Time Dependence of Gel Formation in Lyotropic Nematic Liquid Crystals: From Hours to Weeks.

The combination of lyotropic liquid crystals (LLCs) and low-molecular-weight gelators (LMWGs) for the formation of lyotropic liquid crystal gels (LLC gels) leads to a versatile and complex material combining properties of both parent systems. We gelled the calamitic nematic NC phases of a binary and ternary system using the LMWG 3,5-bis-(5-hexylcarbamoyl-pentoxy)-benzoic acid hexyl ester (BHPB-6). This binary system consists of the surfactant N,N-dimethyl-N-ethyl-1-hexadecylammonium bromide (CDEAB) and water, whereas the ternary system consists of the surfactant N,N,N-trimethyl-N-tetradecylammonium bromide (C14TAB), the cosurfactant n-decanol, and water. Though containing similar surfactants, the gelled NC phases of the binary and ternary systems show differences in their visual and gel properties. The gelled NC phase of the binary system remains clear for several days after preparation, whereas the gelled NC phase of the ternary system turns turbid within 24 h. We investigated the time evolution of the gel strength with oscillation rheology measurements (a) within the first 24 h and (b) up to two weeks after gel formation. The shape of the fibers was investigated over different time scales with freeze fracture electron microscopy (FFEM). We demonstrate that despite their similarities, the two LLC gels also have distinct differences.

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.

Liquid foams as sensors for the detection of biomarkers.

Bioassays are widely used in healthcare to detect and quantify biomarkers, such as molecules or enzymes, which are crucial in monitoring diseases and health conditions. In developed countries, healthcare professionals use specialized reagents and equipment's to perform these bioassays. However, in less-industrialized countries, the creation of low cost, fast, and technically simple bioassays is required. Herein, we propose a simple approach for detecting biochemical markers using host-guest complexes containing a surfactant. When the biochemical marker is present, the host-guest complex is disrupted, releasing the surfactant and producing foam. The read-out mechanism relies on the change of foam volume as function of biomarker concentration. This change is quantifiable by the naked eye and can be measured with a simple ruler. We claim that the use of foams as sensing tool is an attractive, inexpensive, fast, and easy to handle on-site detection method.

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.

New Gelatin-Based Hydrogel Foams for Improved Substrate Conversion of Immobilized Horseradish Peroxidase.

Hydrogel foams provide an aqueous environment that is very attractive for the immobilization of enzymes. To this end, functional polymers are needed that can be converted into hydrogel foams and that enable bioconjugation while maintaining high enzyme activity. The present study demonstrates the potential of biotinylated gelatin methacryloyl (GM10EB) for this purpose. GM10EB is synthesized in a two-step procedure with gelatin methacryloyl (GM10) being the starting point. Successful biotinylation is confirmed by 2,4,6-trinitrobenzene sulfonic acid and 4'-hydroxyazobenzene-2-carboxylic acid/avidin assays. Most importantly, a high methacryloyl group content (DM) is maintained in GM10EB, so that solutions of GM10EB show both a sufficiently low viscosity for microfluidic foaming and a pronounced curing behavior. Thus, foamed and nonfoamed GM10EB hydrogels can be prepared via radical crosslinking of the polymer chains. Within both sample types, biotin groups are available for bioconjugation, as is demonstrated by coupling streptavidin-conjugated horseradish peroxidase to the hydrogels. When assessing the substrate conversion rate rA in foamed hydrogels by enzymatic conversion of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), a value for rA 12 times higher than the value for nonfoamed hydrogels of the same mass is observed. In other words, rA can be successfully tailored by the hydrogel morphology.

PoreScript: Semi-automated pore size algorithm for scaffold characterization.

The scaffold pore size influences many critical physical aspects of tissue engineering, including tissue infiltration, biodegradation rate, and mechanical properties. Manual measurements of pore sizes from scanning electron micrographs using ImageJ/FIJI are commonly used to characterize scaffolds, but these methods are both time-consuming and subject to user bias. Current semi-automated analysis tools are limited by a lack of accessibility or limited sample size in their verification process. The work here describes the development of a new MATLAB algorithm, PoreScript, to address these limitations. The algorithm was verified using three common scaffold fabrication methods (e.g., salt leaching, gas foaming, emulsion templating) with varying pore sizes and shapes to demonstrate the versatility of this new tool. Our results demonstrate that the pore size characterization using PoreScript is comparable to manual pore size measurements. The PoreScript algorithm was further evaluated to determine the effect of user-input and image parameters (relative image magnification, pixel intensity threshold, and pore structure). Overall, this work validates the accuracy of the PoreScript algorithm across several fabrication methods and provides user-guidance for semi-automated image analysis and increased throughput of scaffold characterization.

Mineral plastic foams.

We describe a route to synthesize a mechanically stable, non-flammable poly(acrylic acid)-calcium salt (the so-called mineral plastic) foam whose structure can be tailored. Main steps of the foam synthesis are: (1) foaming of the PAA-containing solution, (2) gelation of the continuous foam phase, and (3) drying of the hydrogel foam. The main challenge was to formulate an aqueous solution with a large amount of poly(acrylic acid), PAA, and calcium to yield a mechanically stable foam. The resulting PAA-based solid foams with pore sizes of around 220 µm can easily be dissolved, i.e. recycled, in an acidic solution.

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).

Experimental evidence of a transition from a sponge-like to a foam-like nanostructure in water-rich L3 phases.

HYPOTHESIS: The micrometer-sized gas bubbles of a liquid foam with a dispersed gas phase of > 74 vol% are polyhedral and surrounded by a continuous aqueous phase. The structure of a water-rich microemulsion with a water phase of > 74 vol% normally consists of oil droplets in water or is bicontinuous. We hypothesize that at these high water contents polyhedral water droplets in oil can also exist. EXPERIMENTS: We (a) carried out phase studies on the water-rich side of the phase diagram of the quaternary system water/NaCl - hexyl methacrylate - AOT, because AOT is known for its propensity to form water-in-oil structures and hexyl methacrylate can be polymerized, (b) measured the electrical conductivities and viscosities, and (c) visualized the nanostructure with freeze-fracture electron microscopy (FFEM). FINDINGS: We found narrow 1-phase regions emanating from the L3 phase of the oil-free water/NaCl - AOT system by adding small amounts of oil. In these regions the conductivities become extremely low and the viscosities are extremely high. In addition, FFEM images clearly show the foam-like nanostructure.

Less is more: Unstable foams clean better than stable foams.

HYPOTHESIS: Foamed surfactant solutions can clean surfaces! We hypothesise that the cleaning efficiency depends on the liquid fraction and on the stability of the foam. We also hypothesise that the cleaning efficiency is the better the smaller the average bubble size is. EXPERIMENTS: The double syringe technique was used to generate foams with varying liquid fractions but the same, very small bubble sizes with and without perfluorohexane in the gas phase. We performed cleaning tests in which the foams were applied to glass substrates contaminated with a fluorescent oil. FINDINGS: We found that unstable foams clean better than stable foams. Three cleaning mechanisms were identified: (1) imbibition at low liquid fractions, (2) wiping, i.e., shifting of the contact line between oil, foam and glass, at all liquid fractions, and (3) drainage at high liquid fractions. The change of the liquid fraction and of the foam stability lead to different combinations of these mechanisms and thus to different cleaning results.

Tailoring and visualising pore openings in gelatin-based hydrogel foams.

HYPOTHESIS: While tailoring the pore diameters in hydrogel foams has been demonstrated in numerous studies, fine control over the diameters of the pore openings is still a challenge. We hypothesise that this can be achieved by controlling the size of the thin films which separate the bubbles in the liquid foam template. If this is the case, systematic changes of the template's gas fraction ϕ (the higher ϕ, the larger are the thin films) will lead to corresponding changes of the pore opening diameter. EXPERIMENTS: Since the size of the thin films depends on both bubble size 〈Db〉 and gas fraction ϕ, we need to decouple both parameters to control the film size. Thus, we generated foams with constant bubble sizes via microfluidics and adjusted the gas fractions via two different techniques. The foams were solidified using UV light. Subsequently, they were analysed with confocal fluorescence microscopy. FINDINGS: We were able to change the pore opening diameter 〈dp〉 at a constant pore diameter 〈Dp〉 by adjusting the gas fraction of the foam template. The obtained 〈dp〉/〈Dp〉 ratios are between those obtained theoretically for disordered foams and FCC ordered foams, respectively.

Emulsion templating: Unexpected morphology of monodisperse macroporous polymers.

HYPOTHESIS: We synthesised monodisperse macroporous polymers via polymerisation of water-in-monomer droplet emulsions and obtained non-spherical pores with layered pore walls. We hypothesise that this morphology is caused by surfactant diffusion and phase separation during polymerisation. EXPERIMENTS: We varied the surfactant mass fraction of the emulsions and polymerised the templates with a water-soluble initiator. From the resulting macroporous polymers we determined the shape of the pores and thickness of the layers via scanning election microscopy. The response of the monomer/surfactant mixture to polymerisation was studied by a ternary phase diagram that simulated polymerisation. FINDINGS: The emergence of non-spherical pores with layered pore walls is indeed caused by surfactant diffusion and phase separation. During polymerisation the surfactant molecules diffuse either to the water/monomer interface or deeper into the continuous monomer phase. The first process results in non-spherical pores, while the second process generates layered pore walls.

Intersurfactant H-bonds between head groups of n-dodecyl-ß-d-maltoside at the air-water interface.

HYPOTHESIS: In previous studies we looked at the foam stability of various surfactants with C12 alkyl chains but different head groups and found that stable foams are only generated if the head groups are capable of forming hydrogen bonds with each other. Despite the consistency of the experimental data with the conclusions drawn from it we had no direct proof for our hypothesis that H-bonds are formed between surfactant head groups. EXPERIMENTS: To fill this gap, i.e. to demonstrate intersurfactant H-bond formation, we chose the non-ionic sugar surfactant n-dodecyl-ß-d-maltoside (ß-C12G2) and used molecular dynamics (MD) simulations as well as grazing-incidence X-ray (GIX) scattering and diffraction to study the surfactant-loaded air-water interface. FINDINGS: (1) In a densely packed monolayer, close to the critical micelle concentration (cmc), each head group of the sugar surfactant is involved in ∼5 intersurfactant H-bonds with other head groups and in ∼5 H-bonds with water molecules. (2) The number of intersurfactant H-bonds decreases, while the number of surfactant-water H-bonds increases with increasing distance between the head groups (below the cmc). (3) Even at very large distances (well below the cmc) there are still intersurfactant H-bonds, which we ascribe to the formation of clusters at the surface. (4) GIX scattering revealed that a homogeneous surfactant monolayer is formed at full coverage (around the cmc), i.e. cluster formation only happens below the cmc.

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.

Monodisperse liquid foams via membrane foaming.

HYPOTHESIS: It is possible to generate fairly monodisperse liquid foams by a dispersion cell, which was originally designed for the generation of fairly monodisperse emulsions. If this is the case, scaling-up the production of monodisperse liquid and solid foams will be no longer a problem. EXPERIMENTS: We used the dispersion cell - a batch process - and examined the influence of stirrer speed, membrane pore diameter and injection rate on the structure of the resulting liquid foams. We used an aqueous surfactant solution as scouting system. Once the experimental conditions were known we generated gelatin-based liquid foams and methacrylate-based foamed emulsions. FINDINGS: We found that (a) the bubble size of the generated liquid foams can be adjusted by varying the membrane pore diameter, (b) no stirrer should be used to obtain monodisperse foams, and (c) the bubble size is not influenced by the air injection rate. Since (i) the output for all investigated systems is up to two orders of magnitude larger compared to microfluidics and (ii) the membrane technology can very easily be scaled-up if run in a continuous process, the use of membrane foaming is expected to be heavily used for the generation of monodisperse liquid and solid foams, respectively.

Formulation and polymerization of foamed 1,4-BDDMA-in-water emulsions.

Emulsion and foam templating allow the synthesis of tailor-made polymer foams. A complementary templating route is foamed emulsion templating. The concept is based on the generation of a monomer-in-water emulsion which is subsequently foamed. After polymerization of the foamed emulsion, one obtains open-cell polymer foams with porous pore walls. In the paper at hand, we generated foamed emulsions and synthesized polymer foams which are based on the monomer 1,4-butanediol dimethacrylate (1,4-BDDMA). The main challenge was to find the optimal composition of the emulsion by varying the components systematically. We will discuss that the composition of the monomer-in-water emulsion is key for the stability of the foamed emulsion and thus for the structure of the resulting polymer foam. The final composition of the continuous phase was found to be 65 vol% 1,4-BDDMA, 30 vol% water and 5 vol% glycerol. We foamed and polymerized this emulsion to check the foamed emulsion's suitability as a template for solid polymer foams. We generated a foamed emulsion with a mean bubble diameter of 151 µm ± 90 µm and obtained a highly porous poly(1,4-BDDMA) foam with a pore mean diameter of 366 µm ± 91 µm. Furthermore, the polymer foam has a "sub-porosity" within the pore walls.

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.

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.

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.

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.