Research Topics

Experimental Methods

Organic Ionic Liquids, Supercooled Liquids, and Liquid Crystals

The study of the liquids as they super cool and approach the glass transition involves affects that span a very broad range of times. The enormous slowing down of the complete structural relaxation with decreasing temperature is the most prominent feature of supercooled liquids. The slowing down is seen in macroscopic observables, like viscosity and long time diffusion, as well as in microscopic properties, like density fluctuations and rotational correlation times. Several phenomenological theories deal with the reduction in rates of processes in supercooled liquids. However, dynamics in supercooled liquids cover such broad ranges of time and amplitude that a full understanding remains an experimental and theoretical challenge.
Room temperature organic ionic liquids (RTILs) have attracted considerable attention due to their potential applications in a broad range of fields. They have good solvating properties and extremely low vapor pressures, which can make them useful solvents for reducing pollution in chemical synthesis. In electrochemical devices like fuel and solar cells, the advantages of RTILs arise from their thermal stability, relatively large liquid range, and the possibility of being used as both the electrolyte and the redox couple. RTILs are also being used in biocatalysis3 and synthesis of nanostructured materials. The exploration of RTILs is relatively new. A systematic study of the physical and chemical properties of this new class of liquids is therefore important for understanding and optimizing their functionality, and further broadening their potential applications. Furthermore, they present a useful counter point to the properties of non-ionic organic liquids for comparison. In terms of dynamics, the question arises as to whether a relatively non-polar liquid such as ortho-terphenyl and an organic ionic liquid, such as N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) are fundamentally different.

The fundamental difficulty in obtaining a detailed picture of the dynamics of complex liquids is that the time scales over which different types of dynamics occur is enormous. In experiments we have conducted, we have followed the orientational dynamics of liquids from the 100 fs to 10 ms time scales, that is, over 8 decades in time. We do this using optical heterodyne detected optical Kerr effect (OHD-OKD) experiments. The OHD-OKE technique is discussed in the Experimental Methods section.

Another class of complex liquids that has dynamics that span a vast range of times scale is nematogens, in the isotropic of nematic phases. The orientational relaxation dynamics of nematogens in the isotropic phase of liquid crystals is complex. Our recent experiments have shown that the dynamics can be divided roughly into two time scales. On a long time scale, the orientational relaxation is exponential and highly temperature dependent. The exponential relaxation is well described by the Landau-de Gennes theory. In the isotropic phase, a liquid crystal sample is macroscopically isotropic but microscopically anisotropic. On a distance scale short compared to a correlation length, x, the local structure is like that of a nematic liquid crystal. As the temperature is lowered toward the nematic-isotropic phase transition temperature, TNI, the correlation length diverges. Below TNI, the liquid crystal is macroscopically ordered. The long time scale exponential orientational relaxation is caused by the decay of the local nematic structure. The decay slows as the correlation length increases, diverging as TNI is approached from above. Until our recent work, the nematogen dynamics on fast time scales had not been addressed in either the isotropic or nematic phases. Like organic supercooled liquid and RTILs, we have found that the short time behavior is characterized by several power laws. The surprising results emerging from our studies of three seemingly distinct types of liquids are the commonalities among them.

Figure 17 show OHD-OKE data for the organic liquid benzophenone on a log plot. The data shown span the time range of ~500 fs to 20 ns. The data can be viewed as a measure of the orientational relaxation of the molecules. The OHD-OKE experiment is a non-resonant technique that is capable of looking at almost any liquid. The slowest time component is an exponential decay. On time scales short compared to the exponential decay, two power laws can be identified. By studying a wide number of liquid, we identified the intermediate power law, which until recently was not described theoretically. As the temperature is lowered toward the glass transition, the exponential decay becomes increasingly long. The intermediate power law is temperature independent. We have applied mode coupling theory (MCT) to the analysis of the temperature dependent data for seven organic liquids, and recently, an RTIL. The initial results show that the RTIL has time and temperature dependences that are almost identical to that of normal non-polar organic liquids.

Figure 18 shows OHD-OKE data for a nematogen, 5-CB, in the isotropic phase. The data span a time range from ~400 fs to 1 ms. This type of data, which we have taken on a number of nematogens, is the first to examine the short time behavior of nematogens in the isotropic phase. The remarkable feature is that the functional form of the data is essentially identical to that of a normal organic liquid. We have extended MCT to a description of nematogens in the isotropic phase. We have also taken the first short and intermediate time scale data in the nematic phase. In the nematic phase, the orientational relaxations of three systems studied have entirely different functional form from the isotropic phase. The nematic phase data is composed of a number of power laws that are common between molecules. We have also made the first measurements on a discotic liquid crystal.

The current challenge is to understand the similarities and differences among distinct families of liquids. The data provide insights into the interplay of structure, intermolecular interactions, and dynamics.