Insights into the Relationship between CO2 Switchability and Basicity

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Insights into the Relationship between CO2-Switchability and Basicity: Examples of Melamine and its Derivatives Hongyao Yin, Yujun Feng, Hanbin Liu, Meng Mu, and Chenghong Fei Langmuir, Just Accepted Manuscript • DOI: 10.1021/la501461n • Publication Date (Web): 04 Aug 2014 Downloaded from http://pubs.acs.org on August 12, 2014

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1

Insights into the Relationship between CO2-

2

Switchability and Basicity: Examples of Melamine

3

and its Derivatives

4 † †§ †§ †§ Hongyao Yin, ,§ Yujun Feng,*,†,‡ Hanbin Liu, , Meng Mu, , and Chenhong Fei ,

5

6 7

†

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR

8 9

China. ‡

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan

10 11

University, Chengdu 610065, PR China. §

University of the Chinese Academy of Sciences, Beijing 100049, PR China.

12 13

Keywords: CO2-switchability; melamine derivatives; basicity; protonation–deprotonation

14

CO2-switchability of melamine and its derivatives

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Abstract

2

Owing to its wide availability, non-toxicity, and low-cost, CO2 working as a trigger to reversibly

3

switch material properties, including polarity, ionic strength, hydrophilicity, viscosity, surface

4

charge, and degree of polymerization or crosslinking, has attracted an increasing attention in

5

recent years. However, quantitative correlation between basicity of these materials and their

6

CO2-switchability has been less documented though it is of great importance for fabricating

7

switchable system. In this work, the “switch-on” and “switch-off” abilities of melamine and its

8

amino-substituted derivatives by introducing and removing CO2 are studied, and then their

9

quantitative relationship with basicity is established, so that performances of other organobases

10

can be quantitatively predicted. These findings are beneficial for forecasting the CO2-stimuli

11

responsive behavior of other organobases and the design of CO2-switchable materials.

12

Introduction

13

Since the pioneering work of Jessop team1 on CO2-switchable solvents, CO2, as a novel and

14

environmentally-benign trigger, has been playing an active role in fabricating switchable

15

surfactants,2,3 solutes,4 polymers,5−7 ionic liquids,8−11organogels,12,13 wormlike micelles,14−16 as

16

well as nano hybrids,17 all of which possess at least one basic group mainly including

17

amidine,1,2,5,18 guanidine3,19 or amine20−23 to react with CO2 in the presence of water. Among

18

these organobases, guanidine is relatively less used because of its “superbase” character which

19

makes it hard to be “switched-off” once it is reacted with CO2.3 Although their basicity is weaker

20

than that of guanidine, amidine-containing compounds are normally difficult to be synthesized

21

and prone to be hydrolytically instable,24,25 and amidine-based monomers display poor

22

polymerization activity, which seriously limits its application in polymers.5 On the contrary, as a CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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mild, stable, and readily available organobase, amines are the most commonly used CO2-

2

switchable functional groups.14,16,26,27 Generally, their protonated products, especially those

3

generated from the reaction of tertiary amines with CO2 in a wet or aqueous environment, are

4

unstable and easy to release CO2 to return to the original state even at room temperature,16

5

indicative of better reversibility and environmentally-benign nature.3 Several earlier

6

studies3,5,25,28,29 described the significant influence that the intrinsic basicity of organobases

7

exerts on their CO2-switchability. And Jessop group,3,28 reported that bicarbonate salts from less

8

basic compounds show better reversibility than those from more basic compounds. However, the

9

quantitative correlation between basicity and switchability has not been established possibly for

10

the lack of homologous compounds with a wide range of basicity, yet it is of great importance

11

for fabricating switchable systems. Furthermore, most of the amines in previous studies are

12

linear compounds that usually bear one single amino group. And cyclic polyamines containing

13

several or even more amino groups are expected to show high sensitivity and effective CO2-

14

switchability, but few studies have been documented so far.20

15

As a typical cyclic organobase, melamine, bearing an s-triazine ring along which three primary

16

amines are evenly distributed on the side-chains,30 is expected to enjoy strong CO2-sensitivity.

17

More importantly, modification by introducing different groups onto the amino nitrogen is

18

readily done to obtain a series of amino-substituted derivatives with different basicities.31 The

19

homologous structure is helpful to correlate their basicity with CO2-switchability. Although

20

melamine and its derivatives can be protonated by such a common acid as halogen acid and the

21

protonated species are widely employed as self-assembly,32 molecular recognition materials,33

22

flame retardants,34 and drugs,35 there are no reports to date, to our knowledge, focusing on

CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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protonation of these cyclic organobases using CO2. If CO2-switchability is imparted on these

2

compounds, “smart” response of these materials can be envisaged.

3

Herein, our primary goal is to evaluate the CO2-switchability of melamine and its amino-

4

substituted derivatives (Scheme 1) by examining the protonation form, protonation degree,

5

reversibility, and next, to establish the relationship between basicity and CO2-switchability for

6

this series of specifically-tailored amine compounds. To this end, methyl group was used to

7

replace one of the hydrogen atoms in the amino nitrogen of the melamine to give N2-methyl-

8

1,3,5-triazine-2,4,6-triamine (2), and then more complicated dimethylamino alkyl groups were

9

used as the substituents to afford N2-(2-(dimethylamino)ethyl)-1,3,5-triazine-2,4,6-triamine (4)

10

and N2-(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (5), respectively. Furthermore,

11

tri-substituted derivatives, N2,N4,N6-trimethyl-1,3,5-triazine-2,4,6-triamine (3), N2,N4,N6-tris(2-

12

(dimethylamino)ethyl)-1,3,5-triazine-2,4,6-triamine

13

(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (7), were prepared for comparison.

14

Different substituents are expected to endow the derivatives with different basicities, thus allow

15

the correlation with their CO2-switchability.

(6)

and

16 17

1

2

18


3

N2,N4,N6-tris(3-

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4

5

2 3

6

7

4

Scheme 1. Molecular structures of melamine and the series of specifically-tailored amino-

5

substituted derivatives used in this work.

6

Experimental Details

7

Materials

8

2-Chloro-4,6-diamino-1,3,5-trazine

(99%),

cyanuric

chloride

(99%),

2-

9

dimethylaminethylamine (99%), 3-dimethylaminopropylamine (99%), methylamine (40wt% in

10

water), were all purchased from Sigma–Aldrich and used without further treatment. Water that

11

was triply distilled by a quartz water purification system was used throughout this study. CO2

12

(≥99.998%) and N2 (99.998%) were used as received. The other reagents and solvents were

13

obtained from Shanghai Chemical Reagent Co., Ltd., and the solvents used in the reactions were

14

distilled prior to use.

15

Synthesis

16

The compounds 2, 4 and 5 were prepared by using 2-chloro-4,6-diamino-1,3,5-trazine to react

17

with methylamine, 2-dimethylaminethylamine and 3-dimethylaminopropylamine, respectively; 3,

18

6 and 7 were obtained in a similar manner but with cyanuric chloride as the starting material


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instead. Detailed synthesis and characterizations of these six compounds are given in the Figures

2

S1–S26 of Supporting Information (SI).

3

Characterizations

4

1

H and

13

C NMR spectra were recorded at 25 °C on a Bruker AV300 NMR spectrometer at

5

300 and 75 MHz, respectively. Chemical shifts (δ) were reported in parts per million (ppm) with

6

reference to the internal standard protons of tetramethylsilane (TMS). The NMR spectra of the

7

CO2-treated compounds were obtained by directly bubbling CO2 into an NMR tube with about 6

8

mg compound in the 0.6 mL solvent.

9 10

ESI–HRMS spectra were obtained with the Bruker Daltonics Data Analysis 3.2 system. Infrared spectra were registered on a Nicolet MX-1E FTIR spectrometer in the scanning range

11

of 4000–400 cm–1 using KBr pellet method.

12

Monitoring of pH Value

13

The variation of pH value of the base aqueous solutions (20 mL, 11.9 mM; all the bases in this

14

work can be fully dissolved in water and display obvious CO2-responsiveness at this

15

concentration) under CO2 bubbling was continuously monitored at 25 °C with a Sartorius basic

16

pH-meter PB–10 (± 0.01) calibrated with standard buffer solutions. The CO2 flow rate was fixed

17

at 3 mL⋅min–1.

18

Determination of pKaH value

19

The pKaH (pKa of the protonated species) values of the seven compounds were determined by

20

titrating 20 mL of 11.9 mM aqueous solutions with 100 mM hydrochloric acid, and the pH was

21

continuously monitored at 25 °C with the same pH meter mentioned above. The pKaH values

22

were obtained by taking the pH readings at the mid-point between two pH jumps.14 CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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Conductivity measurement

2

The conductivity (κ) of 20 mL 11.9 mM base aqueous solution was recorded with an EF30

3

conductometer (Mettler Toledo, USA) at 25 °C while bubbling CO2 or N2 through a syringe

4

needle with inner diameter of 1.5 mm. The large CO2 flow rate leads the switch “on” process too

5

fast and the experimental data are hard to be well recorded, whereas the small N2 flow rate

6

makes the switch “off” process too slow and takes too much time to measure; thus we fixed the

7

CO2 and N2 at different rates, 3 mL⋅min–1 for CO2 and 15 mL⋅min–1 for N2. The conductivity of

8

pure water under the same condition was also monitored as a reference.

9

UV–vis spectroscopy

10

The absorbance of base aqueous solutions with concentration of 1.2×10–5 M was recorded on a

11

double-beam UV–vis spectrophotometer (UV–4802, Unico, USA) at 25 °C, controlled by a

12

water circulating bath, over the wavelength range 190–450 nm.

13

Deprotonation test

14

The deprotonation ability of protonated amines was demonstrated by both of half time (t1/2, the

15

time when protonation degree δ is lowered to half of the maximum value upon the CO2 treatment)

16

and basicity. 20 mL 11.9 mM base aqueous solutions were prepared in 25-mL glass vials without

17

cap, followed by CO2 bubbling until the pH value of solutions remained unchanged. Then N2 at

18

the flow rate of 15 mL⋅min–1 was bubbled into the solutions via a syringe needle with inner

19

diameter of 1.5 mm at the set temperature. The pH was measured at set intervals when the

20

solutions were cooled down to 25 oC and then the protonation degree was calculated according to

21

Eqs. (1) and (2) which appear later.


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Results and Discussion

2

Protonation forms of melamine and its derivatives

3

To study the CO2-switchability of melamine and its derivatives, their protonation forms were

4

investigated firstly. An earlier study36 showed that melamine forms positive ions by assuming a

5

structure with one exocyclic double bond to an =NH2+ group when it is protonated, and the

6

amino-substituted derivatives act in the same manner. Meanwhile, the new conjugated structure

7

of protonated melamine will cause a new absorption band at around 240 nm in UV–vis spectra.37

8

Hence, it is easy to investigate the protonation forms of these melamine derivatives via

9

measuring their UV–vis absorbance upon treatment with CO2. The spectra of compound 1 and 6

10

in aqueous solution (Figure 1) before and after bubbling of CO2 are shown here as an example,

11

and the others can be found in Figure S27 of SI.

12 13

Figure 1. UV–vis spectra of melamine (1) and its derivative (6) in aqueous solution before and

14

after introducing CO2.

15

A new absorbance around 240 nm after bubbling CO2 was observed for the amines except 6

16

and 7, and the initial absorbance was still in existence, implying their melamine structure is CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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partially protonated by carbonic acid. Then the NMR spectroscopy characterization was

2

employed to further confirm the protonation forms. Since the protonated structure of melamine is

3

clear and its amino-substituted derivatives follow the same pattern, the protonation forms of 2, 3

4

as well as the mother structure of 4, 5 are deduced and illustrated in Scheme 2. However, no

5

obvious signal changes were observed in both 1H and 13C NMR spectra of 1, 2 and 3 between the

6

original and those treated by CO2 in wet DMSO-d6. To uncover the cause, the 1H and 13C NMR

7

spectra of compounds 1, 2 and 3 with one equivalent of added HCl (mono-protonation on

8

average) were measured and compared with the initial spectra (Figures S28-30, SI). The signals

9

of these mono-protonated samples experienced an obvious shift in both the 1H and

13

C NMR

10

spectra, demonstrating that the lack of an evident change of NMR spectra upon CO2 addition

11

results from the small protonation degree.

12

In contrast, the changes in both 1H and 13C NMR signals for 4, 5, 6, and 7 are quite obvious.

13

Here take compound 4 as an example for analysis. The peak at 2.08 ppm in the 1H NMR

14

spectrum of 4 (Figure 2a) is attributed to the gemini terminal –CH3 groups (Figure 2b). Two

15

triplets were observed at 2.35 and 3.23 ppm, which were assigned to the carbons 2a and 3a in the

16

methylene group, respectively. After bubbling CO2, obvious change in the chemical shift of 1H

17

NMR spectrum was observed, where the peaks of 1a and 2a have shifted downfield by around

18

0.7 ppm while the peak of 3a has shifted downfield only by 0.2 ppm. For the purpose of

19

comparison, the 1H NMR and

20

examined (Figures S31, SI). The signal of carbons 1a, 2a and 3a in 1H NMR spectrum

21

experienced a more obvious shift than that when CO2 was bubbled, indicating that CO2 cannot

22

fully protonate these tertiary amino groups in the mixed solvent of DMSO-d6 and D2O.

13

C NMR spectra with one equivalent of added HCl were


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Meanwhile, it also demonstrates the protonation only on the tertiary amino group by comparing

2

the 13C NMR spectra. a) 5a

1a

3a N N

1a

3

N

2a H 4a

HCO3 -

NH2 CO2 , H 2O

N N

NH2

5b

1b NH2

5a

3b N NH

1b

N

2b H

N N

4b 5b

NH2

4

5 6

Figure 2. Comparison of NMR spectra of compound 4 in the mixed solvent of DMSO-d6 and

7

D2O (5:1, v/v) with and without CO2 treatment. (a) Numerical assignment of 4 and

8

corresponding protonation form; (b) 1H NMR spectrum; and (c) 13C NMR spectrum.

9

Additionally, a new peak around 160.0 ppm is evidenced in

13

C NMR spectrum (Figure 2c),

10

implying HCO3– was formed. In short, the chemical shifts in both 1H and 13C spectra distinctly

11

demonstrate that tertiary amino moiety in melamine derivatives can react with CO2 in the

12

presence of water to generate corresponding ammonium bicarbonate salt.15,27 CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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The NMR spectra of compounds 5, 6 and 7 also demonstrate that the tertiary amino group can

2

be protonated by carbonic acid and converted to corresponding bicarbonate salts. Their

3

comparative NMR spectra with and without CO2 were listed in Figures S32–S34 of SI. The

4

protonated structure of them are shown in Scheme 2.

5 6

1

2

3

7 8

4

5

9 10

6

7

11

Scheme 2. Protonation forms of melamine and its derivatives.

12

Basicity of melamine and its derivatives


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The key to establishing the quantitative relationship between basicity of organobases and their

2

CO2-switchability is to obtain homologous compounds with gradually varied basicity. Therefore,

3

the basicity of different amines used in this study was determined.

4

Generally, the basicity of organobases is characterized by pKaH value, and the larger the pKaH,

5

the stronger the basicity is. For instance, the pKaH of most small molecule amines, such as N,N-

6

dimethyloctylamine, benzylamine and 4-aminopyridine, is usually centered at 9,3,20,38 while the

7

pKaH value of short-chain alkyl-substituted guanidine derivatives approaches 14 because they are

8

widely recognized as super organobases.3,25,39 The pKaH values of the seven bases and the

9

corresponding reaction sites are shown in Scheme 2. The original titration curves are depicted in

10

Figure S35 of SI.

11

The pKaH values of these amines range from 4.1 to 9.4. It is worth noting that the three tertiary

12

amino groups of compounds 6 and 7 should have three different pKaH values; however, the

13

method used in this work was unable to resolve them, so that they present a merged value.

14

Moreover, the compounds 1, 2, and 3 just have a single pKaH value, implying that only one type

15

of amino group is protonated in this case, while the compounds 4, and 5 have two pKaH values,

16

indicating that two kinds of amino groups are protonated.

17

As a typical weak base, melamine shows pKaH value of about 5.0, which is in good line with

18

the previously reported results.37,40 The pKaH value of the compound 2, 5.3, is also in good

19

agreement with the previously reported data.36,41 According to the protonation forms, the single

20

smaller pKaH value of compounds 1, 2 and 3 in this work is attributed to the protonation of the

21

melamine structure while the larger pKaH value of compound 6 results from the protonated

22

tertiary amines of the three branches. Consequently, it can be concluded that the larger pKaH


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values of 4, 5 and 7 are produced from the protonation of tertiary amines, whereas the smaller

2

pKaH values are stemmed from the protonation of melamine ring as shown in Scheme 2.

3

Based on the above results, the basicity of amino-substituted derivatives increases with the

4

incorporation of electron-donating group such as methyl group on the melamine structure. In

5

comparison of 2 and 3 with 1, the increase of basicity appears to be directly related to the

6

number of electron-donating substituents. On the other hand, the basicity of melamine ring

7

decreases with the connection of electron-withdrawing groups, for example, the protonated

8

dimethylamino alkyl group used here, with amino nitrogen in melamine, and the reduction is also

9

directly related to the number of electron-withdrawing substituents. This can be demonstrated by

10

the pKaH values of 1, 4, 5, 6 and 7, because once the tertiary amine is protonated, it converts into

11

an electron-withdrawing group that exerts a negative effect on the mother structure. By

12

comparing 4, 5, 6 and 7, it is evident that the basicity of melamine ring follows the order 5 (4.7)

13

> 4 (4.5) and 7 (4.1) > 6 (too weak to be measured), which can be explained by the fact that the

14

alkyl chains of 4 and 6 are shorter than those in 5 and 7, so the protonated tertiary amine poses

15

larger negative effect on the melamine structure and leads to the smaller pKaH value.

16

Protonation degree of melamine and its derivatives

17

To uncover their CO2-responsive capability in water, the protonation degree of these bases was

18

determined by titration. Once the dissociation constant (Ka) of conjugated acid and pH value of

19

solution are obtained, the protonation degree (δ) of organobase can be calculated according to

20

Eqs. (1) and (2):5

21

Ka =

[B][H + ] [BH + ]


(1)

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Yin Page 14 [BH + ] δ= × 100% [BH + ]+[B]

1

(2)

2

where [B] and [BH+] are the concentration of non-protonated base and protonated base,

3

respectively.

4

The pH value of 11.9 mM aqueous solutions was continuously monitored at 25 °C during the

5

CO2 bubbling process until saturation, i.e., the pH value remains stable. The calculated

6

protonation degrees are shown in Table 1. It should be noted that the concentration of

7

organobases and the temperature can exert effects on the pH and corresponding protonation

8

degree, thus, the pH and protonation degree discussed below are only valid at the set

9

concentration and temperature mentioned above. The variation of pH with time (the volume of

10

CO2 bubbled) is detailed in Figure S36 of SI.

11

Table 1. Protonation degree of 11.9 mM solutions of melamine and its derivatives at 25 °C Amine

1

2

3

4

5

6

7

pKaH

5.0

5.3

5.8

8.9; 4.5

9.4; 4.7

8.2

8.7; 4.1

pH

5.64

5.70

5.80

6.15

6.24

6.30

6.30

δ (%)

18.6

28.5

50.0

99.8; 2.2

99.9; 2.8

98.7

99.6; 0.6

12 13

Combined with the basicity of these amines, the protonation degree is found to be highly

14

dependent on the basicity. A clear relation is that the larger the basicity, the higher the

15

protonation degree, which is in good line with the Henderson-Hasselbalch equation.42 More

16

interestingly, different basicity of organobases has different protonation degree by CO2,

17

suggesting that a proper base can be chosen to meet the requirement of precise protonation for

18

particular applications. For example, Yuan and co-workers5 reported a diblock copolymer CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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composed of a poly(ethylene oxide) block and a polyacrylamide one bearing an amidine side

2

group (PAD) can be used to fabricate CO2-switchable vesicle with biommimetic “breathing”

3

feature. When dissolved in water, it can self-assemble into vesicles, and cause 37% of the PAD

4

units to be charged upon the bubbling of CO2, which forces the vesicles to swell, whereas the

5

uncharged PAD species still serve as the inner layer to lock the vesicle structure and prevent it

6

from dissociation. The key factor governing the “breathing” vesicle is the precise protonation

7

degree of PAD upon CO2 stimuli. Should the relationship between basicity and protonation

8

degree be known, it undoubtedly helps to choose the right base to meet the accurate protonation

9

degree. Prompted by this understanding, an attempt was made to establish the relationship based

10

on the above data (Figure 3).

11 12

Figure 3. The relationship of basicity and protonation degree for the melamine derivatives in

13

water.

14

As exhibited in Figure 3, the protonation degree is plotted as a function of pKaH to study their

15

inherent relationship. One should bear in mind that such a correlation is generally based on


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Page 16 of 30

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compounds 1, 2 and 3 owing to their conversions to the bicarbonate salts spread over a much

2

wider range, allowing to correlate the protonation degree and basicity.

3 4 5

From the inset in Figure 3, one can find that the protonation degree varies linearly with pKaH below 5.8 as δ = –180.3 + 39.6pKaH

(3)

6

According to eq. (3), the protonation degree can be calculated to be close to 100% when pKaH

7

hits 7.1. In other words, the organobase will be almost entirely protonated in water with bubbling

8

CO2 when its pKaH value is higher than 7.1. But actually, the basicity of 4, 5 and 6, 7 is larger

9

than 7.1, which makes a little difference in the conversion.

10

Reversibility of melamine and its derivatives

11

CO2-switchable bases normally display inconsistent switch “on” and “off” abilities, and

12

different bases usually show different reversibility in light of the reported studies.3,28 Generally,

13

the “switch-on” process upon CO2 bubbling is easier and faster than the “switch-off” one which

14

usually not only requires longer time of inert gas bubbling but also needs higher temperature in

15

some cases, and even irreversible for some strong bases. For instance, our previous study43 found

16

natural sodium erucate can form high-viscoelastic wormlike micelles in water and shift to low-

17

viscosity solutions in short time when CO2 is introduced. However, bubbling N2 while heating

18

cannot make a converse process. On the contrary, streaming CO2 into the solution of N-

19

erucamidopropyl-N,N-dimethylamine gives rise to the high viscoelastic solution, and it can be

20

converted to the initial state only through bubbling air at ambient temperature.16

21

The difference between switch “on” and “off” processes plays a crucial role in fabricating

22

CO2-switchable systems. On the one hand, easy reverse brings less stability; on the other hand,


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difficult conversion is hard to impart switchability. Thus the reversibility of these amines were

2

examined by conductometry in this work (Table 2).

3

As shown in Figure 4, the conductivity of compound 7 in aqueous solution increases

4

significantly upon CO2 streaming, ascending from 850 µS⋅cm–1 to 2730 µS⋅cm–1 (the

5

corresponding figures of other compounds are shown in Figure S37, SI). Displayed in Figure 5

6

are the snapshots in wet dichloromethane when sequentially bubbling CO2 and N2 inside. Since

7

the variation of ionic strength exerts more obvious effect in solubility in organic solvent

8

compared to water for compound 7, dichloromethane instead of water was used here. When CO2

9

is bubbled, the initial solution becomes cloudy, which is ascribed to the dramatic increase of the

10

ionic strength; and then the removal of CO2 by bubbling N2 makes the suspension clear again,

11

which results from the drop of the ionic strength. Importantly, the time needed to “switch-on”

12

amine aqueous solutions is shorter than that of “switch-off” process even the CO2 bubbling rate

13

is smaller than that of N2. For example, it takes only 12 min for the solution of 4 to be protonated,

14

while 79 min is needed in deprotonation process. Interestingly, as depicted in Table 2, the

15

protonation equilibrium time (the time taken to reach a stable conductivity value) by bubbling

16

CO2 for all of these bases is very close, but much different deprotonation equilibrium time by

17

displacing CO2 with N2 is evidenced, manifesting that the location and number of substitutes

18

along the melamine ring make variable reversibility.

19 20

Table 2. Conductivity (κ) of different stages and protonation, deprotonation time of 11.9 mM

21

different bases in pure water


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Page 18 of 30

Yin Page 18 Compound κ0*

1 2 3

tp**

tdep**

κCO2*

κN2*

(µS⋅cm-1)

(µS⋅cm-1)

(µS⋅cm-1) (min)

(min)

water

1

52

10

8

20

1

8

330

26

9

19

5.0

2

5

440

50

13

22

5.3

3

17

550

55

14

35

5.8

4

210

1070

477

12

79

8.9

5

266

1020

486

12

77

9.4

6

580

1770

683

13

105

8.2

7

846

2700

1177

12

105

8.7

pKaH

* κ0, κCO2, κN2 refer to the conductivity of the initial solutions, and those treated by CO2 or N2 to equilibrium (no longer change with time). ** tp and tdep denote the equilibrium time of protonation and deprotonation.

4

Apart from these, it should be noted that the conductivity of these base aqueous solutions has

5

not been entirely switched back to their starting point during the observation time. This might be

6

caused by the incomplete deprotonation and the residual protonated base contributes to the

7

higher conductivity, which implies that it is difficult to completely deplete CO2 using inert gas.

8

Nevertheless, the compounds 1, 2 and 3 show better reversibility than other bases, as supported

9

by the final conductivity values after bubbling N2.


Page 19 of 30

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1 2

Figure 4. Variation of conductivity of compound 7 in water (11.9 mM) when successively

3

treated by CO2 and N2.

4 5

Figure 5. Snapshots of compound 7 in wet dichloromethane.

6

Combining with the basicity of these amines, we can easily explain why conductivity results

7

differ from amine to amine. The maximum conductivity upon bubbling CO2 dramatically

8

increases with the rise of basicity, ascending from 330 µS⋅cm–1 of melamine to 1020 µS⋅cm–1 of

9

compound 5. It should be noted that although compounds 6 and 7 are less basic than compounds

10

4 and 5, respectively, they display much higher conductivity, which originates from the treble

11

CO2-responsive tertiary amino groups they bear. Furthermore, the conductivity of the initial

12

solutions appears to increase from melamine to compound 7. This might stem from their partial

13

protonation by H2O.3 Stronger bases are more easily protonated, which leads the initial CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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Page 20 of 30

Yin Page 20 1

conductivity to increase from compound 1 to compound 5. Similarly, 6 and 7 have treble CO2-

2

sensitive groups compared with 4 and 5, respectively, thus, their initial conductivities are almost

3

three times those of 4 and 5. In addition, although their “switch-on” time treated with CO2 does

4

not show direct relation to basicity, the “switch-off” time by N2 is closely related to it. The

5

changes in conductivity of base solutions derive from the absorption and desorption of CO2. The

6

rate limiting step for the absorption process involves CO2 rather than amine,44 so that the

7

“switch-on” time does not depend on amine choice. Essentially, the distinct performance

8

between switch “on” and “off” processes, as well as the different reversibility originate from the

9

different deprotonation capability. The good deprotonation ability implies good reversibility.

10

To gain insight into the mechanism, the deprotonation ability of these amines was investigated

11

at 25, 40 and 60 °C, respectively, with bubbling N2, and the time when the protontaion degree is

12

reduced to half of the maximum value (t1/2) was used to evaluate their deprotonation capability.

13

As shown in Figure 6a, obvious difference was observed in the deprotonation process among

14

these amines. It is found that the time of CO2 desorption increases with the rise of basicity,

15

especially when the pKaH of amine is larger than 8.2, where the t1/2 experiences a dramatic

16

increase. Furthermore, an increase in temperature evidently leads to the decrease of the t1/2

17

because the high temperature weakens the stability of formed ammonium bicarbonate salts and

18

speeds up their deprotonation. For example, the strongest base among these amines, compound 5,

19

whose t1/2 is more than 300 min at 25 oC and still longer than 200 min at 40 °C; however, it

20

declines to only 126 min when temperature is increased to 60 °C. Such a rise in temperature is

21

clearly an effective approach to improve the reversibility. The reversibility, which plays an

22

essential role in CO2-switchable system, is directly dominated by deprotonation ability, but no

23

methods can quantitatively evaluate it so far. Thus, we manage to establish the relationship


Page 21 of 30

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between deprotonation ability and basicity to estimate the reversibility by fitting the curves

2

depicted in Figure 6a as t1/2 = Ae(–pKaH/a) + Be(–pKaH/b) + t0

3

(4)

4

where A, a, B, b and t0 are constants, the corresponding value under different conditions are

5

given in Table 3. The corresponding fitting curves can be found in Figure 38 of SI.

6

Table 3. Constants of eq.(4) under different conditions T (°C) With N2 bubbling

A

a –10

B

b

t0

–2.25

–5.74

–10

–0.35

–4.11

–10

–0.37

1.95

–6

–0.63

4.21

–6

–0.61

2.87

25

1.37×10

–0.32

0.69

40

0.33

–1.95

4.29×10

60

5.70×10

–10

–0.37

5.70×10

Without N2

40

7.98×10

–6

–0.637

7.98×10

bubbling

60

3.25×10

–6

–0.61

3.25×10

7 8

According to the fitting results, it can be calculated that 594 and 254 min are needed for

9

protonated 5 to release half of the absorbed CO2 in aqueous solution at 25 °C and 40 °C,

10

respectively, so that we did not observe the t1/2 during the test time, which lasts for a maximum

11

of 200 min.

12

Since high temperature deteriorates the stability of protonation products and some CO2-

13

switchable materials are used above room temperature, the stability of the protonated products at

14

different temperature is of importance and deserves to be examined. Thus, we investigated the

15

thermal stability of these protonated amines using the same method at 40 °C and 60 °C,

16

respectively, but without bubbling inert gas. In contrast to the reversibility, the poor

17

deprotonation ability of ammonium bicarbonate salt demonstrates good thermal stability. As

18

shown in Figure 6b, basicity of amines is also found to play a key role in stabilizing their CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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Page 22 of 30

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protonated species. Stronger basicity imparts the ammonium bicarbonate salt with better stability,

2

and the dramatic increase of t1/2 also occurs around the pKaH value of 8.2. Besides, protonated

3

species from compounds 1, 2 and 3 are very unstable as they are much less basic than other

4

amines. As expected, a temperature increase obviously lowers the stability. For instance, the t1/2

5

was not observed even the test time was more than 40 h at 40 °C; however, it decreases to 32 h

6

when heated at 60 °C. Thus, the effect of temperature must be carefully considered when CO2-

7

switchable compounds are used in practical applications. For example, Crespy and co-workers22

8

employed the protonated N,N-dimethyldodecylamine (pKaH 9.97) as the CO2-switchable

9

surfactant to prepare poly(methyl methacrylate) and polystyrene latexes at 40 °C, 60 °C and 80

10

°C. Since the polymerizations were carried out at higher temperatures, CO2-switchable surfactant

11

with larger basicity was employed. If there is a method to evaluate the thermal stability of

12

protonated bases, practical uses will be easier. With this objective in mind, the curves shown in

13

Figure 6b were fitted, and it is found that the fitting results also satisfy eq. (4). The

14

corresponding parameters are shown in Table 3 and the fitting curves can be found in Figure S39

15

of SI. Hence, the thermal stability of other organobases with known basicity can be

16

quantitatively forecasted according to eq. (4).

17

It is worth noting that the inert gas bubbling evidently accelerates the deprotonation of

18

ammonium bicarbonate salts when Figure 6a is compared with Figure 6b. This might be caused

19

by two reasons. First, the rapidly moved inert gas molecules transfer energy to ammonium

20

bicarbonate salts in solution to promote their decomposition. Moreover, the introduction of inert

21

gas can instantly drive the released CO2 out and avoid its re-dissolution.45


Page 23 of 30

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Yin Page 23

1 2

Figure 6. The relationship of basicity and deprotonation capability for the melamine and its

3

derivatives in water. (a) With N2 bubbling; (b) Without N2 bubbling.

4

In addition, the number of CO2-sensitive group in an amine molecule also plays an important

5

role on its switchability. As shown in Table 2, although the basicity of 4 and 5 is larger than that

6

of 6 and 7, the conductivity of compounds 6 and 7 is respectively much higher than that of 4 and

7

5, and the conductivity change between the initial value and final one after treatment of CO2 is

8

also larger than that of 4 and 5. The large difference between the properties before and after CO2

9

bubbling means effective CO2-switchability, thus organobases with more CO2-sensitive groups

10

show more effective CO2-switchability than the organobases with fewer corresponding groups.

11

But more CO2-sensitive groups take longer time to achieve the final equilibrium value in the

12

deprotonation process in comparison 6, 7 with 4, 5.

13

Conclusion

14

A series of amino-substituted melamine derivatives with different basicities was synthesized

15

and characterized. Then CO2-switchability of melamine and these derivatives were tested by

16

examining the protonation forms, protonation degree, and reversibility. Results showed all these CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

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Page 24 of 30

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bases have good CO2-responsiveness. However, they display different switch “on” and “off”

2

capabilities, which were found directly related to their basicity. Based on these results, we

3

established the quantitative relationships of basicity–protonation degree and basicity–

4

deprotonation ability of organobases in water. In addition, organobases containing more

5

responsive groups are found to show higher effective CO2-switchability, but they need more time

6

to achieve the equilibrium in deprotonation process. These findings may be useful to choose

7

appropriate basicity and specific numbers of sensitive groups to design CO2-switchable materials

8

to meet the requirements in practical applications.

9

Acknowledgements

10

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

11

(21273223, 21173207) and Distinguished Youth Fund from Science and Technology Department

12

of Sichuan Province (2010JQ0029).

13

Supporting Information Available

14

FTIR, NMR and HRMS spectrum of compounds and additional results. This material is

15

available free of charge via the Internet at http://pubs.acs.org.

16

Author Information

17

*Corresponding Author. E-mail: [email protected]

18

References

19

(1) Jessop, P. G.; Heldebrant, D. J.; Li, X. W.; Eckert, C. A.; Liotta, C. L. Reversible nonpolar-

20

to-polar solvent. Nature 2005, 436, 1102–1102.

21

(2) Liu, Y. X.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Switchable surfactants.

22

Science 2006, 313, 958–960. CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment

Page 25 of 30

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

Yin Page 25 1

(3) Scott, L. M.; Robert, T.; Harjani, J. R.; Jessop, P. G. Designing the head group of CO2-

2

triggered switchable surfactants. RSC Adv. 2012, 2, 4925–4931.

3

(4) Phan, L.; Jessop, P. G. Switching the hydrophilicity of a solute. Green Chem. 2009, 11, 307–

4

308.

5

(5) Yan, Q.; Zhou, R.; Fu, C. K.; Zhang, H. J.; Yin, Y. W.; Yuan, J. Y. CO2-responsive

6

polymeric vesicles that “breathe”. Angew. Chem. Int. Ed. 2011, 50, 4923–4927.

7

(6) Han, D. H.; Tong, X.; Boissiere, O.; Zhao, Y. General strategy for making CO2-switchable

8

polymers. ACS Macro. Lett. 2012, 1, 57–61.

9

(7) Guo, Z.; Feng, Y.; Wang, Y.; Wang, J.; Wu, Y.; Zhang, Y. A novel smart polymer responsive

10

to CO2. Chem. Commun. 2011, 47, 9348–9350.

11

(8) Li, W. J.; Zhang, Z. F.; Han, B. X.; Hu, S. Q.; Song, J. L.; Xie, Y.; Zhou, X. S. Switching the

12

basicity of ionic liquids by CO2. Green Chem. 2008, 10, 1142–1145.

13

(9) Li, X. Y.; Hou, M. Q.; Zhang, Z. F.; Han, B. X.; Yang, G. Y.; Wang, X. L.; Zou, L. Z.

14

Absorption of CO2 by ionic liquid/polyethylene glycol mixture and the thermodynamic

15

parameters. Green Chem. 2008, 10, 879–884.

16

(10) Wang, C. M.; Mahurin, S. M.; Luo, H. M.; Baker, G. A.; Li, H. R.; Dai, S. Reversible and

17

robust CO2 capture by equimolar task-specific ionic liquid-superbase mixtures. Green Chem.

18

2010, 12, 870–874.

19

(11) Wang, C. M.; Luo, H. M.; Jiang, D. E.; Li, H. R.; Dai, S. Carbon dioxide capture by

20

superbase-derived protic ionic liquids. Angew. Chem. Int. Ed. 2010, 49, 5978–5981.

21

(12) Hoshino, Y.; Imamura, K.; Yue, M. C.; Inoue, G.; Miura, Y. Reversible absorption of CO2

22

triggered by phase transition of amine-containing micro- and nanogel particles. J. Am. Chem. Soc.

23

2012, 134, 18177–18180.


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

Page 26 of 30

Yin Page 26 1

(13) Han, D. H.; Boissiere, O.; Kumar, S.; Tong, X.; Tremblay, L.; Zhao, Y. Two-way CO2-

2

switchable triblock copolymer hydrogels. Macromolecules 2012, 45, 7440–7445.

3

(14) Zhang, Y.; Feng, Y.; Wang, J.; He, S.; Guo, Z.; Chu, Z.; Dreiss, C.. CO2-switchable

4

wormlike micelles. Chem. Commun. 2013, 49, 4902–4904.

5

(15) Zhang, Y.; Feng, Y.; Wang, Y.; Li, X.. CO2-switchable viscoelastic fluids based on a

6

pseudogemini surfactant. Langmuir 2013, 29, 4187–4192.

7

(16) Zhang, Y.; Chu, Z.; Dreiss, C. A.; Wang, Y.; Fei, C.; Feng, Y.. Smart wormlike micelles

8

switched by CO2 and air. Soft Matter 2013, 9, 6217–6221.

9

(17) Guo, Z.; Feng, Y.; He, S.; Qu, M.; Chen, H.; Liu, H.; Wu, Y.; Wang, Y. CO2-responsive

10

“smart” single-walled carbon nanotubes. Adv. Mater. 2013, 25, 584–590.

11

(18) Wang, C. M.; Luo, H. M.; Luo, X. Y.; Li, H. R.; Dai, S. Equimolar CO2 capture by

12

imidazolium-based ionic liquids and superbase systems. Green Chem. 2010, 12, 2019–2023.

13

(19) Quek, J. Y.; Roth, P. J.; Evans, R. A.; Davis, T. P.; Lowe, A. B. Reversible addition-

14

fragmentation chain transfer synthesis of amidine-based, CO2-responsive homo and AB diblock

15

(co)polymers comprised of histamine and their gas-triggered self-assembly in water. J. Polym.

16

Sci. Pol. Chem. 2013, 51, 394–404.

17

(20) Mercer, S. M.; Robert, T.; Dixon, D. V.; Chen, C. S.; Ghoshouni, Z.; Harjani, J. R.;

18

Jahangiri, S.; Peslherbe, G. H.; Jessop, P. G. Design, synthesis, and solution behaviour of small

19

polyamines as switchable water additives. Green Chem. 2012, 14, 832–839.

20

(21) Robert, T.; Mercer, S. M.; Clark, T. J.; Mariampillai, B. E.; Champagne, P.; Cunningham,

21

M. F.; Jessop, P. G. Nitrogen-containing polymers as potent iionogens for aqueous solutions of

22

switchable ionic strength: application to separation of organic liquids and clay particles from

23

water. Green Chem. 2012, 14, 3053–3062.


Page 27 of 30

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

Yin Page 27 1

(22) Zhao, Y.; Landfester, K.; Crespy, D. CO2 responsive reversible aggregation of nanoparticles

2

and formation of nanocapsules with an aqueous core. Soft Matter 2012, 8, 11687–11696.

3

(23) Heldebrant, D. J.; Koech, P. K.; Ang, M. T. C.; Liang, C.; Rainbolt, J. E.; Yonkera, C. R.;

4

Jessop, P. G. Reversible zwitterionic liquids, the reaction of alkanol guanidines, alkanol

5

amidines, and diamines with CO2. Green Chem. 2010, 12, 713–721.

6

(24) Harjani, J. R.; Liang, C.; Jessop, P. G. A synthesis of acetamidines. J. Org. Chem. 2011, 76,

7

1683–1691.

8

(25) Jing, Y. Q.; Thomas, P. D.; Andrew. B. L. Amidine functionality as a stimulus-responsive

9

building block. Chem. Soc. Rev. 2013, 42, 7326–7334.

10

(26) Yan, B.; Han, D. H.; Boissiere, O.; Ayotte, P.; Zhao, Y. Manipulation of block copolymer

11

vesicles using CO2: dissociation or “breathing”. Soft Matter 2013, 9, 2011–2016.

12

(27) Mercer, S. M.; Jessop, P. G. “Switchable water”: aqueous solutions of switchable ionic

13

strength. ChemSusChem 2010, 3, 467–470.

14

(28) Fowler, C. I.; Jessop, P. G.; Cunningham, M. F. Aryl amidine and tertiary amine switchable

15

surfactants and their application in the emulsion polymerization of methyl methacrylate.

16

Macromolecules 2012, 45, 2955−2962.

17

(29) Ceschia, E.; Harjani, J. R.; Liang, C.; Ghoshouni, Z.; Andrea, T.; Brown, R. S.; Jessop, P. G.

18

Switchable anionic surfactants for the remediation of oil-contaminated sand by soil washing.

19

RSC Adv., 2014, 4, 4638−4645.

20

(30) Wang, Y. L.; Mebel, A. M.; Wu, C. J.; Chen, Y. T.; Lin, C. E.; Jiang, J. C. IR Spectroscopy

21

and theoretical vibrational calculation of the melamine molecule. J. Chem. Soc., Faraday Trans.

22

1997, 93, 3445–3451.


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

Page 28 of 30

Yin Page 28 1

(31) Dixon, J. K.; Woodberry, N. T.; Costa, G. W. The dissociation constants of melamine and

2

certain of its compounds. J. Am. Chem. Soc. 1947, 69, 599–603.

3

(32) Shen, J. S.; Cai, Q. G.; Jiang, Y. B.; Zhang, H. W. Anion-triggered melamine based self-

4

sssembly and hydrogel. Chem. Commun. 2010, 46, 6786–6788.

5

(33) Marchi-Artzner, V.; Artzner, F.; Karthaus, O.; Shimomura, M.; Ariga, K.; Kunitake, T.;

6

Lehn, J. M. Molecular recognition between 2,4,6-triaminopyrimidine lipid monolayers and

7

complementary barbituric molecules at the air/water interface: effects of hydrophilic spacer,

8

ionic strength, and pH. Langmuir 1998, 14, 5164–5171.

9

(34) Tan, Y.; Sun, L.; Shao, S.; Fu, J.; Peng, Z. Synthesis and characterization of melamine

10

halogen acid salts and its application as flame retardant. Adv. Mater. Res. 2013, 750–752, 1087–

11

1090.

12

(35) Vidal-Mosquera, M.; Fernandez-Carvajal, A.; Moure, A.; Valente, P.; Planells-Cases, R.;

13

Gonzalez-Ros, J. M.; Bujons, J.; Ferrer-Montiel, A.; Messeguer, A. Triazine-based vanilloid 1

14

receptor open channel blockers: design, synthesis, evaluation, and SAR analysis. J. Med. Chem.

15

2011, 54, 7441–7452.

16

(36) Hirt, R. C.; Schmitt, R. G.; Strauss, H. L.; Koren, J. G. Spectrophotometrically determined

17

ionization constants of derivatives of symmetric triazine. J. Chem. Eng. Data 1961, 6, 610–612.

18

(37) Hirt, R. C.; Schmitt, R. G. Ultraviolet absorption spectra of derivatives of symmetric

19

triazine.-II. Oxo-triazines and their acyclic analogs. Spectrochim. Acta. 1958, 12, 127–138.

20

(38) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito, I.; Koppel, I. A.

21

Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of

22

28 pKa units: unification of different basicity scales. J. Org. Chem. 2005, 70, 1019–1028.


Page 29 of 30

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Yin Page 29 1

(39) Hirt, R. C.; Halverson, F.; King, F. T. Ultraviolet absorption spectra of derivatives of

2

symmetric triazine-III. The guanyl-melamines. Spectrochim. Acta. 1959, 15, 962–968.

3

(40) Dudley, J. R. Cyanuric chloride derivatives. IX. Dissociation constants of substituted

4

melamines and related triazines. J. Am. Chem. Soc. 1951, 73, 3007–3008.

5

(41) Tashiro, T. The Ionization constants of 2-substituted 4,6-diamino-s-triazines: the

6

applicability to the Hammett and Taft equations. J. Heterocyclic Chem. 2002, 39, 615–622.

7

(42) Po, H. N.; Senozan, N. M. Henderson-Hasselbalch equation: its history and limitations. J.

8

Chem. Educ. 2001, 78, 1499–1503.

9

(43) Zhang, Y.; Yin H.; Feng, Y. CO2-responsive anionic wormlike micelles based on natural

10

erucic acid. Green Mater. 2014, 2, 95–103.

11

(44) Jessop, P. G.; Kozycz, L.; Rahami, Z. G.; Schoenmakers, D.; Boyd, A. R.; Wechslerb, D.;

12

Hollandb, A. M. Tertiary amine solvents having switchable hydrophilicity. Green Chem., 2011,

13

13, 619–623.

14

(45) George, M.; Weiss, R. G. Chemically reversible organogels via “latent” gelators. Aliphatic

15

amines with carbon dioxide and their ammonium carbamates. Langmuir 2002, 18, 7124–7135.

16


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Table of Contents

2 3

Quantitative relationship between basicity of melamine and its amino-substituted derivatives and

4

their CO2-switchability was established. The larger the basicity is, the higher the conversation is,

5

the more difficult the deprotonation process is, and the more stable the protonated species is.


Insights into the Relationship between CO2 Switchability and Basicity

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