H.-J. Mögel

Crystal Structure and Formation of the Aluminium Hydroxide Chloride [Al13(OH)24(H2O)24]Cl15 · 13 H2O

A basic aluminium chloride with high chlorine content (basicity 1.85) has been crystallized from a concentrated aqueous solution. Its crystal structure has been determined by X-ray structure analysis. The structure contains tridecameric cations of a novel type consisting only of interconnected AlO6 octahedra. The formation of these polycations is discussed.

Helical Nanofibers of Self-Assembled Bipolar Phospholipids as Template for Gold Nanoparticles

Bipolar phospholipids (bolalipids) represent an exciting class of amphiphilic molecules as they self-assemble in water to distinct structures of nanoscopic dimensions. Reported here are structural details of helical nanofibers, composed of achiral, symmetrical single-chain bolalipids with phosphocholine headgroups. These nanofibers are used as template for the fixation of gold nanoparticles (AuNPs) without prior functionalization. This realization of a metal array on bolalipid nanofibers is one of the rare examples of one-dimensional AuNP arrangements in solution. The loading and the heat of binding of AuNPs are determined applying transmission electron microscopy and isothermal titration calorimetry.

Fractal analysis of physical adsorption on material surfaces

Abstract In order to find the fractal dimension of rough surfaces, adsorption isotherms are studied. Fripiat Gatineau and Van Damme ‘Langmuir 2 (1986) 562’ extended the BET-Formula ‘J. Am. Chem. Soc. 60 (1938) 309’ which was originally designed for flat surfaces. As far as we are aware, this method has never been practically applied. This is mainly due to a term in the formula not expressible in mathematically closed form. Besides, this isotherm displays an inconsistent behavior in the case of surface fractal dimension d =3 ‘Langmuir 4 (1988) 781’. In this paper we present a new modified BET-formula which does not exhibit the inconsistency for d =3 and yields an easy to use algorithm to determine the fractal dimension of a surface from one adsorption isotherm. The fractal dimension of a TiO 2 –ceramic ultrafiltration membrane is determined according to this method from a N 2 adsorption isotherm. The result is compared with the fractal dimension estimated according to a method described by Neimark and Russ ‘J. Phys. Chem. (Transl. Zh. Fiz. Khim.) 64 (1990) 1397’, which uses fractal versions of Kelvin- and Kiselev-equation.

Time dependent viscosity of concentrated alumina suspensions

Abstract Viscometric investigations of concentrated aqueous alumina suspensions with particles smaller than 5 μm have been performed. Experimental flow curves indicate thixotropy in the shear rate interval between γ =20 and 640 s−1. In the range smaller than γ =200 s−1 we found pseudoplastic flow behavior, in the higher range the material shows dilatancy. The non-Newtonian behavior results from a small content of sodium aluminum oxide in the alumina suspension. This gives rise to interparticle forces that can drive the suspension into a gel-like state. The time scale of this process is some days. On the short-time scale of some hours the material ages slowly increasing moderately the apparent viscosity. Studying the relaxation process after a shear rate jump, the shear stress time dependency at constant shear rate follows an exponential law. There is a single particular relaxation time for each shear rate. The relaxation towards a steady state occurs asymptotically over some 103 s. Flow curves calculated from steady state data after relaxation processes are below the experimental flow curves which were measured during some 100 s. The flow curves follow the Herschel–Bulkley formula. The shape of the viscosity curves indicates changes of suspension structure at ca. γ =200 and 400 s−1. At constant shear rates in the interval between γ =400 and 450 s−1 the apparent viscosity of the alumina suspension fluctuates periodically in time in the same manner found for other suspensions. This effect is interpreted as periodic organization of agglomeration and deglomeration processes. Supposing, that the stabilisation energy of agglomerates is of the order of the energy introduced by the mechanical shear field, the observation of oscillations at γ =400 s−1 is in agreement with the drastic slope change in the viscosity curves.

Interactions between spheroidal colloidal particles.

Using Derjaguins approximation, we have evaluated the interaction energy associated with van der Waals, electrostatic, depletion, and capillary forces between colloidal spheroids. If the interaction range between spheroids is distinctly smaller than the lengths of their principal axes, then simple pair potentials that depend on particle distance and orientation can be derived. Attractive interactions between adjacent spheroids favor their parallel alignment. Parallel spheroids can be arranged into a variety of densely packed configurations. All of these configurations turn out to have the same lattice energy. We discuss the implications of this degeneracy with respect to the stability of photonic crystals consisting of spheroids.

Monte Carlo Study of the Self-Assembly of Achiral Bolaform Amphiphiles into Helical Nanofibers

It is shown by coarse-grained off-lattice Monte Carlo simulations that a geometrically induced frustration of the parallel arrangement of rigid achiral bolaform amphiphiles can cause chirality in self-assembled nanostructures. The amphiphilic molecules are represented as rigid linear chains of 8 equally sized hydrophobic spheres (tail) and a hydrophilic sphere (head) at each end. The hydrophilic and hydrophobic spheres differ in size. A very simple interaction scheme consisting of only hard-core repulsion between all spheres and square-well attraction between hydrophobic spheres is sufficient for self-assembly into helical fibers for molecules with head/tail diameter ratios ranging from 1.3 to 1.8.

Orientational transitions of two-dimensional hard rod fluids

‘Scaled particle theory’ is applied to a two-dimensional fluid of hard ellipses. The model predicts a transition from the isotropic alignment of the ellipses to the two-dimensional nematic configuration in accordance with Monte Carlo simulations. By modifying the molecular shape it is possible to consider an ensemble of hard rectangular particles. The phases observed and the character of the orientational transition turn out to be strongly dependent on details of the particle geometry.

Computer Simulations of the Formation of Bile Salt Micelles and Bile Salt/DPPC Mixed Micelles in Aqueous Solutions

Brownian dynamics simulations for a coarse-grained model have been performed to study the formation of micelles from bile salts and mixed micelles with dipalmitoyl-phosphatidylcholine (DPPC) in aqueous solutions. The particular association behavior of bile salts as facial surfactants was shown to be caused by their special molecular architecture with a hydrophilic and a hydrophobic side. The experimentally observed smooth transition into the micellar region with increasing concentration is reproduced. Micelle size distributions have been evaluated at different bile salt concentrations. Typical structures of pure bile salt micelles could be identified. The composition and the structure of mixed micelles have been studied in their dependence on the bile salt/lipid concentration ratio in the aqueous solution. We have found that the bile salt fraction in the mixed micelles increases considerably with increasing bile salt/lipid concentration ratio and decreasing micelle size. The structural and thermodynamic features of micelle formation in the aqueous bile salt solutions with DPPC, which we have studied with the coarse-grained model, are in good qualitative agreement with experimental findings.

Monte Carlo Simulation of Surfactant Adsorption on Hydrophilic Surfaces

Monte Carlo simulations have been carried out to study the adsorption behavior of small flexible amphiphilic molecules on solid surfaces from aqueous solutions. A simple coarse-grained solvent-free off-lattice model, with a square-well pair potential and hard core excluded volume effect, has been used. Adsorption isotherms for weakly and strongly hydrophilic homogeneous surfaces have been determined. The adsorbed layer displays a coexistence region with an upper critical point. Below the critical temperature a densely packed patch coexists with a two-dimensional gas-analogous phase. Above the critical temperature, a percolating network forms at higher surfactant concentrations. Depending on the ratio between the strength of the hydrophobic effect and the adsorption energy, a large variety of associates has been observed. Monolayers, bilayers, admicelles, small clusters, and percolating networks as typical associate structures have been found. In the four-region model, which is extended by the coexistence region, a characteristic adsorbed layer structure for each region can be detected. Intermediate structure types have been produced by variation of the adsorption energy.

Depletion Force between Anisometric Colloidal Particles

A simple mathematical model for the depletion force between two arbitrarily shaped large convex colloidal particles immersed in a suspension of small spherical particles is proposed. Using differential geometry, the interaction potential is expressed in terms of the mean and Gaussian curvature of the particle surfaces. The accuracy of theoretical results is tested by Monte Carlo simulations for parallel and nonparallel circular cylinders. The agreement between theoretical results and simulated data is very good if the density of the depletion agent is not too high.