Water, structural behavior

Two dendrimers based on Fe-porphyrin core carrying peptide-like branches of different sizes have been synthesized in order to have more open and a more densely packed (23) structures [43]. The electrochemical behavior has been examined in CH2C12 and in aqueous solution. In the less polar solvent, the two dendrimers show similar potentials for the Fem/Fen couple, suggesting that the iron porphyrins in both the more open and the more densely packed dendrimers experience similar microenvironments. On the contrary, in water the behavior of the two dendrimers is very different since the reduction from Fem to Fe11 is much easier for the densely packed dendrimer. This result can be explained considering that in the dendrimer with the relatively open structure the aqueous solvation of the iron porphyrin is still possible, whereas in the densely packed one the contact between the heme and the external solvent is signifi-... 

From the frequency measurements of the LB-film-deposited QCM plate in water, the behavior of phospholipid LB films can be classified into three types (i) phospholipids having relatively hydrophilic head groups such as DPPC and DPPG are hydrated and then easily flaked from the substrate in the fluid liquid crystalline state above Tc (ii) DPPE and DPPS having the less hydrophilic head groups are hydrated only near their Tc (iii) cholesterol LB films show relatively large hydration behavior even at low temperatures due to the water penetration into the structure defects in the membrane. 

Over the years, a large number of models of water structure have been developed in an attempt to reconcile all the known physical properties of water and to arrive at a molecular description of water that accounts correctly for its behavior over a large range of thermodynamic conditions. Early models of water structure have been categorized by Fennema (1996) and Ball (2001) into three general types mixture, uniformist, and interstitial. Mixture models are based on the concept of intermolecular hydrogen bonds... 

Chaplin, M. 2004. Water structure and behavior, http //www.sbu.ac.uk/water/ January 8, 2004. 

This behavior suggests that the Na and crions keep their solvation shells intact upon adsorption, with only small changes due to the restriction put on the water structure hy the metal lattice. For example, when an external electric field is applied, stronger ion-metal interactions are able to strip a small part of the coordination shell, but only for large fields. The electric field is also found to decrease the residence time of water molecules in the ion s coordination shell. 

Takayama, C., Akamatsu, M., and Fujita, T. Effects of structure on 1-octanol/water partitioning behavior of aliphatic amines and ammonium ions, Quant. Struct.-Act. Relat., 4 (4) 149-160, 1985. 

It is pertinent at this stage of the discussion to recall various studies (40, 41) on the effect of urea on water structure. It has been shown, for example, that the behavior of urea in water can be explained by an essentially structural model. This conclusion could well be related to our own findings on the behavior of urea in water-THF mixtures, which is qualitatively similar to that of n-Bu4NBr. The predominant structural effect TAS°t > AH°t is here in both cases in the water-rich region, and the preferential solvation of urea by water becomes the predominant effect at higher THF composition, as does the Br ion for n-Bu4NBr. 

Safford and Naumann (128) have shown that the time-of-flight spectra for 4.6M solutions of KF, KC1, CsCl, NaCl, and LiCl show peaks in the inelastic scattering region which coincide both in frequency and shape with the ice-like (structured) frequencies of pure water. Also, solutions of KSCN, KI, KBr, and NaC104 have lattice frequencies where they are found for water although in these cases apparently with less resolution and less intensity. Even an 18.5-M solution of KSCN showed a similar behavior. We take this to suggest that elements of water structure remain in these solutions (as discussed elsewhere in this paper, where we noted that the thermal anomalies occur at approximately the same temperatures, even for relatively concentrated solutions, as where they occur in pure water see also Ref. 103). 

We have mentioned these examples of the effects of nonelectrolytes on water structure only to indicate the important information which may be derived from studies of this type (see also Refs. 50 and 38). Elsewhere, we will discuss the available evidence for discreteness in water structure based on the behavior of aqueous solutions of nonelectrolytes. When combined with the information about solutions of electrolytes (treated here) a better understanding of the structural properties of water in solutions may emerge. 

The importance of carefully considering anomalies when studying the behavior of solute-solvent interactions has been stressed. For aqueous solutions, many anomalous results presented in the literature suggest the existence of some type of discreteness in water structure. Discreteness is consistent with a view of water structure providing distinct sites such as those found in the models of water, implying a broken down ice lattice structure or clathrate hydrate cage-like structures. 

Starting from the mixture model, the structural behavior of water in the presence of dissolved simple ions is discussed from the point of view of defect formation and lattice distortions at interfaces. The observed behavior of the ions and the water lattice is applied to a number of unsolved biological problems in an attempt to elucidate the specific interface phenomena that are characteristic of such systems. 

In the Frank and Evans iceberg model, ice-like structures form around hydrophobic entities, such as methane. In this model, the hydrophobic molecules enhance the local water structure (greater tetrahedral order) compared with pure water. Ordering of the water hydration shell around hydrophobic molecules has been attributed to clathrate-like behavior, in which the water hydration shell is dominated by pentagons compared to bulk liquid water (Franks and Reid, 1973). 

In aqueous solutions of Cm-(EO)n amphiphilic molecules, two interesting features are observed. First, isotropic micellar solutions undergo phase separation on heating. Such behavior is typical of hydrophobic interaction and is also observed for several water-soluble polymers. Hydrophobic interaction results from a change of order in the water structure [54]. Second, at high concentration, liquid crystalline phase behavior is observed with several structures [55]. 

The behavior of hA in real micellar systems is more complex as seen in Fig. 2.12. Similar data have been obtained for several other amphiphiles148,149). The deviations in hA from the standard value at infinite dilution appear clearly below the CMC, but at these concentrations one has a compensating change in the partial molar entropy. This effect might be due to a repulsive interaction between the hydrophobically hydrated alkyl chains leading to a breakdown of the water structure with a concomitant increase in entropy. 

---