Ionic versus temperature

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

Fig. 92. Mass spectral parameters pressure (p in torr) and ionic currents (J in relative units) of Li2NbOF5 (A), Na2NbOFs (B) and K2NbOFs (C) versus temperature in °C (after Agulyansky et ah, [383]). Numbers on the curves correspond to ions as shown in Fig. 91.

A plot of the optical absorbance at 260 nm (the wavelength of maximum light absorption by nucleic acids) versus temperature is known as a melting curve (Fig. 5-45). The absorbance is lower, by up to 40%, for native than for denatured nucleic acids. This hypochromic effect (Chapter 23) is a result of the interaction between the closely stacked bases in the helices of the native molecules. The melting temperature Tm is taken as the midpoint of the increase in absorbance (Fig. 5-45). As the percentage of G + C increases, the nucleic acid becomes more stable toward denaturation because of the three hydrogen bonds in each GC pair. Tm increases almost linearly with increases in the G + C content. In the "standard" citrate buffer (0.15 M NaCl + 0.015 M sodium citrate, pH 7.0) Eq. 5-22 holds. The exact numerical relationship depends strongly upon the ionic composition and pH of the medium.37 72 552 553... 

The third relaxation process is located in the low-frequency region and the temperature interval 50°C to 100°C. The amplitude of this process essentially decreases when the frequency increases, and the maximum of the dielectric permittivity versus temperature has almost no temperature dependence (Fig 15). Finally, the low-frequency ac-conductivity ct demonstrates an S-shape dependency with increasing temperature (Fig. 16), which is typical of percolation [2,143,154]. Note in this regard that at the lowest-frequency limit of the covered frequency band the ac-conductivity can be associated with dc-conductivity cio usually measured at a fixed frequency by traditional conductometry. The dielectric relaxation process here is due to percolation of the apparent dipole moment excitation within the developed fractal structure of the connected pores [153,154,156]. This excitation is associated with the selfdiffusion of the charge carriers in the porous net. Note that as distinct from dynamic percolation in ionic microemulsions, the percolation in porous glasses appears via the transport of the excitation through the geometrical static fractal structure of the porous medium. 

Because the effect of neutralization by monovalent cations on the mechanical relaxations presents interesting features, it is also of interest to explore the effect of divalent cations. A preliminary study of this kind was conducted by Kyu and coworkers (58). Nafion acid of 1155 EW was neutralized with an appropriate BaCl2 solution to prepare a Ba-salt sample. The torsion pendulum results in the form of G, G" and tan 6 versus temperature are depicted in Figure 32. A peak is evident at approximately -90°C in the tan 6 and G" curves. Judging from the peak temperature, this y relaxation is probably caused by the same mechanism as in the acid and the monovalent salt samples described before. The 6 peak occurs at approximately -2O C and thus overlaps slightly with the y peak. The observation of the 6 region at such a low temperature may be due to the presence of residual water which would reduce ionic interactions within the ionic domains and in turn would enhance backbone mobility. 

Transition-metal ions with decreasing softness are capable of coordinating also to the phosphate oxygen atoms. The ionic versus covalent character of these complexes clearly depends on the metal ions involved. In a classic study, examining the melting temperature of double-helical DNA in the presence of dif-... 

FIGURE 20.5 Solubility (grams of solute in lOOg H2O) versus temperature for several ionic compounds. 

Fig. 1. Water content versus temperature phase diagram of water-undecane systems with 12% (w/w) non-ionic surfactant added to the hydrocarbon phase, realm of...

Fig. 16. The molar conductivities derived from ionic conductivity Amp(solid) and NMR ANMR(open) are plotted versus temperature for the four ILs.

It was shown some time ago that in ionic surfactants (such as SDS), the trajectory of CMCs versus temperature intersects the Krafft phase boundary at the CMC Krafft point [7,44,45]. Just as the CMC itself is not a thermodynamic discontinuity [46], there is no kink or cusp in the Krafft boundary at this intersection. Nevertheless, this behavior is important because below the temperature of the CMC Krafft point micellar structure does not exist in equilibrium surfactant solutions. Metastable micellar solutions may, however, easily be formed below the Krafft boundary by cooling concentrated liquid phases [47]. Cooling liquid-crystal phases below the Krafft eutectic typically yields metastable liquid-crystal (not liquid) phases. 

Solubility versus temperature for several ionic compounds. 

For nonionic surfactants of the ethylene oxide type, the temperature effect is opposite to that of ionic surfactants, because of the peculiar temperature dependence of the hydration of ethylene oxide groups. As the temperature increases, the ethylene oxide chain loses its hydration water and the spontaneous curvature decreases. To a good approximation, the spontaneous curvature can be approximated by the first term of expansion in series versus temperature "... 

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