Confined Complexation Reaction of Metal Ions with a Lipophilic

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Confined complexation reaction of metal ion with lipophilic surfactant at water/air interface: A new understanding based on surface experiments and molecular dynamic simulations Pan Sun, Kun Huang, Xinping Wang, Jiemiao Yu, Olivier Diat, and Huizhou Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00452 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Langmuir

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Confined complexation reaction of metal ion with lipophilic surfactant at

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water/air interface: A new understanding based on surface experiments and

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molecular dynamic simulations

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Pan Sun[a,c], Kun Huang*[a,b], Xinping Wang[d], Jiemiao Yu[a], Olivier Diat[e] and

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Huizhou Liu[a]

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a. Key Laboratory of Green Process and Engineering, Institute of Process Engineering,

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Chinese Academy of Sciences, Beijing 100190, P.R. China.

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b. School of Metallurgical and Ecological Engineering, University of Science and

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Technology Beijing, 30 Xueyuan Road, Haidian, Beijing 100083, P.R. China.

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c. University of Chinese Academy of Sciences, Beijing 100049, P.R. China.

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d. Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, P.R.

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China.

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e. Institut De Chimie Séparative de Marcoule, ICSM,CEA, CNRS, UM, ENSCM, BP

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17171, 30207 Bagnols-sur-Cèze, France.

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*Corresponding Author:

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Prof. Dr. Kun HUANG

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School of Metallurgical and Ecological Engineering, University of Science and

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Technology Beijing, 30 Xueyuan Road, Haidian Distinct, Beijing 100083, P. R. China

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Email: [email protected]

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Tel: 86-10-82544910, Fax: 86-10-62554264

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ACS Paragon Plus Environment

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Abstract: Understanding the fundamentals of confined chemical reaction was an

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important subject in various heterogeneous physicochemical processes. Here, we

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investigated the orientation behavior of an amphiphilic ligand, the tri-n-octylphosphine

4

oxide (TOPO), in a compressed monolayer at air/water interface and its impact on the

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complexation reactivity of TOPO molecule with chromate ions at the interface. The

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analysis of SFG and PM-IRRAS experiments combined with surface pressure

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measurements reveal a significant dependence of the adsorption rate and saturated

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concentration of chromate ions on the orientation of TOPO molecules during the

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increase of the surface pressure. In parallel, the analysis of MD simulations indicates

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that the interaction energy between TOPO molecules and chromate ions is strongly

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sensitive to the orientation of TOPO molecules confined at the water/air interface. The

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present work highlights a novel insight into the unique chemical reactivity of molecules

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in a confined microenvironment and it provides a basis for further progresses in

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improving chemical reactivity and selectivity of molecules in a confined environment

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by regulating confinement of molecules.

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Keywords: Amphiphilic ligand; ion complexation; confined space; molecular

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orientation; ligand at water/air interface 2


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Introduction

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Unique behaviors of molecules in a confined space have attracted tremendous

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attentions in the past decades.1,2,3 It has been demonstrated that chemical reactivity of

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molecules within a confined space could be significantly changed, compared to their

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usual case in a free environment.4,5,6 In recent years, more and more novel one-

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dimensional, two-dimensional and even three-dimensional confined materials have

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been developed in order to achieve artificial regulation of the chemical reactivity or

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selectivity in heterogeneous physicochemical processes.7,8 Bao et al reported that Ru

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nanoparticles confined in carbon nanotubes (CNTs) could improve the catalytic activity

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and selectivity in the hydrogenation of cinnamaldehyde (CAL).9 Besides, the

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microenvironment created by coating the catalyst within the surface of 2D materials

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can be used to modulate surface reactivity, such as the oxygen reduction reaction

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activity on Pt(111).10 Moreover, the embedding of Pt nanoparticles in the

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nanocrystalline metal-organic frameworks (nMOFs) could regulate the activity and

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selectivity

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methylcyclopentane (MCP) to acyclic isomer, olefins, cyclohexane, and benzene.11

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Although great efforts have been made to study the nanoconfined effects in various

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physicochemical processes, fundamental understanding of the unique chemical

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reactivity and selectivity of the reaction in a confined space was still limited.

of

catalytic

reactions

such

as

the

gas-phase

conversion

of

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Unlike the molecules in the bulk circumstance, molecules restricted in a confined

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space, always exhibit distinct orientation behaviors.12,13 It has been demonstrated that

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orientation behaviors of molecules in a confined space play a crucial role in determining 3


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their unique chemical reactivity.14 The study on the orientation behaviors of molecules

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in a confined space could then help to understand their unique chemical reactivity.

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However, complex confined environment makes it challenging to in-situ study the

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orientation behaviors of molecules in the confined space. Compared with the one-

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dimensional and three-dimensional confined systems, two-dimensional confined

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spaces are much more easily controllable.14 The orientation behaviors of molecules in

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the two-dimensional confined spaces can be easily controlled by some methods such as

8

the Langmuir−Blodgett technique and could be appropriately modeled in theoretical

9

calculations15,16 and studied experimentally by surface-selective spectroscopy, such as

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X-ray spectroscopy,17 vibrational sum-frequency-generation spectroscopy (SFG),18

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polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS).19

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Although the orientation behavior of molecules in two-dimensional confined space

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were widely studied, its effects on the chemical reactivity of molecules restricted in the

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confined interface still remain elusive.

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Tri-n-octylphosphine oxide (TOPO) was applied widely as the stabilizer in the

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synthesis of nanoparticle20,21,22 and membrane component in the removal of hazardous

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materials from industrial wastewater materials using emulsion liquid membrane

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(ELM).23,24,25 As a typical amphiphilic molecule, TOPO molecules usually adsorb at

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the solid/liquid or liquid/liquid interface, to promote the stabilization of interface,

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hinder the self-assembly of nanoparticles or facilitate the transport of ions across

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interface. The local asymmetric environments across the interfacial region always

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induce orientation behaviors of TOPO molecules at the interface, which would lead to 4


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distinct chemical reactivity compared to the bulk environment. However, little was

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understood about the distinct chemical reactivity of TOPO molecules with different

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orientation. Detailed studies into the effects of orientation behaviors of TOPO

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molecules on their chemical reactivity at the interface could not only help to improve

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the performance of TOPO molecules in the synthesis of nanoparticle and emulsion

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liquid membrane, but also provide a basis for the further understanding of other

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phenomena concerning orientation of molecules induced by the confined environments.

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In present study, two-dimensional TOPO molecular monolayers at the surface of

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water were constructed to investigate the effect of orientation of TOPO molecules on

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their chemical reactivity towards chromate ions. The confinement of TOPO molecules

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at air/water interface was regulated by the surface pressure of TOPO molecular

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monolayers. The corresponding orientation behaviors of TOPO molecules at the

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interface were elucidated by combined application of SFG and PM-IRRAS. The present

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contribution demonstrated that the adsorption rate and saturated concentration of

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chromate ions on the TOPO molecular monolayers would be strongly affected by the

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orientation behaviors of TOPO molecules. In addition, molecular dynamic simulations

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were employed to elucidate the different chemical reactivity of TOPO molecules

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toward chromate ions from a molecular level.

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Experimental Section

20

Materials

5


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Tri-n-octylphosphine oxide (C24H51PO, TOPO) was purchased from Sigma-Aldrich.

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Dichloromethane and Na2CrO4 were purchased from Sinopharm Chemical Reagent Co.,

3

Ltd. China. All the chemicals were used without further purification. For Langmuir

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monolayer experiments, the spreading solutions were prepared by dissolving TOPO in

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freshly dichloromethane. The spreading solutions were stored at 4°C. The stock

6

solution of Na2CrO4 was prepared in double-distilled water.

7

Langmuir Experiment

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The π-A isotherms for TOPO molecules at water/air interface were performed with a

9

NIMA (U.K.) Langmuir trough placed on an antivibration table. The experimental set-

10

up was shown in the Figure S1. In detail, the spreading organic solutions were dropped

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on the surface of water. After 15 min, the barriers begun to compress at a speed of 5

12

mm/min. The surface pressure was then recorded, and presented in the Figure S2.

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To study the interaction between the TOPO monolayer and the ion injected in the

14

sub-phase, the Δπ versus time curves were obtained according to the following

15

experiments. Firstly, TOPO molecular monolayers were spread at the surface of water

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in a Teflon trough. The sub-phase was 73 mL neat water. The TOPO film was then

17

compressed with a computer controlled barrier to a defined surface pressure between

18

1.5 mN/m and 20 mN/m. After a constant pressure had been maintained for 10 min,

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100 ul stock solutions containing 2.48 mol/L chromate ions were then injected through

20

an injection hole into the sub-phase with a Hamilton syringe. Compared to the volume

21

of sub-phase neat water, the volume of stock solutions injected into the sub-phase was

22

negligible. Thus, the influence from the injection of stock solutions on the height of 6


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sub-phase was negligible. The π versus time curves were then recorded, and processed

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to Δπ versus time curves by subtracting the initial surface pressure. Each new injection

3

of stock solutions containing chromate ions were performed for a new monolayer

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preparation at a different surface pressure. All of the experiments were carried out at a

5

constant temperature of 25°C.

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Langmuir-Blodgett Transfer Experiment

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In order to investigate the amount of chromate ion absorbed on the TOPO monolayer

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at equilibrium, the monolayers was transferred to the quartz substrate with a vertical

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dipping device. The quartz substrate was rinsed thoroughly with distilled water and

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ethanol prior to the deposition of LB film.26 In detail, the quartz substrate was first

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submerged in the sub-phase before the spread of TOPO monolayers. As the equilibrium

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of surface pressure was achieved after the injection of stock solutions containing

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chromate ions, the monolayer was transferred onto the quartz substrate by a single up

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trip at speed of 1mm/min. The surface pressures were kept constant during the

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deposition of monolayer. The transfer ratio of LB film for different initial surface

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pressures were listed in table S1. The quartz substrates loaded with monolayers were

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then washed by kerosene/2mol/L NaOH solutions, and the chromate ions were stripped

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from the substrate into the aqueous solutions. The concentration of chromate ions in the

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aqueous solutions were analyzed by OPTIMA 7000DV inductively coupled plasma-

20

optical emission spectrometer (ICP-OES) (PekinElmer, USA). In addition, to eliminate

21

the amount of chromate ions adsorbed on the quart substrate from the sub-phase

22

aqueous solutions, the quartz substrate was also dipped into the solutions containing 7


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chromate ions without spreading of TOPO monolayers, and the substrate was then

2

processed follow as the described above. Thus, the amount of TOPO-Cr complexes in

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the TOPO monolayers could be calculated.

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Sum Frequency Generation Spectroscopy (SFG)

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Sum frequency generation (SFG) vibrational spectra were collected using a custom-

6

designed EKSPLA SFG spectrometer, which has been described in detail by various

7

researchers.27 To summarize, the visible input beam at 0.532 um was generated by

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frequency doubling of part of the fundamental output from an EKSPLA Nd: YAG laser.

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The IR beam, tunable between 1000 and 4300 cm-1 (with a line width < 6 cm-1), was

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obtained from an optical parametric generation/amplification/difference frequency

11

generation (OPG/OPA/DFG) system based on LBO and AgGaS2 crystals. Both beams

12

were pumped by the second harmonic and the fundamental output of the laser and had

13

a pulse width of 30 ps, a repetition rate of 50 Hz. The incident angles of the visible

14

beam and the IR beam were 60°and 55°, respectively and a typical total beam diameter

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of 0.5 mm with respective energies of B230 and B130 mJ at the sample surface. The

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measurements were carried out at room temperature using polarization combinations,

17

namely ssp (SFG output, visible input, and infrared input). In the SFG spectra presented

18

below, each data point represents an average of 500 laser pulses of five individual

19

experiments, and the standard deviation for each data point is less than 3%. All of the

20

experiments were carried out at a constant temperature of 25°C. A nonlinear least-

21

squares routine was used to fit the SFG spectra. The intensity of the SFG signal (ISFG)

22

was calculated according to the equation (1): 8


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I SFG ∝ χ（Ν2R）+

Α0 ω - ω0 + iΓ 0

2

(1)

2

where,

3

susceptibility, ω is the infrared frequency, and ω0, A0, and 0 are the resonant frequency,

4

transition amplitude and homogeneous width, respectively.28

5

Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-

6

IRRAS)

7

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS)

8

measurements were measured with a KSV PMI 550 instrument (KSV Instrument, Ltd.,

9

Helsinki, Finland). The Langmuir trough was set up so that the light beam reaches the

10

monolayer at a fixed incidence angle of 76°. The incoming light is continuously

11

modulated between s and p polarization at a high frequency, which allows for the

12

simultaneous measurement of the spectra for the two polarizations. The difference

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between the spectra provides surface-specific information, and the sum provides the

14

reference spectrum. With the simultaneous measurements, the effect of the water vapor

15

is largely reduced. All of the experiments were carried out at a constant temperature of

16

25°C.

17

Molecular Dynamics Simulations

18

Molecular dynamics simulations were performed with the program NAMD, version

19

2.10.29 The CHARMM force field30 was used for the simulation of ions. Water

20

molecules was described by TIP3P model.31 A simulation solutions box was firstly built

21

and filled with 2370 H2O, 32 HCrO4-, 32 H+, 10 Cl- and 10 Na+. A 500 ps constant-

χ（2NR）

represents the non-resonant contribution to the surface non-linear

9


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NPT simulation (time step 1 fs) was performed to obtain the reasonable size of solutions

2

box, the final size of box is 33.98 × 33.98 × 61.21 Å. Then, a 1ns constant-NVT

3

simulation (time step 1 fs) was performed to pre-equilibrium the system. Two identical

4

TOPO monolayers were placed in the both sides of solution box, and the phosphorus

5

oxygen group of TOPO molecules were inserted into the surface of solution box. The

6

monolayers with different molecule mean area of TOPO molecules were constructed in

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the present study to simulate the different surface pressure of TOPO monolayers. The

8

numbers of TOPO molecule initially put on each plane and corresponding molecule

9

mean area are listed in Table S2. A vacuum environment is preset at both sides of the

10

TOPO monolayers to eliminate the influences from the three-dimensional periodic

11

boundary conditions.

12

Results and Discussion

13

Dynamic adsorption of chromate ions at air/water interface absorbed with

14

different oriented TOPO molecules

15

Figure 1 gives the time-dependence of the increase in surface pressure of TOPO

16

molecular monolayers after injecting stock solutions containing chromate ions. As

17

depicted in Figure 1, an increase in the surface pressure (abbreviated as Δπ) could be

18

observed after injecting chromate ions into the sub-phase. It can be seen that the

19

equilibrium increment of the surface pressure (abbreviated to Δπe) and the time required

20

to reach Δπe were different for different initial surface pressure. Herein, the Δπ versus

10


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time curves were divided into two types according to their change trends, as presented

2

in Figure 1a and Figure 1b.

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Figure 1. Time-dependence of the increase in surface pressure of TOPO molecular

5

monolayers after injecting Cr(VI) solutions for different initial surface pressure and

6

corresponding fitted curves adjusted using equation (2). (The final concentration of chromate

7

ion in the sub-phase solution was 3.4 mmol/L. Aqueous pH was 2.34. The red line

8

represented fitted curves.)

9

As depicted in Figure 1a and 1b, Δπ increased with the contacting time, and reached

10

different equilibrium value, Δπe, for different initial surface pressure. Interestingly, Δπe

11

increased gradually when increasing the initial surface pressure from 1.5 mN/m to 10

12

mN/m, while the time required to reach Δπe decreased. However, when the initial

13

surface pressure was above 10 mN/m, the Δπe decreased gradually as increasing the

14

initial surface pressure, while the time required to reach Δπe increased. The increase of

15

the surface pressure is due to a lateral electrostatic repulsion within the TOPO

16

monolayers, which is caused by the adsorption of chromate ions. Therefore, the increase

17

of the surface pressure can be correlated to the adsorption of ions at the interface. 11


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Fitting analysis of Δπ-time curves

2

Furthermore, the increase of the surface pressure could be attributed to the formation

3

of TOPO-Cr complexes at air/water interface. Hence, it can be used to estimate the

4

saturated concentration of chromate ions adsorbed at the TOPO molecular monolayers.

5

Herein, we used the concentration ratio of TOPO-Cr complexes to total TOPO

6

molecules at equilibrium, ceTOPO-Cr/cTOPO, to represent the saturated concentration of

7

chromate ions on the TOPO molecular monolayers. For this investigation, an

8

adsorption kinetic equation was used to describe the time dependence of Δπ, as shown

9

in equation (2) and detailed in the supporting information.

10

,

∆π = ∆𝜋 𝑒 (1 − 𝑒 −𝑘 𝑡 )

(2)

11

where k’ represents the increase rate of Δπ, which is related to the adsorption rate of

12

chromate ions on the TOPO molecular monolayers. Δπe represents the equilibrium

13

increment of surface pressure.

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Δπ-time curves in Figure 1 were fitted in a rather good agreement with equation (2)

15

and the fitted curves were also depicted as the red line in Figure 1. Figure 2 depicts the

16

variation of k’, ceTOPO-Cr/cTOPO and the occupy area per TOPO molecule, ATOPO, with

17

different initial surface pressure.

18

12


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Figure 2. Plots of the variation of k’, ceTOPO-Cr/cTOPO and the surface per TOPO polar head,

3

ATOPO, before ion injection as a function of the initial monolayer pressure. Three regions, A,

4

B and C, can be defined corresponding to different trend of variation in k’ and ceTOPO-Cr/cTOPO

5

with the increase of initial surface pressure.

6

As shown in Figure 2, both k’ and ceTOPO-Cr/cTOPO first increased gently (zone

7

A), then increased more significantly (zone B) to reach their maximum at the

8

initial surface pressure of 10 mN/m and finally decreased abruptly with the

9

increase of initial surface pressure (zone C). The results indicated that optimal

10

surface per TOPO molecule for the most favourable complexation of TOPO

11

molecules with chromate ions at the interface was about 1 nm2.

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Notably, although the apparent concentration of TOPO molecules at air/water

13

interface increased with increasing surface pressure of molecular monolayers, the

14

complexation reactivity of the functional groups in TOPO molecules with

15

chromate ions did not vary in a monotonous way. We supposed that might be

16

attributed to the changes in orientation behavior of TOPO molecules at the

17

interface during the increase of surface pressure. Thus, knowledge of the

18

orientation behavior of TOPO molecules at the interface is of primary importance 13


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for rationalizing these observed variations in the Figure 2 and is then essential

2

for understanding of the unique ion complexation in respect to TOPO molecules

3

orientation.

4

Orientation behavior of TOPO molecules at the interface

5

The orientation behavior of TOPO molecules at the interface could be

6

described by the orientation of its aliphatic tails and phosphorus functional group,

7

respectively.32 In order to elucidate the changes in the orientation behavior of

8

TOPO molecules at air/water interface during the compression of the TOPO

9

monolayers, interface-selective spectral techniques including Sum Frequency

10

Generation (SFG) and Polarization Modulation Infrared Reflection Absorption

11

Spectroscopy (PM-IRRAS) will be carried out in the present study, as reported

12

in the other works.32,33

13 14

Figure 3. a) SFG spectra of TOPO molecular monolayers at equilibrium after ion injection

15

and for different initial surface pressure (The curves I to X correspond to the different

16

systems submitted to an initial surface pressure of 1.5 mN/m, 2.5 mN/m, 5.0 mN/m, 7.5

17

mN/m, 10.0 mN/m, 12.0 mN/m, 14.0 mN/m, 16.0 mN/m, 18.0 mN/m, 20.0 mN/m, 14


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Langmuir

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respectively). b) Ir+/Id+ of TOPO molecular monolayers under different initial surface

2

pressure. The three zones, A, B and C, already defined in figure 3 were drawn on the graph.

3

Figure 3a depicts the SFG spectra of TOPO molecular monolayers under

4

different surface pressure. The peaks located at about 2840 cm-1 and 2870 cm-1

5

represent the symmetric methylene (CH2) and symmetric methyl (CH3)

6

stretching modes, respectively. It was demonstrated that the intensity ratio of the

7

symmetric methyl (CH3) and symmetric methylene (CH2) stretching modes

8

(Ir+/Id+) can be used as an indicator of the relative order within the aliphatic

9

tails.34,35,36

10

Figure 3b gives the variation of Ir+/Id+ with the increase of surface pressure. As

11

depicted in Figure 3b, an increase in Ir+/Id+ can be observed as increasing the

12

surface pressure, suggesting a change in the orientation of aliphatic tail at the

13

interface. Interestingly, the changing trend of Ir+/Id+ could be roughly divided into

14

three regions, which can characterize different transition of aliphatic tails

15

orientation in the TOPO molecules. In region A, the degree of orientation of the

16

TOPO aliphatic chain is rather low as expected for a gas-analogous phase of the

17

ligands at the interface but very sensitive to the compression. In region B, TOPO

18

molecular monolayer transits into a liquid-expanded phase and the degree of

19

orientation of the aliphatic tails are oriented moderately at the interface. At higher

20

compression, in region C, we can consider that TOPO monolayers is compressed

21

into a liquid-condensed phase characterized by a much ordering state of the

22

aliphatic tails and that varies sensitive to the initial surface pressure. 15


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1 2

Figure 4. a) PM-IRRAS spectra of TOPO molecular monolayers on the surface of water

3

under different surface pressure (The curves I to XI represent the surface pressure of 0.0

4

mN/m, 1.5 mN/m, 2.5 mN/m, 5.0 mN/m, 7.5 mN/m, 10.0 mN/m, 12.0 mN/m, 14.0 mN/m,

5

16.0 mN/m, 18.0 mN/m, 20.0 mN/m, respectively). b) PM-IRRAS intensity of ν ss(P=O) in

6

the TOPO monolayers versus initial surface pressure. The three zones, A, B and C, already

7

defined in figure 2 were drawn on the graph.

8

Furthermore, orientation behavior of the phosphorus functional group in

9

TOPO molecules was investigated by PM-IRRAS, and the results are depicted

10

in Figure 4a. The peaks located at about 1150 cm-1 was attributed to the

11

symmetric stretching vibration band of phosphorus functional group in TOPO

12

molecules.37 It was further verified by the IR spectra of TOPO in the Figure S8

13

in the supporting information. It is clear that the intensity of symmetric stretching

14

vibration band of phosphorus group decreases gradually with the increase of

15

surface pressure, and finally turn from a positive band into a negative band

16

(Figure 4b). According to the selection rule of PM-IRRAS under the

17

experimental conditions, transition moments preferentially oriented in the plane

18

of the interface give intense and upward oriented bands, while perpendicular 16


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Langmuir

1

moments give weaker and downward oriented bands.38 The results depicted in

2

Figure 4b demonstrate that the phosphorus functional group in the TOPO

3

molecules turn gradually from being parallel into perpendicular to the interface,

4

and its orientation behavior could also be divided into three zones, described as

5

A, B and C inserted in Figure 4b and as already defined for the previous

6

investigations.

7

Combining the analysis about the orientation behavior of aliphatic tails and

8

phosphorus functional group in TOPO molecules with the increase of surface

9

pressure, we could conclude that the orientation variation of the TOPO molecules

10

at air/water interface divided into three zones. It affects strongly the adsorption

11

rate of chromate ion and the possible complexes formation between TOPO

12

molecules and the anions. The optimal ratio of ceTOPO-Cr/cTOPO appears to be

13

correlated with a strong orientation of the phosphoric polar group of the TOPO

14

and a not so dense packing of the aliphatic tails: a subtle compromise. However,

15

a detailed study is necessary to elucidate the effects of orientation of TOPO

16

molecules on the chemical reactivity of TOPO molecules toward chromate ions.

17

Molecular dynamic simulations of interaction between oriented TOPO

18

molecules and chromate ions

19

To gain more molecular level understanding of interaction between orientated

20

TOPO molecules and chromate ions, molecular dynamic simulations were

21

carried out in the present study. Here, the orientation of TOPO molecules at the 17


Langmuir 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

1

interface was regulated by the packing density of TOPO molecules at the

2

interface. Snapshot of initial simulation box for the system III was depicted in

3

Figure S4 in the supporting information.

4

Figure 5 gives the distribution probability of the tilt angle θ between the

5

hydrophobic chains of TOPO molecules with respect to the normal of the

6

interface. As can be seen that the mean value of the tilt angle distributions for

7

system I to system IX decrease from 55°to 29°. With the increase of packing

8

density of TOPO molecules within the monolayers, TOPO molecules gradually

9

turned from being parallel to the surface of water into being perpendicular to the

10

surface of water. In addition, the density profiles of TOPO molecules across the

11

air/liquid interface also demonstrated similar behavior of TOPO molecules at the

12

interface during the increase of packing density of TOPO molecules at the

13

interface (seen in the Figure S5 in the supporting information).

14

15

Figure 5. Distribution probabilities of the tilt angle (θ) of the tail in TOPO with respect to the normal

16

of the interface. (System I to IX represent 8, 9, 10, 11, 12, 13, 14, 15, 16 TOPO molecules adsorbed

17

at the air/liquid interface.) 18


Page 18 of 48

Page 19 of 48

1

Figure 6a gives the density profiles of chromate ions near the air/liquid

2

interface. The peak located at about 32.5 Å represent an enriched region of

3

chromate ions near the interface due to the interaction of TOPO molecules with

4

chromate ions. To analyze the evolution of these density profiles, we divided it

5

into two parts, as shown in Figure 6b and Figure 6c. We observe a peak of density

6

at around 34 Å that first increase in intensity as increasing the number of TOPO

7

molecules from 8 to 11, which demonstrated an increase in the adsorption amount

8

of chromate ions at the TOPO molecular monolayers. While the enrich peak of

9

chromate ions gradually be away from the interface as increasing the number of

10

TOPO molecules from 12 to 16, which indicated a decrease in the adsorption

11

amount of chromate ions at the TOPO molecular monolayers. The analysis of

12

density profiles indicated that the adsorption of chromate ions at the TOPO

13

molecular monolayers increased firstly, then decreased gradually with the

14

increase in the density of TOPO molecules at the interface. The simulation results

15

were in consistent with the above experimental results. a

3

0.20 0.15 0.10

0.25

0.05 0.00 25.0

I II III IV V

3

0.25

16

0.30 b

I II III IV V VI VII VIII IX

Density (g/cm )

0.30

Density (g/cm )

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

Langmuir

0.20 0.15 0.10 0.05

27.5

30.0

32.5 35.0 37.5 Distance (Å)

40.0

42.5

0.00 25.0

27.5

30.0

32.5

35.0

Distance (Å)

19


37.5

40.0

42.5

Langmuir

0.25

c

VI VII VIII IX

0.20 3

Density (g/cm )

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

1

0.15 0.10 0.05 0.00 25.0

27.5

30.0

32.5 35.0 Distance (Å)

37.5

40.0

42.5

2

Figure 6. a-c. Density profiles of chromate ions near the air/liquid interface (System I to IX

3

represent 8, 9, 10, 11, 12, 13, 14, 15, 16 TOPO molecules adsorbed at the air/liquid interface); d.

4

Simulation box snapshot of the equilibrium configuration of the system V. (Coloring is as follows:

5

water was represented by cyan transparent box, chromate ions are represented by yellow spheres.

6

Hydrogen ions are represented by green spheres. Sodium ions are represented by blue spheres.

7

Chloride ions are represented by red spheres. The oxygen, phosphorus, carbon, and hydrogen atoms

8

in the TOPO molecules are represented by the red, tan, cyan and white sticks, respectively.)

9

Besides, radical distribution function (RDF) between TOPO molecules and

10

chromate ions were also analyzed for different systems, as depicted in Figure S6.

11

The results indicated that distribution of chromate ions around the O atom of

12

TOPO molecules increased firstly, then decreased gradually, during the increase

13

of the density of TOPO molecules at the interface. It was also inconsistent with

14

the trend in the experimental results above.

15

To obtain more quantitative information about the interaction of TOPO

16

molecules with chromate ions, we analyzed the interaction energy of TOPO

17

molecules with chromate ions. Figure 7 depicts the effect of initial surface

20


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Langmuir

1

pressure of TOPO molecular monolayers on the interaction energy of TOPO

2

molecules with chromate ions.

3 4

Figure 7. Variation of the interaction energy between TOPO molecules and chromate ions as the

5

function of the initial surface pressure of TOPO molecules at the interface.

6

As shown in Figure 7, the interaction energy of TOPO molecules with

7

chromate ions increased firstly, then decreased with increasing the surface

8

pressure. Its change trend was consistent with that of adsorption rate and loaded

9

capacity of chromate ions at the TOPO molecular monolayers, as depicted in

10

Figure 2. The results further demonstrated that the complexation activity of

11

TOPO molecules with chromate ions was strongly sensitive to the orientation

12

behavior of TOPO molecules at the air/water interface. We suggested that

13

specific complexation behavior might be existed at the interface, which was

14

responsible for the unique orientation dependence of complexation activity of

15

TOPO molecules with chromate ions. To clarify the mechanism underlying this

16

phenomenon, it is necessary for us to elucidate the interfacial complexation

17

behavior at the equilibrium state. 21


Langmuir 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

1

Figure 8 gives snapshot of interfacial regions of simulation boxes at the

2

equilibrium state for the systems I, V, and IX. Compared with the system I and

3

system IX, significantly enhanced adsorption of chromate ions at the TOPO

4

monolayers/water interface was observed in the system V. It was consistent with

5

the results of interaction energy in the Figure 7.

6 7

Figure 8. Snapshot of interface regions for the system I (a), V (b) and IX (c). (Coloring is as follows:

8

the chromium, oxygen, hydrogen atoms in the chromate ion are represented by cyan red and white

9

spheres, respectively. Hydrogen ions are represented by green spheres. The oxygen, phosphorus,

10

carbon, and hydrogen atoms in the TOPO molecules are represented by red, tan, cyan and white

11

sticks, respectively. H2O are represented by red-white line.)

12

To understand the variation in adsorption behavior of chromate ions at the

13

TOPO molecular monolayers, it is essential to elucidate the specific

14

complexation mechanism of TOPO molecules with chromate ions in a confined

15

interface. Firstly, the radical distribution function of water molecules around

16

chromate ions was analyzed (seen Figure S7 in the supporting information). The 22


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Langmuir

1

result indicated that hydration radius of chromate ions was about 0.60 nm,

2

suggesting the sectional area of hydrated chromate ions was about 1.13 nm2.

3

Notably, the sectional area of hydrated chromate ions is close to the optimal

4

occupy area per TOPO molecule being beneficial to the complexation with

5

chromate ions as depicted in the Figure 2. Thus, we supposed that the ratio of

6

TOPO molecules to chromate ions in the complex was 1:1 at the most favourable

7

complexation condition. To further understand the effect of confinement on the

8

interaction between TOPO molecules and chromate ions, we analyzed the

9

snapshot of TOPO-Cr complex at equilibrium state for the systems I, V, and IX,

10

as depicted in the Figure 9.

11 12

Figure 9. The complex of TOPO molecules and chromate ion at the interface for the system I (a),

13

V (b) and IX (c). (Coloring is as follows: the chromium, oxygen, hydrogen atoms in the chromate

14

ion are represented by cyan, red and white spheres, respectively. Hydrogen ions are represented by

15

green spheres. The oxygen, phosphorus, carbon, and hydrogen atoms in the TOPO molecules are

16

represented by red, tan, cyan and white sticks, respectively.)

23


Langmuir 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

1

As show in Figure 9a, the complexation reaction occurred between two TOPO

2

molecules and one chromate ions when TOPO molecules were confined in the

3

loose environment. When TOPO molecules were confined in a highly compact

4

environment, chromate ions were excluded from the TOPO molecular

5

monolayers (Figure 9c). Interestingly, the optimal complexation occurred

6

between one TOPO molecules and one chromate ions when TOPO molecules

7

were confined in the appropriately confined environment (Figure 9b). The ratio

8

of TOPO molecules to chromate ions in the complexes was indeed consistent

9

with our previous hypothesis. Notably, the complex of TOPO molecules with

10

chromate ions would combine with another neighboring complex to form a kind

11

of dimer at the interface when TOPO molecules were confined in the

12

appropriately confined environment. These dimerized complexes owned lower

13

energy compared with the individual complexes. Thereby, we suggested that the

14

formation of these dimers at the interface facilitated the adsorption of chromate

15

ions at the TOPO monolayers. When TOPO molecules were too close or too

16

away from each other at a confined interface, the steric hindrance hindered the

17

formation of dimerized complex of TOPO molecules with chromate ions. As a

18

result, low complexation reactivity of TOPO molecules with chromate ions was

19

exhibited in these confined environments. When TOPO molecules were

20

restricted within the appropriately confined environment, the appropriate

21

orientation of TOPO molecules facilitated the formation of dimerized complex

24


Page 24 of 48

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Langmuir

1

of TOPO molecules with chromate ions. Thus, enhanced reactivity of TOPO

2

molecules was exhibited for their complexation with chromate ions.

3

Conclusions

4

In summary, we demonstrated that orientation behavior of TOPO molecules in two-

5

dimensional confined TOPO molecular monolayers has significant influences on the

6

complexation reactivity of TOPO molecules with chromate ions at interfaces. The

7

experimental and molecular dynamic simulations results indicated that TOPO

8

molecules with an appropriate orientation at the confined interface exhibited enhanced

9

complexation reactivity toward the chromate ions. The dimerized complex of TOPO

10

molecules with chromate ions would form in the appropriately confined environment,

11

which facilitated the adsorption of chromate ions at the TOPO molecular monolayers.

12

By contrast, TOPO molecules restricted in a highly compact or loose environment

13

would inhibit the formation of stable dimerized complex, and therefore exhibited poor

14

complexation reactivity to the chromate ions.

15

The present work highlight a general tendency for enhanced interfacial complexation

16

reactivity of amphiphilic molecules with ions under the appropriately confinement of

17

amphiphilic molecules in a confined space. It illustrated the feasible modulation of

18

interface reactions by controlling orientation of molecules at the interfaces, and was

19

excepted to provide new insight for further progresses in developing new strategies to

20

intensify the interfacial reactions, transfer processes across liquid/liquid interface

21

through regulating the interfacial structures. 25


Langmuir 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

1

Supporting information

2

The experimental set-up, adsorption kinetic equation, other simulation results and IR spectra

3

of TOPO were provided in the supporting information.

4

Notes

5

The authors declare no competing financial interest.

6

Acknowledgements

7

This work was financially supported by the National Natural Science Foundation of

8

China (Nos. 51574213, 51074150), and the Key Project of Chinese National Programs

9

for Fundamental Research and Development (973 Program No. 2013CB632602). We

10

thank the Supercomputing Center of Chinese Academy of Sciences for allowing us to

11

use the ScGrid for theoretical calculations.

12

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10 11 12 13 14 15 16 17 18 19 20 21 22 31


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1

TOC GRAPHICS

2

3 4 5

Chemical complexation reactivity of lipophilic extractant and metal ions at water/air

6

interface are regulated by the orientation of lipophilic extractant in the confined two-

7

dimensional space.

8

32


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254x190mm (300 x 300 DPI)


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254x179mm (300 x 300 DPI)


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Langmuir

381x238mm (300 x 300 DPI)


Langmuir 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

254x201mm (300 x 300 DPI)


Page 36 of 48

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Langmuir

254x194mm (300 x 300 DPI)


Langmuir 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

254x202mm (300 x 300 DPI)


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Langmuir

254x182mm (300 x 300 DPI)


Langmuir 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

359x307mm (96 x 96 DPI)


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Langmuir

254x180mm (300 x 300 DPI)


Langmuir 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

304x210mm (300 x 300 DPI)


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Langmuir

304x211mm (300 x 300 DPI)


Langmuir 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

254x177mm (300 x 300 DPI)


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Langmuir

398x213mm (96 x 96 DPI)


Langmuir 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

254x190mm (300 x 300 DPI)


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Langmuir

572x315mm (96 x 96 DPI)


Langmuir 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

455x283mm (96 x 96 DPI)


Page 48 of 48

Confined Complexation Reaction of Metal Ions with a Lipophilic

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