How Much Weaker Are the Effects of Cations than Those of Anions

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Article

How Much Weaker are the Effects of Cations than Anions? : The Effects of K and Cs on the Molecular Organization of Liquid HO +

+

2

Takeshi Morita, Peter Westh, Keiko Nishikawa, and Yoshikata Koga J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp504245c • Publication Date (Web): 07 Jul 2014 Downloaded from http://pubs.acs.org on July 9, 2014

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The Journal of Physical Chemistry

How Much Weaker are the Effects of Cations than Anions? : The Effects of K+ and Cs+ on the Molecular Organization of Liquid H2O

Takeshi Morita,2 Peter Westh,1 Keiko Nishikawa,2 Yoshikata Koga3* 1

NSM Research for Functional Biomaterials, Roskilde University, Roskilde, DK4000, Denmark,

2

Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan

3

Department of Chemistry, The University of British Columbia, Vancouver, V6T 1Z1, and

Suitekijuku, Vancouver V6R 2P5, Canada.

*Corresponding author: [email protected] Tel; 1-604-822-3491, and fax; 1-604-822-2847

1 ACS Paragon Plus Environment


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Abstract We characterized the effects of K+ and Cs+ ions on the molecular organization of H2O by the 1-propanol probing methodology, developed by us earlier (PCCP, 2013, 15, 14548). The results indicated that both ions belong to the class of “hydration center”, which is hydrated by 4.6 ± 0.8 and 1.1 ± 0.5 H2O molecules respectively and leaves the bulk H2O away from hydration shells unperturbed. Together with our earlier results for the total of 7 cations and 11 anions, we display resulting characterization on a two dimensional map and show a quantitative difference in their strength of the effects on H2O between anions and cations.

Keywords: •

1-Propanol (1P) probing, a thermodynamic probing methodology.

•

The effect of an ion on the molecular organization of liquid H2O.

•

Both K+ and Cs+ are “hydration centers” leaving bulk H2O unperturbed.

•

Hydration numbers are 1.1 ± 0.5 for Cs+ and 4.6 ± 0.8 for K+.

•

Quantitative difference in their effects between cations and anions.

•

Qualitative distinction between “kosmotropes” and “chaotropes”.


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Introduction

In general, the cations’ effects on the molecular organization of liquid H2O have been recognized to be weaker than those of anions. Indeed, the Hofmeister rankings of anions are known to be stable from application to application,1-3 while those for cations vary case by case, and at times the ranking reverses completely.4,5 In this study, we first characterize the effect of K+ and Cs+ on H2O by the so-called 1-propanol probing methodology developed by us earlier.6-8 The methodology can characterize the effect of a solute including an individual ion on liquid H2O by two coordinates; hydrophobicity and hydrophilicity. It is based on our observation that within the H2O-rich region where the integrity of liquid H2O is retained, the effects of two solutes turned out to be additive. Thus, we were able to display the solute’s effect on H2O in two dimensional map, as reviewed elsewhere.6-8 We call the most recent one as paper A.6 Briefly, we determine the excess partial molar enthalpy of the probing 1-propanol (1P), H 1EP , in the 0

mixed solvent of H2O and a chosen test sample S, the initial mole fraction of which is xS . We then graphically differentiate the resulting H 1EP data without resorting to a fitting function. We thus obtain what we call the 1P – 1P enthalpic interaction, H 1EP1P , which shows a peak type anomaly reflecting the fact that 1P is a typical hydrophobe. The mole fraction of the peak top is where the mixing scheme starts to change from what is operative in the limited H2O-rich region. In this limited composition, the integrity of liquid H2O, that the hydrogen bonds are fleetingly forming and breaking and yet the hydrogen bond network is bond-percolated, is retained in spite of the presence of 1P. On addition of S within this limited composition, its ‫ݔ‬ଵP -dependence pattern is modified keeping the peak type anomaly intact. Namely, the peak top shifts within the graph, which can be decomposed into the two diagonal components. Their rates per a unit



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0

0

increase in xS are used as two indices, hydrophobicity, a, and hydrophilicity, b. When xS

becomes too large and the hydrogen bond percolated network is destroyed, the ‫ݔ‬ଵP -dependence pattern of H 1EP1P does not show a peak type any longer.6-8

H 1EP1P is defined as,

H 1EP1P =N(∂ H 1EP /∂n1P ) = (1 – ‫ݔ‬ଵP )( ∂ H 1EP /∂‫ݔ‬ଵP ), and

(1)

H 1EP = (∂HE/∂n1P ),

(2)

where HE is the excess enthalpy of the entire system, H 1EP is the excess partial molar enthalpy of 1P, n1P the molar amount of 1P, N the total molar amount including that of S, and ‫ݔ‬ଵP = n1P /N. By applying this methodology, we have identified five different classes of solutes; hydrophobes, hydrophiles, amphiphiles, hydration centers, and hydrophobe-like hydration centers, as detailed in paper A.6 Briefly, we interpreted the molecular level scenario of the effect of solute on H2O, or the mixing scheme, for each class as follows:-

(1) “Hydrophobes” are those that form the hydration shell with somewhat enhanced hydrogen bonds than the normal bulk H2O. More importantly, however, the hydrogen bond probability of bulk H2O away from hydration shells is reduced progressively, but not yet reached the hydrogen bond percolation threshold. (2) “Hydration centers (normal)” also form hydration shell and, more importantly, do not perturb the bulk H2O away from hydration shells. The degree of hydrogen bonding within the hydration shell is not yet known.


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(3) “Hydrophobe-like hydration centers” also form the hydration shell and leaves the bulk H2O away from hydration shells unperturbed at the outset, ‫ݔ‬ଵP = 0. As ‫ݔ‬ଵP increases and the probing 1P consumes bulk H2O by forming its own hydration shells (1P is a typical hydrophobe), they gradually alter the bulk H2O away from two sets of hydration shells in the same way as a stronger “hydrophobes” than the probing 1P does. We note the ions belonging to this class are large anions, SO42 – and tartrate2 −, as we have studies so far. (4) “Hydrophiles”, being as such, show weaker interactions to H2O, not as drastic as the hydrophobes. They form hydrogen bonds directly to the fleetingly and yet permanently existing hydrogen bond network of H2O. By so doing, they pin down the inherent hydrogen bond fluctuation by breaking the H-donor/accepter symmetry enjoyed in pure liquid H2O. (5) “Amphiphiles” behave in H2O in such a manner that their hydrophobic and hydrophilic moieties modify H2O additively. Indeed, this finding is instrumental in devising the present methodology.

We stress that the methodology is applicable only in the limited H2O-rich composition range where the integrity of liquid H2O is retained, and hence the above descriptions of the five classes are true within each limited H2O-rich region. In paper A,6 we showed the results for 11 normal anions and 5 normal cations and did not pay attention to the afore-mentioned general difference in the effects on H2O between anions and cations. Here we first study two more cations, K+ and Cs+, by applying the methodology using Cl – as the counter aion. We then tried to see how much the two sets of ions, cations and anions, differ in their effect on H2O. While the number of ions studied would still be limited, we might



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be able to see a general trend. As detailed in paper A,6 the Cl – chosen here for the counter ion was found to belong to the class of hydration center. It is hydrated by 2.3 ± 0.6 molecules of H2O and more importantly leaves the bulk H2O away from hydration shells unperturbed.6 This finding on Cl − is consistent with that by a femto-second pump probe mid infrared spectroscopic study.9 The coordinates for the cations can be obtained by assuming that the test salts are completely deionized and that the effects of each ions on H2O are also additive within the limited H2O-rich region.

Experimental

KCl (Merck, > 99.5 %), CsCl (Merck, 99.5 %) and 1-propanol (1P) (Sigma-Aldrich, Chromosolv for HPLC, 99.9 %) were used as supplied. H2O was prepared by using Milli-Q system. The stock solutions were prepared using salts from a freshly opened bottle. The mixed solvents for H 1EP measurements, aqueous KCl or CsCl, were prepared by diluting the stock solution with Milli-Q H2O immediately before charging about 1 mL into the cell, into which a 5 µL or 10 µL of 1P was titrated successively up to the resulting mole fraction of 1P of 0.11. Due care was taken to avoid contamination to mildly hygroscopic 1P from atmospheric moisture. A TAM III semi-isothermal titration calorimeter (TA instruments, New Castle, DE, USA) was used to determine the excess partial molar enthalpy of 1P, H 1EP at 25.000 ± 0.005 oC. The time interval between two successive titrations were 50 min for KCl and 30 min for CsCl runs. The uncertainty in H 1EP was estimated as ± 0.03 kJ· mol-1, except for a high salt concentration mixed solvent in which the dissolution rate of 1P into the aqueous salt solution seems to slow down so


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that the dynamic correction inherent in the TAM loses the basic assumption, and the results 0

become sporadic. (See Fig 1 (a) for xS =0.03).

Results and Discussion

The results of the excess partial molar enthalpy of 1P, H 1EP , are listed in Table S1 and plotted in 0

Fig. 1. Fig. 1 (a) shows the results for xS = 0.03014 for KCl sample are sporadic. At this concentration of KCl, the rate of dissolution of 1P into the KCl aqueous solution became slower. This is probably because the ternary system is near a liquid-liquid phase separation. This additional slow dynamics might have upset the basic assumption used in data analysis of TAM 0

calorimeters. Thus, the upper applicable limit of the 1P-probing for KCl is xS = 0.03 for S = 0

KCl. For S = CsCl, at xS = 0.04, the results were sporadic (not shown) and at the end of the run, the cell solution was found turbid indicating that a liquid-liquid phase separation had occurred. 0

Hence xS = 0.04 is the upper applicable limit for S = CsCl. For the results of H 1EP for more 0

dilute xS regions, we drew smooth curves through all the H 1EP data points by aide of a flexible ruler, read the data off the smooth curves at δ‫ݔ‬ଵP =0.004 interval and approximated the partial derivative of eq. (1) with the quotient, δ H 1EP /δ‫ݔ‬ଵP with δ‫ݔ‬ଵP = 0.008. Such approximation was proved to be acceptable earlier.10 Through this graphical differentiation the random error in H 1EP1P increases which is estimated as ± 10 kJ· mol-1, but any systematic errors induced by using a wrong fitting function can be avoided. The results are plotted in Fig. 2. As is evident, the ‫ݔ‬ଵP



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dependence pattern of H 1EP1P remains of peak type assuring that the bulk H2O retains its integrity as liquid H2O, i. e. the hydrogen bond network is still bond-percolated. On addition of the test salt S, the H 1EP1P pattern is squashed to the west (to a lower value of ‫ݔ‬ଵP ) keeping the ordinate value of H 1EP1P unchanged within the estimated uncertainty ± 10 kJ· mol-1 at the outset (‫ݔ‬ଵP = 0) and at the top of the peak (called point X). This behavior is typical for “hydration centers”. Indeed, the fact that the values of H 1EP1P remain the same both at the outset, ‫ݔ‬ଵP = 0, and at the peak top, point X, was an important evidence for the interpretation (2) given in Introduction. The independence of the value of H 1EP1P at the peak top gives the zero hydrophilicity index, b = 0, for both samples.

0

Fig. 3 shows the plots of the ‫ݔ‬ଵP locus of point X shown in Fig. 2 against xS . The 0

westward shift is evidently linear to xS for both S = KCl and CsCl. The slopes therefore will 0

0

provide the values of hydrophobicity, a. The xS intercept in Fig. 3, xS (0), would be the mole fraction at which all the available H2O molecules hydrates a given ion pair. Hence,

xS0 (0) = 1/(1 + nH),

(3)

where nH is the hydration number for the ion pair.

Their coordinates, a, b, nH, etc. after

subtracting the contributions from the counter anion, Cl −, are listed in Table 1. Thus we conclude that both K+ and Cs+ ions belong to the class of hydration center, which is hydrated by 4.6 ± 0.8 and 1.1 ± 0.5 H2O molecules respectively but leaves the bulk H2O away from hydration shells unperturbed. In literature, there are a large number of estimates of what are loosely equivalent of nH.11,12 Clearly, the results for the hydration number depend on the methodology 8 ACS Paragon Plus Environment

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used and the assumptions inherent to the methodology. Here our methodology is based on the observation that within the limited dilute region listed in Table 1, the effects of two solutes are additive. The only another assumption is that ions dissociate completely and that each effects on H2O are also additive.6-8 The hydration numbers given in Table 1 must therefore be regarded as such within the present methodology.

The hydrophobicity/hydrophilicity map, Fig. 4, is defined by H2O itself at the origin and the probing 1P is necessarily at (0, −1). Relative to these two points on the map, each ion is spread out according to its coordinates, hydrophobicity, a, and hydrophobicity, b. The red plots in the figure are for anions and the blue for cations. Although the number of ions characterized so far is limited, the general trend is evident such that cations tend to cluster nearer to the origin(H2O) while anions are spread further out, particularly so for the northwest direction (hydrophobicity) of the map. This means that the effect of anions on H2O is stronger than that of cations, which has been the general observation since the days of Hofmeister.1 To quantify the trend, we define the distance from the origin to each plot, D, by normalizing the xy-coordinates of Fig. 4 arbitrarily as, D = ±[(a/1.25)2 + (b/7000)2]1/2.

(4)

The positive signs are given for those plots in the northwest quadrant including the negative abscissa, and the negative for those in the southwest quadrant including the negative ordinate. The D-values are also listed in Table 1. Accordingly the D-ranking runs for anions from D = +0.80 to – 0.40 with H2O at the null point as, SO42 − > tartrate2 − >F − > C2H5COO − > CH3COO − > Cl − > HCOO − > H2O



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Br − > I – > ClO4 − ~ SCN −,

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(5)

and for cations from D = +0.34 to −0.33 as, Ca2+ > Na+ >NH4+ > H2O > (CH3)4N+ > (C2H5)4N+.

(6)

Both rankings above are almost identical to the Hofmeister ranking given for the stability of polystyrene latex colloids.5 While the cation ranking could be completely reversed in other applications,4 the ranking (5) for anions is not generally inconsistent with those for various processes involving biopolymers.1-3 Thus, the rankings (5) and (6) show the degree of the effect of each ion on H2O quantitatively. Since the present D-ranking resembles the century old Hofmeister series, the ion specific effects on H2O must be important for advancing the understandings of the Hofmeister effects further. A large number of modern non-linear spectroscopic studies suggest that the effect of ions on the bulk H2O away from hydration shells are negligible,9,13-18 and hence the mechanism of the Hofmeister effects ought to be sought in the direct interaction of ions with biopolymer surfaces. After all, biopolymers are giant molecules of colloidal dimension and a single ion interacting a local spot on different parts of biopolymer may not be enough to influence the behaviour of giant biopolymer. It seems to be more natural to consider that the effect of ion on H2O first and then how it would influence the entire body of biopolymer in a holistic manner via the bond percolated hydrogen bond network.

For cations, their effects are evidently smaller than for anions as quantified by D-ranking (6) and displayed in Fig. 4. Thus, the effect of modified H2O by cations on different parts of biopolymer could compete each other and tip the balance of generally weaker effects for the overall effects, resulting in unstable Hofmeister rankings. Another important point to note is that


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if the above D-rankings have some bearing to the Hofmeister effects, they provide not only a quantitative indication but also add a qualitative meanings. Namely, the Hofmeister series have been named as running from “kosmotropes” to “chaotropes” with Cl – at the nominal null point.3 In terms of the five classifications found for normal ions by the present methodology, the “kosmotropes” themselves are ranked from “hydrophobe-like hydration center” to “normal hydration center” and “hydrophobes” mixed up then to H2O at the null point. “Chaotropes”, on the other hand, run from H2O to “hydrophiles”. While the Hofmeister” rankings have been taken as a one-dimensional scale, the qualitative distinction in two-dimension may be an important step towards further understanding of the century old enigma.

Acknowledgement

This work was supported by the Carlsberg Foundation. T. M. thanks MEXT, Japan, for Grants for Excellent Graduate Schools.

Supporting information Table S1 containing the measured values of the excess partial molar enthalpy of 1-propanol, H 1EP . This information is available free of charge via the Internet at http://pubs.acs.org



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Table 1 Hydrophobicity/hydrophilicity indices for normal ions.

Plot # Ions

a

b

nH

0.0

±

D

x(max)

ref

Map basics {0}

H2O

0.0

{4}

1P

-1.00 0.0

20

0.5

0.00

19

0.80

20

Hydration Centers {10}

Na+

-0.30 0.0

5.2

0.24

0.024

21,22

{11}

F−

-0.72 0.0

14

2

0.58

0.004

21

{12}

Cl −

-0.16 0.0

2.3

0.6

0.13

0.024

21, 22

{29}

NH4+

-0.10 0.0

1

1

0.08

0.06

23

{33}

Ca2+

-0.34 0.0

6

2

0.27

0.02

23

{37}

HCOO –

-0.11 0.0

1.2

0.5

0.08

0.04

24

{47}

K+

-0.23 0.0

4.6

0.8

0.18

0.025

This work

{48}

Cs+

-0.10 0.0

1.1

0.5

0.08

0.033

This work

Hydrophobe-like hydration centers {13}

SO42−

-0.72 3890

14

2

0.8

0.01

25

{36}

tartrate2 −

-0.72 2270

14

2

0.66

0.01

25

Hydrophobes {38}

CH3COO −

-0.22 0

3.7

0.7

0.18

0.05

24

{39}

C2H5COO −

-0.48 0

9

2

0.38

0.03

24

(table continued)


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Hydrophiles {14}

Br −

0

-920

-0.13 0.06

21

{15}

I−

0

-2050

-0.29 0.07

21

{28}

(CH3)4N+

0

-1182

-0.17 0.009

23

{34}

ClO4 −

-0.02 -2800

-0.40 0.048

26

{35}

SCN −

-0.02 -2800

-0.40 0.049

26

{46}

(C2H5)4N+

-0.09 -2270

-0.33 0.03

23

(end of Table 1.)



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Koga, Y.; Katayanagi, H.; Davies, J. V.; Kato, H.; Nishikawa, K.; Westh, P. The Effects of Chloride Salts of Some Cations on the Molecular Organization of H2O. Towards Understanding the Hofmeister Series. II. Bull. Chem. Soc. Jpn. 2006, 79, 1347–1354.

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Kondo, T.; Miyazaki, Y.; Inaba, A.; Koga, Y. Effects of Carboxylate Anions on the Molecular Organization of H2O as probed by 1-propanol. J. Phys. Chem. B 2012, 116, 3571–3577.

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Koga, Y.; Kondo, T.; Miyazaki, Y.; Inaba, A. The Effects of Sulphate and Tartrate Ions on the Molecular Organization of Water: Towards Understanding the Hofmeister Series (VI). J. Solution Chem. 2012, 41, 1388–1400.

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Koga, Y.; Westh, P.; Davies, J. V.; Miki, K.; Nishikawa, K.; Katayanagi, H. Toward Understanding the Hofmeister Series. 1. Effects of Sodium Salts of Some Anions on the Molecular Organization of H2O. J. Phys. Chem. A 2004, 108, 8533–8541.



2

-1

0

(a) KCl

-2

-4

-6

E

H1P / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 22

xS0 = 0

-8

0.01376 0.01910 0.02473 0.03014

-10

-12 0.00

0.02

0.04

0.06

0.08

0.10

0.12

x1P Fig. 1 (a) The excess partial molar enthalpy of 1-propanol (1P), H 1EP , for 1P – S – H2O (S = KCl) at 25 oC.


Page 17 of 22

2

-1

0

(b) CsCl

-2

-4

-6

E

H1P / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

xS = 0 -8

0.01735 0.02495 0.03301

-10

-12 0.00

0.02

0.04

0.06

0.08

0.10

0.12

x1P Fig. 1 (b) The excess partial molar enthalpy of 1-propanol (1P), H 1EP , for 1P – S – H2O (S = CsCl) at 25 oC.



300

(a) KCl xS0 = 0

-1

250

200

0.01376 0.01910 0.02473

150

E

H1P1P / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

100

50

0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

x1P Fig. 2 (a). The 1P-1P enthalpic interaction, H 1EP1P , in 1P – S – H2O (S = KCl) at 25 oC. The uncertainty in H 1EP1P was estimated as ± 10 kJ· mol-1. The fact that the peak top shifts westward with its height remains constant within the estimated error, the hydrophobicity index a is the 0

slope of the ‫ݔ‬ଵP locus against xS (S = KCl), (Fig. 3) and the hydrophilicity index b = 0. See text.


Page 19 of 22

300

(b) CsCl

xS 0 = 0 0.01753 0.02495 0.03301

-1

250

200

150

E

H1P1P / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

50

0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

x1P Fig. 2 (b). The 1P-1P enthalpic interaction, H 1EP1P , in 1P – S – H2O (S = CsCl) at 25 oC. The uncertainty in H 1EP1P was estimated as ± 10 kJ· mol-1. The fact that the peak top shifts westward with its height remains constant within the estimated error, the hydrophobicity index a is the 0

slope of the ‫ݔ‬ଵP locus against xS (S = CsCl), (Fig. 3) and the hydrophilicity index b = 0. See text.



0.06

0.05

0.04

x1P(X)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

0.03

0.02

CsCl KCl

0.01

0.00 0.00

0.01

0.02

0.03

0.04

0

xS (S = KCl or CsCl) 0

Fig. 3. The ‫ݔ‬ଵP -loci of point X (peak top) of H 1EP1P shown in Fig. 2 against xS , for S = KCl and CsCl. The slopes give hydrophobicity, a, for each salts.


Page 21 of 22

4000

b (hydrophilicity)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13

36

2000

29

37 0

4

11

3310

39

47

12

48

0 14 28

38

-2000

15

34 35 -4000 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

a (hydrophobicity)

Fig. 4 Hydrophobicity/hydrophilicity map for ions. See Table 1 for Plot #’s. Red plots are for anions and the blue for cations. Plot {34} ClO4 – and plot {35} SCN – are at the same spot on the ordinate. Plot {37} HCOO −, plot {29} NH4+ and plot {48} Cs+ are at the same spot on the abscissa. So are plot {47} K+ and plot {38} CH3COO –.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

b (hydrophilicity)


4000

13

2000

36

0 -2000 -4000

4

3729

Page 22 of 22

48

11

39 33 10 12 0 14 47 38 28 anions red 15 34 cations blue 35

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

a (hydrophobicity)

Table of Content Graphics


How Much Weaker Are the Effects of Cations than Those of Anions

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