Cosima Stubenrauch

Sugar surfactants - aggregation, interfacial, and adsorption phenomena

The latest research work provides us with basic knowledge about some selected properties of systems containing sugar surfactants. Now we are able to understand the behavior of aqueous solutions reasonably well and very recently progress was made with respect to the properties of microemulsions. However, little is still known about the adsorption properties. Apart from the need for more fundamental research, the future task will be to handle more complex systems. Here complex refers to multi-component mixtures used in technical applications, but also to biotechnological applications where the structure of the surfactant is more complex. Promising future developments in this field are outlined.

Disjoining pressure in thin liquid foam and emulsion films - new concepts and perspectives

The present review is a topical survey of the disjoining pressure in thin liquid foam and emulsion films from both the experimental and the theoretical points of view. Section 2 deals with the latest research work on experimental techniques with which the disjoining pressure Π in foam, emulsion, and pseudo-emulsion films can be measured. Although a lot of techniques are available, the question of the origin of the charges at the water/air and the water/oil interfaces of films, which are stabilized by non-ionic surfactants, has not yet been answered. We address this question in section 3, reviewing the latest relevant literature. The relevance of structural forces for the disjoining pressure is outlined in section 4, which focuses on films which are stabilized by surfactant/polyelectrolyte mixtures.

Stability of foam films and surface rheology: An oscillating bubble study at low frequencies

The dilational surface elasticity epsilon and the dilational surface viscosity eta of the two nonionic surfactants n-dodecyl-beta-d-maltoside (beta-C12G2) and tetraethyleneglycol-monodecyl ether (C10E4) were studied using the oscillating drop method. The experiments were carried out at different concentrations and frequencies with an accessible frequency range of 0.005-0.2 Hz. The results are discussed in the light of previous disjoining pressure measurements that demonstrated that the stability of thin liquid films cannot be explained solely by the magnitude of the surface forces. Indeed, a comparison of the results obtained for beta-C12G2 with those obtained for C10E4 reveals a correlation between the stability of the films and the surface dilational elasticity of the respective monolayers.

A disjoining pressure study of n-dodecyl-β-D-maltoside foam films

The disjoining pressure Π as function of film thickness h of aqueous n-dodecyl-β-D-maltoside (β-C12G2) solutions has been measured using a newly assembled thin film pressure balance. The theoretical analysis of the experimental Π(h) curves in terms of the DLVO theory confirms the existence of electrical charges and provides numerical values for their density at the water/air interface of this nonionic sugar surfactant system. The origin of the charges is discussed on the basis of a recently proposed adsorption model for OH− ions. The surface charge density is found to decrease with increasing surfactant concentration, to increase with increasing electrolyte concentration and stay constant in the range 4 < pH < 8. Furthermore, it is shown that the molecular structure of sugar surfactants has no influence on the magnitude of the long-range repulsive forces stabilizing the common black film. However, the structure influences the stability of the film as well as its ability to form a Newton black film. The good agreement between the results presented and those for nonionic alkyl polyglycol ethers (CiEj) suggests a common origin of the properties of thin liquid films stabilized by nonionic surfactants.

Mixtures of n-dodecyl-β-d-maltoside and hexaoxyethylene dodecyl ether — Surface properties, bulk properties, foam films, and foams

Mixtures of the two non-ionic surfactants hexaoxyethylene dodecyl ether (C(12)E(6)) and n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) 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 beta-C(12)G(2) from between thiolated gold surfaces, while for C(12)E(6) low loads are sufficient. Fourthly, the adsorption of C(12)E(6) and beta-C(12)G(2) on hydrophilic silica and titania, respectively, is inverted. While the surface excess of C(12)E(6) is large on silica and negligible on titania, beta-C(12)G(2) 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 C(12)E(6)+beta-C(12)G(2) 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.

Foaming properties of mixtures of a non-ionic (C12DMPO) and an ionic surfactant (C12TAB)

An extensive study of the foaming properties of a surfactant mixture consisting of the non-ionic dodecyldimethyl phosphineoxide (C(12)DMPO) and the cationic dodecyltrimethyl ammonium bromide (C(12)TAB) with mixing ratios of C(12)DMPO:C(12)TAB = 1:0, 50:1, 1:1, 1:50, 0:1 is performed both above and below the critical micelle concentration. Foamability and foam stability were examined using the commercially available FoamScan (sparging), the standardised Ross-Miles (pouring) and a home-built winding (shaking) technique. The focus, however, was on FoamScan measurements as they allowed for the evaluation of the foams liquid content. The foamability and foam stability of C(12)TAB was found to be larger than that of C(12)DMPO. The foamability continually increased with increasing C(12)TAB content in the surfactant mixture, which reflects the reduction of the diffusion relaxation time (i.e. faster adsorption). Where possible correlations are drawn between the foam properties on the one hand and adsorption and foam film properties on the other hand, which were studied previously. Interestingly, the 1:1 mixture shows weak/negligible surfactant interactions but--counterintuitively--an increased foam stability compared to the single surfactant systems. However, at this ratio charge neutralization occurs, which leads to the formation of a Newton Black Film thus suggesting that the foam film type plays an important role in the foam stability.

Bicontinuous microemulsions revisited: a new approach to freeze fracture electron microscopy (FFEM)

Abstract New images by freeze fracture electron microscopy (FFEM) are obtained for microemulsions in the ternary system water–n-octane–C12E5 as function of varying water-to-oil ratio. The surfactant concentration is chosen such that the composition of the sample is close to the X -point, i.e. the point of highest efficiency, where the three- and the one-phase regions meet. As the phase behavior and thus the microstructure of these particular nonionic microemulsions are extremely sensitive to temperature, a precise temperature control of the sample was necessary prior to rapid freezing in liquid ethane. The images presented are the first FFEM images along the trajectory of the middle-phase in phase space. Of particular interest is the structure of the microemulsion on the water-rich side not visualized so far. The images give strong evidence that even at the highest water and oil contents, respectively, elements of a bicontinuous microstructure are visible, which confirms results obtained by means of various other techniques.

Phase Behavior and Microstructure of Microemulsions Containing the Hydrophobic Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate

The phase behavior and microstructure of the ternary system water/1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6))/Triton X-100 was studied as a function of temperature and ionic liquid (IL) mass fraction alpha. In the present study, a hydrophobic IL instead of commonly used organic solvents such as n-alkanes is used. The fish-shaped region is distorted at low and high values of alpha, whereas it is symmetric at intermediate alpha. With increasing alpha, the extension of the three-phase region decreases regarding the surfactant concentration range, whereas it increases regarding the temperature range. For comparison the phase behavior of two ternary water/bmimPF(6)/alkyl oligoethyleneoxide (C(i)E(j)) systems has been investigated. Our results are compared with those obtained for water/n-alkane/C(i)E(j) and IL/n-alkane/C(i)E(j) systems, respectively.

Binary mixtures of β-dodecylmaltoside (β-C12G2) with cationic and non-ionic surfactants: micelle and surface compositions

Surfactants used for practical applications are usually surfactant mixtures because they often exhibit a performance that is superior to the individual surfactants. In the present study binary mixtures of the sugar based surfactant n-dodecyl-β-D-maltoside (β-C12G2) with the cationic surfactant dodecyl trimethylammonium bromide (C12TAB) and the non-ionic hexaethylene-glycol dodecyl ether (C12E6), respectively, were investigated at different bulk mole fractions. Surface tension measurements were used to determine the critical micelle concentration and the surfactant composition at the surface. In addition, the regular solution theory was used to calculate interaction parameters as well as the mole fractions of the individual surfactants in the mixed air-water monolayer and in the mixed micelles, respectively. It was found that n-dodecyl-β-D-maltoside interacts weakly with the cationic surfactant (C12TAB) and that it dominates in both the mixed monolayer and the mixed micelles. On the other hand, β-C12G2 and C12E6 mix ideally in solution. For both surfactant mixtures the surfactant composition at the surface determined by surface tension measurements and by the regular solution theory, respectively, were compared and discussed in detail.

Confinement of linear polymers, surfactants, and particles between interfaces

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.