Even Molar Ratio Effects

LDAO/SDS Interaction. Mixing of cationic and anionic surfactant solutions results In the formation of a mixed species that Is more surface active than the Individual species. The enhanced synergistic effect has been explained (2,3) by showing that a close-packed adsorption of electroneutral R R takes place (R" " and R represent the long chain cation and anion respectively). In the case of Ci2 and C14-DAO, a 1 1 LDAO/SDS molar ratio produces a minimum In surface tension and Is accompanied by an Increase In pH In the bulk solution the association seems to be of the type R R", and the absence of visible precipitate may be attributed to the solubilization of the R R" complex In the solution. In the region where LDAO Is In excess, the structure Is probably [cationic (LDAOH ) anionic (SDS)] nonlonlc (LDAO), while [cationic (LDAOH anionic (SDS)] anionic (SDS) Is formed when SDS Is In excess. Equal molar concentration results In cationic (LDAOH ) anionic (SDS) complex which should favor precipitation. However, at pH >9, there Is no Indication of precipitation (even when the total solute concentration Is 0.35 M). When the pH Is below 9, then precipitation will take place. 

Practically all available iodinated extracellular X-ray contrast agents have been encapsulated into liposomes using different lipids and methods of preparation. Table 1 gives a short and intentionally incomplete overview of some of the approaches. The first liposomal contrast agent preparation that was tested in humans contained diatrizoate [48]. The injected dose was up to 0.5 ml kg k The preparation was effective even in plain radiography where lesions down to 0.8-1.0 cm could be detected in patients. However, adverse events such as fever and hyperthermia, which occurred in 30% of the patients, limited further use. We have incorporated iopromide into MLVs that were prepared from phosphatidyl choline (PC), cholesterol and stearic acid at a molar ratio of 4 5 1 using the ethanol-evaporation technique [44]. The liposomes can be stored freeze-dried and they are reconstituted before use by... 

Organolithium compounds other than butyllithium can be used with no change in the reaction efficiency reduction of the molar ratio of organolithium to alcohol merely slows the transesterification. Even when one-sixth of an equivalent is used, efficient but slow transesterification occurs. In no case has it been found necessary to leave reactions longer than 18 hr or to use temperatures higher than ambient. Ether solvents are far more effective than hydrocarbons, in which slower reactions occur. 

Although the general effect of the addition of bicarbonate was to increase the size of the colloidal species, Lindenbaum and Westfall obtained the opposite effect with citrate addition over the pH range 4-11, as measured by the percent of plutonium (IV) that was ultrafilterable (22). However, their plutonium concentrations were 2 X 10 5Af, and the solutions probably contained true colloids, rather than pseudocolloids, if one accepts Davydovs analysis. Lindenbaum and Westfall concluded that the mechanism of the citrate action was the complexation of plutonium, thereby preventing the formation of hydrolytic polymers. It should be noted, however, that even with a citrate-plutonium molar ratio of 1800 (3.4 X 10 4Af citrate), about 10% of the plutonium still could not pass through the ultrafilter for solutions aged up to four days (22). 

Effect of cations other than TPA has also been studied and the results are given in Table 2 and Fig.3. The molar ratio of additional cation/CTA" was kept at 1.4. It can be seen that cations like tetramethylammonium or tetraethylammonium ions or even sodium also gives highly ordered MCM-41 structure The BET surface area of these samples was found to be >1000 m2/g. Fig.3 shows that the MCM-41 structures obtained with these additional cations have improved hydrothermal stability. The long range ordering was unaffected by 4 days of hydrothermal treatment. Moreover, the surface area and pore volume of the water-treated samples was only marginally lower than that of the calcined samples. 

Figure 2 presents the effect of the various volumetric ratios of water to rapeseed oil on the yield of fatty acids as prepared with both flow- and batch-type reaction systems at 270°C for 20 min. The volumetric ratios of 1/4 and 4 correspond to the molar ratios of 13 and 217, respectively. For the batch-type system, the hydrolysis rate of triglycerides seemed to be affected more by the amount of water, and a slightly better conversion was seen with the flow-type reaction system. Even though the volumetric ratio of 1/4 is equivalent to the molar ratio of 13 in water, which is theoretically higher than its stoichiometry of 3, the formation of fatty acids in both reaction systems was obviously low. In addition, it was found that at a volumetric ratio less than 2/3, it was difficult to separate hydrolysis products from the water portion that contained glycerol. On the other hand, the presence of water in fatty acids would have a negative effect on the methyl esterification reaction (15). 

Paraffin to olefin molar ratios for the C3 and Cy fractions are reported in Table I for two sets of runs in which the degree of mass transfer was varied by changing RPM. Even with a CSTR, data on the effect of mass transfer must be Interpreted carefully. An Increase in mass transfer resistance, caused by decreased agitation, causes a drop in conversion. Usually the consumption ratio of H2/CO is different than the feed ratio and hence a drop in conversion is accompanied by a change in the H2/CO ratio in... 

The reaction of XIII with EtOH in CHjCl EtOH, in 1 1 molar ratio, affords the ethoxy-derivative, [8,8-(q2-dppp)-9-(OEt)-nido-RhSB,H,] (XIV) behavior which is seen in the series of related compounds.12,111 This suggests that the boron vertex at the 9-position in Xm, and the related series of compounds, is prone to nucleophilic substitution reactions. XIV was characterized by NMR spectroscopy, mass spectrometry and X-ray diffraction. In contrast to the parent rhodathiaborane IV, and XIII, compound XIV does not feature fluxional behavior, even at 373 K implying, perhaps, AG values greater than 64 kjmol 1 for a possible dynamic process in XIV. The apparent large difference in the activation energy for the substituted versus the unsubstituted species may be due to either electronic or steric effects. The differences in the structures of XIII and XIV are minimal except for the B(9)-B(10) distances which differ by 0.083 A.4b This tends to suggest a role for this bond in the fluxional process although a purely steric influence of the substituent at the 9-position in XIV cannot be ruled out. 

---