Density, Surface Tension, and Cloud Point of Aqueous Solutions of β

Article pubs.acs.org/jced

Density, Surface Tension, and Cloud Point of Aqueous Solutions of β‑Cyclodextrin-polyethylene Glycol Yuan Liu, Changjun Zou,* Taiyang Wang, and Ming Li College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, P. R. China ABSTRACT: The physical property data (density, surface tension, and cloud point) for aqueous solutions of β-cyclodextrin-polyethylene glycol have been determined at temperatures from (293.15 to 313.15) K and atmospheric pressure. Results show that the density and surface tension decrease as the temperature increases. The density and cloud point increase with the mass fraction of β-CD-PEG, whereas the surface tension decreases. Finally, the density and surface tension experimental data have been correlated with different equations. Specifically, the density increases linearly with the increase of β-CD-PEG mass fraction from 2 % to 10 %; however, the surface tension of β-CD-PEG decreases quickly in the β-CD-PEG mass range of 0.001 % to 0.060 %. When the β-CD-PEG mass fraction is 0.080 % the surface tension of β-CD-PEG shows the minimum value, 56.1 mN·m−1.

1. INTRODUCTION

2. EXPERIMENTAL SECTION 2.1. Preparation of β-CD-PEG. β-CD, PEG400, thionyl chloride, and absolute ethyl alcohol, all described as 99.0+ % pure (analytical grade), were purchased from Kelong Chemical Reagent Factory (Chengdu, China). NaOH and HCl (analytical grade), produced by the same factory, were used as pH modifiers. Table 1 provides information about the source and purity of the chemicals used.

β-Cyclodextrins (β-CDs) are cyclic oligosaccharides consisting of seven glucose units linked by α-1,4 glucosidic bonds with an internal hydrophobic cavity and hydrophilic external surfaces. The special structure gives CDs the ability to complex a wide range of lipophilic guest molecules, which is driven by steric effect, hydrophobicity, and van der Waals forces, dispersive forces, dipole−dipole interactions, electrostatic forces, and hydrogen bonding.1 On the basis of research, CDs and their derivatives have been extensive applied in the fields of agriculture, medicine and health, food and cosmetic industry, and so forth.2,3 Aimed at obtaining better solubility in water with the unique molecular recognition ability, cyclodextrins have been physically combined or chemically conjugated with various hydrophilic polymers.4,5 PEGs have been found to be stable to acid, base, high temperature, high oxidation systems, and reduction systems.6−9 The reasons for the employment of PEG as modifier in our study lie in its low flammability and biodegradability10 and above all a wide application of the surfactant−polymer mixtures11 and cloud point extraction.12 Therefore, the combination of β-CD with PEG has a wider range of applications than β-CD and PEG in industry, such as in the environment, medicine, and petrochemical technology. So, it is valuable to investigate its basic physical properties like density, surface tension, and cloud point. To our best knowledge, several researchers have studied the synthetic process and characterization,13−16 but no report is available on physicochemical properties of this short chain derivative. In this paper, the density, surface tension, and cloud point of aqueous solutions of β-CD-PEG were studied systematically. © XXXX American Chemical Society

Table 1. Sample Information chemical name β-CD PEG400 SOCl2 C2H5OH NaOH HCl

source ChengDu Kelong Chemical Co., Ltd. ChengDu Kelong Chemical Co., Ltd. ChengDu Kelong Chemical Co., Ltd. ChengDu Kelong Chemical Co., Ltd. ChengDu Kelong Chemical Co., Ltd. ChengDu Kelong Chemical Co., Ltd.

initial mass fraction purity

purification method

0.99

none

0.99

none

0.99

none

0.99

none

0.82

none

0.36

none

A synthesis procedure for β-cyclodextrin-polyethylene glycol (β-CD-PEG) is described as follows. Thionyl chloride (8.72 mL) was slowly dropped into the PEG400 (21.5 mL) at room temperature for 5 h, followed by addition of β-CD (68.1 g) which has already been dissolved in an aqueous solution of NaOH at the thermostat (333.15 K) with a magnetic stirrer to Received: July 14, 2014 Accepted: September 23, 2014

A

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Figure 1. 1H NMR spectrum of β-CD-PEG.

container at temperatures from (293.15 to 313.15) K in different mass concentrations with an uncertainty of ± 0.1 K.20 The same method was used to clarify the homogeneity of the mixture in 2.1. Each reported value of the interfacial tension was an average of multiple measurements on the sample was within ± 0.01 mN·m−1. 2.4. Cloud Point. We prepared different mass concentrations from 0.500 % to 14.000 % of product in the closed glass tubes and then immersed the tube into an accurate thermostat water bath. The temperature was raised slowly, and the change in the glass tube was observed, when the clarified liquid became turbid, and we recorded this instantaneous temperature as initial value.21 The temperature around the initial value was increased every 1 K and kept for 30 min. Then, the temperature was raised every 0.1 K for 30 min when it was close to the initial one. The cloud point temperature was recorded at which the mixture became turbid and all the measurements were carried out at least five times, and the average values of the cloud points were reported.22 The uncertainty of temperature was recorded at ± 0.01 K.

mix approximately. After 7 h, the synthesis was stopped and the solution was neutralized to pH 7 by the HCl solution. The polymer was washed and extracted with ethanol to remove water, residual monomers, and initiator. Then, the bridged βCD-PEG was further dried under vacuum oven at 333.15 K for 48 h. Finally, the derivative samples were weighed accurately, determined through an analytical balance with an uncertainty of 0.0001 g. 1H NMR spectrum wave was recorded at 298 K using a Bruker Avance-III HD NMR spectrometer (Switzerland) at 400 MHz. β-CD-PEG was dissolved in D2O solution and tested. 2.2. Density. The densities (ρ) of β-CD-PEG aqueous solution were measured by a vibrating tube densitometer (DMA4500 Anton Paar, Graz, Austria) with the precision of ± 1.0·10−5 g·cm−3.17 Double distilled and deionized water and dry air were used as reference substances to calibrate the densitometer at the atmospheric pressure. The densities of βCD-PEG at temperatures of (293.15, 298.15, 303.15, 308.15, and 313.15) K were tested. The main principle was at first to prepare 5 groups of sample at different mass concentrations (2.000 %, 4.000 %, 6.000 %, 8.000 %, and 10.000 %). The tubes were shaken for 2 min by hand and then placed in an ultrasound machines (JK-50B Jinnike, Hefei, China) for 30 min. The equilibrium state was verified by Turbiscan, e.g., the clear interface was stable and volumes of both phases changed little when the mixture stood for 20 min.12 Each value was an average of five measurements. 2.3. Surface Tension. Surface tension measurements were performed by a ring detachment method using an interfacial tensiometer (ZL-3000, Shangdong, China) which has an uncertainty of ± 0.1 mN·m−1. The platinum ring was thoroughly cleaned and flame-dried before each measurement.18,19 All measurements were carried out in a thermostat

3. RESULTS AND DISCUSSION 3.1. 1H NMR Spectrum. In order to get more information on β-CD-PEG, the 1H NMR spectrum of β-CD-PEG is shown in Figure 1. From Figure 1, each line splitting can correspond with the hydrogen of the sample. δ4.98 ppm is the proton peak of C1 in β-CD, and protons C2, C3, C4, and C5 in the 1,4 glycosidic bond were δ3.78 ppm, δ3.89 ppm, δ3.57 ppm, δ3.84 ppm,23,24 respectively. Proton C6 which connects the skeleton and chain PEG400 is δ3.87 ppm. In addition, protons of PEG400 are from δ3.46 ppm to δ3.49 ppm. 3.2. Density. To validate the experimental methods of measuring density, densities of aqueous solutions of PEG400 B

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Table 2. Comparison of the Measured Liquid Density, ρ, of PEG400 with Value from the Literature25 T/K

293.15

298.15

303.15

C/mol·kg−1

ρ/g·cm−3

ρ(lit.)/g·cm−3

ρ/g·cm−3

ρ(lit.)/g·cm−3

ρ/g·cm−3

ρ(lit.)/g·cm−3

0.1 0.4 0.8 1.2

1.00443 1.02079 1.03871 1.05364

1.00444 1.02079 1.03873 1.05363

1.00317 1.01916 1.03666 1.05113

1.00316 1.01915 1.03666 1.05113

1.00165 1.01730 1.03440 1.04851

1.00165 1.01731 1.03441 1.04850

where ρ is the density in g·cm−3, ω is the mass fraction of polymer, and K1 and K2 are the equation parameters. The straight lines in Figure 2 correspond to the values correlated by eq 1, and the correlation coefficients all exceed 0.99 with the equation parameters listed in Table 4. In order to correlation of density with mass fraction and temperature, the statistics analysis system (SAS) was used to fit the model:

with different concentrations were measured at (293.15, 298.15, and 303.15) K and compared with the values from the literature,25 as shown in Table 2. The measured data for the aqueous PEG400 solution are in good agreement with those reported in the literature. The densities of β-CD-PEG aqueous solutions were measured in a range of temperatures from (293.15 to 313.15) K, in the scope of β-CD-PEG mass fraction ω from 2.000 % to 10.000 %. Results are presented in Table 3 and Figure 2. The density of β-CD-PEG aqueous solutions decreased with an increase in temperature but increased with an increase in mass fraction of β-CD-PEG.26

ρ = 1.46951 + 0.01869ω − 0.00151T

where ρ is the density in g·cm−3, ω is the mass fraction of βCD-PEG, and T is the temperature in K. Besides, the correlation coefficient of eq 2 is 0.9979. 3.3. Surface Tension. The surface tension for aqueous solutions of β-CD-PEG at various temperatures and different mass concentrations are listed in Table 5. The variation in the surface tension of β-CD-PEG with the changes of concentration is shown in Figure 3. The γ vs ln ω curves were obtained in two segments: a linear decrease followed by a plateau at 56.1 mN·m−1; the intercept of the two parts corresponds to the critical micelle concentration (CMC) at the weight concentration 0.08%. The surface tension of PEG400 is 44.9 mN·m−1 at 303.15 K and is same as the value reported by Wu et al.29 which is smaller than that of β-CD-PEG. The points of surface tension of β-CD-PEG at mass concentration between 0.004 % and 0.060 % are fitted to eq 3.30−32

Table 3. Densities ρ for β-CD-PEG Aqueous Solutions from T = (293.15 to 313.15) K, and β-CD-PEG Mass Fraction ω = (2.000 to 10.000) %a T /K ω/% 2.000 4.000 6.000 8.000 10.000

293.15

298.15

303.15

308.15

313.15

1.06759 1.10479 1.14139 1.18152 1.21633

ρ/g·cm−3 1.05459 1.04679 1.09203 1.08418 1.12852 1.12167 1.16525 1.15965 1.20174 1.19665

1.04237 1.07877 1.11647 1.15416 1.19137

1.03487 1.07223 1.11009 1.14932 1.18586

(2)

a Abbreviations: ω, β-CD-PEG mass fraction; T, temperature; ρ, density. The relative standard uncertainties of ur (ω) = ± 0.001, ur (T) = ± 0.1 K, and ur (ρ) = ± 0.00001 g·cm−3

γ = K1 + K 2 ln ω

(3)

where γ is the surface tension, ω is the mass fraction of solute, and K1 and K2 are equation parameters. After regression of the values in Figure 3, the results are shown in Table 6. In addition, all the correlation coefficients reach 0.99. As for the dependence of surface tension of β-CD-PEG aqueous solution upon temperature, the results are shown in Figure 4. It was observed that surface tension also decreased with temperature significantly. However, the temperature influences obviously the surface tension in the low concentration of β-CD-PEG and the decrease rate of surface tension with temperature became smaller at β-CD-PEG mass fraction ω of 0.060 % and higher. The surface tension decreased with temperature linearly with a slope at −0.30 mN·m−1·K−1 at 0.001 %, while the slope decreased to −0.01 mN·m−1·K−1 at 0.100 %. This trend suggested that the temperature plays an important role in the low mass fraction. When the mass fraction reached a certain value (0.060 % in this work), the temperature showed a limited effect upon molecular association.27 3.4. Cloud Point. The hydrogen bonds form between the nonionic surfactant and water.31 Once the temperature exceeds a certain temperature (cloud point), the solution turns turbid and phase separates because the hydrogen bonding of water is insufficient to maintain the link to the ether oxygen atom. Otherwise, when the temperature is below a certain point, the mixture reform homogeneous again. This temperature becomes

Figure 2. Densities ρ for β-CD-PEG aqueous solutions at selected mass fraction ω and temperatures T of β-CD-PEG: points, experimental data; line, eq 1

The density values for β-CD-PEG are correlated as a function of mass fraction following eq 1.27,28 ρ = K1 + K 2ω (1) C

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Table 4. Density Parameters K1 and K2 (eq 1) for the System T/K 293.15

298.15

303.15

308.15

313.15

K1

K2

K1

K2

K1

K2

K1

K2

K1

K2

1.030

0.019

1.018

0.018

1.009

0.019

1.004

0.019

0.997

0.019

Table 5. Surface Tensions γ for β-CD-PEG Aqueous Solutions from T = (293.15 to 313.15) K, and β-CD-PEG Mass Fraction ω = (0.001 to 0.100) %a T/K ω/% 0.001 0.002 0.004 0.006 0.008 0.010 0.020 0.040 0.060 0.080 0.090 0.100

293.15

298.15

303.15

γ/mN·m−1 68.2 66.7 66.3 65.1 64.3 63.4 63.3 62.5 62.5 61.7 61.8 61.2 59.7 59.5 58.0 57.8 57.1 57.0 56.3 56.4 56.3 56.2 56.1 56.1

69.4 67.6 65.2 64.2 63.3 62.6 60.4 58.3 57.4 56.5 56.4 56.2

308.15

313.15

65.2 63.8 62.3 61.5 60.9 60.4 59.0 57.5 56.8 56.1 56.1 56.0

63.4 62.2 61.0 60.3 59.9 59.5 58.3 57.2 56.6 56.0 56.0 56.0

Figure 4. Surface tensions γ for β-CD-PEG aqueous solutions at selected temperatures T and mass fraction ω of β-CD-PEG: points, experimental data; line, liner fit.

a Abbreviations: ω, β-CD-PEG mass fraction; T, temperature; γ, surface tension. The relative standard uncertainties of ur (ω) = ± 0.001, ur (T) = ± 0.1 K, and ur (γ) = ± 0.1 mN·m−1

reason is that the ether linkage and hydroxyl of PEG is more than those belonging to products in the same mass fraction. Second, the cloud points are a relative constant at mass fraction from 0.500 % to 2.000 %. At the same time, the cloud points decrease with the increase of polymer with the mass fraction higher than 2.000 %. For this phenomenon, on the one hand, the free water has a reduction in solution with the polymer’s increase. On the other hand, the number of micelles in solution increases, the probability to encounter with each other increases, and the micellar aggregations become easier, leading to a decrease in the cloud point of the solution.34

4. CONCLUSIONS This paper reports some new experimental data for aqueous solutions of β-CD-PEG at temperatures from (293.15 to 313.15) K and different range of the mass fractions of β-CDPEG. The following conclusions can be drawn from the comprehensive analysis of this paper. The density and cloud point increase with an increase of mass fraction, but the surface tension of the solution decreases with β-CD-PEG mass fraction. The density and surface tension for β-CD-PEG aqueous solutions decrease with solution temperature. However, the temperature showed a limited effect upon surface tension when the β-CD-PEG mass fraction reaches 0.060 %. The product has an obvious property of surfactant and the CMC value is weight concentration 0.080 %.

Figure 3. Surface tensions γ for β-CD-PEG aqueous solutions at selected mass fraction ω and temperatures T of β-CD-PEG: points, experimental data; line, eq 3

the cloud point.33 The cloud points for aqueous solutions of βCD-PEG at different concentrations are shown in Table 7. The values show that the two change trends of the cloud points. First, the cloud points of polymer are below the PEG400’s. The

Table 6. Surface Tension Parameters K1 and K2 (eq 3) for the System T/K 293.15

298.15

303.15

308.15

313.15

K1

K2

K1

K2

K1

K2

K1

K2

K1

K2

49.319

−6.631

49.835

−6.011

50.577

−5.319

51.181

−4.623

52.105

−3.694

D

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Table 7. Cloud Points CP for β-CD-PEG Aqueous Solutions at β-CD-PEG Mass Fraction ω = (0.001 to 14.000) %a ω/% CPβ‑CD‑PEG/K CPPEG/K a

0.500

1.000

2.000

4.000

6.000

8.000

10.000

12.000

14.000

358.36 ≥ 373.15

358.35

358.35

353.55

349.45

342.05

338.35

336.55

333.35

Abbreviations: ω, β-CD-PEG mass fraction; CP, cloud point. The relative standard uncertainties of ur(ω) = ± 0.001, ur(CP) = ± 0.01 K.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 02883037327. Fax: +86 02883037305. Funding

This work was financially supported by the National Natural Science Foundation of China, China National Petroleum Corporation Petrochemical Unite Funded Project (U1262111). Notes

The authors declare no competing financial interest.

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F

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