Organic liquids, dielectric measurements

Values of the dielectric constants for the liquid state of some fatty acids and esters are given in Table 8.16. Dielectric measurements are particularly useful for studies of molecular motion. Organic crystals without rotational disorder exhibit dielectric constants in the range 2-3 with a sharp increase at the melting point. If a crystal form with rotation of the molecules is formed from the melt, however, there is an increase in the dielectric constant related to the rotational freedom of the molecular dipoles in the applied field. 

Bonhote and co-workers [10] reported that ILs containing triflate, perfluorocar-boxylate, and bistrifylimide anions were miscible with liquids of medium to high dielectric constant (e), including short-chain alcohols, ketones, dichloromethane, and THF, while being immiscible with low dielectric constant materials such as alkanes, dioxane, toluene, and diethyl ether. It was noted that ethyl acetate (e = 6.04) is miscible with the less-polar bistrifylimide and triflate ILs, and only partially miscible with more polar ILs containing carboxylate anions. Brennecke [15] has described miscibility measurements for a series of organic solvents with ILs with complementary results based on bulk properties. 

The most common measure of polarity used by chemists in general is that of dielectric constant. It has been measured for most molecular liquids and is widely available in reference texts. However, direct measurement, which requires a nonconducting medium, is not available for ionic liquids. Other methods to determine the polarities of ionic liquids have been used and are the subject of this chapter. However, these are early days and little has been reported on ionic liquids themselves. I have therefore included the literature on higher melting point organic salts, which has proven to be very informative. 

An interface between two immiscible electrolyte solutions (ITIES) is formed between two liqnid solvents of a low mutual miscibility (typically, <1% by weight), each containing an electrolyte. One of these solvents is usually water and the other one is a polar organic solvent of a moderate or high relative dielectric constant (permittivity). The latter requirement is a condition for at least partial dissociation of dissolved electrolyte(s) into ions, which thus can ensure the electric conductivity of the liquid phase. A list of the solvents commonly used in electrochemical measurements at ITIES is given in Table 32.1. 

Physical properties of the solvent are used to describe polarity scales. These include both bulk properties, such as dielectric constant (relative permittivity), refractive index, latent heat of fusion, and vaporization, and molecular properties, such as dipole moment. A second set of polarity assessments has used measures of the chemical interactions between solvents and convenient reference solutes (see table 3.2). Polarity is a subjective phenomenon. (To a synthetic organic chemist, dichloromethane may be a polar solvent, whereas to an inorganic chemist, who is used to water, liquid ammonia, and concentrated sulfuric acid, dichloromethane has low polarity.)... 

On the other hand, C0CI2 is insoluble in pure liquid SbClg since cation stabilization cannot take place. The majority of solvents that are extensively used in solution chemistry (particularly in the field of organic chemistry) are typical EPD solvents. Gutmann (25-28) has introduced the so-called donor number or donicity (DN) as a measure of the EPD properties of donor solvents. This is defined as the negative AH values for formation of the 1 1 adduct of the EPD with SbClg as reference standard EPA in a dilute solution of 1,2-dichloroethane. Donicities for various solvents are listed in Table I together with their dielectric constants e. 

The early applications of the piezoelectric crystal detectors were limited to the measurement in the gas phase, because of the common impression that stable oscillation cannot be obtained in the liquid phase. However, recent advances in PZ research have shown that quartz crystals can oscillate in contact with solution, and several studies have been reported addressing the theoretical aspects of the oscillating frequency of piezoelectric crystals in solution. Nomura and Okuhara (92) demonstrated that the frequency change of a crystal immersed in an organic solvent depends on the density and viscosity of the solvent, and was not affected by the dielectric constant ... 

The liquid liquid interface is an extremely thin liquid region with a few nanometers thickness, where it is predicted that properties such as cohesive energy density, electrical potential, dielectric constant, and viscosity are drastically changed along with the coordinate from an organic phase to an aqueous phase. Therefore, various specific chemical phenomena, which are not observed in bulk liquid phases, occur at liquid-liquid interfaces. However, the nature of the liquid liquid interface and its chemical function are still not fully understood. This situation is mainly due to the lack of suitable experimental methods, for the determination of the chemical species adsorbed at the interface and for the measurement of chemical reaction rates at the interface [1,2]. Recently, some new methods were developed in our laboratory [3], which attained a breakthrough in the study of interfacial reactions. 

This book is intended to serve as a reference and/or textbook on the topic of impedance spectroscopy, with special emphasis on its application to solid materials. The goal was to produce a text that would be useful to both the novice and the expert in IS. To this end, the book is organized so that each individual chapter stands on its own. It is intended to be useful to the materials scientist or electrochemist, student or professional, who is planning an IS study of a solid state system and who may have had little previous experience with impedance measurements. Such a reader will find an outline of basic theory, various applications of impedance spectroscopy, and a discussion of experimental methods and data analysis, with examples and appropriate references. It is hoped that the more advanced reader will also find this book valuable as a review and summary of the literature up to the time of writing, with a discussion of current theoretical and experimental issues. A considerable amount of the material in the book is applicable not only to solid ionic systems but also to the electrical response of liquid electrolytes as well as to sohd ones, to electronic as well as to ionic conductors, and even to dielectric response. 

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