Amplification of Hofmeister Effect by Alcohols - The Journal of Physical

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Amplification of Hofmeister Effect by Alcohols Yun Xu, and Guangming Liu J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2014 Downloaded from http://pubs.acs.org on June 17, 2014

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The Journal of Physical Chemistry

Amplification of Hofmeister Effect by Alcohols Yun Xu and Guangming Liu* Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, P. R. China 230026

1

ACS Paragon Plus Environment


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Abstract. In the present work, we have demonstrated that Hofmeister effect can be amplified by adding alcohols to aqueous solutions. The lower critical solution temperature behavior of poly(N-isopropylacrylamide) has been employed as the model system to study the amplification of Hofmeister effect. The alcohols can more effectively amplify the Hofmeister effect following the series methanol < ethanol < 1-propanol < 2-propanol for the monohydric alcohols and following the series D-sorbitol ≈ xylitol ≈ meso-erythritol < glycerol < ethylene glycol < methanol for the polyhydric alcohols. Our study reveals that the relative extent of amplification of Hofmeister effect is determined by the stability of the water/alcohol complex, which is strongly dependent on the chemical structure of alcohols. The more stable solvent complex formed via stronger hydrogen bonds can more effectively differentiate the anions through the anion-solvent complex interactions, resulting in a stronger amplification of Hofmeister effect. This study provides an alternative method to tune the relative strength of Hofmeister effect besides salt concentration.

Keywords: Specific ion effect, Polymer, Phase transition, Solvent complex, Hydrophobic hydration

*To whom correspondence should be addressed. Email: [email protected] 2


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Introduction Specific ion effect or Hofmeister effect is ubiquitous in biological and chemical systems.1-10 Several models have been proposed to clarify the mechanism of Hofmeister effect. Collins has proposed a concept of matching water affinities to explain the ion specificity.4 Kunz et al. have suggested that the nature of surfaces may have significant influences on the specific ion effect.6 Ninham et al. have found that the specific ion effect could be better understood by including electrostatic, dispersion, and cavity contributions in a continuum model.11,12 It is well known that the relative strength of specific ion effect increases with increasing salt concentration.2,7,13 In other words, Hofmeister effect can be amplified by the continuous addition of salt to aqueous solutions. However, the usage of high salt concentration may be limited in some systems, for example, the high salt concentration would lead to aggregation or precipitation of proteins, polymers, and colloids in aqueous solutions.14-16 Thus, it is important to exploit other methods to tune the relative strength of Hofmeister effect. Our previous studies have shown that the specific ion effect can also be amplified by adding methanol (MeOH) or ethylene glycol (EG) to aqueous solutions when keeping the salt concentration constant.17,18 We have also suggested that the amplification of ion specificity by MeOH or EG is attributed to the enlarging difference in ion-solvent complex interactions between the ions with increasing alcohol concentration.17,18 Considering that the formation of solvent complexes between water and alcohol molecules is strongly dependent on the alcohol structure, it is expected that the amplification of Hofmeister effect should be significantly 3



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influenced by the chemical structure of alcohols. However, the mechanism of how alcohol structure influences the amplification of Hofmeister effect is still unknown, which would hinder the utilization of alcohols to modulate the specific ion effect. Poly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer, which has a lower critical solution temperature (LCST) behavior.19 In general, the PNIPAM solution becomes cloudy at the temperature above the LCST due to the aggregation of PNIPAM chains in the solution (Figure S1, Supporting Information). In the present study, the LCST behavior of PNIPAM has been employed as the model system to study the amplification of specific anion effect by alcohols. We have employed monohydric and polyhydric alcohols with different chemical structures to conduct the studies. We find that the relative extent of amplification of anion specificity is strongly dependent on the alcohol structure. Our aim is to clarify the mechanism of the influence of alcohol structure on the amplification of Hofmeister effect.

Experimental Section Materials. Isopropylacrylamide (NIPAM, Aldrich) was recrystallized three times in a mixture of toluene and hexane (65/35, v/v). 1,4-dioxane was refluxed over Na, and then distilled under vacuum before use. 4,4’-Azobis(iso-butyronitrile) (AIBN) was recrystallized three times from ethanol. Sodium chloride (NaCl, 99.5%), sodium nitrate (NaNO3, 99.0%), sodium acetate (CH3COONa, 99.0%), sodium thiocyanate (NaSCN, 98.5%), and all the alcohols (AR grade) were purchased from Sinopharm or

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Aladdin and used as received. The water used was purified by filtration through a Millipore Gradient system after distillation, giving a resistivity of 18.2 MΩ cm. PNIPAM Preparation. Chain transfer agent cyanoisopropyl dithiobenzoate (CPDB) was synthesized following the previous method.20 PNIPAM was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization in dioxane at 70 ºC for ~ 20 h with CPDB as the chain-transfer agent and AIBN as the initiator. The sample was precipitated three times from tetrahydrofuran (THF) into diethyl ether for purification. The number-average molar mass (Mn ~ 1.6×104 g mol-1) and the polydispersity index (Mw/Mn ~ 1.2) of the sample were measured by gel permeation chromatography (Waters 1515) using monodisperse polystyrene as the calibration standard and THF as the eluent with a flow rate of 1.0 mL min-1. Cloud Point Measurements. Cloud points were determined by monitoring the turbidity of solutions heated at a rate of 0.1 ºC min-1 during the measurements of LCST using a UNICO 2802PCS UV/visible spectrophotometer with the wavelength set to 500 nm. The temperature of the cell was controlled using a circulating temperature controlled bath with an accuracy of ± 0.1 ºC and monitored by an electronic thermometer. For measurements below 10 ºC, the cell holder was flushed with nitrogen to prevent moisture condensation on the cell surface. The concentration of PNIPAM in turbidity measurements was fixed at 1.0 mg mL-1 and the salt concentration was fixed at 0.3 M. In turbidity studies, we chose to use the temperature at which the transmittance equalled the average of the transmittance values of the high

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transmittance (low turbidity) and low transmittance (high turbidity) plateaus as the LCST (Figure S2). Raman Spectra Measurements. In the Raman spectra measurements, a CW laser source (Coherent, Verdi-5W, 532nm), a monochromator with a 600 grooves/mm grating, and a liquid-nitrogen-cooled CCD detector (Acton Research, Triple-Pro) were used to record the spontaneous Raman spectrum.21 Liquid samples in a quartz cell (Spectrocell, 10 × 10 mm2) were excited by the laser. The Raman scattering photons were collected at 180°geometry relative to the incident laser beam with a pair of f = 5 and 20 cm lenses and imaged through a polarization scrambler onto the entrance slit of the mono-chromator for spectral dispersion. The laser power employed was 4 W. The spectral resolution was determined to be ∼ 3.0 cm−1. All the Raman spectra were measured at ∼25 °C.

Results and Discussion In the present work, we have employed nine types of monohydric and polyhydric alcohols with different chemical structures to conduct the investigations on the amplification of specific anion effect. The chemical structures and physical properties of the employed alcohols in this study are presented in Table 1. For the monohydric alcohols, the length and shape of the hydrophobic alkyl group are varied but the number of the hydroxyl group is kept at one. For the polyhydric alcohols, the ratio of the number of hydroxyl group to the number of methylene group keeps constant at one but the total number of the hydroxyl group is gradually varied. It is anticipated 6


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that the alcohols with different chemical structures will interact with water molecules differently and amplify the Hofmeister effect to different extents.

Table 1. Chemical structures and physical properties of the employed alcohols.

chemical name Methanol Ethanol 1-Propanol 2-Propanol Ethylene glycol Glycerol

meso-Erythritol

Xylitol

D-sorbitol

a

MeOH EtOH 1-PrOH 2-PrOH

molar mass (g· mol−1) 32.0 46.1 60.1 60.1

solubility in water at 25 ºC (mol/mol) miscible miscible miscible miscible

EG

62.1

miscible

GLY

92.1

miscible

m-ERY

122.1

9%a

XYL

152.2

18%b

D-SOR

182.2

19%c

abbreviation

structure

Ref.22, bRef.23, and cRef.24

Figure 1 shows the change in LCST of PNIPAM in the salt-free solutions as a function of the molar fraction of alcohols (x) obtained from the turbidity measurements. For all the alcohols, the LCST decreases with increasing x, indicating that the addition of alcohols decreases the solubility of PNIPAM. It is known that PNIPAM chains can be solvated by both free solvent molecules and solvent 7



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complexes in water-alcohol mixtures.25,26 The solvent complexes are generally considered as poorer solvents compared with the free solvent molecules.25,26 Specifically, the solubilization of PNIPAM in pure water or alcohol is due to the hydrogen bonding between PNIPAM and water or between PNIPAM and alcohol, which dominates over the self-association of PNIPAM chains induced by the hydrophobic interactions. In the water-alcohol mixtures, water and alcohol molecules form solvent complexes. The water/alcohol complexes have less hydrogen bonding sites, such that the hydrogen bonding between the solvent complexes and PNIPAM is overwhelmed by the hydrophobic interactions of PNIPAM backbone, giving rise to a desolvation of PNIPAM, and resulting in a decrease in LCST with the addition of alcohol to water.26 Thus, the LCST decreases with increasing x for all the alcohols, suggesting that the addition of alcohols leads water to be a poorer solvent for PNIPAM due to the formation of water-alcohol complexes. As x increases, the LCST decreases more rapidly following the series MeOH < EtOH < 2-PrOH < 1-PrOH in Figure 1a for the monohydric alcohols and following the series MeOH < EG < GLY < m-ERY < XYL < D-SOR in Figure 1b for the polyhydric alcohols. Note that the addition of some alcohols to aqueous solutions will significantly decrease the LCST to a temperature well below zero degree in the presence of some anions (Figure S2, Supporting Information). Consequently, it was not possible to determine the LCST of PNIPAM at the high molar fraction of alcohols due to the fast moisture condensation on the cell surface during turbidity measurements. For m-ERY, the low solubility of such kind of polyhydric alcohol also limits the conduction of experiments at the high 8


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molar fraction of alcohol.

(a)

30

LCST / C

30

LCST / C

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20 10

MeOH EtOH 2-PrOH

0

0

20 10 0

1-PrOH

2

4

(b)

MeOH EG GLY m-ERY XYL D-SOR

0

6

5

10

15

x % (mol/mol)

x % (mol/mol)

Figure 1. (a) LCST of PNIPAM in the salt-free solutions as a function of the molar fraction of monohydric alcohols (x). (b) LCST of PNIPAM in the salt-free solutions as a function of the molar fraction of polyhydric alcohols (x). Here, the PNIPAM concentration is fixed at 1.0 mg mL-1.

It has been reported that both the hydroxyl and the hydrocarbon groups have strong influences on the formation of solvent complexes between water and alcohol molecules.26-31 The alcohol molecules can form hydrogen bonds with water molecules through the hydroxyl groups.26-28,32 Meanwhile, the hydrophobic hydrocarbon groups can also be hydrated by water molecules accompanied by the formation of hydration shell with the clathrate-like structure around the nonpolar groups.33-35 Thus, if the formation of solvent complexes of one type of alcohol needs a larger amount of water molecules, the addition of the alcohol would produce a more significant disturbance to the hydration of PNIPAM, thereby leading to a more rapid decrease in LCST with increasing x.33 All the monohydric alcohols only have one hydroxyl group at the end 9



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of the molecules. Thus, the difference in the formation of solvent complexes between the monohydric alcohols is mainly attributed to the hydrophobic hydration of the different hydrocarbon groups. It has been reported that the amount of water molecules required for the hydrophobic hydration increases following the order MeOH < EtOH < PrOH.35,36 For the cases of 1-PrOH and 2-PrOH, the difference in the hydrophobic group between them is the molecular shape. 1-PrOH has a linear hydrocarbon group, whereas 2-PrOH possesses a more spherical shape-like hydrocarbon group. It has been suggested that the more spherical hydrophobic group of 2-PrOH is more miscible with water molecules as the cage structure can be formed by water molecules without a significant distortion of the normal angle between hydrogen bonds of water molecules.37 Thus, the hydrophobic hydration of 1-PrOH would need more water molecules compared with the hydrophobic hydration of 2-PrOH.35,36 As a result, the addition of monohydric alcohols would produce a more significant disturbance to the hydration of PNIPAM following the series MeOH < EtOH < 2-PrOH < 1-PrOH and the LCST decreases more rapidly with increasing x following the same series. In Figure 1b, the formation of solvent complexes between polyhydric alcohols and water molecules is mainly dominated by the hydration of the hydroxyl groups.27,28,32 From MeOH to D-SOR, the number of the hydroxyl group of each molecule increases from one to six. Therefore, the amount of water molecules required to hydrate the polyhydric alcohols increases following the series MeOH < EG < GLY < m-ERY < XYL < D-SOR and the polyhydric alcohols can more significantly disturb the hydration of PNIPAM chains following the same series. This is why the LCST 10


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decreases more rapidly with increasing x following this series. Nevertheless, this effect may become weaker with increasing size of polyhydric alcohol because more internal hydrogen bonds are formed and the interactions between the hydroxyl group and water molecules are weakened with increasing size of polyhydric alcohol.32 Therefore, there might be a limiting alcohol size effect on the LCST behavior of PNIPAM after the number of hydroxyl group of polyhydric alcohol reaches ~ 6, as reflected by the fact that the LCST of PNIPAM for XYL as a function of x lies close with that for D-SOR (Figure 1b). In addition, the LCST of PNIPAM obtained from the differential scanning calorimetry measurements also exhibits a similar change with that in Figure 1 (Figure S3, Supporting Information).

36

LCST / C

32

28

Anion type

OCH 3C O

Cl -

Sa

N

lt-

fre

e

O 3

24 SC N-

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Figure 2. LCST of PNIPAM as a function of anion type in aqueous solutions with Na+ as the common cation, where the PNIPAM concentration is fixed at 1.0 mg mL-1 and the salt concentration is fixed at 0.3 M.

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Figure 2 shows the LCST of PNIPAM decreases following the series SCN- > salt-free > NO3- > Cl- > CH3COO- in the aqueous solutions, which is consistent with the previous observation that SCN- exhibits the salting-in effect but the other anions exhibit the salting-out effect.2,17,18,38 More specifically, the salting-in effect is mainly caused by the direct binding of SCN- on the PNIPAM surface, whereas the salting-out effect is induced by either the destabilization of hydrogen bonding between PNIPAM and water via the anionic polarization or the increase in the PNIPAM/water interfacial tension due to increasing anion concentration.2 As discussed above, the addition of alcohols to the aqueous solution will decrease the solubility of PNIPAM due to the formation of solvent complexes between alcohol and water molecules. Since the different types of anions would exhibit different interactions with the solvent complexes, it is expected that the addition of alcohols to aqueous solutions will also influence the relative strength of the specific anion effect.

Figure 3. (a) ΔLCST of PNIPAM as a function of the molar fraction of monohydric alcohols (x) for the different anions with Na+ as the common cation. (b) ΔLCST of PNIPAM as a function of the molar fraction of polyhydric alcohols (x) for the 12


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different anions with Na+ as the common cation. Here, the PNIPAM concentration is fixed at 1.0 mg mL-1 and the salt concentration is fixed at 0.3 M. The values of ΔLCST are obtained using the LCST for SCN- as the reference.

Figure 3 shows the change in ΔLCST of PNIPAM as a function of x in the presence of different anions. Here, ΔLCST is the difference in LCST between the anions and is calculated by subtracting the LCST for the other anions from the LCST for SCN -. Namely, the values of ΔLCST are obtained using the LCST for SCN- as the reference. In Figure 3a, the difference in ΔLCST between the anions becomes more obvious with increasing x for all the monohydric alcohols, indicating that the anion specificity is amplified by the addition of alcohols. More interestingly, the relative extent of amplification of anion specificity increases following the series MeOH < EtOH < 1-PrOH < 2-PrOH at the same x. In other words, the monohydric alcohols can more effectively amplify the anion specificity following this series. In Figure 3b, the anion specificity can also be amplified by the addition of polyhydric alcohols and the polyhydric alcohols can more effectively amplify the anion specificity following the series D-SOR ≈ XYL ≈ m-ERY < GLY < EG < MeOH. A possible explanation for the amplification of specific anion effect by alcohols is the local salt concentration around the PNIPAM chains may be enhanced through the addition of alcohols to aqueous solutions because the hydration of alcohol molecules consumes some free water molecules. Therefore, the anion specificity is expected to be amplified with increasing x as the Hofmeister effect can be amplified with 13



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increasing salt concentration. However, this hypothesis cannot explain why 2-PrOH can more effectively amplify the anion specificity than that of 1-PrOH because the hydration of the latter requires more water molecules than that of the former. Similarly, it can also not explain why the polyhydric alcohols can more effectively amplify the anion specificity following the series D-SOR ≈ XYL ≈ m-ERY < GLY < EG < MeOH. Alternatively, the amplification of anion specificity by alcohols could be attributed to the enlarging difference in the anion-solvent complex interactions between the anions with increasing x.17,18 More specifically, a dynamic equilibrium exists between the free solvent molecules and the solvent complexes in the water-alcohol mixtures. The shift of this equilibrium is influenced by the anion-complex interactions. Generally, a more chaotropic anion exhibits a “softer” character.39 Thus, the more chaotropic anion possessing a higher polarizability would have stronger interactions with the nonpolar group of the alcohols via dispersion forces.40 That is, the more chaotropic anion can more effectively disrupt the solvent complexes.41-43 As a result, the increasing concentration of solvent complexes with the addition of alcohols would enlarge the difference in the anion-solvent complex interactions between the anions, giving rise to the increasing difference in the solubility of PNIPAM between the anions by shifting the equilibrium between the free solvent molecules and the solvent complexes, and resulting in the amplification of anion specificity. On the other hand, if water molecules and alcohol molecules form more stable solvent complexes via stronger hydrogen bonds, the solvent complexes will more 14


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effectively differentiate the anions via the anion-solvent complex interactions. We have investigated the Raman spectra of the water-alcohol mixtures at x of 5% to exemplify the change in the relative proportion between the strong and the weak hydrogen bonds formed in the solvent mixtures for the different types of alcohols (Figure 4a and 4b). It is known that the low frequency band (~ 3240 cm-1) and the high frequency band (~ 3400 cm-1) in the Raman spectra are respectively ascribed to the hydroxyl groups in strong and weak hydrogen bond networks in water-alcohol mixtures.44 Moreover, the variation in the ratio of the intensity of low frequency band to the intensity of high frequency band (I3240/I3400) reflects the change in the relative proportion between the strong and the weak hydrogen bonds formed in the solvent mixtures.44,45

(a)

MeOH EtOH 1-PrOH 2-PrOH

80

40

150

Intensity / a.u.

120

Intensity / a.u.

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

MeOH EG GLY m-ERY XYL D-SOR

(b)

100

50

0 3060

3240

3420

3600

3060

-1

3240

3420

3600 -1

Raman shift / cm

Raman shift / cm

Figure 4. (a) Raman spectra of the water-monohydric alcohol mixtures in the OH stretching vibration region at the molar fraction of monohydric alcohol (x) of 5%. (b) Raman spectra of the water-polyhydric alcohol mixtures in the OH stretching vibration region at the molar fraction of polyhydric alcohol (x) of 5%. For clarity, the

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spectra corresponding to the different alcohols have been displaced relative to each other.

In Figure 5a, the ratio of I3240/I3400 at x of 5% increases following the series MeOH < EtOH < 1-PrOH < 2-PrOH. As reported previously, the hydrophobic hydration of the hydrocarbon group of alcohols is achieved by forming stable clathrate structure via strong hydrogen bonds.46 Thus, a stronger hydrophobic hydration will generate a higher value of the relative proportion of strong hydrogen bond to weak hydrogen bond. In comparison with 1-PrOH, 2-PrOH with the more spherical hydrophobic group is more miscible with the water molecules within the clathrate structure, which is favorable for forming more stable clathrate structures via strong hydrogen bonds.37 Thus, the ratio of I3240/I3400 increases following the series MeOH < EtOH < 1-PrOH < 2-PrOH, suggesting that more stable solvent complexes are formed following this series. Consequently, the monohydric alcohols can more effectively amplify the anion specificity following this series because the more stable solvent complexes can more effectively differentiate the anions via the anion-solvent complex interactions by shifting the equilibrium between the free solvent molecules and the solvent complexes.

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1.05

(b)

(a)

1.04

1.03

I3240/I3400

1.02 2-PrOH

I3240/I3400

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EtOH

MeOH EG

1.01

GLY m-ERY

1.00

1-PrOH

MeOH

0.99

1.02

Alcohol Type

XYL D-SOR

Alcohol Type

Figure 5. (a) Change in the ratio of the intensity of low frequency band to the intensity of high frequency band (I3240/I3400) as a function of the type of monohydric alcohol at x of 5%. (b) Change in the ratio of the intensity of low frequency band to the intensity of high frequency band (I3240/I3400) as a function of the type of polyhydric alcohol at x of 5%.

In Figure 5b, the ratio of I3240/I3400 decreases following the series MeOH > EG > GLY > m-ERY > XYL > D-SOR, suggesting that the relative proportion of strong hydrogen bonds in the solvent mixtures decreases following this series. From MeOH to D-SOR along this series, the extent of hydrophobic hydration gradually decreases due to the increasing number of hydroxyl group of each molecule.47 Meanwhile, the ability of per hydroxyl group to form hydrogen bonds with surrounding water molecules also decreases from MeOH to D-SOR because more internal hydrogen bonds are formed with increasing number of hydroxyl group per polyhydric alcohol.28,32 As a result, water and polyhydric alcohol molecules would form more stable solvent complexes following the series D-SOR < XYL < m-ERY < GLY < EG < MeOH, which is exactly the same as the result observed in Figure 5b. As the more 17



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stable solvent complexes can more effectively differentiate the anions via the anion-solvent complex interactions, the polyhydric alcohols can more effectively amplify the anion specificity following the series D-SOR ≈ XYL ≈ m-ERY < GLY < EG < MeOH. Note that no obvious amplification of anion specificity can be observed in the range of x less than 10% for m-ERY, XYL, and D-SOR, which might be because the relatively unstable solvent complexes formed by these alcohols have relatively weak ability to differentiate the anions through the anion-solvent complex interactions.

Conclusion The specific anion effect on the LCST behavior of PNIPAM can be amplified by both monohydric and polyhydric alcohols. The amplification of anion specificity is dominated by the stability of the solvent complex rather than the extent of hydration of the alcohols in the water-alcohol mixtures. The stability of solvent complex is dependent on the chemical structure of alcohols. The more stable solvent complex formed by water and alcohol molecules can more effectively amplify the Hofmeister effect. We believe that this study will open a new avenue to tune the relative strength of Hofmeister effect.

Acknowledgements The financial support of National Program on Key Basic Research Project (2012CB933800), the National Natural Science Foundation of China (21374110, 18


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91127042) is acknowledged.

Supporting Information: Temperature dependence of transmittance of PNIPAM solution and the differential scanning calorimetry results are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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Amplification of Hofmeister Effect by Alcohols - The Journal of Physical

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