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Cosima Stubenrauch

Publications and source records attributed to Cosima Stubenrauch.

11 recordsLinked to original sources
PubMed2006

Short-range interactions between non-ionic surfactant layers.

Short-range interactions between surfactant and lipid layers are of great importance in technical applications in complex fluids such as foams, dispersions and emulsions, as well as in the formulation and performance of dispersants, detergents and flocculants. It is also of utmost importance in biological systems where interactions between biomembranes influence a range of processes. The field of short-range interactions has been thoroughly investigated during the past 30 years, following the emergence of a number of techniques to measure interaction forces. Thus, our understanding has increased considerably and it is timely to summarize relevant knowledge accumulated in this area. In this review we focus on the nature of short-range interactions between non-ionic and zwitterionic surfactant and lipid layers exposing their polar groups to the surrounding medium. We discuss the complex interplay of short-range (van der Waals, hydration, steric and other) forces based on recent theoretical and experimental results.

Emulsions↗
PubMed2006

Disconnected lamellar phases (Lalpha) in pseudobinary water-non-ionic surfactant systems: a general phenomenon.

This study provides new experimental evidence for the disconnection of the lamellar phase (L(alpha)) in pseudobinary water-non-ionic surfactant systems. To prove that the disconnection is indeed a general phenomenon the phase behavior of the pseudobinary system water-pentaethylene glycol dodecyl ether/hexaethylene glycol dodecyl ether (H(2)O-C(12)E(5)/C(12)E(6)) was investigated as a function of the surfactant composition delta and the total surfactant concentration gamma. At a fixed gamma of 0.10 the extension of the highly diluted L(alpha) phase shrank continuously with increasing amount of C(12)E(6), i.e., increasing delta, until it disappeared at delta=0.60. The gamma-T phase diagram of this particular surfactant mixture was found to have a disconnected L(alpha) phase. For the first time, SAXS measurements were carried out to monitor structural changes related to the disconnection. For this purpose the interlayer spacing d and the effective cross-sectional area a(s) were determined from the SAXS data along characteristic paths through the L(alpha) phase.

Journal Article↗
PubMed2005

Interactions between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-beta-D-maltoside.

The forces acting between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were investigated at concentrations below and above the critical micelle concentration. The long-range and adhesive forces were measured with a bimorph surface force apparatus (MASIF). It was found that the effect of hydrodynamic interactions had to be taken into account for an accurate determination of the short-range static interactions. The results were compared with disjoining pressure versus thickness curves that were obtained earlier with a thin film pressure balance (TFPB). This comparison led to the conclusion that the charges observed at the air-water interface are not due to charged species present in the surfactant sample. In addition, it was observed that the stability of thin liquid films crucially depends on the surfactant's bulk concentration (c) and thus on the packing density in the adsorbed layer. The force barrier preventing removal of the surfactant layer from between two solid-liquid interfaces increases with increasing c, while for foam films it is the stability of the Newton black film that increases with c. Finally, the results obtained for beta-C(12)G(2) were compared with those obtained for the homologue n-decyl-beta-d-maltoside (beta-C(10)G(2)) as well as with those obtained for nonionic surfactants with polyoxyethylene moieties as polar groups.

Adsorption↗
PubMed2005

New equation of state for thin foam films.

We recently constructed phase diagrams for thin foam films stabilized with nonionic surfactants by treating disjoining pressure (Pi) versus thickness (h) curves of foam films like p-V(m) isotherms of real gases. However, the correlations were qualitative rather than quantitative. In the present paper, an empirically derived equation of state for a quantitative description of foam film properties is proposed. New experimental data are provided which clearly illustrate under which conditions (a) subcritical, critical, and supercritical films are formed, (b) the common black film (CBF) to Newton black film (NBF) transition is reversible, and (c) CBF and NBF coexist.

Journal Article↗
PubMed2005

Influence of amphiphilic block copolymers on lyotropic liquid crystals in water-oil-surfactant systems.

In ternary water-oil-nonionic alkyl polyglycol ether (C(i)E(j)) microemulsions, an increase in efficiency is always accompanied by the formation of a lamellar (L(alpha)) phase. The addition of an amphiphilic block copolymer to the ternary base system increases the efficiency of the microemulsion drastically while suppressing--at least partly--the formation of the L(alpha) phase. However, amphiphilic block copolymers can be used not only to suppress the formation of lyotropic liquid crystals but also for the opposite effect, namely, to induce their formation. To understand to what extent the increase in efficiency is accompanied by the formation of lyotropic liquid crystals, we studied phase diagrams of water-n-alkane-n-alkyl polyglycol ethers (C(i)E(j))-PEPX-PEOY at a constant volume fraction of oil in the water/oil mixture. Using polymers of the poly(ethylene propylene)-copoly(ethylene oxide) type, with M(PEP) = X kg mol(-1) and M(PEO) = Y kg mol(-1), we determined phase diagrams as a function of the polymer concentration, size, and symmetry. Moreover, the influence of a particular polymer mixture was studied, which turned out to be the best system if both a high efficiency and a low tendency to form an L(alpha) phase are needed.

Journal Article↗
PubMed2005

Foams and foam films stabilized by CnTAB: influence of the chain length and of impurities.

Quantitative comparison of foam films and the corresponding foams is very demanding. One problem is the fact that investigations of foam films are usually performed at constant capillary pressures P, whereas in foams P is a function of the height of the foam column. A way out of this dilemma is to examine films and foams at the same P. The method of choice for the foam films is the thin film pressure balance (TFPB), whereas the corresponding investigation of foams is based on the foam pressure drop technique (FPDT). An extensive TFPB study on foam films stabilized by the cationic alkyltrimethylammonium bromides C(n)TAB with n=10, 12, 14, and 16 was performed by Bergeron. For this series a steep increase of the foam film stability was observed when the chain length was increased from n=12 to n=14. Moreover, the influence of impurities was found to be limited to the films stabilized by C(12)TAB. In order to study the correlation between the properties of films and foams, the present study deals with the respective foam properties investigated with the FPDT. It was found that both the steep increase in the film stability and the influence of impurities are also reflected in the properties of the foam.

Journal Article↗
PubMed2005

What do a foam film and a real gas have in common?

The stability of well-drained quasistatic foam films (thickness <100 nm) is usually discussed in terms of surface forces, which create an excess pressure normal to the film interfaces, called the disjoining pressure pi The disjoining pressure is the sum of repulsive electrostatic (pi(elec)), attractive van der Waals (pi(vdW)), and repulsive steric (pi(sr)) forces on the assumption that structural forces can be neglected. On the basis of these forces two different types of thin foam films are distinguished, namely common black films (CBF), which are mainly stabilized by pi(elec), and Newton black films (NBF), the stability of which is determined by pi(sr),With a thin-film pressure balance (TFPB) the thickness h of a foam film can be measured as a function of the applied pressure from which the disjoining pressure pi can be calculated. A thorough analysis of the results published so far reveals that the pi-h curves of nonionic surfactants measured at different surfactant concentrations resemble p-V(m) isotherms of a real gas measured at different temperatures. On the basis of these observations the van der Waals description of a real gas can be applied to foam films and a phase diagram for a foam film was constructed using the Maxwell construction.

Journal Article↗
PubMed2004

Phase diagrams of nonionic foam films: construction by means of disjoining pressure versus thickness curves.

The thickness h of foam films can be measured as a function of the disjoining pressure Pi using a thin film pressure balance. Experimental Pi-h curves of foam films stabilized with nonionic surfactants measured at various concentrations resemble the p-V(m) isotherms of real gases measured at various temperatures (p is the pressure and V(m) is the molar volume of the gas). This observation led us to adopt the van der Waals approach for describing real gases to thin foam films, where the thickness h takes the role of V(m) and the disjoining pressure Pi replaces the ordinary pressure p. Our analysis results in a phase diagram for a thin foam film with spinodal, binodal as well as a critical point. The thicker common black film corresponds to the gas phase and the compact Newton black film for which the two surfaces are in direct contact corresponds to the dense liquid. We show that the tuning parameter for the phase behavior of the film is the surface charge density, which means that Pi-h curves should not be referred to as isotherms. In addition to the equilibrium properties the driving force for the phase transition from a common black film to a Newton black film or vice versa is calculated. We discuss how this transition can be controlled experimentally.

Journal Article↗
PubMed2004

Phase diagrams of nonionic foam films: new interpretation of disjoining pressure vs thickness curves.

Recently we constructed phase diagrams for thin foam films stabilized by a nonionic surfactant. The idea was born by synopsis of various disjoining pressure (pi) versus thickness (h) curves of foam films resembling p-Vm isotherms of real gases. The new concept of interpreting the pi-h curves of foam films in terms of phase diagrams allows us to describe experimental observations much more precisely. Three logical consequences will be discussed here to illustrate the strength of this approach. First, the observation is explained that common black films (CBF) rupture or form a Newton black film (NBF) within a certain pressure range rather than at a defined pressure. Both observations can be rationalized by invoking a nucleation process of holes or of the thinner NBF, respectively, in close analogy to the vapor to liquid condensation. Second, the question whether the CBF to NBF transition is discrete or continuous is answered by analyzing under which conditions the supercritical state of a foam film can be reached. Third, the evidence of corresponding states is discussed.

Letter↗
PubMed2004

Interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 and the origin of surface charges at the air/water interface.

The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces were chosen. The interactions between the air/water interfaces were measured with a thin film pressure balance in terms of disjoining pressure as a function of film thickness. The interactions between the solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the nature of the surface on the interaction forces was investigated at surfactant concentrations below and above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged silanated glass and the charged air/water interface results in a lowering of the surface charge density. The reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at the air/water interface as well as its decrease with increasing surfactant concentration is discussed and a new mechanism for generation of OH- ions at this particular interface is proposed.

Journal Article↗