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Hydration of Cations: a Key to Understanding of Specific Cation Effects on Aggregation Behaviors of PEO-PPO-PEO Triblock Copolymers Jacob C

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Article pubs.acs.org/JPCB Hydration of Cations: A Key to Understanding of Specific Cation Effects on Aggregation Behaviors of PEO-PPO-PEO Triblock Copolymers Jacob C. Lutter, Tsung-yu Wu, and Yanjie Zhang* Department of Chemistry and Biochemistry, James Madison University, Harrisonburg, Virginia 22807, United States ABSTRACT: This work reports results from the interactions of a series of monovalent and divalent cations with a triblock copolymer, poly(ethylene oxide)-b-poly(propylene oxide)-b- poly(ethylene oxide) (PEO-PPO-PEO). Phase transition temperatures of the polymer in the presence of chloride salts with six monovalent and eight divalent cations were measured using an automated melting point apparatus. The polymer undergoes a two-step phase transition, consisting of micellization of the polymer followed by aggregation of the micelles, in the presence of all the salts studied herein. The results suggest that hydration of cations plays a key role in determining the interactions between the cations and the polymer. The modulation of the phase transition temperature of the polymer by cations can be explained as a balance between three interactions: direct binding of cations to the oxygen in the polymer chains, cations sharing one water molecule with the polymer in their hydration layer, and cations interacting with the polymer via two water molecules. + + + + + + Monovalent cations Na ,K,Rb, and Cs do not bind to the polymer, while Li and NH4 and all the divalent cations investigated including Mg2+,Ca2+,Sr2+,Ba2+,Co2+,Ni2+,Cu2+, and Cd2+ bind to the polymer. The effects of the cations correlate well with their hydration thermodynamic properties. Mechanisms for cation−polymer interactions are discussed. ■ INTRODUCTION that the Hofmeister effects are related to the ability of ions to partition between the nonpolar surface of the protein and bulk Specific ion effects on the physicochemical properties of − − 28 30 aqueous processes such as polymer and protein aggregation,1 8 solution. In addition, the relative polarization of anions has − − ff protein folding,9 11 and enzyme activity12 15 follow an been found to contribute to the e ects of the Hofmeister − 24,39−41 empirical trend that is called the Hofmeister series.16 21 This series. In some cases, the Hofmeister series is reversed series was first discovered over 125 years ago by a Czech or partially reversed when ion concentration, surface charge, or 6,20,42−45 & protein scientist, Franz Hofmeister, who tested the effects of a surface polarity are altered. An article in C E News, “Hofmeister Still Mystifies”, highlighted the current status of variety of inorganic salts on the solubility and conformational 32 stability of proteins.16,17 Typical orders for the ability of anions research on the mechanisms of the Hofmeister series. 19−21 ff and cations to salt proteins out of solution are as follows: The e ects of anions on uncharged systems are stronger than cations. Therefore, the studies of the effects of cations are more 2−−2 2 − −−−− CO3 >> SO4 S23 O > H24 PO >>> F Cl Br difficult than similar studies of anions. Cations and anions are ff >∼NO−− > I > SCN − di erent in several ways. Cations are usually smaller in size and 3 more strongly hydrated as compared to anions of similar ++++++22 + + 46 NH4 >>>>>> Cs Rb K Na Li Ca > Mg mass. The hydration of anions is more local than cations; thus, water molecules in the hydration layer of the ions can Although the Hofmeister effects are general phenomena in 47 ff approach closer to anions than cations. Cations are repelled many di erent systems, the molecular level mechanisms of the 33,47 Hofmeister series are not well understood. Explanations on the from hydrophobic surfaces to a larger extent than anions. A molecular origin of the Hofmeister series started to emerge in few recent papers from Cremer and co-workers reported 1,19,22−31 thermodynamic and spectroscopic investigations of cation the past 10 years. Recent studies suggest that multiple ff mechanisms are involved in a variety of systems.32 Direct ion− e ects on the aggregation of peptides, interactions with negatively charged hydrophilic surfaces, and their binding to biomacromolecule interactions are believed to be the 35,36,48 predominant force responsible for the effects of the Hofmeister amide groups. Cremer et al. found weak binding of − − 48 series.1 3,33 37 NMR studies and molecular dynamic simu- cations to amide groups. Interestingly, molecular dynamic − fi lations have shown that weakly hydrated anions such as SCN simulations showed that there was signi cant association of and I− bind to the amide nitrogen on the backbone and adjacent α-carbon, while strongly hydrated anions such as Received: June 9, 2013 2− SO4 are repelled from the backbone and the hydrophobic Revised: August 8, 2013 side chains of an elastin-like peptide.38 Record et al. proposed Published: August 9, 2013 © 2013 American Chemical Society 10132 dx.doi.org/10.1021/jp405709x | J. Phys. Chem. B 2013, 117, 10132−10141 The Journal of Physical Chemistry B Article monovalent cations such as Li+,Na+, and K+ with carboxylate H O with a minimum resistivity of 18 MΩ·cm produced from a − 2 groups or protein anionic surface charges.49 54 Moreover, the NANOpure Ultrapure Water System (Barnstead, Dubuque, IA) specific roles of cations in protein stability, enzymatic catalysis, was used to prepare all the polymer and salt solutions. Stock − and aqueous solubility of amino acids have been reported.55 57 solutions of L44 at 20 mg/mL in water were prepared at room To further explore the interactions of cations with macro- temperature. Aliquots of these solutions were mixed with water molecules, we study the effects of a series of 14 cations on the or inorganic salt solutions to the desired concentrations. The phase behaviors of a triblock copolymer, poly(ethylene oxide)- final polymer concentration for all measurements was 10 mg/ b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO- mL. The concentrations of inorganic salts ranged from 0 to 2.0 PEO). PEO-PPO-PEO block copolymers have a hydrophobic M. Due to low salt solubility, measurements at 2.0 M salt PPO block in the middle and two hydrophilic PEO blocks at concentration were not carried out for BaCl2, SrCl2, CoCl2, and the ends. These polymers undergo a two-step phase transition − NiCl2. as they are heated in aqueous solutions.58 60 PEO-PPO-PEO The phase transition temperatures of L44 solutions in the block copolymers represent uncharged macromolecules with no presence of chloride salts were measured with an automated carboxylate moieties. The particular polymer we used in this melting point apparatus (Optimelt MPA100, Stanford Research study is L44. L44 is a liquid at room temperature and has a System).4 Briefly, three capillary tubes each filled with 10 μLof PPO block with molecular weight on the order of 1200 and 40 sample were placed into the apparatus and heated at a rate of 2 wt % of PEO block. Previously, we investigated the specific °C/min. To ensure both phase transitions were observed, we anion effects on the phase transition of L44 and proposed the studied the temperature range from 35 to 95 °C. Real-time underlying mechanisms for how anions interact with the images of the samples were continuously captured by a built-in 4 polymer and further modulate its phase behaviors. Our studies camera. Digital image processing was used to produce plots of showed that the polymer underwent a two-step phase transition scattering intensity as a function of temperature. Phase in the presence of poorly hydrated anions. The first step is the transition temperatures were recorded as the initial break micellization, where the hydrophobic PPO blocks interact with points of these curves.61 Micellization temperatures were each other to form the core of the micelles. The second step is recorded as the first phase transition that occurs at lower the aggregation of the micelles induced by the dehydration of temperature; cloud point temperatures were recorded as the ff the PEO blocks. The e ects of poorly hydrated anions were second phase transition that appears at higher temperature. The correlated to the changes in the interfacial tension at the phase transition temperature at each salt concentration was the polymer/aqueous interfaces and anion binding to the hydro- average value of six repeated measurements. Typical standard phobic portion of the polymer. On the other hand, L44 exhibits errors of the measurements are about ±0.05 to ±0.10 °C. a single-step aggregation in the presence of well hydrated anions. The effects of strongly hydrated anions were correlated RESULTS AND DISCUSSION to polarization of hydration water around the hydrophilic ■ moieties. In our current study, we choose chloride as a Phase Transition Temperature of PEO-PPO-PEO as a common anion for all 14 cations employed. Chloride anion Function of Salt Concentration. Fourteen chloride salts, interacts with the polymer solely through changing the LiCl, NaCl, KCl, RbCl, CsCl, NH4Cl, MgCl2, CaCl2, SrCl2, interfacial tension at the polymer/aqueous interfaces as BaCl2, CoCl2, NiCl2, CuCl2, and CdCl2, were employed to shown in Figure 1. We find that the hydration of cations is a investigate the specific cation effects on the phase behaviors of a PEO-PPO-PEO block copolymer, L44. Our previous studies have shown that L44 underwent a two-step phase transition in the presence of NaCl.4 Switching the cation from a sodium ion to the cations listed above does not alter the two-step feature of the phase transition. The first phase transition that occurs at lower temperature is micelle formation due to the dehydration of PPO blocks. The second phase transition at higher Figure 1. Schematic illustration of the effect of chloride anion on the phase transition of PEO-PPO-PEO.
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