Rauzah Hashim

Computer simulation studies of anisotropic systems

The Lebwohl-Lasher model of a nematogen provides a simple system with which to study the order-disorder transition and to examine the properties of the nematic phase. We have investigated the model containing 203 particles over a small temperature range characteristic of that for real nematogens using the Monte Carlo method to evaluate the internal energy, the heat capacity and the second rank orientational order parameter. Our results are in reasonable agreement with those of other simulations and are compared with the predictions of theories based on a cluster expansion of the free energy. The temperature dependence of the order parameter is used to investigate the validity of the singlet orientational distribution function predicted by the Maier-Saupe theory. The potential of mean torque is found to be linear in the order parameter as required by the theory although the orientational free energy is in error. The simulated properties are in reasonable but not complete accord with those of a real nematog...

Carbohydrate liquid crystals: structure–property relationship of thermotropic and lyotropic glycolipids

Abstract Glycolipids have important biological functions in nature: as membrane formation, mediators of cell–cell-recognitions and various specialized functions. Thousands of very complex glycoconjugates have been described by the biochemistry research, whereas the liquid crystal research in the previous decades only recognizes the structure/property relationships of more simple alkylated sugars, e.g. most of the natural materials have never been tested for their mesomorphous properties and the relation between the physical state of matter and the biological functions. This is the challenge of the present research. The bicontinuous cubic phase seems to have a high importance for cell fusion, transportation processes and drug delivery. Glycolipids also have a high potential to form chiral nanostructures, e.g. nanotubes, fibers, gels, bicontinuous and discontinuous cubic phases, and lyotropic and thermotropic cholesteric phases, etc.

Nature-like synthetic alkyl branched-chain glycolipids: a review on chemical structure and self-assembly properties

This review presents the structure/property relationship of two classes of synthetic branched-chain glycolipids, namely the Guerbet and isoprenoid glycosides, in particular highlighting the importance of chain branching to their self-assembly properties. Alkyl chain branching in glycolipids tends to stabilise the reversed type phases as well as a lamellar (Lα) phase. These glycolipids exhibit several reversed bicontinuous cubic phases ( ), the reversed hexagonal phase (H II) and the lamellar phase. The detailed structures of the phases are governed by the balance between the hydrophobic chain bulkiness and the headgroup interaction, including hydrogen bonding. Among the isoprenoid glycolipids, an aqueous isoprenoid xyloside exhibits a bicontinuous cubic phase with a diamond (Pn3m) space group at full hydration and that of a glucoside adopts the stable reversed micellar cubic phase of an Fd3m space group. In the series of Guerbet glycolipids, the ones with longer chains may form stable bicontinuous cubic phases (diamond and Schwarz primitive) in dry condition, as well as the gyroid (Ia3d) in excess water. The phase behaviours of these synthetic glycolipids are comparable to those observed in other natural glycolipids, thus making them potentially useful for applications in both nano-electronics and biomedicine, as therapeutic delivery systems.

The Saupe ordering matrices for solutes in uniaxial liquid crystals

A theory is described for U(θ2), the potential of mean torque of rigid solutes at infinite dilution in a uniaxial liquid crystal phase. The general form of U(θ2), is an infinite expansion of modified spherical harmonics CLn (θ2), and truncation at the second rank terms produces a practical form for U(θ2) which is used to calculate Sxx-Syy and Szz , the principal elements of the Saupe ordering matrix of a biaxial solute. The theory predicts that the dependence of Sxx-Syy on Szz for a particular solute-solvent mixture is determined entirely by λ, a parameter describing the departure from cylindrical symmetry of the potential of mean torque, and which is independent of temperature. Furthermore, if U(θ2) is determined entirely by dispersion forces then λ is predicted to be independent of the solvent and to depend entirely on the anisotropy of the electric polarizability tensor of the solute. These predictions are tested by determining the values of Sxx-Syy and Szz by analysing proton N.M.R. spectra of a solut...

Solute alignment in liquid crystal solvents The Saupe ordering matrix for anthracene dissolved in uniaxial liquid crystals

Abstract We have described a theory for U, the potential of mean torque of rigid solutes at infinite dilution in a uniaxial liquid crystal phase; this may be used to calculate (S xx - S yy) and S zz, the principal elements of the Saupe ordering matrix. In its simplest form U(ω) contains only second-rank terms and the dependence of the biaxiality (S xx - S yy) is determined by ω, a parameter which describes the departure of the potential of mean torque from cylindrical symmetry, and is predicted to be temperature independent. If dispersion forces are responsible for the magnitude of the orientational order parameter then ω should be independent of the solvent and depend only on the anisotropy in the electric polarizability of the solute. Indeed, this independence should result for any pair potential which can be factorized into a product of solute and solvent properties. These predictions are tested here by determining values of S zz and (S xx - S yy) for anthracene-d 10 as a solute in several liquid cryst...

Quantitative analysis of the packing of alkyl glycosides: a comparison of linear and branched alkyl chains

In an attempt to relate the geometry of glycolipid assemblies with molecular packing constraints, the surface areas per molecule for straight and branched-chain alkyl glycosides with varying chain length are calculated. Effects of temperature, water content, sugar size and paraffin chain length are analysed based on closest packing assumption. The results show a continuous increase of the interface between the hydrophilic and the hydrophobic domain per molecule with growing dominance in bulkiness of either domain, until it reaches a maximum in hexagonal phases. The surface area per molecule, on the other hand, exhibits a sudden jump upon the phase transition from a lamellar to a hexagonal phase, reflecting different values of the packing parameter in both assemblies. This increase is primarily based on the assembly, rather than on molecule-based domain sizes. Therefore, estimations of molecular region sizes can serve only to determine the principal ability of compounds to form certain phases, but not predict the actual phase exhibited under given conditions. Within straight-chain glycosides the surface area per molecule is practically constant, whereas it increases with growing chain length for branched-chain analogues. This can be explained with differences in the volume–length ratio of the hydrocarbon domain.

Thermotropic and lyotropic liquid crystalline phases of Guerbet branched-chain -D-glucosides

The effect of chain branching on glycolipid thermotropic and lyotropic phases was investigated for a series of synthetic β-D-glucosides derived from Guerbet alcohols, whose total hydrocarbon chain length ranged from C8 to C24. The compounds, which can be viewed as isosteric mimics for glycoglycerolipids, were synthesised in high purity and their liquid crystalline phases were studied using optical polarising microscopy (OPM), and small-angle X-ray diffraction. When dry, the shortest compound (total C8) exhibits a monotropic Lα phase while longer ones (C16 and C20) adopt inverse hexagonal HII phases. The C24 compound forms an ordered lamellar phase at room temperature, but exhibits a metastable HII phase upon cooling. Curiously the intermediate chain length homologue (C12) adopts an isotropic inverse micellar (L2) phase in the dry state over the range of temperatures studied. Upon hydration, the C8 compound dissolves, and the C12 compound forms a fluid lamellar Lα phase. The C16 Guerbet glucoside (i.e. β-Glc-C10C6) exhibits an inverse bicontinuous cubic phase of space group Ia3d in excess water, never previously observed in branched-chain lipids, and very seldom observed in excess water. The C20 compound remains in the HII phase upon hydrating, with the lattice parameter swelling substantially.

Amphiphilic designer nano-carriers for controlled release: from drug delivery to diagnostics

Vesicles formed by self-assembly of lipids and surfactants are increasingly recognised as carriers for drug delivery applications in disease targeting and many other biomedical-related areas, demonstrable by the growing number of significant publications. This manuscript reviews important facets of lipid-based vesicles as drug carriers and their surface modifications to achieve controlled release and selective cell targeting. We cover both the more commonly used ionic phospholipid vesicle carriers and the rapidly growing field of non-ionic vesicles/niosomes using self-assembly of uncharged amphiphilic molecules, which could be formed by using sugar surfactants or glycolipids, sorbitan esters, and polyoxyethylene alkyl ethers. Due to their lower cost, biodegradability, low-toxicity, low-immunogenicity and specific sugar-cell recognition, much attention would be devoted to glycolipid bio-surfactants as potential carriers for targeted delivery. Specifically, our review points to the design consideration of lipid and surfactant nano-carriers based on critical packing parameter, membrane curvature, and the effects of hydrophobic chain structures. We have also dedicated a section of this review to summarise some novel applications of various lipid liquid crystal phases in drug delivery, and how in turn these are related to chemical structures of the lipid entities. The final section of this review outlines the application of lipid vesicles as delivery agents for diagnostic imaging.

Computer simulation studies of anisotropic systems. XXII. An equimolar mixture of rods and discs: A biaxial nematic?

Abstract In principle, binary mixtures of rod-like and disc-like particles should exhibit a biaxial nematic phase, but in practice phase separation into two uniaxial nematic phases prevents this. Here, we report the results of a computer simulation study of an equimolar mixture of rods and discs in which phase separation is not allowed. The particles are confined to the sites of a simple cubic lattice in which each rod is surrounded by six discs and vice versa. Neighbouring particles interact such that they prefer to align with their respective symmetry axes orthogonal to each other. In contrast, the interaction between next nearest neighbours, which are either rods or discs, is such that their symmetry axes tend to be parallel. Monte Carlo simulations of this model mixture show that an orientationally ordered phase exists at low temperatures. This nematic phase has overall uniaxial symmetry and the particles have a negative second rank orientational order parameter, indicating that they tend to align at ...

Influence of nonionic branched-chain alkyl glycosides on a model nano-emulsion for drug delivery systems

The effect of incorporating new nonionic glycolipid surfactants on the properties of a model water/nonionic surfactant/oil nano-emulsion system was investigated using branched-chain alkyl glycosides: 2-hexyldecyl-β(/α)-D-glucoside (2-HDG) and 2-hexyldecyl-β(/α)-D-maltoside (2-HDM), whose structures are closely related to glycero-glycolipids. Both 2-HDG and 2-HDM have an identical hydrophobic chain (C16), but the former consists a monosaccharide glucose head group, in contrast to the latter which has a disaccharide maltose unit. Consequently, their hydrophilic-lipophilic balance (HLB) is different. The results obtained have shown that these branched-chain alkyl glycosides affect differently the stability of the nano-emulsions. Compared to the model nano-emulsion, the presence of 2-HDG reduces the oil droplet size, whereas 2-HDM modify the properties of the model nano-emulsion system in terms of its droplet size and storage time stability at high temperature. These nano-emulsions have been proven capable of encapsulating ketoprofen, showing a fast release of almost 100% in 24h. Thus, both synthetically prepared branched-chain alkyl glycosides with mono- and disaccharide sugar head groups are suitable as nano-emulsion stabilizing agents and as drug delivery systems in the future.