Comprehensive Computational and Experimental Analysis of

Apr 25, 2017 - To provide insights into the aggregation behavior, hydration tendency and variation in phase transition temperature produced by the add...
2 downloads 22 Views 5MB Size
Download PDF
Article pubs.acs.org/JPCB

Comprehensive Computational and Experimental Analysis of Biomaterial toward the Behavior of Imidazolium-Based Ionic Liquids: An Interplay between Hydrophilic and Hydrophobic Interactions Reddicherla Umapathi,† Suresh B. Vepuri,‡,§ Pannuru Venkatesu,*,† and Mahmoud E. Soliman§ †

Department of Chemistry, University of Delhi, Delhi 110007, India K L College of Pharmacy, K L University, Guntur 522 502, India § Discipline of Pharmaceutical Sciences, School of Health Sciences, University of KwaZulu-Natal (UKZN), Westville Campus, Durban 4000, South Africa ‡

S Supporting Information *

ABSTRACT: To provide insights into the aggregation behavior, hydration tendency and variation in phase transition temperature produced by the addition of ionic liquids (ILs) to poly(N-isopropylacrylamide) (PNIPAM) aqueous solution, systematic physicochemical studies, and molecular dynamic simulations were carried out. The influence of ILs possessing the same [Cl]− anion and a set of cations [Cnmim]+ with increasing alkyl chain length such as 1-ethyl-3-methylimidazolium ([Emim]+), 1-allyl-3-methylimidazolium ([Amim]+), 1butyl-3-methylimidazolium ([Bmim]+), 1-hexyl-3-methylimidazolium ([Hmim] + ), 1-benzyl-3-methylimidazolium ([Bzmim]+), and 1-decyl-3-methylimidazolium ([Dmim]+) on the phase transition of PNIPAM was monitored by the aid of UV−visible absorption spectra, fluorescence intensity spectra, viscosity (η), dynamic light scattering (DLS), and Fourier transform infrared (FTIR) spectroscopy. Furthermore, to interpret the direct images and surface morphologies of the PNIPAM−IL aggregates, we performed field emission scanning electron microscopy (FESEM). The overall specific ranking of ILs in preserving the hydration layer around the PNIPAM aqueous solution was [Emim][Cl] > [Amim][Cl] > [Bmim][Cl] > [Hmim][Cl] > [Bzmim][Cl] > [Dmim][Cl]. Moreover, to investigate the molecular mechanism behind the change in the lower critical solution temperature (LCST) of the polymer in the presence of the ILs, a molecular dynamics (MD) study was performed. The MD simulation has clearly shown the reduction in hydration shell of the polymer after interacting with the ILs at their respective LCST. MD study revealed significant changes in polymer conformation because of IL interactions and strongly supports the experimental observation of polymer phase transition at a temperature lower than typical LCST for all the studied ILs. The driving force for concomitant sharp configurational transition has been attributed to the displacement of water molecules on the polymer surface by the ILs because of their hydrophobic interaction with the polymer.

■

INTRODUCTION Interest in the coil-to-globule transition behavior of thermoresponsive smart polymers such as poly(N-isopropylacrylamide) (PNIPAM), poly(vinylmethyether) (PVME), and poly(Nvinylcaprolactam) (PVCL) originates largely from its biocompatibility, biodegradability, and low toxicity, as they enable the development of more and more complex responsive materials.1−6 Thermoresponsive polymers are well-known and widely studied because of their wide usage in different fields such as protein folding, cardiac repair, bioseparation and biosensors surface modification, peptide separation, protein drug release, biomolecule separations, drug delivery, sensors, biotechnology, enzyme immobilization, tumor-targeting, drug transport, DNA packing, drug release, interchain complexation, and tissue engineering.7−21 These pivotal findings have prompted researchers from different scientific fields to © 2017 American Chemical Society

investigate the phase behavior of thermoresponsive polymers by a variety of biophysical techniques.22−25 The phase transition temperature of thermoresponsive polymers has been divided into two types based on their solubility: the lower critical solution temperature (LCST), which marks the point when the hydrated polymer structure starts to collapse, and the upper critical solution temperature (UCST), at which the solubility of the thermoresponsive polymers increases.1,26 The LCST is the most commonly studied phenomenon, since all water-soluble polymers with intermediate hydrophilicity exhibit a phase transition at a certain temperature. Naturally, PNIPAM is a widely used Received: March 8, 2017 Revised: April 24, 2017 Published: April 25, 2017 4909

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

[CH3COO]−, and [HSO4]− on the hydration layer of PNIPAM aqueous solution and found that the decrease in phase transition temperature is more pronounced in the case of kosmotropes than that of chaotropes at all studied concentrations.23 Several molecular modeling studies at the atomistic level have been employed to understand the phase transition of PNIPAM using various force fields such as OPLS, PCFF, AMBER-94, and DRIEDING force field.54−57 Molecular simulation studies of PNIPAM using 3, 5, 10, 30, and 50 monomers were previously reported.54−56 30- and 50-monomer PNIPAM molecules were found to show significant change in conformation at the LCST during phase transition, indicating these as ideal lengths for the polymer modeling.55,56 However, simulation of a single polymer molecule was shown to be less affected by the temperature changes than in the presence of multiple molecules.55 The radius of gyration (Rg) and polymer end-to-end distance were used to estimate the folding phenomena and conformational changes during phase transition.54−57 Changes in the number of water molecules in the first hydration shell and the number of polymer hydrogen bonds with these water molecules as a consequence to the LCST are also estimated to correlate with experimental phase transition observation.54,56 A mesoscopic study using a dissipative particle dynamics approach was used to evaluate the phase transition of several polymers in various ionic liquids such as [BMIM][PF6],58,59 [C2MIM][NTf2],60 and [EMIM][TFSI].61 Apparently, studies of effects of with different cations (with increasing alkyl chain length of cation) with a fixed anion of ILs on the phase transition of thermoresponsive polymers are lacking in the literature. Moreover, no conclusive experimental and molecular dynamic simulation results have explored the influence of imidazolium-based ILs on the LCST of PNIPAM. In addition, significantly less knowledge is available on the concentration mediated aggregation process of polymers by the addition of ILs. Therefore, a deeper understanding of the mechanism governing the aggregation of polymer in ILs is required. Furthermore, an adequate knowledge of the phase behavior of thermoresponsive polymers with imidazolium-based ILs with different cations to explore the Hofmeister series effects on the LCST of PNIPAM is essentially required to clarify the nature of molecular interactions and to design and engineer new biophysical, biomedical, biochemical, and biotechnological processes. In this connection, we have put our efforts into understanding the effects of imidazolium-based ILs with the same Cl− anion and a set of cations [Cnmim]+ with increasing alkyl chain length such as 1-ethyl-3-methylimidazolium ([Emim]+), 1-allyl3-methylimidazolium ([Amim]+), 1-butyl-3-methylimidazolium ([Bmim]+), 1-hexyl-3-methylimidazolium ([Hmim]+), 1-benzyl-3-methylimidazolium ([Bzmim]+), and 1-decyl-3-methylimidazolium ([Dmim]+) on the phase transition temperature of PNIPAM aqueous solution. The influence of the phase transition process of PNIPAM in the presence of different ILs is elucidated by UV−visible absorption spectra, fluorescence intensity spectra, viscosity (η), dynamic light scattering (DLS), and Fourier transform infrared (FTIR) spectroscopy analysis. Further, to achieve the morphological visualization of these systems, we employed field emission scanning electron microscopy (FESEM). We also look into the consequences of the Hofmeister series effects on the LCST of PNIPAM. Furthermore, we present a comprehensive computational work to study the conformational behavior of PNIPAM in IL

thermoresponsive smart polymer that displays a LCST of 33 °C, and this LCST falls within the range of the human body temperature. Obviously, this advantage is offering PNIPAM highly suitable biomaterial for a variety of biomedical applications. The phase transition temperature of thermoresponsive polymers which can be tuned by copolymerization or by adding various additives has been studied extensively by both experiments27−31 and computer simulations.28,32−36 The investigations related to the interactions between polymers and ionic liquids (ILs) are comparatively more important as compared to those of other additives because of their excellent advantages in polymer science.4 The main driving force for the interaction of these biomolecules is their large surface area to mass ratio.1,3,4 On the other hand, the phase transition and swelling/deswelling behavior are also controlled by the introduction of charged groups (e.g., carboxyl, sulfonic, and an amino group) to PNIPAM microgel networks. For instance, the results of Chang et al.37 and Sun et al.38 indicate that the volume phase transition temperature (VPTT) of poly(Nisopropylacrylamide-co-acrylic acid) hydrogel is slightly higher than that of PNIPAM. Moreover, the charge density within the PNIPAM may affect the VPTT of P(NIPAM-co-MAA) microgel and also its dynamic or rheological properties.39−42 The phase transition temperature of polymer in combination with ILs has been closely investigated for membrane applications and designing new polymeric smart materials, owing to the versatile structure designability and unique properties of ILs. The scope and utility of ILs with polymers is an important area of ongoing investigation for the design of novel classes of catalytic membranes,43 organic thin film transistors,44 gas separation membranes,45 solid-state electrolytes,46,47 supported catalyst,48 polymer electrolyte membrane,49 dye-sensitized solar cells,50,51 lithium ion batteries,52 and metal ion removal.53 The solidification of polymer can be properly tuned in the presence of ILs by the possibility of selection of different cations and anions.4 Imidazolium-based ILs are able to form hydrogen bonds with the functional groups present in the polymer. In view of the growing importance of advanced applications of ILs in polymer science4 and in versatile scientific fields, it is desirable to summarize the effect of ILs on the hydration behavior of thermoresponsive polymers. After the exhaustive literature survey, it was revealed that only very few researchers have reported phase transition studies of polymers with imidazolium-based ILs.20,22−24 For instance, in 2011, Reddy et al.20 investigated the effect of 1benzyl-3-methylimidazolium tetrafluoroborate ([Bzmim][BF4]) on the LCST of PNIPAM. Later, in 2012, Wang et al.22 reported the role of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) on the phase transition behavior of PNIPAM. The experimental results from Reddy et al.20 and Wang et al.22 elucidated that the LCST of PNIPAM is decreased due to hydrophobic collapse/aggregation and preferential interactions in both cases. Interestingly, the LCST values of PNIPAM disappear due to the formation of a stable interaction network via intra- and intermolecular hydrogen bonding at higher concentrations of [Bmim][BF4].20 Irrespective of their different cations, they showed the same effect on the LCST of PNIPAM; however, the degree of variation in LCST is observed to be significantly more for [Bmim][BF4] as compared to [Bzmim][BF4]. In 2014, we examined the influence of ILs containing the same cation, 1butyl-3-methylimidazolium [Bmim]+, and commonly used anions such as [SCN] − , [BF 4 ] − , [I] − , [Br] − , [Cl] − , 4910

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B solution below and above the LCST. The present finding can pave the way for selection of ILs for the tailoring of the thermoresponsive behavior of stimuli responsive polymers.

the simulation studies, as this length was reported to be ideal for studying the polymer phase transition. The effect of IL interaction on PNIPAM structure at the LCST was also studied using MD simulation. All of the simulation studies were performed using Biovia’s Materials Studio (MS) software. The forcite module in MS was employed for geometry optimization and dynamics study. The COMPASS force field was used for the energy calculations throughout the simulations. For MD simulation, the polymer structure in extended chain form was placed in a cubic box with dimensions 150 × 150 × 150 Å3. To study the IL effect, the molecular structures of [Emim][Cl], [Bmim][Cl], [Hmim][Cl], [Dmim][Cl], [Amim][Cl], and [Bzmim][Cl] were included in separate studies. The structure was then solvated by adding water molecules at a density of 1 g· cm−3. The packing protocol of the amorphous module in MS was used to produce a solvated polymer and polymer−IL system with optimized structure placements and cell geometry. Prior to MD simulation, all of the solvated systems were energy minimized with both the steepest descent and conjugate gradient algorithms (the maximum number of steps and the energy step size of the two kinds of energy minimization were set to 5,00,000 and 0.01 nm). Afterward, 100 ps of NVT equilibrium and 200 ps of NPT equilibrium were conducted sequentially. A productive MD simulation was then carried out on the above equilibrated system in the NPT ensemble. The time step was set to 1 fs in all of the MD simulations. The temperature and pressure were controlled by using a Nosé thermostat and Benderson barostat, respectively. Periodic boundary conditions were used in all dimensions, and the long-range electrostatic interactions were treated using the particle mesh Ewald (PME) method with a vDW cutoff (12 Å). The total simulation time was 20 ns, and the coordinates were saved every 1 ps interval. The trajectories were visualized and analyzed to study the conformational changes and molecular interactions. The phenomena of phase transition in terms of structural dynamics in PNIPAM were evaluated by studying the trajectory of conformations obtained in production dynamics. Coil to globule transformation of polymer structure during phase transition was estimated by measuring the radius of gyration (Rg) over a trajectory of polymer conformations obtained in a production run. The Rg was calculated as the arithmetic mean over the trajectory together with its standard deviation. Changes in polymer solvation structure during phase transition were determined in terms of the number of water molecules (Nw) in the first hydration shell of the polymer. The first hydration shell was defined as the layer encompassing the residues at a radial distance of 3.5 Å from the polymer atoms. The phenomena of intramolecular hydrogen bonds during polymer collapse were also evaluated as an estimate of polymer folding at the LCST. The simulations were performed on a remote high-performance computer at the Centre for High Performance Computing (CHPC) in Cape Town (South Africa).

■

EXPERIMENTAL SECTION Materials. The PNIPAM (Mn = 20000−25000), ANS, and six ILs, [Emim][Cl] (≥98.0%), [Amim][Cl] (≥97.0%), [Bmim][Cl] (≥99.0%), [Hmim][Cl] (≥97.0%), [Bzmim][Cl] (97.0%), and [Dmim][Cl] (≥97.0%), were all purchased from Sigma-Aldrich Chemical Co. and used without further purification. Preparation of the PNIPAM in the Absence and Presence of Ionic Liquid Samples. All of the samples in the present study were prepared in distilled deionized water with a resistivity of 18.3 Ω cm. All of the sample mixtures were filtered with a 0.22 μm disposal filter (Millipore, Millex-GS) through a syringe before being used for the experiments and were incubated for a few hours at 25 °C in order to obtain complete dissolution before performing experiments. The final polymer concentration for all measurements was 7 mg/mL. The weighed amounts of ILs at various concentrations (5, 10, and 15 mg/mL) were added directly to the aqueous polymer solution. The concentration of the external probe used for the UV−vis absorption and fluorescence study was kept at 2 × 10−5 M to avoid the probe interference in the measurements. The sample solutions were stored in a cool place and kept in a container tightly to prevent water absorption. Instrumentation and Measurements. Ultraviolet−visible absorption spectra of 1,8-anilinonapthalene sulfonic acid (ANS) in aqueous PNIPAM solution in the absence and in the presence of ILs were recorded from 250 to 450 nm by means of a double beam UV−visible spectrophotometer (UV-1800, Shimadzu Co., Japan) at room temperature. Fluorescence intensity measurements of the sample solutions were carried out using a Cary Eclipse fluorescence spectrophotometer (Varian optical spectroscopy instruments, Mulgrave, Victoria, Australia) with an intense xenon flash lamp as a light source. The hydrodynamic radius (Rh) of all samples was measured by using dynamic light scattering (DLS), Zetasizer Nano ZS90 (Malvern Instruments Ltd., U.K.), equipped with He−Ne (4 mW, 632.8 nm). The viscosities (η) of all of the samples are measured using a sine-wave vibro viscometer (model SV-10, A&D Company Limited, Japan) with an uncertainty of 1%. The η of sample solutions was collected in the wide temperature range by using a circulating temperature control water bath (LAUDA alpha 6, Japan) with an accuracy of temperature of ±0.02 K. The Fourier transform infrared (FTIR) spectrum was recorded on an iS 50 FT-IR (Thermo-Fisher scientific) spectrometer. The bubble-free samples were placed into an IR cell with two ZnSe windows. A chromel−alumel K-type thermocouple was provided for continuous monitoring of the temperature inside the sample chamber. Each IR spectrum reported here was an average of 200 scans using a spectral resolution of 4 cm−1. Field emission scanning electron microscopy (FESEM) studies were carried out using a MIRA3 TESCAN electron microscope operating at 10 kV. All of the reported values are an average of three measurements of the sample. The detailed information on instrumentation and measurements used in the present study has been delineated in our previous papers.23,55 Molecular Dynamics Study. Molecular dynamics simulation was performed on an atomistic model of a PNIPAM structure below and above the LCST. A 35-mer was selected for

■

RESULTS AND DISCUSSION UV−Visible Spectroscopic Analysis of the Phase Transition Behavior of PNIPAM in the Presence of Imidazolium-Based ILs. To illustrate the effect of ILs on the phase transition behavior of PNIPAM, we initially employed UV−visible spectroscopy, which is a powerful tool to study the changes in absorption and emission bands of polymers. The literature reveals that the changes in the hydrated structure of the polymer are followed with the shift in the wavelength in the 4911

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B near-UV region.23 The absorption spectra of 1,8-anilinonapthalene sulfonic acid (ANS) in aqueous solutions containing PNIPAM in the presence of 5 mg mL−1 concentration of various imidazolium-based ILs with increasing alkyl chain length of cation at room temperature are displayed in Figure 1.

Figure 2. Steady state fluorescence emission spectra of ANS in PNIPAM aqueous solution without and with ILs {IL free (black line), [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line), and [Dmim][Cl] (yellow line)}, at 25 °C. The concentration of ILs is 5 mg/mL.

wavelength range, indicating the absence of a hydrophobic environment for ANS molecules. However, a marked enhancement in intensity was observed with the blue shift in λmax, upon the addition of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl]. This appreciable change in the intensity of ANS with PNIPAM and with the addition of these ILs implies the formation of the hydrophobic microenvironment from the collapse of the hydrated structure of PNIPAM. However, the extent of change in emission intensity was strongly influenced by the type of the cation of IL. A similar type of enhancement in intensity of ANS in PNIPAM with sodium cholate (NaC) has been observed during the hydrophobic collapse of polymer.25 The maximum intensity with a hypsochromic shift was observed with the addition of [Dmim][Cl], while the minimum intensity with hypsochromic shift was observed for [Emim][Cl]. To ascertain the effect of IL concentration on the phase transition behavior of PNIPAM aqueous solution, the higher concentrations of ILs (10 and 15 mg/mL) are also studied and the obtained results are provided in Figures S3 and S4. As shown in Figures S3 and S4, the fluorescence intensity was further increased with increasing concentration of ILs. The characteristic differences in intensities, as shown in Figure 2, were expected as a result of the differences in the interactions between the polymer chain and ions of ILs. Figure 2 explicitly elucidates that the addition of ILs with increasing alkyl chain length of cation to the PNIPAM aqueous solution shows varied behavior irrespective of their same anions and different cations. The steady state fluorescence results elucidate the IL-induced structural changes of PNIPAM through the changes in ANS fluorescence intensities. Additionally, we have measured the temperature dependent fluorescence intensities of PNIPAM aqueous solution in the absence and in the presence of ILs as a function of IL concentration. In Figure 3, we have summarized the fluorescence emission spectra of PNIPAM aqueous solution with and without ILs at 5 mg/mL IL concentration as a function of temperature. Moreover, the modulations in the LCST have been varied to different extents with the increase in the concentration of ILs from 5 to 10 and 15 mg mL−1, as shown in Figures S5 and S6. As shown in Figure 3, there is no increase in intensity until ∼33.0 °C, which indicates that the

Figure 1. UV−visible absorbance spectra of ANS in PNIPAM aqueous solution without and with ILs {IL free (black line), [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line)}, and [Dmim][Cl] (yellow line)}, at 25 °C. The concentration of ILs is 5 mg/mL.

The two main characteristics of the absorption spectra peaks are distinctly observable from Figure 1 at λmax 270 and 370 nm.62 As shown in Figure 1, the absorption of ANS in a polymer aqueous solution is minimal in the absence of ILs; however, the ANS absorption is increased and eventually reaches a maximum with the addition of 5 mg/mL concentration of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl]. This appreciable increase in absorbance indicates the changes in the environment of ANS contributed by the change in hydration states of PNIPAM with the addition of ILs. It is evident from Figure 1 that the absorbance was being altered significantly from one IL to another IL depending on the alkyl chain length of the cation. The absorbance was further increased with increasing concentration of ILs (10 and 15 mg mL−1), as delineated in Figures S1 and S2, which is a direct result of the increasing conformational changes of PNIPAM at the higher concentration of ILs. Fluorescence Spectroscopic Analysis of the Phase Transition Behavior of PNIPAM in the Presence of Imidazolium-Based ILs. ANS was used as an extrinsic fluorescent probe in the current study. The concentration of ANS was taken as 2 × 10−5 M so that there would be a negligible effect of the probe on the PNIPAM aggregation process. ANS shows either red or blue shift depending upon the increase or decrease in local polarity and mobility. Steadystate fluorescence spectroscopy was used to see the changes occurring in the hydration layer of PNIPAM aqueous solution in the presence of the ILs. The fluorescence spectra of ANS in PNIPAM aqueous solution as well as in the presence of 5 mg mL−1 IL concentration at 25 °C with 360 nm excitation wavelength have been presented in Figure 2. From Figure 2, it is evident that ANS in PNIPAM aqueous solution in the absence of ILs has exhibited relatively less intensity in the entire 4912

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

significantly larger on the PNIPAM collapse state. From Figures 3, S5, and S6, the results show that the ILs [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl] behave as “destroyers” for the hydration layer of PNIPAM aqueous solution at all studied concentrations by changing the well swollen PNIPAM to collapse into a shrunken structure by a “salting out” mechanism. Dynamic Light Scattering Analysis of the Phase Transition Behavior of PNIPAM in the Presence of Imidazolium-Based ILs. To confirm the effect of ILs on the phase transition behavior of PNIPAM aqueous solution, DLS measurements were performed as a function of temperature. The hydrodynamic diameter (dH) of molecular aggregates is known as a fingerprint for detecting the changes in hydration behavior of PNIPAM in an aqueous medium.23,24,63 Figure 4

Figure 3. Temperature dependent fluorescence emission spectra of ANS in PNIPAM aqueous solution without and with ILs {IL free (black line), [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line), and [Dmim][Cl] (yellow line)}, in the temperature range 25−40 °C. The concentration of ILs is 5 mg/mL.

polymer remains in a coil conformation; particularly monomers are interlinked with the water molecules. Further heating of this solution leads to a sudden enhancement in intensity, indicating the disruption of hydrogen bonds between monomers of polymer and water molecules which results in hydrophobic collapse of PNIPAM chains. After experiencing the hydrophobic collapse process at the LCST, PNIPAM chains can supply the hydrophobic environment for ANS molecules and thereby reduce the mobility, and finally lead to the enhancement of intensity. In Figure 3, we further noticed the change in the LCST with the addition of ILs. Obviously, a sudden change in fluorescence intensity was shifted toward lower temperatures from 33.0 °C (IL free aqueous solution) to 32.5, 32.0, 31.7, 31.0, 30.6, and 30.2 °C, upon addition of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl], respectively. The extent of change in the LCST is in agreement with the normal Hofmeister series from [Emim]+, [Bmim]+, [Hmim]+, to [Dmim]+. The results further show that ILs depress the LCST to different extents depending upon their alkyl chain length of cation and ability to break hydrogen bonds between monomers of polymer and water molecules. This ensures the intensity enhancement is due to the polymer aggregation but not due to the interaction of ILs and probe. To confirm this phenomenon, we have further performed the temperature dependent fluorescence spectroscopy measurements for ANS in aqueous ILs without PNIPAM over a temperature range of 20−50 °C. The obtained results are presented in Figure S7. The results in Figure S7 show that the PNIPAM is solely responsible for the aggregated state of polymer. The results in Figure 3 explicitly explained that the fluorescence intensity of PNIPAM was found to increase sharply in the presence of ILs even at low temperatures. This change in fluorescence intensity in response to ILs might be due to the difference in the behavior of ILs on the hydrated state of PNIPAM. Here, ILs not only depleted water molecules from the vicinity of polar amide groups but also from the apolar isopropyl moieties. It can be seen from Figure 3 that the effect of [Emim]+ on the hydrophobic collapse of PNIPAM is rather small and does not contribute significantly to PNIPAM collapse, whereas the larger cation [Dmim]+ contributes

Figure 4. Hydrodynamic diameter, dH, of PNIPAM in the absence and presence of ILs {IL free (black line), [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line), and [Dmim][Cl] (yellow line)}, in the temperature range 25−40 °C. The concentration of ILs is 5 mg/ mL.

explicitly uncovers the effect of ILs on the dH of PNIPAM aqueous solution at a concentration of 5 mg mL−1. The DLS intensity distribution graphs of PNIPAM at higher concentrations (10 and 15 mg/mL) of ILs have been shown in the Supporting Information (Figures S8 and S9). As shown in Figure 4, there is no substantial change in dH until 33 °C; however, a dramatic increase in the dH value was observed for PNIPAM in aqueous solution at 33 °C, which reflects the enhancement of the collapsed state and this point is called the cloud point, phase transition temperature, or LCST. This dramatic increase in dH at the LCST indicates the aggregation of polymer chains resulting from dehydration upon heating. This aggregation of the polymer chains results in the transformation of a transparent solution into a milky and opaque solution at the LCST. Furthermore, with the addition of ILs to PNIPAM aqueous solution, we observed a noticeable change in dH at 32.5, 32.0, 31.7, 31.0, 30.6, and 30.2 °C in the presence of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl], respectively. This indicates that the process of altering hydrogen bonds between the amide group of the polymer and water molecules is significantly prompted by the counterions (cations) of IL. This appreciable effect has been more pronounced to different 4913

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

summarized in Figure 5. The decrease in η value clearly reveals that the IL presumably exerts its effect on the destruction of hydrogen bonds between polymer and water molecules and tends to lead to conformational changes in PNIPAM. However, the substantial modulations in the decrease of η with the addition of 5 mg mL−1 concentration of imidazolium family ILs were found approximately at 32.5, 32.0, 31.7, 31.0, 30.6, and 30.2 °C in the presence of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl], respectively. Further, with the addition of 10 and 15 mg mL−1 ILs to PNIPAM aqueous solution, the modulations have been varied to different extents with further diminishment in η values, as shown in Figures S10 and S11. This clearly specifies that the majority of the PNIPAM molecules underwent conformational changes which leads to the formation of turbidity. The influence of ILs on the temperature dependence of PNIPAM displays a pattern very similar to Figures 3 and 4 of fluorescence and DLS results. The same family of ILs with different alkyl chain lengths is showing modulations in the phase transition temperature of PNIPAM to different extents; η values of PNIPAM with the addition of ILs decreased with the increase in the alkyl chain length of the cation. By analyzing Figures 5, S10, and S11, it is revealed that the modulations in the LCST are closely related to their structures. The imidazolium-based ILs decreased the LCST of PNIPAM in the following order: [Emim][Cl] > [Bmim][Cl] > [Hmim][Cl] > [Dmim][Cl], which is consistent with the Hofmeister series of cations. On the other hand, the overall specific ranking of ILs with increasing alkyl chain length of cations in decreasing the LCST of PNIPAM follows the order of [Emim][Cl] > [Amim][Cl] > [Bmim][Cl] > [Hmim][Cl] > [Bzmim][Cl] > [Dmim][Cl]. The maximum and minimum extent of reduction in η values was observed in the presence of [Emim][Cl] and [Dmim][Cl], respectively. The above analysis clearly indicates that the structure of ILs influences the η and phase transition behavior of PNIPAM aqueous solution. FTIR Spectroscopic Analysis of the Molecular Interactions between PNIPAM and ILs. FTIR spectroscopy provides thriving information about the molecular interactions between PNIPAM and ILs. D2O rather than H2O was used as a solvent for the FTIR measurements to prevent the overlap of the O−H band of water molecules at around 1560 cm−1 with the carbonyl bond. For characterizing the effects of ILs on the hydration behavior of PNIPAM in D2O, FTIR spectra of PNIPAM in imidazolium ILs with increasing alkyl chain length of cation at 5 mg mL−1 were measured at room temperature. In Figure 6, we present the obtained FTIR spectra of PNIPAM in ILs (for other concentrations, see Figures S12 and S13). Any conformational change in the polymer as a consequence of the disturbances in the hydration layer is best substantiated by monitoring the shift of amide bands. Here, the variation in hydration behavior of the polymer in the presence of ILs was described on the basis of the shifts in the amide I and II bands, as these bands are sensitive to bonding. The principal contribution of the amide I band is due to CO···DO D hydrogen bonds, whereas the amide II band is due to ND deformation prompted by the D2O molecules64,65 in the presence of ions of ILs. A typical FTIR spectrum of PNIPAM in ILs shown in the range from 1400 to 1650 cm−1 was mainly considered to probe the molecular mechanism behind the phase transition behavior. From a close inspection of Figure 6, one can find the amide I and II bands at 1623 and 1460 cm−1 in

extents with the addition of higher concentrations of ILs to PNIPAM aqueous solution, as shown in Figures S8 and S9. The substantial increase in dH at the LCST is due to the formation of large aggregates. As a result of the varied behavior of intraand intermolecular hydrogen bonds among the amide group of polymer and ions of ILs. Figures 4, S8, and S9 clearly indicate that, with the addition of ILs with increasing alkyl chain length of cation, the LCST of PNIPAM shifts toward lower temperatures. This reflects that these ILs act as “destroyers” for the hydration layer around the PNIPAM and thereby provoke the hydrophobic collapse of PNIPAM chains through hydrogen bonding, electrostatic interactions, hydrophobic forces, ion pair formation, and molecular interactions. The extent of change in dH of PNIPAM with the addition of different cations of IL to PNIPAM aqueous solution is found to be consistent with the Hofmeister series of cations and with associated changes in molecular aggregates for [Emim][Cl], [Bmim][Cl], [Hmim][Cl], and [Dmim][Cl] ILs. All of the added ILs increased the size of molecular aggregates of PNIPAM at their respective LCST values. The reason behind this increase in aggregates at all studied concentrations of ILs would be different on the basis of the various structural alterations and molecular mechanisms brought by these ILs at different concentration ranges. However, the extent of increase in the size of molecular aggregates varies from cation to cation depending upon its water structure making and breaking tendency. Viscosity Analysis of the Phase Transition Behavior of PNIPAM in the Presence of Imidazolium-Based ILs. Investigation of the effect of ILs on the viscosity (η) of PNIPAM aqueous solution leads to a better understanding of the coil-to-globule transition of the polymer aqueous solutions. The η values of PNIPAM aqueous solutions in the presence of various ILs were measured over a broad range of temperature. The miscibility of PNIPAM can be affected by the addition of ILs. Figure 5 depicts the temperature and cation type dependent η of PNIPAM aqueous solution. The η in the absence of ILs starts to decrease slightly from 1.43 m·Pas at 25 °C, by increasing the temperature, and the sudden substantial decrease in η was observed at 1.33 m·Pas at 33 °C. This point is considered as the LCST which is an indication of conformational change of hydrated compact to globule structure, as

Figure 5. Viscosity of the PNIPAM aqueous solution without and with ILs {IL free (black line), [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line), and [Dmim][Cl] (yellow line)}, at a temperature range of 25−40 °C. The concentration of ILs is 5 mg/mL. 4914

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

Figure 6. FTIR spectra of the PNIPAM in D2O solution without and with ILs {IL free (black line), [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line), and [Dmim][Cl] (yellow line)}, at 25 °C. The concentration of ILs is 5 mg/mL.

Figure 7. FESEM micrographs of (a) aqueous solution of PNIPAM without ionic liquid, (b−d) in the presence of [Emim][Cl], (e−g) in the presence of [Dmim][Cl].

D2O. The amide I and II bands provide valuable information regarding the hydrogen bonding of the amide group. As can be seen from the spectral variations in Figure 6, the wavenumber of the amide group of PNIPAM experiences a decrease in the absorbance with the addition of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl] ILs, respectively. This decrease in the absorbance is an indication of the transformation of well hydrated amide groups toward dehydration. A similar type of decrease in absorbance was observed when the Hofmeister series of anions is added to the PNIPAM due to dehydration of the amide group.23 This decrease in absorbance in the amide region is due to molecular interactions triggered by the presence of ILs. Thus, the present FT-IR results unveil that the ILs act as structure “destroyers” for the PNIPAM hydration system. Further, from these results, one can easily understand that the hydration behavior of PNIPAM depends on the chemical composition of the ILs. Morphological Changes of PNIPAM with Added ILs. Our prior UV−vis absorption spectra, fluorescence intensity spectra, DLS, η, and FTIR measurements uncovered that the type of cation of ILs has a great influence on the phase

transition behavior of PNIPAM. Further, in order to gain deeper knowledge on the influence of varying cation identity in altering the surface morphology of PNIPAM, we have collected the field emission scanning electron microscopy (FESEM) images in the presence of [Emim][Cl] (strongest inducer for hydrophobic collapse of PNIPAM) and [Dmim][Cl] (weakest inducer for hydrophobic collapse of PNIPAM) ILs to PNIPAM aqueous solution at room temperature. The obtained morphologies were portrayed in Figure 7. It could be observed from the images that the morphologies of pure polymer and polymer in the presence of the studied ILs are varied. This indicates that the polymer undergoes aggregation. The FESEM micrograph (Figure 7a) of PNIPAM aqueous solution without IL clearly exemplifies the coiled structure of PNIPAM, However, in the presence of different concentrations of [Emim][Cl] and [Dmim][Cl] (Figure 7b−g), hydrogen bonds between the polymer and water molecules are ruptured by the cations of these ILs; thereby, the polymer undergoes hydrophobic collapse and ultimately contributes to the nanoscale range aggregation of PNIPAM molecules. However, a significant morphological change was observed concomitant with hydrogen bonds, hydrophobic/hydrophilic balance 4915

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

Figure 8. Variation in LCST values of PNIPAM (black line) and in various imidazolium ILs which are obtained from various biophysical techniques with [Emim][Cl] (red line), [Amim][Cl] (green line), [Bmim][Cl] (blue line), [Hmim][Cl] (cyan line), [Bzmim][Cl] (pink line), and [Dmim][Cl] (yellow line).

Figure 9. Radius of gyration (Rg) associated with PNIPAM in the presence and absence of the ionic liquids.

value at 33 °C can be attributed to the phase transformation of the polymer to globule conformation. The number of water molecules (Nw) associated with the first hydration shell of the PNIPAM at 20 and 33 °C was also investigated. The mean Nw for PNIPAM at 20 °C was found to be 285 ± 1.41, whereas the mean Nw at 33 °C was decreased to 265 ± 9.67 (Figure 11). Examining the dynamics trajectory and the final PNIPAM conformations, as shown in Figure 12, a slightly folded structure was observed above 32 °C and an extended coil structure was observed at 20 °C. From the similar studies reported by others, it was observed that a complete phenomenon of PNIPAM globule transformation at LCST could not be achieved even at larger simulation time.54−56 As the focus in the present study is on how the presence of ILs such as [Emim][Cl], [Bmim][Cl], [Hmim][Cl], [Dmim][Cl],

variation among PNIPAM chains, water molecules, and ILs. The FESEM micrographs revealed that ILs played a major role in modulating the phase transition temperature of PNIPAM. The results obtained from various experimental techniques have been pictorially represented in the form of a bar diagram in Figure 8a (5 mg/mL IL concentration) and Figure 8b (5, 10, and 15 mg/mL IL concentration). Molecular Dynamics Study. MD simulations of PNIPAM-35mer in water were previously conducted at 33 and 20 °C above and below its LCST of 32 °C. The radius of gyration (Rg) of PNIPAM in water during the 20 ns MD has been decreased, and the mean Rg for PNIPAM at 33 °C was found to be 13.99 ± 1.52 Å during the simulation (Figures 9 and 10). The higher Rg at 20 °C was due to the extended coil conformation for the polymer, whereas a relative decrease in Rg 4916

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

Figure 10. Average value of the radius of gyration (Rg) associated with PNIPAM in the presence and absence of the ionic liquids.

Figure 11. Average number of water molecules (Nw) associated with the first hydration shell of PNIPAM in the presence and absence of the ionic liquids.

Figure 12. Representation of phase transition of PNIPAM at below and above the LCST by means of coil and globule conformational change as observed in MD simulation in the absence of ILs.

[Amim][Cl], and [Bzmim][Cl] act as structure “destroyers” for the PNIPAM hydration system, we simulated the transition of PNIPAM at the experimentally observed LCST in the presence of the above IL structures. MD simulations of PNIPAM in the presence of a molecule of [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], and [Bzmim][Cl] and [Dmim][Cl] were carried out subsequently at their LCST temperatures of 32.5, 32.0, 31.7, 31.0, 30.6, and 30.2 °C, respectively, as observed in the experimental study. MD simulations of PNIPAM-35mer in the presence of the ILs in solution have demonstrated a destabilization effect on the PNIPAM solvation structure. Figure 9 shows the evolution of

Rg for PNIPAM in the presence of various ILs like [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl]. In the presence of [Emim], the mean radius of gyration (Rg) of PNIPAM in water during the 20 ns MD simulation has been decreased to 10.24 ± 1.85, as depicted in Figure 9. The mean Rg values for PNIPAM in the presence of Bmim, Hmim, Dmim, Amim, and Bzmim during the simulation were found to be 10.69 ± 2.56, 11.33 ± 2.93, 11.67 ± 1.83, 11.88 ± 2.64, and 12.47 ± 2.43, respectively, which are also far lower than the mean Rg of PNIPAM alone in water, as shown in Figure 10. A significant decrease in Nw at the first hydration shell of the PNIPAM was also observed as an effect of IL 4917

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

Figure 13. Interaction of Emim (A), Bmim (B), Hmim (C), Dmim (D), Amim (E), and Bzmim (F) (stick models) with first hydration shell water molecules of PNIPAM (ball and stick model). Intermolecular interactions that include intramolecular hydrogen bonds in PNIPAM and ionic interactions between the IL and water molecules are shown as blue dashed lines.

Figure 14. Important snapshots from the PNIPAM−Dmim MD trajectory that show the evolution of the globule state for PNIPAM-35mer (ball and stick model) in the presence of Dmim (stick model) by means of hydrophobic interactions.

The conformational change of PNIPAM from extended coil to highly folded globule structure as observed in the presence of the ILs is a crucial finding of the study. Hydrophobic Effect of Emim, Bmim, Hmim, and Dmim on PNIPAM LCST. This rapid structural transformation in the presence of ILs clearly indicates the premature phase transition for PNIPAM and decrease in the LCST in the presence of ILs, such as Emim, Bmim, Hmim, and Dmim. Intermolecular bonding analysis of the PNIPAM and saturated alkyl imidazolium ion complexes revealed a little contribution of imidazolium ion and a major contribution of the hydrophobic alkyl chain in phase transition. In the case of Emim, the electron deficient pi system of the imidazolium cation interacts

interaction during the MD simulation. In the presence of Emim, Bmim, Hmim, Dmim, Amim, and Bzmim, the average Nw values for PNIPAM during the 20 ns simulation were found to be 255 ± 8.6, 253 ± 9.2, 251 ± 8.3, 248 ± 8.6, 250 ± 9.5, and 252 ± 9.1, respectively, as shown in Figure 11. Figure 13 shows the final solvation structures for PNIPAM in the presence of Emim, Bmim, Hmim, Dmim, Amim, and Bzmim. Examining the dynamics trajectory and the final PNIPAM conformations, a profoundly folded structure for PNIPAM was executed in the presence of the above ILs in solution, as shown in Figure 13. This dynamic structural phenomenon is critical and also a major difference observed between the PNIPAM simulations conducted in the presence and absence of the ILs. 4918

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B

Amim = Bzmim) clearly indicates a compact folding for PNIPAM in the presence of Emim when compared with other ILs. A significant difference in the number of intramolecular hydrogen bonds was observed between the intact PNIPAM and its IL affected globule structure, as shown in Figure 13. Intramolecular hydrogen bonding is a critical phenomenon in protein folding to produce a stable tertiary structure. Similarly, in the present simulation, under the environmental stress in the form of temperature and destabilized hydration, the polymer attained a stable globule conformation by intramolecular hydrogen bonding. Most importantly, the intramolecular hydrogen bonds are significantly higher in the PNIPAM globule structure in the presence of Emim. As the polymer folding is facilitated by the intramolecular bonding of the nearby groups, an increase in the number of intramolecular bonds produces a highly coiled structure for PNIPAM with a lesser Rg value. Hence, in the case of Emim, the polymer folding is tight and eventually produced a lesser Rg value for the globule state. This could be because the alkyl chain in Emim is relatively shorter and did not interfere with the internal folding space. In the case of longer alkyl chains such as with Bmim, Hmim, and Dmim, the hydrophobic chain deeply intrudes with internal folding space by the hydrophobic interaction in between the alkyl groups of isopropyl regions of the PNIAPM. This could cause a significant distance problem to the nearby groups to interact, and thus, the compactness of folding will eventually decrease. As observed from the final structures of simulation, the number of intramolecular hydrogen bonds in PNIPAM (20 °C), PNIPAM (33 °C), PNIPAM−Emim, PNIPAM−Bmim, PNIPAM−Hmim, PNIPAM−Dmim, PNIPAM−Amim, and PNIPAM−Bzmim was 4, 5, 12, 11, 10, 10, 8, and 8, respectively. Therefore, as the alkyl chain length is increased, the ILs show a profound decreasing effect on the PNIPAM LCST. This is clearly supported by the MD simulation study, where the hydrophobic interaction between the IL and PNIPAM caused disturbance in the hydration shell and subsequently brought conformational changes in PNIPAM at lower temperatures than the LCST. In the case of ILs where there is unsaturated substitution, the hydration shell is altered by means of ion− dipole interactions between the imidazolium cation and the water molecules. Above the LCST, Amim and Bzmim are shown to be interacting with the carbonyl functional groups of PNIPAM by means of ionic−dipole interaction. These cause significant conformational changes in the polymer to produce a globule state. In summary, MD simulations reveal that, basically, the PNIPAM LCST was decreased because of strong hydrophobic interaction of Emim, Bmim, Hmim, and Dmim with PNIPAM isopropyl groups, whereas the effect of Amim and Bzmim is by virtue of their stabilized ion−dipole interactions. The results from various biophysical techniques and molecular dynamic simulations have potentially proved that the hydration behavior of PNIPAM in the presence of the imidazolium-based ILs showed IL-induced aggregation of PNIPAM. This suggests that ILs have slowly led to structural changes in PNIPAM depending on the structural and molecular attributes at different concentration ranges. The obtained variations (in LCST values) in the hydration layer of PNIPAM in ILs have been pictorially represented in Figure 8 and Figure S14. It is apparent from Figure 8a that the effect of the 5 mg/ mL concentration of ILs on the hydration layer of PNIPAM is little. However, with the addition of 10 and 15 mg/mL

with the PNIPAM hydration shell water molecule and carbonyl oxygen of PNIPAM simultaneously by means of ion−dipole interaction. Interestingly, as the alkyl chain length increases, the hydrophobicity factor becomes the major influencing element in the phase transition. While the ion−dipole interactions were observed as major phase transition elements in Emim, the hydrophobic phenomenon was observed as a crucial factor for phase transition in the cases of Bmim, Hmim, and Dmim. Taking the Dmim MD trajectory as an example, a brief representation of the IL hydrophobic effect on the PNIPAM solvation structure was depicted in Figure 14. During the simulation, initially, the alkyl chains of the IL and PNIPAM were differentially solvated while the imidazolium ion displayed interactions with PNIPAM hydration shell water molecules. As the simulation proceeded, at about 2 ns, the hydrophobic tail of the IL was brought close to the isopropyl groups of the PNIPAM structure. This builds a hydrophobic association between PNIPAM and IL by significantly displacing the water molecules around these hydrophobic regions at about 4 ns. This entropy favored intrusion of the hydrophobic tail into the hydrophobic regions of the PNIPAM causes a dramatic shift in the PNIPAM coil conformation to globule conformation, as observed from parts a to d in Figure 14. Similarly, Bmim and Hmim also influenced the conformational change in PNIPAM structure strongly by means of the hydrophobic aggregation in contrast to the Emim ion−dipole effect on PNIPAM phase transition. Overall, in the presence of the saturated alkyl chain substituted imidazolium cations such as Emim, Bmim, Hmim, and Dmim, the phase transition temperature of PNIPAM was altered significantly because of hydrophobic interactions. Ionic Effect of Amim and Bzmim on the PNIPAM LCST. In contrast to the effect of ILs with saturated substitution such as Emim, Bmim, Hmim, and Dmim over the PNIPAM LCST, the ILs with unsaturated substitution such as Amim and Bzmim have shown a dehydration effect on the PNIPAM hydration shell and cause a decrease in the LCST. In general, the interaction of water molecules with ionic substances is thermodynamically stable when compared to hydrophobic substances. In the present IL study, the positive charge on the imidazoilium cation has a critical role to interact with water molecules by means of ionic interactions. Amim and Bzmim have unsaturated substitutions on the imidazolium cation. The pi electrons in this unsaturated substitution are able to stabilize the cation charge by means of resonance and strongly support its ionic interaction. Therefore, Amim and Bzmim could significantly withdraw the water molecules from the PNIPAM hydration shell by means of ion interactions. Further, the Amim and Bzmim could also interact with PNIPAM by means of ion− dipole interactions similar to Emim and initiate phase transition. Parts c and d of Figure 13 clearly show the interaction of Amim and Bzmim with water molecules by means of ionic interactions and a phase transition interaction with PNIPAM by means of ion−dipole interaction. The Rg as obtained in the simulation study was also comparable with dH of the PNIPAM−IL aggregates as observed in the experimental study. The Rg of PNIPAM−IL aggregates as obtained in MD simulation was in the order of Emim < Bmim < Hmim < Dmim < Amim < Bzmim. The same order was seen with particle diameter in the DLS experiment. Therefore, the phase transition state in the simulation study is perfectly correlated with the experimental observations. Further, the order of intramolecular hydrogen bonds in the globule state of PNIPAM (Emim > Bmim > Hmim = Dmim > 4919

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B concentration of ILs, the effect on the hydration layer of PNIPAM is drastic (Figure S14). The combined pictorial representation of 5, 10, and 15 mg/mL concentration effects has been given in Figure 8b. The degree of deviation in the LCST of PNIPAM varies from cation to cation depending upon its water structure making and breaking tendency. Figure 8b signifies the decrease in LCST values (destruction for the hydration layer) with the addition of ILs [Emim][Cl], [Amim][Cl], [Bmim][Cl], [Hmim][Cl], [Bzmim][Cl], and [Dmim][Cl]. This exemplifies that increasing the alkyl chain length of the cation of IL is having an intense influence on the hydration behavior of PNIPAM. The obtained experimental and molecular dynamic simulations results revealed that the ILs behave as “destroyers” for the hydration layer of PNIPAM aqueous solution at all concentrations by causing the well swollen PNIPAM to collapse into a shrunken globular structure by a “salting out” mechanism. The order of cations of IL for collapsing the hydration behavior of PNIPAM aqueous solution is corroborating with the normal Hofmeister series of cations of ILs.

■

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ac.in. Phone: +91-11-27666646-142. Fax: +91-11-2766 6605.

CONCLUSIONS

ORCID

In this work, we used a series of ILs having different cations ([Emim]+, [Amim]+, [Bmim]+, [Hmim]+, [Bzmim]+, and [Dmim]+) with fixed anion (Cl−) to explore the influence of ILs on the phase transition behavior of PNIPAM aqueous solution. The sophisticated UV−vis absorption spectra, fluorescence intensity spectra, η, DLS, FTIR, and FESEM techniques were used to quantify the cation effects. Our experimental results demonstrated that the addition of ILs decreased the LCST toward lower temperatures with the enhancement in temperature. The extent of the response of PNIPAM to solute IL is consistent with the Hofmeister series, with [Emim][Cl] being the weakest inducer of hydrophobic collapse of PNIPAM and [Dmim][Cl] the strongest inducer of hydrophobic collapse of PNIPAM in water. Interestingly, the observed results elucidate that the LCST of PNIPAM is decreasing with increasing alkyl chain length of the cation of ILs. However, the mechanism of modulating the LCST toward lower temperatures is different based on their molecular and structural variations. The overall specific ranking of ILs in preserving the hydration layer around the PNIPAM aqueous solution was [Emim][Cl] > [Amim][Cl] > [Bmim][Cl] > [Hmim][Cl] > [Bzmim][Cl] > [Dmim][Cl]. The molecular dynamics simulations also confirmed the influence of the cation structure on PNIPAM phase transition at the LCST. The simulation study produced a detailed insight on the mechanism of IL effect on PNIPAM molecular conformation changes by revealing ion−dipole interactions between the cation and PNIPAM functional groups. The order of hydrodynamic diameter as observed from DLS experiment is corroborated by the order of Rg of PNIPAM aggregates as obtained in simulation study. Further, the Nw values from the simulation study confirmed the specific ranking of ILs in preserving the hydration layer around the PNIPAM. Overall, the experimental and simulation studies collectively revealed the mechanism of IL behavior in reducing the LCST of PNIPAM.

■

UV−visible absorbance spectra (Figures S1 and S2), steady state fluorescence emission spectra (Figures S3 and S4), temperature dependent fluorescence emission spectra of ANS in PNIPAM aqueous solution without and with ILs (Figures S5 and S6), temperature dependent fluorescence emission spectra of ANS aqueous solution without and with ILs (Figure S7), hydrodynamic diameter, dH, of PNIPAM in the absence and presence of ILs (Figures S8 and S9), viscosity of the PNIPAM aqueous solution without and with ILs (Figures S10 and S11), FTIR spectra of the PNIPAM in D2O solution without and with ILs (Figures S12 and S13), and LCST of PNIPAM as a function of cation type in aqueous solutions with Cl− as the common anion (Figure S14) (the concentration of ILs is 10 and 15 mg/ mL) (PDF)

Pannuru Venkatesu: 0000-0002-8926-2861 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support from the Department of Science and Technology (DST), New Delhi, India (Grant No. SB/SI/PC-109/2012). We are grateful to the Department of Physics, University of Delhi, Delhi, India, for providing FESEM. We are extremely grateful to Centre for High Performance Computing (CHPC) facility in Cape Town, South Africa, for providing access to the required computational software.

■

REFERENCES

(1) Halperin, A.; Kröger, M.; Winnik, F. M. Poly(N-isopropylacrylamide) phase diagrams: Fifty years of research. Angew. Chem., Int. Ed. 2015, 54, 15342−15367. (2) Roy, D.; Brooks, W. L. A.; Sumerlin, B. S. New directions in thermoresponsive polymers. Chem. Soc. Rev. 2013, 42, 7214−7243. (3) Ueki, T. Stimuli-responsive polymers in ionic liquids. Polym. J. 2014, 46, 646−655. (4) Lu, J.; Yan, F.; Texter, J. Advanced applications of ionic liquids in polymer science. Prog. Polym. Sci. 2009, 34, 431−448. (5) Cortez-Lemus, N. A.; Claverie, A. L. Poly(N-vinylcaprolactam), a comprehensive review on a thermoresponsive polymer becoming popular. Prog. Polym. Sci. 2016, 53, 1−51. (6) Roy, D.; Cambre, J. N.; Sumerlin, B. S. Future perspectives and recent advances in stimuli-responsive materials. Prog. Polym. Sci. 2010, 35, 278−301. (7) Johnson, R. P.; Jeong, Y.; John, J. V.; Chung, C.; Kang, D. H.; Selvaraj, M.; Suh, H.; Kim, I. Dual stimuli-responsive poly(Nisopropylacrylamide)-b-poly(L-histidine) chimeric materials for the controlled delivery of doxorubicin into liver carcinoma. Biomacromolecules 2013, 14, 1434−1444. (8) Kobayashi, J.; Kikuchi, A.; Sakai, K.; Okano, T. Cross-linked thermo responsive anionic polymer-grafted surfaces to separate bioactive basic peptides. Anal. Chem. 2003, 75, 3244−3249. (9) Ahn, S. J.; Kaholek, M.; Lee, W. K.; Mattina, B. L.; Labean, T. H.; Zauscher, S. Surface initiated polymerization on nano patterns fabricated by electron beam lithography. Adv. Mater. 2004, 16, 2141−2145.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b02208. 4920

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B (10) Qin, S.; Geng, Y.; Discher, D. E.; Yang, S. Temperaturecontrolled assembly and release from polymer vesicles of poly(ethyleneoxide)-block-poly(N-isopropylacrylamide). Adv. Mater. 2006, 18, 2905−2909. (11) Naito, H.; Takewa, Y.; Mizuno, T.; Ohya, S.; Nakayama, Y.; Tatsumi, E.; Kitamura, S.; Takano, H.; Taniguchi, S.; Taenaka, Y. Three-dimensional cardiac tissue engineering using a thermoresponsive artificial extracellular matrix. ASAIO J. 2004, 50, 344−348. (12) Nelson, D. M.; Leeson, Z.; Ma, C. E.; Wagner, W. R. Extended and sequential delivery of protein from injectable thermoresponsive hydrogels. J. Biomed. Mater. Res., Part A 2012, 100, 776−785. (13) Zhou, P.; Yu, S. B.; Liu, Z. H.; Hu, J. M.; Deng, Y. Z. Electrophoretic separation of DNA using a new matrix in uncoated capillaries. Journal of Chromatography A 2005, 1083, 173−178. (14) Zhang, J. T.; Huang, S. W.; Cheng, S. X.; Zhuo, R. X. Preparation and properties of poly(N-isopropylacrylamide)/poly(Nisopropylacrylamide) interpenetrating polymer networks for drug delivery. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1249−1254. (15) Hu, J.; Liu, S. Responsive polymers for detection and sensing applications: current status and future developments. Macromolecules 2010, 43, 8315−8330. (16) Durocher, S.; Rezaee, A.; Hamm, C.; Rangan, C.; Mittler, S.; Mutus, B. Disulfide-linked, gold nanoparticle based reagent for detecting small molecular weight thiols. J. Am. Chem. Soc. 2009, 131, 2475−2477. (17) Harada, A.; Johnin, K.; Kawamura, A.; Kono, K. Preparation of temperature-responsive polymer gels physically immobilizing coreshell type bioconjugates. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5942−5948. (18) Yamamoto, K.; Mizutani, Y.; Kitagawa, T. Nanosecond temperature jump and time-resolved raman study of thermal unfolding of ribonuclease A. Biophys. J. 2000, 79, 485−495. (19) Callender, R.; Dyer, R. B. Probing protein dynamics using temperature jump relaxation spectroscopy. Curr. Opin. Struct. Biol. 2002, 12, 628−633. (20) Reddy, P. M.; Venkatesu, P. Ionic liquid modifies the Lower Critical Solution Temperature (LCST) of Poly(N-isopropylacrylamide) in Aqueous Solution. J. Phys. Chem. B 2011, 115, 4752−4757. (21) Tan, H.; Ramirez, C. M.; Miljkovic, N.; Li, H.; Rubin, J. P.; Marra, K. G. Thermosensitive injectable hyaluronic acid hydrogel for adipose tissue engineering. Biomaterials 2009, 30, 6844−6853. (22) Wang, Z.; Wu, P. The influence of ionic liquid on phase separation of poly(N-isopropylacrylamide) aqueous solution. RSC Adv. 2012, 2, 7099−7108. (23) Reddy, P. M.; Umapathi, R.; Venkatesu, P. Interactions of ionic liquids with hydration layer of poly(N-isopropylacrylamide): comprehensive analysis of biophysical techniques results. Phys. Chem. Chem. Phys. 2014, 16, 10708−10718. (24) Umapathi, R.; Mkhize, T. Y.; Venkatesu, P.; Deenadayalu, N. The influence of various alkylammonium-based ionic liquids on the hydration state of temperature-responsive polymer. J. Mol. Liq. 2017, 225, 186−194. (25) Kumar, A. C.; Bohidar, H. B.; Mishra, A. K. The effect of sodium cholate aggregates on thermoreversible gelation of PNIPAM. Colloids Surf., B 2009, 70, 60−67. (26) Heskins, M.; Guillet, J. E. Solution Properties of Poly(Nisopropylacrylamide). J. Macromol. Sci., Chem. 1968, 2, 1441−1455. (27) Zhang, Y.; Furyk, S.; Sagle, L. B.; Cho, Y.; Bergbreiter, D. E.; Cremer, P. S. Effects of Hofmeister Anions on the LCST of PNIPAM as a Function of Molecular Weight. J. Phys. Chem. C 2007, 111, 8916− 8924. (28) Umapathi, R.; Reddy, P. M.; Kumar, A.; Venkatesu, P.; Chang, C. J. The biological stimuli for governing the phase transition temperature of the “smart” polymer PNIPAM in water. Colloids Surf., B 2015, 135, 588−595. (29) Tanaka, F.; Koga, T.; Winnik, F. M. Temperature-Responsive Polymers in Mixed Solvents: Competitive Hydrogen Bonds Cause Cononsolvency. Phys. Rev. Lett. 2008, 101, 028302−028304.

(30) Sagle, L. B.; Zhang, Y.; Litosh, V. A.; Chen, X.; Cho, Y.; Cremer, P. S. Investigating the hydrogen-bonding model of urea denaturation. J. Am. Chem. Soc. 2009, 131, 9304−9310. (31) Pang, J.; Yang, H.; Ma, J.; Cheng, R. Solvation Behaviors of NIsopropylacrylamide in Water/Methanol Mixtures Revealed by Molecular Dynamics Simulations. J. Phys. Chem. B 2010, 114, 8652− 8658. (32) de Oliveira, T. E.; Mukherji, D.; Kremer, K.; Netz, P. A. Effects of stereochemistry and copolymerization on the LCST of PNIPAM. J. Chem. Phys. 2017, 146, 034904. (33) Liu, L.; Kou, R.; Liu, G. Ion specificities of artificial macromolecules. Soft Matter 2017, 13, 68−80. (34) Tah, I.; Mondal, J. How does a hydrophobic macromolecule respond to a mixed osmolyte environment? J. Phys. Chem. B 2016, 120, 10969−10978. (35) Schroer, M. A.; Michalowsky, J.; Fischer, B.; Smiatek, J.; Grubel, G. Stabilizing effect of TMAO on globular PNIPAM states: preferential attraction induces preferential hydration. Phys. Chem. Chem. Phys. 2016, 18, 31459−31470. (36) Rodríguez-Roper, F.; Rötzscher, P.; van der Vegt, N. F. A. Comparison of different TMAO force fields and their impact on the folding equilibrium of a hydrophobic Polymer. J. Phys. Chem. B 2016, 120, 8757−8767. (37) Chang, C. J.; Reddy, P. M.; Hsieh, S. R.; Huang, H. Influence of imidazolium based green solvents on volume phase transition temperature of crosslinked poly(N-isopropylacrylamide-co-acrylic acid) hydrogel. Soft Matter 2015, 11, 785−792. (38) Sun, S.; Hu, J.; Tang, H.; Wu, P. Spectral interpretation of thermally irreversible recovery of poly(N-isopropylacrylamide-coacrylic acid) hydrogel. Phys. Chem. Chem. Phys. 2011, 13, 5061−5067. (39) Ueki, T.; Nakamura, Y.; Usui, R.; Kitazawa, Y.; So, S.; Lodge, T. P.; Watanabe, M. Photoreversible gelation of a triblock copolymer in an ionic liquid. Angew. Chem., Int. Ed. 2015, 54, 3018−3022. (40) Hirose, H.; Shibayama, M. Kinetics of volume phase transition in poly (N-isopropylacrylamide-co-acrylic acid) gels. Macromolecules 1998, 31, 5336−5342. (41) Kratz, K.; Hellweg, T.; Eimer, W. Influence of charge density on the swelling of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels. Colloids Surf., A 2000, 170, 137−149. (42) Ye, Y.; Shanguan, Y.; Song, Y.; Zheng, Q. Influence of charge density on rheological properties and dehydration dynamics of weakly charged poly(N-isopropylacrylamide) during phase transition. Polymer 2014, 55, 2445−2454. (43) Snedden, P.; Cooper, A. I.; Scott, K.; Winterton, N. Crosslinked polymer-ionic liquid composite materials. Macromolecules 2003, 36, 4549−4556. (44) Kim, S. H.; Hong, K.; Xie, W.; Lee, K. H.; Zhang, S.; Lodge, T. P.; Frisbie, C. D. Electrolyte-gated transistors for organic and printed electronics. Adv. Mater. 2013, 25, 1822−1846. (45) Scovazzo, P.; Kieft, J.; Finan, D.; Koval, C.; Dubois, D.; Noble, R. Gas separations using non-hexafluorophosphate [PF6] anion supported ionic liquid membranes. J. Membr. Sci. 2004, 238, 57−63. (46) Ueki, T.; Watanabe, M. Polymers in ionic liquids: dawn of neoteric solvents and innovative materials. Bull. Chem. Soc. Jpn. 2012, 85, 33−50. (47) Ueki, T.; Watanabe, M. Macromolecules in ionic liquids: progress, challenges, and opportunities. Macromolecules 2008, 41, 3739−3749. (48) Snedden, P.; Cooper, A. I.; Scott, K.; Winterton, N. Cross linked polymer-ionic liquid composite materials. Macromolecules 2003, 36 (12), 4549−4556. (49) Susan, M. A. B. H.; Kaneko, T.; Noda, A.; Watanabe, M. Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes. J. Am. Chem. Soc. 2005, 127, 4976−4983. (50) Lewandowski, A.; Swiderska-Mocek, A. Ionic liquids as electrolytes for Li-ion batteries - An overview of electrochemical studies. J. Power Sources 2009, 194, 601−609. 4921

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922

Article

The Journal of Physical Chemistry B (51) Freitas, F. S.; de Freitas, J. N.; Ito, B. I.; de Paoli, M. A.; Nogueira, A. F. Electrochemical and structural characterization of polymer gel electrolytes based on a PEO copolymer and an imidazolium-based ionic liquid for cye-sensitized solar cells. ACS Appl. Mater. Interfaces 2009, 1, 2870−2877. (52) Soni, S. S.; Fadadu, K. B.; Vekariya, R. L.; Debgupta, J.; Patel, K. D.; Gibaud, A.; Aswal, V. K. Effect of self-assembly on triiodide diffusion in water based polymer gel electrolytes: An application in dye solar cell. J. Colloid Interface Sci. 2014, 425, 110−117. (53) de los Rios, A. P.; Fernandez, F. J. H.; Lozano, L. J.; Sanchez, S.; Moreno, J. I.; Godınez, C. Removal of metal ions from aqueous solutions by extraction with ionic liquids. J. Chem. Eng. Data 2010, 55, 605−608. (54) Du, H.; Wickramasinghe, R.; Qian, X. Effects of Salt on the Lower Critical Solution Temperature of Poly (N-Isopropylacrylamide). J. Phys. Chem. B 2010, 114, 16594−16604. (55) Alaghemandi, M.; Spohr, E. Molecular Dynamics Investigation of the Thermo-Responsive Polymer Poly(N-isopropylacrylamide). Macromol. Theory Simul. 2012, 21, 106−112. (56) Walter, J.; Sehrt, J.; Vrabec, J.; Hasse, H. Molecular dynamics and experimental study of conformation change of poly(Nisopropylacrylamide) hydrogels in mixtures of water and methanol. J. Phys. Chem. B 2012, 116, 5251−5259. (57) Min, S. H.; Kwak, S. K.; Kim, B. Atomistic simulation for coil-toglobule transition of poly(2-dimethylaminoethyl methacrylate). Soft Matter 2015, 11, 2423−2433. (58) Soto-Figueroa, C.; Hidalgoa, M. R. R; Vicente, L. Dissipative particle dynamics simulation of the micellization−demicellization process and micellar shuttle of a diblock copolymer in a biphasic system (water/ionic-liquid). Soft Matter 2012, 8, 1871−1877. (59) Mai, J.; Sun, D.; Li, L.; Zhou, J. Phase Behavior of an Amphiphilic Block Copolymer in Ionic Liquid: A Dissipative Particle Dynamics Study. J. Chem. Eng. Data 2016, 61, 3998−4005. (60) Lai, H.; Wang, Z.; Wu, P. Structural evolution in a biphasic system: poly(N-isopropylacrylamide) transfer from water to hydrophobic ionic liquid. RSC Adv. 2012, 2, 11850−11857. (61) Bai, Z.; Nagy, M. W.; Zhao, B.; Lodge, T. P. Thermoreversible Partitioning of Poly(ethylene oxide)s between Water and a Hydrophobic Ionic Liquid. Langmuir 2014, 30, 8201−8208. (62) Beyaz, A.; Woon, S. O.; Reddy, V. P. Ionic liquids as modulators of the critical micelle concentration of sodium dodecyl sulfate. Colloids Surf., B 2004, 35, 119−124. (63) Umapathi, R.; Venkatesu, P. Solution behaviour of triblock copolymer in the presence of ionic liquids: a comparative study of two ionic liquids possessing different cations with same anion. ACS Sustainable Chem. Eng. 2016, 4, 2412−2421. (64) Sun, B.; Lin, Y.; Wu, P.; Siesler, H. W. A FTIR and 2D-IR spectroscopic study on the microdynamics phase separation mechanism of the poly(N-isopropylacrylamide) aqueous solution. Macromolecules 2008, 41, 1512−1520. (65) Kesselman, E.; Ramon, O.; Berkovici, R.; Paz, Y. ATR-FTIR studies on the effect of strong salting-out salts on the phase separation scenario in aqueous solutions of poly(N-isopropylacrylamide) [PNIPA]. Polym. Adv. Technol. 2002, 13, 982−991.

4922

DOI: 10.1021/acs.jpcb.7b02208 J. Phys. Chem. B 2017, 121, 4909−4922