Hofineister series

An anion s lipophilicity plays an important role when the anion-selective electrodes involving ionophores with no functionalities are applied to the recognition of anions. The selectivity order for this recognition is known as the Hofineister series ... 

Therefore, to prepare a selective anion sensor for less lipophilic anions in the series, the selective interactions between the ionophores and the targeted anion should be considered. Truly selective nitrate electrodes were developed by the electrochemical deposition of PPy onto glassy carbon electrodes in the presence of NaNOa [131]. As seen in Figure 8.79, the selectivity pattern of PPy(N03 ) electrodes exhibited a significant deviation firom the Hofineister series. The electrodes showed selectivity especially for nitrate and thiocyanate, while the commercially available nitrate-selective electrodes were affected by the more lipophilic anions including iodide and perchlorate. 

Piima MC, Sails A, Monduzzi M, Ninham BW (2005) Hofineister series The hydrolytic activity of Aspergillus niger lipase depends on specific anion effects. J Phys Chem B 109 5406... 

Bostrom M, Tavares FW, Finet S, Skouri-Panet F, Tardieu A, Ninham BW (2005) Why forces between proteins follow different Hofineister series for pH above and below pi. Biophys Chem 117 217-224... 

Zhang Y, Cremer PS (2009) The inverse and direct Hofineister series for lysozyme. Proc Natl Acad Sci 106 15249-15253. doi 10.1073/pnas.0907616106... 

Collins KD (2004) Ions from the Hofineister series and osmolytes effects on proteins in solution and in the crystallization process. Methods 34 3(X)-311... 

Vlachy N, Jagoda-Cwiklik B, Vacha R, Touraud D, Jungwirth P, Kunz W (2009) Hofineister series and specific interaction of charged headgroups with aqueous ions. Adv Colloid Surf Sci 146 42-47... 

The selectivity of membrane-based ISEs is somewhat controlled by the relative lipophilicity of the ions. In particular, the response of ISEs based on anion-exchangers, such as tridodecylmethylammonium chloride (TDMAC), that simply form ion pairs with anions follows the Hofineister series or classical response lipophilic organic anions > CIO > SCN > T > N03" > Br > NOj > Cf > HC03 > HPO/, SO/ . Analogously, ISEs based on lipophilic borates, such as potassium tetrakis(p-chlorophenyl)borate (KTpClPB), which forms ion pairs with cations, respond to cations in the following order Ag > Hg > Cs > Rb, NH/ > > Lf... 

Cation properties that could be relevant to their ordering in the Hofineister series, arbitrarily arranged according to their surface charge densities, zJ Tur, are shown in Table 8.13. The reader confronted with a cation Hofmeister series for some biophysical phenomenon may attempt to correlate it with the listed cation properties. 

In Hofmeister s papers, only an ordering of salts and not of isolated ions is given. Such a different ordering, usually known as Hofineister series , appeared only many years later. 

Fig. 1. A typical ordering of cations and anions in Hofineister series.

However, when looking in more details into the series, some unexplained inconsistencies appear lithium has an unexpectedly small salting-out effect and perchloric acid is even a salting-in species. Nonetheless, in total roughly a Hofineister series is found. 

Of course, this is a somewhat simplified picmre. In real systems these effects are always coupled. Nevertheless, it gives a clear hint of the molecular reasons for the Hofineister series and elucidates that more than one effect must be taken into account to explain such series. 

Aroti A, Leontidis E, Dubois M, Zemb T. (2007) Effects ofmonovalent anions of the Hofineister series on DPPC lipid bilayers. Part 1. Osmotic Stress experiments and in-pane equation of state. Biophys/93 1580-1590. 

Leontidis E, Aroti A, Belloni L. (2008) DPPC liquid expanded monolayers as model systems to understand the anionic Hofineister series 1. A tale of models. JPhys Chem B 113 1447-1459. 

Ion specific effects generally follow direct or reverse order of the so-called Hofineister series, which for monovalent anions is, SCN > CIO4 1 > Br- > Cl- > F. In many cases, interfacial effects appear to play a key role. In the simplest case of ions at the air-water interface for example, the specificity is directly reflected in the stability of foams or in the surface tension and surface potential of aqueous salt solutions. In biology, beyond protein crystallisation, the enzymatic activity can be controlled by addition of alkali halides with large differences between fluoride, chloride, bromide, or iodide. Specific types of cations are also identified together with... 

Cacace M, Landau E, Ramsden J. (1997) The Hofineister series Salt and solvent effects on interfadal phenomena. Quarterly Reviews of Biophysics 30 241-277. Hall D, Darke P. (1995) Activation of the herpes simplex virus type 1 protease. JBiol Chem 270 22697-22700. 

Nevertheless, some problems remained. Although MD simulations very roughly predicted the right Hofineister series of surface tensions, such simulation techniques are heavy to carry out. Further, the simulation box is small, leading to possible artefacts in the calculation of the Gibbs integral. Due to the statistical noise, thermodynamic properties cannot... 

In this respect the approach by Ninham and Yaminsky is much easier to use. In principle the influence of solvent structure can be taken into account within the DLVO model by using a convenient Lifshitz-like ansatz. There, all non-electrostatic interactions are taken into account via frequency summations over all electromagnetic interactions that take place in the solutions. If done rigorously, the result should be more or less exact. As a proof of principle, Bostrom and Ninham made a first attempt in this direction. The classical DLVO ansatz was replaced by a modified Poisson-Boitzmann (PB) equation, in which a simplified so-called dispersion term was added to the electrostatic interaction. In this way ion specificity came in quite naturally via the polarisability and the ionisation potential of the ions. However, it turned out that this first-order approximation of the non-electrostatic interactions was not sufficient to predict the Hofineister series of surface tension. Heavier ions such as iodide had to be supposed to have smaller polarisabilities compared to smaller ions such as chloride. Although the exact polarisabilities of ions in water are still under debate, this is not physical. 

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