Dielectric behaviour of H-bonded liquids and amorphous and crystalline solids

Optimization of gliclazide loaded alginate-gelatin beads employing central composite design

Objective: The aim of this study was to optimize the formulation of alginate-gelatin (AL-GL) beads containing gliclazide (GLZ) employing design of experiments (DOE).Significance: DOE enabled identification of the interaction between the studied factors, deep understanding of GLZ release pattern and acceleration of the optimization process.Methods: A three-factor, three-level face centered design was employed. The effects of GLZ content (GLZ%, X₁), polymer ratio (AL:GL ratio, X₂), crosslinker concentration (glutaraldehyde, GA%, X₃), and their interaction on incorporation efficiency (IE) and release rate were studied. The optimized formulation was prepared using numerical optimization and evaluated by DSC, FT-IR, SEM and release rate studies.Results: Increasing GA% (X₃) decreased IE (Y₁) with the highest magnitude of effect among the studied factors. On the other hand, increasing alginate content in AL:GL ratio (X₂) increased IE (Y₁). The amount of GLZ released Q_0.5h, Q_2h(pH 1.2) and Q_4h(pH 7.4) decreased by increasing GLZ% (X₁) and AL:GL ratio (X₂). Both drug content and AL:GL ratio appeared to affect water penetration into the gel matrix and drug release. Generally, there was a direct relationship between GA% (X₃) and GLZ release in pH 1.2 (Q_0.5h and Q_2h). However, in pH 7.4 (Q_4h), increasing GA% decreased GLZ release. In addition, increasing GA% caused deviation from zero-order release model. The actual responses of the optimized formulation were in close agreement with the predicted ones.Conclusion: The selected factors and their levels studied in the optimization design were useful for tailoring the anticipated formulation characteristics and GLZ release pattern.

Dielectric studies of water clusters in cyclodextrins: Relevance to the transition between slow and fast forms of thrombin

Cyclodextrins are useful models in the study of hydrogen bonded water clusters. In alpha-cyclodextrin hexahydrate (alpha-CD.6H2O), water molecules are ordered and occupy well-defined positions whereas in the larger beta-cyclodextrin dodecahydrate (beta-CD.12H2O), there is considerable disorder with water molecules freely arranged over several possible sites. Here it is shown that beta-CD exhibits substantial structural flexibility and proton mobility compared with alpha-CD which is relatively very rigid and exhibits negligible short-range protonic conduction. These properties are directly controlled by the effective dielectric constant of the molecule, which is determined by the rotational freedom of water molecules in the hydrogen bond network. This model may be relevant to proteins where water clusters of this kind are found on the protein surface and occasionally in the protein interior. The case of thrombin, an allosteric enzyme incorporating a network of 20 internal hydrogen bonded water molecules, is discussed.

On the nonlinear variation of dc conductivity with dielectric relaxation time

The long-known observations that dc conductivity sigma(dc) of an ultraviscous liquid varies nonlinearly with the dielectric relaxation time tau, and the slope of the log sigma(dc) against log tau plot deviates from -1 are currently seen as two of the violations of the Debye-Stokes-Einstein equation. Here we provide a formalism using a zeroth order Bjerrum description for ion association to show that in addition to its variation with temperature T and pressure P, impurity ion population varies with a liquid's equilibrium dielectric permittivity. Inclusion of this electrostatic effect modifies the Debye-Stokes-Einstein equation to log(sigma(dc)tau)=constant+log alpha, where alpha is the T and P-dependent degree of ionic dissociation of an electrolytic impurity. Variation of a liquid's shear modulus with T and P would add to the nonlinearity of sigma(dc)-tau relation, as would a nonequivalence of the shear and dielectric relaxation times, proton transfer along the hydrogen bonds, or occurrence of another chemical process. This is illustrated by using the data for ultraviscous acetaminophen-aspirin liquid.

Dielectric relaxation spectroscopy and some applications in the pharmaceutical sciences

With a few exceptions, dielectric relaxation spectroscopy (DRS) has been largely neglected by pharmaceutical scientists, despite the potential for this technique as a noninvasive and rapid method for the structural characterization and quality control of pharmaceutical materials. DRS determines both the magnitude and time dependency of electrical polarization (i.e. the separation of localized charge distributions) by either measuring the ability of the material to pass alternating current (frequency domain DRS) or by investigating the current that flows on application of a step voltage (time domain DRS). DRS is thus (i) sensitive to molecular mobility and structure, (ii) non-invasive, and (iii) employs only mild stresses (a weak electromagnetic field) in order to measure the sample properties. The technique covers a broad-band frequency window (from 10(-5) to 10(11) Hz) and therefore enables the investigation of a diverse range of processes, from slow and hindered macromolecular vibrations and restricted charge transfer processes (such as proton conductivity in nearly dry systems) to the relatively fast reorientations of small molecules or side chain groups. The dielectric response provides information on (i) structural characteristics of polymers, gels, proteins, and emulsions, (ii) the interfacial properties of molecular films, (iii) membrane properties, (iv) water content and states of water (and the effects of water as a plasticizer), and (v) lyophilization of biomolecules. This review article details the basis of dielectric theory and the principles of measuring dielectric properties (including a comprehensive account of measurement artifacts), and gives some applications of DRS to the pharmaceutical sciences.

Dielectric analysis in the characterization of amorphous pharmaceutical solids. 1. Molecular mobility in poly(vinylpyrrolidone)-water systems in the glassy state

The effect of water on the relaxation behavior below the glass transition temperature (beta-relaxation) of an amorphous powder, poly(vinylpyrrolidone) (PVP, MW 30,000), was studied by subjecting the sample to dielectric analysis in the frequency range from 20 Hz to 20 kHz. The material stored at 0% relative humidity (RH) (containing 0.05% w/w H2O) exhibited a frequency dependent second-order beta-relaxation (T beta = -56 degrees C at 500 Hz). The peak frequency-temperature data could be fitted to the Arrhenius equation, yielding an activation energy (Ea) of 36.5 kJ mol-1. Water was found to significantly lower T beta, increase the dielectric loss, and increase Ea. The initial decrease in T beta was found to be quite significant, as little as 7% w/w H2O lowering T beta by 26 degrees C, followed by a more gradual decrease. PVP exposed to 69% RH (containing approximately 31% w/w H2O) exhibited T beta at -104 degrees C with an activation energy of 46.3 kJ mol-1. The observations that the beta relaxation was poorly visible when the water content was 0.05% w/w and that the change in Ea was from a low to a high value as the temperature is decreased suggest that thermally activated rotational diffusion of water molecules plays a major role in the beta-relaxation of PVP containing moderate to high water contents. The rate of increase in activation energy as a function of H2O/PVP mole ratio exhibited a minimum at unity, suggesting that water binding to one site on PVP has a distinct effect on the activation energy.