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.

In Situ Ultra-Small- and Small-Angle X-ray Scattering Study of ZnO Nanoparticle Formation and Growth through Chemical Bath Deposition in the Presence of Polyvinylpyrrolidone.

ZnO inverse opals combine the outstanding properties of the semiconductor ZnO with the high surface area of the open-porous framework, making them valuable photonic and catalysis support materials. One route to produce inverse opals is to mineralize the voids of close-packed polymer nanoparticle templates by chemical bath deposition (CBD) using a ZnO precursor solution, followed by template removal. To ensure synthesis control, the formation and growth of ZnO nanoparticles in a precursor solution containing the organic additive polyvinylpyrrolidone (PVP) was investigated by in situ ultra-small- and small-angle X-ray scattering (USAXS/SAXS). Before that, we studied the precursor solution by in-house SAXS at T = 25 °C, revealing the presence of a PVP network with semiflexible chain behavior. Heating the precursor solution to 58 °C or 63 °C initiates the formation of small ZnO nanoparticles that cluster together, as shown by complementary transmission electron microscopy images (TEM) taken after synthesis. The underlying kinetics of this process could be deciphered by quantitatively analyzing the USAXS/SAXS data considering the scattering contributions of particles, clusters, and the PVP network. A nearly quantitative description of both the nucleation and growth period could be achieved using the two-step Finke-Watzky model with slow, continuous nucleation followed by autocatalytic growth.

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).

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.

Micellar Lyotropic Nematic Gels.

Lyotropic liquid crystal (LLC) gels are a new class of liquid crystal (LC) networks that combine the anisotropy of micellar LLCs with the mechanical stability of a gel. However, so far, only micellar LLC gels with lamellar and hexagonal structures have been obtained by the addition of gelators to LLCs. Here, the first examples of lyotropic nematic gels are presented. The key to obtain these nematic gels is the use of gelators that have a non-amphiphilic molecular structure and thus leave the size and shape of the micellar aggregates essentially unchanged. By adding these gelators to lyotropic nematic phases, an easy and reproducible way to obtain large amounts of lyotropic nematic gels is established. These nematic gels preserve the long-range orientational order and optical birefringence of a lyotropic nematic phase but have the mechanical stability of a gel. LLC nematic gels are promising new materials for elastic and anisotropic hydrogels to be applied as water-based stimuli-responsive actuators and sensors.

Aescin - a natural soap for the formation of lipid nanodiscs with tunable size.

The saponin ß-aescin from the seed extract of the horse chestnut tree Aesculus hippocastanum has demonstrated a beneficial role in clinical therapy which is in part related to its strong interaction with biological membranes. In this context the present work investigates the self-assembly of nm-sized discoidal lipid nanoparticles composed of ß-aescin and the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The discoidal lipid nanoparticles reassemble from small discs into larger discs, ribbons and finally stacks of sheets upon heating from gel-phase to fluid phase DMPC. The morphological transition of the lipid nano-particles is mainly triggered by the phospholipid phase state change. The final morphology depends on the phospholipid-to-saponin ratio and the actual temperature. The study is conducted by small-angle X-ray scattering (SAXS) and transmission (TEM) and freeze fracture electron microscopy (FFEM) are used to cover larger length scales. Two different models, representing a disc and ribbon-like shape are applied to the SAXS data, evaluating possible geometries and molecular mixing of the nano-particles. The stacked sheets are analysed by the Caillé theory.

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 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.

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.

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.

On how hydrogen bonds affect foam stability.

Do intermolecular H-bonds between surfactant head groups play a role for foam stability? From the literature on the foam stability of various surfactants with C12 alkyl chains but different head groups a clear picture emerges: stable foams are only generated when hydrogen bonds can form between the head groups, i.e. when the polar head group has a hydrogen bond donor and a proton acceptor. Stable foams can therefore be generated with surfactants having a sugar unit, a glycine, an amine oxide (at pH~5), or a carboxylic acid (at pH~pKa) as polar head group. On the other hand, aqueous foams stabilized with surfactants having oligo(ethylene oxide), phosphine oxide, quaternary ammonium, sulfate, sarcosine, amine oxide (at pH≠5), or carboxylic acid (at pH≠pKa) are not very stable. These observations suggest that hydrogen bonds between neighbouring molecules at the surface enhance foam stability. Formation of hydrogen bonds between surfactant head groups gives rise to a short-range attractive interaction that may restrict the surfactant's mobility while providing a more elastic surfactant layer which can counteract deformations. To support our hypothesis we carried out a systematic foaming study of two types of surfactants, one of them being capable of forming H-bonds and the other one not. Generating foams of all surfactants mentioned above with the same foaming conditions we found that stable foams are obtained when the head group is capable of forming intersurfactant H-bonds. The outcome of this study constitutes a new step towards the implementation of H-bonds in the future design of surfactants.

Phosphine oxide surfactants revisited.

This review summarizes everything we currently know about the nonionic surfactants alkyl dimethyl (C(n)DMPO) and alkyl diethyl (C(n)DEPO) phosphine oxide (PO surfactants). The review starts with the synthesis and the general properties (Section 2) of these compounds and continues with their interfacial properties (Section 3) such as surface tension, surface rheology, interfacial tension and adsorption at solid surfaces. We discuss studies on thin liquid films and foams stabilized by PO surfactants (Section 4) as well as studies on their self-assembly into lyotropic liquid crystals and microemulsions, respectively (Section 5). We aim at encouraging colleagues from both academia and industry to take on board PO surfactants whenever possible and feasible because of their broad variety of excellent properties.

Gelled Lyotropic Liquid Crystals.

In our previous work we were able to prove that gelled bicontinuous microemulsions are a novel type of orthogonal self-assembled system. The study at hand aims at complementing our previous work by answering the question of whether gelled lyotropic liquid crystals are also orthogonal self-assembled systems. For this purpose we studied the same system, namely, water-n-decane/12-hydroxyoctadecanoic acid (12-HOA)-n-decyl tetraoxyethylene glycol ether (C10E4). The phase boundaries of the nongelled and the gelled lyotropic liquid crystals were determined visually and with (2)H NMR spectroscopy. Oscillating shear measurements revealed that the absolute values of the storage and loss moduli of the gelled liquid crystalline (LC) phases do not differ very much from those of the binary organogel. While both the phase behavior and the rheological properties of the LC phases support the hypothesis that gelled lyotropic liquid crystals are orthogonal self-assembled systems, freeze-fracture electron microscopy (FFEM) seems to indicate an influence of the gel network on the structure of the Lα phase and vice versa.

Effects of protonation on foaming properties of dodecyldimethylamine oxide solutions: a pH-study.

The critical micelle concentration (cmc), the surface excess (Γ), as well as the micelle aggregation number (m) of the surfactant dodecyldimethylamine oxide (C12DMAO) have been reported to strongly depend on the pH-value of the aqueous surfactant solution. At high ionic strength, the cmc displays a minimum, while both Γ and m have a maximum at a pH-value close to the pKa of the surfactant. These experimental observations have been explained as being due to specific hydrogen bonds between the head groups, which are formed once the surfactant is partly or fully protonated. This investigation addresses the question of whether the pH also affects the foaming properties of C12DMAO solutions. To answer this question we measured the foamability and the foam stability of C12DMAO solutions at a fixed C12DMAO concentration of 5 cmc for five different pH-values, namely pH = 2, 3, 5, 8, and 10. We found that the foamability is hardly affected by the pH-value, while the foam stability strongly depends on the pH. As is the case for the above mentioned properties, the foam stability also displays an extremum in the studied pH-range, namely a maximum at pH = 5. We discuss our results in terms of the hydrogen bond hypothesis and show that this hypothesis indeed is in line with the observed trend for the foam stability. Moreover, we discuss that hydrogen bond formation may rationalize how the molecular structure of a surfactant affects foam stability.

Dilational surface rheology studies of n-dodecyl-ß-D-maltoside, hexaoxyethylene dodecyl ether, and their 1:1 mixture.

It is time to review latest activities on the dilational surface rheology of the two nonionic surfactants n-dodecyl-ß-D-maltoside (ß-C12G2) and hexaoxyethylene dodecyl ether (C12E6) and their 1:1 mixture as a lot of different data generated with different techniques have been published in the last years. As the data are scattered throughout different papers and were generated with different techniques, we carried out an extensive study with one technique, which we will use as reference for the discussion of different data sets. We found that the results are in most of the cases in line with already published data as regards the general trends. However, a quantitative comparison reveals differences, which may result in different interpretations of the data. In the review at hand, we summarize, compare and discuss our latest and previously published data.