Hydration and Intermolecular Interactions in Carboxylic Acids Текст научной статьи по специальности «Химические науки»

Condensed Matter and Interphases (Kondensirovannye sredy i mezhfaznye granitsy)

Original article

DOI: https://doi.org/10.17308/kcmf.2020.22/2998 ISSN 1606-867X

Received 09 June 2020 elSSN 2687-0711

Accepted 15 August 2020 Published online 30 September 2020

Hydration and Intermolecular Interactions in Carboxylic Acids

© 2020 V. F. Selemeneva, O. B. Rudakovb, N. V. Mironenkoa, S. I. Karpov8, V. N. Semenova, N. A. BelanovaEa, L. A. Sinyaevaa, A. N. Lukina

aVoronezh State University,

1 Universitetskaya pl., Voronezh 394018, Russian Federation bVoronezh State Technical University,

84 ul. 20-Letiya Oktyabrya, Voronezh 394006, Russian Federation Abstract

At the moment, the most accurate and reliable information about intermolecular interactions in low-molecular compounds and their polymer analogues can be obtained by means of combined UV, visible, and IR spectroscopy. However, this combination is not always used when interpreting the results of intermolecular interactions in carboxylic acids. Therefore, the aim of our study was to investigate the intermolecular interactions in carboxylic acids and their hydration properties using the UV, visible, and IR spectroscopy.

The article presents the results of the investigation of intermolecular interactions and hydration in carboxylic acids by means of UV, visible, and IR spectroscopy, and the microscopic study of the swelling/contraction curves of the beads of the sorbents with slightly acidic -COOH groups in exchange reactions of R-COOH + NaOH ^ R-COONa+ + H2O. The study revealed that in water dimers, the total energy of hydrogen bonds is determined by the Coulomb, exchange, charge transfer, polarization, and dispersion components. In our study we also tested the formulas for the calculation of the energy of the H-bond, enthalpy, the force constants of the H-bond, and the elongation of the covalent bond. The article suggests a formula for estimation of the distance RCH2...O. The calculations of the length of H-bonds between the donor and the acceptor of the proton based on the information about stretching vibrations in the IR spectra of carboxylic acids. The article demonstrates the possibility of the formation of five- and six-membered cycles, resulting from the formation of H-bonds between CH2 groups of the chain and -COOH end groups of carboxylic acids.

The characteristic electron and vibrational frequencies in the UV and IR spectra were used to determine the intermolecular interactions in ion exchangers CB-2 and CB-4. The microscopic and microphotographic study of the swelling of certain beads of carboxylic cationites help us to register the presence of the external shell R-COO...Me+ and the internal shell R-COOH during the exchange reactions: R-COOH + Me+ + OH- ^ R-COO...Me+ + H2O.

Keywords: UV-vis spectroscopy, IR spectroscopy, visible spectroscopy, carboxylic acids, intermolecular interactions. Funding: The work was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of the government order to higher education institutions in the sphere of scientific research for years 2020-2022, project No. FZGU-2020-0044.

For citation: Selemenev V. F., Rudakov O. B., Mironenko N. V., Karpov S. I., Semenov V. N., Belanova N. A., Sinyaeva L. A., Lukin A. N. Hydration and intermolecular interactions in carboxylic acids. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2020; 22(3): 373-387. DOI: https://doi.org/10.17308/kcmf.2020.22/2998

El Natalya A. Belanova, e-mail: belanovana@mail.ru

The content is available under Creative Commons Attribution 4.0 License.

1. Introduction

When assessing an analytical method, we should take into account the three most important parameters: its accuracy, sensitivity, and cost. Unfortunately, in the modern industrial world there are few people who realise that without analytical chemistry it is difficult to combine these criteria when assessing industrial technological solutions. Indeed, analytics, being just a supplementary discipline at first, is now becoming a popular interdisciplinary branch. This indicates the restoration of analytical chemistry as an independent scientific field.

Most of the information about the structure of the initial components and the physicochemical properties of target products can be obtained by means of spectroscopy. Therefore, spectroscopy is the study of the interaction between matter and light. The electromagnetic spectrum covers electromagnetic waves ranging from high-energy cosmic radiation to X-rays, ultraviolet, visible light, infrared, and low-energy radio waves [1-4]. g-rays, with the wavelength of just 10-11 cm, are emitted during nuclear reactions. X-rays (with the wavelengths of 10-8 cm) are formed as a result of electronic transitions between the inner shells of the so-called core electrons (Table 1).

The electronic excitation of molecules, atom vibrations, and excitation of molecular rotation are possible within a single spectrum region (a relatively small one) with the wavelengths ranging from 10-1 to 10-6 cm. This region of the UV, visible light, and IR radiation is referred to as the "optical spectrum". When wavelengths are

shorter than 800 nm (i.e. the UV and visible light), the radiation energy is high enough to interact with the electrons in molecules [1-3]. In this case, only the valence electrons of compounds are excited. The strongly bonded s electrons (Table 1) of simple (single) bonds absorb radiation of shorter wavelengths than weakly bonded n electrons of multiple bonds and n (nonbonding) electrons of heteroatoms.

The vibrations of molecules are excited by lower energy as compared to the ultraviolet and visible light regions of the spectrum, i.e. at longer waves. The molecular rotations are also excited, and therefore we can call these spectra rotational-vibrational [1-4]. To rotate a molecule along the three inertial axes x, y, z, minimal radiant energy is required. Therefore, at wavelengths of more than 800 nm, the rotational, vibrational, and electronic spectra are observed.

At the moment, the most accurate and reliable information about intermolecular interactions in low-molecular compounds and their polymer analogues can be obtained by means of combined UV, visible, and IR spectroscopy. However, this combination is not always used when interpreting the results of intermolecular interactions in carboxylic acids. The effect of the solvent (solvation or hydration) as one of the factors determining the character of intermolecular interactions in solutions and polymers is also rarely taken into account. Therefore, the aim of our study was to investigate the intermolecular interactions in carboxylic acids and their hydration properties using UV, visible, and IR spectroscopy.

Table 1. Classification of the spectral regions

Spectrum Interaction Spectral region Wavelength

X-ray spectrum Inner electrons X-rays 0.01-1.0 nm

Electrons of the s-bonds UV in vacuum 10-190 nm

(sp3-orbitals)

Electron spectrum Electrons of the n-bonds (sp2-orbitals) UV 190-380 nm

n electrons (sp-orbitals) Visible region 380-800 nm

Vibration spectrum Higher harmonic vibrations Near IR 0.8-2.5 ]im

Molecular vibrations Mid-IR 2.5-50 ]im

Rotation spectrum Rotation of the Far IR 50-500 ]im

molecules Microwaves 0.5-3.0 mm

s, s* bonding and antibonding orbitals; p, p* bonding and antibonding orbitals

2. Experimental

The objectives of the study determined the use of the spectroscopy methods commonly applied for detecting various structures in solutions of target substances. The UV spectra were registered using the Shimadzu 2401 spectrophotometer. Each spectrum was interpreted based on a single wavelength. The absorbance was measured at the maximum peak of the spectrum, followed by the calculation of the concentration [3-7].

The character of the swelling kinetic curves f = V/V (V, V is the volume of the bead at the

' t n x t' n

moment t and the initial volume in the H shape) of the sorbents with slightly acidic -COOH groups was used as a criterion for the transitions taking place in the ion exchanger phase. Therefore, we used the microscopic methodology [8, 9] to study the interaction between the sorbent and the -COOH groups. The character of the swelling kinetic curves (concentrations of separate beads of the sorbent) was studied in special plexiglass cuvettes using the MIR-12 and MBI-6 microscopes. The size of the beads was registered with a precision of up to ±0.002 mm.

The IR spectra of liquids were recorded during the analysis of either a pure substance or its mixtures with solvents. Solid samples were prepared in the form of oil suspensions, thin films on the surface of NaCl, KBr, and CaF2 wafers, and pressed tablets with alkali metals halides. The samples were ground in an agate mortar so that the size of the particles was no more than 0.5 pm. The samples were prepared for the analysis using the methodology described in [5]. The spectra were registered by means of the Bruker Vertex 70v vacuum FTIR spectrometer using a "Platina" single-pass ATR adapter. The preparation of samples for the IR spectroscopy most commonly involves pressing them together with KBr [6-9]. To produce the tablets, the press die described in [6] was used until the KBr crystals formed a translucent matrix with the powder of the studied substance regularly distributed within it.

The dependence of the energy of the H-bond (Eint) and its components on the distance R(O...O) is demonstrated in Fig. 1. The curves were calculated for the water dimer [10] and they demonstrate that, when the distance is large, the Coulomb energy (Ecoul) of two

neighbouring molecules with intact electron shells is predominant [2, 4].

Near the equilibrium, the energy of the H-bond is determined by the Coulomb (Ecoul) and exchange (Eex) contributions [2, 4, 10], where Eex is bonded taking into account the equality of the electrons of the interacting molecules, when their wave functions overlap.

Besides the Coulomb E . and exchange E

coul ° ex

contributions, the energy of the H-bond (Eint) is also contributed to by the following components: the polarisation interaction energy (Eind); the charge transfer energy (Echt) reducing the H-bond energy as a result of the redistribution of the electron density within the subsystem (polarisation) and between the subsystems (charge transfer) [2,4,10]; and the dispersion energy (Edisp), which takes into account the correlation in the transmission of electrons of different molecules. Therefore,

E. t = E ,+ E + E. d+ E.+ Ed. + E(n * 3). (1)

int coul ex ind cht disp v '

Depending on the value, the energies of the H-bond are classified as weak, medium, and strong [7]. The formation of the hydrogen bond significantly alters the properties of the molecules

Fig. 1. The dependence of the total H-bond energy Eint and its individual components in the H2O dimer on R(O.O) [7, 8]

of the associates R1A-H...BR2 (particularly of the A...H group). Our experiment demonstrated (Fig. 2, Table 2) the reduction in the frequency of stretching vibrations of the A-H (RxO.OR2) bond in the 10-2600 cm-1 range depending on the strength of the H-bond.

Fig. 2. The correlation between O-H-bond vibration frequency and the equilibrium distance R0(O...O) in crystals [8]

It should be noted that the characteristic frequencies of absorption of the groups of atoms in carboxylic acids obtained in [11] (Table 3) are similar to those presented in this article (Table 4, Figs. 3 and 4).

However, the results shown in Table 4 and Figs. 3 and 4 allowed us to determine the stretching vibrations and deformation vibrations to be certain functional groups and suggest a new description of the formation mechanisms of cyclic structures with H-bonds in carboxylic acids.

It is somewhat difficult to interpret the IR spectra of carboxylic acids, when the absorption bands of -COOH and -CH2 groups overlap (Table 5). Therefore, we performed a preliminary comparison of the UV spectra (Fig. 3a and 4c) demonstrating the electronic transitions (scheme 1) with the characteristic vibrations of the groups in the IR spectra (Fig. 3a, 3b, 3c).

Scheme (1) demonstrates that the peaks at 2962 and 2834 cm-1 are characteristic for -OH-bonds in associates with carboxylic groups and for -CH2- in "hydrophobic" chains of carboxylic acids [2-6]. For -CH2, C-O, and -OH in COOH groups the peaks at 1525, 1379, 1300, 1245, 960-944, and 737 cm-1 appeared to be joint absorption bands. This, although making

Table 2. Formulas used to calculate the parameters of the hydrogen bridge using the values of the shift in the IR spectra

No. Parameter Symbol Unit of measurement Formula Source

1 Energy of the H-bond EH kJ/mol -Av / vOH = EH • 1.6 10-2 [1]*

2 Enthalpy AH kJ/mol -AH = 2.9-AA1/2; Av = [AA1/2f • 80 [2]

3 Force constant of the H-bond KH cm-2 Kh=(5.5±1.2)-104-£h

4 Force constant of the OH-bond KOH cm-2 -K0H= 8.63(5.5±1.2)-104-£H - 12.879-106

R OH...O À Av = 4.4 103(2.84 - RO^O)

Length of the hydrogen bridge R OH...N À Av = 6.92 102(3.04 - RO...n)

5 R NH...O À Av = 5.48 102(3.21 - RN...O) [3, 4]

R NH...H À Av = 1.05 103(3.38 - RN...n)

R CH2...O À Av = 0.89 102(3.42 - RCH...O)

6 Elongation of the covalent bond AroH À ArOH = 5.3 102 Av

* v° for ROH O=3700 cm-1; v° for Roh...n = 3400 cm-1 [2, 6] ; v° for Rnh...o = 3550 cm-1; v° for Rh...h = 3300 cm-1

[1, 2, 5, 6]; v° for RC

3200 cm-1 [1, 2, 5, 6].

Table 3. Characteristic frequencies of absorption of the groups of atoms of fatty acids [2-8]

Vibrational frequency v, cm-1 Av*, cm-1 Assignment of vibrations

[7, 9, 13] Stearic acid Oleic acid

3010 2962 2956 3008 -2; -6 v =CH - in RCH=CHR'(trans); CH3(v ) 3V as'

2925 2911 2918 -7; -14 v *CH2 as 2

2853 2843 2844 -9; -10 v **CH3; v CH2 s 3 s 2

2726 2686 2883 -40; -43 -OH in COOH (bonded)

1720 1698 1705 -22; -12 v C=O in COOH

1467 1421 1400 1471 1431 1413 1472 1433 1413 4; 5; 10;12; 13 S*as CH3; sciss. CH2 in the chain 8os CH2 (scissoring); 8 , if CH2 is with C=O; CHin -C=CH as 2

1375 1379 1382 4; 7 8 CH3; 8 -O-H s 3'

1300 1312 1316 12; 16 CH2 wagging; -COOH in dimers

1245-1180 1252-1182 1266-1192 Group of bands CH3; CH2 rocking stretching with an unbranched chain; with COOH end groups

1125-1120 1104 1104 -21; -21 vOH in C-O groups with five or six-membered cycles

935 944; 886 953; 895 9; 18 Wide bands nonplanar 8 vibrations of OH in COOH

750; 720 817;737 817; 744 67; 67 (CH2)n rocking stretching

- 608 612 8 CH in cycles

* v, S are stretching and deformation vibrations respectively;

** as, s are asymmetrical and symmetrical stretching vibrations respectively.

the bands C=C and lactone C=0 (1800-1785 cm-1 and 1735 cm-1 respectively) [2-5].

It is interesting that there are clear groups of absorption bands at 1433-1104 cm-1 (Table 4, scheme 1). This characterises the transition from planar conformation (scissoring - and rocking g stretching) in hydrophobic tails to the nonplanar conformation (wagging gw and twisting gt stretching) caused by the mixture of the stretching vibrations vs and vas of OH polar groups with gr and g deformation vibrations of CH2 groups of the methylene chain [3, 5, 8]. Each of these transitions is accompanied by the absorption of a photon of a certain value. The c^c* transition requires the most energy (scheme 1). The corresponding absorption bands are observed in the vacuum UV region (l < 200 nm).

the interpretation of the spectra difficult, has a positive effect. For example, it allows determining the form (open or lactone) of levulinicc acid, if the spectrum includes the bands 3260, 2970, 2930, 2870, 2850, 1720, 1705, and 900 cm-1 [7].

O CH3 H

CH

CH3-C—CH2—CH—COOH

C=O peaks are characteristic for the open structure in ketonic (1720 cm-1) and carboxylic (1705 cm-1) acid; 3260 cm-1 (v OH), and 900 cm-1 (S OH). Frequencies 2970, 2930 cm-1 and 2870, 2850 cm-1 are determined to be stretching vibrations of methyl CH3 and methylene CH2 groups. The cyclic structure should be demonstrated by

Table 4. Characteristic frequencies of absorption of the groups of atoms of carboxylic acids [2-8]

Vibrational frequency v, cm-1

[2-8] Acetic acid Av, cm-1 Stearic acid Av, cm-1 Oleic acid Av, cm-1 Assignment of vibrations

3550 3583 -33 Free OH group

3005 3028 -23 3125 -120 3052 -47 bonded by 2 H-bonds (v CH2);

2925 2925 0 2927 -2 2918 +7 bonded by 3 H-bonds (v CH3);

2853 2857 +4 2843 -10 2844 -9 bonded by OH in dimers (v CH2);

2726 2686 -40 2726 0 2683 -43 OH-bonded in dimers

1720 1700 -20 1680 -40 1692 -28 v C=O in COOH;

- - - 1600 - 1606 - vas COO;

1525 - - 1525 0 1551 +26 v COO-

1430 1466 1438 +33 +8 1480 1400 +50 -30 1472 1435 +42 +5 vs COO", ACH2 scissoring scissoring in chains, if CH2 is next to C=O or C-CH=CH2

1375* 1300 1245 1350 1300 1266 +25 0 -21 1379 1320 1263 -4 -20 -18 1380 1305 1238 -5 -5 +7 5 CH3 in the dimeric ring; gw wagging; -COOH in dimers; W Q = v C-O; 5 CH2 twisting in the dimeric ring

1192 1192 0 1195 -3 1192 0 gr CH2 with an unbranched chain with end COOH

1125 1104 -21 1104 -21 1104 -21 vOH in C-OH groups with five or six-membered cycles

Any OH group; gt twisting

in the dimeric ring; planar

940 940 0 926 +14 953 -13 stretching vibrations Q(C-C) = vs rocking; gr CH2 of the methylene chain

750 744 +6 728 +22 756 -6 g OCO; g CCC; g OCC in dimeric

646 642 +4 636 +10 638 +8 rings of carboxylic acids

Q = vs; v

Designations for the vibrations: - symmetric and asymmetric deformation vibrations; 65; 5as - deformation scissoring, symmetric and asymmetric stretching; gw; gt; yr - wagging, twisting, and rocking deformation vibrations; A - deformation vibrations of the skeleton of the chain; v C=O=q=Q=gOCO; gOCC; gCCC -vibrations in the dimeric ring; gOCC; gCCC - nonplanar vibrations in the stretched chain

^transition from the planar to the nonplanar conformation due to the shift of v OH polar vibration with gt and g in the methylene chain of the -CH„- groups

Scheme 1

The absorption of light in the visible region and absorption of the UV light in the near UV

region indicate the n and n electronic states. The following functional groups

C=O

C=N— —

=\ =<°

CHO

NH C=C—C=O

are called chromophoric and cause the absorption in the UV region [2-6].

2.5 , tee

1.5

\ 1

1 -

\ 304

0.5 -

307

0

0,9 0,80,7 0,60,50,40,3 0,2-

150

200

0,1

250

300

350 X.i

400

o t

Ov

o 2

•o —

\

3200

2700

700

1200

700

v, cm"

Fig. 3. UV (a) and IR spectra (b) of acetic (1) and oleic acid (2)

It should be noted that such transitions are one of the causes of the formation of dimers by -COOH end groups:

R—C

o--h—O,

"H

I

2 H ^H-

O-H-O'

/

,C—R

C

H3C— (CH2)n-2 —CH

^H-O Scheme 2

C—OH

as well as the formation of five-membered cycles due to the appearance of hydrogen bonds between CH2 methyl groups of hydrophobic chains and oxygen in carboxylic groups [2, 3, 11]:

Scheme 3

a

b

0,8 -1

700

2800 1300 v, cm1 800

Fig. 4. UV and IR spectra of stearic (a) and oleic (b) acids. UV spectra of stearic (1) and oleic (2) acids (c)

Jg£

240

\ 1

188

307

'3*43 ^v-.

150

250

350 A., nm

End of Fig. 4

This is demonstrated by the peaks 1192, 1104, 940, 744, and 638 cm-1. Intense bands at 182 nm (n^o* transitions), 240 nm (p^p* transitions), and 305-343 (n^p transitions) in the UV spectra of carboxylic acids (Fig. 3a, 4b) indicate the possibility of the formation of cyclic structures in the solutions and the mentioned bands prove the accuracy of the description of their structure (scheme 2 and 3) based on the results of the IR spectroscopy.

Besides experiments with monomers, UV and IR spectroscopy can be used in experiments with polymers containing carboxylic groups [36]. The most interesting examples of this group of polymers are poly(methyl methacrylate) and poly(methacrylic acid):

CH„

-CH,-C-

COOCH

CH

-CH,-C-

COOH

In the IR spectra of poly(methacrylic acid), the bands at 1750 and 1700 cm1 are assigned to the vibrations of carbonyl in monomer and dimer groups, and the bands at 3540 and

2650 cm-1 are assigned to the vibrations of free and bonded hydroxyl groups [3, 5, 6]. Peaks 1490 and 1460 cm-1 indicate the presence of o and o

r s as

(deformation vibrations) of the methyl group =CH2. In the spectra of copolymers of metacrylic acid with acrylonitrile, a shift in the location of the bands is observed at 1263 and 1167 cm-1, caused by the vibrations of the C-O-bond [4, 5, 11].

The IR spectra of syndiotactic poly(methyl methacrylate) reveal a group of bands in the 3460-2835 cm-1 region, caused by the vibrations of free and bonded hydroxyl groups [3, 5, 6, 9]. It should be noted that the peaks 2948-2835 cm-1 also characterise the vibrations of methyl and methylene groups [3, 6, 7] (Table 6). The variability of the bands at 1270, 1240, 1190, 1172, and 1163 cm-1 is accounted for by intramolecular interactions, since the multiplet is also observed in the spectrum of diluted solutions, i.e. without intermolecular interaction.

Similar to the monomers of carboxylic acids, poly(methyl methacrylate) demonstrates clear peaks at 1430-910 cm-1, indicating the transition from planar conformation (scissoring ps and rocking yr) in hydrophobic tails to the nonplanar conformation (wagging gw and twisting gt) caused by the mixture of the stretching vibrations

c

n

n

Table 5. Assignment of close absorption bands of -COOH- and -CH2 groups in the IR spectra of carboxylic acids

Vibrations of COOH groups v, cm-1 Vibrations of CH2 groups v, cm-1

3580 - free OH

3028 - OH with two H-bonds 3008 =CH in RCH=CHR' (trans)*

2962 - OH with three H-bonds 2962 v CH2 as 2

2843 - OH in dimers 2843 v CH2 s2

1700 - C=O in COOH- groups

1525 - C=O in COO- groups 1480 ACH2 next to C=O

1379 - C-O in dimers 1375 s C+H3 in the dimeric ring

1300 - COOH in dimers 1300 gw CH2 wagging stretching in the ring

1245 - C-OH in the dimeric ring 1245 gt twisting in the ring

1104 - C-OH in five-membered cycles 1122 gr rocking next to COOH

944 - OH groups in COOH 960 gw; gt; gr in the dimeric ring

737 - OCC in the dimeric ring 638 gOCO; gCCO in dimers with COOH

Assignment of vibrations: Q = vs; vas - symmetric and asymmetric stretching;

j = p = ss; sas - deformation scissoring, symmetric and asymmetric stretching; gw; gt; yr - wagging, twisting, and rocking stretching; A - deformation vibrations of the skeleton of the chain;

v C=O=q=QC=O; gOCO; gOCC; gCCC -vibrations in the dimeric ring; nonplanar vibrations in the completely stretched chain.

vs and vas of the polar groups with gr- and gt deformation vibrations of CH2groups of the methylene chain (Fig 5, Table 6) [3, 5, 8, 9]. Thus, dimeric rings are formed in polyacrylates and poly(methyl methacrylates) due to intramolecular interactions. The transition form the planar to the nonplanar conformation is also observed in the polymer chain.

Microscopic and microphotographic methods of studying polymers with carboxyl functional groups are supplementary methods used to visualise the process of swelling and contraction of carboxyl sorbents during their contact with aqueous solutions [13, 14]. The objects of our study were carboxylic cationites CB-2x2 and CB-4 (scheme 7):

CH,

-CH2-C-

COOH

CH-CH,

n \\ //

—CH—CH—CH CH2

HOOC COOH

CH-CH2

-HC-CH2

CB-2

CB-4

Fig. 5b demonstrates that the IR spectra of the sorbents are close to the IR spectra of poly(methyl methacrylates) (Fig. 5a), namely in the vibration areas of the carboxylic groups, formation of dimers, and intramolecular H-bonds. There is a difference in the vibration areas characterising the presence of divinyl benzene: 1310, 1053, 922, 896, and 700 cm-1.

The microscopic study of the swelling of certain beads of carboxylic cationites, the kinetic curves of volume change, and the swelling diagrams demonstrated that slightly acidic ion exchangers have the minimal volume in the H form as compared to the salt forms (Fig. 6). This effect is accounted for by the formation of associates presented as dimer cycles with methyl -CH2 groups

2

n

Fig. 5. IR spectra of poly(methyl methacrylate) (5a) CB-2 and CB-4 (5b): 1 - CB-2; 2 - CB-4

1.3 -

1.1 -

0.9

/

a

-- 4

3

...... 1

/ s/s

N ' S V N' , '

x v ^ ,.•■

V. 4 x

/ V. -V

_ 4 ~~~ 3

■"•f2

10

20

т. min

30

U 1

u -

l -

RCOOH RCOOMe

H70

RCOO'Me^

HCl 50 30 MeCl 50 70

0 100 0

II

70 30

50 50

HC1 MeCl

Fig. 6. The swelling curves of the CB-2 cationite (a) and the bead volume change of the cation exchanger CB-4 (b): 1 - R-COOH; 2 - R-COOLi; 3 - R-COONa; 4 - R-COOK f = V/Vn, where V, Vn are the bead volume at the moment t and initial bead volume in the H form respectively. A scheme of the exchange processes RCOOH+MeOH (I) and RCOOMe+HCl (II) (c)

the internal shell R-COOH (Fig. 6c). The exchange reaction is accompanied by the swelling of the bead. The amount of water transported by the ions during the reaction is not sufficient for the hydration of the cationite in salt form, which results in further introduction of water from the solution into the sorbent (Fig. 6c) [15-19].

During the R-COO- + Me+ + H+ + Cl- ^ R-COOH.. .Me++ Cl- transition, in the first stages of the exchange process the -COO- groups located on the bead's surface absorb H+ ions from the solution

and as intramolecular H-bonds formed due to the p^p* transitions in benzol cycles [1-6,15-21].

When the reaction takes place in the surface layers of the cationite bead, the carboxylic groups are ionised through the reaction:

R-COOH + Me+ + OH- ^ R-COO...Me+ + H2O.

During this reaction, the metal ions neutralise the negative charge R-СОО-. As a result, two shells are formed in the bead of the carboxylic cationite: the external shell R-COO...Me+ and

b

c

Table 6. Poly(methyl methacrylate) IR spectrum

Wave number, cm-1 Vibrations

3460 Free OH groups; 2 (n C=0)

3368 Bonded OH groups; nas CH(+ v (CH3-0)

3002 ns(CH3-°)+ nas(L-CH3)+ v (L-CH3) + nas(CH()

2920 Combination tone bonded with CH3 in an ester group

2835 Id.

1730 n C=0 in COO- groups

1483 8as (CH3-0); A CH( next to C=0

1465* sas (CH3-0); A CH( scissoring stretching next to C-CH=CH(

1452 1438 8 (CH(); Ss (CH3-0);

1388 ss (CH3-0); s+(CH3) in the dimeric ring

1300 gw wagging; -C00H in dimers

1270 1260 nas(C-C-0); Q = n(C-0); sCH2= yt(CH2) in the dimeric ring

1190 gr(CH2) rocking; nC-C)+ s(CH) groups

1172 ss in esters (intramolecular)

1150 n(C-C) mixed with deformation vibrations sCH

1063 To ^e

988 967 v (C-0-C)= gr(CH3-0)+ ^(L-CH^+g in the ring; gr(L-CH3) mixed with g in the dimeric ring;

749 638 gr(CH2) mixed with n(C-C)+ s (0C0)+ n(0CC) in the dimeric rings with C00H groups

* P, gw; v Yr - scissoring, wagging, rocking, and twisting vibrations; A - deformation vibrations of the skeleton of the chain.

and become non-dissociated RCOOH groups. Thus, the bead is once more divided into two zones (Fig. 6c). Then the H+ ions diffuse through the outer shell towards the boundary and replace the metal ions which are, in turn, eliminated from the cationite bead. The process can be described as interdiffusion between the hydroxonium ions and metal ions through the layer of the cationite in the H form. Processes (II) and (I) are accompanied by contraction and swelling respectively (Fig. 6c) as well as by the migration of the solvent either from or to the sorbent phase. Both processes have a clear boundary visible through a microscope. Processes I and II are indirectly proved by the calculation of the elongation of covalent bonds r (A-H) as the function of R(A...B) for bonds R(O...O) R(O...N); R(N...O); R(N.N), and R(CH2...O), according to the results of the IR spectroscopy (Fig. 7). Each curve was obtained by shifting curve 1 (Fig. 7) horizontally according to the van der Waals radii

Req(O-O) = Rexp(A-B) + 2r„(0) - r„(A) - r„(B)];

[W(A-H) = req(°-H) + rc(A) - rc(°)L

where r is the van der Waals radius, r is the

u ' c

covalent radius.

The vertical shift is performed taking into account the covalent radii. Taking into account the processes in (2), we obtained an equation for calculating R(CH2...O), based on the value of the band shift An:

An° for R(CH2...O) = 3200 cm-1; An = 0.89-103(3.42 - RCH. O).

Similar interactions accompanied by the formation of H-bonds between hydrophobic CH groups and hydrophilic C=0, N-H, C^H (in acetylene), and S-H groups were described earlier in [2, 4], which makes it necessary to explain the term "hydrophobic interactions". The term "hydrophobic interactions" was introduced to describe the joint effect of the London dispersion

Fig. 7. The values of the covalent bonds r(A-H) calculated as a function of R (A...B) for the associates 1 -R(O...O); 2 - R(O...N); 3 - R(N...O); 4 - R(N...N); 5 -R(CH2...O). Curves 1' represent the dependence between R(O...O) and Avs

forces (forces resulting from the formation of the instantaneous dipoles), the van der Waals forces (orientation, induction, and steric repulsion), and hydrogen bonds on the processes taking place in aqueous solutions [1, 2, 8]. The nature of these interactions is similar to that of other intermolecular (noncovalent) interactions, although in some cases they are characterised by small enthalpy changes [1, 2, 4], calculated based on the values of Av (curve 1' in Fig. 7). It is noteworthy that the shape of the curves showing the dependencies for [R(O...O) - r(A-H)] and [R(O...O) - Avs], are symbatic. This proves the correctness of the methods used and accuracy of the calculations presented in this article.

4. Conclusions

The article described the method and analysed the results of the combined use of UV-vis and IR spectroscopy, as well as the microscopic and microphotographic studies of intermolecular interactions and hydration properties of acetic, stearic, and oleic acids, and carboxylated cationites CB-2 and CB-4 in the exchange reactions R-COOH + NaOH ~ R-COO- j Na+ + H2O. The energy of the hydrogen bond (EH), the enthalpy (AH), the force constant of the H-bond (KH) and OH-bond (K0H), and the elongation of the covalent bond (Ar0H) were calculated based on the results of the IR spectroscopy for intermolecular interactions of

carboxylic acids in solutions. The article suggested a method for calculating the length of the H-bond and RCH2 O between the donor (CH2 group) of the non-polar chains of fatty acids and the acceptor (O- in COOH groups) resulting from the formation of cyclic structures in carboxylic acids.

The microscopic method was used to obtain the swelling/contraction curves for the beads of the cationites CB-2 and CB-4. The first description of the formation of two boundaries (shells) in the beads of carboxylic cationites during the ion exchange reaction R-COOH + NaOH ~ R-COO- j Na+ + H2O was provided.

The study demonstrated that the combined use of the UV, IR, and visible spectroscopy is the most effective for studying the intermolecular bonds and hydration properties in solutions and polymers.

Conflict of interests

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

References

1. Tinoco I., Sauer K., Wang J.C., Puglisi J. D., Harbison G., Rovnyak D. Physical Chemistry: Principles and Applications in Biological Sciences. (4th Ed.). Prentice Hall; 2002. 704 p.

2. Pimentel G. C., McClellan A. L. The hydrogen bond. San Francisco: W.H. Freeman; 1960. 475 p.

3. Dehant I., Danz R., Kimmer W., Shmolke R. Infrakrasnaja spektroskopija polimerov. [Infrared spectroscopy of polymers]. Moscow: Himiya Publ.; 1976. 472 p. (in Russ.)

4. Bokker Ju. Spektroskopie. Wurzburg: Vogel Buchverlag; 1997. 526 p.

5. Zundel G. Hydration andintermolecular interaction. Germany by Verlag Chemie; 1969. 324 p.

6. Ugljanskaja V. A., Chikin G. A., Selemenev V. F., Zav'jalova T. A. Infrakrasnaja spektroskopija ionoob-mennyh materialov. [Infrared spectroscopy of ion exchange materials]. Voronezh: VGU Publ.; 1989. 208 p. (in Russ.)

7. Kazicyna L. A., Kupletskaja N. B. Primenenie IK-, UF-, JaMR-spektroskopii v organicheskoj himii [Applications of IR, UV, and NMR spectroscopy in organic chemistry]. Ucheb. posobie dlja vuzov. Moscow: Vyssh. Shkola Publ.; 1971. 248 p. (in Russ.)

8. Selemenev V. F., Kotova D. L., Oros G. Ju., Hohlov V. Ju. Supersaturation processes and methods for obtaining amino acids on ion-exchangers. Sorbcionnye i hromatograficheskie processy = Sorption

and Chromatographic Processes. 2013;13(5): 623-633. Available at: https://journals.vsu.ru/sorpchrom/ article/view/1684/1740 (In Russ., abstract in Eng.)

9. Selemenev V. F., Hohlov V. Ju., Bobreshova O. V., Aristov I. V., Kotova D. L. Fiziko-himicheskie osnovy sorbcionnyh i membrannyh metodov vydelenija i razdelenija aminokislot [Physicochemical fundamentals of sorption and membrane methods for the isolation and separation of amino acids]. Voronezh. VGU Publ.; 2002. 300 p. (in Russ.)

10. Babkov L. M., Puchkovskaja G. A., Makaren-ko S. P., Gavrilko T. A. IK-spektroskopija molekuljarnyh kristallov s vodorodnymi svjazjami [IR spectroscopy of molecular crystals with hydrogen bonds]. Kiev: Naukova Dumka Publ.; 1989. 160 p. (in Russ.)

11. Selemenev V. F., Nazarova A. A., Sinjaeva L. A., Zjablov A. N., Popov V. N. Interaction processes with the participation of higher carboxylic acids. Sorbcionnye i hromatograficheskie process = Sorption and Chromatographic Processes. 2013;13(3): 307-312. Available at: https://journals.vsu.ru/sorpchrom/ article/view/1647/1702 (In Russ., abstract in Eng.)

12. Mur R., Flik Dzh. Vlijanie koncentracii vody na mehanicheskie i reostaticheskie svojstva polimetil-metakrilata. V kn.: Voda v polimerah [The effect of water concentration on the mechanical and rheostatic properties of polymethyl methacrylate. In: Water in polymers. Moscow: Mir Publ.; 1984: 513-527. (in Russ.)

13. Shamritskaja I. P., Matveeva M. V. A microphotographic method for studying the kinetics of the swelling of ion-exchange resins. Teorija i praktika sorbcionnyh processov = Theory and practice of sorption processes. Voronezh: Izd. VGU Publ.; 1971. vol. 5. p. 61-64. (in Russ.)

14. Broejr M., Bjura E., Fukson A. Izmenenie ob'ema pri svyazyvanii vody fibrillami volos. V kn.: Voda v polimerah [A change in volume occurring when water binds to hair fibrils. In: Water in polymers]. Moscow: Mir Publ.; 1984. p. 304-314. (in Russ.)

15. Shtykov S. N. Ljuminescentnyj analiz v organizovannyh sredah. V kn. Problemy analiticheskoj himii [Luminescent analysis in organised media. In: Problems of analytical chemistry]. Moscow: Nauka Publ., 2015. vol. 19. p. 121-155. (in Russ.).

16. Erdey-Gruz T. Grundlagen der Strukrur der Materie. Leipzig: Teubner; 1967. 498 p. DOI: https:// doi.org/10.1007/978-3-663-02531-3

17. Dawson R., Elliot D. C., Elliot W., Jones K. M. Data for biochemical research. London: Oxford Science Pub.; 1986. 543 p.

18. Maurice P. Environmental surfaces and interfaces from the nanoscale to the global scale. Wiley; 2013. 540 p.

19. Bokker J. Chromatographie. Instrumentale analytik: mit chromatographie und kapillarelectrophorese. Wurzburg: Vogel Buchverlag; 1997. 472 p.

20. Dzhatdoeva A. A., Polimova A. M., Proskurnina E. V., Vladimirov Y. A., Proskurnin M. A.

Determination of lipids and their oxidation products by IR spectrometry. Journal of Analytical Chemistry. 2016;71(6): 542-548. DOI: https://doi.org/10.7868/ S0044450216060050

21. Max J.-J., Chapados C. Infrared spectroscopy of aqueous carboxylic acids: Comparison between different acids and their salts. J. Phys. Chem. A. 2004;108: 3324-3337. DOI: https://doi.org/10.1021/jp036401t

Information about the authors

Vladimir F. Selemenev, DSc in Chemistry, Professor, Department of Analytical Chemistry, Voronezh State University, Voronezh, Russian Federation; e-mail: common@chem.vsu.ru. ORCID iD: https://orcid. org/0000-0002-5061-2588.

OlegB. Rudakov, DSc in Chemistry, Professor, Head of the Department of Chemistry and Chemical Technology of Materials, Voronezh State Technica University, Voronezh, Russian Federation; e-mail: rudakov@vgasu.vrn.ru. ORCID iD: https://orcid.org/ 0000-0003-2527-2857.

Natalya V. Mironenko, PhD in Chemistry, lecturer, Department of Analytical Chemistry, Voronezh State University, Voronezh, Russian Federation; e-mail: natashamir@yandex.ru. ORCID iD: https://orcid.org/ 0000-0002-3049-6647.

Sergey I. Karpov, PhD in Chemistry, Associate Professor, Department of Analytical Chemistry, Voronezh State University, Voronezh, Russian Federation; e-mail: karsiv@mail.ru. ORCID iD: https://orcid.org/ 0000-0001-8469-7236.

Victor N. Semenov, DSc in Chemistry, Professor, Head of the Department of General and Inorganic Chemistry, Voronezh State University, Voronezh, Russian Federation; e-mail: semenov@chem.vsu.ru. ORCID iD: https://orcid.org/ 0000-0002-4247-5667.

Natalya. A. Belanova, PhD in Chemistry, lecturer, Department of Analytical Chemistry, Voronezh State University, Voronezh, Russian Federation; e-mail: belanovana@mail.ru. ORCID iD: https://orcid.org/ 0000-0002-3869-7160.

Liliia A. Sinyaeva, PhD in Chemistry, lead engineer at the Department of Analytical Chemistry, Voronezh State University, Russian Federation; e-mail: liliya. sinyaevavsu@mail.ru. ORCID iD: https://orcid.org/ 0000-0002-7378-346X.

Anatoly N. Lukin, PhD in Physics and Mathematics, Associate Professor, Department of Solid State and Nanostructure Physics, Voronezh State University, Voronezh, Russian Federation; e-mail: ckp_49@mail. ru. ORCID iD: https://orcid.org/0000-0001-6521-8009.

All authors have read and approved the final manuscript.

Translated by Yulia Dymant

Edited and proofread by Simon Cox