Real-time monitor on the release of salicylic acid from chitosan gel beads by means of dielectric spectroscopy

The release processes of salicylic acid (SA) from the chitosan gel beads (CGB) with different crosslinking densities were monitored in real time by means of dielectric spectroscopy. Distinct dielectric relaxations attributed to interfacial polarization were observed, which exhibited obvious dependences on the crosslinking density and the release time. In line with Hanai’s equation, the properties of CGB and release media were quantitatively determined from the dielectric profiles. Based on these properties and dielectric behaviors, the release profiles were analyzed. It was found that only the dispersion of CGB with the lowest crosslinking density exhibited release behavior, while other dispersions showed an obvious adsorption process following a burst of drug diffusion. The total loading and release amount of SA were determined and compared, and the influence of crosslinking degree on the drug loading and release was discussed. In addition, the release behavior was confirmed to be diffusionally controlled, and the diffusion coefficient of SA was determined.     

Dielectric relaxation behavior of colloidal suspensions of palladium nanoparticle chains dispersed in PVP/EG solution

Dielectric measurements were carried out on colloidal suspensions of palladium nanoparticle chains dispersed in poly(vinyl pyrrolidone)/ethylene glycol (PVP/EG) solution with different particle volume fractions, and dielectric relaxation with relaxation time distribution and small relaxation amplitude was observed in the frequency range from 10(5) to 10(7) Hz. By means of the method based on logarithmic derivative of the dielectric constant and a numerical Kramers-Kronig transform method, two dielectric relaxations were confirmed and dielectric parameters were determined from the dielectric spectra. The dielectric parameters showed a strong dependence on the volume fraction of palladium nanoparticle chain. Through analyzing limiting conductivity at low frequency, the authors found the conductance percolation phenomenon of the suspensions, and the threshold volume fraction is about 0.18. It was concluded from analyzing the dielectric parameters that the high frequency dielectric relaxation results from interfacial polarization and the low frequency dielectric relaxation is a consequence of counterion polarization. They also found that the dispersion state of the palladium nanoparticle chain in PVP/EG solution is dependent on the particle volume fraction, and this may shed some light on a better application of this kind of materials.     

Dielectric spectroscopy of Anabaena 7120 protoplast suspensions

The dielectric spectroscopy of Anabaena 7120 protoplast suspensions has been investigated over the frequency range of 40 Hz-110 MHz. The protoplast suspensions showed a complicated dielectric dispersion consisting of at least four distinct sub-dispersions with the increasing frequencies due to the Maxwell-Wagner interfacial polarization. The double-shell model, in which an equivalent shell of thylakoid was assumed inside the cytoplasm, was adopted to describe the special morphology of the protoplast. Under the assumption that the conductivity of plasmalemma was negligible and the conductivity of the equivalent shell was 0.1 microS/cm, we attempted to estimate the dielectric properties of various protoplast components by fitting theoretical curve to experimental data. The relative permittivity of the plasmalemma epsilon(mem) was estimated to be 6.5+/-0.5, and the permittivity of the equivalent shell of thylakoid epsilon(thy) was estimated to be about 3.2+/-0.2. The permittivity epsilon(cyt) and conductivity kappa(cyt) of the cytoplasm were estimated to be 60 and 0.88+/-0.11 mS/cm, respectively. The permittivity epsilon(nuc) and conductivity kappa(nuc) of the nucleoplasmic region were determined to be 100 and 0.13+/-0.02 mS/cm, respectively.     

Dielectric properties of aqueous zwitterionic liposome suspensions

The dielectric spectra of aqueous suspensions of unilamellar liposomial vesicles built up by zwitterionic phospholipids (dipalmitoylphosphatidyl-choline, DPPC) were measured over the frequency range extending from 1 kHz to 10 MHz, where the interfacial polarization effects, due to the highly heterogeneous properties of the system, prevail. The dielectric parameters, i.e., the permittivity epsilon'(omega) and the electrical conductivity sigma(omega), have been analyzed in terms of dielectric models based on the effective medium approximation theory, considering the contribution associated with the bulk ion diffusion on both sides of the aqueous interfaces. The zwitterionic character of the lipidic bilayer has been modeled by introducing an "apparent" surface charge density at both the inner and outer aqueous interface, which causes a tangential ion diffusion similar to the one occurring in charged colloidal particle suspensions. A good agreement with the experimental results has been found for all the liposomes investigated, with size ranging from 100 to 1000 nm in diameter, and the most relevant parameters have briefly discussed in the light of the effective medium approximation theory.     

Effect of deacetylation rate on gelation kinetics of konjac glucomannan

Effect of deacetylation rate on the gelation behaviors on addition of sodium carbonate for native and acetylated konjac glucomannan (KGM) samples with a degree of acetylation (DA) range of 1.38-10.1 wt.% synthesized using acetic anhydride in the presence of pyridine as catalyst was studied by dynamic viscoelastic measurements. At a fixed alkaline concentration (C(Na)), both the critical gelation times (t(cr)) and the plateau values of storage moduli (G'(sat)) of the KGM gels increased with increasing DA. While at a fixed ratio of alkaline concentrations to values of DA (C(Na)/DA), the similar t(cr) and (G'(sat)) values independent of DA were observed. On the whole, increasing KGM concentration or temperature shortened the gelation time and enhanced the elastic modulus for KGM gel. The effect of deacetylation rate related to the C(Na)/DA on the gelation kinetics of the KGM samples were discussed.     

An investigation on the high-frequency dielectric dispersion of concentrated ion-exchange resin beads suspensions

In this paper, dielectric measurements were carried out on concentrated suspensions of D₃₅₄ ion-exchange resin (IER) beads dispersed in electrolyte solutions. The distinct high-frequency dielectric behavior occurring in the megahertz frequency range was interpreted based on the understanding of the interparticle interaction and the properties of the constitute phases. The results indicated that the dominant parameter of continuous phase influencing HFDD is the solution concentration after full Donnan equilibrium, while the dominant parameter of dispersed phase influencing HFDD is the fixed charge density. In addition, properties of the dispersed IER beads including electric conductivity and permittivity were obtained in terms of the Hanai’s method.     

壳聚糖微球悬浊液的介电谱研究: 弛豫机制的解释和内部信息的获得

结合DLVO理论和壳聚糖微球的特点, 提出了适合实验数据的电动力学模型, 并分析了浓厚分散系的界面极化弛豫的微观机制. 进而测量了不同粒径壳聚糖微球悬浊液的介电谱, 发现在10～100 MHz频率范围内均出现明显的介电弛豫现象. 利用上述模型合理解释了该弛豫现象以及微球粒径对界面极化弛豫的影响, 结果也证明了我们提出的模型的适当性. 此外, 利用Hanai方法, 通过介电参数准确计算出了各悬浊液的内部信息, 并且分析了这些实时信息的合理性. 研究结果从实验和理论两方面展示了介电谱方法在即时获取壳聚糖内部信息上的独到优势.     

Time-dependent amorphization of ice at 0.8-0.9 GPa

Thermal conductivity measurements show that ice continues to amorphize for several days when kept at a fixed pressure p in the 0.79-0.88 GPa range, and fixed temperature T in the 127-130 K range. Thermal conductivity kappa decreases according to a stretched exponential in time, and its limiting long time value kappa( infinity ) varies with p and T. At 0.8 GPa and 128 K, kappa( infinity ) remains 2.5 times the value observed for high-density amorph. Consequences of these findings for our understanding of amorphization are discussed.     

Dielectric analysis of sheep erythrocyte ghost. Examination of applicability of dielectric mixture equations

The purpose of this study is to confirm the applicability of dielectric mixture equations in dielectric analysis of biological cell suspensions. Two dielectric mixture equations, the Pauly-Schwan (P-S) equation and the Hanai-Asami-Koizumi (H-A-K) equation were tested using sheep erythrocyte ghosts whose internal solution is identical with the external solution. Dielectric measurements were carried out for the ghost suspensions over a frequency range 10 kHz to 100 MHz; a single dielectric relaxation was found between 100 kHz and 10 MHz. From the dielectric relaxation, the conductivity and permittivity of the ghost interior and the capacitance of the cell membrane were calculated following the P-S and H-A-K equations. When the H-A-K equation was employed (and as expected from the property of the ghosts), the estimated internal conductivity was in good agreement with the external conductivity at volume fractions up to about 0.7. With the P-S equation, on the other hand, the same results as above were obtained but only at low volume fractions below about 0.3. In addition, the H-A-K equation provided a better simulation for the observed relaxation curves than did the P-S equation, especially at high volume fractions. It is, therefore, concluded that the H-A-K equation is applicable to a wider range of volume fraction than is the P-S equation.     

Transition from dilute to concentrated electrokinetic behavior in the dielectric spectra of a colloidal suspension

Dielectric spectroscopy is used to measure the complex permittivity of 200 and 100 nm diameter polystyrene latex suspended in potassium chloride (KCl) solutions over the frequency range 10(4)-10(7) Hz as a function of particle volume fraction (?) and ionic strength. Dilute suspension dielectric spectra are in excellent agreement with electrokinetic theory. A volume fraction dependence of the dielectric increment is observed for low electrolyte concentrations (0.01, 0.05, and 0.1 mM) above ? ≈ 0.02. This deviation from the dilute theory occurs at a critical frequency ω* that is a function of volume fraction, particle size, and ionic strength. The dielectric increment of suspensions at the highest salt concentration (1 mM) shows no volume fraction dependence up to ? = 0.09. Values of ω* are collapsed onto a master curve that accounts for the length and time scales of ion migration between neighboring particles. The measured conductivity increment is independent of volume fraction and agrees with theory after accounting for added counterions and nonspecific adsorption.     

Dielectric analysis of macroporous anion-exchange resin beads suspensions

Dielectric measurements were carried out for suspensions of D354 anion-exchange beads dispersed in electrolyte solutions at different concentrations, and distinct Maxwell-Wagner dielectric relaxations were observed around 10(6) Hz. Through fitting the experimental data we obtained the dielectric parameters of the suspensions, and then we calculated the phase parameters from the dielectric parameters and the measured volume fractions by Hanai's method. In light of the present understanding of the interfacial properties, and with the information obtained from the phase parameters, we satisfactorily interpreted the concentration dependences of the dielectric parameters. It is concluded that Hanai's method is an effective tool for obtaining the properties of dispersed particles; the properties of the electrical double layer, which are mainly decided by the properties of the electrolyte solution, predict the dielectric behavior of suspensions with conducting particles. The dielectric relaxation spectroscopy (DRS), based on the M-W mechanism, is also a very sensitive tool for probing the properties of the liquid/solid interface.     

Dielectric relaxation spectroscopy of macroporous IER beads suspensions dispersed in primary alcohols and water–ethanol mixtures

Dielectric measurements were first carried out on suspensions of ion-exchange resin beads dispersed in primary alcohols and water–ethanol mixtures in the frequency range 40 Hz–110 MHz. Due to the large bead radius, only Maxwell–Wagner (M–W) dielectric relaxations were observed. Regular dielectric behaviors were observed and phase parameters concerning constituent phases’ properties were determined through dielectric analysis, which revealed that the properties and the dispersing state of the bead are strongly dependent on the properties of dispersion medium. It is also found that dry beads cannot be completely soaked by alcohols with long aliphatic chain, and that suspension in water-rich mixture has similar dielectric behavior as in pure water due to the mixture's molecular construction. Then the dielectric behaviors of the following suspensions were measured as a function of time: suspensions of beads that have been equilibrated with water/ethanol redispersed in ethanol/water. The dielectric behaviors showed remarkable time dependency, characterized by distinct transitions on the curves of time dependent relaxation parameters. Based on the above understandings, the time dependent dielectric behaviors were analyzed in detail. It is showed that the time dependency directly reflected such processes as ion diffusion, solvent diffusion and solvent uptake that the systems undergo.     

On the dielectric relaxation of biological cell suspensions: the effect of the membrane electrical conductivity

Due to the mismatch of the electrical parameters (the permittivity ?' and the electrical conductivity σ) of the membrane of a biological cell with the ones of the cytosol and the extracellular medium, biological cell suspensions are the site, under the influence of an external electric field, of large dielectric relaxations in the radiowave frequency range. However, a point still remains controversial, i.e., whether or not the value of membrane conductivity σ(s) might be extracted from the de-convolution of the dielectric spectra or otherwise if it would be more reasonable to assign to the membrane conductivity a value equal to zero. This point is not to be considered with superficiality since it concerns an a priori choice which ultimately influences the values of the electrical parameters deduced from this technique. As far as this point is concerned, the opinion of the researchers in this field diverges. We believe that, at least within certain limits, the membrane conductivity can be deduced from the shape of the relaxation spectra. We substantiate this thesis with two different examples concerning the first a suspension of human normal erythrocyte cells and the second a suspension of human lymphocyte cells. In both cases, by means of an accurate fitting procedure based on the Levenberg-Marquardt method for complex functions, we can evaluate the membrane conductivity σ(s) with its associated uncertainty. The knowledge of the membrane electrical conductivity will favor the investigation of different ion transport mechanisms across the cell membrane.     

NaA沸石离子交换过程的介电弛豫谱研究

在40 Hz~110 MHz频率段对不同Ca²⁺交换度的NaA沸石堆积体系进行了介电测量, 并利用Cole-Cole公式及Hanai方法对介电参数和相参数进行拟合与解析. 结果表明, 随着Ca²⁺交换度的不断增高, 堆积体系的介电增量、弛豫频率、弛豫分布系数以及粒子的电导率均有不同程度的降低, 而粒子的介电常数保持不变. 通过综合分析弛豫变化规律与离子交换度的内在联系发现, 离子交换后Ca²⁺选择六元环占位, 同时六元环和八元环位置的相邻2个Na⁺被置换; 占据六元环的Ca²⁺与八元环位置的Na⁺对沸石粒子的极化贡献等价; Ca²⁺的进入导致沸石孔道内微观电场多样化.     

反渗透膜UTC-70在水溶液中的介电谱及其解析

根据平面层状体系介电弛豫理论研究了反渗透膜UTC-70在各种浓度氯化钠和氯化钾溶液中的介电弛豫行为.利用计算机拟合的方法得到膜/溶液体系的介电参数,并由此计算得到了UTC-70膜相和水溶液相的相参数,获得了反映反渗透膜UTC-70荷电情况的信息及其与电解质溶液浓度的关系,介电解析的结果解释了介电弛豫的产生机制.     

Electrical Conductivity and Dielectric Dispersion Phenomena of Concentrated TiO(2) Suspensions

The complex conductivity of concentrated TiO(2) suspensions (up to 50 vol%) was measured over a large frequency range (1 MHz-1.8 GHz), as a function of grain volume content. These measurements highlight relaxation phenomena in the intermediate frequency range, which are associated with the dispersed powder. These phenomena were previously noted in O'Brien's theoretical developments. A quantitative data analysis was made, in terms of time constant distribution, using the CONTIN software, from Provencher. As a result, a complex process is highlighted, including two main mechanisms in different ranges of time constants. Particularly, we showed good agreement between the higher frequency mechanism and the O'Brien theory. From Dukhin's lambda ratios, which are ratios of grain surface to bulk electrolyte conductances, we calculated the electrolyte conductivity as a function of powder concentrations. At higher powder concentrations, these values are different from those measured with centrifuged liquids. Copyright 2000 Academic Press.     

Dielectric analysis of a nanoscale particle in an aqueous solution of low electrolyte concentration

The dielectric spectra of aqueous suspensions of nanoscale silica particles (8 and 24 nm) with low electrolyte concentrations were investigated as a function of the particle concentration. Obvious dispersions observed in the frequency range of 10-10(5) kHz are explained by the multiple effects of the interfacial polarization and the polarization of counterions by using a two-step model and the corresponding dielectric analytical method arising from the combination of Hanai's method and O'Konski's theory. The phase parameters, which reflect the inner properties of constituent phases of the system, are calculated and discussed in detail. The validity of the two-step model was tested in terms of the standard electrokinetic model deduced from pure theories.     

非导电微球悬浊系的介电谱——弛豫机制的考察和界面信息的解析

研究了不同浓度电解质溶液中聚苯乙烯微球悬浊液的介电谱, 发现在40 Hz～110 MHz频率范围内出现了两个明显的弛豫. 在介电模型基础上对弛豫原因的理论分析结果表明, 千赫兹频域出现的弛豫是由粒子附近双电层中对离子的扩散所致, 兆赫兹附近出现的弛豫源于空间电荷在固/液界面的累积. 应用Hanai方法计算出体系内部的相参数, 获得了微球/溶液界面的电信息, 并给出了合理解释. 理论计算结果验证了模型和方法的适用性. 实验采用透析法调制样品, 有效地防止了体系内部粒子二次团聚的发生.