Advances in Colloid and Interface Science (v.155, #1-2)

Foreword of the SOCON co-ordinator by Cosima Stubenrauch (1-2).

Mixtures of n-dodecyl-β-d-maltoside and hexaoxyethylene dodecyl ether — Surface properties, bulk properties, foam films, and foams by C. Stubenrauch; P.M. Claesson; M. Rutland; E. Manev; I. Johansson; J.S. Pedersen; D. Langevin; D. Blunk; C.D. Bain (5-18).
Mixtures of the two non-ionic surfactants hexaoxyethylene dodecyl ether (C12E6) and n-dodecyl-β-d-maltoside (β-C12G2) were studied with regard to surface properties, bulk properties, foam films, and foams. The reason for studying a mixture of an ethylene oxide (C i E j ) and a sugar (C n G m ) based surfactant is that despite being non-ionic, these two surfactants behave quite differently. Firstly, the physico-chemical properties of aqueous solutions of C n G m surfactants are less temperature-sensitive than those of C i E j solutions. Secondly, the surface charge density q 0 of foam films stabilized by C n G m surfactants is pH insensitive down to the so-called isoelectric point, while that of foam films stabilized by C i E j surfactants changes linearly with the pH. The third difference is related to interaction forces between solid surfaces. Under equilibrium conditions very high forces are needed to expel β-C12G2 from between thiolated gold surfaces, while for C12E6 low loads are sufficient. Fourthly, the adsorption of C12E6 and β-C12G2 on hydrophilic silica and titania, respectively, is inverted. While the surface excess of C12E6 is large on silica and negligible on titania, β-C12G2 adsorbs very little on silica but has a large surface excess on titania. What is the reason for this different behaviour? Under similar conditions and for comparable head group sizes, it was found that the hydration of C i E j surfactants is one order of magnitude higher but on average much weaker than that of C n G m surfactants. Moreover, C n G m surfactants possess a rigid maltoside unit, while C i E j surfactants have a very flexible hydrophilic part. Indeed, most of the different properties mentioned above can be explained by the different hydration and the head group flexibilities. The intriguing question of how mixtures of C i E j and C n G m surfactants would behave arises organically. Thus various properties of C12E6  + β-C12G2 mixtures in aqueous solution have been studied with a focus on the 1:1 mixture. The results are compared with those of the single surfactants and are discussed accordingly.
Keywords: Surfactant mixtures; Non-ionic surfactants; Surface properties; Phase diagrams; Foams; Foam films;

Confinement of linear polymers, surfactants, and particles between interfaces by R. von Klitzing; E. Thormann; T. Nylander; D. Langevin; C. Stubenrauch (19-31).
The review addresses the effect of geometrical confinement on the structure formation of colloidal dispersions like particle suspensions, (non)micellar surfactant solutions, polyelectrolyte solutions and mixed dispersions. The dispersions are entrapped either between two fluid interfaces (foam film) in a Thin Film Pressure Balance (TFPB) or between two solid interfaces in a Colloidal Probe Atomic Force Microscope (Colloidal Probe AFM) or a Surface Force Apparatus (SFA). The oscillating concentration profile in front of the surface leads to an oscillating force during film thinning. It is shown that the characteristic lengths like the distance between particles, the distance between micelles, or the mesh size of the polymer network remain the same during the confining process. The influence of different parameters like ionic strength, molecular structure, and the properties of the outer surfaces on the structure formation are reported. The confinement of mixed dispersions might lead to phase separation and capillary condensation, which in turn causes a pronounced attraction between the two opposing film surfaces.
Keywords: Linear polymers; Surfactants; Thin liquid films; Structure formation under confinement; Structural forces; Stratification;

Complexes of surfactants with oppositely charged polymers at surfaces and in bulk by C.D. Bain; P.M. Claesson; D. Langevin; R. Meszaros; T. Nylander; C. Stubenrauch; S. Titmuss; R. von Klitzing (32-49).
Addition of surfactants to aqueous solutions of polyelectrolytes carrying an opposite charge causes the spontaneous formation of complexes in the bulk phase in certain concentration ranges. Under some conditions, compact monodisperse multichain complexes are obtained in the bulk. The size of these complexes depends on the mixing procedure and it can be varied in a controlled way from nanometers up to micrometers. The complexes exhibit microstructures analoguous to those of the precipitates formed at higher concentrations. In other cases, however, the bulk complexes are large, soft and polydisperse. In most cases, the dispersions are only kinetically stable and exhibit pronounced non-equilibrium features.Association at air-water interfaces readily occurs, even at very small concentrations. When the surfactant concentration is small, the surface complexes are usually made of a surfactant monolayer to which the polymer binds and adsorbs in a flat-like configuration. However, under some conditions, thicker layers can be found, with bulk complexes sticking to the surface. The association at solid-water interfaces is more complex and depends on the specific interactions between surfactants, polymers and the surface. However, the behaviour can be understood if distinctions between hydrophilic surfaces and hydrophobic surfaces are made. Note that the behaviour at air-water interfaces is closer to that of hydrophobic than that of hydrophilic solid surfaces.The relation between bulk and surface complexation will be discussed in this review. The emphasis will be given to the results obtained by the teams of the EC-funded Marie Curie RTN “SOCON”.
Keywords: Surfactant-polymer complexes; Charged polyelectrolytes; Surfactant-polymer interactions;

Bottle-brush polymers: Adsorption at surfaces and interactions with surfactants by P.M. Claesson; R. Makuska; I. Varga; R. Meszaros; S. Titmuss; P. Linse; J. Skov Pedersen; Cosima Stubenrauch (50-57).
Solution and adsorption properties of both charged and uncharged bottle-brush polymers have been investigated. The solution conformation and interactions in solution have been investigated by small-angle scattering techniques. The association of the bottle-brush polymers with anionic surfactants has also been studied. Surfactant binding isotherm measurements, NMR, surface tension measurements, as well as SAXS, SANS and light scattering techniques were utilized for understanding the association behaviour in bulk solutions. The adsorption of the bottle-brush polymers onto oppositely charged surfaces has been explored using a battery of techniques, including reflectometry, ellipsometry, quartz crystal microbalance, and neutron reflectivity. The combination of these techniques allowed determination of adsorbed mass, layer thickness, water content, and structural changes occurring during layer formation. The adsorption onto mica was found to be very different to that on silica, and an explanation for this was sought by employing a lattice mean-field theory. The model was able to reproduce a number of salient experimental features characterizing the adsorption of the bottle-brush polymers over a wide range of compositions, spanning from uncharged bottle-brushes to linear polyelectrolytes. This allowed us to shed light on the importance of electrostatic surface properties and non-electrostatic surface-polymer affinity for the adsorption. The interactions between bottle-brush polymers and anionic surfactants in adsorbed layers have also been elucidated using ellipsometry, neutron reflectivity and surface force measurements.
Keywords: Bottle-brush polymers; Synthesis; Solution properties; Adsorption; Interactions with surfactants; Surface forces; Self-consistent lattice mean-field model;