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

Self-Assembly of Lamellae-in-Lamellae by Double-Tail Cationic Surfactants

The molecular structures of surfactants play a pivotal role in influencing their self-assembly behaviors. In this work, using simulations and experiments, an unconventional hierarchically layered structure in the didodecyldimethylammonium bromide (DDAB)/water binary system: lamellae-in-lamellae is revealed, a new self-assembly structure in surfactant system. This self-assembly structure refers to a lamellar structure with a shorter periodic length (inner lamellae) embedded in a lamellar phase with a longer periodic length (outer lamellae). The normal vectors of these two lamellar regions orient perpendicularly. In addition, it is observed that this lamellar-in-lamellar phase disappears when the two tails of the cationic surfactants become longer. The formation of the lamellar-in-lamellar architecture arises from multiple interacting factors. The key element is that the short tails of the DDAB surfactants enhance hydrophilicity and rigidity, which facilitates the formation of the inner lamellae. Moreover, the lateral monolayer of the inner lamellae provides shielding from the water and prompts the formation of the outer lamellae. These findings indicate that molecular structures and flexibility can profoundly redirect the hierarchical self-assembly behaviors in amphiphilic systems. More broadly, this work presents a new strategy to deliberately program hierarchical nanomaterials by designing specific surfactant molecules to act as tunable scaffolds, reactors, and carriers.

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.

Manipulating supramolecular gels with surfactants: Interfacial and non-interfacial mechanisms

Gel is a class of self-supporting soft materials with applications in many fields. Fast, controllable gelation, micro/nano structure and suitable rheological properties are essential considerations for the design of gels for specific applications. Many methods can be used to control these parameters, among which the additive approach is convenient as it is a simple physical mixing process with significant advantages, such as avoidance of pH change and external energy fields (ultrasound, UV light and others). Although surfactants are widely used to control the formation of many materials, particularly nanomaterials, their effects on gelation are less known. This review summarizes the studies that utilized different surfactants to control the formation, structure, and properties of molecular and silk fibroin gels. The mechanisms of surfactants, which are interfacial and non-interfacial effects, are classified and discussed. Knowledge and technical gaps are identified, and perspectives for further research are outlined. This review is expected to inspire increasing research interest in using surfactants for designing/fabricating gels with desirable formation kinetics, structure, properties and functionalities.

Interpenetrated Biosurfactant-Biopolymer Orthogonal Hydrogels: The Biosurfactant's Phase Controls the Hydrogel's Mechanics

Controlling the viscoelastic properties of hydrogels is a challenge for many applications. Low molecular weight gelators (LMWGs) like bile salts and glycolipids and biopolymers like chitosan and alginate are good candidates for developing fully biobased hybrid hydrogels that combine the advantages of both components. Biopolymers lead to enhanced mechanics, while LMWGs add functionality. In this work, hybrid hydrogels are composed of biopolymers (gelatin, chitosan, and alginate) and microbial glycolipid bioamphiphiles, known as biosurfactants. Besides their biocompatibility and natural origin, bioamphiphiles can present chameleonic behavior, as pH and ions control their phase diagram in water around neutrality under strongly diluted conditions (<5 wt%). The glycolipid used in this work behaves like a surfactant (micellar phase) at high pH or like a phospholipid (vesicle phase) at low pH. Moreover, at neutral-to-alkaline pH in the presence of calcium, it behaves like a gelator (fiber phase). The impact of each of these phases on the elastic properties of biopolymers is explored by means of oscillatory rheology, while the hybrid structure is studied by small angle X-ray scattering. The micellar and vesicular phases reduce the elastic properties of the hydrogels, while the fiber phase has the opposite effect; it enhances the hydrogel's strength by forming an interpenetrated biopolymer-LMWG network.

Hydrogel Formation by Glutamic-acid-based Organogelator Using Surfactant-mediated Gelation

Hydrogels formed by low-molecular-weight gelators have reversible sol-gel transition and responsiveness to various stimuli, and are used in cosmetics and drug applications. It is challenging to obtain hydrogels using novel gelators because subtle differences in their molecular architecture affect gelation. Organogelators (which form organogels) are insoluble in water, and their use as hydrogelators has not previously been considered. However, a surfactant-mediated gelation method was reported in which organogelators were solubilized in water by surfactants to form hydrogels using 12-hydroxyoctadecanoic acid. To investigate whether this method can be applied with other organogelators, the formation of hydrogel using a glutamic-acid-based organogelator was studied here. Hydrogels were formed by solubilizing 1:1 mixtures of glutamate-based organogelators, N-lauroyl-L-glutamic acid dibuthylamide, and N-2-ethylhexanoyl-L-glutamic acid dibutylamide in aqueous micellar solutions of anionic surfactant (sodium lauroyl glutamate) and cationic surfactant (cetyltrimethylammonium chloride). The minimum gelation concentration of the hydrogel was ~0.2-0.6 wt%. By changing the molar fraction of cetyltrimethylammonium chloride in the mixed surfactant, either spherical or wormlike micelles were formed. The hydrogel with wormlike micelles had a higher sol-gel transition temperature than that with spherical micelles and formed fine self-assembled fibrillar networks. Additionally, the hydrogel with the spherical micelles was elastic, whereas that with wormlike micelles was viscoelastic, suggesting that networks of the organogelators and wormlike micelles coexisted in the hydrogel from the wormlike micellar solution. Moreover, the hydrogel suppressed the reduction in the storage modulus at higher temperatures compared with the micellar aqueous solution, indicating that the elastic properties of the organogelator networks were maintained at high temperatures. The gel fibers of the hydrogel partially formed a loosely aggregated structure as the temperature increased, the fibers bundled via hydrophobic interactions, and new cross-linking points formed spontaneously. This phenomenon corresponded with an inflection point in the temperature-dependent storage modulus of the hydrogel.

Phase Change Material with Gelation Imparting Shape Stability

Blending two gelators with different chemistries (12-hydroxystearic acid and a bis-urea derivative, Millithix MT-800) was used to impart shape stability to CrodaTherm 29, a bio-based phase change material (PCM), melting/crystallizing at near-ambient temperature. The gelators immobilized the PCM by forming an interpenetrating fibrillar network. 15 wt % concentration of the gelators was found to be effective in preventing liquid PCM leakage. In order to improve the mechanical properties and thermal conductivity (TC) of the PCM, gelation of suspensions of multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GnPs) in a molten material was done at concentrations exceeding their percolation thresholds. Compared to pristine PCM, the gelled PCM containing 3.0 wt % of GnPs demonstrated a shorter crystallization time, ∼1.5-fold increase in strength, improved stability, and ∼65% increase in TC. At the same time, PCM filled with up to 0.6 wt % of MWCNTs had diminished strength and increased leakage with a slight TC improvement. Gelation of PCM did not significantly alter its thermal behavior, but it did change its crystalline morphology. The developed shape-stable PCMs may have a wide range of applications in ambient temperature solar-thermal installations, for example, temperature-controlled greenhouses, net zero-energy buildings, and water heaters.

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).

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.

Synergistic structures in lyotropic lamellar gels

In this work we present a systematic study on the microstructure of soft materials which combine the anisotropy of lyotropic liquid crystals with the mechanical stability of a physical gel. Systematic small-angle neutron (SANS) and X-ray (SAXS) scattering experiments were successfully used to characterize the lyotropic lamellar phase (L_α) of the system D₂O -n-decanol - SDS which was gelled by two low molecular weight organogelators, 1,3:2,4-dibenzylidene-d-sorbitol (DBS) and 12-hydroxyoctadecanoic acid (12-HOA). Surprisingly, a pronounced shoulder appeared in the scattering curves of the lamellar phase gelled with 12-HOA, whereas the curves of the DBS-gelled L_α phase remained almost unchanged compared to the ones of the gelator-free L_α phase. The appearance of this additional shoulder strongly indicates the formation of a synergistic structure, which neither exists in the gelator-free L_α phase nor in the isotropic binary gel. By comparing the thicknesses of the 12-HOA (25-30 nm) and DBS (4-8 nm) gel fibers with the lamellar repeat distance (7.5 nm), we suggest that the synergistic structure originates from the minimization of the elastic free energy of the lamellar phase. In the case of 12-HOA, where the fiber diameter is significantly larger than the lamellar repeat distance, energetically unfavored layer ends can be prevented, when the layers cylindrically enclose the gel fibers. Interestingly, such structures mimic similar schemes found in neural cells, where axons are surrounded by lamellar myelin sheets.

Effect of the Cationic Head Group on Cationic Surfactant-Based Surfactant Mediated Gelation (SMG)

The surfactant-mediated gelation (SMG) method allows us to formulate hydrogels using a water-insoluble organogelator. In this study, we formulated hydrogels using three cationic surfactants, hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), and hexadecylpyridinium chloride (CPC)] and an organogelator (12-hydroxyoctadecanoic acid (12-HOA), and studied their structures and mechanical properties. A fiber-like structure similar to that found in the 12-HOA-based organogels was observed by optical microscopy. Small- and wide-angle X-ray scattering profiles showed Bragg peaks derived from the long- and short-spacing of the crystalline structures in the gel fibers and a correlation peak from the surfactant micelles in the small-angle region. Furthermore, the formation of micelles in the hydrogels was confirmed by UV-vis spectroscopic measurements of the gel samples in the presence of Rhodamine 6G. We concluded that the hydrogels prepared by the SMG method in the present systems are orthogonal molecular assembled systems in which two different molecular assembled structures coexist. Among the three surfactant systems, the CTAB system presented the lowest critical gelation concentration and highest sol-gel transition temperature and viscoelasticity. These differences in gel fiber formation and gel properties were discussed from the viewpoint of the degree of solubilization of the gelator molecules in micelles coexisting with gel fibers and diffusion of the gelator molecules in the gel formation process.

The Evaluations of Menthol and Propylene Glycol on the Transdermal Delivery System of Dual Drug-Loaded Lyotropic Liquid Crystalline Gels

This study aimed to evaluate the effects of two different structural alcohol permeation enhancers (menthol and propylene glycol) on the internal structure and in vitro properties of the dual drug-loaded lyotropic liquid crystalline (LLC) gels. The LLC gels were prepared and characterized by polarized light microscopy, small-angle X-ray scattering, differential scanning calorimetry, attenuated total reflectance-Fourier transform infrared spectrum, and rheology. Based on the results, the inner structure of the gels was Q_II mesophase and exhibited a pseudoplastic fluid behavior. The level of internal order in the LLC mesophase would be affected by introduced 2 wt% menthol (MEN) and propylene glycol (PG). The in vitro release experiment showed that the release behavior of sinomenine hydrochloride (SH) and cinnamaldehyde (CA) from the LLC system was dominated by Fickian diffusion (n < 0.43). MEN and PG had the opposite effects on the release of hydrophilic SH, while the MEN and PG both increased the release of lipophilic drug CA. Furthermore, in vitro permeation studies indicated that MEN and PG could both improve the skin permeability of SH and CA, and MEN displayed more pronounced enhancement. All the samples showed no skin irritation on the normal rat skin. Collectively, in our research, monoterpenoid MEN exhibited a better penetration-promoting effect than straight-chain fatty alcohol PG on the dual drug-loaded LLC system.

Self-assembled toron-like structures in inverse nematic gels

A novel form of nematic gel (N-gel) wherein bright flower-like domains (BFDs) rich in gelator fibres are embedded in a matrix of liquid crystal (LC) molecules has been reported. These gels which we denote as inverse N-gels are unlike typical N-gels in which the LC is encapsulated within an aggregated network of gelator molecules. The self-organization of the helical gelator fibres within the BFDs leads to the creation of localized toron-like structures that are topologically protected due to their skyrmion director profile. Optical and confocal microscopy have been used to deduce the LC director configuration, in order to understand possible intermolecular interactions that can lead to the formation of the twisted structures and the inverse N-gels.

Single-molecule lamellar hydrogels from bolaform microbial glucolipids

Lipid lamellar hydrogels are rare soft fluids composed of a phospholipid lamellar phase instead of fibrillar networks. The mechanical properties of these materials are controlled by defects, induced by local accumulation of a polymer or surfactant in a classical lipid bilayer. Herein we report a new class of lipid lamellar hydrogels composed of one single bolaform glycosylated lipid obtained by fermentation. The lipid is self-organized into flat interdigitated membranes, stabilized by electrostatic repulsive forces and stacked in micrometer-sized lamellar domains. The defects in the membranes and the interconnection of the lamellar domains are responsible, from the nano- to the micrometer scales, for the elastic properties of the hydrogels. The lamellar structure is probed by combining small angle X-ray and neutron scattering (SAXS, SANS), the defect-rich lamellar domains are visualized by polarized light microscopy while the elastic properties are studied by oscillatory rheology. The latter show that both storage G' and loss G'' moduli scale as a weak power-law of the frequency, that can be fitted with fractional rheology models. The hydrogels possess rheo-thinning properties with second-scale recovery. We also show that ionic strength is not only necessary, as one could expect, to control the interactions in the lamellar phase but, most importantly, it directly controls the elastic properties of the lamellar gels.

Effects of pH, temperature and shear on the structure-property relationship of lamellar hydrogels from microbial glucolipids probed by in situ rheo-SAXS

Lipid lamellar hydrogels are a class of soft materials composed of a defectuous lipid lamellar phase, where defects are classically stabilized by polymer or surfactant inclusions in lipid membranes. We have recently shown that bolaform microbial glucolipids, composed of a single glucose headgroup and a C18:0 fatty acid, with the carboxylic acid group located opposite to glucose, spontaneously form lamellar hydrogels at room temperature below pH 8. In this work, we combine rheology with small angle X-ray scattering (SAXS), rheo-SAXS, to correlate, in situ, the structural and mechanical properties of microbial glycolipid lamellar hydrogels upon application of three different stimuli: pH, temperature and a shear rate. In all cases we find unusual structural features of the lamellar phase if compared to classical phospholipid lamellar structures: reducing pH from alkaline to acidic induces a sol-to-gel transition during which an increasing elastic modulus is associated with an oscillatory evolution of lamellar d(100) spacing; temperature above Tm and increasing shear induce the formation of spherulitic crumpled domains, instead of a classically-expected lamellar-to-vesicle or lamellar-to-onion phase transitions.

How an organogelator can gelate water: gelation transfer from oil to water induced by a nanoemulsion

A hydrogel can be formed by an organogelator in the presence of a nanoemulsion. It is expected that this is due to a gelation transfer from oil to water. The system started with an oil-in-water nanoemulsion prepared according to a phase inversion temperature (PIT) process. Into this nanoemulsion consisting of Kolliphor® RH40 and Brij® L4 as surfactants, and Miglyol® 812 as oil and water, we introduced the organogelator 12-hydroxyoctadecanoic acid (12-HOA) in the oil phase. After cooling at room temperature, a slow reversible gelation of the water phase occurred with persistence of the nanoemulsion. This thermally reversible system was investigated using various techniques (rheology, turbidimetry, optical and electron microscopies, scattering techniques). Successive stages appeared during the cooling process after the nanoemulsion formation, corresponding to the migration and self-assembly of the organogelator from the oil nanodroplets to the water phase. According to our measurements and the known self-assembly of 12-HOA, a mechanism explaining the formation of the gelled nanoemulsion is proposed.

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 L_α 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 d_LLC 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 L_α 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.

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 L_α 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 T_sol-gel of the gelled LLCs. At η = 0.0075, all T_sol-gel values are below the LLC-to-isotropic phase transition temperatures T_LLC-iso, that is, the LLCs are formed first while cooling down, followed by gel formation. At η = 0.015, however, T_sol-gel > T_LLC-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?

Gelation of Lyotropic Liquid-Crystal Phases-The Interplay between Liquid Crystalline Order and Physical Gel Formation

We present a systematical investigation of gelled lyotropic liquid crystals (LLCs). This new class of soft materials combines the anisotropy of LLCs with the mechanical stability of a physical gel. The studied LLC system consists of sodium dodecyl sulfate as a surfactant, n-decanol as a cosurfactant, and water as a solvent. At room temperature, four liquid crystalline phases (lamellar L_α, nematic N_d and N_c, and hexagonal H₁) are formed depending on the composition. We were successful in gelling the lyotropic lamellar phase with the low-molecular-weight organogelator 12-hydroxyoctadecanoic acid (12-HOA). The obtained gelled lamellar phase shows optical birefringence, elastic response, and no macroscopic flow. However, we were not able to obtain gels with hexagonal or nematic structure. These findings can be explained twofold. When gelling the hexagonal phase, the long-range hexagonal order was destroyed and an isotropic gel was formed. The reason might be the incompatibility between the gel fiber network and the two-dimensional long-range translational order of the cylindrical micelles in the hexagonal phase. Otherwise, the lyotropic nematic phase was transformed into an anisotropic gel with the lamellar structure during gelation. Evidently, the addition of the gelator 12-HOA to the lyotropic system considerably widens the lamellar regime because the integration of the surface-active 12-HOA gelator molecules into the nematic micelles flattens out the micelle curvature. We further investigated the successfully gelated L_α phase to examine the impacts of the gel network and the remaining monomeric gelator on both the structure and properties of the gelled lamellar phase. Small-angle X-ray scattering results showed an arrested lamellar layer spacing in the gelled state, which indicates a higher translational order for the gelled lamellar phases in comparison with their gelator-free counterparts.

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, T_sol-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 T_sol-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.

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 β-d-glucopyranoside (β-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.

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.

The Twofold Role of 12-Hydroxyoctadecanoic Acid (12-HOA) in a Ternary Water-Surfactant-12-HOA System: Gelator and Co-Surfactant

Gelled lyotropic liquid crystals can be formed by adding a gelator to a mixture of surfactant and solvent. If the gel network and the liquid-crystalline phase coexist without influencing each other, the self-assembly is called orthogonal. In this study, the influence of the organogelator 12-hydroxyoctadecanoic acid (12-HOA) on the lamellar and hexagonal liquid crystalline phases of the binary system H₂O–C₁₂E₇ (heptaethylene glycol monododecyl ether) is investigated. More precisely, we added 12-HOA at mass fractions from 0.015 to 0.05 and studied the resulting phase diagram of the system H₂O–C₁₂E₇ by visual observation of birefringence and by ²H NMR spectroscopy. In addition, the dynamic shear moduli of the samples were measured in order to examine their gel character. The results show that 12-HOA is partly acting as co-surfactant, manifested by the destabilization of the hexagonal phase and the stabilization of the lamellar phase. The higher the total surfactant concentration, the more 12-HOA is incorporated in the surfactant layer. Accordingly, its gelation capacity is substantially reduced in the surfactant solution compared to the system 12-HOA–n-decane, and large amounts of gelator are required for gels to form, especially in the lamellar phase.