Gudrdate delagua mansa,the Spanish translation of the known proverb,beware of still waters,is atheaterpiece of the great Golden Age Spanish writer Pedro Calderon dela Barcabased on astory ofloveand honor.Thistitleisperfectly suitedto any work devoted to the understanding of water, this vital,essential,fascinating,surprising,and eccentric small molecule that continues to preserve an aura of mystery around it,as if to remind us that without it, life(as we understand it) is not possible. The water molecule can be considered as a very simple one,given that it is constituted by only one oxygen andtwo custodian hydrogen atomswith ashape that recalls avery popular cartoon character,but we have to be aware when watermoleculescondense intoliquid and solid states through hydrogen bonding because this simplicity is only apparent. Below such an apparent simplicity a complex and mysterious creature is hidden.It is a sort of complex simplicity fully justifying the term,Guardate delagua mansa.We could find a parallelism between water and Fermat's last theorem, where no integer solution exists for the equation x"+y"= z",forn>2.It appears to be a rather simple problem but it took more than 300 years to solve it!Water is perhaps the only compound having a biography(P.Ball,Life's Marix: A Biography of Water,University of California Press,2001)and many books,articles,reviews,and so on,have been and continue to be published focused on diverse aspects.I strongly recommend M. Chaplin's and US Geological Survey webpages http://www.Isbu.ac.uk/water/and http://www.usgs.gov/,respectively.However,anin- troductory book oninterfacial watertrying toreachabroadmultidisciplinary audience was missing in my view,and this was the pivotal motivation,orexcuse,that drove me to conceive the work you are now reading. Among the myriad published works devoted to water and interfacial water, only a partial selection of thememerge in this book,as the tip of an iceberg.The rest, those below the surface,indeed contribute to the general knowledge,but this book is not intended as a review of all published material.That would have little sense for a book because of its intrinsic static nature in printed version in a quite dynamic and active subject.I just mention that more than 1.5×10°articles have been published on water since 1900,according to online bibliographic research platforms,and that about 4 and 18% of them contain the terms"interface"and"surface", respectively. Water at interfaces is thus a visible part of the water iceberg (only about 10% of the volume of an iceberg emerges).This book is conceived to go from basic simplified concepts toward more complex issues, increasing the degree of complexity. We start in Chapter I with isolated water molecules, because it is fundamental to know its properties in the absence of intermolecular interactions. Then we allow them to interact through van der Waals interactions, then through hydrogen bonding to build liquids and solids,and we sub- merge in Chapter 2 into the(pristine)interfaces built by water in a broad sense, including flat liquid/vapor,liquid/oil,and liquid/solid interfaces.Chapters3 and5are devoted to interfaces involving both ideal and real surfaces,respectively,and Chap- ter4 discusses the affinity of water to surfaces (hydrophobicity and hydrophilicity). Finally, Chapter6 deals with the interaction of water with biomolecules.Interfaces are considered in a broad sense and at different length scales: the zero-dimensional case of water sequestering small molecules (clathrate hydrates), water surrounding macromolecules,liquid water wetting (or not) extended flat surfaces,and so on.Finally,Iwouldliketoacknowledgemanypeople whohave participated todifferent extent during the preparation of this book,but to the tip of the iceberg I would first invite E. Canadell and C. Rovira for extended Hückel and DFT calculations, M. M. Conde and C.Vega for MD simulations，M.V. Feråndez-Serra for discussions on DFT calculations，and T.Hernåndez and A. Santos for help on documentation. I would skip my family,my wife Montse and my sons Roger and Marc,from the uncomfortable,cold,and slippery honorary iceberg tip and reserve for them a more hospitable place for their warm permanent support.

Preface ix

Abbreviations and Symbols xi

Chapter 1 An Introduction to Water 1

1.1 Where Does Water Come From? 2

1.2 Molecular Structure of Isolated Water Molecules 4

      1.2.1A Nonlinear Polar Molecule 4

      1.2.2 Electronic Structure 6

      1.2.3Vibrational Structure 13

1.3 Hydrogen Bonding Off: van der Waals Interactions 17

1.4 Hydrogen Bonding On: Condensed Water 21

      1.4.1Water Clusters 21

      1.4.2 Solid Water 26

      1.4.3 Liquid Water 39

      1.4.4 Computer Water 43

1.5 Elixir of Life 48

1.6 Summary 50

apter 2 Interfaces of Condensed Pure Water 59

2.1 Liquid Water 59

      2.1.1Vapor/Liquid Interfaces 60

      2.1.2 Water/Oil Interfaces 69

2.2 Solid Water 72

      2.2.1Solid/Vacuum Interface 72

      2.2.2 Solid/Liquid/Vapor Interfaces 75

2.3 Summary 82

Chapter 3 Water on Ideal Solid Surfaces 87

3.1 Single Water Molecules and Clusters 87

      3.1.1 Single Water Moleculer on Metallic Surfaces 87

      3.1.2 Small Clusters on Surfaces 96

3.2 Substrate-Induced Structuring of Mono-and Bilayers 99

      3.2.1Interfacial Registry 99

      3.2.2 Inorganic Surfaces with Hexagonal Symmetry 100

      3.2.3 Surfaces with Non-Hexagonal Symmetry 117

3.3 Substrate-Induced Structuring of Water Multilayers 123

      3.3.1Unpolarized Substrates 123

      3.3.2 Polarized Substrates: Electrofreezing 124

3.4 Confined Water 126

3.4.1 2D Confinement 126

3.4.2 1D Confinement 131

3.4.3 Electrochemical Nanopatterning 134

3.5 When Ions Come on the Scene 135

      3.5.1 lon Hydration 135

      3.5.2 Electrical Double Layer 141

      3.5.3 Dissolution of an Ionic Surface 143

3.6 Summary 145

Chapter 4 Hydrophobicity and Hydrophilicity 155

4.1 Wetting and Contact Angle 55

4.2 Hydrophobicity at Different Length Scales 156

      4.2.1Clathrate Hydrates 157

      4.2.2 Extended Surfaces 158

      4.2.3 Superhydrophobicity 163

4.3 Amphiphilicity 170

      4.3.1Janus Particles 170

      4.3.2 Janus Surfaces 71

      4.3.3 Tunable Wetting 176

4.4 The Role of Dipoles 179

4.5 Summary 182

Chapter 5 Water on Real Solid Surfaces 189

5.1 Water Purification 89

5.2 Water Splitting 194

5.3 Atmospheric Agents 196

5.4 Capillary Adhesion 200

5.5 Water Desorption in Vacum Systems 202

5.6 Summary 204

Chapter 6 Water/Biomolecule Interfaces 209

6.1 Is Water a Biomolecule? 209

6.2 Proteins 211

      6.2.1Water-Peptide Interactions 211

      6.2.2 Hydrophobic Forces and Hydration 213

      6.2.3Antifreeze and Ice Nucleating Proteins 217

6.3 Nucleic Acids and DNA 220

6.4 Biological Membranes 222

      6.4.1Water/Phospholipid Interfaces 222

      6.4.2Water Channels: Aquaporins 225

6.5 Summary 226

Appendix A Buoyancy and Surface Tension 231

Appendix B Capillary Forces 233

Appendix C Marangoni-Bénard Patterns 235

Index 237

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