Recent advances in the description of the structure of water, the hydrophobic effect, and the like-dissolves-like rule

Following a critical survey of the vast recent literature. the state of the art may be summarized as follows: (A) Water structure. The key is appreciating the next-nearest neighbour asp ect. Thus, liquid water may be conceived as a fluctuating mixture of broadl y two groups of structure elements: (i) an open ice-I-h-type outer neighbou r O . . .O bonding at about 4.5 Angstrom and (ii) a dense ice-II-type outer neighbour O . . .O bonding at about 3.4 Angstrom. On the other hand, the n earest neighbour O . . .O distance of about 2.8 Angstrom and the number of these neighbors (4) is very similar in the solid and liquid state. The char acterization of the two states may be directed either by the geometry of th e H-bonds (more linear H-bonds in (i) and more bent H-bonds in (ii) or by t he bonding forces operating (H-bonding favours the ordered open state (i), oxygen-oxygen interactions favour the random dense state (ii). Basically, t he nature of liquid water can be understood in terms of a competition betwe en H-bond (Coulomb) and dispersion (van der Waals) forces. Since the bondin g characteristics in crystalline phases carry over to the liquid state, any molecular dynamics (MD) model of the liquid would have first of all to rep roduce well the ice polymorph structures under appropriate thermodynamic co nditions. (B) Hydrophobic effect. The two classic approaches, i.e. the clathrate cage model and the cavity-based model, appear to be just different perspectives on the same physics. The particular features of water are (i) the small mo lecular size or, more specifically, the small size of the space between wat er molecules and the low expansibility, and (ii) the structure of the water molecule with the same number of donor and acceptor sites arranged tetrahe drally. Due to (i), cavity formation is particularly demanding, and this is the main contributor to the hydrophobic effect. This is mitigated by the c apability of water, due to (ii), to form a cage around a nonpolar solute wi thout sacrificing much of the H-bonding; rather, H-bonding networks are sta bilized by the presence of guest molecules. In view of the tangential orien tation of the first-sphere waters, such a cage can be compared with an elas ticated net effecting strong solute-solvent dispersive interactions, render ing the solubility of nonpolar gases exothermic at room temperature. Furthe rmore, cavity formation largely determines the excess entropy, whereas disp ersive forces determine the excess enthalpy. This gives rise to compensatio n behaviour when the solute size varies. Whereas an increase in solute size enhances the cavity formation energy, polarizability is also increased, an d this leads to stronger solute-water interaction. Unfortunately, present m odels of cavity formation predict positional entropies that are far in exce ss of the experimental entropies so that orientational contributions due to cage formation are hard to accommodate. (C) Like-dissolves-like rule. The number of exceptions is dramatically redu ced if the term polarity is given a broader meaning. Instead of identifying it solely with dipolarity, it should also include higher multipolar proper ties, in particular quadrupolarity. Quadrupolar solvent effects on solvatio n and reactivity are receiving increasing attention, particularly in low di electric solvents.