS) and methyloleate (MO), characterized by having the same chain length but a different interface orientation.It was shown that the main interactions occurring are those between the hydrophobic chains, and consequently that compatibility between the substances depends essentially on their having the same interface orientation.     

Consistency of surface mechanical properties of spread protein layers at the liquid–air interface at different spreading conditions

Π/A isotherms of spread β-lactoglobulin and β-casein at the air–water interface are measured under different spreading conditions. While the isotherms do not show drastic effects of the spreading concentration and the compression rate the interfacial shear rheological behaviour is significantly influenced. In particular, the shear viscosity of β-lactoglobulin layers depend directly on the spreading concentration. Significant viscosity increase is obtained at larger surface pressures when the spreading concentration is increased. In contrast the shear rheology of the spread β-casein layers can be normalised by plotting the viscosities as a function of the surface pressure Π. The different behaviour is discussed in terms of denaturation of the β-lactoglobulin during the monolayer formation process by adsorption from the spread thin protein solution layer.     

Synthesis and Characterization of Self-Assembling Some Thiol Surfactants on Gold Nanoparticles

The purpose of this work is to study the self-assembling of some synthesized thiol surfactants namely (mercaptopropane-, mercaptohexane-, mercaptooctane-, and mercaptodecane sodium sulfonate) on the fabricated gold nanoparticles. The self-assembling of these surfactants on gold nanoparticles characterized using different techniques such as FTIR spectroscopy, UV spectroscopy, and transmission electron microscopy (TEM). Spectroscopic evidence suggests that the synthesized thiol surfactants have been attached to the gold nanoparticles. The effect of self-assembling of these surfactants on the size of the gold nanoparticles was studied using TEM images. The growth of the gold nanoparticles was investigated with respect to the increase of alkyl chain in the synthesized thiol surfactants. The results show that the stabilization of gold nanoparticles was affected by the increase in alkyl chain length of these surfactants. The effect of gold nanoparticles on the interfacial tension and the emulsion stability of these surfactants with crude oil was studied.     

Interfacial dilational rheology properties of films formed at the oil/water interface by reaction between tetrameric acid and calcium ion

This article aims to determine the applicability of interfacial dilational rheology to study the formation of viscoelastic film at the oil/water interface by reaction between tetrameric acids ARN and calcium ions, and to determine the influence of asphaltenes and naphthenic acids (NA) on this film. It was first found that the formation of viscoelastic film by reaction between ARN and calcium ions is easily observed by dilational rheology: Significantly high values of E′ (130?mN/m) were measured for this system at low ARN concentration (10?µM). These values are at least 5 to 10 times higher than values obtained for ARN without Ca²⁺ or other crude oil components such as asphaltenes and naphthenic acids.

The influence of asphaltenes and NA on the viscoelastic film formation has been studied. When asphaltenes or NA are present, the interfacial viscoelastic film is weakened: There is a gradual decrease of E′ and E″ when the asphaltenes or NA concentration increases. These two components can therefore inhibit the ARN/Ca²⁺ film formation. This decrease is similar to the one previously observed by shear rheology. Several explanations are proposed.     

Thin surfactant layers at the solid interface

The formation of surfactant layers on solid surfaces is an important process in many industrial applications. The structure of these layers influences the properties of the solid in processing and use. We summarize the literature data on the structure of surfactant layers at the solid/liquid interface and our own results characterizing the interactions between solid surfaces and solutions or emulsions. Ideas are presented for the processes taking place at the solid surface during drying by heat treatment. An outlook is given of how to investigate surfactant layers on dry surfaces. Received: 11 July 1999/Accepted: 2 May 2000     

Carpetlike dense‐layer formation in a polyelectrolyte brush at the air/water interface

A carpetlike dense‐layer formation between a hydrophobic layer and a polyelectrolyte brush layer has been found in the monolayers of an ionic amphiphilic diblock copolymer, poly(1,1‐diethylsilacyclobutane)_m‐block‐poly(methacrylic acid)_n, on a water surface by an X‐ray reflectivity technique. By detailed analysis, we have found that the hydrophilic layer under the water is not a simple layer but is divided into two layers, that is, a carpetlike dense methacrylic acid (MAA) layer near the hydrophobic layer and a polyelectrolyte brush layer. We have also confirmed that a well‐established polyelectrolyte brush is formed only for the m:n = 43:81 polymer monolayer: For m:n = 40:10 and m:n = 45:60 polymer monolayers, only a dense MAA layer is formed. This dense‐layer formation should be the origin of the interesting hydrophobic‐layer thickness variation previously reported; The hydrophobic‐layer thickness takes a minimum as a function of the hydrophilic chain length at any surface pressure studied. An overview of the data for three samples with different chain lengths (m:n = 40:10, 45:60, or 43:81) has shown that the thickness of this dense layer is 10–20 Å and is independent of the surface pressure and polymerization degree of poly(methacrylic acid) (PMAA) in the range studied. This dense‐layer formation is explained by the reasonable speculation that contact with PMAA is thermodynamically more stable than direct contact with water for the diethylsilacyclobutane (Et₂SB) layer on water. In this sense, the dense layer acts like a carpet for the hydrophobic Et₂SB layer, and a 10–20‐Å thickness could be a critical value for the carpet. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1921–1928, 2003     

Mechanical properties of protein adsorption layers at the air/water and oil/water interface: A comparison in light of the thermodynamical stability of proteins

Over the last decades numerous studies on the interfacial rheological response of protein adsorption layers have been published. The comparison of these studies and the retrieval of a common parameter to compare protein interfacial activity are hampered by the fact that different boundary conditions (e.g. physico-chemical, instrumental, interfacial) were used. In the present work we review previous studies and attempt a unifying approach for the comparison between bulk protein properties and their adsorption films. Among many common food grade proteins we chose bovine serum albumin, β-lactoglobulin and lysozyme for their difference in thermodynamic stability and studied their adsorption at the air/water and limonene/water interface. In order to achieve this we have i) systematically analyzed protein adsorption kinetics in terms of surface pressure rise using a drop profile analysis tensiometer and ii) we addressed the interfacial layer properties under shear stress using an interfacial shear rheometer under the same experimental conditions. We could show that thermodynamically less stable proteins adsorb generally faster and yield films with higher shear rheological properties at air/water interface. The same proteins showed an analog behavior when adsorbing at the limonene/water interface but at slower rates.     

Tailoring the interfacial assembly of colloidal particles by engineering the mechanical properties of the interface

Fluid interfaces can be used as a platform for promoting the direct and spontaneous self-assembly of colloidal particles, where the driving force is the reduction in interfacial energy. In addition, fluid interfaces allow fine-tuning of the particles ensemble by an external force, such as the presence of an imposed interfacial flow, or by engineering the interparticle interactions dictated by the interplay of interfacial forces. As a consequence, a wide-ranging set of interfacial structures can be achieved from liquid-like layers, which can flow under stress, to amorphous solids that are able to sustain static stress. Here, far from a comprehensive overview of the interfacial assembly of colloidal particles, different ways of tailoring it by rationally designing the rheological properties of the interface are provided, with a focus on experimental and theoretical methods and model systems that have been recently exploited. In particular, ligand-coated nanoparticles, with a strong emphasis on the effect of the ligands on the interfacial structure and the rheological properties, and soft nanogel particles, in which an environmental factor, such as the temperature, drives to different interfacial structures and mechanical responses will be further discussed.     

Synthesis and rheological investigation of self‐healable deferoxamine grafted alginate hydrogel

Deferoxamine grafted alginate (SA‐DFA) was successfully synthesized via amidation of sodium alginate with deferoxamine mesylate as determined by H‐NMR and elemental analysis. SA‐DFA with different graft yield was obtained by adjusting the ratio of sodium alginate and deferoxamine mesylate. It was found that aqueous solution of SA‐DFA could form hydrogel spontaneously due to hydrogen bonding interactions, which also endowed the SA‐DFA hydrogel with self‐healing capability. The healing efficiency of SA‐DFA hydrogels ranged from 53.64 to 90.16%. In addition, surface morphologies of SA‐DFA hydrogels before/after self‐healing process were demonstrated by SEM images. We anticipated that such self‐healable alginate hydrogel would be applied in the field of wound healing. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 856–865     

Macromolecular conformations at the water-air interface: Interactions between alpha and beta conformations of polypeptides

Bidimensional miscibility between alpha and beta conformations of polypeptides was investigated at the water-air interface in the 15°–30°C temperature range. The polypeptides were poly--methyl-L-glutamate (PGMG), poly--benzyl-L-glutamate (PGBG) and poly--benzyl-L-aspartate (PBBA). The polypeptide conformations, alpha or beta, were checked by IR spectroscopy using the MIR technique.The spreading isotherms for mixed monolayers alpha-PGMG/alpha-PGBG and beta-PGMG/beta-PBBA showed bidimensional miscibility both for alpha-alpha and beta-beta mixtures.For the alpha-alpha system, attractive interactions among the polypeptide alphahelices were found (G_mix<0) and the driving factor appeared to be the entropic one (packing). Compressibility moduli and surface potential measurements showed a fluidification effect of alpha-PGBG on mixed monolayers. In the case of beta-beta mixed monolayers, ideal behaviour was observed and no fluidification effect detected.Scanning electron micrographs made on collapsed monolayers showed hexagonal structures for alpha-alpha mixtures and no well-defined or characterized features for the beta-beta system.     

New terpyridine derivatives of thiacalix[4]arenes in solution and at the water-air interface

New di- and tetrasubstituted thiacalix[4]arenes in cone and 1,3-alternate configurations containing terpyridine and triazole fragments at the lower rim of the macrocycle, as well as model triazole-terpyridine, are synthesized. The structures and purity of the compounds are confirmed by NMR spectroscopy and high-resolution MALDI mass spectrometry. The compounds form luminescent complexes with lanthanide cations (europium and terbium) in a chloroform-methanol (10: 1) system during fluorimetric titration. The possibility of antenna ef ect during fluorescence sensitization in lanthanides by the thiacalixarene ligands is demonstrated. Successful immobilization of the terpyridine receptors on a solid substrate makes it possible to consider the Langmuir-Blodgett films on the basis of these receptors as promising adsorbents of lanthanide from nonaqueous media or organic salts and also provides new opportunities for the investigation of the luminescence characteristics of lanthanide complexes in thin films.

     

Formation of n-alkane layers at the vapor/water interface

We present a study on the initial wetting behaviors of two low molecular weight alkanes, heptane and octane, at the vapor/water interface using both neutron and X-ray reflectometry. Combined X-ray and neutron reflectivity studies data showed that a uniform film, which has never been reported, was formed continuously at 25 degrees C. As the adsorptive deposition continued, each adsorbed film was saturated at a specific equilibrium thickness: 48 and 36 A for deuterated heptane and octane, respectively, and 21 A for hydrogenated octane. The thickness of the adsorbed layer measured by neutron reflectivity is in agreement with that measured using X-ray reflectivity. Our observations of continuous and saturated adsorption behaviors are analyzed qualitatively using a kinetic adsorption model.     

Hydrogels from the Assembly of SAA/Elastin-Inspired Peptides Reveal Non-Canonical Nanotopologies

Peptide-based hydrogels are of great interest in the biomedical field according to their biocompatibility, simple structure and tunable properties via sequence modification. In recent years, multicomponent assembly of peptides have expanded the possibilities to produce more versatile hydrogels, by blending gelating peptides with different type of peptides to add new features. In the present study, the assembly of gelating P5 peptide SFFSF blended with P21 peptide, SFFSFGVPGVGVPGVGSFFSF, an elastin-inspired peptides or, alternatively, with FF dipeptide, was investigated by oscillatory rheology and different microscopy techniques in order to shed light on the nanotopologies formed by the self-assembled peptide mixtures. Our data show that, depending on the added peptides, cooperative or disruptive assembly can be observed giving rise to distinct nanotopologies to which correspond different mechanical properties that could be exploited to fabricate materials with desired properties.     

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air-water interface

The structural and dynamic characteristics of dioctadecyldimethylammonium bromide (DODAB) monolayers on a pure water subphase were investigated by surface film balance, Brewster angle microscopy, and relaxation in area and surface pressure at constant surface pressure and area, respectively. The first compression-expansion cycle of the monolayer is not reversible and the second pi-A compression isotherm deviates to larger molecular areas relative to the first one. At a microscopic level this hysteresis may be assigned to an irreversible hydration of the ammonium groups of DODAB. The morphology and reflectivity of DODAB monolayers during compression and expansion on the monolayer depend on the monolayer history. Bright domains randomly dispersed were observed during compression before collapse. Surprisingly, this random distribution of domains changes into a fractal-like structure during the monolayer expansion in a narrow range of surface pressures. This morphology does not form when the monolayer is previously compressed above the collapse surface pressure. 2D foam-like structure is often observed when the film is expanded at maximum area. Relaxation phenomena in DODAB monolayers are attributed to monolayer reorganization and nucleation of liquid-condensed domains from the liquid-expanded phase. These time-dependent processes are irreversible.