Letter to the editors

The sense of twist in the fibrous aggregates from 12-hydroxystearic acid and its alkali metal soaps

Fatty acid crystallization is a complex process initiated by super cooling leading to nucleation and subsequent crystal growth. Each of these stages is greatly influenced by heat transfer, i.e., cooling rate, and mass transfer, i.e., viscosity, presence of shear, etc. During the assembly of the fatty acids to form a colloidal fat crystal network, there are numerous levels of structure starting with the lamella, which are the smallest structural unit influenced by the order fatty acids or triglycerides. Lamella stack without any crystal imperfections to domains, and these domains stack to form primary crystals. The primary crystals arrange into flocs that then assemble into crystals and then finally into the supramolecular colloidal fat crystal network. Every levels of structure is greatly influenced by the heat and mass transfer conditions driving crystallization and each level of structure is a determinate of the final physical properties of the fat crystal network.

Various compounds based on the structural leitmotif of 12-hydroxy stearic acid (HSA) were studied with respect to their ability to form organogels. They were modified by ethoxylation in order to avoid the acid group of HSA, which is unwanted for many of the applications of organogels. In this paper, it is shown that the rheological performance of organogels depends strongly on the extent of ethoxylation, exhibiting an optimum at intermediate degrees of ethoxylation. Furthermore, we reveal that the ability for gelation as well as the mechanical properties are substantially reduced by the presence of stearic acid (SA) in the original reaction mixture, which is a typical contamination of HSA. This is quantified by the amount of gelator required for gelation and the elastic moduli observed for the gels. At the same time the mesoscopic structure, as probed by small-angle neutron scattering (SANS), is almost unchanged for different degrees of ethoxylation or the addition of SA – and similarly thick fibres are observed, while the viscoelastic parameters evolve. Accordingly the elastic efficiency of the individual structural units is responsible for the observed changes in the gelation properties. These findings are relevant for the application of such low molecular weight organogelators in practical formulations, as one can optimise the rheological properties of organogelators by appropriately choosing the degree of ethoxylation.

Bilayers and monolayers are excellent models of biological membranes. The constituents of the biological membranes such as lipids, cholesterols and proteins are chiral. Chiral molecules are abundant in nature (protein, nucleic acid and lipid). It is obvious that relationship between chirality and morphology (as well as function) of biological membrane is of interest for its fundamental importance and has technological implication regarding various membrane functions. The recent years have witnessed that a number of experimental studies in biomimetic systems have shown fascinating morphologies where chirality of the constituent molecule has decisive influence. Significant progress is made towards the understanding of these systems from the theoretical and computational studies. Helfrich's concept of intrinsic force arising from chirality is a milestone in understanding the biomimetic system such as bilayer and the related concepts, further progresses in molecular understanding made in recent years and experimental studies revealing the influence of chirality on morphology are the focus of the present review. Helfrich's concept of intrinsic force arising due to chirality is useful in understanding two-dimensional bilayers and one-dimensional monolayers and related mimetic systems. Various experimental techniques are used, which can probe the molecular architecture of these mimetic systems at different length scales and both macroscopic (thermodynamic) as well as microscopic (molecular) theories are developed. These studies are aimed to understand the role of chirality in the molecular interaction when the corresponding molecule is present in an aggregate. When one looks into the variety of morphologies exhibited by three-dimensional bilayer and two-dimensional monolayer, the later types of systems are more exotic in the sense that they show more diversity and interesting chiral discrimination. Helfrich's concept of intrinsic force may be considered useful in both cases. The intrinsic force due to chirality is the decisive factor in determining morphology which is explained by molecular approaches. Finally, biological and technological implications of such morphological variations are briefly mentioned.

This chapter focuses on Hydroxystearic Acid (HSA) Oleogels. It discusses production, crystal structure, nucleation behavior, crystal growth, and applications of 12-HAS. The elastic properties of 12HSA organogels result from the formation of both permanent and transient junction zones within these soft materials. The permanent junction zones arise from crystallographic mismatches while the transient junction zones are a consequence of fiber entanglement. The non-covalent interactions responsible for the occurrence of transient junction zones in 12HSA arise from hydroxyl-hydroxyl hydrogen bonding. This may result from the preferential arrangement of carboxylic acid groups to dimerize within the fiber, causing a reduction of the fiber-solvent interfacial tension. Recently, 12HSA has been investigated as a delivery system for pharmaceuticals and nutraceuticals. The rate of release of biologically active compounds from food matrices and their bioavailability are crucial in the development of functional foods. For functional foods, it is often desirable for the bioactive to have limited solubility and not be released immediately after consumption. Instead, it is desirable to control the rate of release and hence the rate at which it enters the bloodstream. 12HSA organogels have been shown to slow the rate of beta carotene micellarisation using an in-vivo model for digestion.

Various porous scaffolds utilizing an organogel were prepared by particulate-leaching method. The porous organogels were made of biodegradable, non-toxic ingredients like soybean oil or caprylic/capric triglyceride as the organic liquids and 12-hydroxystearic acid as the gelator. The scaffolds possessed an effective porosity of 56–65%, and good pore interconnectivity with an average pore size from 220 to 290 μm. The biodegradability of such materials was evaluated and lipases were able to totally degrade the scaffolds. The porosity of the material associated with high draining led to suitable scaffolds which were evaluated for CHO cell viability and proliferation using the MTT test. This evaluation was performed over a period of 3 weeks and showed a greater ability to promote cell proliferation for the soybean oil based scaffold than for the caprylic/capric triglyceride one. The histological investigations revealed that this scaffold was able to promote cell colonization and attachment and could induce the production of collagen.