A Disjoining Pressure Study of Foam Films Stabilized by Mixtures of Nonionic and Ionic Surfactants

Studying the disjoining pressure Π as a function of the film thickness h (Π−h curves) of foam films stabilized by ionic and nonionic surfactants, one finds that the surface charge density q0 of films stabilized by ionic surfactants increases with increasing surfactant concentration, while the opposite holds true for nonionic surfactants. Thus, it should be possible to tune the surface charge density with mixtures of nonionic and ionic surfactants. To address this question, we studied foam films stabilized by aqueous solutions of surfactant mixtures. The mixtures consisted of the nonionic β-dodecylmaltoside (β-C12G2) and the cationic dodecyl trimethylammonium bromide (C12TAB) with mixing ratios of β-C12G2/C12TAB = 1:0, 50:1, 1:1, 1:50, 0:1. The addition of small amounts of C12TAB to β-C12G2 first neutralizes the negative surface charge of the β-C12G2 films and finally leads to a charge reversal from negatively to positively charged surfaces. On the other hand, by adding small amounts of β-C12G2 to C12TAB, one observes the formation of stable CBFs which was also observed for the pure C12TAB. However, in contrast to the pure C12TAB, the resulting Π−h curves for the mixtures cannot be described with the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory; the slope of the curves is too steep, and it barely changes with changing electrolyte concentration. A possible explanation for this observation will be given and discussed

Gelling Lyotropic Liquid Crystals with the Organogelator 1,3:2,4-Dibenzylidene‑d‑sorbitol Part II: Microstructure

This study deals with the gelation of lyotropic liquid crystals (LLCs) of the binary system H2O–heptaethylene glycol monododecyl ether (C12E7). The Lα and H1 phases are gelled with the organogelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS). The microstructure of the gelled LLCs is compared to those of the binary counterparts, i.e., the pure LLCs and the binary gel ethylene glycol–DBS. We present the first examples of gelled lyotropic liquid crystals (LLCs) formed by two different ways upon cooling: (1) At a DBS mass fraction of η = 0.015, the gel is formed first, followed by LLC formation. (2) At η = 0.0075, the LLC is formed first, followed by gel formation. Addressing LLC and gel formation in different orders, the influence of the LLC on the gel network and vice versa can be examined. Independent of which structure is formed first, the interlayer spacing dLLC of the LLCs is only slightly larger in the presence of the gel network compared to the nongelled counterparts. Likewise, the influence of the LLCs on the gel fibers is independent of the chronology of the gel and LLC formation. For both ways, the gel fibers are twisted and arranged in bundles parallel to the bilayers of the Lα phase and the cylindrical micelles of the H1 phase. Whereas the twisted structure of the gel fibers in ethylene glycol is retained in the presence of the LLCs, the arrangement in bundles is not observed in the binary gels. In the latter case, randomly distributed single fibers which are also slightly thinner are detected. However, we observed the fiber bundles independent of whether the gel network is formed in the isotropic phase or in the LLC and argue that the difference is caused by different interactions of organogelator DBS with the system H2O–C12E7 than with ethylene glycol. In summary, we found that both the surfactant and the gelator molecules self-assemble in the presence of each other, leading to the coexistence of an LLC and a gel network. This is what is called orthogonal self-assembly

Diving into the Finestructure of Macroporous Polymer Foams Synthesized via Emulsion Templating: A Phase Diagram Study

During our studies on emulsion-templated monodisperse polymer foams we found significant differences in the finestructure if the locus of initiation is changed. This motivated us to study the phase behavior of the liquid template. Our studies indicate that the template consists of droplets of three different length scales: The water droplets generated via microfluidics (∼70 μm) are surrounded by a continuous phase in which a w/o emulsion (≤100 nm) coexists with a w/o <i>micro</i>emulsion (∼5 nm). We speculate that the w/o-emulsion droplets act as seeds for the porous finestructure observed in AIBN-initiated polymer foams. We have experimental evidence that the w/o emulsion inverts to an o/w emulsion with progressing polymerization. This explains the granular texture observed in KPS-initiated polymer foams. The control of the finestructure is important in the preparation of tailor-made polymer foams because it directly impacts the material’s density and thus, in turn, its mechanical stability

On How Surfactant Depletion during Foam Generation Influences Foam Properties

Although it is known that foaming a surfactant solution results in a depletion of the surfactant in the bulk phase, this effect is often overlooked and has never been quantified. Therefore, the influence of surfactant depletion on foam properties using solutions of the two nonionic surfactants, <i>n</i>-dodecyl-β-d-maltoside (β-C₁₂G₂) and hexaethyleneglycol monododecyl ether (C₁₂E₆), were investigated. These investigations were conducted in two steps. First, different foam volumes were generated with the same surfactant solution at a concentration of <i>c</i> = 2 cmc. It was found that the higher the foam volume, the larger the surfactant depletion. Second, two different bulk concentrations (<i>c</i> = 2 and 1.33 cmc) were used for the generation of 50 and 110 mL of foam, respectively. For a foam volume of 50 mL, no differences were observed, whereas generating 110 mL led to different results. The surfactant loss in the bulk solution was measured via surface tension measurements and then compared to the results of purely geometric considerations that take into account the amount of interface created in the foam. Both results were in very good agreement, which means that surfactant depletion can be calculated in the way suggested here. Under conditions where depletion plays a role, our approach can also be used to estimate the bubble size of a foam of known volume by measuring the surfactant concentration in the bulk solution after foaming

Monte Carlo Simulation of the Size and Composition of Bimetallic Nanoparticles Synthesized in Water in Oil Microemulsions

Structural features of bimetallic nanoparticles synthesized in w/o microemulsion have been examined by using a coarse model solved by Monte Carlo simulations. The microemulsion was modeled as spherical droplets containing either two metal salts or reducing agent, and the processes occurring during the collision of the droplets and leading to nucleation and growth of the bimetallic nanoparticles were determined by a set of variables. The bimetallic nanoparticle structure is mainly determined by the difference in the reduction rates of the two metal ions and the excess of reducing agent. An intermetallic structure is always obtained when both reduction reactions take place at about the same rate. When the metal ions have very different reduction potentials, a core−shell to intermetallic structure transition is found at increasing the excess of the reducing agent. An enhancement of the intermetallic structure at the expense of the core−shell, one can be obtained either by decreasing the concentration of both metal salts or by increasing the interdroplet exchange rates. Predictions of the Monte Carlo simulations at large differences in the reduction rates of both metal ions compare well with the experimental data on the size and structure of PtBi nanoparticles, leading to prediction of the experimental conditions for designing specific bimetallic structures

Syntheses, Amphitropic Liquid Crystallinity, and Surface Activity of New Inositol-Based Amphiphiles

Carbohydrates are interesting starting materials for scientific and industrial syntheses as they allow a versatile chemistry. Moreover, they are of natural origin and environmentally benign. During the past few years, inositol, a rather “exotic” carbohydrate, and its derivatives have gained increasing attention. Here, we describe the syntheses of new regiochemically defined inositol monoethers and monoesters as well as regioisomeric inositol ester mixtures and investigate their amphitropic liquid crystallinity. Furthermore, first results on their surface activity in aqueous solutions are given and compared with classical sugar surfactants

Microemulsions with the Ionic Liquid Ethylammonium Nitrate: Phase Behavior, Composition, and Microstructure

In this study, we investigate properties of microemulsions which consist of the ionic liquid (IL) ethylammonium nitrate (EAN), the nonionic surfactant C₁₂E₃ and an <i>n</i>-alkane, namely <i>n</i>-dodecane or <i>n-</i>octane. The compositions of the coexisting phases are calculated from the densities and volumes of the respective phases. Since the interfacial tension between the water-rich and the oil-rich phase in traditional microemulsions (containing water and oil) relates to the microstructure, spinning drop tensiometry is used to measure the interfacial tension σ_ab and to estimate the domain sizes. Finally, measuring the self-diffusion coefficients of all components via the Fourier Transform Pulsed Gradient Spin Echo (FTPGSE) NMR technique allowed distinguishing between continuous and discrete structures. Our results indicate that the general principles underlying water–<i>n</i>-alkane–C_iE_j microemulsions can indeed be transferred to oil-in-EAN droplet and the respective bicontinuous microemulsions, while differences are observed for EAN-in-oil droplet microemulsions

Interface Adsorption versus Bulk Micellization of Surfactants: Insights from Molecular Simulations

Surfactants play essential roles in many commonplace applications and industrial processes. Although significant progress has been made over the past decades with regard to model-based predictions of the behavior of surfactants, important challenges have remained. Notably, the characteristic time scales of surfactant exchange among micelles, interfaces, and the bulk solution typically exceed the time scales currently accessible with atomistic molecular dynamics (MD) simulations. Here, we circumvent this problem by introducing a framework that combines the general thermodynamic principles of self-assembly and interfacial adsorption with atomistic MD simulations. This approach provides a full thermodynamic description based on equal chemical potentials and connects the surfactant bulk concentration, the experimental control parameter, with the surfactant surface density, the suitable control parameter in MD simulations. Self-consistency is demonstrated for the nonionic surfactant C12EO6 (hexaethylene glycol monododecyl ether) at an alkane/water interface, for which the adsorption and pressure isotherms are computed. The agreement between the simulation results and experiments is semiquantitative. A detailed analysis reveals that the used atomistic model captures well the interactions between surfactants at the interface but less so their adsorption affinities to the interface and incorporation into micelles. Based on a comparison with other recent studies that pursued similar modeling challenges, we conclude that the current atomistic models systematically overestimate the surfactant affinities to aggregates, which calls for improved models in the future

Highly Ordered Gelatin Methacryloyl Hydrogel Foams with Tunable Pore Size

In this study, we present a fast and convenient liquid foam templating route to generate gelatin methacryloyl (GM) foams. Microfluidic bubbling was used to generate monodisperse liquid foams with bubble sizes ranging from 220 to 390 μm. The continuous phase contained 20 wt % GM and 0.7 wt % lithium phenyl-2,4,6-trimethylbenzoylphosphinate as photoinitiator. Gelation was achieved via UV-initiated radical cross-linking of GM. After cross-linking, the hydrogel foams were either swollen in water or freeze-dried. The pore sizes of the dry foams were 15–20% smaller than the bubble sizes of the liquid templates, whereas the pore sizes of the swollen porous hydrogels were in the range of the bubble sizes of the liquid templates. Compared to commonly used methods for the fabrication of biopolymer scaffolds, our route neither involves cryogenic treatment nor toxic chemicals or organic solvents and potentially allows for the photoencapsulation of cells