Critical association

At first, one would tend to reconsider conventional crossover due to mean-field criticality associated with long-range interactions in terms of the refined theories. Conventional crossover conforms to the first case mentioned—that is, small u with the correlation length of the critical fluctuations to be larger than 0. However, in the latter case one expects smooth crossover with slowly and monotonously varying critical exponents, as observed in nonionic fluids. Thus, the sharp and nonmonotonous behavior cannot be reconciled with one length scale only. 

Mixed micelles, comprising both polymerized and free surfactants, are formed once the critical association concentration (CAC) of the free am-phiphiles is reached. The CAC is typically much smaller than the CMC for the formation of free micelles. As the fraction of unpolymerized surfactants within the mixed micelles grows, their structure approaches that of free micelles. 

Once Halbert made the critical association between Nutrimaster and Firemaster, the mystery was solved. By April 29, 197 Fries had established via both QC and MS (mass spectrometry) that Halbert s feed did indeed contain PBB. 

Amphiphilic poly(ethylene glycol)-alkyl dextran ethers are emerging as vehicles in the oral delivery of poorly water soluble drugs [251,268,269]. They form polymer micelles of low critical association concentrations (CAC) and small micelle sizes in aqueous solution. Particulate delivery systems lead to an enhancement of the absorption efficiency and bioavailability of highly hpophihc drugs orally applied, and provide the drug with some level of pro-... 

Commensurate with the criticality associated with sterile products, the global regulatory community has established a substantial number of the basic requirements that firms are expected to adhere to in the manufacture of sterile products. The most extensive of these are those defined by the Food and Drug Administration... 

The development of amphiphilic and aggregation behaviour as a fiinction of dendrimer generation of PS-t/enstudied with several diflferent techniques the amphiphilic character at a toluene-water interface was investigated with conductivity measurements, and at a water-air interface with monolayer experiments. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to examine the aggregates formed by the different generations in aqueous solutions, while the critical association concentrations were determined with the pyrene-probe luminescence technique. 

Many techniques can be used for the determination of critical association concentrations (CACs), however, not all of them are sensitive enough to detect the onset of aggregation if this occm i at very low concentrations. Since the CACs of block copolymers are usually much lower than those of low molecular mass surfactants [54], we used pyrene as a fluorescent probe and calculated the effective CACs from the changes in the spectral characteristics of pyrene [55] as a function of surfactant concentration. If we represent the intensity of the emission spectra as a function of the block copolymer concentration, we directly obtain C4C, [56]. From the excitation spectra we obtain CACi by representing the ratio /340//335 vs. log C. 

Hydrophobically modified polymers can associate in aqueous media to form micelle-like structures above their critical association concentrations (CACs). The nanosized self-aggregates were prepared using modified natural polysaccharides such as pullulan, curdlan, and glycol chitosan. The modified polysaccharides provide excellent biocompatibility, biodegradability, low immunogenicity, and biological activities. 

Polymeric micelles have critical association concentrations that can be modified by the polymer structures. The critical concentrations are lower than those in the case of surfactant-based micelles enabling the stability of the polymeric micelles in the circulation [18]. Polymeric micelles have been used successfully for dmg targeting intravenously. 

In diluted AP polymer solution, the polymer exists in the form of single molecules, and the association is weak. Thus, the AP viscosity is not much different from HPAM viscosity. When AP concentration reaches a certain level, the molecules have more chance of contacting each other, thus the association becomes strong, and viscosity sharply increases. This concentration is called critical associating concentration. Table 5.9 shows that when the polymer concentration was below 400 mg/L, AP viscosities were lower than HPAM. Above... 

On the other hand, the anisotropy within the unoriented film can also be investigated via the ratio of two absorptions at 1783 cm" and 742 cm", which represents the in and out-of-plane bending of the carbonyl group, respectively. Recently, it has been observed that the absorption of the IR beam yields different spectrum intensities at these two bands for BPDA-PFMB and their copolyimide films (5), and the ratio between 1783 cm" and 742 cm" absorption bands is critically associated with the degree of in-plane orientation. 

We may interpret the concentration dependence of y in the presence of a polymer as follows. At a certain concentration, often termed the critical association concentration (CAC) and denoted T in Figure 20.1, there... 

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