Ion association approach

The present author has developed a novel method called ion-association method. This is also a simple and versatile method for the preparation of ion-based organic dye nanoparticles in pure aqueous solution by the ion association approach [23]. It utilizes the control of hydrophilicity/hydrophobicity of the ionic material itself via ion-pair formation for example, addition of a cationic target dye solution into aqueous solution containing a certain kind of hydrophobic anions forms an electrically neutral ion-pair because of the strong electrostatic attraction, followed by aggregation of ion-pair species originated from van der Waals attractive interactions between them to produce nuclei and the subsequent nanoparticles (Fig. 3). In this case, hydrophobic but water-soluble anions, such as tetraphenyl-borate (TPB) or its derivatives (tetrakis(4-fluorophenyl)borate (TFPB), tetrakis [3,5-... 

There are several limitations which lead to the discrepancies in Tables IV-X. First of all, no model will be better than the assumptions upon which it is based. The models compiled in this survey are based on the ion association approach whose general reliability rests on several non-thermodynamic assumptions. For example, the use of activity coefficients to describe the non-ideal behavior of aqueous electrolytes reflects our uncertain knowledge of ionic interactions and as a consequence we must approximate activity coefficients with semi-empirical equations. In addition, the assumption of ion association may be a naive representation of the true interactions of "ions" in aqueous solutions. If a consistent and comprehensive theory of electrolyte solutions were available along with a consistent set of thermodynamic data then our aqueous models should be in excellent agreement for most systems. Until such a theory is provided we should expect the type of results shown in Tables IV-X. No degree of computational or numerical sophistication can improve upon the basic chemical model which is utilized. 

Ion-Association and lon-Hvdration. Aqueous solutions of electrolytes have been chemically described using a variety of theories. The original theoretical approach used by geochemists to model aqueous systems was based on the concept of ion-pairing or ion-association. The ion association approach as described by Carrels and Thompson (1) accurately depicted the speciation of seawater and later many other aqueous solutions. This approach was subsequently found to be inadequate for defining the chemistry of more complex and more concentrated aqueous solutions or those solutions near the critical point of water. This deficiency led to the use of other theoretical approaches to describe these systems, such as the ion-interaction, mean salt, and ion-hydration theories. 

It was possible to estimate parameters for OH complexes of K" ", Li" ", and Na in the fitting process. Of these complexes, the WATEQ and amended WATEQ models included a OH complex for Ba, and the value for the stability constant was similar to that calculated by the fitting process. In the fit model, the hi (Equation 6) parameters for the OH complexes of K, Li", and Na were 0.56, 0.4, and 0.42 respectively. The physical significance of these large h values is not clear. When the association reactions were written in terms of OH , the stability constants were -0.09,0.44, and 0.06, respectively. Calculations indicate that, in a 1.0 molal KOH solution, the concentration of the KOH complex is 0.14 molal. Additional experimental evidence from titrations or spectroscopy is needed to determine whether this ion pair actually exists and whether this concentration is reasonable. If there is no evidence that the OH salts are partially associated in solution, the large stability constants may indicate that specific-ion interactions are involved that can not be accounted for by the ion-association approach. 

Nucleation in a cloud chamber is an important experimental tool to understand nucleation processes. Such nucleation by ions can arise in atmospheric physics theoretical analysis has been made [62, 63] and there are interesting differences in the nucleating ability of positive and negative ions [64]. In water vapor, it appears that the full heat of solvation of an ion is approached after only 5-10 water molecules have associated with... 

The central engine of this data workflow is the process of spectral deconvolution. During spectral deconvolution, sets of multiply charged ions associated with particular proteins are reduced to a simplified spectrum representing the neutral mass forms of those proteins. Our laboratory makes use of a maximum entropy-based approach to spectral deconvolution (Ferrige et al., 1992a and b) that attempts to identify the most likely distribution of neutral masses that accounts for all data within the m/z mass spectrum. With this approach, quantitative peak intensity information is retained from the source spectrum, and meaningful intensity differences can be obtained by comparison of LC/MS runs acquired and processed under similar conditions. 

Fig. 3 Concept of the ion-association method for fabricating ion-based organic dye nanoparticles in pure aqueous media. The approach is based on ion-pair formation between the ionic dye (for example, cationic dye) and the hydrophobic counterion that is soluble in water [for example, tetraphenylborate (TPB) or its derivative anion], which gives rise to a hydrophobic phase in water. For preparation, organic cosolvent is unnecessary. The size of the dye nanoparticles can be controlled by adjusting the interionic interaction between the dye cation and the associative hydrophobic counteranion...

Dissertation Washington University St. Louis, 1979]. From data for the analogous cobalt(III) complex with perchlorate and hexafluorophosphate as anions, he found association constants of about 900 and 300, with distances of closest approach of 5 or 6 8, respectively. These values seem reasonable. For the iron(II) compound with the hexafluorophosphate or perchlorate, the ion association constants were too small to measure (< - 50). 

The standard hydrocarbon model, data analysis approach 2, does a poor job of estimating in this system (1.83 eV). This has led us to recognize that the case of an atomic ion associating with a neutral is exceptional because of the small number of rotational degrees of freedom of the reactants. With an appropriate correction for this effect, the standard hydrocarbon model estimate is lowered to about 1.5 eV, which is an entirely acceptable estimate. 

This approach applies only when we are certain that the substrate is mainly in the form of the free ion at the lowest anion concentrations. This is true in the chloride exchange of cw-[Co en2 Cl2]+ in methanol and we can safely conclude that the mechanism is unimolecular (8, 9. 10, 11, 26, 27). This condition did not exist when we studied the displacement of water in trans-[Co en2N02H20]+2 by anions where, because of the large ion association constants, none of the substrate was in the free ion form under reaction conditions. However, in the reaction between trans-[Co en2N02Br]+ and thiocyanate in sulfolane, the substrate was mainly in the free ion form. The observed second-order kinetic form was fully consistent with assigning a bimolecular mechanism to the rearrangement of the ion pair. 

With the knowledge that 14 can activate aldehydes in 1, the role of 1 in the reaction was explored further. Specifically, the relative rates of C—H bond activation and guest ejection, and the possibility of ion association with 1, were investigated. The hydrophobic nature of 14 could allow for ion association on the exterior of 1, which would be both cn t h al pi cal I y favorable due to the cation-it interaction, and entropically favorable due to the partial desolvation of 14. To explore these questions, 14 was irreversibly trapped in solution by a large phosphine, which coordinates to the iridium complex and thereby inhibits encapsulation. Two different trapping phosphines were used. The first, triphenylphosphine tris-sulfonate sodium salt (TPPTS), is a trianionic water-soluble phosphine and should not be able to approach the highly anionic 1, thereby only trapping the iridium complex that has diffused away from 1. The second phosphine, l,3,5-triaza-7-phosphaadamantane (PTA), is a water-soluble neutral phosphine that should be able to intercept an ion-associated iridium complex. 

An alternative approach to ion association was proposed by Fuoss. He defined the ion-pair as two oppositely charged ions that are in contact, i.e. at a distance of r=a, and derived the following equation for the ion association constant ... 

There are five reactions that deal with ion associations (numbers 8 to 12 in Table 3.3). There are, of course, many more such associations in concentrated electrolyte solutions. But the Pitzer approach allows one to either explicitly identify an ion association (Table 3.3) or to implicitly include the interaction effect in the interaction coefficients (B, C, minor components of the aqueous phase. 

Reference Electrodes for Use in Nonpolar Solvents. Solvents such as dichloromethane (not highly polar) present special problems. Their low dielectric constants promote extensive ion association, and cell resistances tend to be large. For this reason they are often used in mixtures with more polar solvents. Because dichloromethane and other nonpolar solvents are not miscible with water, use of an aqueous reference electrode with such solvents is not practical unless a salt bridge with some mutually miscible solvent is used. A better approach is to use a reference electrode of known reliability prepared in a solvent miscible with dichloromethane or to use the reference electrode based on the half-cell in dichloromethane.88... 

Another attempt to go beyond the cell model proceeds with the Debye-Hiickel-Bjerrum theory [38]. The linearized PB equation is used as a starting point, however ion association is inserted by hand to correct for the non-linear couplings. This approach incorporates rod-rod interactions and should thus account for full solution properties. For the case of added salt the theory predicts an osmotic coefficient below the Manning limiting value, which is much too low. The same is true for a simplified version of the salt free case. 

The analysis of brines perhaps deserves special mention as the high sodium chloride concentrations are extremely unfavourable for electrothermal atomisation and most troublesome in flame analysis. The preferred approach is probably solvent extraction with either oxine or APDC to remove the trace metals into a small volume of MIBK for flame atomisation or chloroform for electrothermal cells. Care must be taken to avoid interference from chloro-complexes in the extraction, and if this is suspected an ion-association extraction of these complexes might be preferable. 

This approach has been used successfully to derive the stability constants of a number of ionic complexes such as NiS04 [105] and metal cation polyhalides [106, 107], as well as of molecular associations [108, 109]. Figure 4.11 shows the difference in the A.3 vs C variations for Niz+ perchlorate (no ion association) and NiS04 (ion association the sulfate anion has no action on t3) due to the lesser availability of the odd electrons of the metal ions, the Ps spin conversion reaction is less efficient when NiS04 ion pairs are present. 

An understanding of the concentration dependence of activity coefficients required the postulation of the concepts of ion-pair formation and complex formation. Certain structural questions, however, could not be answered unequivocally by these considerations alone. For instance, it was not possible to decide whether pure Cou-lombic or chemical forces were involved in the process of ion association, i.e., whether the associated entities were ion pairs or complexes. The approach has been to postulate one of these types of association, then to work out the effect of such an association on the value of the activity coefficient, and finally to compare the observed and calculated values. Proceeding on this basis, it is inevitable that the postulate will always stand in need of confirmation because the path from postulate to fact is indirect. 

In the development of the theory of Debye and Hiickel, it was assumed that two main changes had to be made in the ion size (represented by the distance of closest approach a) and the diminution of the available waters due to hydration. Ion association was taken into account also. 

The nature of the analyte interactions with liophilic ions could be electrostatic attraction, ion association, or dispersive-type interactions. Most probably all mentioned types are present. Ion association is essentially the same as an ion-pairing used in a general form of time-dependent interionic formation with the average lifetime on the level of 10 sec in water-organic solution with dielectric constant between 30 and 40. With increase of the water content in the mobile phase, the dielectric constant increases and approaches 80 (water) this decrease the lifetime of ion-associated complexes to approximately 10 sec, which is still about four orders of magnitude longer than average molecular vibration time. 

---