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Jun 15, 2010 - Single-chain mechanics of poly(N-isopropyl-acrylamide) in the water/methanol mixed solvent. Xiangchao Pang , Kefeng Wang , Shuxun Cui...
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Solvation Behaviors of N-Isopropylacrylamide in Water/Methanol Mixtures Revealed by Molecular Dynamics Simulations Juan Pang,†,§ Hu Yang,*,†,§ Jing Ma,*,‡,§ and Rongshi Cheng†,§,| Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing, 210093, People’s Republic of China, Institute of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing, 210093, People’s Republic of China, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing, 210093, People’s Republic of China, and Polymer Institute, College of Material Science and Engineering, South China UniVersity of Technology, Guangzhou, 510640, People’s Republic of China ReceiVed: January 26, 2010; ReVised Manuscript ReceiVed: June 1, 2010

Solvation behaviors of N-isopropylacrylamide (NIPAM) in water/methanol mixtures are investigated by molecular dynamics simulations. The results indicate that NIPAM-solvent interactions are weakened with the increase of methanol mole fractions (xmethanol) from 0.25 to 0.80, rationalizing the reentrant coil-to-globuleto-coil transition behaviors of poly(N-isopropylacrylamide) in the mixed solvents. Interestingly, hydrogenbonded water/methanol clusters are abundant in binary mixed solvents, leading to the decrement of NIPAM-solvent interactions. To better understand the intermolecular interactions in the water/methanol complex clusters, the structures of pure water and methanol clusters are also studied for a comparison. Although the amount of water clusters decreases with an increase in xmethanol, the structure of water clusters stays stable, and hydrogen-bonded networks are not essentially disrupted. As for methanol molecules, they prefer to form short nonbranched chainlike hydrogen-bonded clusters. However, most of the chainlike hydrogen-bonded methanol clusters are broken in water-rich solutions, leaving the little probability for the formation of dimeric and trimeric methanol clusters. 1. Introduction Solvation behavior of a polymer in solution is an interesting and fundamental topic in both theory and experiment. The solvation-desolvation process is always coinstantaneous with the conformational transition of polymer chain,1–3 such as a reversible coil-to-globule and coil-to-helix transition, which implicates many living phenomena, for example, protein folding and DNA packing.4 Poly(N-isopropylacrylamide) (PNIPAM)5–18 is a thermosensitive polymer that is well-known for its lower critical solution temperature (LCST) around 32 °C in aqueous solution. The coil-to-globule transition of the PNIPAM chain occurs when the temperature is raised above the LCST. The nature of the transition is revealed as the hydrophilic interaction changes to the hydrophobic interaction. In addition to the temperature’s inducing the collapse of the polymer chain, PNIPAM is insoluble in a proper mixture of water and methanol, despite both water and methanol being good solvents for PNIPAM at low temperature. The reentrant coil-to-globule-to-coil transition of PNIPAM will take place in a water/methanol mixture with an increase in the methanol content at room temperature19–24 (cf. Scheme 1), as demonstrated by light scattering,25,26 quartz crystal microbalance (QCM) measurements,27 fluorescence spectra,28 and H-1 MAS NMR.29 However, the mechanism of * Corresponding authors. E-mails: (H.Y.) [email protected], (J.M.) [email protected]. † Department of Polymer Science and Engineering, Nanjing University. ‡ Institute of Theoretical and Computational Chemistry, Nanjing University. § Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University. | South China University of Technology.

SCHEME 1: Reentrant Coil-to-Globule-to-Coil Transition of a PNIPAM Chain with Increasing Methanol Content, xmethanol, in Water/Methanol Mixtures at Room Temperature

this transition is still not well established. It is believed that the transition is implicative to the solvation effect. Two potential mechanisms have been proposed. One suggested that the transition should be induced by the preferential adsorption of methanol molecules on PNIPAM.30 However, our theoretical calculations well demonstrate that water is as stable as methanol for staying in the vicinity of PNIPAM. The other ascribed it to the formation of water/methanol clusters,25 and this view has been accepted by more and more researchers. Previous studies on water/methanol mixtures by both experiment31 and computer simulations32 have supported the immiscibility of the binary solution on a microscopic scale. It implies that, even without PNIPAM, water can form certain clusters with methanol. Obviously, the combination of water and methanol must lead to more complicated structures, which have been widely investigated in the past decades both experimentally33,34 and

10.1021/jp100743k  2010 American Chemical Society Published on Web 06/15/2010

N-Isopropylacrylamide in Water/Methanol Mixtures theoretically.35–43 Zhang and Wu25 have proposed a simple pentagon model for water/methanol clusters on the basis of their experimental observations. However, various possible geometries of such water/methanol clusters are still not fully explored. In addition to water/methanol clusters, pure water/water and methanol/methanol clusters can also be formed in their mixtures and may affect the conformational change of polymer chain. In fact, the structures of pure water and methanol, respectively, are not simple at all. It is generally accepted that water prefers to form a tetrahedral hydrogen-bonded structure44 or so-called iceberg structure. Methanol, comprising hydrogen-bonded chains and rings,32,45–47 differs only in the presence of a methyl group in place of a proton and exhibits a distinctly different structure from water. Undoubtedly, the understanding of various solvent configurations in their mixed solvents is crucial to understanding the nature of a solvent’s inducing the collapse of a polymer chain. As noted, PNIPAM has carbonyl and amine groups that can form hydrogen bonds with either water or methanol molecules. The monomer of PNIPAM (NIPAM) in water/methanol binary solvents is selected in this article to demonstrate the hydrogen bonding interactions and solvation behavior of NIPAM affected by solvent components. Our results indicate that NIPAM-solvent interactions are weakened in water/methanol mixtures. The various solvent configurations, including pure water, pure methanol, and their complexes, are also investigated. It is revealed that hydrogen-bonded networks of water clusters are not essentially disrupted, but methanol clusters in the dominant chainlike hydrogen-bonded forms are broken in water-rich mixtures. Remarkably, hydrogen-bonded water/methanol clusters are abundant in the range of xmethanol from 0.25 to 0.80, so that NIPAM-solvent interactions are weakened. Detailed analyses of the solvent structures are helpful for understanding the nature of the reentrance coil-to-globule-to-coil transition of PNIPAM in water/methanol mixtures. 2. Computational Details 2.1. Validations of Force Field. The performance of MD simulations is largely dependent on the selection of force field. The polymer consistent force field (PCFF)48 was adopted in the present work. To validate the applicability of PCFF in describing the intermolecular interactions between NIPAM and solvent molecules, the potential energy curves of (C)O · · · HW and (N)H · · · OW in NIPAM-water interactions were tested in the frameworks of PCFF and quantum mechanism (QM) methods, respectively. The basis set superposition error (BSSE)49 was corrected by the counterpoise method50 in the QM calculations. The results are depicted in Figure 1. In addition, the potential energies for (C)O · · · HM, and (N)H · · · OM of NIPAM-methanol interactions were tested, too, with the results shown in Figure S1 of the Supporting Information. It can be seen that PCFF potentials are qualitatively consistent with the B3LYP/631+G** ones near the equilibrium locations of dimers, so a PCFF force field can give a reasonable description of the interaction between NIPAM and solvent molecules of both water and methanol. 2.2. MD Simulations. All molecular dynamics (MD) simulations were carried out by using the discover module in Materials Studio package51 in cubic boxes with periodic boundary conditions in three directions. The MD simulations were performed in an NVT ensemble at 293.15 K by using an Andersen thermostat52 with a time step of 1 fs applying the Verlet velocity algorithm.53 A cutoff of 12.5 Å was employed in the evaluation of Ewald sums54 for nonbonded interactions.

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Figure 1. Evolution of the interaction potentials of dimers of (a) (C)O · · · HW and (b) (N)H · · · OW as a function of the intermolecular distances, rO · · · H and rNH · · · O. The interaction energy was calculated as following: ∆E ) E dimer - (ENIPAM + EH2O). The data were obtained from B3LYP/6-31+G**, and PCFF calculations, respectively. The BSSE was corrected by the counterpoise method in the QM calculations.

TABLE 1: Simulation Details of the Number of Solvents Corresponding to a NIPAM Molecule, Length of the Simulation Box, And Density of Solutiona system

NH2O

NCH3OH

L (Å)

F (g/cm3)

WAT 17M 25M 50M 60M 80M MET

548 385 324 173 193 56 0

0 77 108 173 129 224 243

25.51 25.50 25.53 25.50 25.51 25.06 25.52

1.0 0.96 0.94 0.88 0.86 0.83 0.79

a

The concentration of solution is assumed to be 0.1 M.

The detailed information of the number of solvent molecules, cell parameters, and density are tabulated in Table 1. The concentration of systems was set to 0.1 M. Seven different solvent environments were investigated for NIPAM, including a NIPAM molecule solvated in water (labeled WAT), in water/methanol binary solvents with a methanol mole fraction of 0.17, 0.25, 0.50, 0.60, and 0.80 (17M, 25M, 50M, 60 M and 80M, respectively), and in methanol (MET). MD simulations (3 ns) were performed after the equilibrium stage, and the last 1 ns trajectories were collected every 100 fs for the statistical analysis of NIPAM-solvent interactions as well as the solvent-solvent interactions. The two ends of NIPAM were terminated with one hydrogen atom and one methyl group,55 respectively, and were fully optimized at B3LYP/6-31+G* level. The most stable conformation of NIPAM was picked out from numerous conformers (Table S1 of the Supporting Information). Figure S2 of the Supporting Information shows the radial distribution functions (RDFs) of (C)O · · · HW, (C)O · · · OW, and (N)H · · · OW obtained from two simulations performed of NIPAM in water. One contained a fully flexible solute and the other contained the NIPAM with fixed amide group. The nearly overlapping curves in Figure S2 show that freezing the geometry of the amide group

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Figure 2. Radial distribution functions: (a) (C)O · · · HW for carbonyl oxygen of NIPAM to water hydrogen; (b) (C)O · · · HM for carbonyl oxygen of NIPAM to methanol hydrogen; (c) (N)H · · · OW for amine hydrogen of NIPAM to water oxygen; and (d) (N)H · · · OM for amine hydrogen of NIPAM to methanol oxygen in water/methanol mixtures.

of NIPAM has little effect on the hydration structure around the OdC-N-H of NIPAM. A dihedral angle of OdC-N-H of fully flexible NIPAM was also monitored over 3 ns MD simulations in the aqueous solution. The results are shown in Figure S3 of the Supporting Information, in which the dihedral angle of OdC-N-H fluctuates around 180° throughout the simulation. Therefore, we take the fixed geometry of solute in our MD simulations in the following study of WAT, 17M, 25M, 50M, 60M, 80M, and MET, respectively. 3. Results and Discussion 3.1. Local Solute-Solvent Interactions. The local solvation environment around NIPAM in water/methanol mixtures was investigated by MD simulations. The interest here was focused on carbonyl oxygen and amine hydrogen atoms of NIPAM. The methyl groups are involved in much weaker C-H · · · OW and C-H · · · OM interactions, which are not discussed in this article. The local solute-solvent interactions according to the changes in components of binary solvents can be characterized by the radial distribution function (RDF), g(r), which displays the distribution of solvent molecules around solute (Figures 2a-d). By using a definition of hydrogen bonding using a set of geometric criteria: rO · · · O e 3.5 Å, rO · · · H e 2.6 Å, and ∠H-O · · · O e 30°56 (an analogous criterion was applied for the (N)H · · · O pair; see Figure S4 of the Supporting Information), we estimate distributions and numbers of hydrogen bonding between solute and solvents (Figures 3 and 5). For more insights into the three-dimensional local solvation structures of water and methanol around NIPAM, spatial distribution functions (SDFs)57 are also computed (Figure 4). NIPAM Interacts with SolWents: (C)O Ws (N)H. The ability of (C)O and (N)H groups of NIPAM to interact with solvent molecules is different, as revealed by the RDFs of (C)O and (N)H. There are much more structured solvent molecules around (C)O than around (N)H. The prominent first peak in (C)O · · · H RDFs (Figure 2a and b), with a maximum at ∼1.8 Å, evidently

Figure 3. The distribution of hydrogen bonding number per atom, P(n). (a) PCO(n): (C)O · · · H hydrogen bonding. (b) PNH(n): (N)H · · · O hydrogen bonding for NIPAM in water/methanol mixtures.

shows the hydrogen bonding between (C)O and neighboring solvents, although in the (N)H case, the first peak of (N)H · · · O RDFs (Figure 2c and d) is not as sharply defined as the corresponding (C)O · · · H pair and centers at about 2.3 Å, indicating relatively weaker interactions of (N)H · · · O. More interestingly, (C)O interacts with solvent molecules in the form of multihydrogen bonding, but (N)H forms singlehydrogen bonding with solvent molecules. The distributions of hydrogen bonding number per atom, P(n), where n is the number

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Figure 4. SDFs around carbonyl and amide groups of NIPAM: (a) oxygen (green) and hydrogen (white) atoms of water in WAT and (b) oxygen (red), hydrogen (white), and carbon (gray) atoms of methanol in MET. The oxygen and nitrogen atoms of NIPAM are shown in red and blue, respectively. The isosurface of SDFs is drawn at thresholds of 1.0.

J. Phys. Chem. B, Vol. 114, No. 26, 2010 8655 in the first solvation shell. The enhanced structure of water in the first solvation shell of (C)O group is accompanied by a decrease in the methanol structure, as illustrated by the RDFs in Figure 2: the first peak of (C)O · · · HW (Figure 2a) in any ratio mixed solvent is higher than that in WAT, whereas (C)O · · · HM (Figure 2b) stands in distinct contrast. Furthermore, the geometrical arrangement is given by spatial distribution functions (SDFs) drawn by the gOpenMol package58 in Figure 4. SDF gives the probability of finding an atom in the threedimensional space around a center molecule, in contrast to the average values given by RDF.59 The pictures in the three dimensional illustration clearly show that both water and methanol molecules are in spherelike distributions in the vicinity of the carbonyl group of NIPAM. However, the molecular orientation of methanol is more rigidly defined than that of water: methanol carbon, oxygen, and hydrogen atoms show almost isolated distributions from each other, whereas outer distributions of water hydrogen atoms share partly common space regions with that of water oxygen. This may result from the coexistence of (C)O · · · H-O-H and (C)O · · · H2-O interaction in water solvents. A quite different picture is presented in the solvent molecules around (N)H group. Both water and methanol are enriched significantly at concentrated solutions in the vicinity of the amine group of NIPAM, as shown in Figure 2c and d, respectively. Consistent with RDFs, the spatial distribution of the solvents in the vicinity of amine group of NIPAM is less than that around the (C)O group (Figure 4). Moreover, the outshoots of water oxygen distributions at both sides again suggest that the water orientation is not as well-defined as methanol around the (N)H group. The Weakening of NIPAM-SolWent Interaction in the Mixed SolWents. The total hydrogen bonding number, nHB, is calculated from PCO(n) and PNH(n). The contributions of hydrogen bonding of the NIPAM-water (HBW) and NIPAMmethanol (HBM) to nHB are also evaluated, as shown in Figure 5a. As expected, with increasing xmethanol, HBM increases, and HBW decreases, but the sum of HBM and HBW, that is, nHB, does not obey a linear dependence on xmethanol, as a theoretical expected (the dashed line shown in Figure 5a). The minimum of nHB nHB appears in the system of 50 M with a value of ∼1.5. The expected is calculated further by difference (∆n) of nHB and nHB following eq 1.

∆n )

Figure 5. (a) Number of hydrogen bonding between NIPAM and solvent molecules: 9, nHB; O, HBW; 4, HBM; the dashed line is nexpected . HB expected (b) The percentage of the difference between nHB and nHB with xmethanol increasing.

of hydrogen bonding per atom, are calculated and summarized in Figure 3. For (C)O, P(1) and P(2) are almost the same in water-rich solvents, whereas when xmethanol is increasing, P(1) increases significantly. On the other hand, it is notable that, P(0) distributes a considerable proportion in 25M, 50M, and 60M, but for (N)H, P(0) accounts for 30-50% throughout the entire methanol contents. Local SolWent Configurations: Water Ws Methanol. In addition to the different solvation behaviors of the functional groups of NIPAM, the two types of solvents perform differently

expected nHB - nHB expected nHB

× 100%

(1)

The data of ∆n are shown in Figure 5b, which indicate that the interactions between NIPAM and solvent molecules are weakened in the mixed solvents with xmethanol ranging from 0.25 to 0.80, and desolvation behavior evidently takes place. The maximum weakening effect reaches 23.2% in 50M. On the basis of a previous report,25 the reentrance of the globule-to-coil transition of PNIPAM in water/methanol mixtures occurred around xmethanol of 0.5. 3.2. Solvent-Solvent Interactions. As mentioned above, the abnormal solvation behaviors of NIPAM in water/methanol mixtures are induced just by the change of the component ratio of mixed solvents, so it is significant to survey the various configurations of solvents in different ratios. The postulation of the incomplete mixing of water and methanol on a microscopic scale is tested by RDFs (Figure 6). Three types of hydrogen-bonded clusters, including pure water clusters (CW-W),

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Figure 6. RDFs for (a) C · · · C, M · · · M (methyl hydrogen) and O · · · O of methanol molecules, (b) OW · · · OW, and (c) OM · · · OW in water/methanol mixtures.

Figure 7. Distribution of the hydrogen-bonded clusters in water/ methanol mixtures: 9, singleton (single water or methanol which has not hydrogen-bonded with other molecules); O, water hydrogen-bonded clusters; 4, methanol hydrogen-bonded clusters; and b, water/methanol hydrogen-bonded clusters in the systems of different xmethanol.

pure methanol clusters (CM-M), and water/methanol complexes clusters (CW-M) are studied in detail (Figures 7-10 and S5-6 of the Supporting Information). Microimmiscibility of Water and Methanol. The methanolmethanol correlation is given by gCC(r), gMM(r) (M represents methyl hydrogen here) and gOMOM(r) (Figure 6a), water/water correlation is given by gOWOW(r) (Figure 6b), and methanolwater correlation is given by gOMOW(r) (Figure 6c). The first peaks in gCC(r) and gMM(r) become lower as xmethanol increases, whereas, in contrast, the height of the first peak of gOMOM(r) increases (Figure 6a). The curves confirm that as methanol molecules are added into the mixture, they tend to aggregate with each other by pushing methyl groups slightly farther apart and drawing hydroxyl groups closer together. It is propitious to the formation of hydrogen-bonded methanol clusters. Furthermore, enriched local solvent structures of water/water and water/methanol clusters are suggested in mixed solvents, since the first peaks of both gOWOW(r) (Figure

6b) and gOMOW(r) (Figure 6c) become higher with an increase in the methanol content. The curves in Figure 6 provide evidence for the microimmiscibility of water and methanol in the solutions. Our results for NIPAM solvation systems agree well with the findings of previous studies31,32 on a 7:3 mol fraction methanol aqueous solution. Clusters are formed among solvent molecules via hydrogen bonding, such as water clusters, methanol clusters and water/methanol complex clusters. Abundance of Water/Methanol Clusters. Distribution of hydrogen-bonded clusters among mixed solvents was studied further. The same geometric criteria of hydrogen bonding of rO · · · O e 3.5 Å, rO · · · H e 2.6 Å, and ∠H-O · · · O e 30° were adopted. Solvent molecules that cannot form hydrogen bonding with others are recorded as singletons. The distributions of various solvent clusters in water/methanol mixtures with different xmethanol are presented in Figure 7. Water-methanol complex clusters (CW-M) are found to be abundant in the mixtures. For convenience, the formula of CW-M is written as (H2O)m(CH3OH)n, and the values of n/(m + n) are calculated and plotted to xmethanol, as shown in Figure S5 of the Supporting Information. It is found that the values of n/(m + n) correlate well with xmethanol, which indicates that the components of the water/methanol clusters change linearly with an increase in the methanol content. The clustering of (H2O)m(CH3OH)n can be clearly seen in snapshots taken from the simulation boxes of 17M, 25M, 50M, 60M, and 80M, respectively, as shown in Figure 8. One may notice that the cluster changes from a polyhedral arrangement (Figure 8a) to a chainlike structure (Figure 8e) as the methanol content increases. This is not surprising because a water molecule has one more proton than a methanol molecule, thus allowing a larger possibility to form intermolecular hydrogen

N-Isopropylacrylamide in Water/Methanol Mixtures

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Figure 10. Distributions of nonbranched, chainlike hydrogen-bonded methanol clusters in MET. The sum of all the data points in this figure is 68%. Some snapshots were taken out from the simulation. Blue dashed lines are denoted hydrogen bonding.

Figure 8. Snapshots of some water/methanol hydrogen-bonded clusters of (H2O)m(CH3OH)n that were taken from MD simulations of systems of 17M, 25M, 50M, 60M, and 80M. Gray lines represent water molecules, sticks represent methanol O-H bonds (oxygen in red and hydrogen in white), and dashed lines indicate the presence of a hydrogen bonding between neighboring molecules.

Figure 9. Distributions of O-O-O angles for hydrogen-bonded water trimers in systems of WAT, 17M, 25M, 50M, 60M, and 80M. The O-O-O angles are formed with a central water molecule as the vertex and the two nearest oxygen atoms belonging to two other water molecules that are both hydrogen-bonded to the vertex molecule.

bonding. In a methanol-rich solution, the local structure very closely resembles that found in pure methanol.60 Water Hydrogen-Bonded Network. Though the amount of CW-W decreases evidently with an increase in the methanol content, the hydrogen-bonded network of water clusters is not essentially disrupted. This can be seen from the distributions of the O-O-O angle of hydrogen-bonded water trimers, as shown in Figure 9. The O-O-O angle is formed with a central water molecule as the vertex and the two nearest oxygen atoms belonging to two other water molecules that are both hydrogenbonded to the vertex molecule.61 Interestingly, the distributions of the O-O-O angle of water are little affected by the addition of methanol. The O-O-O angle ranges from 40° to 180°, and the most probable distribution is at ∼100° (close to the tetrahedral angle of about 109.5°), which indicates that even in

methanol-rich solutions, water hydrogen-bonded clusters still exist and are stable enough. Short Chainlike Methanol Clusters. Consistent with previous studies,62 methanol molecules tend to form chainlike polymeric units. The distribution of nonbranched chainlike methanol clusters (mc%), with their units ranging from 1 to 10, is statistically calculated in MET, presented in Figure 10. Here, mc% is calculated using eq 2:

mc% )

units × nmethanol-clusters × 100% nmethanol

(2)

In eq 2, units represents the size of the nonbranched chainlike methanol clusters, ranging from 1 to 10; nmethanol-clusters is the number of nonbranched chainlike methanol clusters; and nmethanol is the total number of methanol molecules. Surprisingly, a sum up to ∼68% methanol molecules favors formation of short, nonbranched, chainlike clusters. The selected snapshots of methanol clusters in Figure 10 illuminate well the characteristics of hydrogen-bonding. Scarcely any ring structures are found in MET, which seems inconsistent with the findings of Allison et al.,33 who claim that ring structures are a numerous in methanol solution. The conflict may be partly a result of the different criteria of hydrogen bonding that were adopted. They solely used the distance criterion of rO · · · O e 3.5 Å. In the present work, we think the bond angle also plays an important role in identifying the hydrogen bonding. Table S2 of the Supporting Information shows the molecular numbers of methanol singletons in WAT, 17M, 25M, 50M, 60M, 80M, and MET calculated by using these two different geometric criteria. As seen from Table S2, the stricter criteria using both distance and bond angle show a distinctly larger distribution of methanol singletons in the systems. Furthermore, methanol clusters are easily broken into shorter chains as xmethanol decreases. As shown in Figure S6 of the Supporting Information, methanol clusters of chain units longer than three are rare in water-rich mixtures. 4. Conclusions In this work, solvation behaviors of a monomer of PNIPAM in water/methanol mixtures with various component ratios are studied by molecular dynamics simulations. It is found that the desolvation process of NIPAM will take place in the systems with methanol contents ranging from 0.25 to 0.80, in which the number of hydrogen bonding between NIPAM and solvent molecules evidently decreases. Detailed analyses of the various

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configurations of solvents reveal that water/methanol clusters are abundant in solutions in the same range of xmethanol, which results in the desolvation of NIPAM and leads to the collapse of the PNIPAM chain. Acknowledgment. Financial support from the National Science Foundation of China (NSFC) Project (Grants 50633030 and 20825312). Part of the computer resources were provided by the Institute of Theoretical and Computational Chemistry of Nanjing University. Supporting Information Available: The energy and geometrical parameters of conformers of NIPAM optimized at the B3LYP/6-31+G(d) level (Table S1); the comparison of molecular numbers of methanol singletons in WAT, 17M, 25M, 50M, 60M, 80M, and MET by using different geometric criteria (Table S2); evolution of the interaction potentials between solute and solvent as a function of the intermolecular distances (Figure S1); radial distribution functions obtained from the two simulations of NIPAM in WAT, one containing a fully flexible solute and the other containing the NIPAM of a fixed amide group (Figure S2); distribution of dihedral angle of OdC-N-H of a fully flexible NIPAM that was monitored over 3 ns MD simulations in the aqueous solution (Figure S3); sketch of the geometric criteria of hydrogen bonding between NIPAM and solvent molecules (Figure S4); proportion of methanol in the water/methanol hydrogen-bonded clusters of (H2O)m(CH3OH)n (Figure S5); and percentage of methanol that forms nonbranched linear clusters with other methanol molecules via hydrogen bonding (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wu, C.; Wang, X. Phys. ReV. Lett. 1998, 80, 4092. (b) Wu, C.; Qiu, X. Phys. ReV. Lett. 1998, 80, 620. (2) Cheng, R. S.; Yang, H.; Yan, X.; Wang, Z. L.; Li, L. Chem. J. Chin. UniV. 2001, 22, 1262. (3) Yang, H.; Cheng, R. S.; Xie, H. F.; Wang, Z. L. Polymer 2005, 46, 7557. (4) Baysal, B. M.; Karasz, F. E. Macromol. Theory Simul. 2003, 12, 627. (5) Scarpa, J. S.; Mueller, D. D.; Klotz, I. M. J. Am. Chem. Soc. 1967, 89, 6024. (6) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (7) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154. (8) Dhara, D.; Chatterji, P. R. J.M.S.-ReV. Macromol. Chem. Phys. 2000, C40, 51. (9) Li, M.; Jiang, M.; Zhang, Y.; Fang, Q. Macromolecules 1997, 30, 470. (10) Zhang, Y. B.; Li, M.; Fang, Q.; Zhang, Y. X.; Jiang, M.; Wu, C. Macromolecules 1998, 31, 2527. (11) Wu, C.; Zhou, S. Q. Macromolecules 1996, 29, 1574. (12) Zhou, K. J.; Lu, Y. J.; Li, J. F.; Shen, L.; Zhang, G. Z.; Xie, Z. W.; Wu, C. Macromolecules 2008, 41, 8927. (13) Ye, X. D.; Lu, Y. J.; Shen, L.; Ding, Y. W.; Liu, S. L.; Zhang, G. Z.; Wu, C. Macromolecules 2007, 40, 4750. (14) Qiu, X. P.; Tanaka, F.; Winnik, F. M. Macromolecules 2007, 40, 7069. (15) Winnik, F. M. Macromolecules 1990, 23, 233. (16) Kujawa, P.; Winnik, F. M. Macromolecules 2001, 34, 4130.

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