Ion Pair Formation between Tertiary Aliphatic Amines and Perchlorate

Article pubs.acs.org/JPCB

Ion Pair Formation between Tertiary Aliphatic Amines and Perchlorate in the Biphasic Water/Dichloromethane System Denis Badocco,† Valerio Di Marco,† Alfonso Venzo,‡ Marco Frasconi,† Diego Frezzato,† and Paolo Pastore*,† †

Department of Chemical Sciences, University of Padova, Via F. Marzolo 1, 35131 Padova, Italy CNR, ICMATE, Institute of Condensed Matter Chemistry and Technologies for Energy, Via F. Marzolo 1, 35131 Padova, Italy

‡

S Supporting Information *

ABSTRACT: The ability of aliphatic amines (AAs), namely, tripropylamine (TPrA), trisobutylamine (TisoBuA), and tributylamine (TBuA), to form ion pairs with perchlorate anion (ClO4−) in biphasic aqueous/dichloromethane (CH2Cl2) mixtures containing ClO4− 0.1 M has been demonstrated by GC with flame ionization (FID) and mass detectors (MS) and by NMR measurements. The extraction efficiency of the AAs to the organic phase was modeled by equations that were used to fit the experimental GC data, allowing us to determine values for KP (partition constant of the free AA), KIP (formation constant of the ion pair), and KIP P (partition constant of the ion pair) for TPrA, TisoBuA, and TBuA at 25 °C. Ion pairs were shown to form in CH2Cl2 also when ClO4− is replaced by other inorganic anions, like NO3−, ClO3−, Cl−, H2PO4−, and IO3−. No ion pairs formed when CH2Cl2 was replaced by n-hexane, suggesting that aliphatic amine ion pairs can form in polar organic solvents but not in nonpolar ones.

■

INTRODUCTION Aliphatic amines (AAs) are widely used in the chemical and pharmaceutical industry. Short alkyl chain (C1−C6) AAs are important intermediates for the production of herbicides, pesticides, dyes, for the synthesis of amino-compounds,1 and in the production of quaternary ammonium salts through the Menshutkin reaction between tertiary amines and alkyl halides.2 Tertiary AAs are used to synthesize a monomer with dimethyl methacrylate, which by polymerization gives a resin for dental implants.3 Tertiary AAs are also used as chain terminators in the block copolymer syntheses.4,5 AAs with longer chains are surfactants due to their high superficial activity, and they are involved in the production of softeners,1 in the separation (by flotation) of nonmetallic materials such as feldspars, phosphates, and phyllosilicates, and in the extraction of carboxylic acids (e.g., acetic and citric) from aqueous solutions to organic solvents.6 AAs are used as corrosion regulators and in the protection and finishing of metallic surfaces,7 as rubber stabilizers (as they inhibit oxidation and promote vulcanization), as catalysts in the syntheses of epoxy resins,8 and to modulate the pore dimensions in the preparation of mesoporous materials.9 Some AAs are playing important roles in several scientific fields. For example, trioctylamine is used as solvent and mild reducing agent in the solvothermal synthesis of nanocrystalline Cu2O from copper acetate or copper acetylacetonate10 and as solvent for the extraction of organic compounds in fuel obtained by biomass pyrolysis.11 Tripropylamine is a coreagent in electrochemiluminescence reactions, and it is used to detect and identify [Ru(bipy)3]2+-marked DNA fragments.12 © XXXX American Chemical Society

This very broad usage of AAs is (at least partly) due to their ability to partially solubilize in both aqueous solutions and organic solvents, as they bear an amino hydrophilic head and an alkyl lipophilic tail. If an aqueous solution containing AAs is put into contact with an organic phase, then the neutral form R3N can therefore partly solubilize in the latter according to the partition equilibrium

R3N(aq) ⇌ R3N(o)

(1)

where the subscripts “o” and “aq” denote organic and aqueous phase. The AA water solubility and their organic/aqueous partition are modulated by the nature and the number of amino substituents. Comprehensive lists of experimental water solubilities for many AAs are reported in the literature.13,14 In our recent paper, pH and ionic strength were demonstrated to strongly affect AA water solubility.15 The pH control is necessary because of the acid−base property of the amino group, which can protonate/deprotonate in water R3NH+ + H 2O ⇌ R3N + H3O+

(2)

The ionic strength control is necessary because it affects the activity coefficients of the ionic species in equilibrium 2. Other than pH and ionic strength, there is another property that can affect water solubility and organic/aqueous partition of AA: the formation of ion pairs. Received: May 25, 2017 Revised: August 29, 2017 Published: September 15, 2017 A

DOI: 10.1021/acs.jpcb.7b05088 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B Ion pairs between anions and cations have been investigated and can be useful in several fields,16 among which the catalytic17 and the pharmaceutical ones18 appear to be the most investigated. For example, recent efforts are directed to improve drug absorption by forming ion pairs.18 Also, AAs are well known to form ion pairs under some conditions19 if they are in the protonated form, and the idea to use ion pairs to improve AA extraction in lipophilic matrices has been applied.20,21 However, to the best of our knowledge, the relation among ion pairs, water solubility, and organic/aqueous partition has not been thoroughly investigated and also not theoretically formulated for these compounds. If the AA R3NH+ and the anion X− form the ion pair R3NH+| − X in aqueous solution, then the following reaction occurs R3NH+(aq) + X−(aq) ⇌ R3NH+|X−(aq)

K IP =

KPIP =

(3)

(4)

Equilibria 3 and 4 can affect the concentration of AAs in aqueous and organic phases, and they must be considered for a correct evaluation of AA water solubility. In this paper, the ion pair formation involving selected tertiary AAs is studied in water/n-hexane and in water/ dichloromethane biphasic (two-liquid) mixtures. The considered AAs are tripropylamine (TPrA), trisobutylamine (TisoBuA), and tributylamine (TBuA): They are moderately to poorly soluble in water, and they have been chosen because literature reports scattered and incoherent solubility results for them.13−15 The anion chosen for this study is ClO4−, as it is commonly employed for the ionic strength and pH adjustment. Perchlorate was added as sodium salt in the aqueous phase at a concentration of 0.1 M, which was sufficiently high to ensure the constancy of the ionic strength and sufficiently low to allow the activity coefficients in water not to deviate significantly from unit. The formation of the ion pairs between AA and ClO4− is followed by chromatography (GC-FID and GC−MS) and by 1 H NMR and 13C NMR, employing both monodimensional as well as bidimensional TOCSY and phase-sensitive NOESY measurements.

A I + 0.2z ion 2I 1+B I

(9)

n

(10)

i=1

where ci is the molar concentration of the ionic species i, zi is its charge, and n is the number of ionic species in solution. Under the experimental conditions, the coefficients γ(aq) ion are found to differ very slightly from 1 (see below) so that the null-ionicstrength approximation is safely applicable. If an AA is contained in an aqueous/organic biphasic mixture and only the reactions 1 and 2 take place (i.e., no ion pair is formed), then the total number of moles of AA, nAA, is given by

THEORETICAL MODEL The equilibrium constants of reactions 1−4 can be thermodynamically defined as explained in the Supporting Information. In the following, the “stoichiometric constants” defined in terms of molar concentrations (except for the species H3O+) are considered

nAA = [R3N]o Vo + [R3NH+]aq Vaq + [R3N]aq Vaq

(11)

where V0 is the volume of organic phase and Vaq is the volume of aqueous phase. The extracted fraction of AA in the organic phase, ϕR3N, is defined by the number of moles of R3N in the organic phase divided by nAA

(5)

ϕR N = 3

[R3N]aq a H3O+ [R3NH+]aq

(8)

I = 0.5 ∑ cizi2

■

Ka =

[R3NH+|X−]o [R3NH+|X−]aq

where A = 0.5102 M−1/2 at 25 °C and B is a parameter that depends on the size of the ion (B ≈ 1.5 M−1/2 is considered valid for the protonated AAs at 25 °C),27 zion is the charge of the ionic species under consideration (in units of electron charge), and I is the ionic strength expressed by

EXPERIMENTAL SECTION The Experimental Section is reported in the Supporting Information.

[R3N]o [R3N]aq

(7)

(aq) log γion = −z ion 2

■

KP =

[R3NH+]aq [X−]aq

where KP is the organic/aqueous partition constant of the AAs, Ka is the AA acidity constant, aH3O+ is the proton activity in water (pH = −log aH3O+), KIP is the formation constant of the ion pair in the aqueous phase, and KIP P is the organic/aqueous partition constant of the ion pair. Because KP, Ka, KIP and KPIP depend on the activity coefficients of the chemical species in the specific phase, they are not true constants. However, the low concentration of the species and the moderately low value of ionic strength realized under the present experimental conditions make it licit to refer to the ideal situation of “infinite dilution” and null ionic strength. In such a limit, the parameters defined in eqs 5−8 become effective constants. The constant KP still bears a dependence on the infinite-dilution activity coefficient of R3N dissolved in the organic solvent, γR(o)3N, which generally differs from 1; the value of such a coefficient could be estimated, for instance, by means of semiempiric models like UNIFAC.22,23 Concerning Ka and KIP, which are related to equilibria involving ionic species in the aqueous phase, the applicability of the nullionic-strength approximation has been checked by computing the activity coefficients by means of the Debye−Hückel model24 corrected as reported by Robinson, Truesdell et al.25,26

If the aqueous phase is put into contact with an organic phase, then R3NH+|X−(aq) can solubilize in the latter as it is a neutral species, according to the partition equilibrium R3NH+|X−(aq) ⇌ R3NH+|X−(o)

[R3NH+|X−]aq

[R3N]o Vo [R3N]o Vo + [R3NH+]aq Vaq + [R3N]aq Vaq

(12)

For the lipophilic AAs under investigation KP ≫ 1 (see below), that is, [R3N]aq ≪ [R3N]o, so that the [R3N]aqVaq term in the

(6) B

DOI: 10.1021/acs.jpcb.7b05088 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B The total number of X−, nX, is given by

denominator of eq 12 can be neglected. By substituting eqs 5 and 6 into 12, the extracted fraction becomes 1 ϕR N = Vaq 10−pH 3 1 + V KK (13) o

nX = [R3NH+|X−]o Vo + [X−]aq Vaq + [R3NH+|X−]aq Vaq (16)

Under the experimental conditions of the present work, where nX ≫ nAA (see Experimental Section in the Supporting Information), eq 16 simplifies to

a P

Equation 13 as a function of pH has a typical sigmoid form (see e.g. Figure 1). At acidic pH, the second term of the

nX ≈ [X−]aq Vaq

(17)

The extracted fraction of free amine in the organic phase, ϕR3N, is now given by

(

)

ϕR N = [R3N]o Vo /([R3N]o Vo + [R3NH+|X−]o Vo 3

+ [R3NH+]aq Vaq + [R3NH+|X−]aq Vaq + [R3N)aq Vaq)

(18)

By substituting eqs 5−8 and 17 into eq 18 and again neglecting the [R3N]aqVaq term, the extracted fraction becomes ϕR N =

Figure 1. Experimental values for ϕR3N obtained by the GC measurements of aqueous/n-C6H14 biphasic mixtures at various pH values after extraction of a n-C6H14 solution of TPrA 2.67 mM (△), TisoBuA 2.23 mM (○), or TBuA 2.06 mM (□). White symbols refer to aqueous solutions without ClO4− and black symbols refer to aqueous solutions containing ClO4− 0.1 M. The lines represent fittings obtained by eq 13.

3

(19)

The shape of eq 19 as a function of pH is the same as for eq 13 (sigmoid), with limiting values of 0 and 1 at acidic and basic pH, respectively. KIP and KIP P can be obtained by fitting the experimental values (eq 14) with eq 19 if pH, nX, V0, Vaq, Ka, and KP are known. As it will be seen below, in some cases experimental values for an extracted fraction of ion pair in the organic phase, ϕR3NH+|X−, can be obtained by the GC measurements

denominator may become much larger than 1, so that ϕR3N has a limiting value equal to zero: Here AA is totally protonated and it cannot solubilize in the organic phase. At basic pH, the second term of the denominator may become much smaller than 1, so that ϕR3N has a limiting value equal to 1: Here AA is totally deprotonated (neutral) and a complete extraction in the organic phase is obtained if KP is sufficiently large. Experimental values for ϕR3N are obtained from GC-FID measurements by measuring the peak area of the AAs and of the internal standard according to A R3N AIS ′ ϕR N = 3 AIS A R′ 3N (14)

ϕR NH+|X− = 3

A R3NH+|X− AIS ′ AIS A R′ 3N

(20)

where AR3NH+|X− represents the peak area of the ion pair after the partition with the aqueous phase. ϕR3NH+|X− is defined as the number of moles of ion pair in the organic phase divided by nAA given in eq 15

( ) /([R N] V + [R NH |X ] V

ϕR NH+|X− = [R3NH+|X−]o Vo 3

+

In eq 14, AR3N and AIS represent the peak area of free AAs and of the internal standard after the partition with the aqueous phase and A′R3N and A′IS are the peak area of the same AAs and of the internal standard before the partition. In eq 13, V0 and Vaq are known, pH is measured, and Ka for each AA is reported in the literature (pKa values obtained in our previous work15 were considered: 10.65 for TPrA, 10.32 for TisoBuA, 10.90 for TBuA). Thus the fitting of the experimental values (eq 14) by using this equation can allow us to determine KP. The previous theoretical treatment is slightly more complicated if an ion pair between AA and an anion X− forms in the aqueous phase through reaction 3 and if the ion pair can solubilize in the organic phase through reaction 4. The total number of AA moles, nAA, is given by

3

o o

3

−

o o

+ [R3NH+]aq Vaq + [R3NH+|X−]aq Vaq + [R3N]aq Vaq

)

(21)

By using eqs 5−8 with the approximation (eq 17) and again neglecting the [R3N]aqVaq term, eq 21 becomes ϕR NH+|X− = 3

1 1+

Vaq VoKPIP

+

Vaq KPIPK IPnX

(

Vaq Vo

)

+ 10 pHK aKP

(22)

The profile of ϕR3NH+|X− versus pH is a sigmoid with an opposite behavior with respect to that of ϕR3N because the ion pair forms at acidic pH, so that the zero limiting value is observed at basic pH. The limiting value of ϕR3NH+|X− at acidic pH is lower than 1 because the ion pair can be only partially extracted from the aqueous phase: The exact limiting value

nAA = [R3N]o Vo + [R3NH+|X−]o Vo + [R3NH+]aq Vaq + [R3NH+|X−]aq Vaq + [R3N]aq Vaq

1 V ⎛ (KPIP + Vaqo )KIPnX ⎞⎟ 10−pH Vaq 1 + K K ⎜⎜ V + ⎟ Vaq P a ⎝ o ⎠

(15) C

DOI: 10.1021/acs.jpcb.7b05088 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B depends on Vaq, V0, nX, KIP P , and KP (see second and third terms of the denominator in eq 22). In any case, KIP and KIP P can be obtained by fitting the experimental values (eq 20) with eq 22 if the other parameters of this equation are known. Finally, when the experimental values do not allow us to discriminate between ϕR3N and ϕR3NH+|X−, that is, if only the sum of free AA and ion pair is determined, then the theoretical function that fits the experimental data is given by the sum of eqs 19 and 22. In this case, the limiting value at basic pH is one, whereas at acidic pH it is larger than zero.

■

RESULTS AND DISCUSSION Partition Constants and Ion Pair Formation Constants. Biphasic aqueous/n-C6H14 mixtures have been considered first, and the organic phase after partition of the AA from aqueous solution has been analyzed by GC-FID. Figure 1 reports the experimental values for ϕR3N at various pH values of the aqueous phase (symbols). White symbols refer to extractions from aqueous solutions without ClO4−, and black symbols refer to extractions from aqueous solutions containing ClO4− 0.1 M. It can be observed that all ϕR3N curves have the limiting value of 1 at basic pH: This confirms the correctness of neglecting the [R3N]aqVaq term in deriving eq 13. The partition profiles ϕR3N versus pH in the absence or in the presence of ClO4− are practically undistinguishable: No ion pair formation is detected under these conditions in the organic phase. The partition profiles undergo just a slightly shift to higher pH values (by ca. 0.1 unit) in the presence of ClO4−; this effect can be ascribed to the change of the ionic strength (using eq 9, log γR3NH+ = −0.0894 with I = 0.1 M, whereas γR3NH+ ≈ 1 in the absence of ClO4− and at nonextreme pH values). Fitting with eq 13 (lines in Figure 1) allows us to obtain the KP values in the presence and in the absence of ClO4− for the considered AA. Data are resumed in Table 1.

Figure 2. Experimental values for ϕR3N (symbols) obtained by the GC measurements of aqueous/CH2Cl2 mixtures at various pH values after extraction of a CH2Cl2 solution of TPrA 2.67 mM (△), TisoBuA 2.23 mM (○), or TBuA 2.06 mM (□) in the absence of ClO4−. The lines represent fittings obtained by eq 13.

Table 2. KP Values of the AAs for an Aqueous/CH2Cl2 Mixture in the Absence of ClO4− in the Aqueous Phasea,b

a

KP (sKp) [ClO4−] = 0

KP (sKp) [ClO4−] = 0.1 M

TPrA TisoBuA TBuA

2.65 (0.14) × 10 1.10 (0.04) × 107 1.52 (0.07) × 105

2.35 (0.13) × 103 1.03 (0.07) × 107 1.50 (0.06) × 105

3

KP (sKp)

TPrA TisoBuA TBuA

0.56 (0.02) × 10 0.60 (0.03) × 107 1.18 (0.05) × 105 3

0.67 0.26 0.11

sKp represents the standard deviation of KP. bΔlog(KP) represents the difference observed with respect to the aqueous/n-C6H14 mixture (Table 1).

a

Figure 3a reports the experimental chromatograms obtained in the CH2Cl2 phase after aqueous/CH2Cl2 partition at two different pH values in the presence of ClO4− 0.1 M. At the most basic pH (12.5), the three expected peaks (apart that of the internal standard) were observed, but when the pH was lowered to less basic pH (8.6), a fourth peak appeared at a retention time of 10.3 min, after that of TBuA (8.7 min). When the same solution was analyzed by GC−MS, the two peaks at 8.7 and 10.3 min could be both identified and attributed to TBuA. Figure 3b reports the experimental values obtained by these GC measurements. In this Figure some unusual features can be observed. First, the fourth experimental data set, belonging to the additional peak eluted at 10.3 min (black symbols), has an opposite behavior with respect to the other three sets, as it decreases with pH instead of increasing. Second, two of the other three data sets (white symbols) show values at acidic pH values, which are significantly larger than zero. These findings indicate that each AA is efficiently extracted in the organic phase even if it is completely protonated. Third, the inflection points for TisoBuA and especially for TBuA are significantly shifted (up to 1−3 pH units) with respect to the corresponding ones obtained in the absence of ClO4− 0.1 M (Figure 2). No similar behavior was observed in n-C6H14 (Figure 1). To explain these experimental results, the formation of the ion pair R3NH+|ClO4− by each AA was supposed. Under this hypothesis, the fourth GC peak (at 10.3 min) is attributed to the ion pair of TBuA, which in MS fragmented to give free TBuA. The black symbols in Figure 3b therefore represent ϕR3NH+|X− values. The ion pairs of the two other amines (TPrA and TisoBuA) did not give resolved GC peaks, but their formation is indicated by the nonzero limiting value of ϕ at acidic pH. Therefore, it is assumed that the ion pairs have the

Table 1. KP Values of the AAs for Aqueous/n-C6H14 Biphasic Mixtures in the Absence and in the Presence of ClO4− 0.1 M in the Aqueous Phasea amine

Δlog(KP)

amine

sKp represents the standard deviation of KP.

KP values are generally rather high, and they increase passing from TPrA to TBuA and to TisoBuA, that is, by increasing the lipophilicity of the AA aliphatic chain. A t-test on the KP values obtained in the absence and in the presence of ClO4− demonstrates that the two data sets are statistically equivalent (with a 95% probability). No ion pair formation is therefore needed to explain the experimental results. Figure 2 reports the experimental values and the fitting lines for ϕR3N obtained by the GC measurements in CH2Cl2 after extraction from an aqueous phase in the absence of ClO4−. Fitting with eq 13 gave the KP values reported in Table 2. If results of Tables 1 and 2 are compared, then lower KP values are observed in CH2Cl2. This solvent effect appears to be more relevant for TPrA, and it can be justified by the larger polarity of CH2Cl2 with respect to n-C6H14. D

DOI: 10.1021/acs.jpcb.7b05088 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 3. (a) GC-FID chromatograms obtained for a CH2Cl2 phase after aqueous/organic partition in the presence of ClO4− 0.1 M. Aqueous solution was at pH 12.5 (dotted line) and pH 8.6 (full line). The CH2Cl2 solution before partition contained TPrA 0.7 mM, TisoBuA 0.7 mM, TBuA 0.8 mM, and p-xylene 1.05 mM. (b) Experimental values for ϕ obtained from GC-FID at various pH values. Symbols refer to TPrA (△), TisoBuA (○), and TBuA 0.8 mM (■ and □). The lines represent fittings obtained by eqs 19 (TBuA, white symbols) (eq 22) (black symbols) and the sum of eqs 19 and 22 (TPrA and TisoBuA).

solution containing NaClO4 0.1 M and at pH 3 (by HClO4). Table 4 resumes the chemical shift values obtained for a set of protons and carbons. Proton/carbon atoms are numbered in the second column according to their chain closeness to the nitrogen atom; for example, “1” is the CH2 bound to the amino group. Figure 4 reports as example the 1H NMR spectrum for TisoBuA after partition and the atom numbering for this AA. In this spectrum, the typical 1H pattern of the isobutyl chain can be observed: a doublet at 1.14 ppm, a multiplet with 9 peaks at 2.17 ppm, and a doublet of doublets at 3.14 ppm. These three signals have an integration ratio of 6:1:2, and they are assigned to CH3 (labeled “3” in the structure of Figure 4b), CH (“2”), and CH2 (“1”), respectively. When the solution before partition was analyzed, the same peaks were observed, but they are shifted upfield: Δδ values are larger for protons closer to the AA nitrogen. This indicates that an electron withdrawing occurred in the proximity of the nitrogen after partition. After partition a new peak appears at 7 ppm as a broad triplet with an integration ratio equal to 1. This new signal can be attributed to the charged ammonium proton (N+−H), which explains the Δδ trends observed on the alkyl chains. This assignment was confirmed with the assistance of data from 2D spectra (Figures 2S and 3S in the Supporting Information, which refer to TisoBuA). A TOCSY spectrum evidenced the chain correlation between the signal at 7 ppm and those of the isobutyl group and with the signals of the “1” protons as well, and a NOESY phase-sensitive spectrum indicates the spatial closeness between the proton resonating at 7 ppm and the protons “2” (dipolar correlation, antiphase with respect to the diagonal). NOESY spectra also show the closeness of water to the ion pair, indicating a R3NH+|ClO4−· H2O solvation. The largest correlation with water was observed for the N+−H proton, as expected. Similar Δδ trends and the appearance of a new proton signal at large ppm values have been observed also in the NMR spectra of TPrA and TBuA (in the Supporting Information, Figures 4S and 5S refer to TPrA). On the basis of these results,

same GC retention time as that of free TPrA and TisoBuA, respectively. For this reason, the white symbols for TPrA and TIsoBuA are the sum of free AA and ion pair (ϕR3NH+|X− + ϕR3N). To fit the experimental points of Figure 3b, three different equations were accordingly used. Equation 19 was used for the ϕR3N values obtained for free TBuA (white symbols, squares), whereas eq 22 fitted the ϕR3NH+|X− obtained for the ion pair formed by TBuA (black symbols). The sum of eqs 19 and 22 was used to fit the other experimental points. Values of KIP and KIP P given by the fittings are reported in Table 3. Table 3. KIP and KIP P Values of the AAs for a Biphasic Aqueous/CH2Cl2 Mixture in the Presence of ClO4− 0.1 M (see Figure 3)a,b KIP (sKIP) (M−1) IP KIP P (sKP ) a b

TPrA

TisoBuA

TBuA

10.1 (3.0) 3.02 (0.81)

5.0 (1.0) × 102 9.0 (1.1)

1.02 (0.33) × 103 5.5 (1.5)

KP values requested by the fitting procedure are reported in Table 2. sKIP and sKIPP represent the standard deviations of KIP and KIP P.

Values of KIP P are above 1 for all AAs, indicating that the ion pairs are preferentially in the organic phase. The most stable ion pair (KIP = 1.02 × 103) is formed by TBuA. This stability, and possibly also kinetic reasons, may explain why only for TBuA could the ion pair “survive” under GC conditions and give a peak resolved from that of free TBuA. The good-fitting quality appreciable in Figure 3 for ϕR3NH+|X− with eq 22 strongly suggests by itself the correctness of the whole model employed, in particular, the assumption of ion pair formation. Nevertheless, an independent identification of R3NH+|ClO4− was obtained by NMR. Formation of Ion Pairs in the Organic Phase. 1H NMR and 13C NMR measurements have been performed on AA solutions in CD2Cl2 before and after partition with an aqueous E

DOI: 10.1021/acs.jpcb.7b05088 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Table 4. Chemical Shift Values (δH and δC, ppm) Observed in CD2Cl2 for the Proton and Carbon Atoms of the Examined AA R3NH+|ClO4−b

R3Na amines TPrA

TisoBuA

TBuA

proton/carbon 1 2 3 H+ 1 2 3 H+ 1 2 3 4 H+

c

δH

δC

δH

δC

ΔδHd

ΔδCd

2.35 1.44 0.89

56.46 20.53 11.78

55.1 17.5 11.2

0.73 0.38 0.17

−1.36 −3.03 −0.58

2.04 1.71 0.91

64.90 26.70 20.88

62.1 24.1 20.4

1.1 0.46 0.23

−2.8 −2.6 −0.48

2.38 1.41 1.32 0.93

53.99 29.55 20.81 13.98

3.08 1.82 1.06 8.00 3.14 2.17 1.14 7.00 3.11 1.75 1.46 1.04 8.00

53.3 25.4 19.8 13.2

0.73 0.34 0.14 0.11

−0.69 −4.15 −1.01 −0.78

Refer to solutions of R3N before partition with aqueous solution. bRefer to solutions of R3NH+|ClO4− after partition with an aqueous solution containing NaClO4 0.1 M at pH 3 (by HClO4). cFor the proton/carbon numbering refer to Figure 4b. dδ differences (ΔδH and ΔδC) between chemical shifts before and after partition. a

Figure 4. (a) 1H NMR spectrum of TisoBuA in CD2Cl2 after partition with an aqueous solution containing NaClO4 0.1 M and at pH 3 (by HClO4). Peak at 7 ppm is enlarged for clarity. (b) Details of the 1H NMR spectrum at high fields and atom numbering of TisoBuA.

the formation of an ion pair between R3NH+ and the only negative available anion, ClO4−, must be assumed. Formation of Ion Pairs between TisoBuA and Other Inorganic Anions. The formation of ion pairs between TisoBuA and other inorganic anions, that is, NO3−, ClO3−, Cl−, H2PO4−, and IO3−, was evaluated by measuring the fraction of AA extracted in the organic phase (CH2Cl2) after partition with an aqueous phase at pH 1.5 containing the sodium or the potassium salt at a 0.1 M concentration and TisoBuA 3 mM. Under these acidic conditions, AAs can solubilize in the organic phase as ion pair, as seen in the previous sections, and values of ϕR3NH+|X− are obtained. These values are summarized in Table 5. The extracted fraction is maximum in the presence of ClO4−, indicating that this ion forms the most stable ion pair or that its ion pair has the largest partition constant in the organic phase. Nonzero extracted fractions were also obtained for the other examined anions. Analytical Application to the Analysis of Perchlorate. The results obtained for ClO4− suggest the possibility to

Table 5. Fraction of TisoBuA (ϕR3NH+|X−) Extracted in the Organic Phase (CH2Cl2) after Partition with an Aqueous Phase at pH 1.5 Containing the Indicated Salts at a 0.1 M Concentration and TisoBuA 3 mM salt

ϕR3NH+|X−

salt

ϕR3NH+|X−

NaClO4 NaNO3 KClO3

0.322 0.131 0.079

KCl KH2PO4 KIO3

0.019 0.012 0.007

determine the ClO4− content in aqueous solutions by extracting its AA ion pair, R3NH+|ClO4−, to an organic phase, and analyzing it by GC-FID. The most suitable AA to be used for this purpose seems to be TisoBuA, as it displayed the largest KIP P values (Table 3). The chosen organic phase is CH2Cl2. Figure 6S (Supporting Information) reports the ratio between AR3NH+|X− and AIS as a function of the total ClO4− concentration in the aqueous phase. The limit of quantification (LOQ)28 for ClO4− calculated from this graph is 200 ppm, F

DOI: 10.1021/acs.jpcb.7b05088 J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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which indeed is not very low. A better (lower) LOQ could, in principle, be obtained by replacing the FID detector with a nitrogen−phosphorus one (NPD).29

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +390498275182. Fax: +390498275175.

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CONCLUSIONS In this work, the ability of selected tertiary AAs (TPrA, TisoBuA, TBuA) to form ion pairs with ClO4− in biphasic aqueous/CH2Cl2 mixtures has been demonstrated by GC and NMR measurements. Some experimental evidence supports the ion pair formation. First, AAs are not present in the organic phase when the pH of aqueous solution is acidic and no ClO4− is added, whereas they are significantly transferred (with high extraction efficiency) to the CH2Cl2 phase when the aqueous solution is added with ClO4− 0.1 M. Second, one additional GC peak was observed, together with those expected for the AAs, when the aqueous solution was added with ClO4− 0.1 M. Spectrometric mass measurements of this peak indicated that it is TBuA. As another GC peak was obtained for “free” TBuA, the additional peak was attributed to its ion pair. Third, the good-fitting qualities of the experimental data substantiate the goodness of the model employed, which included the formation of the ion pair and its partition between aqueous and organic phase. Fourth, NMR spectra in CD2Cl2 before and after partition showed the presence of a R3NH+ group in the organic phase after partition and no evidence of such species before partition. The ion pair formation was shown to affect the distribution of AAs between organic and aqueous phases. Taking into account all equilibria involved, values for KP (partition constant of the free AA), KIP (formation constant of the ion pair in the aqueous phase), and KIP P (partition constant of the ion pair) were obtained for TPrA, TisoBuA, and TBuA in biphasic aqueous/CH2Cl2 mixtures at 25 °C. Ion pairs form with TisoBuA also if ClO4− is replaced by other inorganic anions, like NO3−, ClO3−, Cl−, H2PO4−, and IO3−. The maximum effect on the extracted fraction at acidic pH is, however, observed for ClO4−. The formation of ion pairs between AAs and ClO4− can be used for the analytical determination of ClO4− by GC-FID. The LOQ of the method involving TisoBuA was calculated to be 200 ppm, and, in principle, it could be improved by using a more suitable detector than FID. Conversely, no ion pairs are observed when n-C6H14 is used as organic phase, as the GC-FID data can be interpreted by simply considering the partition of free AAs and by accounting for ionic strength changes. The absence of ion pairs in n-C6H14 might be due to a very low affinity between the polar compound R3NH+|ClO4− and the nonpolar n-C6H14 solvent. Actually, KIP P values were relatively low for CH2Cl2, and they are expected to be much lower for n-C6H14. These results suggest that AA ion pairs can form in polar organic solvents but not in nonpolar ones.

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Article

ORCID

Valerio Di Marco: 0000-0001-6108-746X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS University of Padova is gratefully acknowledged for financial support (DOR Pastore 2016).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b05088. Experimental (chemicals, apparatus, procedure, references). Figures 1S−6S. Relation between thermodynamic and stoichiometric KP, Ka, KIP, and KIP P constants. (PDF) G

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