propylene carbonate mhc lmc vbc
2.13 an overall view
The sie trends across a range of experiments for a range of solvents are summarised in Table 13. This table also includes the data for solvents that have not been discussed in the previous sections because of the little amount of data available: proh,eg, meno2,aceand mecn.
Whilst in some cases the data is limited, the survey reveals some inter- esting information. The sie observed are different for different solvents. It would be desirable to relate the sie to the properties of the solvent, how- ever at this stage the complexity and extensive variation seen insieacross a range of solvents and a range of experiments does not allow for simple gen- eralisations. Additionally, in many experiments the confidence with which the siecan be labelled as a Hofmeister series or a lyotropic series is dimin- ished due to the limited availability of data.
The lmc and vbc experiments are particularly interesting as the data is the most extensive across the range of solvents for these experiments. In comparing the trends observed in the non-aqueous solvents to those in water, it is apparent that the ammonium cations should be treated separately to the alkali metal cations, as in no case did the overall ordering of the ions coincide between water and a non-aqueous solvent, but in many cases the ordering did coincide when the classes were treated separately. Considering the lmc of cations, in general the ammonium class of cations follows the Hofmeister series and the alkali metal cations follow the lyotropic series. It should be highlighted here that the lyotropic numbers are not available for the ammonium class of cations, therefore the ordering of these ions can only be identified as either ‘Hofmeister series’ or ‘other’. For the anions, a reverse Hofmeister series is observed when a series is evident, but not in all cases. It is expected that the lmc will be strongly influenced by the size and solvation of an ion and for this reason the tetraalkylammonium cations might be expected to behave differently to other ions because of their size (i.e. the increase in size due to the addition of methyl groups in the four lateral chains is not compensated by the change in solvation or other properties of the ion, as discussed in Section 2.3). As with the lmc experiments, thevbcexperiments show agreements with both the sub-series of cations. The vbc experiments reveal that the cations follow the same trends as in water for both of the series considered separately foretoh,dmf (two ions only),dmso,pc(two ions) andec. However, for three solvent, the vbc ordering of the ammonium cations agrees with that in water, whereas the ordering of the alkali metal cations does not: inmeoh andfa, the alkali metal cations follow a different trend (other than a lyotropic or Hofmeister series), and for nmf, the alkali metal cations series is reversed compared
2.13 an overall view 31
Table 13: Summary of thesie series seen for a range of experiments in water and non-aqueous solvents. Protic solvents are in bold, aprotic solvents are it- alicised. Series with three or less ions have been indicated in parentheses.
cations
solvent mrt rdd mhc lmc vbc
R4N+† M+‡ R4N+ M+ R4N+ M+
water R-HS other R-HS R-lyo HS lyo R-HS R-lyo
meoh R-HS (3) other R-HS HS HS lyo R-HS (3) other
etoh / / R-HS HS HS lyo R-HS R-lyo
fa R-lyo other§ / / HS lyo R-HS (3) other
nmf R-lyo other§ / HS (3) HS lyo R-HS lyo
dmf / / ? (2) R-HS HS HS lyo R-HS / (2)
dmso R-lyo / / HS HS (3) lyo R-HS R-lyo
pc / / R-HS HS HS lyo R-HS / (2) ec / / / / / / / R-lyo proh / / R-HS / (2) HS lyo / / eg R-lyo / / / HS lyo / / meno2 / / / R-lyo HS lyo / / ace / / / / HS lyo R-HS / (2)
mecn / / ? (2) R-HS other HS lyo R-HS other
anions
solvent mrt rdd mhc lmc vbc
water HS R-HS other other other
meoh HS (3) HS R-HS R-HS HS
etoh / / R-HS R-HS / (2)
fa HS (3) / / other HS
nmf HS (3) / R-HS (3) other HS (3)
dmf / / other other /
dmso / (2) / other other other
pc / / other R-HS HS (3)
ec / / / / HS (3)
proh / / R-HS R-HS /
eg HS (3) / / HS /
meno2 / / other R-HS /
ace / / / other other
mecn / / HS R-HS HS
†ammonium cations, where R=H, (CH3), (C2H5), (C3H7) or (C4H9).
‡alkali metals.
to that in water. That is, in nmf the forward lyotropic series is observed, whereas the reverselyotropic series is observed in water. Nmf andfa also show reversed sie series with respect to water in the rddexperiment. The cause of series reversal in water has attracted some interest and has been related to salt concentration and surface charge (Parsons, Boström, Maceina et al., 2010). This investigation reveals that the solvent itself can also cause series reversal. It remains an interesting and important challenge to explain these phenomena.
A similar situation exists for the mhc data: the ammonium cations con- sistently follow a reverse Hofmeister ordering across solvents, but the al- kali metal cations show a lyotropic trend only in water and meno2, and by
contrast the rest of the non-aqueous solvents follows a forward Hofmeister series.
For anions, the ordering observed in water is seldom reflected by the non- aqueous solvents. It is very interesting that both in the lmc and vbc case, a wider agreement of series is seen for cations rather than anions. This highlights that, with respect to those properties, the solvents behave simil- arly with cations and less so with anions. One might expect the opposite as stronger sieare usually observed for anions.
2.14 summary
There is evidence of the Hofmeister series (forward or reverse) for the anions in all solvents except dmf, dmso, meno2 and ace and for the cations in
all solvents apart from ec. Though it is notable that the Hofmeister series is not uniformly observed for a particular experiment across the solvents and no particular effect of the solvent being protic is evident. Notably, in meoh the anions follow the Hofmeister series in the mhc, lmc and vbc experiments even though this is not the case for these experiments in water. Also the other non-aqueous solvents can show a Hofmeister series in one of these experiments, where it has not been observed in water. Whereas for the cations, Hofmeister or lyotropic series are evident in a number of exper- iments with good consistency across solvents. Clearly the situation is very complex, and explanations for this situation are difficult to find. However, recognition that these series exist in non-aqueous solvents brings into focus the depth of complexity of sieevident when a wider range of solvents are observed and highlights that there is a substantial degree of similarity insie present across a range of solvents, albeit with some outstanding differences. The available evidence indicates that all solvents on occasion will exhibit the Hofmeister series and the lyotropic series and no particular property of a solvent can be correlated to the Hofmeister effect or the lyotropic series.
2.14 summary 33
Therefore, we can conclude that neither series can be attributed to the prop- erties of a solvent. The significant challenge is then to determine why par- ticular experiments reveal the Hofmeister series or lyotropic series in one solvent and not another solvent. Resolution of this may well require a deep understanding of the full range of sie. This question is addressed in the following chapters.
It also emerges that, in order to achieve a comprehensive picture of this phenomenon in non-aqueous solvents, many more experiments must be per- formed and theoretical investigations pursued. This is no small task and requires a renewed interest in the fundamental and systematic investigation of the properties of non-aqueous electrolyte solutions. It is desirable that the- orists engaged in tackling the complexity of sieextend their considerations to include non-aqueous solvents. I argue that explanations for siein aque- ous systems can be rigorously tested by assessing their utility in explaining sieobserved in other solvents.