Thermodynamics solubilization

A, B, C, D depend on the extracted protein and are functions of AGo, oc, e, A i, n, pi, Na, z. Their numerical value have been calculated from experimental data on solubilization of ribonuclease and concanavalin A in AOT/isooctane with a good correlation to the model equation. The great interest of this model is that all the assumptions necessary for its elaboration make it very simple, and at the same time, a promising tool of quantification of protein solubilization thermodynamics, even if some further refinements are still needed. It can be noted that there are more parameters than can be adjusted from experimental data. As a consequence, the model can provide no value for n, related to the micelle size, which could have permitted an interesting comparison with that predicted by Caselli et al. s model. 

M. E. Vitha, A. J. Dallas, P. W. Carr, Study of water-sodium dodecyl sulfate micellar solubilization thermodynamics for several solute homolog series by headspace gas chromatography, J. Phys. Chem., 1996, 100, 5050-5062. 

For the polymer PVDF (in the solid state), the monomer C2H2F2 is a good solvent even at high T andp (over 500 K and 200 MPa, respectively) in this respect the comparison with the solubilization thermodynamics of inert N2 in PVDF is of practical interest since PVDF is a major polymeric material in numerous industrial applications. Furthermore, because the monomer C2H2F2 is the major component of... 

Detaded thermodynamic and mechanistic analyses of solubilization and related mechanisms are given in References 17 and 82—84. These works show that under proper circumstances, solubilization can make a significant contribution to ody-sod removal. 

Generally, solubilization occurs spontaneously when the pure solubilizate contacts the solution of reversed micelles. Often, vigorous stirring consistently reduces the time necessary to obtain complete solubihzation and thermodynamically stable systems. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

Adding compounds solubilized in DMSO to aqueous medium as part of a discovery solubility assay can lead to two types of solubility assay with different uses. At one extreme, the quantity of DMSO is kept very low (<1%). At this low level of DMSO, the solubility is only slightly affected by the DMSO content. For example, data from a poster by Ricerca Ltd. [11] suggest that a DMSO content of 1% should not elevate apparent solubility by more than about 65%. At 5% DMSO, this group reported an average solubility increase of 145% due to the DMSO content. Solubility in an early discovery assay containing one percent DMSO can however exceed thermodynamic solubility by much more than 65%. However, this is very likely due to the time scale. Studies by the Avdeef (plon Inc.) group show a close approximation of early discovery solubility (quantitated by UV) to literature ther-... 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

The most useful characteristic of the micelle arises from its inner (alkyl chain) part (Figure 3.17). The inner part consists of alkyl groups that are closely packed. It is known that these clusters behave as liquid paraffin (Cn H2n+2). The alkyl chains are thus not fully extended. Hence, one would expect that this inner hydrophobic part of the micelle should exhibit properties that are common to alkanes, such as ability to solubilize all kinds of water-insoluble organic compounds. The solute enters the alkyl core of the micelle and it swells. Equilibrium is reached when the ratio between moles soluteimoles detergent is reached corresponding to the thermodynamic value. 

Below CMC the amount dissolved remains constant, which corresponds to its solubility in pure water. The slope of the plot above CMC corresponds to 14 mole SDS 1 mol naphthalene. It is seen that, at the CMC, the solubility of naphthalene abruptly increases. This is because all micelles can solubilize water-insoluble organic compounds. A more useful analysis can be carried out by considering the thermodynamics of this solubilization process. 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

Other molecular thermodynamic models for protein-reverse micelle complexes have also emerged. Bratko et al. [171] presented a model for phase transfer of proteins in RMs. The shell and core model was combined with the Poisson-Boltzmann approximation for the protein-RM complex and for the protein-free RM. The increase in entropy of counterions released from RMs on solubilization of a protein was the main contribution to the decrease in free energy of com-plexation. Good agreement was found with SANS results of Sheu et al. [151] for cytochrome C solubilization and the effect of electrolytes on it. However, this model assumes that filled and empty RMs are of the same size, independent of salt strength and pH, which is not true according to experimental evidence available since then. 

The structure and thermodynamics of formation of mixed micelles is of great theoretical interest. Micelles are also present and often integrally involved in practical processes. For example, in a small pore volume surfactant flooding process (sometimes called micellar flooding), the solution injected into an oil field generally contains 5-12 weight X surfactant (i) and the surfactant is predominately in micellar form in the reservoir water. In detergency, solubilization can be... 

Nucleic acids can also be solubilized in reverse micelles, including ribosomes and plasmids, (Imre and Luisi, 1982 Palazzo and Luisi, 1992 Pietrini and Luisi, 2002 2004 Ousfuri et al, 2005), which also gives rise to a series of interesting structural and thermodynamic questions. In particular, high-molecular-weight... 

The failure of the theory indicates that soy protein behavior is more complex than Melander and Horvath s (7, ) model. In deriving their simple theory, Melander and Horvath assumed that the changes in hydrophobic surface area, in the dipole moment, and in the net charge of the protein upon solubilization are invariant with respect to salt species or salt concentration. In other words, they assumed that the soluble proteins have the same thermodynamic state regardless of salt species or salt concentration. The results for soy proteins can be treated in the framework of the Melander and Horvath s theory if it is expanded to allow the exposed surface area, the dipole moment, and the net protein charge to be functions of the salt species and the salt concentration. 

Above the CMC, a number of solutes that would normally be insoluble or only slightly soluble in water dissolve extensively in surfactant solutions. The process is called solubilization, the substance dissolved is called the solubilizate, and —in this context —the surfactant is called the solubilizer. The result is a thermodynamically stable, isotropic solution in which the solubilizate is somehow taken up by micelles since the enhancement of solubility begins at the CMC. This observation, in fact, provides one method for determining the CMC of a surfactant it... 

---