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Paper Статья
DOI: 10.6060/mhc170518r
Disperse Systems Based on a Dodecyl Derivative of p-Sulfonatocalix[6]arene: Self-Organization and Physicochemical Properties in a Wide Range of Concentrations and Temperatures
Irina S. Ryzhkina,a@ Yulia V. Kiseleva,a Lyaisan I. Murtazina,a Svetlana E. Solovieva,a Nikolay G. Manin,b and Alexander I. Konovalova
aA.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, 420088 Kazan, Russian Federation
bG.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, 153045 Ivanovo, Russian Federation @Corresponding author E-mail: ryzhkina@iopc.ru
Using a complex of physicochemical methods it has been shown for the first time that the diluted solutions (1011 -10-3 М) of dodecyl derivative ofp-sulfonatocalix[6]arene are disperse systems, where the role of a disperse phase is played by the supramolecular structures of different nature - micelles (above 610-4 М), domains (110-4-110-7 М) and nanoassociates (110-9-110-8 М). The disperse phase's parameters change nonmonotonically as systems dilution, what results in the decrease of surface tension at concentrations lower than the critical micelle concentration and determines the differences in systems' behaviour at various temperatures (15-60 °C) and hold up times (after 8 months).
Keywords: Sulfonatocalix[6]arene, dodecyl derivative, aqueous disperse systems, self-organization, physicochemical properties.
Дисперсные системы на основе додецильного производного п-сульфонатокаликс[6]арена: самоорганизация и физико-химические свойства в широком интервале концентраций и температур
И. С. Рыжкина,а@ Ю. В. Киселева,а Л. И. Муртазина,а С. Е. Соловьева,а Н. Г. Манин,ь А. И. Коновалов'1
Федеральное государственное бюджетное учреждение науки Институт органической и физической химии им. А.Е. Арбузова Казанского научного центра Российской академии наук, 420088 Казань, Российская Федерация ъФедеральное государственное бюджетное учреждение науки Институт химии растворов им. Г.А. Крестова Российской академии наук, 153045 Иваново, Российская Федерация @E-mail: ryzhkina@iopc.ru
Впервые комплексом физико-химических методов показано, что разбавленные растворы (1011-103 М) додецильного производного п-сульфонатокаликс[6]арена представляют собой дисперсные системы, дисперсной фазой в которых являются супрамолекулярные структуры различной природы - мицеллы (выше 610 4 М), домены (110-4-110-7 М) и наноассоциаты (1109-1108 М). Параметры дисперсной фазы немонотонно изменяются по мере разбавления систем, что вызывает снижение поверхностного натяжения при концентрациях, ниже критической концентрации мицеллообразования, а также обусловливает отличия в поведении систем при различных температурах (15-60 °С) и времени выдерживания (по истечении 8 месяцев).
Ключевые слова: Сульфонатокаликс[6]арен, додецильное производное, водные дисперсные системы, самоорганизация, физико-химические свойства.
Introduction
A new recently developed experimental approach to studying highly diluted aqueous solutions (10"20-10-6 M) has allowed to explain for the first time the reasons that account for the emergence of nonmonotonic changes of their physicochemical and some of the biological properties.11"31 As a result of a comparative study of self-organization and properties of a diverse series of biologically active sub" stances' solutions, prepared using the method of successive decimal dilution and exposed both to natural conditions (being kept on the laboratory bench) and to the full absence of external low-frequency electromagnetic fields (EMF) (being kept in a permalloy container), it has been shown that the diluted solutions of many biologically active substances (BAS) are in fact the disperse systems, where the role of the disperse phase is played by supramolecular ensembles of different nature - at first these are the small substance aggregates and supramolecular domains, while with dilu" tion below the threshold concentration (Cth) they reorganize into nanoassociates of hundreds of nm in size. It is shown that it is this kind of reorganization that is responsible for the nonmonotonic concentration dependencies of the systems' physicochemical properties, and not the decrease in the ini" tial substance's concentration. In case of the systems with the low initial substance's concentration, the term "concentration" involves that it is a calculated value, used to characterize systems prepared as a result of dilution of an initial system of a known molar concentration.
The supramolecular domains and nanoassociates are formed with the participation of water molecules, but they consist of them in a different ratio, e.g. the nanoassociates they are clearly dominant, being a sensor for the low-frequency EMFs.[4] The size of hundreds of nanometers in the case of the nanoassociates points out that millions of water molecules take part in their formation. A specified structure of the BAS is also required for their formation, apart from the presence of external EMFs and the special preparation procedure.[1]
Utilizing the above-described original approach for studying the disperse systems allowed for an experimentally proved differentiation of the disperse phase's nature, which often has a similar size of hundreds of nanometers, and it also can be useful for studying the supramolecular systems containing BAS of complex structure, e.g. the systems based on an intensively studied in the last decades calixarenes,[5"7] for which a complicated nonmonotonic change of the physi-cochemical properties is often observed with the decrease in concentration. Studies[8-10] managed to explain the causes of nonmonotonic changes in the concentration dependencies of the specific electrical conductivity, surface tension, and bioeffects in water-organic systems based on amphiphilic calix[4]resorcinols (10-4-10-7 M) and amphiphilic acetylhy-drazide derivative of 4-tert-butyltetrathiacalix[4]arene in the cone and 1,3-alternate conformations (10-4-10-8 M) from the perspective of formation and rearrangement of supramolecular domains and nanoassociates.
Calixarenes are synthetically available complexly organized tridimensional macrocyclic phenols that model the receptor, transport and catalytic functions of complex biomolecules.[1112] Calixarenes containing hydrophilic and
hydrophobic groups are the amphiphilic molecules, capable of self-organizing and forming supramolecular ensembles of different kinds.[51112] In the last decades much attention is given to exploring synthesis methods, the properties and possible use cases of />-sulfonatocalix[n]arenes (SCA) and their derivatives.[51113-19] One of the most promising usage scenarios lies in the biomedical sphere.[20,21] This is aided by the SCA's high solubility in water, low toxicity[20] and availability for functionalization, which allows for controlling the physicochemical properties by changing the nature of the substituting groups and the number of aromatic rings. For example, appending the alkyl radicals to the SCA molecule results in the change of its hydrophilic-lipophilic balance and the emergence of the surfactant properties.[13-15] The increased number of aromatic rings translates into the conformational flexibility of the macrocycle,[511] leading to the emergence of an adaptive capability of substrate fixation (flexible "adjustment"). However, despite the large number of publications dedicated to />-sulfonatocalix[n]arenes, the processes of self-organization and the physicochemical properties of the alkylated SCA's solutions in a wide range of concentrations (including the low ones), have not been yet systematically studied.
The aim of this work was to study the self-organization and properties of a water-soluble dodecyl derivative of the />-sulfonatocalix[6]arene (1) that has a vast scope of applications[5-7] using a complex of physicochemical methods (dynamic light scattering - DLS, nanoparticle tracking analysis - NTA, tensiometry, differential scanning calorimetry - DSC), which can help to find the concentration ranges of different disperse phases' formation, define their behavior patterns and explain the properties of 1 solutions in a wide range of concentrations (110-11-110-3 M), temperatures (10-90 °C) and storage times.
Experimental
The synthesis of the calixarene 1 was carried out following a known procedure.[6] The data of IR and 1H NMR spectra of 1 corresponded to the previously published ones.[6] The preparation and study of systems based on 1 was carried out using a fresh bidistilled water, specific conductivity of which was no more than 1.5 ^Sm/cm. The systems based on 1 were prepared by the method of successive decimal dilutions from an initial substrate's solution of concentration of 110-2 M analogous to.[1] The particles' size (the effective hydrodynamic diameter of kinetically labile particles at the maximum of the distribution curve, D) was determined by the method of dynamic light scattering (DLS) on a highsensitive analyzer Zetasizer Nano ZS ("Malvern Instruments", Great Britain). With the purpose of particles' visualization as well as determining their size and quantity in 1 mL of the solution, the systems based on 1 have been elucidated using the nanoparticles tracking analysis method (NTA) on the NanoSight LM 10 analyzer ("NanoSight Instruments", Great Britain) in the HS-BF configuration analogous to.[22] The surface tension (a) in the water-air system was studied using the Du Nouy anchor-ring method. The measurement of a by the Du Nouy method was carried out on a high-precision Sigma 702 ET tensiometer (KSV Instruments), equipped with a thermostatically-controlled jacket, whose resolving power corresponds to 0.001 mN/m, using a platinum-iridium ring with a radius of 9.545 mm and a wire radius of 0.185 mm.
Disperse Systems Based on />-Suffonatocalix[6]arene
1: n=6, R=Dod
The study of the EMF influence on the self-organization and properties of aqueous solutions was carried out in the following way - the 1-based system of each examined concentration in the course of 24 hours was simultaneously exposed to: 1) natural conditions on the laboratory bench, 2) screening conditions of a three-layer permalloy container which protects its contents against the influence of external EMFs.
The heat capacities of the diluted aqueous systems based on 1 with concentrations of 110-3, 110-4, 110-5, 110-6, 110-7, 110-8 and 110-9 M were elucidated on an adiabatic differential scanning microcalorimeter Skal1 in the temperature range of 10-90 °C, the scanning speed being equal to 1 K/min and the step of 0.5 °C in three repeated measurements. Additionally, after eight months from the preparation of 1 solutions the measurements of their heat capacities were repeated under the same conditions. Experimental data were processed using the standard programs of Excel software package, particle size measurement errors - <15 %, errors in the measurement of physicochemical properties - <3 %.
Results and Discussion
The study of the systems based on 1 using the DLS method has shown that at the concentration of 110-3 M the particle size distribution is bimodal, the sizes being approximately 35 and 270 nm. As stated before,[6] the former are micelles, and the critical micelle concentration (CMC) is 6 10-4 M. In the concentration range from 110-9 to 110-4 M the distribution has a monomodal character, there are particles of a nonmonotonically changing size from
Figure 1. Dependence of the size of particles (D) (1) formed in aqueous solutions of 1, and the surface tension (a) (2) of systems exposed to natural conditions, on the concentration, 25 °C. The inset shows the dependence of the particle size (1) and the surface tension (2) of systems held in hypoelectromagnetic
conditions on the concentration of 1, 25 concentration.
C. C. is the threshold
140 to 380 nm forming in the system (Figure 1, curve 1). The limit of particle detection in the 1-based systems by the DLS method is reached at 110-9 М. The maximum particle size value is at concentrations of 110-4 and 110-8 М, the minimum - at 110-6 М.
The studies of systems based on 1, exposed both to natural and hypoelectromagnetic conditions, have shown that the threshold concentration is 110-7 М (Figure 1, inset, curve 1). This means that particles of a size of hundreds of nm, formed in the interval 110-7-110-3 M, are domains, and in the interval 110-9-110-8 M - nanoassociates. Thus, the 1 solutions are disperse systems, the role of the disperse phase in which is played by supramolecular structures of different nature - micelles (above CMC 6 10-4 M), domains (above 110-7 M), and nanoassociates (110-9-110-8 M).
The formation of particles with a size of hundreds of nm in the disperse systems based on diluted solutions of 1 was confirmed by the NTA method, which makes it possible to determine the size and number of particles in 1 ml of the solution, and also to visualize them.[2223] The NTA method can be used for particles in the range from 10 nm to 1 ^m, with the number of particles from 107 to 109 in a milliliter of solution. For the study of systems based on 1 using the NTA method, concentrations of 110-5, 110-6, 110-8, 1T0-9 M were chosen. It was found that particles in the range of tens and hundreds of nm are visualized in all the solutions studied (Figure 2). The number of particles in a milliliter of solution at 110-5 M (Figure 2a) exceeds 1010, what makes it impossible to accurately determine their size by the NTA method. At 110-6, 110-8, 110-9 M, the number of particles in 1 ml of the solution in order of decreasing concentration of 1 is 4T08, 8T08, 2T08, their average diameter being 80, 170 and 150 nm (Figure 2b-d). The number of particles in bidistilled water is less than 1T06 in 1 ml, which lies outside the sensitivity range of the device. Thus, it was discovered that nanoassociates of the largest size and in the largest amount are formed in dilute solutions of 1 at a concentration of about 1T0-8 M.
An important fact is that when the calculated concentration of 1 decreases by an order of magnitude from 110-5 to 1T0"6 M, within which domains exist, the number of particles in ml decreases by almost two orders of magnitude. A further decrease in the concentration of 1 by two orders of magnitude from 1T0"6 to 1T0-8 M and again by an order of magnitude to 1T0-9 M, within which the nanoassociates are formed, is accompanied by a nonmonotonic change in the number of particles by a factor of 2-4. On the one hand, the obtained result confirms that the self-organization of highly dilute solutions is a complex nonlinear process that is not directly related to the concentration of the solute, and on the other hand it indicates that the formation of an unusual dispersed phase - nanoassociates of tens and hundreds of nm size - is primarily due to molecules of water with minimal participation of molecules of the substance.
A study of surface tension in the concentration range 110-15-110-3 M showed (Figure 1, curve 2) that the surface tension isotherm has a stepped nature, containing two plateaus in the CMC region (6 10-4 M) and in the region of domains' formation (110-4-410-4 M) (Figure 1, curve 2). In addition, in the interval 5 10-8-3 10-9 M with an extremum at 6 10-9 M, i.е. approximately in the region of formation
Figure 2. Images of aqueous solutions of 1 obtained by the NTA method at concentrations of 1 equal to M0-5 M (a), M0-6 M (b), M0-8 M (c), M0-9 M (d), 25 ° C.
of nanoassociates with the maximum size and number, a sharp decrease in the c of the solution to 57 mN/m is also observed.
In systems maintained under hypoelectromagnetic conditions, the region of local reduction of c at a concentration of 6 10-9 M and a plateau in the region of 110-4-4 10-4 M disappears on the surface tension isotherm (Figure 1, inset, curve 2), i.e. the surface tension isotherm acquires a classical form with one inflection in the CMC region, which is inherent in the solution of surfactants in which micelles are formed.[24]
The results obtained jointly by the methods of DLS, NTA and tensiometry about the size, number of domains and nanoassociates and surface tension in the interval 110"9—110"3 M make it possible to make an assumption about the causes of the appearance of an unusual type of surface tension isotherm in the low concentration region. Reduction of c in the intervals of domains and nanoassociates formation, i.e. at 110-4-410-4 M and 6 10-9 M, is associated with a significant rearrangement that the disperse system undergoes with dilution. In the region of these concentrations, the domain size increases sharply, and in the case of nanoassociates not only the size, but also the number, which probably leads to the expulsion of amphiphilic molecules of 1 from the dispersion medium to the water-air interface and to a decrease in the surface tension of the system (Figure 1, curves 1 and 2). Thus, as the dilution proceeds, a structural rearrangement of the dispersed system as a whole takes place, i.e. both of the dispersed phase and the dispersion medium, which is most pronounced in the concentration ranges of about 110-4-110-5 M and 110-7-110-9 M.
In order to obtain additional information on the structural rearrangement of the system 1 and the properties of the dispersed phase in various ranges of concentrations and temperatures, the heat capacity of aqueous systems
based on 1 in the concentration range 110-3-110-9 M in the range of 10-90 °C was studied by differential scanning microcalorimetry, as well as of 1 systems, subjected to long-term storage (for 8 months). From the experimentally observed specific heat capacity of the solution (cp), the excess partial specific heat of 1 in an aqueous solution ( cP,2) was found and analyzed, calculated analogous to [2526], which is the differential characteristic of the dissolved substance that most clearly reflects the structural rearrangements in the solution with respect to the dissolved substance, associated
Figure 3. Dependences of the excess partial specific heat Cp^ of aqueous solutions of 1 on temperature at concentrations equal to MO-4 M (1), 110-5 M (2), 10-7 M (3), 10-8 M (4). For solutions
with a concentration of MO-4 M and 110-5 M, the dimension of
p
Cp,2 is in J/g-K.
Disperse Systems Based on />-Sulfonatocalix[6]arene
with the formation and destruction of the associates of "dissolved substance - solute" and "dissolved substance -solvent" kinds.
The dependence of cP,2 on the temperature at various concentrations of 1 is shown in Figure 3. As can be seen from the data in this figure, the value of cP,2 varies differently with different concentrations with the temperature growth, being both positive (curve 1, 2) and negative (curve 3, 4), which may be due to the structural rearrangements in the system in different concentration intervals.
The curve of the temperature dependence of cP,2 (Figure 3) reflects structural transitions in the system, excluding information on the temperature dependences of the heat capacity of the initial and final (after structural changes) states. The temperature dependence of the heat capacity before and after the peak is practically linear, and the change in the heat capacity between the initial and final transition states is close to zero. As can be seen from the data in Figure 3, with a decrease in the concentration of 1 from 10-4 M to 10-8 M, a change in cP2 as a function of temperature is observed in different sign and magnitude, probably related to the different nature of the domains and nanoassociates formed in the region of these concentrations.
At concentrations of 110-4 M and 110-5 M, the cp 2 value is negative in the range from 15 °C to 80 °C, with an absolute value of cP,2 at a lower solution concentration of 110-5 M being substantially higher (cp 2 <-151 J/gK) than at 110-4 M (cp, 2 <-4.4 J/g-K) (Figure 3, curves 1, 2). The observed decrease in the heat capacity of 1 with increasing temperature, most pronounced for 110-5 M, can be due to the thermally induced domain rearrangement, which leads to the formation of more ordered structures.
As follows from the data in Figure 1, at 25 °C with a decrease in the concentration of 1 by an order of magnitude from 110-4 M to 110-5 M, a sharp decrease in the domain size from 380 to 140 nm occurs as well as the change in the properties of the system, whose surface tension at 110-5 M increases to a value of 72 mN/m, which corresponds to the c of water. The different state of the dispersed system at 110-4 M to 1 • 10-5 M probably causes a different kind of curves cP, 2 =fT) at these concentrations of 1. The domains formed at 110-5 M are more sensitive to an increase in temperature, under the influence of which considerable changes are undergone, which may be due to a decrease in the number of molecules of 1 and an increase in the fraction of water structures in the domain, i.e. changing its packing density and size.[22] In this case, a temperature of ~50 °C, corresponding to the minimum value of cP,2 leads to the greatest structural organization of domains in solution.
A further decrease in the concentration of 1 leads, as the temperature increases, to an increase in the positive value of cP,2 (Figure 3, curves 3, 4), reaching the maximum at 110-8 M (at 110-9 M cEp;2 ~0). As can be seen from the data in Figure 1, at 25 °C the system undergoes significant rearrangements with extrema in the size of nanoassociates and surface tension also at the concentration of 110-8 M. Thus, the observed increase in cP2 with increasing temperature, most pronounced for 110-8 M, can be due to the rearrangement of nanoassociates, accompanied by opposing effects, compared with the domain formation interval.
The change in the sign of the cP,2 curve with increasing temperature upon transition from the systems in which the domains are formed to the systems in which nanoassociates are formed is probably associated not only with the different nature of nanoassociates and domains, but also with various processes taking place in the system as a whole, which can exert significant influence on the behavior of systems when interacting with biomembranes. For example, the study of dilute solutions of potassium phenozane antioxidant (PP) by the DSC method showed that the PP solutions used in the range of formation of domains and nanoassociates[2,3] change the thermostability of the protein fraction of plasma membranes in the opposite direction - in the domain formation range, it increases, and in the nanoassociation formation region it is reduced.[27]
Figure 4. Dependence of cp,2 of aqueous solutions on temperature eight months later after preparation of solutions at concentrations equal to 110-4 M (1), 110-5 M (2), MO-6 M (3), MO"7 M (4), 110-8 M (5).
Both domains and nanoassociates, starting at t>55 °C, are destroyed, the maximum deviations from additivity are observed in both cases in the 50-60 °C interval. Therefore,
E
the peak on the cP,2 =f(T) curve for aqueous systems based on 1 in the range from 110-5 M to 110-8 M at 50-55 °C is a criterion for the presence of domains and nanoassociates in the solution formed by the dissolved substance and water structures, as well as the criterion of their temperature stability. All changes in cP,2 appear up to 60 °C, and at a higher temperature they start to level or disappear, what indicates the destruction of nanoassociates and domains with increasing temperature. Probably, nanoassociates are stronger ( cP,2 ~120 kJ/gK) than domains ( cBP2 ~150 J/g-K).
At the end of eight months after their preparation, the systems based on 1 retained a positive deviation of the
excess partial specific heat of the solute with increasing temperature in the concentration range 110-6-110-8 M (Figure 4, curves 3-5). In contrast to the freshly prepared systems (Figure 3), positive deviations in the concentration range 110-4-1T0-5 M were also observed with a temperature increase of cp,2 <0.75 J/gK for 110-4 and cp2 <3.8 J/g K for 110-5 M. The increase in the positive value of cP,2 in the concentration range 110-6-110-8 M indicates the stability of nanoassociates in time, and the absence of cP,2 <0 in the concentration range from 110-4 M to 110-5 M - on lower temporal stability of the formed domains.
Conclusions
Thus, as a result of a research which studied the self-organization and properties of a dodecyl derivative of />-sulfonatocalix[6]arene (1) in a wide range of calculated concentrations 110-11-1T0-3 M utilizing a complex of physicochemical methods it has been shown for the first time that such solutions are disperse systems, where the role of a disperse phase is played by the supramolecular structures of different nature - micelles (above 610-4 M), domains (110-4-110-7 M) and nanoassociates (110-9-110-8 M). The parameters of the dispersed phase vary nonmonotonically with dilution, what causes differences in the behavior of the systems at different temperatures (15-60 °C) and aging time (after 8 months).
Acknowledgements. The authors thank E.G. Evtushenko (Lomonosov Moscow State University, Chemistry Department) for help in conducting measurements by the NTA method.
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Received 15.05.2017 Accepted 26.05.2017
