Part II Ions, Hydration, and Transport

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1 Part II Ions, Hydration, and Transport Aqueous solutions containing ions are ubiquitous in the world around us and their properties and behavior influence a variety of processes. In biology, regulation of ionic transport and ph is critical for the functioning of biological cells and tissues and for the transmission of nerve impulses. This regulation is accomplished by proteins called ion channels that transport or pump specific ions in and out of the cell. In geology, dissolution and deposition of minerals and their precipitation can sculpt rocks, shape landscapes, erode mountains, and generate intriguing features such as stalagmites and stalactites. The stability of colloidal solutions is influenced by ions in the solution into which they are dispersed; addition of alum that dissociates into multivalent ions is widely used to clean water by flocculation of colloidal impurities. The manner in which certain materials interact with ions is also critical for realization of technologies such as seawater desalination by reverse osmosis using semi-permeable membranes. While ions in aqueous solutions play such a critical role in natural as well as artificial systems, only 100 years ago the nature of ionic solutions was largely unknown. The term ion was first coined by Michael Faraday in 1834, who observed that something (unknown to him at the time) carried electrical current through a solution in an electrolytic cell. Ion derives from Greek šo which means to go (from one electrode to another) [1]. The first major advance in understanding of the nature of ionic solutions was made by Svante Arrhenius [2]. In his doctoral thesis submitted in 1884, Arrhenius pointed out that although neither salts nor pure water are very good conductors of electricity, solutions of salts in water are. He was therefore the first person to posit that electrolytes separate into ions when in solution. This idea failed to impress his professors and earned him a low grade in his thesis, but the same thesis later won him the Nobel Prize in Chemistry in 1903! Rapid developments in the understanding of the nature and behavior of ionic solutions occurred in the first decade of the twentieth century, when Nernst and others probed the question as to whether ions are associated with water molecules, i.e. whether ions are hydrated [3]. The experiments carried out to probe ion hydration seem surprisingly crude in today s days of advanced instrumentation: The basic hypothesis was that if water molecules were associated with ions, then water molecules will move

2 78 II Ions, Hydration, and Transport along with ions and get concentrated at the electrodes in an electrolysis cell! The concentration of water at electrode surfaces (i.e. depletion of a dissolved nonelectrolyte) suggested that indeed ions carry water molecules. As more evidence of the nature of electrolytes and hydration of ions was emerging, Born s theory of ion solvation published in 1920 was successful in qualitatively explaining the energies of dissolution of different salts; the theory explained how the dielectric shielding of charges allows ions to be dissolved in water [4]. A major conceptual advance occurred in the same year, when Latimer and Rodebush published a paper that pointed out the existence of the hydrogen bond [5]. They wrote :::a free pair of electrons on one water molecule might be able to exert sufficient force on a hydrogen held by a pair of electrons on another water molecule to bind the two molecules together ::: Such combination need not be limited to the formation of double or triple molecules. Indeed the liquid may be made up of large aggregates of molecules, continually breaking up and reforming under the influence of thermal agitation. It is now well-appreciated how hydrogen bonding in water plays a critical role in governing the structure and behavior of water and biomolecules, but it was only half a century later that sufficient progress was made to fully appreciate Latimer s conjecture. Three years after Latimer and Born s papers, Debye and Hückel proposed their famous theory that accounted for electrostatic interactions between ions in solution, which explained the deviation of electrolyte behavior from ideality [6]. In parallel, the nature of the interface between water and electrolyte solutions was also being investigated. Louis Guoy in 1910 and David Chapman in 1913 had developed the diffuse electrical double layer model, which described the distribution of ions in solution next to a charged solid surface, forming a double layer of charges (one in solution and the other on the solid surface). In 1924, Otto Stern proposed a new model for the electric double layer, which comprised a layer of surface-adsorbed ions first proposed by Helmholtz in addition to the diffuse double layer concepts developed by Guoy and Chapman. Thus, we can say that the foundations of today s understanding of the structure of water and ionic solutions, as well as the field of electrokinetics, were laid in the first quarter of the twentieth century. Later significant developments in understanding of the nature of ionic solutions and their behavior at interfaces included the DLVO theory of colloidal stability in the 1940s, which explained how the balance between van der Waals attraction and electrostatic repulsion determined the stability of colloids. Even with these developments, much debate remained about the structure of water and hydration of ions well into the 1970s [7]. During this period several theories of the structure of water were proposed, with debates in the early years ranging from whether water molecule was (H 2 O), (H 2 O) 2, (H 2 O) 3 or a mixture of these forms [8], to the polywater controversy in the 1960s. By the 1970s, new tools such as NMR, vibrational spectroscopy, and surface force apparatus, as well as computer simulation techniques were being employed to study the structure of water and ionic solutions. The realization occurred that the structure of water was dynamic, and it made sense to talk about correlations between the relative positions of water molecules or atoms with respect to an ion or water molecule ( radial distribution

3 II Ions, Hydration, and Transport 79 function ), rather than talk about a fixed number of water molecules bound to an ion [9]. The same decade saw the first measurements of ionic currents through single ion channels in cell membranes by Sakmann and Neher [10], (which won them the Nobel Prize in 1991), and direct measurements of forces between surfaces that quantified the effects of van der Waals, electrostatic, and hydration [11]. The 1990s saw the discovery of the aquaporin water transporter channels by Peter Agre, which also led to a Nobel Prize in 2003 shared with MacKinnon who elucidated the mechanism of potassium ion channels. Electron microscopy and crystallography helped shed light on the structure and function of ion and water channels in cell membranes, with the interesting mechanism of selective water transport in aquaporin being revealed only in 2001 [12]. These historical developments have led to our current understanding of ions and hydration, in which two important length scales that affect ionic transport have emerged [13]. The first length scale concerns the size of the hydrated ions, most effectively described using the radial distribution function of water molecules around the ion. This length scale is on the order of 1 nm. The second important length scale concerns the range of electrostatic interactions in solution. This length scale is characterized by the Debye length, which depends on the ionic strength of the solution and ranges from about 1 m in deionized water to less than 1 nm in concentrated salt solutions. Transport of ions and water in confined systems changes dramatically as either of these two length scales is approached. For example, if channel geometries approach the nanometer length scale, hydration effects can significantly alter transport properties. Such systems include carbon nanotubes, small nanochannels or nanopores, and porous materials such as clays, zeolites, graphene, and polymers that have nanometer-scale conduits or spaces. In contrast to hydration effects, electrostatic and electrokinetic effects can play a role in larger channels or conduits approaching the millimeter length scale. The layer of ions next to a charged surface moves in response to electric fields, leading to electroosmotic flows and other electrokinetic effects in microchannels and porous media. In smaller channels, the surface charge can govern ionic transport and co-ions are excluded [13]. Transport of ions and water in confined systems is thus intimately connected to electrokinetic and hydration effects that occur at small length scales. In perspective, it is only in the last two to three decades that sufficient computational resources and advanced experimental techniques have been employed for understanding the structure and interactions of ions and water. On the experimental side, characterization, control, and interrogation of interfacial phenomena are difficult. Going beyond the continuum double-layer theories to account for the behavior of ionic solutions at interfaces involves complexities that often necessitate simulations. At the same time, the first decade of the twenty-first century has seen a burst of research on nanoscale and microscale systems that often operate in aqueous solutions containing ions where interfaces play a major role. As a result, tools and techniques for controlling the structure of matter at the nanoscale have become ubiquitous. Research has also shifted much of its focus from basic to applied, with interdisciplinary methods and collaborations becoming the norm rather than exceptions. Examples of problems being tackled in this new era range from how to

4 80 II Ions, Hydration, and Transport design a drug molecule to bind a target protein in physiological solution, to flow of water in carbon nanotubes for filtration, to control of surfaces of biosensors. The molecular structures and behaviors of these systems are varied, and there is no simple analytical theory that enables quantitative prediction of their behavior. Simulations have become important in advancing our understanding of the behavior of ionic solutions and interfaces, complementing and helping the interpretation of experimental results. The following chapters in this section describe various studies concerning the behavior of ions and water molecules in systems ranging from nanoscale devices to geological length scales, where confinement at the length scales of electrostatic or hydration effects dominates the observed transport behavior. The chapter by Duan describes deviations of ionic mobilities from bulk values observed in 2- nm slit-like nanochannels. This behavior is attributed to the fact that the channel height is comparable to the hydration length scales, possibly affecting the structure of water and ionic hydration. The chapter by Karnik outlines transport of ions through nanoscale defects in graphene, a recently discovered and much-studied 2-dimensional material that holds promise for applications in electronics, energy storage, membranes, and other areas. The chapter by Kalinichev describes advances in computational modeling of the structure and dynamics of water at crystal surfaces. The final chapter in this section by Shilov provides an analytical treatment of the siesmoelectric effect, where vibrations in the earth result in corresponding electric potential oscillations due to the interactions between the charged surfaces in the soil or rock and the ionic solutions contained therein. These chapters illustrate the diverse range of ionic and interfacial phenomena in confined systems being investigated in the twenty-first century using tools, resources, and conceptual frameworks that were probably unimaginable just a century ago. Rohit Karnik References 1. Online Etymology Dictionary (2013) 2. Arrhenius G, Caldwell K, Wold S (2008) A tribute to the memory of Svante Arrhenius ( ) a scientist ahead of his time. %B6gtidssammankomst/Minnesskrift% pdf 3. Washburn EW (1909) The hydration of ions determined by transference experiments in the presence of a non-electrolyte. J Am Chem Soc 31: Born M (1920) Volumes and hydration warmth of ions. Z Phys 1: Latimer WM, Rodebush WH (1920) Polarity and ionization from the standpoint of the Lewis theory of valence. J Am Chem Soc 42: Debye P, Huckel E (1923) The theory of electrolytes I. The lowering of the freezing point and related occurrences. Phys Z 24: Hinton JF, Amis ES (1971) Solvation numbers of ions. Chem Rev 71: Chadwell HM (1927) The molecular structure of water. Chem Rev 4: Enderby JE (1995) Ion solvation via neutron-scattering. Chem Soc Rev 24:

5 II Ions, Hydration, and Transport Neher E, Sakmann B (1976) Single-channel currents recorded from membrane of denervated frog muscle-fibers. Nature 260: Israelachvili JN, Adams GE (1978) Measurement of forces between two mica surfaces in aqueous electrolyte solutions in the range nm. J Chem Soc Faraday Trans 1 74: de Groot BL, Grubmuller H (2001) Water permeation across biological membranes: mechanism and dynamics of aquaporin-1 and GlpF. Science 294: Sparreboom W, van den Berg A, Eijkel JC (2009) Principles and applications of nanofluidic transport. Nat Nanotechnol 4: