Polar water molecules orientation

Solid sodium chloride dissolves as its ions are surrounded by solvent water molecules. Note how the polar water molecules orient themselves differently around the positive and negative ions. 

The work of adhesion is influenced by the orientation of the molecules at the interface. For example, with the help of Table A.4.1 and Eq. (A.4.8), the work of adhesion of n-decane-water (corresponding to a paraffinic oil-water system) and of glycerol-water can be computed to be 40 10 3 J nr2 and 56x 10 3 J nr2, respectively. It requires more work to separate the polar glycerol molecules (oriented with the OH groups toward the water) from the water phase than the nonpolar hydrocarbon molecules. For paraffinic oils Woo is about 44 mj nr2, for water Www is 144 mj nr2, and for glycerol Woo is 127 mJ nr2. 

As you might expect, the favored orientation of a polar molecule in the presence of ions is one where the positive end of the dipole is near an anion and the negative end of the dipole is near a cation. The magnitude of the interaction energy E depends on the charge on the ion z, on the strength of the dipole as measured by its dipole moment /x, and on the inverse square of the distance r from the ion to the dipole E = z/x/r2. Ion-dipole forces are particularly important in aqueous solutions of ionic substances such as NaCl, in which polar water molecules surround the ions. We ll explore this point in more detail in the next chapter. 

Figure 3.1 Schematic representations of a) a water molecule orientation near a nonpolar CHs-group, which is optimal if none of the hydrogen atoms or electron pairs is directed toward the nonpolar group ( = 0) b) contour line diagrams of three polar molecules with the first inner line of a solvation energy o/O kcal/mol, the second line of 1 kcal, the third line of 2 kcal/mol e/c and c) of the hydrophobic effect. Upon association of hydrophobic particles water or other solvent molecules are released. Entropy grows.

A major weakness of the theory is that (9,29) does not explain the observed difference in behavior of positive and negative ions for water vapor condensation observed by Wilson and other investigators. This difference is usually attributed to the polar character of water molecules. Highly polar water molecules form an oriented surface layer that is probably modified depending on the magnitude and polarity of the ionic charge(s) on the droplet. We also note that as shown by the moleculardynamics calculations, theories based on bulk material properties such as the dielectric constant are likely to break down for very small droplets. 

The energy of attraction between an ion and a wate molecule is due to an ion dipole force. Water molecules are polar, so they tend to orient with respect to nearby ions. In the case of a positive ion (Li, for example), water molecules orient with their oxygen atoms (the negative ends of the molecular dipoles) toward the ion. In the case of a negative ion (for instance, F ), wato molecules orient with their hydrogen atoms (the positive ends of the molecular dipoles) toward the ion (see Figure 12.8). 

In a micelle, the nonpolar tails of soap molecules (or of other molecules that have properties that are similar to soap) are oriented inward (where they can interact with one another), and the ionic heads are oriented outward (where they can interact with the polar water molecules). 

In addition, SFG spectroscopy can be used to indirectly detect ion distributions at charged interfaces using water vibrational signatures. The strength of the SFG response depends on the number of oriented water molecules. At a charged aqueous interface, the electric field at the surface aligns the polar water molecules, which in turn increases the SFG response. This enhancement of the water vibrational signal can be used to indirectly detect the depth of the electric field in the solution, which consequently depends on the ion distribution in the vicinity of the interface. 

---