Polysaccharide/Surfactant complexes at the air-water interface - effect of the charge density on interfacial and foaming behaviors

Stability and Structural Analysis Using Small-Angle Neutron Scattering for Foam of Homogeneous Polyoxyethylene-Type Nonionic Surfactants with Multibranched Chains

Small-angle neutron scattering can provide insight into the microstructure of the surfactant-stabilized foam. In this study, small-angle neutron scattering in combination with other techniques was employed to determine the microstructure of the foams stabilized using novel homogeneous polyoxyethylene (EO) alkyl ether-type nonionic surfactants with multibranched double chains (bC7-bC9EO12). Similarly, homogeneous EO-type nonionic surfactants with linear double chains (C8-C8EO12) and a linear single chain (C18EO12) were used. bC7-bC9EO12 and C8-C8EO12 surfactants with branched hydrophobic chains or double chains increased the foam stability and suppressed the draining. Furthermore, they formed rod-like micelles, and C18EO12 formed spherical micelles in the bulk solution. The foam film containing the plateau border contained micelles identical with those found in the bulk solution. For bC7-bC9EO12 and C8-C8EO12, the average radius of the bubbles immediately after foaming was of the order of hundreds of μm. Finally, these radii grew to the order of thousands of μm. Thus, a significant correlation was observed between the micellar structure and the stability of these foams.

Structure formation of PNIPAM microgels in foams and foam films

Responsive aqueous foams are very interesting from a fundamental point of view and for various applications like foam flooding or foam flotation. In this study thermoresponsive microgels (MGs) made from poly(N-isopropyl-acrylamide) (PNIPAM) with varying cross-linker content, are used as foam stabilisers. The foams obtained are thermoresponsive and can be destabilised by increasing the temperature. The structuring of MGs inside the foam films is investigated with small-angle neutron scattering and in a thin film pressure balance. The foam films are inhomogeneous and form a network-like structure, in which thin and MG depleted zones with a thickness of ca. 30 nm are interspersed in a continuous network of thick MG containing areas with a thickness of several 100 nm. The thickness of this continuous network is related to the elastic modulus of the individual MGs, which was determined by atomic force microscopy indentation experiments. Both, the elastic moduli and foam film thicknesses, indicate a correlation to the network elasticity of the MGs predicted by the affine network model.

A new model to describe small-angle neutron scattering from foams

The modelling of scattering data from foams is very challenging due to the complex structure of foams and is therefore often reduced to the fitting of single peak positions or feature mimicking. This article presents a more elaborate model to describe the small-angle neutron scattering (SANS) data from foams. The model takes into account the geometry of the foam bubbles and is based on an incoherent superposition of the reflectivity curves arising from the foam films and the small-angle scattering (SAS) contribution from the plateau borders. The model is capable of describing the complete scattering curve of a foam stabilized by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. different drainage states, and provides information on the thickness distribution of liquid films inside the foam. The mean film thickness decreases with decreasing water content because of drainage, from 28 to 22 nm, while the polydispersity increases. These results are in good agreement with the film thicknesses of individual horizontal foam films studied with a thin-film pressure balance.

Friction Dynamics of Foams under Nonlinear Motion

Foams are viscoelastic soft materials with complex mechanical properties. Here, we evaluated the friction dynamics of foams between acrylic plates using a sinusoidal motion friction evaluation system and we found some interesting characteristics under accelerated conditions. On a typical solid surface, a symmetrical friction profile, in which static and kinetic frictions are observed, is obtained under reciprocating nonlinear motion. Meanwhile, significant lubricant effects and velocity-dependent friction profiles without static friction were observed in foams. The friction force in foams increased in proportion to the power of velocity, with a power index of <1. These characteristic and dynamic phenomena in foams were observed in this study. They had been caused by the formation of a thick lubricant film and various dissipative modes including surfactant diffusion, viscous dissipation, and wall slip of bubbles. Moreover, the addition of a thickener increased the friction force and the delay time of friction response and improved the foam durability against normal force and shear. These findings are useful for understanding dynamic phenomena in soft materials.

Multiscale Microstructure, Composition, and Stability of Surfactant/Polymer Foams

Inclusion of polymer additives is a known strategy to improve foam stability, but questions persist about the amount of polymer incorporated in the foam and the resulting structural changes that impact material performance. Here, we study these questions in sodium dodecyl sulfate (SDS)/hydroxypropyl methylcellulose (HPMC) foams using a combination of flow injection QTOF mass spectrometry and small-angle neutron scattering (SANS) measurements leveraging contrast matching. Mass spectrometry results demonstrate polymer incorporation and retention in the foam during drainage by measuring the HPMC-to-SDS ratio. The results confirm a ratio matching the parent solution and stability over the time of our measurements. The SANS measurements leverage precise contrast matching to reveal detailed descriptions of the micellar structure (size, shape, and aggregation number) along with the foam film thickness. The presence of HPMC leads to thicker films, correlating with increased foam stability over the first 15-20 min after foam production. Taken together, mass spectrometry and SANS present a structural and compositional picture of SDS/HPMC foams and an approach amenable to systematic study for foams, gathering mechanistic insights and providing formulation guidance for rational foam design.

Microstructural Characterization of Foam Formed by a Hydroxy Group-Containing Amino Acid Surfactant Using Small-Angle Neutron Scattering

Small-angle neutron scattering, which has not been extensively utilized for foam characterization, can provide important insights into the microstructure of surfactant-stabilized foam. Small-angle neutron scattering in combination with several other techniques was herein employed to determine the microstructure of foams stabilized by hydroxy group-containing (C₁₂-EtOH-βAla) and hydroxy group-free (C₁₂-Me-βAla) surfactants of the amino acid type. Hydroxy group introduction at the amide nitrogen had no effect on the foam film thickness (∼26 nm in both cases) but increased the foam stability and suppressed draining, as hydrogen bonding between hydroxy groups and carboxylate ions increased the foam film strength. Moreover, the obtained foam films were shown to contain micelles identical to those in the bulk solution.

Segregation versus Interdigitation in Highly Dynamic Polymer/Surfactant Layers

Many polymer/surfactant formulations involve a trapped kinetic state that provides some beneficial character to the formulation. However, the vast majority of studies on formulations focus on equilibrium states. Here, nanoscale structures present at dynamic interfaces in the form of air-in-water foams are explored, stabilised by mixtures of commonly used non-ionic, surface active block copolymers (Pluronic^®) and small molecule ionic surfactants (sodium dodecylsulfate, SDS, and dodecyltrimethylammonium bromide, C₁₂TAB). Transient foams formed from binary mixtures of these surfactants shows considerable changes in stability which correlate with the strength of the solution interaction which delineate the interfacial structures. Weak solution interactions reflective of distinct coexisting micellar structures in solution lead to segregated layers at the foam interface, whereas strong solution interactions lead to mixed structures both in bulk solution, forming interdigitated layers at the interface.

Micelle-Mediated Self-Assembly of Microfibers Bridging Millimeter-Scale Gap To Form Three-Dimensional-Ordered Polysaccharide Membranes

Micelle-mediated three-dimensional-ordered polysaccharide membranes are constructed by introducing cationic/anionic surfactant into a liquid crystalline polysaccharide solution. Upon drying mixtures of the polysaccharide solution with the surfactant such as cetyltrimethylammonium bromide or sodium dodecyl sulfate (SDS), the polymeric microfibers deposit as a nucleus to form a membrane, bridging millimeter-scale gap with high probability. In particular, in a solution with SDS micellar structures, the microscale fibers with diameter ∼1 μm disassemble into nanoscale fibers with diameter ∼50 nm. This transformation allows the polymeric network to become finer in nanoscale, and the vertical membrane is formed much more easily than that from a pure polysaccharide solution. Furthermore, it is clarified that the vertical membrane has been successfully formed with three-dimensionally ordered microstructures with a linearly oriented and layered structure. This method will shed light on the preparation of hybrid materials with biocompatibility and responsivity to stimuli such as magnetics, electrics, and optics via hybridization with nanomaterials dispersed by surfactants.

Effect of Alkyl Tail Length of Alpha Olefin Sulfonates on Foam Properties

The effect of alkyl tail length on the foam properties of alpha olefin sulfonates (AOS) in aqueous solutions at various concentrations was investigated by measurements of the foamability, foam stability and bubble size and numbers, obtained from conductivity and image analyses techniques based on FoamScan foam analyzer. It was found that foamability and foam stability of Cn AOS (n = 14–16, 16–18, 20–24) first increase and then decrease with increase in the carbon chain length, showing an optimum for a length of n = 16–18. The foamability and foam stability of AOS increase with increasing of surfactant concentration. This is due to the fact that the adsorbed quantity of surfactant molecules increases at the air/water interface with the increase of concentration. In addition, it was found that the bubble size produced by C16–18 AOS is smaller than that of C14–16 AOS.

Assembly of small molecule surfactants at highly dynamic air-water interfaces

Small-angle neutron scattering has been used to probe the interfacial structure of foams stabilised by small molecule surfactants at concentrations well below their critical micelle concentration. The data for wet foams showed a pronounced Q^-4 dependence at low Q and noticeable inflexions over the mid Q range. These features were found to be dependent on the surfactant structure (mainly the alkyl chain length) with various inflexions across the measured Q range as a function of the chain length but independent of factors such as concentration and foam age/height. By contrast, foam stability (for C < CMC) was significantly different at this experimental range. Drained foams showed different yet equally characteristic features, including additional peaks attributed to the formation of classical micellar structures. Together, these features suggest the dynamic air-water interface is not as simple as often depicted, indeed the data have been successfully described by a model consisting paracrystalline stacks (multilayer) of adsorbed surfactant layers; a structure that we believe is induced by the dynamic nature of the air-water interface in a foam.

Probing foam with neutrons

Foams are multiscale materials that have an enormous number of uses. As the relevant structural length-scales span from a few nanometres up to millimetres a number of characterisation methods need to be combined to obtain the full material structure. In this review we explain how foams can be explored using Small Angle Neutron Scattering (SANS). We remind the reader of the basics of SANS and contrast variation before we describe the different types of experiments that have been carried out on foams emphasising the specific role of neutrons in learning about the systems. To date SANS has been used to measure different foam structural parameters, such as the film thickness and the bubble size. Several studies have also been carried out to elucidate the organisation of the stabilising objects in the bulk solution. Finally we show how SANS measurements can be used to measure foam composition. Some of the accessible information is unique to SANS experiments, but as the method is still not very widely used on foams the review is also aimed to act as an introduction on how to carry out such measurements on foams.

Polymer-surfactant systems in bulk and at fluid interfaces

The interest of polymer-surfactant systems has undergone a spectacular development in the last thirty years due to their complex behavior and their importance in different industrial sectors. The importance can be mainly associated with the rich phase behavior of these mixtures that confers a wide range of physico-chemical properties to the complexes formed by polymers and surfactants, both in bulk and at the interfaces. This latter aspect is especially relevant because of the use of their mixture for the stabilization of dispersed systems such as foams and emulsions, with an increasing interest in several fields such as cosmetic, food science or fabrication of controlled drug delivery structures. This review presents a comprehensive analysis of different aspects related to the phase behavior of these mixtures and their intriguing behavior after adsorption at the liquid/air interface. A discussion of some physical properties of the bulk is also included. The discussion clearly points out that much more work is needed for obtaining the necessary insights for designing polymer-surfactant mixtures for specific applications.

Enhancement of foamability and foam stability induced by interactions between a hyperbranched exopolysaccharide and a zwitterionic surfactant dodecyl sulfobetaine

Weak hydrogen bonding and electrostatic interactions between a zwitterionic surfactant dodecyl sulfobetaine (DSB) and a hyperbranched exopolysaccharide (EPS) enhanced considerably the stability and foamability of EPS/DSB foam.

Specific salt and pH effects on foam film of a pH sensitive surfactant

Steady state foams made of a pH sensitive surfactant, nonaoxyethylene oleylether carboxylic acid, with ion complexing properties was studied using small angle neutron scattering (SANS). The effect of pH variation and salt addition on the foam film thickness was investigated and discussed in terms of the influent parameters stabilizing the foam such as surface properties and electrostatic effects determined by tensiometry and zeta potential measurements. The decrease in the film thickness by adding mono (Na(+)) and divalent (Ca(2+)) salts is classically explained by screening of the double layer in foam films (transverse interactions). On the contrary, addition of acid or complexing ion (Nd(3+)) results in an increase in the film thickness and can be analyzed in terms of cohesive forces between surfactants at the liquid/gas interface (lateral interactions). pH and specific salt effects revealed that foams produced by nonaoxyethylene oleylether carboxylic acid are of interest in the potential use of this surfactant in ion separation process.

Foams stabilized by multilamellar polyglycerol ester self-assemblies

The importance of surfactant self-assemblies in foam stabilization is well-known. The aim of the current study was to investigate the self-assemblies of the nonionic surfactant polyglycerol ester (PGE) in bulk solutions, at the interface and within foams, using a combined approach of small-angle neutron scattering, neutron reflectivity, and electron microscopy. PGE bulk solutions contain vesicles as well as open lamellar structures. Upon heating of the solutions the lamellar spacing increases, with significant differences in the presence of NaCl or CaCl(2) as compared to the standard solution. The adsorption of the multilamellar structures present in the bulk solutions lead to a multilayered film at the air-water interface. The ordering within this film was increased as a result of a 20% area compression mimicking a coalescence event. Finally, PGE foams were shown to be stabilized not only by strong interfacial films but also by agglomerated self-assemblies within the interstitial areas of the foams.

Proteins, polysaccharides, and their complexes used as stabilizers for emulsions: alternatives to synthetic surfactants in the pharmaceutical field?

Emulsions are widely used in pharmaceutics for the encapsulation, solubilization, entrapment, and controlled delivery of active ingredients. In order to answer the increasing demand for clean label excipients, natural polymers can replace the potentially irritative synthetic surfactants used in emulsion formulation. Indeed, biopolymers are currently used in the food industry to stabilize emulsions, and they appear as promising candidates in the pharmaceutical field too. All proteins and some polysaccharides are able to adsorb at a globule surface, thus decreasing the interfacial tension and enhancing the interfacial elasticity. However, most polysaccharides stabilize emulsions simply by increasing the viscosity of the continuous phase. Proteins and polysaccharides may also be associated either through covalent bonding or electrostatic interactions. The combination of the properties of these biopolymers under appropriate conditions leads to increased emulsion stability. Alternative layers of oppositely charged biopolymers can also be formed around the globules to obtain multi-layered "membranes". These layers can provide electrostatic and steric stabilization thus improving thermal stability and resistance to external treatment. The novel biopolymer-stabilized emulsions have a great potential in the pharmaceutical field for encapsulation, controlled digestion, and targeted release although several challenging issues such as storage and bacteriological concerns still need to be addressed.

Dilational surface visco-elasticity of polyelectrolyte/surfactant solutions: formation of heterogeneous adsorption layers

Recent application of the methods of surface dilational rheology to solutions of the complexes between synthetic polyelectrolytes and oppositely charged surfactants (PSC) gave a possibility to determine some steps of the adsorption layer formation and to discover an abrupt transition connected with the formation of microaggregates at the liquid surface. The kinetic dependencies of the dynamic surface elasticity are always monotonous at low surfactant concentrations but can have one or two local maxima in the range beyond the critical aggregation concentration. The first maximum is accompanied by the generation of higher harmonics of induced surface tension oscillations and caused by heterogeneities in the adsorption layer. The formation of a multilayered structure at the surface for some systems leads to the second maximum in the dynamic surface elasticity. The hydrophobicity and charge density of a polymer chain influence strongly the surface structure, resulting in a variety of dynamic surface properties of PSC solutions. Optical methods and atomic force microscopy give additional information for the systems under consideration. Experimental results and existing theoretical frameworks are reviewed with emphasis on the general features of all studied PSC systems.

Spontaneous formation of hierarchically structured curly films of nickel carbonate hydrate through drying

Novel curly nickel carbonate hydrate film superstructures can be prepared for the first time via a facile drying process of the films formed on air/solution interface in the presence of double hydrophilic copolymer or polyelectrolyte additives. As-prepared curly film patches with average edge sizes of several hundred micrometers display adjustable curly features along different orientation. The coiling up degree of the film edges is strongly dependent on the polymer concentration in bulk solution. Most of these curly structures have a relatively smooth outer surface; however, the microstructures of the outer surface of curly films formed show porous network-like features. In addition, using different kinds of nickel salts can produce distinct curly film samples. A possible formation mechanism of the curly film structure has been proposed. The multiple interaction modes between nickel salt precursors and polymer can favor the self-organization of the film formed at the air/solution interface. This approach is expected to be extended for producing a variety of curly hierarchical structures.