Microheterogeneity and CO2 Switchability of N,N

Subscriber access provided by UNIV OF LOUISIANA

B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution 2

Microheterogeneity and CO Switchability of N, NDimethylcyclohexylamine-Water Binary Mixtures Xueqian Guan, Zhiyu Huang, Hongsheng Lu, and Dejun Sun J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b12060 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

The Journal of Physical Chemistry

Microheterogeneity and CO2 Switchability of N, NDimethylcyclohexylamine–Water Binary Mixtures Xueqian Guan,† Zhiyu Huang,†* Hongsheng Lu,†* Dejun Sun‡*

†College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, P. R. China

‡Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan, 250100, P. R. China

ABSTRACT: Binary mixtures of water and organic solvents are described as the aqueous solutions of organic solvents, which are usually spatially heterogeneous on the scale of a few molecular sizes but homogeneous on longer length scales, i.e., microheterogeneity. For the water–organic solvent binary mixtures with microheterogeneity, most organic solvents are miscible with water at any ratio. Interestingly, some slightly water-miscible organic 1 ACS Paragon Plus Environment

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

Page 2 of 28

solvents can also be used to prepare binary mixtures with microheterogeneity. In this study, N, N-dimethylcyclohexylamine (DMCHA) was used to prepare binary mixtures with microheterogeneity and CO2 switchability. With the help of conductivity, Fourier-transform infrared (FT-IR) spectroscopy, ultraviolet–visible (UV-Vis) spectroscopy and dynamic light scattering (DLS) measurements, we found that water molecules are hydrogen-bonded together to form clusters over the range of water content 9 to 27 wt%, exhibiting microheterogeneity in the binary mixture. The size of water clusters increases slightly with increasing water content. What’s more, the DMCHA–water mixtures can be reversibly split into two phases by alternate bubbling of CO2 and N2, possessing excellent CO2 switchability. The binary mixtures can be used as reaction media for the synthesis of CaCO3 nanoparticles. Binary mixtures with microheterogeneity can also be formed under high salinity or high temperature conditions, or be prepared using other slightly watermiscible organic solvents, opening up more interesting possibilities about the binary mixtures with microheterogeneity.

1. INTRODUCTION

2 ACS Paragon Plus Environment

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


Binary liquid mixtures of water and organic solvents are described as the aqueous solutions of organic solvent that exhibit microheterogeneity but macrohomogeneity.1 Water–organic solvent mixtures with microheterogeneity are monophasic solutions with tunable properties, usually used as reaction media for chemical synthesis and solvent extraction.2 Typically, the organic solvents in such binary mixtures with microheterogeneity are short-chain amphiphiles and cannot form micelles in bulk solution.3 The binary mixtures with microheterogeneity have attracted considerable attention due to their anomalous thermodynamic and transport properties, such as density, diffusion coefficient, excess entropy and surface tension.4 Recently, experimental and computational studies have proved that microheterogeneity may be responsible for anomalous properties of the binary mixtures.3-10 Microheterogeneity describes the phenomenon that molecules of each component are preferably surrounded by the same kind of molecules. Molecules of water or organic solvent tend to form “clusters” of microscopic size without visible phase separation in binary mixtures with microheterogeneity.10 A prominent example is the tert-butanol–water mixture with pronounced cluster structures that have been studied by the reference interaction site model (RISM) theory and light-scattering technique.11-12 The existence of small clusters indicates that the mesoscale structure of binary mixtures with microheterogeneity is quite similar to that of surfactant-free microemulsions (denoted as SFMEs).13-15 Nevertheless, the microheterogeneous mixtures are described as “microemulsionlike” systems rather than microemulsions. Kunz et al.16-17 emphasized that SFMEs are ternary mixtures with well-defined domains in the pre-Ouzo region, whereas the binary mixtures with microheterogeneity contain many clusters of different shapes and sizes without any preferred



Page 4 of 28

size. Compared with SFMEs, the binary mixtures with microheterogeneity display unique features, such as simpler formula, good recovery and higher separation factors.18 In addition, the binary mixtures with microheterogeneity, also known as “design solvents”, require only changes in composition to achieve desired performance.4 Recently, many organic solvents have been reported to prepare binary mixtures with microheterogeneity, such as C1-C4 monohydric alcohols,17 dimethyl sulfoxide,19 dioxane9 and acetonitrile.20-22 Generally, clusters in binary mixtures with microheterogeneity are assembled by hydrogen bonds of water molecules, or by dipole-dipole or hydrophobic interactions of organic solvent molecules. Furthermore, the molecules on the surface of clusters are hydrogen-bonded with the molecules of another component around the clusters. For example, about 87% of water molecules exist as hydrogen-bonded strings and clusters in the methanol-water mixture (7:3 molar ratio), with water clusters binding to surrounding methanol hydroxyl groups by hydrogen bonding.23 Three-dimensional clusters of both acetonitrile (AN) and water molecules were observed in the AN–water mixtures at intermediate AN concentration.24 Summarizing the reported water–organic solvent mixtures with microheterogeneity, it is clear that most organic solvents are always miscible with water at any ratio. In this case, however, the range of binary mixtures compositions is confined to a narrow region and sometimes cannot meet industry demands. More interesting possibilities about the binary mixtures with microheterogeneity should be explored. Investigation of the binary mixtures with microheterogeneity suggests that the interaction between the organic solvent and water is a necessary condition for the formation of clusters. The major existing intermolecular interaction is the water–organic solvent hydrogen bonding.23-28 In order to expand the range of binary mixtures compositions, we assume that the 4 ACS Paragon Plus Environment

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


microheterogeneous mixture can also be composed of water and a slightly water-miscible organic solvent. So far, few studies have focused on such binary mixtures with microheterogeneity. Fortunately, in this study, amphiphilic fatty acids, alcohols or amines, such as N, Ndimethylcyclohexylamine (DMCHA), have been successfully used to prepare binary mixtures with microheterogeneity, which supports the hypothesis. All of them are slightly miscible with water, and their functional groups can bind water molecules through hydrogen bonding. Accordingly, the binary mixtures with microheterogeneity remain monophasic at a water content below the critical value but become biphasic above it. In this paper, binary mixtures with microheterogeneity were prepared using DMCHA and water. DMCHA is a hydrophobic tertiary amine with Log KOW value of 2.04 and water solubility of about 10 g/L. The microheterogeneity of DMCHA–water mixtures was characterized by means of conductivity, FT-IR and UV-Vis spectra and dynamic light scattering (DLS). Furthermore, DMCHA is a prominent CO2 switchable hydrophilicity solvent (SHS) and therefore the DMCHAwater mixtures exhibit excellent CO2 switchability.29-31 This CO2 switchability enhances the potential of DMCHA–water mixtures in technological applications such as solvent extraction, chemical reactions and nanoparticle synthesis. Herein, the DMCHA–water mixture can be used as a reaction medium to synthesize nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Materials N, N-dimethylcyclohexylamine (DMCHA, 98 wt%), Nile red (95 wt%) and methylene blue trihydrate were purchased from Aladdin Reagent Co., Ltd., China. The water content in DMCHA is 0.051 wt%, which is very low and neglected in the experiments. Calcium chloride (CaCl2, 96 5 ACS Paragon Plus Environment


Page 6 of 28

wt%) was purchased from Sinopharm Chemical Reagent Co. Ltd., China. All the reagents were used as received. Deionized water was used in all the experiments. 2.2. Preparation of DMCHA–Water Mixtures The DMCHA–water mixtures with different water mass fractions (XW) were prepared by simply mixing DMCHA and deionized water under mechanical stirring. The mixtures were filtered with 0.22 μm Nylon 66 filters to remove dust particles. The operating temperature was about 17 ℃. 2.3. Characterization of DMCHA–Water Mixtures 2.3.1. Conductivity Conductivity of stock solutions was determined using a DDS-307 conductivity meter under gentle stirring at 17 ℃. 2.3.2. Fourier–transform infrared (FT-IR) spectroscopy FT-IR spectra of stock solutions with different XW were recorded in the spectral region of 4000400 cm-1 using a BRUKER ALPAH-T spectrometer equipped with CaF2 windows. 2.3.3. Ultraviolet–visible (UV-Vis) spectroscopy UV-Vis absorption spectra were acquired over the wavelength range 190-800 nm using a HITACHI U-4100 model spectrophotometer with quartz cells. Nile red and methylene blue were used as two optical probes. The maximum absorption wavelength of the two dyes is significantly affected by the polarity of probing environment. 2.3.4. Clusters size determination 6 ACS Paragon Plus Environment

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


The clusters size and size distribution of the DMCHA–water mixtures were measured using a DLS apparatus (Zetasizer Nano ZEN3600, Malvern Instruments Ltd., U.K.). The DLS measurement was carried out at 17 ℃ with a fixed scattering angle of 90°. 2.4. Interfacial Tension (IFT) Measurement The dynamic IFT of DMCHA and aqueous phase was measured with a tensiometer (Tracker, Teclis) by the pendant drop method. As the solubility of DMCHA in pure water is about 10 g/L, here the pure water was replaced by the DMCHA-saturated aqueous solution. The latter was prepared by mixing equal amounts of DMCHA and pure water, and the aqueous phase was separated using a separating funnel. A drop of aqueous solution was created with syringe in a cuvette filled with DMCHA at 17 ℃. Images of drop were captured and analyzed by the Laplace equation to obtain IFT. 2.5. Nucleus Magnetic Resonance (NMR) Spectroscopy 1H

NMR and 13C NMR spectra were measured using a Bruker Avance 400 spectrometer at 400.3

and 100.7 MHz, respectively. DMCHA and the upper organic phase after CO2 bubbling were dissolved in CDCl3, while the lower aqueous phase was dissolved in D2O. 2.6. Synthesis and Characterization of CaCO3 Nanoparticles CaCO3 nanoparticles were produced with the DMCHA–water mixture as microreactor. First, the binary mixture was prepared by mixing DMCHA and 0.05 M CaCl2 aqueous solution. The XW was fixed at 20 wt%. Then CO2 was bubbled into the mixture for 10 min, generating CaCO3 precipitate particles in lower phase. The particles were dispersed in deionized water and then



Page 8 of 28

separated by centrifugal sedimentation. This process was repeated at least three times. Finally, the particles were dried in a vacuum. The morphological characteristics of CaCO3 nanoparticles were examined using a JEOL JEM1400 transmission electron microscope (TEM) operated at 120 kV. The nanoparticles were dispersed in pure water by ultrasonic, and then a drop of solution was loaded onto a carbon-coated copper grid placed on the filter paper. The images were recorded on a Gatan multiscan CCD and processed with digital micrograph.

3. RESULTS AND DISSCUSSION 3.1. Microheterogeneity of DMCHA–Water Mixtures At XW less than 27 wt%, the DMCHA–water mixtures exhibit transparent monophasic appearance. Further addition of water can result in a macroscopic phase separation. Here, the conductivity of the monophasic mixtures was measured to explore the structuring of DMCHA– water mixtures in a XW range of 0-27 wt%. As shown in Figure 1, for the DMCHA–water mixtures with XW less than 9 wt%, the conductivity increases slightly with increasing XW, indicating that most water exist as isolated molecules in the DMCHA–water mixtures. With XW greater than 9 wt%, however, the conductivity increases sharply with the increase in XW, suggesting the occurrence of percolative conduction phenomenon. A similar trend of increasing conductivity with an increase in XW has been described at the low water content of the classical microemulsions.3233 Generally,

the percolation phenomenon occurs in inhomogeneous media, meaning the formation

of clusters. 34 Therefore, we infer that water clusters exist in the DMCHA–water mixtures over the range of XW 9-27 wt%. It is noteworthy that the critical point of 9 wt% is lower than the corresponding XW (12.4 wt%) of the DMCHA–water equimolar mixture. For the DMCHA–water 8 ACS Paragon Plus Environment

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


mixtures, water–water interaction (hydrogen bonding), DMCHA–water interaction (hydrogen bonding and dipole–dipole interaction), and DMCHA–DMCHA interaction (dipole–dipole interaction) are all different. Thus, the DMCHA–water equimolar mixture is a non-ideal mixture with incomplete mixing,35 in which both isolated water molecules and water clusters coexist. The existence of clusters has also been described in acetonitrile–water equimolar mixture36 and alcohol–water equimolar mixture.37

Figure 1. Conductivity of DMCHA–water mixtures as a function of XW at 17 ℃. Inset: Photograph of DMCHA–water mixtures containing hydrophilic methylene blue with XW of (a) 1.1, (b) 8.8 and (c) 20.7 wt%, respectively. With the addition of water, the color variation of methylene blue solutions in Figure 1 implies the change of microenvironment of the DMCHA–water mixtures. UV-Vis absorption spectra of the DMCHA–water mixtures with different XW were measured using hydrophilic methylene blue and hydrophobic Nile red as optical probes. The maximum absorption wavelength λmax of methylene blue in water was 664±1 nm, and λmax of Nile red in DMCHA was 503±2 nm (Figure S1). For XW in the range of 0-9 wt%, absorption peaks near λmax in Figure 2a are negligible with very low absorbance, revealing that water molecules are not assembled together, but exist in the 9 ACS Paragon Plus Environment


Page 10 of 28

form of isolated molecules. Upon increasing XW above 9 wt%, strong broad peaks can be observed at about 641 nm and show a clear red shift with increasing XW, suggesting the presence of “bulk” water. In this case, water molecules are partially assembled into hydrogen-bonded clusters in the DMCHA, exhibiting microheterogeneity. The increased λmax with increasing XW corresponds to a greater polarity of the microenvironment, which reflects the growth of water clusters. A similar increasing trend of λmax with increasing XW was also observed in the UV-Vis spectra of Methyl orange (MO) in the W/O microemulsions.38 All the λmax of methylene blue in the mixtures are below 664 nm, verifying that the microenvironment polarity of DMCHA–water mixtures is less than that of pure water (Figure S1a). Furthermore, the IFT value of DMCHA–water is 1.72 mN·m1

(Figure S2). The low IFT value demonstrates that some DMCHA molecules interact with the

water molecules on the surface of clusters and thus adsorb at the interface.39 The adsorption behavior of DMCHA is in line with that of co-solvent in SFMEs.40 For the UV-Vis spectra of Nile red in DMCHA–water mixtures (Figure 2b), the λmax of Nile red increases gradually with increasing XW, which means that the polarity of the mixture increases gradually with the addition of water. Furthermore, all the λmax of Nile red in the DMCHA-water mixtures are greater than that of pure DMCHA (Figure S1b), indicating the greater polarity of DMCHA–water mixtures than the polarity of DMCHA.


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


Figure 2. UV-Vis absorption spectra of DMCHA–water mixtures with different XW. The optical probes used in the spectra are (a) methylene blue and (b) Nile red, respectively. XW in spectra (a) are 0.9, 2.2, 3.3, 4.9, 7.0, 9.4, 11.5, and 13.6 wt%. XW in spectra (b) are 1.0, 2.4, 4.2, 5.9, 7.9, 10.8, 13.1, and 14.2 wt%. FT-IR absorption spectra of DMCHA–water mixtures with different XW were determined to investigate the nature of molecular association. At XW less than 3 wt%, small sharp peaks “f” appear near 3681 cm-1 in the spectra of Figure 3a, proving the existence of “free” water with no aggregation in the DMCHA–water mixtures.41 The “free” water molecules interact with DMCHA molecules through dipole–dipole interaction (C…O). At XW higher than 3 wt%, both small sharp 11 ACS Paragon Plus Environment


Page 12 of 28

peaks “f” and strong broad peaks “g” were observed in the spectra of Figure 3a, which implies the appearance of both dipole structure and hydrogen-bonded network structure.42 Similarly, at XW greater than 3 wt%, the appearance of strong broad peaks “h” centered at 1648 cm-1 in Figure 3b also demonstrates the formation of hydrogen-bonded network structure in the DMCHA–water mixtures. With increasing XW, the intensity of peak “g” and peak “h” increases monotonically and the intensity of peak “f” remains almost unchanged, suggesting that the number of hydrogenbonded water molecules increases while that of “free” ones remains almost unchanged. According to the results of conductivity, FT-IR and UV-Vis absorption spectra, two composition regions can be distinguished in the DMCHA–water mixtures: at XW less than 9 wt%, water molecules exist as isolated molecules that interact with surrounding DMCHA molecules by both hydrogen bonding and dipole-dipole interaction; upon increasing the XW above 9 wt%, the additional water molecules are assembled into clusters (i.e. “bulk” water) and thus microheterogeneity occurs in the mixtures.


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


Figure 3. FT-IR absorption spectra of DMCHA–water mixtures with different XW in the (a) O-H stretching and (b) O-H bending region. XW in both spectra are 0, 1.4, 3.1, 5.4, 9.7, 11.9, 14.7, and 20.0 wt%. DLS measurements were performed to identify the size of water clusters in the mixtures with different XW. As shown in Figure 4, the size of water clusters increases slightly with increasing XW. This is in qualitative agreement with the result of UV-Vis spectra that water clusters grow gradually with increasing XW. A similar increasing trend of droplets size with the increase in XW was also described in microemulsions of water/eugenol/ethanol system and water/Brij35/[bmim][PF6] system.43-44 The size of all water clusters in Figure 4 ranges from 0.5 to 4 nm, 13 ACS Paragon Plus Environment


Page 14 of 28

which is the same order of magnitude as the clusters size in the acetonitrile–water mixture24 and SFME of water/ethanol/n-octanol system.45

Figure 4. Size distribution of water clusters in DMCHA–water mixtures with different XW at 17 ℃. In addition, such binary mixtures with microheterogeneity can also be prepared using other slightly water-miscible organic solvents. We found that many organic solvents, including npentanol, n-hexanol, n-octanol, triethylamine, 1-hexanamine and n-hexanoic acid, could form binary mixtures with micro-heterogeneity by mixing with water. All these organic solvents can hydrogen bond with water. On the contrary, for some organic solvents such as benzene, toluene, petroleum ether, chloroform and dichloromethane that cannot hydrogen bond with water, they couldn’t mix with water to prepare binary mixtures with microheterogeneity and macrohomogeneity. Existence of hydrogen bonding between the organic solvents and water is a necessary condition for cluster formation in binary mixtures with microheterogeneity.1, 3, 46, 47 For C5-C8 alcohol–water mixtures, the monophasic region becomes narrower with the increase in the hydrophobic chain length of alcohol (Figure S3). What’s more, the binary mixtures with


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


microheterogeneity can also be formed under different environments. For DMCHA–water mixtures or n-pentanol–water mixtures, monophasic mixtures could be formed under 0.1 M NaCl, KCl, or CaCl2 at 17 ℃. Moreover, the DMCHA–pure water mixtures remain monophasic over a wide temperature range. The monophasic region of the DMCHA–water mixtures is significantly narrowed with increasing temperature (Figure S4). 3.2. CO2 Switchability of DMCHA–Water Mixtures DMCHA has been reported to be a prominent CO2 switchable hydrophilicity solvent (SHS).2931

For the DMCHA–water mixture with high XW (e.g. 60 wt%), it behaves as a biphasic solution

in ambient air and becomes a monophasic solution upon exposure to CO2 (Figure S5). Surprisingly, in this study, the microheterogeneous mixtures get the opposite CO2 switchable behavior. Over a range of XW from 9 to 27 wt%, all the DMCHA–water mixtures are monophasic in ambient air but become biphasic upon exposure to CO2 (Figure 5). This is the first example of the use of CO2 to cause phase splitting in DMCHA–water mixtures, rather than reverse phase splitting. For the DMCHA–water binary mixture with XW of 20 wt%, CO2 was predominantly entrapped in water clusters and existed in the form of carbonic acid. Then DMCHA reacted with carbonic acid to form N, N-dimethylcyclohexylammonium bicarbonate.29 Due to the insufficient amount of water (XW = 20 wt%), only part of DMCHA is protonated and thus becomes watermiscible, while the rest of DMCHA keeps neutral and hydrophobic. Consequently, the monophasic mixture split into two transparent phases under CO2 atmosphere. Most of Nile red dye entered the upper organic phase, and a distinct interface between the two phases was observed in Figure 5b. The protonated DMCHA transferred into the lower aqueous phase, leading to the increase in pH (approximately 8.97) and volume of the aqueous phase (Figure S6). According to the pKa of DMCHA and the corresponding species distribution (Figure S7), about 95% protonated DMCHA 15 ACS Paragon Plus Environment


Page 16 of 28

and 5% neutral DMCHA were present in aqueous phase. Removal of CO2 by bubbling N2 at 60 ℃, however, reversed the reaction and facilitated the recovery of the monophasic mixture (Figure 5). The size distribution of the formed water clusters after one cycle almost coincides with that of the original one (Figure S8).

Figure 5. Photographs of the CO2 switchable mixtures of DMCHA and water with XW of 9, 15, 20 and 27 wt%, respectively. (a) The original DMCHA–water mixtures or the reformed monophasic solutions by N2 bubbling; (b) The biphasic solutions treated with CO2. The mixtures were stained by oil-soluble Nile red. CO2 was bubbled at 17 ℃ for 10 min while N2 was bubbled at 60 ○C for 80 min. The flow rates of both gases were fixed at 200 mL/min. For the biphasic solutions after CO2 bubbling, the components in the separated organic phase and aqueous phase were characterized by 1H NMR and

13C

NMR spectroscopy. As shown in

Figure 6, the 1H NMR and 13C NMR spectra of the organic phase completely coincide with those of DMCHA, revealing that the upper organic phase is neutral DMCHA. In the 1H NMR spectrum 16 ACS Paragon Plus Environment

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


of the aqueous phase, however, the signals of protons Ha, Hb and Hc slightly shift upfield, and the signals of protons Hd and He near the ammonium headgroups obviously shift downfield. These results indicate the protonation of DMCHA. Furthermore, the signal of carbon C6 at ~160 ppm in the 13C NMR spectrum of aqueous phase assigns to the HCO3- ion. Therefore, the lower aqueous phase is a mixture of water and ammonium bicarbonate.



Page 18 of 28

Figure 6. (a)1H NMR and (b)13C NMR spectra of DMCHA, the separated organic phase and aqueous phase. The organic phase and aqueous phase were obtained by bubbling CO2 into the DMCHA–water mixture with XW of 20 wt% at 17 ℃. The microheterogeneity and CO2 switchability make the DMCHA–water mixture appealing as a microreactor for preparing nanoparticles. Here, CaCO3 nanoparticles were prepared using the binary mixture of DMCHA and CaCl2 aqueous solution. After CO2 bubbling, CaCl2 in water clusters reacted with carbonic acid and DMCHA to generate CaCO3. Precipitate particles were observed in the lower phase and separated out directly by filtration. TEM image in Figure 7 shows the morphology of discrete spherical CaCO3 particles with sizes in the range 1-10 nm. The formation mechanism of nanoparticle is close to that of microemulsion approach.48-50 More importantly, DMCHA in binary mixture can be recovered and subsequently recycled according to its switchable characteristic (Figure S9), which is preferred from an ecological and economic point of view.

Figure 7. Illustration of the preparation process of CaCO3 nanoparticles. (a) Initially, DMCHA mixed with CaCl2 aqueous solution, forming an optically transparent monophasic mixture with XW of 20 wt%; (b) Then CO2 was bubbled into the monophasic mixture, generating CaCO3 precipitate nanoparticles in aqueous phase; (c) TEM image of CaCO3 nanoparticles obtained after filtration. The mixtures in (a) and (b) were stained by Nile red. All the experimental operating temperatures were 17 ℃. 18 ACS Paragon Plus Environment

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


4. CONCLUSIONS Binary mixtures with microheterogeneity and CO2 switchability were prepared using water and the slightly water-miscible DMCHA. The microheterogeneity of DMCHA–water mixtures was characterized by means of conductivity, DLS, FT-IR and UV-Vis spectra. The water molecules tend to form hydrogen-bonded clusters over the XW range 9-27 wt%, exhibiting microheterogeneity in the binary mixtures. The water molecules on the surface of clusters interact with surrounding DMCHA molecules by both hydrogen bonding and dipole–dipole interaction. The water clusters grow gradually with increasing XW. In addition, the DMCHA–water mixtures possess excellent CO2 switchable characteristic because DMCHA is a prominent CO2 switchable hydrophilicity solvent (SHS). The monophasic mixtures with XW in the range of 9-27 wt% can be switched to biphasic ones and back again by alternate bubbling of CO2 and N2. This is the first example of the use of CO2 to cause phase splitting in DMCHA-water mixtures, rather than reverse phase splitting. The DMCHA–water mixture can be used as a reaction medium for the preparation of CaCO3 nanoparticles. Binary mixtures with microheterogeneity can also be prepared under high salinity or high temperature conditions, or be prepared using other slightly water-miscible fatty acids, alcohols or amines. Our study expands the range of binary mixtures compositions, advancing a better understanding of the formation mechanism of the binary mixtures with microheterogeneity. ASSOCIATED CONTENT

Supporting Information



Page 20 of 28

UV-Vis absorption spectra of methylene blue and Nile red (Figure S1); Dynamic IFT of DMCHA–water at 17 ℃ (Figure S2); The maximum XW value for binary mixtures of water and alcohols with different carbon numbers (Figure S3); The maximum XW value for DMCHA–water mixtures at different temperatures (Figure S4); The phase behavior of the DMCHA–water mixture with XW of 60 wt% (Figure S5); The phase behavior of the DMCHA–water mixture with XW of 20 wt% (Figure S6); Species distribution of DMCHA (Figure S7); Size distribution of water clusters in the original DMCHA–water mixture and in the mixture treated with CO2/N2 (Figure S8); Illustration of recovering and recycling DMCHA in the preparation process of CaCO3 nanoparticles (Figure S9).

AUTHOR INFORMATION

Corresponding Author *E-mail (Z. H.): [email protected].

*E-mail (H. L.): [email protected].

*E-mail (D. S.): [email protected].


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


ORCID Zhiyu Huang: 0000-0002-9115-5571 Hongsheng Lu: 0000-0003-3201-0855 Dejun Sun: 0000-0003-0841-1501 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (NSFC, No. 21403173 and 21333005).

REFERENCES (1) Mountain, R. D. Microstructure and Hydrogen Bonding in Water−Acetonitrile Mixtures. J. Phys. Chem. B 2010, 114, 16460–16464. (2) Hoogenboom, R.; Thijs, H. M.; Wouters, D.; Hoeppener, S.; Schubert, U. S. Tuning Solution Polymer Properties by Binary Water–Ethanol Solvent Mixtures. Soft Matter 2008, 4, 103–107. (3) Subramanian, D.; Boughter, C. T.; Klauda, J. B.; Hammouda, B.; Anisimov, M. A. Mesoscale Inhomogeneities in Aqueous Solutions of Small Amphiphilic Molecules. Faraday Discuss. 2013, 167, 217–238.



Page 22 of 28

(4) Banik, D.; Roy, A.; Kundu, N.; Sarkar, N. Picosecond Solvation and Rotational Dynamics: An Attempt to Reinvestigate the Mystery of Alcohol–Water Binary Mixtures. J. Phys. Chem. B 2015, 119, 9905–9919. (5) Mizuno, K.; Miyashita, Y.; Shindo, Y.; Ogawa, H. NMR and FT-IR Studies of Hydrogen Bonds in Ethanol–Water Mixtures. J. Phys. Chem. 1995, 99, 3225–3228. (6) Zhong, Y.; Patel, S. Electrostatic Polarization Effects and Hydrophobic Hydration in Ethanol–Water Solutions from Molecular Dynamics Simulations. J. Phys. Chem. B 2008, 113, 767–778. (7) Oldiges, C.; Wittler, K.; Tönsing, T.; Alijah, A. MD Calculated Structural Properties of Clusters in Liquid Acetonitrile/Water Mixtures with Various Contents of Acetonitrile. J. Phys. Chem. A 2002, 106, 7147–7154. (8) Jalili, S.; Akhavan, M. Molecular Dynamics Simulation Study of Association in Trifluoroethanol/Water Mixtures. J Comput. Phys. 2010, 31, 286–294. (9) Takamuku, T.; Noguchi, Y.; Nakano, M.; Matsugami, M.; Iwase, H.; Otomo, T. Microinhomogeneity for Aqueous Mixtures of Water–Miscible Organic Solvents. J. Ceram. Soc. Jpn. 2007, 115, 861–866. (10) Marcus, Y. The Structure and Interactions in Binary Acetonitrile + Water Mixtures. J. Phys. Org. Chem. 2012, 25, 1072–1085. (11) Yoshida, K.; Yamaguchi, T.; Kovalenko, A.; Hirata, F. Structure of Tert-Butyl Alcohol– Water Mixtures Studied by the RISM Theory. J. Phys. Chem. B 2002, 106, 5042–5049. (12) Subramanian, D.; Anisimov, M. A. Resolving the Mystery of Aqueous Solutions of Tertiary Butyl Alcohol. J. Phys. Chem. B 2011, 115, 9179–9183.


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


(13) Schöttl, S.; Touraud, D.; Kunz, W.; Zemb, T.; Horinek, D. Consistent Definitions of “the Interface” in Surfactant-Free Micellar Aggregates. Colloids Surf. A 2015, 480, 222–227. (14) Zemb, T. N.; Klossek, M.; Lopian, T.; Marcus, J.; Schöettl, S.; Horinek, D.; Prevost, S. F.; Touraud, D.; Diat, O.; Marcelja, S.; Kunz, W. How to Explain Microemulsions Formed by Solvent Mixtures without Conventional Surfactants. PNAS 2016, 113, 4260–4265. (15) Hou, W.; Xu, J. Surfactant-Free Microemulsions. Curr. Opin. Colloid Interface Sci. 2016, 25, 67–74. (16) Bošković, P.; Sokol, V.; Zemb, T.; Touraud, D.; Kunz, W. Weak Micelle-Like Aggregation in Ternary Liquid Mixtures as Revealed by Conductivity, Surface Tension, and Light Scattering. J. Phys. Chem. B 2015, 119, 9933–9939. (17) Buchecker, T.; Krickl, S.; Winkler, R.; Grillo, I.; Bauduin, P.; Touraud, D.; Pfitzner, A.; Kunz, W. The Impact of the Structuring of Hydrotropes in Water on the Mesoscale Solubilization of a Third Hydrophobic Component. Phys. Chem. Chem. Phys. 2017, 19, 1806–1816. (18) Dhapte, V.; Mehta, P. Advances in Hydrotropic Solutions: An Updated Review. St Petersburg Polytech Univ J: Phys Math 2015, 1, 424–435. (19) Shin, D. N.; Wijnen, J. W.; Engberts, J. B.; Wakisaka, A. On the Origin of Microheterogeneity: A Mass Spectrometric Study of Dimethyl Sulfoxide–Water Binary Mixture. J. Phys. Chem. B 2001, 105, 6759–6762. (20) Jamroz, D.; Stangret, J.; Lindgren, J. An Infrared Spectroscopic Study of the Preferential Solvation in Water–Acetonitrile Mixtures. J. Am. Chem. Soc. 1993,115, 6165–6168. (21) Bertie, J. E.; Lan, Z. Liquid Water-Acetonitrile Mixtures at 25 °C: The Hydrogen-Bonded Structure Studied through Infrared Absolute Integrated Absorption Intensities. J. Phys. Chem. B 1997, 101, 4111–4119.



Page 24 of 28

(22) Kovacs, H.; Laaksonen, A. Molecular Dynamics Simulation and NMR Study of Water– Acetonitrile Mixtures. J. Am. Chem. Soc. 1991,113, 5596–5605. (23) Dixit, S.; Crain, J.; Poon, W. C. K.; Finney, J. L.; Soper, A. K. Molecular Segregation Observed in a Concentrated Alcohol–Water Solution. Nature 2002, 416, 829–832. (24) Takamuku, T.; Tabata, M.; Yamaguchi, A.; Nishimoto, J. Liquid Structure of Acetonitrile– Water Mixtures by X-Ray Diffraction and Infrared Spectroscopy. J. Phys. Chem. B 1998, 102, 8880–8888. (25) Zoranić, L.; Mazighi, R.; Sokolić, F.; Perera, A. Concentration Fluctuations and Microheterogeneity in Aqueous Amide Mixtures. J. Chem. Phys. 2009, 130, 124315. (26) Roux, G.; Roberts, D.; Perron, G.; Desnoyers, J. E. Microheterogeneity in Aqueous– Organic Solutions: Heat Capacities, Volumes and Expansibilities of Some Alcohols, Aminoalcohol and Tertiary Amines in Water. J. Solution Chem. 1980, 9, 629–647. (27) Zoranić, L.; Mazighi, R.; Sokolić, F.; Perera, A. On the Microheterogeneity in Neat and Aqueous Amides: A molecular Dynamics Study. J. Phys. Chem. C 2007, 111, 15586–15595. (28) Gereben, O.; Pusztai, L. Investigation of the Structure of Ethanol–Water Mixtures by Molecular Dynamics Simulation I: Analyses Concerning the Hydrogen-Bonded Pairs. J. Phys. Chem. B 2015, 119, 3070–3084. (29) Jessop, P. G.; Kozycz, L.; Rahami, Z. G.; Schoenmakers, D.; Boyd, A. R.; Wechsler, D.; Holland, A. M. Tertiary Amine Solvents Having Switchable Hydrophilicity. Green Chem. 2011, 13, 619–623. (30) Vanderveen, J. R.; Canales, R. I.; Quan, Y.; Chalifoux, C. B.; Stadtherr, M. A.; Brennecke, J. F.; Jessop, P. G. Non-Random Two-Liquid Modelling of Switchable–Hydrophilicity Solvent


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


Systems: N, N-Dimethylcyclohexanamine, Water, and Toluene. Fluid Phase Equilibr. 2016, 409, 150–156. (31) Liu, D.; Huang, Z.; Suo, Y.; Zhu, P.; Tan, J.; Lu, H. CO2‑Responsive Surfactant-Free Microemulsion. Langmuir 2018, 34, 8910–8916. (32) Mays, H. Dynamics and Energetics of Droplet Aggregation in Percolating AOT Water-inOil Microemulsions. J. Phys. Chem. B 1997, 101, 10271–10280. (33) Klossek, M. L.; Marcus, J.; Touraud, D.; Kunz, W. Highly Water Dilutable Green Microemulsions. Colloids Surf. A 2014, 442, 105–110. (34) Lagourette, B.; Peyrelasse, J.; Boned, C.; Clausse, M. Percolative Conduction in Microemulsion Type Systems. Nature 1979, 281, 60–62. (35) Wakisaka, A.; Abdoul-Carime, H.; Yamamoto, Y.; Kiyozumi, Y. Non-ideality of Binary Mixtures Water–Methanol and Wate–Acetonitrile from the Viewpoint of Clustering Structure. J. Chem. Soc., Faraday Trans. 1998, 94, 369–374. (36) Chen, J.; Sit, P. H. L. Ab Initio Study of the Structural Properties of Acetonitrile–Water Mixtures. Chem. Phys. 2015, 457, 87–97. (37) Krishna, R.; van Baten, J. M. Highlighting Pitfalls in the Maxwell–Stefan Modeling of Water–Alcohol Mixture Permeation Across Pervaporation Membranes. J. Membrane Sci. 2010, 360, 476–482. (38) Xu, J.; Yin, A.; Zhao, J.; Li, D.; Hou, W. Surfactant-Free Microemulsion Composed of Oleic Acid, n-Propanol, and H2O. J. Phys. Chem. B 2013, 117, 450–456. (39) Ren, G.; Wang, L.; Chen, Q.; Xu, Z.; Xu, Jian.; Sun, D. pH Switchable Emulsions Based on Dynamic Covalent Surfactants. Langmuir 2017, 33, 3040–3046.



Page 26 of 28

(40) Kunz, W.; Holmberg, K.; Zemb, T. Hydrotropes. Curr. Opin. Colloid Interface Sci. 2016, 22, 99–107. (41) Jain, T. K.; Varshney, M.; Maitra, A. Structural Studies of Aerosol OT Reverse Micellar Aggregates by FT-IR Spectroscopy. J. Phys. Chem. 1989, 93, 7409–7416. (42) Grimaldi, N.; Rojas, P. E.; Stehle, S.; Cordoba, A.; Schweins, R.; Sala, S.; Luelsdorf, S.; Pina, D.; Veciana, J.; Faraudo, J.; Triolo, A.; Braeuer, A. S.; Ventosa, N. Pressure-Responsive, Surfactant-Free CO2‑Based Nanostructured Fluids. Acs Nano 2017, 11, 10774–10784. (43) Rai, R.; Pandey, S. Evidence of Water-in-Ionic Liquid Microemulsion Formation by Nonionic Surfactant Brij-35. Langmuir 2014, 30, 10156–10160. (44) Lucia, A.; Argudo, P. G.; Guzmán, E.; Rubio, R. G.; Ortega, F. Formation of Surfactant Free Microemulsions in the Ternary System Water/Eugenol/Ethanol. Colloids Surf. A 2016, 133, 133–140. (45) Klossek, M. L.; Touraud, D.; Zemb, T.; Kunz, W. Structure and Solubility in SurfactantFree Microemulsions. Chemphyschem 2012, 13, 4116–4119. (46) Lenton, S.; Rhys, N. H.; Towey, J. J.; Soper, A. K.; Dougan, L. Temperature-Dependent Segregation in Alcohol–Water Binary Mixtures Is Driven by Water Clustering. J. Phys. Chem. B 2018, 122, 7884–7894. (47) Artola, P. A.; Raihane, A.; Crauste-Thibierge, C.; Merlet, D.; Emo, M.; Alba-Simionesco, C.; Rousseau, B. Limit of Miscibility and Nanophase Separation in Associated Mixtures. J. Phys. Chem. B 2013, 117, 9718–9727. (48) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. Reverse Micelle Based Formation of BaCO3 Nanowires. J. Phys. Chem. B 1997, 101, 3460–3463.


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


(49) Martıńez-Rodrıǵuez, R. A.; Vidal-Iglesias, F. J.; Solla-Gulloń, J.; Cabrera, C. R.; Feliu, J. M. Synthesis of Pt Nanoparticles in Water-in-Oil Microemulsion: Effect of HCl on Their Surface Structure. J. Am. Chem. Soc. 2014, 136, 1280–1283. (50) Bandyopadhyaya, R.; Kumar, R.; Gandhi, K. S. Modeling of Precipitation in Reverse Micellar Systems. Langmuir 1997, 13, 3610–3620.



Page 28 of 28

TOC Graphic


Microheterogeneity and CO2 Switchability of N,N

Recommend Documents