Micro viscosity

The present study demonstrates the utility of the above probe in describing the constitution of aggregates of certain alkarylsulfonates in hydrocarbon media. It also demonstrates the use of the probe technique in measuring the micropolarity of these same aggregates as a function of distance from the polar core. The micro-viscosity of inverted or normal micelles in the past has been estimated only as an average value of either the polar or non-polar regions (6). 

Because / esc is directly proportional to / difn the in-cage combination product yield can be related directly to the solvent micro viscosity, rjm- Thus, plots of In [100/(2 x (5)-3b + 2-BN + 4-BN)in cage-l] versus In (l/r)n using data from irradiations of (/ )-3b in the -alkanes are linear, with positive slopes, 0.47 0.10 at 296K and 0.6 0.1 at 333K. Unfortunately, the number of data points is too small to determine whether the two slopes are truly different. Regardless, the positive slopes indicate that the sum of the in-cage combination rate constants, kj, + k, is less sensitive to micro-... 

The correlation time, in Eq. (4) is generally used in the rotational diffusion model of a liquid, which is concerned with the reorientational motion of a molecule as being impelled by a viscosity-related frictional force (Stokes-Einstein-Debye model). Gierer and Wirtz have introduced the idea of a micro viscosity, The reorientational... 

As noted in Section II.E, both layers decrease the absorption rate, especially of large water-soluble molecules, since the diffusion coefficient is directly proportional to the reciprocal values of both the hydrodynamic radius and the viscosity of the layers. About the influence of absorption enhancers on mucus rheology very little is known. Bile salts and phospholipids are known to reduce the viscoelasticity of bronchial mucus and probably also of mucus layers in the gastrointestinal tract [40], The results should be interpreted with caution since it is not the observed (macro)viscosity that determines the diffusion rate in polymer networks but the much lower (micro)viscosity between the polymer chains [41]. 

Quite large enhancements in the rate of photochemical reaction have been observed in heterogeneous environments such as those that occur in aqueous micelle solutions or surface semiconductors (Cooper W.J. and Herr. F.L., I 987). The ways that micelles may influence solute chemical reactivity have been sumimarized below. These influences include cage, localization-compartmentalization, micro viscosity, polarity, pre-orientation, counterion and local electric field effects. 

Figure 1. Dose-response diagram for the effect of ichfan on micro viscosity of the erythrocyte (A), and of the cell of Ehrlich ascitic carcinoma (B). Probe I (light) (2,2,6,6-tetramethyl-4-capryloyl-oxypiperidin-l-oxyl) is localized in the surface layer of the membrane lipid bilayer Probe II (dark) (5,6-benzo-2,2,6,6-tetramethyl-l,2.3,4-tetrahydro-y-carbolin-3-oxyl) in the deep near-protein sites of lipids.

Time-resolved emission spectra Although there have been several attempts to simplify the characterisation of the SR process, the determination of time-resolved emission spectra (TRES) is certainly the most general and most precise way to quantitatively describe the solvent response. The time-resolved emission spectra are usually determined by spectral reconstruction [96, 97, 106]. The time-resolved emission spectrum at a given time t is calculated from the wavelength dependent time-resolved decays by relative normalization to the steady-state spectrum [107]. By fitting the TRES at different times t by the empirical log-normal function, the emission maximum frequencies i (t) (or 2(t) see Fig. 6.26) and the total Stokes-shift Ac (or A2) are usually derived [106]. Since c(t) contains both information about the polarity (Ac) and the viscosity of the reported environment, the spectral shift c(t) may be normalized to the total shift Ac. The resulting correlation functions C(t) (Eq. (7)) describe the time course of the solvent response and allow for comparison of the SR-kinetic and, thus, of relative micro-viscosities, reported from environments of different polarities [96, 97, 106, 108, 109, 116, 117, 122]... 

Solvent relaxation probes used for the characterisation of micro-viscosities and polarities are listed in Tab. 6.11. They are characterized hy a large change in the dipole moment Afi upon electronic excitation. 

Thus, the time-resolved measurement of such membrane probes contains information on the dynamics of the hindered probe rotation, often interpreted as the micro-viscosity, and about the hindrance of this rotation, usually interpreted as the static packing arrangement of the lipids or the so-called membrane order [136, 137]. Fluorescence polarisation studies in membranes, however, exhibit some major limitations the experimentally determined steady-state and time-re-solved anisotropies characterize the motional restrictions of the reporter molecule itself and give therefore only indirect information about the dye environment, with the consequence that, if the probe is bound covalently to the lipid (TMA-DPH), this attachment may dominate the recorded depolarisation behaviour. The membrane order parameters obtained from freely mobile probes like (DPH) result from a broad distribution of localisation within the hydrophobic interior, the detailed characterisation of which reveals inherent ambiguities [138]. 

Fluorescence anisotropy measurements have been used extensively to study biological membranes. These studies have their origin in the early studies of microviscosity of micelles and n mbranes. Tlie basic idea was to measure the aniscKr( y of a flucrophore in a refnence solvent of known viscosiQ and then in the membrane. The micro-viscosity of the membrane was then estimated by comparison with the viscosity calibration curve. 

The electrical conductivity of systems containing electrolytes does not show any appreciable change at the sol-gel transformation, despite the tremendous change in viscosity. This proves that the mobility of the ions is not hampered and, hence, that the gel must contain an anastomising system of capillary spaces filled with the unchanged solvent. The micro-viscosity of the system has remained unchanged. 

A fiuorescent-photochrome method of quantifying the orientation of solid-phase antibodies immobilized on a silica plate was proposed [9]. The method is based on the measurement of fiuorescence quenching by a quencher in solution, rates of trans-cis photoisomerization and photodestruction of a stilbene-labeled hapten in an antibody binding site. These experimental parameters enable a quantitative description of the order of binding sites of antibodies immobilized on a surface and can be used to characterize the micro viscosity and steric hindrance in the vicinity of the binding site. Furthermore, a theoretical method for the determination of the depth of immersion of the fiuorescent label in a two-phase system was developed [9]. 

Another approach to overcome MDR involves the use of polymeric surfactants, such as Pluronic block copolymers, as chemosensitizing agents. The affect of Pluronic block copolymers on MDR cells has two components first, the polymers alter membrane micro viscosity and, second, they cause ATP depletion selectively in MDR cells Both effects... 

The intra-micellar collision rate between probe and quencher depends not only on the size but also on the shape of the micelles, an effect that can also be investigated since depends on N as IW", where a = 1 for spherical micelles and larger for other geometries. However, a complication of interpreting kq in terms of micellar shape is that it depends not only on the aggregate size and shape but also on the micro-viscosity of the environment for probe and quencher. 

Dietrich T, Ereitag A, Schlecht U. New micro viscosity sensor—A novel analytical tool for online monitoring of polymerization reactions in a micro reaction plant. Chem Eng J 2010 160 823-826. 

The forward reaction of Equation 3.4 is diffusion controlled and consequently its rate will vary inversely with the viscosity of the medium. The ratio /e/f m is commonly used as a measure of the ease of excimer formation, and /m being the excimer and monomer emission, respectively. Excimer formation in micellar systems requires at least two probe molecules per micelle for the reaction of Equation 3.4 to occur within the micelles. The ratio /e//m is thus dependent on the distribution of probe molecules among the micelles which is assumed to follow a Poisson distribution. At the commonly used probe/surfactant molar ratio of 0.01, Zachariasse [13] calculates from Poisson statistics that 27% of sodium dodecyl sulphate (NaDS) micelles are more than singly occupied. There is difficulty in the interpretation of the fluorescence data since excimer emission occurs alongside the partly quenched monomer fluorescence in doubly or higher occupied micelles, whereas singly occupied micelles show only unquenched monomer fluorescence. This situation leads to uncertainty in the calculated microviscosity and may explain the anomalous value of 150 cP proposed by Pownall and Smith [11] for the micro viscosity of the micellar core of hexadecyltrimethylammonium bromide. 

A number of attempts have been made to account for the discrepancies between correlation times calculated from the Debye-Stokes-Einstein equation and the correlation times observed by magnetic resonance spectroscopy. A micro viscosity correction factor (/ ) has been introduced in the Debye-Stokes-Einstein equation in which rj is reduced by... 

After being incorporated into the bilayer, the collision process of excited states and the rotational and diffusional freedom of the sensitizer molecule are consistently slowed down as a consequence of the increasing micro-viscosity [46]. 

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