Influence of a CO2-switchable additive on the surface and foaming properties of a cationic non-switchable surfactant.

Switchable materials in general and CO2-switchable materials in particular are of great interest in environmental research. The replacement of common non-switchable materials (solutions, solvents, surfactants, etc.) with their switchable counterparts has a great potential to make processes more environmentally friendly by enhancing reusability and circularity and thus reducing energy costs and material consumption. Inspired by this, the present work deals with the surface and foaming properties of aqueous solutions of a non-switchable surfactant in presence of a CO2-switchable additive. A 1 : 1 and a 1 : 5 (molar ratios) mixture of the non-switchable surfactant C14TAB (tetradecyltrimethylammonium bromide) and the CO2-switchable additive TMBDA (N,N,N,N-tetramethyl-1,4-butanediamine) were investigated. It was found that surface properties, foamability, and foam stability can be changed by switching the additive with CO2 as a trigger. This observation can be explained by the fact that TMBDA is surface active in its unprotonated, i.e. neutral form, which disturbs the tight packing of the surfactant molecules on the surface. As a consequence, foams generated with surfactant solutions containing the neutral TMBDA are less stable than their TMBDA-free counterparts. On the other hand, the switched diprotonated additive is a 2 : 1 electrolyte with hardly any surface activity and thus does not affect surface and foam properties.

From water-rich to oil-rich gelled non-toxic microemulsions.

Gelled non-toxic microemulsions have great potential in transdermal drug delivery: the microemulsion provides an optimum solubilizing capacity for drugs and promotes drug permeation through the skin barrier, while the gel network provides mechanical stability. We have formulated such a gelled non-toxic microemulsion consisting of H2O - isopropyl myristate (IPM) - Plantacare 1200 UP (technical-grade alkyl polyglucoside with an average composition of C12G1.4) - 1,2-octanediol in the presence of the low molecular weight gelator (LMWG) 1,3:2,4-dibenzylidene-d-sorbitol (DBS) at an oil-to-water ratio of φ = 0.50. The study at hand aimed to develop gelled non-toxic microemulsions that can contain both oil- and water-soluble drugs and are either water- or oil-based, depending on the application. To accomplish this, we varied the oil-to-water ratio from being water-rich to oil-rich, i.e. 0.2 ≤ φ ≤ 0.8. Phase studies were carried out along the middle phase trajectory, and a suitable LMWG was identified for all φ-ratios. Electrical conductivity measurements showed that the structure can be tuned from water- to oil-continuous by adjusting the amount of 1,2-octanediol and φ-ratios. The existence of the gel network was visualized by freeze-fracture electron microscopy (FFEM) at three different φ-ratios. We found that all systems from φ = 0.35 to φ = 0.80 form strong gels with nearly the same rheological behavior, while the system with φ = 0.20 is a much weaker gel. We attribute this behavior on the one hand to the microemulsion microstructure and on the other hand to the solvent-dependent gelation properties of DBS, which can be described by the Hansen solubility parameters (HSPs).

Formulation of Gelled Non-toxic Bicontinuous Microemulsions Stabilized by Highly Efficient Alkanoyl Methylglucamides.

Gelled non-toxic bicontinuous microemulsions have a great potential for transdermal drug delivery as the microemulsion facilitates the solubilization of both hydrophilic and hydrophobic drugs, while the gel network provides mechanical stability and thus an easy application on the skin. In our previous study, we formulated a gelled non-toxic bicontinuous microemulsion: we gelled the system H2O-isopropyl myristate (IPM)-Plantacare 1200 UP (C12G1.4)-1,2-octanediol with the low molecular weight organogelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS). However, a large amount of Plantacare 1200 UP (12 wt %) is needed to form a bicontinuous microemulsion. To solve this problem, we studied a new class of surfactants, namely, alkanoyl methylglucamides (MEGA), which have been rarely used for the formulation of microemulsions. The phase behavior of microemulsions stabilized by MEGA-8/10, MEGA-12/14-PC, and MEGA-12/14-HC was compared with that of systems stabilized by alkyl polyglucosides. We found that even with 2 wt % MEGA-12/14-HC, a bicontinuous microemulsion can be formed, which is 1/6 of the amount of Plantacare 1200 UP. The bicontinuous microstructure of the non-toxic microemulsion H2O-IPM-MEGA-12/14-HC-1,2-octanediol was confirmed by small-angle neutron scattering. Furthermore, the phase boundaries remained unchanged when gelled by DBS. The rheological properties of the gel were studied by oscillatory shear rheometry. Finally, freeze-fracture electron microscopy images show the coexistence of gel fibers and bicontinuous oil and water domains. These results suggest that the new gelled non-toxic bicontinuous microemulsion is an orthogonal self-assembled system.

Gelling Lyotropic Liquid Crystals with the Organogelator 1,3:2,4-Dibenzylidene-d-sorbitol Part I: Phase Studies and Sol-Gel Transitions.

Gelled lyotropic liquid crystals (gelled LLCs) are the combination of an LLC and a gel network. One method for obtaining gelled LLCs is the addition of a low molecular weight gelator, which forms gels via self-assembled fibrillar networks, to an aqueous surfactant solution. A potent gelator for LLCs is the LMW organogelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS). This gelator gels the lamellar Lα phase, the bicontinuous cubic V1 phase, and the hexagonal H1 phase of the system H2O-C12E7 (heptaethylene glycol monododecyl ether) without influencing the phase boundaries as visual phase studies and rheometry show. Varying the DBS mass fraction η, one can adjust the sol-gel transition temperature Tsol-gel of the gelled LLCs. At η = 0.0075, all Tsol-gel values are below the LLC-to-isotropic phase transition temperatures TLLC-iso, that is, the LLCs are formed first while cooling down, followed by gel formation. At η = 0.015, however, Tsol-gel > TLLC-iso, that is, the gel is formed in an isotropic solvent, which becomes an LLC while cooling down. The system H2O-C12E7 is the first where an adjustment of the gelator concentration allowed us to decouple gel and LLC formation for all three LLCs, that is, gel and LLC formation happen one after the other and not simultaneously. This allows us to study whether the structure and thus the properties of gelled LLCs can be manipulated by the order of gel and LLC formation. We discuss our findings in light of the following question: are our gelled LLCs truly orthogonal self-assembled systems, that is, do the LLCs and the gel network form and coexist independently?

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.

How Promoting and Breaking Intersurfactant H-Bonds Impact Foam Stability.

On the basis of previous results revealing that intersurfactant H-bonds improve foam stability, we now focus on how foams stabilized by two different N-acyl amino acid surfactants are affected by different salts (NaF, NaCl, NaSCN), which can promote or break intersurfactant H-bonds. The chosen surfactants, namely, sodium N-lauroyl sarcosinate (C12SarcNa) and sodium N-lauroyl glycinate (C12GlyNa), differ only by one methyl group at the nitrogen of the amide bond that blocks intersurfactant H-bonds in the case of C12SarcNa. The salts were chosen because they are kosmotropic (NaF), chaotropic (NaSCN), and in between (NaCl) and thus influence the formation of an H-bond network in different ways. Surface tension measurements showed that the addition of salts decreased the cmcs of both surfactants and increased the packing density, as expected. Moreover, in presence of the salts, the head groups of the H-bond forming surfactant C12GlyNa were more tightly packed at the surface than the C12SarcNa head groups. The effect of the salts on foam stability was studied by analysis of the foam height, the foam liquid fraction, and by image analysis of the foam structure. As expected, the salts had no significant effect on foams stabilized by C12SarcNa, which is unable to form intersurfactant H-bonds. In contrast, the stability of C12GlyNa-containing foams followed the trend NaF > NaCl > NaSCN, which is in agreement with NaF promoting and NaSCN breaking intersurfactant H-bonds. Surface rheology measurements allowed us to correlate foam stability with surface elasticity. This study provides new insights into the importance of H-bond promoters and breakers, which should be used in the future design of tailor-made surfactants.

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.

How Cellulose Nanofibrils Affect Bulk, Surface, and Foam Properties of Anionic Surfactant Solutions.

We study the generation and decay of aqueous foams stabilized by sodium dodecyl sulfate (SDS) in the presence of unmodified cellulose nanofibrils (CNF). Together with the rheology of aqueous suspensions containing CNF and SDS, the interfacial/colloidal interactions are determined by quartz crystal microgravimetry with dissipation monitoring, surface plasmon resonance, and isothermal titration calorimetry. The results are used to explain the properties of the air/water interface (interfacial activity and dilatational moduli determined from oscillating air bubbles) and of the bulk (steady-state flow, oscillatory shear, and capillary thinning). These properties are finally correlated to the foamability and to the foam stability. The latter was studied as a function of time by monitoring the foam volume, the liquid fraction, and the bubble size distribution. The shear-thinning effect of CNF is found to facilitate foam formation at SDS concentrations above the critical micelle concentration (cSDS ≥ cmc). Compared with foams stabilized by pure SDS, the presence of CNF enhances the viscosity and elasticity of the continuous phase as well as of the air/water interface. The CNF-containing foams have higher liquid fractions, larger initial bubble sizes, and better stability. Due to charge screening effects caused by sodium counter ions and depletion attraction caused by SDS micelles, especially at high SDS concentrations, CNF forms aggregates in the Plateau borders and nodes of the foam, thus slowing down liquid drainage and bubble growth and improving foam stability. Overall, our findings advance the understanding of the role of CNF in foam generation and stabilization.

Surface Activity and Foaming Capacity of Aggregates Formed between an Anionic Surfactant and Non-Cellulosics Leached from Wood Fibers.

This study relates to the release of non-cellulosic components (cell wall heteropolysaccharides, lignin, and extractives) from swollen wood fibers in the presence of an anionic surfactant (sodium dodecyl sulfate, SDS) at submicellar concentrations. Highly surface-active aggregates form between SDS and the leached, non-cellulosic components, which otherwise do not occur in the presence of cationic or nonionic surfactants. The in situ and efficient generation of liquid foams in the presence of the leached species is demonstrated. The foaming capacity and foam stability, as well as the foam's structure, are determined as a function of the composition of the aqueous suspension. The results indicate that naturally occurring components bound to wood fibers are extractable solely with aqueous solutions of the anionic surfactant. Moreover, they can form surface-active aggregates that have a high foaming capacity. The results further our understanding of residual cell wall components and their role in the generation of foams.

Gelled non-toxic microemulsions: phase behavior & rheology.

Bicontinuous microemulsions gelled with a low molecular weight gelator have been shown to be an orthogonally self-assembled system. With the mechanical stability provided by the gel network, gelled non-toxic bicontinuous microemulsions have the potential to be an efficient transdermal drug delivery carrier. However, up to now no suitable system has been formulated for transdermal drug delivery. To fill this gap, we formulated and characterized a gelled non-toxic bicontinuous microemulsion suitable for the mentioned application. Starting from a previously studied scouting system, namely, H2O-n-octane-n-octyl ß-d-glucopyranoside (ß-C8G1)-1-octanol, the co-surfactant and the oil were replaced by non-toxic components. Subsequently, the expensive pure surfactant was replaced by cheap technical-grade surfactants (Plantacare® series) to make the system economical. Having formulated the non-toxic microemulsion H2O-IPM-Plantacare 1200 UP-1,2-octanediol, three low molecular weight gelators were studied with regard to the gelation of both the scouting system and the non-toxic system. The chosen gelators were 12-hydroxyoctadecanoic acid (12-HOA), 1,3:2,4-dibenzylidene-d-sorbitol (DBS), and N,N'-dibenzoyl-l-cystine (DBC). We found that only DBS gels the non-toxic microemulsion. The gelled non-toxic bicontinuous microemulsion H2O-IPM-Plantacare 1200 UP-1,2-octanediol was characterized with oscillatory shear rheometry and small-angle neutron scattering (SANS) at a DBS concentration of 0.3 wt% to verify that the system is indeed a gel and that the microstructure of the microemulsion is not altered by the gel network.

Hydrogelation with a water-insoluble organogelator - surfactant mediated gelation (SMG).

The low-molecular-weight gelator (LMG) 12-hydroxyoctadecanoic acid (12-HOA) is insoluble in water, but can be solubilized in surfactant micelles. We therefore solubilized 12-HOA at 80 °C in an aqueous solution of cetyltrimethylammonium bromide (CTAB) containing spherical micelles. On cooling this system down to room temperature, a hydrogel is obtained. We will refer to this process as "surfactant-mediated gelation" (SMG). The hydrogels were formed at a lower 12-HOA concentration when sodium salicylate (NaSal) was added to the CTAB system, which induced the formation of wormlike micelles. Hydrogels obtained by SMG from spherical and wormlike micelles are referred to as gelled micellar phases (GMs) and gelled wormlike micellar phases (GWLMs), respectively. Optical microscopy and transmission electron microscopy (TEM) showed that 12-HOA forms self-assembled fibrillar networks (SAFiNs) in both GMs and GWLMs. The sol-gel transition temperature, Tsol-gel, of the GWLM samples was higher than that of the GM samples. Dynamic rheological measurements revealed gel properties (G' > G'' at all angular frequencies) for both gels; however, a higher viscoelasticity was observed for the GWLM samples, which in turn, was reflected in the higher Tsol-gel. Small- and wide-angle X-ray scattering (SWAXS) showed that micelles and gel fibers coexist in the GM and GWLM samples. Our study demonstrates the gelation of aqueous micellar solutions with water-insoluble LMGs.

Tuning gelled lyotropic liquid crystals (LLCs) - probing the influence of different low molecular weight gelators on the phase diagram of the system H2O/NaCl-Genapol LA070.

Gelled lyotropic liquid crystals (LLCs) are highly tunable multi-component materials. By studying a selection of low molecular weight gelators (LMWGs), we find gelators that form self-assembled gels in LLCs without influencing their phase boundaries. We studied the system H2O/NaCl-Genapol LA070 in the presence of (a) the organogelators 12-hydroxyoctadecanoic acid (12-HOA) and 1,3:2,4-dibenzylidene-d-sorbitol (DBS) and (b) the hydrogelators N,N'-dibenzoyl-l-cystine (DBC) and a tris-amido-cyclohexane derivative (HG1). Visual phase studies and oscillation shear frequency sweeps confirmed that 12-HOA acts as co-surfactant (stabilizing the lamellar Lα phase and destabilizing the hexagonal H1 phase), thus preventing gelation. Conversely, DBS was a potent gelator for LLCs, with the phase boundaries un-influenced by the presence of DBS; gelled lamellar Lα, and softly-gelled hexagonal H1 phases are formed. For the hydrogelator DBC, the LLC phase boundaries were only slightly altered, but no gelled LLCs were formed. For the hydrogelator HG1, however, the phase boundaries were unaffected while gelled lamellar Lα and softly-gelled hexagonal H1 phases were formed. Temperature-dependent rheology measurements demonstrated that by changing the DBS or the HG1 concentration, the sol-gel transition temperature of the gelled lamellar Lα phase can be adjusted such that (a) Tsol-gel is below the Lα-isotropic phase transition (DBS, HG1 mass fraction η = 0.0075) and (b) Tsol-gel is above the gelled Lα-isotropic phase transition (DBS, HG1 η = 0.015). This opens the possibility of temporal materials control by addressing phase transitions in different orders. As this system contains oil and water, both the organogelator DBS and the hydrogelator HG1 can gel these LLCs, but this clearly does not apply to all organogelators/hydrogelators. The study indicates that careful optimization of LMWGs is required to avoid interaction with the surfactant layer and to optimize the Tsol-gel value, which is important for the application of LMWGs in gelled LLCs.

Methacrylate-based polymer foams with controllable connectivity, pore shape, pore size and polydispersity.

Polymer foams are becoming increasingly important in industry, especially biodegradable polymer foams are in demand. Depending on the application, polymer foams need to have characteristic properties, which include connectivity and polydispersity. We show how polymer foams with tailor-made structures can be synthesized from water-in-monomer emulsions which were generated via microfluidics. As monomer we used 1,4-butanediol dimethacrylate (1,4-BDDMA). Firstly, we synthesised monodisperse open- and closed-cell poly(1,4-BDDMA) foams with either spherical or hexagonal pore shapes by varying the locus of initiation. Secondly, we were able to control the pore diameters and obtained polymer foams of both connectivities and pore shapes with pore sizes from ∼70 µm up to ∼120 µm by means of one microfluidic chip. Finally, we synthesized poly(1,4-BDDMA) foams with controllable polydispersity. Here, the mean droplet diameter was the same as that of the monodisperse counterparts in order to be able to compare the properties of the resulting polymer foams.

3D-Printing of Functionally Graded Porous Materials Using On-Demand Reconfigurable Microfluidics.

Tailoring the morphology of macroporous structures remains one of the biggest challenges in material synthesis. Herein, we present an innovative approach for the fabrication of custom macroporous materials in which pore size varies throughout the structure by up to an order of magnitude. We employed a valve-based flow-focusing junction (vFF) in which the size of the orifice can be adjusted in real-time (within tens of milliseconds) to generate foams with on-line controlled bubble size. We used the junction to fabricate layered and smoothly graded porous structures with pore size varying in the range of 80-800 µm. Additionally, we mounted the vFF on top of an extrusion printer and 3D-printed constructs characterized by a predefined 3D geometry and a controlled, spatially varying internal porous architecture, such as a model of a bone. The presented technology opens new possibilities in macroporous material synthesis with potential applications ranging from tissue engineering to aerospace industry and construction.

Generation of Solid Foams with Controlled Polydispersity Using Microfluidics.

Many properties of solid foams depend on the distribution of the pore sizes and their organization in space. However, these two parameters are very difficult to control with most traditional foaming techniques. Here we show how microfluidics can be used to tune the polydispersity of the foams (mono- vs different polydispersities) and the spatial organization of the pores (ordered vs disordered). For this purpose, the microfluidic flow-focusing technique was modified such that the gas pressure oscillates periodically, which translates into periodically oscillating bubble sizes in the liquid foam template. The liquid foams were generated from chitosan solutions and then gelled via cross-linking with genipin before we freeze-dried them to obtain a solid foam with a specific structure. The study at hand fills two existing scientific gaps. On the one hand, we present a novel approach for the generation of foams with controlled polydispersity. On the other hand, we obtained a solid foam with a new structure for foam templating consisting of rhombic dodecahedra. The controlled variation of the foam's structure will allow studying systematically structure-property relations. Moreover, being fully biobased, this type of solid foam is a suitable candidate for applications in tissue engineering.

Microstructure of ionic liquid (EAN)-rich and oil-rich microemulsions studied by SANS.

In a previous study we investigated the phase behavior of microemulsions consisting of the ionic liquid ethylammonium nitrate (EAN), an n-alkane and a nonionic alkyl polyglycolether (CiEj). We found the same general trends as for the aqueous counterparts, i.e. a transition from an oil-in-EAN microemulsion via a bicontinuous microemulsion to an EAN-in-oil microemulsion with increasing temperature. However, unlike what happens in the corresponding aqueous systems, in EAN-in-oil microemulsions only a very small amount of EAN was detected by NMR-measurements. This is why we investigated the phase behavior and microstructure of EAN-rich n-dodecane-in-EAN microemulsions and oil-rich EAN-in-n-octane microemulsions. We found that the ionic liquid emulsification failure boundary has an extraordinarily small slope, which suggests that the amphiphilic film loses its ability to solubilize EAN with an increase in temperature by only a few degrees. The analysis of the small angle neutron scattering (SANS) curves unambiguously shows that this behavior is due to the fact that the EAN molecules form a substructure with a characteristic length scale of Λ ≈ 8 Å inside the EAN-in-oil droplets. In more detail, the analysis of the SANS data with the GIFT method revealed a transition from spherical to cylindrical structures approaching the respective critical endpoint temperatures. By using the respective form factors and combining them with a Gaussian spatial intensity distribution to account for the EAN sub-structure we were able to describe the scattering curves nearly quantitatively.

Emulsion and Foam Templating-Promising Routes to Tailor-Made Porous Polymers.

Emulsions, foams, and foamed emulsions have been used successfully as templates for the synthesis of macroporous polymers. Based on this knowledge this Minireview presents strategies to use, optimise, and upscale these templating methods to synthesise tailor-made porous polymers. The uniqueness of such polymers lies in the ability to tailor their structures and, therefore, their properties. However, systematic studies on structure-property relations are lacking mainly because the templating scientific community is "split into two": the polydisperse and monodisperse camps. Thus, it is time to build a bridge between the camps, that is, to synthesise porous polymers with very different structures from the same precursors to determine the relationship between the structure and the properties.

Creating Honeycomb Structures in Porous Polymers by Osmotic Transport.

Understanding why honeycombs are shaped the way they are has moved biologists, physicists, chemists, and mathematicians alike. It was only recently that the honeycombs' shape "at birth" was included in the ongoing discussions: at birth, the cells are spherical but then transform into the well-known hexagons. It was proposed that a flow of wax-driven by surface tension effects-is the reason for this transformation. Our recent work on synthetic polymer foams with honeycomb-like structures points towards a very different mechanism. Just like in honeycomb cells, we observe that a spherical "initial state" transforms into a hexagon-shaped "final state" during polymerization. We have experimental evidence that a concentration gradient arises during polymerization, which transports monomers such that the spherical template becomes a honeycomb structure with walls of homogeneous thickness. The knowledge about this mechanism suggests promising strategies for the development of lightweight materials with optimized mechanical properties.

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 microemulsion (∼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.

Gelling Lamellar Phases of the Binary System Water-Didodecyldimethylammonium Bromide with an Organogelator.

Does the presence of a gel network influence the properties of a lyotropic liquid crystal? Does the replacement of oil by a lyotropic liquid crystal influence the properties of an organogel? To answer these questions we study gelled lyotropic liquid crystals (LLC). In the present study we show that it is possible to gel the lamellar phase of the binary system water-didodecyl dimethylammonium bromide (2C12DAB) with the organogelator 12-hydroxyoctadecanoic acid (12-HOA). We compare various properties of the gelled LLC phases with the "parent systems", i.e., with the binary organogel consisting of n-decane-12-HOA and with the nongelled LC phases, respectively. Optical and electron microscopy, differential scanning calorimetry (DSC), rheometry, as well as small and wide-angle X-ray scattering (SWAXS) proved the coexistence of an Lα phase and a 12-HOA gel network in the gelled Lα phase. However, a small influence of the Lα phase on the gel properties was seen, namely slightly lower sol-gel transition temperatures and viscoelastic moduli of the gelled Lα phase compared to the binary gel. On the other hand, the presence of the gel also has an influence on the Lα phase: the interlayer spacing of the surfactant bilayers in the gelled Lα phases is slightly larger compared to the nongelled Lα phases, which is due to mixing part of the 12-HOA molecules in the Lα bilayers. Despite this mutual influence the structures of both the Lα phase and the gel network are hardly disturbed in the gelled Lα phase, i.e., that the self-assembly of the surfactant and of the gelator molecules clearly occur in an orthogonal way.