Microdomains solubilization

The formation of a microphase structure can be sensitively detected by using hydrophobic fluorescent probes. Hydrophobic microdomains tend to solubilize hydrophobic small molecules present together in aqueous solution. For example, diphenylhexatriene (DHT) is hydrophobically bound to the St aggregates in ASt-x in aqueous solution and, as a result, the fluorescence intensity is greatly enhanced. Figure 9 shows the fluorescence intensity of DHT in the presence of ASt-x relative to the intensity in its absence (I/I0) as a function of the ASt-x concentration [29],... 

Contaminants bound to colloids also may lead to an increase in the apparent solubility of the compounds. Most colloidal phases are effective sorbents of low-solubility contaminants, due to their large surface area. For example. Fig. 8.21 depicts the solubilization of p-nitrophenol into hydrophobic microdomains, which defines the trace metal level in the groundwater of a coastal watershed (Sanudo-Wilhelmy et al. 2002). The authors emphasize that the (heavy) metals contained in the colloidal size fraction in some instances may reach more than 50% of what is considered dissolved metal this should be considered to properly understand the cycling of metals and carbon in the subsurface water. 

The appearance of the excimer peak has been attributed to the formation of a premicelle or nascent micelle aggregate, which because of its small size and hydrophobic nature allows the solubilized excited pyrene to interact with the ground-state pyrene to form an excimer. The peak vanishes in the plateau due to the difficulties for the ground-state pyrene to encounter the excited species in the relatively large microdomain of the micelle. This observation led Sahoo et al. [123] to propose a step-wise formation for the micelles (Scheme 2, Fig. 16)... 

The spatial distribution of homopolymer within lamellar domains formed by the same PS-PB-PS triblock blended with homopolymer has been determined (Kimishima et al. 1995). Blends with PS or PAMS with molecular weights lower than that of the PS in the diblock were investigated using SAXS and TEM. It was found that both PS and PAMS were solubilized in the PS microdomains however, the PAMS was not uniformly distributed, tending to be localized at the domain centres. This localization was diminished on increasing the temperature. In contrast, the PS homopolymer was uniformly distributed. The localization of the PAMS was ascribed to repulsive interactions between PS chains and the PAMS homopolymer, this effect decreasing with increasing temperature. 

These results imply that homopolymer PS is not always miscible with the PS blocks of the copolymer, i.e. confinement of PS to an interface in a block copolymer can lead to immiscibility with homopolymer PS (Hashimoto et al. 1990). This has been interpreted in terms of the enthalpic and entropic contributions to the free energy (Hasegawa and Hashimoto 1996). For a < 1 uniform solubilization increases the translational entropy of the homopolymer, but chain stretching in the homopolymer and in the PS chain of the diblock leads to a decrease in conformational entropy. At the same time, the lateral swelling of microdomains leads... 

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

The surface activity and solubihzation capacity of amphoteric polysoaps were studied [172,177,178]. The surface activity of zwitterionic polysoaps is diminished by added salt due to their antipolyelectrolyte character. The sequence to solubihze hydrophobic dyes is often mid-tail > head > tail-end geometry. An extended study on the solubilization abihty of hydrophobically modified polybetaines can be found in [232]. The surface activity of the cyclocopolymers containing the pH-responsive hydrophobic monomer WAf-diallyl-N-methylamine and the salt-sensitive sulfobetaine monomer 3-(WAf-diallyl-N-methylammonio) propanesulfonate was utilized to solubilize p-cresol within microdomains [233]. Other studies corroborate the general picture [182,183]. 

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for the first time for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. 

The simplest information obtained by qualitative solubilization is the existence (or the absence) of hydrophobic microdomains, using probes which are sensitive to the polarity of the environment. Such solubilization experiments are... 

Templating involves - in the first step - filling or coating of the template. This can be achieved by solubilization of a compound into the template interior or distributing it aroimd the template. For the desired distribution of the precursor it is advantageous to use secondary valence forces to partition the substance in the desired microdomain or at the desired interface. Commonly used forces comprise ... 

The fluorescence decay rate constants, k , of several fluorescent probes in various systems (Table I) provide further evidence for hydrophobic microdomains in PA-I8K2 aqueous solutions. All the decays are single exponential decays except for C PN in aqueous solutions of PMA at pH 8. All the values in PA-I8K2 aqueous solutions at pH 4, 8, and 10 are similar to those found in hexanol and much smaller than those in water and aqueous solutions of PMA at pH 8. In summary, PA-I8K2 readily solubilizes pyrene and some positively and negatively charged derivatives of pyrene, especially long-chain derivatives. 

These data indicate that the environment of CnPN in PMA and in PA-I8K2, both at pH 8, where both polyelectrolytes are almost fully ionized, is quite different. C PN experiences a water phase in PMA, and the interaction between CnPN and PMA is electrostatic in nature. In PA-I8K2 solution, CijPN was solubilized in polymer hydrophobic microdomains because of a hydrophobic interaction between the long hydrocarbon chains and because of electrostatic bonding. 

They act as super solvents of drug. They can solubilize hydrophilic and lipophilic drugs including drugs that are relatively insoluble in both aqueous and hydrophobic solvents. This is due to the existence of microdomains of different polarity within the same singlephase solution. 

In aqueous solution, this copolymer adopts a pseudo-micellar conformation, i.e. the macromolecules form hydrophobic microdomains capable of solubilizing organic compounds that are sparingly soluble in water. Table 14.1 presents typical systems explored in this work. 

Kunieda intensified the studies on phase behavior and formation of microemulsions in mixed-surfactant systems [66-76], in order to understand the relationship between maximum solubilization of microemulsions and surfactant distribution of mixed surfactants at the water/oil interface in the microemulsion phase. He developed a method to calculate the net composition of each surfactant at the interface in the bicontinuous microemulsions assuming that the monomeric solubihty of each surfactant in oil is the same as in the oil microdomain of the microemulsions [69]. Using this approach, the distribution of surfactants in the different domains of bicontinuous microemulsions (Figure 9) could be quantified [70-75], even if the complete microstracture of these systems was not completely elucidated. 

A hydrophobically modified water-soluble polymer (HM-polymer) can be viewed as a modified surfactant. It forms micelles, or hydrophobic microdomains, on its own at very low concentrations (intramolecularly, at infinite dilution) and these micelles can solubilize hydrophobic molecules. Furthermore, an HM-polymer and a surfactant in general have a strong tendency to form mixed micelles in a similar way as two surfactants. Two stoichiometries are important for HM-polymer-surfactant systems, i.e. the alkyl chain stoichiometry and the charge stoichiometry. 

Homopolymer/ copolymer Solubilization of homopolymer by copolymer. The homopolymer A dissolves in microdomains of A-blocks of the A-B copolymer, until a limit of miscibility is reached. Above this limit, excess of homopolymer forms macroscopic domains Hashimoto et al. 1990 Tanaka et al. 1991... 

In so far as microdomains in water are concerned, partially quaternized polytertiary amines with pendant tertiary amine groups possess a property of special interest. The deprotonated microdomains can be readily destabilized by adding strong acids, which cause protonation of tertiary amine residues and thus solubilize the polymer chains conventionally. The destabilization occurs at a constant pH through cooperative chemical and conformational transitions of the all-or-none type for a large range of the degree of protonation.2... 

The destabilization is absolutely reversible and the microdomains reform immediately by adding bases which cause deprotonation of the formerly protonated tertiary amine residues. Such behavior is of special interest if temporary trapping of hydrophobic water-insoluble species by solubilization in the microdomains is desired. Indeed, the trapped species should be releasable instantaneously and at constant pH by adding at most a stoichiometric amount of strong acid. 

Previous publications [136, 137] reviewed models proposed for HS achieved with the diisocyanate MDI and extended with BDO. A detailed morphological analysis [137] of a series of such materials showed that the hard microdomain structure was in qualitative agreement with the model proposed by Koberstein and Stein [136], This model was based on the partial solubilization of short HS into the soft microphase. HS shorter than the critical length for microphase separation were presumed to remain within the soft microphase while longer segments aggregated into lamellar hard microdomains of thickness proportional to the critical sequence... 

Homopolymers A and B become solubilized in the microdomains of the like components of the copolymer. This situation is more likely to be found at high copolymer content. 

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