The Hofmeister effect and the behaviour of water at interfaces

Abstract

Starting from known properties of non-specific salt effects on the surface tension at an air-water interface, we propose the first general, detailed qualitative molecular mechanism for the origins of ion-specific (Hofmeister) effects on the surface potential difference at an air-water interface; this mechanism suggests a simple model for the behaviour of water at all interfaces (including water-solute interfaces), regardless of whether the non-aqueous component is neutral or charged, polar or non-polar. Specifically, water near an isolated interface is conceptually divided into three layers, each layer being I water-molecule thick. We propose that the solute determines the behaviour of the adjacent first interfacial water layer (I1); that the bulk solution determines the behaviour of the third interfacial water layer (I3), and that both I1 and I3 compete for hydrogen-bonding interactions with the intervening water layer (I2), which can be thought of as a transition layer. The model requires that a polar kosmotrope (polar water-structure maker) interact with I1 more strongly than would bulk water in its place; that a chaotrope (water-structure breaker) interact with I1 somewhat less strongly than would bulk water in its place; and that a non-polar kosmotrope (non-polar water-structure maker) interact with I1 much less strongly than would bulk water in its place. We introduce two simple new postulates to describe the behaviour of I1 water molecules in aqueous solution. The first, the 'relative competition' postulate, states that an I1 water molecule, in maximizing its free energy (--delta G), will favour those of its highly directional polar (hydrogen-bonding) interactions with its immediate neighbours for which the maximum pairwise enthalpy of interaction (--delta H) is greatest; that is, it will favour the strongest interactions. We describe such behaviour as 'compliant', since an I1 water molecule will continually adjust its position to maximize these strong interactions. Its behaviour towards its remaining immediate neighbours, with whom it interacts relatively weakly (but still favourably), we describe as 'recalcitrant', since it will be unable to adjust its position to maximize simultaneously these interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of Sodium Salts of Lyotropic Anions on Low-Temperature, Ordered Lipid Monolayers

“…NaCl, NaBr and NaI were baked in an oven at 300 C for 2 hours, while ΝaNO 3 was baked at 250 ºC. NaClO 4 .H 2 O, NaSCN, NaPF 6 and ΝaTPB were not thermally treated prior to solution preparation (they are not very stable upon heating). Salt solutions were prepared using ultrapure water (specific resistance of 18.2 MΩ cm) produced by an Αrium 611UV (Sartorius) reverse osmosis unit.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In the last decade, the mechanism of specific ionic action in several systems has been considerably clarified through intensive research effort in this area (see recent reviews, journal issues, and books on the topic [6][7][8][9][10][11][12][13][14][15][16][17][18] ). It has been established that specific ion action involves a fine balance of interactions between water (the usual solvent), ions, and a solute or a surface, depending on the system.…”

“…As seen in Figure 1, the monolayer is quite ordered at this temperature, and does not exhibit the LE-LC phase coexistence, which is the major feature of isotherms obtained at higher temperatures. We have used NaCl, NaBr, NaNO 3 , NaSCN, NaClO 4 , NaPF 6 , and sodium tetraphenylborate (NaTPB) as electrolytes, to span the greatest possible range of the lyotropic series. Chloride was found to have a very marginal interaction with DPPC monolayers at room temperature, [1][2][3][4] and is in fact found in many phenomena to exhibit "indifferent" behavior and to be at the border between kosmotropic and chaotropic anions.…”

NMR Structures of Salt-Refolded Forms of the 434-Repressor DNA-Binding Domain in 6 M Urea

“…The difference in the refolding of proteins from acidunfolded states or from denaturant-unfolded states is based on an electrostatic or lyotropic stabilization mechanism, respectively (16)(17)(18)(19). In electrostatic stabilization, anions interact specifically with positively charged groups of the protein and thus modify its internal charge distribution.…”

“…Lyotropic effects depend on water-salt interactions that change the water structure, while the protein is assumed to be a passive component. It is especially intriguing to investigate salt refolding of proteins in highly concentrated denaturant solutions, since, as in the case of A-state stabilization, the anion of the salt might largely determine the physical-chemical characteristics of the refolded state (16,18,19).…”