Multi-Pole Approach to Structural Science

May 10 - 13, 2015

Hydration patterns in supramolecular systems
Janusz Lipkowski

Hydration modes in supramolecular systems can be divided into two categories: hydrophilic and hydrophobic. Both types cooperate and, while the former is commonly discussed in structural papers, the latter is less well known. For this reason this part will be discussed here in more detail.

Hydrophobic hydration is the hydration of hydrophobic molecules and surfaces. Hydrophobic hydration (for reviews see [1]) produces a reduction in density and an increase in the heat capacity [2]. The expanded network causes the density decrease whereas the ordered bonds must be bent on increasing the temperature, so affecting the heat capacity. Hydrophobic hydration is accompanied by a negative enthalpy change, due in part to the multiple van der Waals interactions between water and the hydrophobic material, a negative entropy change due to the increased order in the surrounding water and positive heat capacity change (CP) due to the negative enthalpy change (i.e. the stronger hydrogen bonds at the surface). For example, adding CH2 groups to aliphatic alcohols increases the heat produced on solution (ΔH/CH2 = -5.4 kJ mol-1) but causes a greater decrease in the entropy (-TΔS/CH2 = +7.1 kJ mol-1) [this introductory text was taken form the ref. 3].

In its pure form the structure of hydrophobic hydration is observed in clathrate hydrates. In numerous examples a combination of hydrophobic and hydrophilic hydration has been found.

In the present paper a series of x-ray structures will be used to demostrate the characteristic of hydrophobic bonding of organic and metallorganic species. These include macrocyclic crown polyethers and diaza-crowns, cyclodextrins, cucurbiturils and selected metal complexes of the compounds listed above. The hydrophilic counterpart will be illustrated using x-ray structures of selected strongly ionic compounds.

A variety of interesting topologies were found and hydrogen bonding geometries characterized using statistics of bond lengths and angles.

Some dynamic aspects of selected structures will be illustrated with the use of temperature dependence of lattice parameters of highly hydrated structures (CD complexes). Open, zeolite-like structures, demonstrate fast solvent exchange in the systems, what can be observed microscopically as cracking of crystalline samples. This phenomenon will be displayed with the use of short microscope movies.

[1] (a) V. V. Yaminsky and E. A. Vogler, Hydrophobic hydration, Curr. Opin. Colloid Interface Sci. 6 (2001) 342-349; (b) B. Widom, P. Bhimalapuram and K. Koga, The hydrophobic effect, Phys. Chem. Chem. Phys. 5 (2003) 3085-3093 (G. Graziano, Comment on "The hydrophobic effect" by B. Widom, P. Bhimalapuram and K. Koga, Phys. Chem. Chem. Phys., 2003, 5, 3085, Phys. Chem. Chem. Phys. 6 (2004) 4527-4528.) (c) D. Chandler, Interfaces and the driving force of hydrophobic assembly, Nature 437 (2005) 640-647.
[2] V. Gutmann, Fundamental considerations about liquid water, Pure Appl. Chem. 63 (1991) 1715-1724
[3] (Martin Chaplin, “Water Structure and Science”,