A dipole moment and conformations of poly -N-vinylpyrrolidone and of its complex with c60 fullerene in aqueous solutions Текст научной статьи по специальности «Химические науки»

UDC 537.226:544.163.2

T.P. Stepanova ', T.D. Anan'eva ', E.D. Karpenko 2, V.M. Kapralova 2

1 Institute of Macromolecular Compounds RAS 31 Bolshoy Ave. V.O., St. Petersburg, 199004, Russia

2 St. Petersburg State Polytechnical University, 29 Politekhnicheskaya St., St. Petersburg, 195251, Russia

A DIPOLE MOMENT AND CONFORMATIONS OF POLY-N-VINYLPYRROLIDONE AND OF ITS COMPLEx WITH C60 FULLERENE IN AQUEOUS SOLUTIONS

Т.П. Степанова, Т.Д. Ананьева, Е.Д. Карпенко, В.М. Капралова

дипольный момент и информационные свойства

ПОЛИВИНИЛПИРРОЛИДОНА И ЕГО КОМПЛЕКСА С ФУЛЛЕРЕНОМ C60 В РАСТВОРЕ В ВОДЕ

The study of temperature dependences of dipole moments and conformational properties of poly-N-vinylpyrrolidone (PVP) and polymer complex of PVP with C60 fullerene (PVP + FC60) was carried out in dilute aqueous solutions at 293—313 K. It was shown that dipole moment values for PVP and PVP + FC60 were 24—32 D and 18 D, correspondingly. The presence of the molecular group —N—C=0 in each monomer unit near the macromolecule backbone promotes the formation of helical blocks in macromolecular coils. In external electric field, the non-alternating projections on the direction of this field and on the vectors connecting the neighboring segments of the macromolecule appear. It was demonstrated that the changes in the characteristics of the dielectric polarization could be explained by structuring in the coils of PVP and PVP + FC60 in aqueous solutions.

DIELECTRIC PERMITTIVITY, DIPOLE MOMENT, HELICAL BLOCKS IN COILS, ASSOCIATION, SOLUTION.

Проведено исследование дипольных моментов поли-N-винилпирролидона (ПВП) и на его основе полимерного комплекса с фуллереном С60 (ПВП + С60). Показано, что значения дипольного момента ПВП и ПВП + С60 в водных растворах в условиях бесконечного разбавления велики и составляют 24 — 32 Д и 18 Д (Т = 293 — 313 K) соответственно. Анализ экспериментальных результатов указывает на специфические конформационные свойства макромолекул исследованных полимеров. Наличие молекулярной группы —N — C = O в каждом мономерном звене у хребта макромолекулы обусловливает возникновение спирализованных блоков внутри макромолекулы. В этом случае во внешнем электрическом поле возникает неальтернирующая составляющая дипольного момента на направление спирали. суммарный эффект векторного сложения этих дипольных моментов и взаимоориентация спирализованных блоков в статистическом клубке определяют высокие значения дипольных моментов ПВП и ПВП + С60 в водных растворах.

ДИЭЛЕКТРИЧЕСКАЯ ПРОНИЦАЕМОСТЬ, ДИПОЛЬНЫЙ МОМЕНТ, СПИРАЛИЗОВАН-НЫЕ БЛОКИ, АССОЦИАЦИЯ, РАСТВОР.

Polyvinylpyrrolidone (PVP) is known to be one of amphiphilic polymers which are inclined to self-organization into various aggregates in

both polar and non-polar environment [1]. Stable aggregate formation in the solutions of amphiphilic polymers is due to hydrophobic

(-CH2— CH-)

N. ^O

o

PVP structure

and ionic interactions resulting in H-bonds, charge transfer complexes, coordination complexes, etc. PVP ability to form complexes as well as its other physical, chemical and biological properties make this polymer suitable for technological and biomedical applications. PVP structural formula is given in scheme. The PVP solubility in water is mostly due to the lactam cycle.

Recently, the synthesis of PVP with biological or technogenic inclusions of macromolecular scale has been described, and the main attention was paid to PVP and fullerene C60 (FC60) complexes using in medical applications. The use of fullerenes in medicine faces significant difficulties because of their almost total water insolubility, though it could be overcome by using fullerene non-covalent complexes with water soluble polymers like PVP. In this case, the electronic structure and therefore the properties of fullerene molecular clusters are disturbed minimally in contrast to covalent bonding. It is obvious that physical and chemical properties of non-covalent complexes are determined by the proportion of fullerenes and polymer in the complex [2]. It is well-known that fullerene molecule has a form of truncated icosahedron with the surface consisting of hexagons and pentagons connected by single and double bonds. The molecule FC60 diameter is shown to be approximately 10.2 A [3].

The purpose of the present paper is to investigate and compare the dipole moments and conformational properties of PVP and PVP + FC60 complexes in aqueous solutions under the conditions of infinite dilution.

To produce the PVP + FC60 complex, PVP with molecular mass of 24000 produced by «Fluka» and C60 fullerene from the company «Fullerene technologies» (St.-Petersburg) with fullerene content of 99.5 % were used [4 — 7]. PVP used for this work is the white-

yellow powder with softening temperature as high as 160 °C, density d20 = 1.9 g/cm3 [1] and refraction index nD20 = 1.58 (for a film).

The solutions of PVP in chloroform (50 mg/ml) and of fullerene in o-dichloro-benzene (0.8 mg/ml) were mixed with the volume proportion 2 : 1, then the solvents were evaporated in vacuum at 35 — 40 °C, and the solid residual was dried. To remove free fullerenes, the solid residual was stirred into water and filtered through the filter paper. This procedure was followed by water vacuum evaporation at the same temperature.

Fullerene concentration in the complex was estimated by complex destruction in 2 mg/ml solution in the mixture of ethanol and toluene 1 : 9. Fullerene concentration in the mixture of solvents was measured by UV spectroscopy method [7] and brought into correlation with PVP concentration.

The capacity of solution was measured in the cell with platinum electrodes [9] using measuring the bridge E7-12 at the frequency of 1 MHz with the error of 0.001 pf. The capacity of the used cell was 4.53 pf.

The solutions of PVP and PVP + FC60 in distilled water for further measurements were prepared using gravimetrical method.

Dipole moments of PVP molecules and PVP + FC60 complexes in aqueous solutions were estimated according to Buckingham theory of statistical polarization [8] for two-component systems:

(812 -№12 +1) V (n2 -1)(2812 +1) Vr

v12 A ,2\ V x1

3s,,

(2s12 + n2 )

(n -1)(2s12 +1)

(2si2 + n2)

(4nN

,2\ 2 2

Vx =

(I)

a 2 2

Mlefxi + M2efX2-

3kT

Here e is dielectric permittivity; n — refraction index; V — molar volume, V = Mv

1 1 s

(vs — specific volume, M — molar mass); x — mole per cent, T — absolute temperature, NA — Avogadro number, k — Boltzmann constant, — effective dipole moment (indexes 1, 2 h 12 are for solvent, dissolved substance and solution, accordingly).

According to the statistical theory of dielectric polarization,

m 2 = vg, (2)

where g is a correlation parameter for dipole moments orientation characterizing short-range interactions; v — dipole moment of a polar molecule related to its dipole moment in vacuum v0 by

h =

n2 + 2 2s + 1

3 2s + n'

•hr

(3)

Dipole moments v and v2e/ were calculated graphically by extrapolation of the concentration dependence of summary orientation polarization S12 to infinite dilution:

Si2 =

h1efX1 + M-2e/X2'

(4)

S12 = + - )x2 = a + bx• (5)

Dipole moments (v/X =o and (^/L =o were calculated using the parameters a and2 b by the equations

« = WL.=0 = (/

dSn 2 I 2 .

^</L = 0 - M'1 /

b =

dx

(6) (7a)

(Hi/L2=o+b-

(7b)

Taking into account that at infinite dilution e12|x =0 = e1,dipole moments of the components 1 an2 d 2 were defined by

hi = (ho, 1 • gi)1/2 =

h = (h0,

=

3( f=0) (2S1 + n2) (n + 2)(2s1 +1)

3( /=0 )1/2(2s1 + n2)

(n22 +2)(2s1 + 1) "

(8)

(9)

Using the results of the measurement of permittivity and density of the water used as a solvent, the dipole moment of liquid water molecule ^ was calculated by Onsager formula [8]:

(s - n2)(2s + n2) 4nNA

.(10)

s(n2 + 2)2 3 M 3kT

Here e, n, p, M are dielectric permittivity, refraction index, density and molecular mass of water, respectively.

Refraction indexes of the polymer and solvent were determined at fixed temperatures

from the molar refraction RD by

=4+1 m . (11)

n + 2 p

Molar refraction was taken as a sum of atomic refractions and bond increments [12].

PVP monomeric unit molecular mass and molecular refraction were estimated to be MPVP = 111 and Rdpvp = 27.445. Molecular mass and molecular refraction of the hypothetical monomeric unit of PVP + FC60 complex were calculated as average values for a macromolecule containing PVP and FC60 in the proportion of 0.8 mole % FC60 to 100g PVP. Also, using values MFC60 = 720, RDFC60 = 230.4 [3] for molecular

cluster FC60, values for complex МP

DPVP+FC60

PVP+FC60

29.773 were

0 2

10 .12

x, 10* . mol mol

b) z 82

80

78

76

74

■ 1

♦ 2

A 3

▼ 4

♦ J

0 2 4 6 8

x. 10", mol/mol

Fig. 1. Dielectric permittivity concentration dependence of PVP (a) and PVP+FC60 (b) aqueous solutions at T °C: 20(i); 25(2); 30 (3); 35(4); 40(5)

L> = 0

Table 1

Temperature dependences of water, PVP and PVP + FC60 characteristics

T °c v1, cm3/g «12 v2, cm3/g n 2 2 PVP n 2 2 PVP+FС60

20 1.0018 1.7778 0.8333 2.26579 2.40719

25 1.0030 1.7765 0.8343 2.26354 2.40411

30 1.0044 1.7750 0.8353 2.26139 2.40224

35 1.0060 1.7731 0.8363 2.26925 2.39978

40 1.0078 1.7712 0.8373 2.25711 2.39733

Dipole moment estimation total error was A^ = ±0.05 D.

Dielectric permittivity concentration dependences for PVP and PVP + FC60 aqueous solutions in the temperature range of 20 — 40 °C are given in Fig. 1. Concentrations used are small enough (x2 < 1.16-10-3 mole\mole and х2 < 0.8-10-3 mole\mole respectively) to make it possible to extrapolate e12 to infinite dilution situation and to apply dielectric polarization theories for dipole moment calculations.

Molar orientation polarization S12 values were calculated by (1) using permittivity (see Fig. 1), specific volume and refraction indexes (Table 1). As an example, Fig. 2 shows concentration dependences of S12 for aqueous

solutions of PVP and PVP+FC,. at 25 °C. Values

60

SU,D 16.8

16.4

16.0

15.6

15.2

0 2 4 6 8 10 12 14

X, 10\ mol/mol

Fig. 2. Molar orientation polarization S12 concentration dependence of PVP (1) and PVP + FC60 (2) aqueous solutions at 25 °C

Table 2

Molar orientation polarization S temperature dependences for PVP and PVP + FC aqueous solutions

T °c PVP pvp+FC60

x2 = 2.27-10-4 x2 = 8.1410-4 x2 = 0.00116 x2 = 1.9410-4 x2 = 3.87-10-4 x2 = 7.76-10-4

20 25 30 35 40 15.592 15.540 15.483 15.416 15.339 16.344 16.394 16.439 16.481 16.516 16.716 16.880 17.044 17.207 17.222 15.326 15.272 15.261 15.242 15.220 15.464 15.461 15.453 15.439 15.417 15.752 15.754 15.749 15.739 15.723

Table 3

Dipole moment and Kirkwood factor of water in solutions of PVP and PVP + FC60

T °c

Onsager factor (3)

PVP solution

(^12)lx2=0 (6)

Vlef (6)

PVP + Fc60 solution

h Kirkwood (^2)1*2=0 ^1f h Kirkwood

(8) factor (2) (6) (6) (8) factor (2)

3.12 2.88 15.182 3.89 3.10 2.85

3.11 2.86 15.126 3.89 3.10 2.84

3.10 2.84 15.113 3.89 3.10 2.84

3.09 2.82 15.093 3.89 3.10 2.84

3.08 2.81 15.067 3.88 3.10 2.83

20 25 30 35 40

1.254 1.253 1.253 1.252 1.251

15.327 15.216 15.097 14.967 14.879

3.91 3.90 3.89 3.87 3.86

Table 4

Dipole moment and Onsager factor of PVP and PVP + FC60 in the water

PVP PVP+FC60

T °c b = [Ö(J12 ) /0X2)11x2=0 (7a) fb) Onsager factor (3) ^2 (9) b = [Ö(S12 ) /5X2)11x2=0 (7a) fb) Onsager factor (3) (9)

20 25 30 35 40 1213.2 1438.6 1668.9 1909.0 2017.4 35.05 38.13 41.04 43.86 45.08 1.412 1.411 1.410 1.410 1.410 24.8 27.0 29.1 31.1 32.0 733.2 817.3 827.3 842.0 853.0 27.37 28.85 29.02 29.28 29.46 1.456 1.456 1.456 1.456 1.456 18.79 19.82 19.93 20.11 20.24

of S12 at x2 = 0, increments of S12 concentration dependences and dipole moments per monomer unit for PVP and PVP+FC60 calculated by (2)—(9) are presented in Tables 2 — 4.

Dipole moments and Kirkwood factor temperature dependences of PVP and PVP + FC60 as well as of water and N-methylpyrrolidone (NMP) which is low molecular weight analog of PVP monomeric unit are compared in Fig. 3, 4.

It is significant that the values of dipole moments and Kirkwood factor for liquid water calculated according to Buckingham theory are in good agreement with earlier published values. Thus, calculated dipole moments are to be considered correct and adequate.

Dipole moment and Kirkwood factor temperature dependences were discussed in the previous paper [10]. Dipole moments of NMP in the water (4.12 D) and non-

polar solvents dioxane (4.06 D) and benzene (4.09 D) at 20 °C are close to each other within the experimental error confirming the fact that the NMP molecules do not disturb the local structure of water. The life time of hydrated shell seems to be too short to lead to significant polarization effects.

Dipole moments of macromolecules of PVP and PVP + FC60 are rather large being 25 and 18 D, respectively. It can be seen from Fig. 3 that the dipole moments of PVP increase noticeably with temperature, then reach a plateau of approximately 30 D at 40 °C whereas the dipole moments of PVP + FC60 complex stay nearly constant. We suppose that the groups —N—C=0— located in each monomeric unit near the macromolecule main chain, mostly in trans-conformation, determine the formation of helical blocks stabilized by donor-acceptor interactions inside the macromolecular coils.

Fig. 3. Dipole moment temperature dependence of water (1, 2) (see also Table 3); N-methylpyrrolidone aqueous solution (3) [11]; PVP + FC60 (4) and PVP (5) (Table 4).

8 50

40

30

20

3

2

1

0

g = (M.o )2 / (Mm™ )

g = (M water in the continuum )2/(Mo)2

g = (Mn -

N-MP in water ' / V r^O

)2/(Mo)2

□ i

a 2

a 3

V 4

O 5

20 25 30 35 40 T,°C

Fig. 4. Kirkwood factor g = / temperature dependence for water in solutions of PVP and PVP + FC60 at infinite dilution (1, 2); NMP (3) [11], PVP+FC60 (4) and PVP (5) in the water

Fig. 5. Formation of non-alternating projections of dipole moment ^ on helix axis vector h and electrical field vector E for the chain segment with -+N—C-0—group trans-conformation. Thin arrows depict constant dipole moment of —N—C=0— groups near the main chain, bold arrows — induced dipole moments of these groups in donor-acceptor form directions (1, 2, 6); thin lines — movement fluctuation direction of dipole moments of —N—C=0— groups which do not create induced dipole moments (3, 4, 5)

In this case, non-alternating projection of the —N+—C=0- group dipole moment on the helix axis appears in the external electrical field as

depicted in Fig. 5 similar to the matter discussed in [15]. As the temperature rises, hydrophobic interactions tend to become stronger due to H-bonds of hydrophilic segments, and water molecules life time decreases. Further structuring inside the coil takes place because of the helical blocks orientation ordering. Large values of PVP and PVP+FC60 dipole moments in aqueous solutions are determined by induced dipole moments vectorial addition and helical blocks association.

Less values of PVP + FC60 complex dipole moment are caused by fullerene molecules implantation into macromolecular coils, which makes helical blocks formation more difficult. In the macromolecular coil of PVP + FC60, complex helical blocks seem to be shorter than in PVP macromolecules. Distinctive for fullerene, donor-acceptor interactions [16] with PVP monomeric units (so called lactam traps) lead to monomeric unit dipole moments compensation, helical blocks shortening and, thus, macromolecule dipole moment decrease. coordination number of polar groups taking part in correlating orientation can be evaluated by Kirkwood factor (see Fig. 4) as approximately 3 for liquid water, approximately 1 for liquid NMP or NMP aqueous solution, 36 for PVP and 24 for PVP + FC60 infinitely diluted aqueous solutions.

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СТЕПАНОВА Тамара Павловна — кандидат физико-математических наук, старший научный сотрудник Института высокомолекулярных соединений РАН. 199004, Россия, г. Санкт-Петербург, Большой пр. В.О., 31 [email protected]

АНАНЬЕВА Татьяна Дмитриевна — кандидат химических наук, старший научный сотрудник Института высокомолекулярных соединений РАН.

199004, Россия, г. Санкт-Петербург, Большой пр. В.О., 31 [email protected]

КАРПЕНКО Елена Драгановна — студентка Санкт-Петербургского государственного политехнического университета.

195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. [email protected]

КАПРАЛОВА Виктория Маратовна — кандидат физико-математических наук, доцент кафедры интегральной электроники Санкт-Петербургского государственного политехнического университета. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. [email protected]

© St. Petersburg State Polytechnical University, 2013