IntroductionWater is the most abundant molecule on the earth, covering 70% of its surface. Life processes on this planet are crucially dependent on the presence of water. It is often recognized that water has tremendous benefits as a green extraction solvent because it is not only inexpensive and environmentally benign but it is also nonflammable and nontoxic, providing opportunities for clean processing and pollution prevention. The water molecule contains three nuclei, one modestly heavy (oxygen), and two light ones (hydrogen). Because of water's electronic structure, the oxygen atom has a slight negative charge on it and the hydrogen atoms have a slightly positive charge [1]. When water molecules are closed together, their positive and negative regions are attracted, these attractive forces are known as hydrogen bonds. The energy associated with a hydrogen bond is 8-40 kJ/mol. The O-H bond lengths are 0.9572 Å and the average HOH bond angle is 104.52 ∘ , slightly smaller than the tetrahedral angle (109.5 ∘ ). The molecule is very small with a hard sphere diameter of 2.75 Å. The smallness of the water molecule has important consequences for the hydration of solutes. The contours of the total electron density in the HOH plane of the water molecule indicate a shape not far from spherical. The water molecule has a dipole moment of 1.85 D with two partial positive charges on the hydrogen atoms and a single zone of negative charge on the oxygen atom.It is well known that water molecules are strongly associated in the liquid phase by H-bonding. Hydrogen bonds result dominantly from electrostatic interactions with a light contribution of covalent bond formation. The H-bond interactions in liquid water constitute a 3-D H-bond network with localized and structured clustering. Therefore, the liquid structure is not determined by hard-core repulsions between the individual water molecules but rather by intermolecular and directional H-bonding. The broad variation of H-bond angles further demonstrates the noncrystalline arrangement of the water molecules and appears to indicate that some bond angle distortion in the H-bonds is rather easily allowed in the liquid. It is important to recognize that intermolecular interaction Green Extraction of Natural Products: Theory and Practice, First Edition. Edited by Farid Chemat and Jochen Strube.