Clustering of water

The largest protonated cluster of water molecules yet definitively characterized is the discrete unit lHi306l formed serendipitously when the cage compound [(CyHin)3(NH)2Cll Cl was crystallized from a 10% aqueous hydrochloric acid solution. The structure of the cage cation is shown in Fig. 14.14 and the unit cell contains 4 [C9H,8)3(NH)2aiCUHnOfiiai- The hydrated proton features a short. symmetrical O-H-0 bond at the centre of symmetry und 4 longer unsymmetrical O-H - 0 bonds to 4... 

Gas hydrates are an ice-like material which is constituted of methane molecules encaged in a cluster of water molecules and held together by hydrogen bonds. This material occurs in large underground deposits found beneath the ocean floor on continental margins and in places north of the arctic circle such as Siberia. It is estimated that gas hydrate deposits contain twice as much carbon as all other fossil fuels on earth. This source, if proven feasible for recovery, could be a future energy as well as chemical source for petrochemicals. 

As one can see from the Figure, clusters of water molecules across the groove are formed. 

Because at equilibrium virtually all the HCl molecules have donated their protons to water, HCl is classified as a strong acid. The proton transfer reaction essentially goes to completion. The H30+ ion is called the hydronium ion. It is strongly hydrated in solution, and there is some evidence that a better representation of the species is H904+ (or even larger clusters of water molecules attached to a proton). A hydrogen ion in water is sometimes represented as H + (aq), but we must remember that H+ does not exist by itself in water and that H CC is a better representation. 

Figure 2.40 shows the unsteady flow upstream of the ONE in one of the parallel micro-channels of d = 130 pm at = 228kW/m, m = 0.044 g/s (Hetsroni et al. 2001b). In this part of the micro-channel single-phase water flow was mainly observed. Clusters of water appeared as a jet, penetrating the bulk of the water (Fig. 2.40a). The vapor jet moved in the upstream direction, and the space that it occupied increased (Fig. 2.40b). In Fig. 2.40a,b the flow moved from bottom to top. These pictures were obtained at the same part of the micro-channel but not simultaneously. The time interval between events shown in Fig. 2.40a and Fig. 2.40b is 0.055 s. As a result, the vapor accumulated in the inlet plenum and led to increased inlet temperature and to increased temperature and pressure fluctuations.

The transferred proton is actually associated with a cluster of water molecules. Protons exist in solution not only as H30, but also as multimers such as H502 and... 

Figure 12-5 illustrates the solvation of Na and Cl" ions as NaCl dissolves in water. A cluster of water molecules surrounds each ion in solution. Notice how the water molecules are oriented so that their dipole moments align with charges of the ions. The partially negative oxygen atoms of water molecules point toward Na cations, whereas the partially positive hydrogen atoms of water molecules point toward Cl" anions. 

Estrin, D. A., Pagheri, G., Corongiu, G., Clementi, E., 1996, Small Clusters of Water Molecules Using Density Functional Theory , J. Phys. Chem., 100, 8701. 

Besides water dimer, larger clusters of water molecules were extensively investigated by means of the DFT calculations87 111 114 127 128. Laasonen et al.113 studied the structure, the energies, and the vibrational frequencies of small water clusters (up to eight molecules)... 

The second example concerns the lithium ion, either considered in a cluster of water molecules or in aqueous solution. The idealized solution at infinite dilution of a lithium ion (without counter-ion) predicts six molecules of water in the first solvation shell if one uses pair-wise 2-body interactions, but the same type of computation predicts four molecules of water when 3-body effects are included. The computations were performed at room temperature. We have performed cluster computations for the Li fTO), system, with n = 1,2,3,4,5 and 6, using a density functional program developed in our laboratory. When we compute the most stable configuration for the pentamer complex Li+( starting from the most stable config-... 

One way to include these local quantum chemical effects is to perform ab initio calculations on an HOD molecule in a cluster of water molecules, possibly in the field of the point charges of the water molecules surrounding the cluster. In 1991 Hermansson generated such clusters from a Monte Carlo simulation of the liquid, and for each one she determined the relevant Bom Oppenheimer potential and the vibrational frequencies. The transition-dipole-weigh ted histogram of frequencies was in rough agreement with the experimental IR spectrum for H0D/D20 [130],... 

The dipole moment of the adsorbed water molecules is estimated to be = 0.22 D (unit of D = 3.36 x 10 ° C m) from the slope of the observed curves shown in Fig. 5-25. Since this dipole moment is nearly one tenth of the dipole moment of gaseous water molecules (m = 1.84 D), the dipole of the adsorbed water molecules on the silver surface is suggested to be aligned almost parallel to the metal surface by forming hydrogen-bonded two-dimensional clusters of water molecules. On the other hand, bromine molecules are in the state of dissociative adsorption on the silver surface, producing adsorbed bromine atoms which receive electrons... 

My feelings are that we often think of water as a completely inert and non active medium but it is useful to remember that the hydrogen bonds have a half-life of only seconds and that clusters of water molecules are constantly being broken and reformed and there is always a small proportion of D2 and H2O2 in normal water along with dissolved H2 and O or O2 because of nuclear "accidents" in this activity. 

At lower q, in the larger size range Jt/g 30 A the pores are empty. At ( 0.08 A, p q) even becomes shghtly negative. Such behaviour can be understood in terms of clusters of water molecules that exhibit water-air interfaces. Figure 5.2 illustrates the qualitative difference in the adsorption characteristics of hexane and water. 

FIGURE 1.5 In water, ions are hydrated that is, they are surrounded by a cluster of water molecules bonded loosely to the ion. There is a constant interchange between the bonded water molecules and those in the bulk solvent. Note that a cation (a) is surrounded by water molecules with the O atoms closer to the ion, whereas an anion (b) has water molecules attached through their hydrogen atoms. The number of hydrating molecules depends on the size of the ion, but for most ions, it is approximately six. 

A somewhat intermediate view has also been adopted by Horne and Birkett (SO), who also propose a multilayer model of hydration where both the firmly bonded, first hydration layer and the disordered zone of the Frank-Wen model are accepted. However, they suggest the existence of a second layer of water molecules (separating the primary hydration shell and the disordered zone) around the ion, consisting of rarified or extended clusters of water molecules with density less than waters but definitely not of Ice-I like structure. We return to this aspect later. In this connection, compare also our discussion of the studies by Vaslow (150), Griffith and Scheraga (67), and Luz and Yagil (103). 

An ionization reaction often decreases the entropy of a solution, instead of increasing it as one might at first expect, because clustering of water molecules around the ions can result in a net decrease in the number of free water molecules. 

Hydrate nucleation is the process during which small clusters of water and gas (hydrate nuclei) grow and disperse in an attempt to achieve critical size for continued growth. The nucleation step is a microscopic phenomenon involving tens to thousands of molecules (Mullin, 1993, p. 173) and is difficult to observe experimentally. Current hypotheses for hydrate nucleation are based upon the better-known phenomena of water freezing, the dissolution of hydrocarbons in water, and computer simulations of both phenomena. Evidence from experiments shows that nucleation is a statistically probable (not deterministically certain see Section 3.1.3) process. 

---