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Sep 29, 2015 - conductivity of 18.3 MΩ·cm on a Hitech-Sciencetool Master-Q laboratory .... hydroxyl groups that can also act as initiators via the SnCl4 catalyst.
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Preparation and Characterization of Polymeric Surfactants Based on Epoxidized Soybean Oil Grafted Hydroxyethyl Cellulose Xujuan Huang,† He Liu,*,† Shibin Shang,*,†,‡ Xiaoping Rao,†,‡ and Jie Song§ †

Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Key Laboratory of Biomass Energy and Material, National Engineering Laboratory for Biomass Chemical Utilization, Key and Laboratory on Forest Chemical Engineering, State Forestry Administration, Nanjing, Jiangsu Province 210042, China ‡ Institute of New Technology of Forestry, Chinese Academy of Forestry, Beijing 100091, China § Department of Chemistry and Biochemistry, University of MichiganFlint, Flint, Michigan 48502, United States ABSTRACT: Epoxidized soybean oil (ESO) grafted hydroxyethyl cellulose (HEC) was prepared via ring-opening polymerization, in which the hydroxyl groups of HEC acted as initiators and the polymeric ESO were covalently bonded to the HEC. Hydrolysis of ESO-grafted HEC (ESO-HEC) was performed with sodium hydroxide, and the hydrolyzed ESO-HEC (H-ESO-HEC) products were characterized via Fourier transform infrared (FT-IR) and 1H and 13C nuclear magnetic resonance (NMR) spectroscopies, high-temperature gel permeation chromatography (HT-GPC), and differential scanning calorimetry (DSC). The results indicated that ring-opening polymerization of ESO occurred with the hydroxyl groups of HEC as initiators. The molecular weights of the H-ESO-HEC products were varied by adjusting the mass ratio of HEC and ESO. Through neutralizing the carboxylic acid of H-ESO-HEC with sodium hydroxide, novel polymeric surfactants (H-ESO-HEC-Na) were obtained, and the effects of polymeric surfactants on the surface tension of water were investigated as a function of concentration of H-ESO-HEC-Na. The H-ESO-HEC-Na was effective at lowering the surface tension of water to 26.33 mN/m, and the critical micelle concentration (CMC) value decreased from 1.053 to 0.157 g/L with increases in molecular weights of the polymeric surfactants. Rheological measurements indicated that the H-ESO-HEC-Na solutions changed from pseudoplastic property to Newtonian with increasing shear rate. KEYWORDS: hydroxyethyl cellulose, epoxidized soybean oil, polymeric surfactants



comonomer styrene (St) in aqueous solution.12 The C10−C14 alkyl cellulose ester sulfate surfactants were prepared by hydrophilic sulfonation and hydrophobic esterification.17 Further attempts to substitute the environmentally dangerous petrochemical products as well as schemes to increase the hydrophobicity of cellulose derivatives with fatty acids to synthesize cellulose-based surfactants have been investigated.18,19 Grafting fatty acid chains to cellulose increases the hydrophobicity of the ester derivatives even at a low degree of esterification. Although fatty acid esters are found in nature as triglycerides and inexpensive as acylation reagents, the improvement of surface-active properties of fatty acid-esterified cellulose derivatives is limited.1 According to the literature, the surface tension of water was decreased from 72.8 to 46−65 mN/m by fatty acid-esterified cellulose derivatives.18 However, it is well known that saponified fatty acid esters are an important surfactant and effective at lowering the surface tension of water from 20 to 40 mN/m.20,21 In addition, the surface tension of polymerized saponified fatty acid ester is 20.5−39.6 mN/m.22 This study suggests that, versus saponified fatty acid esters, polymerized saponified fatty acid esters have similar utility at lowering the surface tension of water. Saponified fatty acid-

INTRODUCTION Polymeric surfactants are important for a variety of industrial applications. Increasing interest in natural, renewable, and biodegradable materials makes natural polymers like cellulose and its derivatives attractive raw materials for the preparation of biopolymeric surfactants.1 Cellulose-based polymeric surfactants have drawn much attention in the last decades because they present many novel performances such as low cost, biodegradation, associative properties in water, rheological properties, and surface-active properties that can control foaming or emulsion stability.2−7 Landoll did pioneering work on cellulose-based polymeric surfactants in 1980s.8,9 Since then, various cellulose-based polymeric surfactants have been synthesized including nonionic, ionic, fluorocarbon, and amphoteric.10−15 Cellulose-based polymeric surfactants were generally synthesized by modification of hydrophilic cellulose backbones with hydrophobic long alkyl chains. Hydroxyl groups always have chemical handles that can react with hydrophobic chains containing epoxide, halide, acyl halide, isocyanate, or anhydride for the preparation of amphiphilic cellulose. Polymeric surfactants based on hydroxyethyl cellulose (HEC) were prepared by reaction of a long-chain terminal epoxide with the hydroxyl groups of HEC in an alkaline slurry.9 Partial hydrophobization of carboxymethyl cellulose with C12−C18 alkyl halides was carried out in the presence of sodium hydroxide as a catalyst in DMF/H2O (1:1).16 The HEC-based copolymer was prepared via ultraviolet irradiation by copolymerizing HEC with hexadecyl acrylate and © 2015 American Chemical Society

Received: Revised: Accepted: Published: 9062

May 24, 2015 September 16, 2015 September 28, 2015 September 29, 2015 DOI: 10.1021/acs.jafc.5b03765 J. Agric. Food Chem. 2015, 63, 9062−9068

Article

Journal of Agricultural and Food Chemistry

Figure 1. Synthetic route of H-ESO-HEC polymeric surfactants.

novel polymeric surfactants (H-ESO-HEC-Na) were obtained, and the effects of polymeric surfactants on the surface tension of water were investigated as a function of the concentration of H-ESO-HEC-Na. In addition, the rheological behaviors of the H-ESO-HEC were investigated.

grafted cellulose surfactants formed via cellulose derivatives as backbones are polymerized saponified fatty acid surfactants. Therefore, cellulose chains grafting saponified fatty acid esters to form ionic surfactantsinstead of fatty acid-esterified cellulose surfactantsmay become an effective method for synthesis of cellulose-based surfactants with highly surfaceactive properties. Fortunately, the chemical reactivities of some fatty acids isolated from vegetable oils can achieve this vision. Soybean oil is one of the most abundant renewable vegetable oils in the world.23 It possesses three unsaturated fatty acids, oleic (23%), linoleic (54%), and linolenic (8%). It contains 1, 2, and 3 double bonds, respectively.24 The double bonds can be used in many reactions including addition reaction,25 oxidation reaction,26 and polymerization reaction.27 More significantly, the expoxidation reaction makes the soybean oil become epoxidized soybean oil (ESO). A ring-opening reaction of ESO with different alcohols was carried out in the presence of Amberlyst 15 (Dry) as a catalyst.28 Furthermore, the ringopening polymerization of ESO can be catalyzed by boron trifluoride diethyl etherate (BF3·OEt2) in liquid carbon dioxide and methylene chloride.26,29 The ESO-based polymeric surfactants in particular were prepared by hydrolysis of polymerized ESO with a base.22 These interesting works suggest that hydroxyl groups of cellulose can initiate ringopening polymerization of ESO in the presence of catalysts. Moreover, novel saponified fatty acid-modified cellulose-based polymeric surfactants are obtained by saponification of the ESO-grafted cellulose products in which fatty acid chains are combined with cellulose by an ether linkage at the middle site of a fatty acid molecular chain. To prove this assumption, modification of HEC by grafting ESO was investigated and hydrolysis of ESO-grafted HEC (ESO-HEC) was performed with sodium hydroxide (Figure 1). The hydrolyzed ESO-HEC (H-ESO-HEC) products were characterized with Fourier transform infrared (FT-IR) and 1H and 13C nuclear magnetic resonance (NMR) spectroscopies, high-temperature gel permeation chromatography (HT-GPC), and differential scanning calorimetry (DSC). After neutralizing the carboxylic acid of H-ESO-HEC with sodium hydroxide,



MATERIALS AND METHODS

Materials. Hydroxyethyl cellulose (HEC), epoxidized soybean oil (ESO), and stannic chloride (SnCl4) were purchased from Aladdin Industrial Co., Ltd. Dimethyl sulfoxide (DMSO) was purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH), hydrochloric acid (HCl), and ethyl acetate were obtained from Nanjing Chemical Reagent Co., Ltd. Deionized water was purified to a conductivity of 18.3 MΩ·cm on a Hitech-Sciencetool Master-Q laboratory water purification system (Shanghai Hetai Reagent Co., Ltd.). All chemicals were used without further purification. Synthesis of ESO-Grafted HEC Derivatives (ESO-HEC). HEC (1.0 g) was dissolved in 40 mL of DMSO under stirring at room temperature for 2 h and then transferred into a three-necked roundbottomed flask. ESO (1.0 g) was added to the HEC solution. The mixture was stirred by mechanical agitation at room temperature. A mixture of 10 μL of SnCl4 and 10 mL of DMSO was then added dropwise into the reactor, and the reaction was maintained for 30 min. The resulting products were washed sequentially with deionized water to remove DMSO and dried under vacuum at 50 °C. Finally, the ESOgrafted HEC derivatives (ESO-HEC) were obtained. Hydrolysis of ESO-HEC (H-ESO-HEC). The ESO-HEC (2.0 g) was added to a flask with 50 mL of 0.6 M NaOH solution. The reactor was heated to 100 °C with agitation for 12 h under a condensing condition. After reaction, the solution was separated by filtration and the aqueous solution was removed by reduced pressure distillation. The solid product was washed with ethyl acetate three times to remove the glycerin. The solid product was then redispersed in aqueous solution, and the solution was adjusted to pH 5−6 by 0.1 M HCl. The solution became turbid, and the product appeared as a floating oil. The product was extracted by ethyl acetate, and the organic phase was washed with hot distilled water three times. Finally, the H-ESO-HEC product was purified by reduced pressure distillation. The H-ESOHEC-Na was obtained by neutralizing the carboxylic acid of H-ESOHEC with sodium hydroxide. FT-IR. Fourier transform infrared (FT-IR) spectra were used to confirm the structure of the H-ESO-HEC. These spectra were 9063

DOI: 10.1021/acs.jafc.5b03765 J. Agric. Food Chem. 2015, 63, 9062−9068

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Journal of Agricultural and Food Chemistry Table 1. Effect of Reaction Condition on Yield and Carboxyl Content and the HT-GPC Data of H-ESO-HEC HT-GPC data samples

HEC:ESO (mass ratio)

yielda (g/g)

HEC H-ESO-HEC-I H-ESO-HEC-II H-ESO-HEC-III H-ESO-HEC-IV H-ESO-HEC-V

1:1 1:2 1:3 1:4 1:5

1.2 1.6 1.9 2.1 2.7

carboxyl contentb (mmol/g) 0 1.89 2.12 2.35 2.72 2.80

± ± ± ± ±

Mn × 105 (Daltons)

Mw × 105 (Daltons)

PDIc

0.7 1.48 1.55 3.77 4.19 4.58

2.28 2.71 3.31 5.50 6.61 8.28

3.20 1.80 2.10 1.46 1.57 1.80

0.13 0.06 0.10 0.03 0.11

a

Expressed as grams of the H-ESO-HEC per gram of HEC (on dry mass basis). bExpressed as millimoles of carboxyl per gram of H-ESO-HEC (on dry mass basis). cDetermined by Mw/Mn.

obtained on a Thermo Scientific Nicolet IS10 spectrometer with the attenuated total reflectance (ATR) method. The spectra were recorded over the range 4000−500 cm−1 at 4 cm−1 resolution and averaged over 16 scans per sample. NMR. 1H NMR and 13C NMR spectra for H-ESO-HEC were recorded at 40 °C on a BRUKER AV400 spectrometer (Bruker, Rheinstetten, Germany) operating at frequencies of 400.13 and 100.61 MHz, respectively. Deuterated dimethyl sulfoxide (DMSO-d6) and tetramethylsilicane (TMS) were used as the solvent and the internal standard, respectively. The chemical shift values were referenced to the signals of DMSO-d6 and TMS. HT-GPC. Molecular weight distribution curves and relative values of number-average (Mn) and weight-average (Mw) molecular weight of H-ESO-HEC were determined by HT-GPC (HT-GPC Module 350A, Viscotek; GPC equipped with I-MBHMW-3078 HT-GPC column) at 60 °C. The HT-GPC instrument was equipped with a refractive index detector, viscosity detector, and small-angle light scattering detector. The flow rate of the carrier solvent (HPLC-grade DMSO) was 1.0 mL/min. Samples were filtered over microfilters with a pore size of 0.2 μm (Nylon, Millex-HN 13 mm Syringes Filters, Millipore). The results were obtained from OmniSEC software. Conductimetry. The carboxyl content of H-ESO-HEC was determined by conductometric titrations.30 The H-ESO-HEC (30− 40 mg) was suspended into 15 mL of 0.01 M hydrochloric acid solution. After 2 h of stirring, the suspensions were titrated with 0.01 M NaOH. The carboxyl content of the sample was determined from the conductivity and VNaOH curve. The carboxyl content is calculated by the following equation

carboxyl content(mmol/g) =

c(V2 − V1) m

concentration of aqueous H-ESO-HEC-Na solution was tested in triplicatethis was done automatically by the instrument. The concentration at which the equilibrium surface tension of the polymeric surfactants stopped decreasing with concentration and plateaued is the critical micelle concentration (CMC).22 From the inflection point of the plot, the critical micelle concentration (CMC) and minimum surface tension (γcmc) were derived. All data were obtained from the OneAttention software. Krafft Point (KP). A 20 mL amount of 0.5 wt % surfactant aqueous solution (at least quintuplicate the CMC of studied surfactant) was prepared in an alkaline solution and added to a beaker. The beaker was then heated, and the particular temperature was recorded as KP when the solution transformed from turbid to pellucid.31 Hydrophile Lipophile Balance (HLB) Value. The emulsions were prepared using a mixture of the nonionic surfactants Tween 60 and Span 80 to satisfy the proper HLB values for optimum emulsification conditions. The mixed HLB values were calculated with the following equation

HLBmix = HLBT × T% + HLBS × S%

(2)

Here, HLBmix, HLBT, and HLBS are the HLB values of the mixed surfactants, Tween 60 (14.9), and Span 80 (4.3), respectively. The T% and S% are the mass percentages of Tween 60 and Span 80 in the mixed surfactants, respectively.32 All HLB values used were measured at 25 °C. Rheology. Rheological measurements were performed at a rotational rheometer HAAKE MARS II equipped with a measuring geometry named PP60 Ti (diameter, d = 50 mm). The H-ESO-HECNa-III was selected to evaluate the rheological properties. H-ESOHEC-Na-III solutions at three different concentrations (0.1, 1, and 5 g/L) were prepared to measure the shear viscosity that changed with the shear rate ranging from 0.01 to 100 s−1.15 The rheological measurements were carried out at 40 °C. All data were obtained using the HAKKE RheoWin data manager software. Statistical Method. Origin Pro 9.0 was used to calculate the standard deviation of carboxyl content and the surface tension of HESO-HEC. The standard deviation was calculated with the following equation

(1)

Here, V1 and V2 are the volumes of NaOH (mL) in the first and second inflection points of titration curves, respectively. c is the concentration of NaOH (mmol/L), and m is the weight of oven-dried sample (mg). To minimize errors, titrations were duplicated at least three times, and the average value was retained for the discussion. DSC. To determine the ring-opening polymerization of ESO, the DSC measurements of the H-ESO-HEC were performed on a Diamond DSC at 40 mL/min dry nitrogen as the purge gas. Typically, ∼4 mg of the H-ESO-HEC was accurately weighed in an aluminum pan and sealed with pin-perforated lids. The sample was cooled to −50 °C at a rate of 10 °C/min, and a refrigerated cooling system was used to equilibrate the sample at −50 °C for 5 min. Data were recorded while the oven temperature was raised from −50 to 0 °C at 10 °C/ min. Surface Tension. The surface tension of H-ESO-HEC-Na was measured at 25 °C using the Wilhelmy plate T107 (width, 19.44 mm; thickness, 0.1 mm; height, 65 mm; circumference, 39.08) on a Sigma 701 Automatic Surface Tensiometer. The instrument was calibrated against pure water before measurements were made. The concentration of H-ESO-HEC-Na surfactant was gradually increased by dispersing stock solution into the measurement cell. The software automatically determined the surface tension as a function of the bulk concentration. The surface tension data were plotted against the HESO-HEC-Na concentration in water from 0.0003 to 2.0 g/L. Each

n

S=

∑i = 1 (Xi − X̅ )2 n−1

(3)

Here, S is the standard deviation of a sample, Xi is the ith value in a sample, X̅ is the average number of a sample, and n is the total number of a sample.



RESULTS AND DISCUSSION Figure 1 outlines a simplified mechanism for the entire reaction. The ESO-HEC was prepared via ring-opening polymerization, in which the hydroxyl groups of HEC acted as initiators. In addition, the ring-opening polymerization generated new hydroxyl groups that can also act as initiators via the SnCl4 catalyst. The polymeric ESO were covalently bound to the HEC, and hydrolysis of ESO-HEC (H-ESO-HEC) was 9064

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Journal of Agricultural and Food Chemistry obtained with sodium hydroxide. As shown in Table 1, H-ESOHEC products with different molecular weights were obtained by adjusting the mass ratio of HEC and ESO in graft reaction. The graft reaction occurred at room temperature with a short reaction time because of the high activity of the SnCl4 catalyst. The yields of the H-ESO-HEC were measured and ranged from 1.2 to 2.7 g per gram of HEC as the amount of ESO increased in the graft polymerization reaction. In addition, H-ESO-HECNa samples with different molecular weights were obtained by neutralizing carboxylic acid of H-ESO-HEC with sodium hydroxide. Spectroscopic Identification of the Structures of HESO-HEC. In Figure 2, the FT-IR spectrum of H-ESO-HEC-III

Figure 3. 1H NMR and sample in DMSO-d6.

Figure 2. FT-IR spectra of HEC and H-ESO-HEC-III.

13

C NMR spectrum of H-ESO-HEC-III

Table 2. 1H and 13C NMR Signals of H-ESO-HEC

attested to the presence of ESO chains onto the HEC by appearance of the characteristic absorption bands assigned to the carbonyl functions at 1710 cm−1 versus the HEC. The spectral peak at 2920 cm−1 was attributed to the −CH3 group of the alkane chains of ESO. Also, peaks at 716 cm−1 indicate the vibration of −(CH2)n− (n ≥ 4).33 No significant change in hydroxyl group signal at 3360 cm−1 was noticed because the hydroxyl groups of the HEC acted as initiators and were consumed in the grafting reaction. The ring-opening polymerization generated new hydroxyl groups simultaneouslythese are obvious from Figure 1. These results indicated that the HESO-HEC was prepared successfully. The introduction of the ESO on HEC chains is further confirmed by the 1H and 13C NMR spectra of the H-ESOHEC-III in Figure 3. In 1H NMR spectrum, the clusters of signal peaks ranged from δ 0.8 to 1.6. These were related to the protons of the alkane chains of ESO on HEC. The peaks at δ 2.1 corresponded to the side groups −OH of the HEC. The alkane chains were produced by the ring-open reaction. The clusters of signal peaks in the range from δ 3.2 to 5.0 were attributed to the chemical shifts of anhydroglucose unit (AGU) protons. In the 13C NMR spectrum, the possible assignments of the peaks were found according to the previous literature.34,35 The inserted region at δ 176 correspond to carbonyl signals of the carboxylate groups.36 The C-1 carbon resonance in AGU was poor at δ 101. The clusters of signal peaks in the range from δ 62 to 76 contributed to the chemical shifts of AGU carbons. Table 2 offers detailed information. Distinct peaks were observed at δ 62. There are attributed to the C-6 substituted by the hydroxyethylation and connected to the

1

13

H NMR

C NMR

signal

δ (ppm)

signal

δ (ppm)

−OH −(CH2)n−CH3 −(CH2)n−CH3 CH2−O−CH2 CH−OH AGU−H1 AGU−H2 AGU−H3 AGU−H4 AGU−H5 AGU−H6

2.10 0.90 1.30−1.50 2.80 3.20 4.75 3.50 3.55 3.30 3.65 3.45

−COOH −CH3 −CH2− CH2−O−CH2 CH−OH AGU−C1 AGU−C2 AGU−C3 AGU−C4 AGU−C5 AGU−C6

176 15 27−35 83 80 101 74 70 76 73 62

ether bond that is obtained by graft polymerization. The clusters of peaks in the range from δ 10 to 35 corresponding to the carbon signals of ESO chains demonstrated that the graft reaction was successful. 1H NMR and 13C NMR spectroscopy confirmed the characteristic groups of H-ESO-HEC-III. Molecular Weight and Carboxyl Content of H-ESOHEC. The molecular weight value of H-ESO-HEC was determined by HT-GPC. The HT-GPC profile signals of the refractive index detector are shown in Figure 4. This shows that the H-ESO-HEC with different molecular weights had a similar tendency of distribution and moved toward the higher molecular weight regions as the amount of ESO increased in the graft reaction. As listed in Table 1, the Mn of H-ESO-HEC (I−V) ranged from 1.48 × 105 to 4.58 × 105 Da and the Mw of 9065

DOI: 10.1021/acs.jafc.5b03765 J. Agric. Food Chem. 2015, 63, 9062−9068

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Journal of Agricultural and Food Chemistry

Figure 6. DSC curves of HEC and H-ESO-HEC samples.

Figure 4. GPC profile signal of the refractive index of H-ESO-HEC samples.

HEC did not exhibit glass transition, and there was no polymerization of ESO on HEC in H-ESO-HEC-I. Previous literature reported that the products obtained by ring-opening polymerization of ESO had Tg.26 The Tg values of H-ESOHEC-II, H-ESO-HEC-III, H-ESO-HEC-IV, and H-ESO-HECV were −25.81, −23.74, −22.24, and −21.48 °C, respectively. As expected, increasing the amount of ESO in the graft reaction resulted in more ESO being grafted on HEC. The ring-opening polymerization reaction took place in the graft reactionthis was consistent with the GPC results. Furthermore, the Tg of HESO-HEC samples became higher as the molecular weights of H-ESO-HEC increased. Surface Tension of Aqueous H-ESO-HEC Polymeric Surfactants. The H-ESO-HEC products were added to a beaker containing the required amount of NaOH to neutralize all of the acidic protons from carboxylic acid groups. This was then placed in a 60 °C water bath and stirred with a glass rod until the samples were dissolved.28,29 A series of aqueous solutions of the H-ESO-HEC-Na were prepared, and their surface tensions were investigated at room temperature. It is well known that amphiphilic polymers with a suitable hydrophilic−hydrophobic balance can form a micelle structure when exposed to a selective solvent.37 As shown in Figure 7, the surface tension of H-ESO-HEC-Na in water decreased markedly at low concentration at first. When the concentration was higher, the surface tension reduced slightly. Finally, the

H-ESO-HEC (I−V) ranged from 2.71 × 105 to 8.28 × 105 Da along with the mass ratio of HEC and ESO ranged from 1:1 to 1:5, respectively. The conductometric titration curve to determine the carboxyl content of H-ESO-HEC-III was divided into three parts in Figure 5. The first part showed the presence

Figure 5. Conductometric titration curve of H-ESO-HEC-III.

of a strong acid corresponding to excess HCl. The second part where the conductivity remains constant showed a weak acid corresponding to the carboxyl content. The last part showed that strong base corresponded to an excess of sodium hydroxide. V1 and V2 are volumes of NaOH (mL) in the first and second inflection points of the titration curves, respectively. Finally, the carboxyl content can be calculated by eq 1. The carboxyl contents of H-ESO-HEC (I−V) were 1.89−2.80 mmol/g (Table 1). We concluded that the carboxyl content was closely associated with the molecular weight. As the amount of ESO increased in graft reaction, the carboxyl content increased according to the Mn of H-ESO-HEC. In addition, the polydispersity indexes (PDI) of H-ESO-HEC (I−V) ranged from 1.46 to 2.1. This indicates that the H-ESO-HEC samples had a wide distribution (Figure 4). DSC Analysis. The ring-opening polymerization of ESO grafting on HEC was confirmed by DSC in which the glass transition temperature (Tg) was determined to be the temperature at the inflection point. Figure 6 clearly illustrates that there was no Tg in HEC and H-ESO-HEC-I because the

Figure 7. Effect of H-ESO-HEC-Na concentration in water on equilibrium surface tension of water. 9066

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Journal of Agricultural and Food Chemistry surface tension was held nearly constant because its concentration was sufficiently high. As shown in Table 3, the minimum surface tensions of HESO-HEC-Na (I−V) were almost the same and ranged from Table 3. CMC, γcmc, KP, and HLB Values of Samples samples HEC H-ESO-Na H-ESO-HEC-Na-I H-ESO-HEC-Na-II H-ESO-HEC-Na-III H-ESO-HEC-Na-IV H-ESO-HEC-Na-V

CMC (g/L)

γcmc (mN/m)

KP (°C)

HLB value

1.105 1.053 0.702 0.514 0.402 0.157

54.68 34.28 26.33 28.88 27.83 26.37 26.94

± ± ± ± ± ± ±

18 39 42 45 52 68

9.6−10.6 9.6−10.6 9.6−10.6 9.6−10.6 9.6−10.6 9.6−10.6

1.20 0.93 0.56 0.99 0.61 0.50 1.26

Figure 8. Apparent viscosity of the H-ESO-HEC-Na-III solution as a function of shear rate.

26.33 to 28.88 mN/m. The H-ESO-HEC-Na displayed efficient surface properties versus HEC and the hydrolyzed ESO at 54.68 and 34.28 mN/m, respectively. In addition, the previous studies reported that the surface tension of fatty acid-esterified cellulose derivatives ranged between 46 and 65 mN/m.17,18 Different efficiency of these fatty acid-modified cellulose polymeric surfactants in lowering surface tensions of water was attributed to the status of fatty acid in polymeric surfactants. In our study, hydroxyl groups of HEC acted as initiators for ring-opening polymerization of ESO. The fatty acid chains were combined with HEC by ether linkage at the middle site of the fatty acid molecular chain. In this structure, both the hydrophobic alkanes and the hydrophilic carboxyl were introduced into HEC. The simultaneous introduction of hydrophobic alkanes and hydrophilic carboxyl groups improved the hydrophilic−hydrophobic balance of HEC and provided an opportunity to form micelles in water. The CMC values of HESO-HEC-Na were decreased from 1.053 to 0.157 g/L along with the obviously increasing molecular weight of H-ESOHEC-Na. As the molecular weights increased, the H-ESOHEC-Na displayed greater propensity to form micelles because of the flexible molecular chains. This indicated that the efficiency in reducing surface tension was significantly enhanced when the molecular weight of H-ESO-HEC-Na increased. The KP of the H-ESO-HEC-Na ranged from 39 to 68 °C. It increased as a function of molecular weight accordingly. The HLB values of the samples ranged from 9.6 to 10.6. Surface Rheological Properties. The apparent viscosities of the H-ESO-HEC-Na aqueous solution as a function of shear rate at different concentrations were investigated. The results of H-ESO-HEC-Na-III displayed the same properties as the other products (Figure 8). The curves were not smooth at the lowshear rate because of instrument error and the complete minor viscosity. Therefore, the curve of the 0.1 g/L H-ESO-HEC-NaIII can largely be considered a straight line. The curve of the 0.1 g/L H-ESO-HEC-Na-III below the CMC demonstrated that the solution behaved like water due to the low number of species H-ESO-HEC-Na-III in solution. The curves of the 1.0 and 5.0 g/L H-ESO-HEC-Na-III above the CMC that formed micelles in aqueous solution can be divided into two parts. First, the viscosity of the H-ESO-HEC-Na-III solution decreased as the shear rate increased, and the H-ESO-HECNa-III solution exhibited a pseudoplastic property at the lowshear rate.38 This was because micelles in aqueous solution were damaged, and the macromolecules experienced conformational change.39 Its winding structure was separated under the

action of shearing force and arranged along the flow direction along with the increasing shear rate.40 Sequentially, the viscosity remained constant when the shear rate reached a certain level. It behaved like a Newtonian property.41 This is because the winding structure was completely destroyed, and the molecular orientation reached a limiting condition. In summary, ESO-grafted HECs were prepared via ringopening polymerization, and five novel polymeric surfactants H-ESO-HEC-Na (I−V) were obtained by neutralization of HESO-HEC with sodium hydroxide. The molecular weights of H-ESO-HEC products were varied by adjusting the mass ratio of HEC and ESO in the grafting reaction. The synthesized polymeric surfactants H-ESO-HEC-Na exhibited higher activities in reducing water surface tension. These ranged from 26.33 to 28.88 mN/m in comparison to 34.28 mN/m for hydrolyzed ESO and 54.68 mN/m for HEC. In addition, the efficiency in reducing water surface tension was definitely enhanced when the molecular weights of H-ESO-HEC increased. Rheological measurements indicated that the HESO-HEC-Na solutions changed from a pseudoplastic state to Newtonian with increasing shear rate. This study may facilitate an increase in the use of renewable and biodegradable materials with improved properties as polymeric surfactants.



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

The authors express their gratitude for the financial support from the Natural Science Foundation of Jiangsu Province of China (BK2012063 and BK20140973), Special Fund for Basic Scientific Research Business of Central Public Research Institutes (CAFINT2012C05), and National Natural Science Foundation of China (31200446). Notes

The authors declare no competing financial interest.



REFERENCES

(1) Tomanova, V.; Srokova, I.; Ebringerova, A.; Sasinkova, V. Surface-Active and Associative Properties of Ionic Polymeric Surfactants Based on Carboxymethylcellulose. Polym. Eng. Sci. 2011, 51, 1476−1483.

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DOI: 10.1021/acs.jafc.5b03765 J. Agric. Food Chem. 2015, 63, 9062−9068

Article

Journal of Agricultural and Food Chemistry

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DOI: 10.1021/acs.jafc.5b03765 J. Agric. Food Chem. 2015, 63, 9062−9068