Orientational dynamics

Several groups conducted SD experiments on the reverse micelle system. They all found a slow component in the SD in the nanosecond timescale. The timescale was found to depend on the radius of the water pool rw and molar ratio of water to surfactant Wo. Sarkar and co-workers studied the SD of C480 in AOT- -heptane-water microemulsions. They observed a distinct rise in the nanosecond timescale at the red end of the emission spectra. They observed that in a small water pool (Wq = 4, = 8 A) the solvation time was 8 ns, wltile for a large water pool (Wq = 32, = 

64 A) the response was bimodal with a fast component of 1.7 ns and a slower component of 12 ns. Obviously, these studies missed all of the ultrafast solvation, which occurs in the picosecond (or faster) timescale [10]. 

Water in and around micelles, reverse micelles, and micro emulsions 

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Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

Pantelides, C. C., and Barton, P. I., Equation-oriented dynamic simulation. Current status and future perspectives. Comput. Chem. Eng. 17. S263 (1993). 

The first column indicates the possible orientations. Dynamic indicates that the site is not fixed at one side of the membrane. Whether or not binding would be measured is indicates by the + and — signs, respectively. The last line in the table gives the results from the actual experiment [30]. RSO, right-side-out ISO, inside-out. 

Translational and Orientational Dynamics Near the Ideal Glass Transition. 

Fluorescence anisotropy is generally used to provide information about the dipolar orientational dynamics occurring after excitation of a system. This technique has successfully been used to probe ultrafast dynamics of energy transfer in organic conjugated dendrimers. The detected emission intensities Tar and Ter for parallel and perpendicularly polarized excitation respectively, were used to construct an observable emission anisotropy R(t) in accordance with the equation [121] ... 

In Fig. 1 pump-probe transients of solutions of 6 M NaBr and 1 M NaCl in HDO D20 are presented. The probe was polarized at the magic angle (54.7°) with respect to the pump polarization to avoid the measurements being affected by the orientational dynamics of the water molecules. For solutions of NaCl, the largest amplitude is observed for pump and probe frequencies of approximately 3450 cm-1, whereas for solutions of NaBr and... 

In Fig. 2. R is presented for a solution of 3 M NaCl in HD0 D20 at different temperatures. All signals show an overall non-exponential decay, but are close to a single exponential for delays >3 ps. After this delay time, the signals only represent the orientational dynamics of the HDO molecules in the first solvation shell of the Cl- ion, thanks to the difference in lifetimes of the O-H- -0 and the O-H- -Cl- vibrations. At 27 °C, the orientational relaxation time constant ror of these HDO molecules is 9.6 0.6 ps. which is quite long in comparison with the value of Tor of 2.6 ps of HDO molecules in a solution of HDO in D20 [12],... 

In Fig. 3, the orientational diffusion time constants ror of the first solvation shell of the halogenie anions CD. Br, and D are presented as a function of temperature. From the observation that ror is shorter than rc, it follows that the orientational dynamics of the HDO molecules in the first solvation shell of the Cl ion must result from motions that do not contribute to the spectral diffusion, i.e. that do not affect the length of the O-H- -Cl hydrogen bond. Hence, the observed reorientation represents the orientational diffusion of the complete solvation structure. Also shown in Fig. 3 are fits to the data using the relation between ror and the temperature T that follows from the Stokes-Einstein relation for orientational diffusion ... 

In Fig. 5, the measured decay of R for solutions of 0 M. 1 M, and 3 M Mg(C104)2 in HD0 H20 is shown. In these measurements, the OD- -0 band was pumped and probed at the 0—>1 transition using pump and probe pulses at 2500 cm-1. We used the O-D vibration to probe, the bulk water orientational dynamics, because, due to its long vibrational lifetime of 2 ps, this vibration allows us to measure the anisotropy decay over a much longer delay time interval than when the O-H vibration is probed. 

The findings shown in Figs. 2, 3 and 4 show that the orientational mobility of water in ionic solutions is extremely inhomogeneous. In the first solvation shell, the orientational dynamics are observed to be at least a factor of 5 slower than in bulk liquid water, whereas beyond this shell, there is no measurable difference with the bulk. 

Applications of optical methods to study dilute colloidal dispersions subject to flow were pioneered by Mason and coworkers. These authors used simple turbidity measurements to follow the orientation dynamics of ellipsoidal particles during transient shear flow experiments [175,176], In addition, the superposition of shear and electric fields were studied. The goal of this work was to verify the predictions of theories predicting the orientation distributions of prolate and oblate particles, such as that discussed in section 7.2.I.2. This simple technique clearly demonstrated the phenomena of particle rotations within Jeffery orbits, as well as the effects of Brownian motion and particle size distributions. The method employed a parallel plate flow cell with the light sent down the velocity gradient axis. 

Case Study 4 Local Orientational Dynamics - Two Dimensional Raman Scattering... 

This outline of the response theory has for simplicity been limited to molecules with axial symmetry of y and Aa and to the field on, field off cases, but can be extended in both respects without basic difficulties. Detailed comparisons with experiment have not yet been made, but it already is clear that Kerr effect relaxation data can now provide more valuable and better defined information about orientational dynamics of biopolymers and other molecules than was previously possible. With the increasing accuracy and time resolution of digital methods, it should be possible to study not only slow overall rotations of large molecules (microseconds or longer) but small conformational effects and small molecule reorientations on nano and picosecond time scales. Moreover, one can anticipate the possibilities, for simple problems at least, of extending response theory to other quadratic and higher order effects of strong electric fields on observable responses. 

There is a long history of using Rayleigh-wing and related spectroscopies to study orientational dynamics of symmetric-top liquids (31). Liquids that have been studied include carbon disulfide (11,15,26,36-43), acetonitrile (26,44-47), benzene [(11,32,45,48-52), hexafluorobenzene (32,48,52), 1,3,5-trifluorobenzene (52)], mesitylene (32,44,50,52), chloroform (45,48,53), and methyl iodide (34,45,54,55). 

Complementary spin-lattice relaxation measurements corroborate the observations made using the 2H line-shape measurements. Based on these measurements the low temperature relaxation times are dramatically shorter in the intercalated sample as compared to the bulk, indicating enhanced polymer re-orientation dynamics in the intercalated samples. Furthermore, the temperature dependence of the relaxation time in the bulk and intercalated sample show dramatic differences. While the relaxation time for the intercalated sample passes smoothly from low to high temperatures, the bulk sample shows a break between the crystalline state and melt state, with the melt state relaxation times at least one order of magnitude faster than those observed in the intercalated sample at the same temperature. 

It is anticipated that orientational dynamics of a host-guest complex produced by molecular recognition are different from those of a host or guest. Therefore, time-resolved fluorescence spectroscopy under TIR conditions has a possibility to sense molecular recognition at a water/oil interface. The system studied is a model of flavoenzymes as shown in Figure 12.9. 

Generally, the introduction of apolar molecules (such as hydrocarbons or noble gases), or apolar residues in otherwise polar molecules (such as alkyl side chains in biopolymers) into water leads to a reduction of the degrees of freedom (spatial, orientational, dynamic) of the neighbouring water molecules. This effect is called the hydro-phobic effect or hydrophobic hydration [176], Hydrophobic means water-fearing . It should be noted that the interaction between hydrophobic molecules and water molecules is actually attractive because of the dispersion interactions. However, the water/ water interaction is much more attractive. Water molecules simply love themselves too much to let some other compounds get in the way [26b] Therefore, from the point of view of the water molecules, the term hydrophobic is rather a misnomer it would be better to refer to water as being lipophobic . 

Orientational dynamics is also affected with T/ and T2, correlation times relevant to first and second Legendre polynomial for a vector defining the... 

Figures 4.18 and 4.19 show the time evolution of Abs// and Absj of PUR-1 and PUR-3, respectively, during and after linearly polarized irradiation for different irradiation power values. The dynamics of photo-orientation of PUR-2 (not shown) resemble those of PUR-1, and PUR-4 shows a photo-orientation dynamical behavior (not shown) similar to that of PUR-3 vide infra). When irradiation starts at time t = 5 minutes, anisotropy occurs and...

FIG. 4.18 Photo-orientation dynamics of PUR-1. Absorbance, normalized by the absori nce value before irradiation. The irradiation light is turned on and off at 5 and 10 minutes. The numbers 1-3 indicate and increasing irradiation intensity the value of which is ven in units of mW/cm with the corresponding sample s absorbance prior to irradiation in units of OD (value between brackets). The inset shows and expanded view of the first few seconds of photo-orientation. 

FIG. 4.19 Photo-orientation dynamics of PUR-3.The samples OD prior to irradiation was 0.3. 

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