A stochastic principle behind polar properties of condensed molecular matter

A statistical mechanics view leads to the conclusion that polar molecules allowed to populate a degree of freedom for orientational disorder in a condensed phase thermalize into a bi-polar state featuring zero net polarity. In cases of orientational disorder polar order of condensed molecular matter can only exist in corresponding sectors of opposite average polarities. Channel type inclusion compounds, single component molecular crystals, solid solutions, optically anomalous crystals, inorganic ionic crystals, biomimetic crystals and biological tissues investigated by scanning pyroelectric and phase sensitive second harmonic generation microscopy all showed domains of opposite polarities in their final grown state. For reported polar molecular crystal structures it is assumed that kinetic hindrance along one direction of the polar axis is preventing the formation of a bi-polar state, thus allowing for a kinetically controlled mono-domain state. In this review we summarize theoretical and experimental findings leading to far reaching conclusions on the polar state of solid molecular matter. “… no stationary state … of a system has an electrical dipole moment.” P. W. Anderson, Science, 1972, 177, 393.

Symmetry and the polar state of condensed molecular matter

Polar molecular crystals seem to contradict a quantum mechanical statement, according to which no stationary state of a system features a permanent electrical polarization. By stationary we understand here an ensemble for which thermal averaging applies. In the language of statistical mechanics we have thus to ask for the thermal expectation value of the polarization in molecular crystals. Nucleation aggregates and growing crystal surfaces can provide a single degree of freedom for polar molecules required to average the polarization. By means of group theoretical reasoning and Monte Carlo simulations we show that such systems thermalize into a bi-polar state featuring zero bulk polarity. A two domain, i.e. bipolar state is obtained because boundaries are setting up opposing effective electrical fields. Described phenomena can be understood as a process of partial ergodicity-restoring. Experimentally, a bi-polar state of molecular crystals was demonstrated using phase sensitive second harmonic generation and scanning pyroelectric microscopy

Pyroelectric and piezoelectric scanning microscopy applied to reveal the bipolar state of 4-iodo-4′-nitrobiphenyl (INBP)

Two recent scanning probe techniques were applied to investigate the bipolar twin state of 4-iodo-4′-nitrobiphenyl (INBP) crystals. Solution grown crystals of INBP show typically a morphology which does not express that of a mono-domain polar structure (Fdd2, mm2). From previous X-ray diffraction a twinning volume ratio of [similar]70 : 30 is now explained by two unipolar domains (Flack parameter: 0.075(29)) of opposite orientation of the molecular dipoles, joined by a transition zone showing a width of [similar]140 μm. Scanning pyroelectric microscopy (SPEM) demonstrates a continuous transition of the polarization P from +P into −P across the zone. Application of piezoelectric force microscopy (PFM) confirms unipolar alignment of INBP molecules down to a resolution of [similar]20 nm. A previously proposed real structure for INBP crystals built from lamellae with antiparallel alignment is thus rejected. Anomalous X-ray scattering was used to determine the absolute molecular orientation in the two domains. End faces of the polar axis 2 are thus made up by NO2 groups. Using a previously determined negative pyroelectric coefficient pc leads to a confirmation also by a SPEM analysis. Calculated values for functional group interactions (DA), (AA), (DD) and the stochastic theory of polarity formation allow us to predict that NO2 groups should terminate corresponding faces. Following the present analysis, INBP may represent a first example undergoing dipole reversal upon growth to end up in a bipolar state.

Synthesis of bone-like micro-porous calcium phosphate/iota-carrageenan composites by gel diffusion

Brushite and octacalcium phosphate (OCP) crystals are well-known precursors of hydroxylapatite (HAp), the main mineral found in bone. In this report, we present a new method for biomimicking brushite and OCP using single and double diffusion techniques. Brushite and OCP crystals were grown in an iota-carrageenan gel. The aggregates were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and thermal gravimetric analysis (TGA). SEM revealed different morphologies of brushite crystals from highly porous aggregates to plate-shaped forms. OCP crystals grown in iota-carrageenan showed a porous spherical shape different from brushite growth forms. The XRD method demonstrated that the single-diffusion method favors the formation of monoclinic brushite. In contrast, the double diffusion method was found to promote the formation of the triclinic octacalcium phosphate OCP phase. By combining the different parameters for crystal growth in carrageenan, such as ion concentration, gel pH and gel density, it is possible to modify the morphology of composite crystals, change the phase of calcium phosphate and modulate the amount of carrageenan inclusion in crystals. This study suggests that iota-carrageenan is a high-molecular-weight polysaccharide that is potentially applicable for controlling calcium phosphate crystallization.

Glutamic acid inducing kidney stone biomimicry by a brushite/gelatin composite

Brushite is a well known precursor of calcium oxalate monohydrate, the main mineral found in kidney stones having a monoclinic crystal structure. Here, we present a new method for biomimicking brushite using a single tube diffusion technique for gel growth. Brushite crystals were grown by precipitation of calcium hydrogen phosphate hydrate in a gelatin/glutamic acid network. They are compared with those produced in gel in the presence of urea. The aggregates were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and thermal gravimetric analysis (TGA). SEM revealed a change of morphology by glutamic acid from spherulitic growth to plate-shaped and mushroom-like forms consisting of crystal plates and highly ordered prismatic needles, respectively. Furthermore, brushite crystals grown in a gelatin/glutamic acid/urea network showed needle-shaped morphology being different from other brushite growth forms. The XRD method showed that cell parameters for brushite specimens were slightly larger than those of the American Mineral Society reference structure. The mushroom-like biomimetic composite bears a strong resemblance to the brushite kidney stones which may open up new future treatment options for crystal deposition diseases. Hence, suitable diets from glutamic acid rich foods could be recommended to inhibit and control brushite kidney stones.

Mineralization of calcium phosphate crystals in starch template inducing a brushite kidney stone biomimetic composite

Dicalcium phosphate dihydrate (brushite) and octacalcium phosphate (OCP) crystals are precursors of hydroxyapatite (HAp) for tooth enamel, dentine, and bones formation in living organisms. Here, we introduce a new method for biomimicking brushite and OCP in starch using single and double diffusion techniques. Brushite and OCP crystals were grown by precipitation in starch after gelation. The obtained materials were analyzed by infrared spectroscopy (IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and confocal laser scanning microscopy (CLSM). IR spectra demonstrate starch inclusion by peak shifts in the 2900–3500 cm–1 region. SEM showed two different morphologies: plate-shaped and needle-like crystals. Calcium phosphate/starch aggregates bear strong resemblance to prismatic brushite kidney stones. This may open up a clue to understand the mechanism of kidney stone formation.

A universal description of ultraslow glass dynamics

The dynamics of glass is of importance in materials science but its nature has not yet been fully understood. Here we report that a verification of the temperature dependencies of the primary relaxation time or viscosity in the ultraslowing/ultraviscous domain of glass-forming systems can be carried out via the analysis of the inverse of the Dyre-Olsen temperature index. The subsequent analysis of experimental data indicates the possibility of the self-consistent description of glass-forming low-molecular-weight liquids, polymers, liquid crystals, orientationally disordered crystals and Ising spin-glass-like systems, as well as the prevalence of equations associated with the 'finite temperature divergence'. All these lead to a new formula for the configurational entropy in glass-forming systems. Furthermore, a link to the dominated local symmetry for a given glass former is identified here. Results obtained show a new relationship between the glass transition and critical phenomena.

Distortion-sensitive insight into the pretransitional behavior of 4- n -octyloxy-4′-cyanobiphenyl (8OCB)

Results of studies of the static and dynamic dielectric properties in rod-like 4-n-octyloxy-4'-cyanobiphenyl (8OCB) with isotropic (I)–nematic (N)–smectic A (SmA)–crystal (Cr) mesomorphism, combined with measurements of the low-frequency nonlinear dielectric effect and heat capacity are presented. The analysis is supported by the derivative-based and distortion-sensitive transformation of experimental data. Evidence for the I–N and N–SmA pretransitional anomalies, indicating the influence of tricritical behavior, is shown. It has also been found that neither the N phase nor the SmA phase are uniform and hallmarks of fluid–fluid crossovers can be detected. The dynamics, tested via the evolution of the primary relaxation time, is clearly non-Arrhenius and described via τ(T) = τc(T−TC)−phgr. In the immediate vicinity of the I–N transition a novel anomaly has been found: Δτ ∝ 1/(T − T*), where T* is the temperature of the virtual continuous transition and Δτ is the excess over the 'background behavior'. Experimental results are confronted with the comprehensive Landau–de Gennes theory based modeling.