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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Synthesis and Physicochemical Properties of Novel PhenylContaining Sulfobetaine Surfactants Shifeng Gao,† Zhaozheng Song,*,† Fang Lan,† Jianping Zhao,‡ Tianpeng Xu,§ Yunpeng Du,† and Qingzhe Jiang*,†,∥ †

State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, PR China CNOOC EnerTech-Safety & Environmental Protection Company, Tianjin 300457, PR China § Department of Production Optimization, China Oilfield Services Limited, Tianjin 300459, PR China ∥ School of International Trade and Economics, University of International Business and Economics, Beijing 100029, PR China

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‡

S Supporting Information *

ABSTRACT: A novel series of sulfobetaine surfactants, 3-((4-(alkoxy)-3,5-dimethylbenzyl) dimethylammonio)-2-hydroxypropane-1-sulfonate (CnOBSb, n = 12, 14, and 16), were synthesized for the first time. The chemical structures of products were characterized by high-resolution mass spectra, 1H NMR, and 13C NMR spectrometry. The physicochemical properties of CnOBSb were studied by thermogravimetric analysis, equilibrium surface tension measurement, steady-state fluorescence, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The results demonstrated that surface tension values of CnOBSb are in the range of 28.32−30.14 mN/m at a fairly low critical micelle concentration (cmc). The adsorption and micellization properties of CnOBSb were dependent on the hydrocarbon chain length and presence of phenyl group. The results of DLS and cryo-TEM measurements revealed that CnOBSb molecules spontaneously formed vesicles in aqueous solution above the cmc. Furthermore, the application properties indicated that CnOBSb possessed superior interfacial tension capability, outstanding wetting ability, and good foaming and emulsifying properties.

1. INTRODUCTION

physicochemical properties of surfactants have been widely investigated. The majority of studies are focused on varying the hydrophobic tail,9−11 hydrophilic headgroup,12−14 and spacer group.15 Noteworthily, the phenyl-containing sulfobetaine surfactants exhibit superior performance in surface activities and interfacial tensions (IFTs) compared with the alkyl chain analogues.9,16,17 For instance, Hu et al.9 reported that the cmc, γcmc, and Acmc values of benzyl-substituted alkyl sulfobetaine were lower than those of alkyl sulfobetaine (ASB) owing to the increase of hydrophobic nature. Cao et al.17 found that the benzyl-substituted betaine showed lower IFT against n-alkane in comparison with linear betaine (ASB). Recently, owing to the complex synthesis route, only few phenyl-containing sulfobetaine surfactants have been synthesized and studied so

Zwitterionic surfactants, consisting of two opposite charged headgroups in one molecule, have aroused tremendous interest of researchers over the past years owing to superior physicochemical properties, such as good biodegradability, good water solubility, high salt and temperature resistance, and skin compatibility.1−3 Sulfobetaine surfactants with a cationic quaternary ammonium group and sulfonate group in the hydrophilic headgroup are an important type of zwitterionic surfactants. Because of their distinctive properties, they have wide potential utilization in pharmaceuticals, detergent formulations, and chemical flooding agents.4−8 Therefore, nowadays, investigation on sulfobetaine surfactants is still an attractive subject for researchers. As is well known, the molecular structures of sulfobetaine surfactants play a dominant role in affecting the physicochemical properties. Thus, many researches have been devoted to designing and synthesizing new types of sulfobetaine surfactants, and the effects of molecular structure on © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

April 9, 2019 August 7, 2019 August 12, 2019 August 12, 2019 DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthetic Pathway of CnOBSb (n = 12, 14, and 16)

far.9 Thus, the synthesis of phenyl-containing betaine surfactants in an efficient way is still desirable. Further, it is necessary to investigate the physicochemical and application properties of this type of sulfobetaine surfactants. In this study, a series of novel phenyl-containing sulfobetaine surfactants 3-((4-(alkoxy)-3,5-dimethylbenzyl)dimethylammonio)-2-hydroxy-propane-1-sulfonate (CnOBSb, n = 12, 14, and 16) were synthesized and characterized. A number of measurements were carried out with the purpose of clarifying the various effects including molecular structures and the surfactant concentration on the physicochemical properties. It would be of great significance in enriching the understanding of the structure−property relationship of sulfobetaine surfactants more comprehensively. Furthermore, the surface activities and interfacial behavior of CnOBSb were evaluated by IFT measurement, wetting ability, foaming stability, and emulsifying property for potential application in industrial and household fields.

under reduced pressure, and then, the crude product was dissolved in hot methanol. After the residue was filtered and the solvent was evaporated, the crude product was repeatedly recrystallized from ethanol to obtain sodium 3-(dimethylamino)-2-hydroxypropane-1-sulfonate as white powder. The yield was 92%, 1H NMR (500 MHz, DMSO-d6): δ 2.14 (s, 6H, −N(CH3)2), 2.19−2.26 (m, 2H, −CH2−N(CH3)2), 2.39−2.71 (m, 2H, −CH2−CH(OH)−CH2−N(CH3)2), 3.90−3.95 (m, H, −CH2−CH(OH)−CH2−N(CH3)2), 4.83 (s, H, −CH2− CH(OH)−CH2−N(CH3)2). 13C NMR (125 MHz, DMSOd6): δ 45.88, 55.86, 64.48, 65.46. HRMS (ESI+) m/z: calculated for C5H13NaNO4S+ (M + H), 206.0457; found, 206.0456. 2.2.2. Synthesis of 3-((4-(Alkoxy)-3,5-dimethylbenzyl)dimethylammonio)-2-hydroxy-propane-1-sulfonate (CnOBSb). CnOBMCl (0.01 mol, 3.38−4.22 g) was dissolved in around 30 mL hot ethanol, and then, 3-(dimethylamino)-2hydroxypropane-1-sulfonate (0.012 mol, 2.46 g) and sodium bicarbonate (0.01 mol, 0.84 g) were added into the stirred solution. The reaction mixture was refluxed for about 8 h. Then, the residue was filtered, and the filtrate was evaporated under vacuum, the resulting waxy crude betaine surfactants were recrystallized from mixtures of methanol and acetone. The product was isolated and dried under reduced pressure to give CnOBSb as a white solid. 2.2.2.1. 3-((4-(Dodecyloxy)-3,5-dimethylbenzyl)dimethylammonio)-2-hydroxy-propa-ne-1-sulfonate (C12OBSb). White solid, yield (65%), 1H NMR (500 MHz, CDCl3): δ 0.86−0.89 (t, 3H, −CH3), 1.26−1.36 (m, 16H, −O−CH2−CH2−CH2−(CH2)8−CH3), 1.44−1.50 (m, 2H, −O−CH2−CH2−CH2−(CH2)8−CH3), 1.76−1.81 (m, 2H, −O−CH2−CH2−CH2−(CH2)8−CH3), 2.24 (s, 6H, ArCH3), 3.13−3.17 (d, 6H, −CH2−N+(−CH3)2−CH2−CH(OH)−CH2SO3−), 3.04−3.19 (m, 2H, −CH2−N+(−CH3)2− CH2−CH(OH)−CH2SO3−), 3.71−3.74 (t, 2H, −O−CH2− CH2−CH2−(CH2)8−CH3), 3.52−3.96 (m, 2H, −CH2− N+(−CH3)2−CH2−CH (OH)−CH2SO3−), 4.48−4.58 (m, 2H, Ar-CH2−N+(−CH3)2−CH2−), 4.86 (s, H, −CH2− N+(−CH3)2−CH2−CH(OH)CH2−SO3−), 7.14 (s, 2H, metaArH). 13C NMR (125 MHz, CDCl3): δ 14.1, 16.3, 22.6, 26.1, 29.3, 29.5, 29.6, 29.7, 30.4, 31.9, 50.4, 54.8, 63.7, 68.0, 69.2, 72.4, 122.1, 132.0, 133.7, 158.1. HRMS (ESI+) m/z: calculated for C26H48NO5S+ (M + H), 486.3247; found, 486.3242. 2.2.2.2. 3-((4-(Tetradecyloxy)-3,5-dimethylbenzyl)dimethylammonio)-2-hydroxyprop-ane Sulfonate (C14OBSb). White solid, yield (61%), 1H NMR (500 MHz, CDCl3): δ 0.86−0.89 (t, 3H, −CH3), 1.26−1.36 (m, 20H, −O−CH2−CH2−CH2−(CH2)10−CH3), 1.42−1.50 (m, 2H, −O−CH2−CH2−CH2−(CH2)10−CH3), 1.74−1.80 (m, 2H, −O−CH2−CH2−CH2−(CH2)10−CH3), 2.22 (s, 6H, Ar-CH3),

2. EXPERIMENTAL METHODS 2.1. Materials. 1-Bromododecane (98%), 1-bromotetradecane (98%), and 1-bromohexadecane (97%) were purchased from Aladdin Co. (Shanghai, China). Sodium 3-chloro-2hydroxypropane-1-sulfonate (98%) was supplied by Xiya Reagent Co. (Chengdu, China). Dimethylamine aqueous solution (33%), sodium hydroxide (99%), and sodium bicarbonate (99%) were obtained from Tianjin Huchen chemical plant (China). All organic solvents including methanol, acetic acid, ethanol, petroleum ether, and acetone were of analytical grade. 2.2. Synthesis. The novel phenyl-containing sulfobetaine surfactants have been synthesized following a synthetic pathway in Scheme 1. 1H and 13C NMR spectra were obtained on a BRUKER AVANCE III in DMSO-d6 (for 1H, δ 2.49; for 13 C, δ 39.6) and CDCl3 (for 1H, δ 7.26; for 13C, δ 77.00), operated at 500 and 125 MHz, respectively. The highresolution mass spectra (HRMS) analysis was measured with a Bruker APEX_ULTRA94 FT-ICR-MS spectrometer. The 1H and 13C NMR spectra of products were shown in the Supporting Information (Figures S1−S4). The synthesis of 2-(alkoxy)-1,3-dimethylbenzene (CnOB) and 5-(chloromethy)-2-(alkoxy)-1,3-dimethylbenzene (CnOBMCl) with n = 12, 14, and 16 was reported in our previous article.18 2.2.1. Synthesis of Sodium 3-(Dimethylamino)-2-hydroxypropane-1-sulfonate. The solution of sodium 3-chloro-2hydroxypropane-1-sulfonate (0.1 mol, 19.6 g) in deionized water (50 mL) was slowly added dropwise into 33% dimethylamine aqueous solution containing sodium hydroxide (0.2 mol, 27 g). The reaction mixture was stirred for approximate 12 h at 40 °C. The solvent was evaporated B

DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

ji γ −γ zy 0 0 ΔGads = ΔGmic −jjj 0 cmc zzz j Γcmc z k {

3.08−3.20 (d, 6H, −CH2−N+(−CH3)2−CH2−CH(OH)− CH2SO3−), 3.13−3.18 (m, 2H, −CH2−N+(−CH3)2−CH2− CH(OH)−CH2SO3−), 3.70−3.72 (t, 2H, −O−CH2−CH2− CH 2 −(CH 2 ) 8 −CH 3 ), 3.53−3.90 (m, 2H, −CH 2 − N+(−CH3)2−CH2−CH(OH)−CH2SO3−), 4.49−4.59 (m, 2H, Ar-CH2−N+(−CH3)2−CH2−), 4.88 (s, H, −CH2−N+ (−CH3)2−CH2−CH(OH)CH2−SO3−), 7.14 (s, 2H, metaArH). 13C NMR (125 MHz, CDCl3): δ 14.1, 16.3, 22.6, 26.1, 29.3, 29.5, 29.6, 29.6, 29.6, 30.4, 31.9, 50.4, 54.8, 63.7, 68.0, 69.2, 72.4, 122.2, 132.0, 133.7, 158.1. HRMS (ESI+) m/z: calculated for C28H52NO5S+ (M + H), 514.3560; found, 514.3559. 2.2.2.3. 3-((4-(Hexadecyloxy)-3,5-dimethylbenzyl)dimethylammonio)-2-hydroxyprop-ane Sulfonate (C16OBSb). White solid, yield (58%), 1H NMR (500 MHz, CDCl3): δ 0.86−0.89 (t, 3H, −CH3), 1.26−1.36 (m, 24H, −O−CH2−CH2−CH2−(CH2)12−CH3), 1.44−1.50 (m, 2H, −O−CH2−CH2−CH2−(CH2)12−CH3), 1.76−1.80 (m, 2H, −O−CH2−CH2−CH2−(CH2)12−CH3), 2.23 (s, 6H, Ar-CH3), 3.12−3.16 (d, 6H, −CH2−N+(−CH3)2−CH2−CH(OH)− CH2SO3−), 3.06−3.19 (m, 2H, −CH2−N+(−CH3)2−CH2− CH(OH)−CH2SO3−), 3.70−3.73 (t, 2H, −O−CH2−CH2− CH 2 −(CH 2 ) 8 −CH 3 ), 3.52−3.94 (m, 2H, −CH 2 − N+(−CH3)2−CH2−CH(OH)−CH2SO3−), 4.48−4.57 (m, 2H, Ar-CH2−N+(−CH3)2−CH2−), 4.86 (s, H, −CH2− N+(−CH3)2−CH2−CH(OH)CH2−SO3−), 7.13 (s, 2H, metaArH). 13C NMR (125 MHz, CDCl3): δ 14.1, 16.3, 22.6, 26.1, 29.3, 29.5, 29.6, 29.7, 30.4, 31.9, 50.4, 54.9, 63.7, 68.0, 69.2, 72.4, 122.2, 132.0, 133.7, 158.1. HRMS (ESI+) m/z: calculated for C30H56NO5S+ (M + H), 542.3873; found, 542.3872. 2.3. Measurements. 2.3.1. Thermal Stability Measurements. The thermal stability of sulfobetaine surfactants was performed by a HTG-3 thermogravimetric analyzer in a nitrogen atmosphere at a flow rate of 50 mL min−1. All samples were heated in aluminum pans at a heating rate of 10 °C min−1. 2.3.2. Surface Tension Measurements. The surface tension measurements were performed with a BZY-2 automatic tensiometer by the Wilhelmy plate method at 25.0 ± 0.1 °C. Each sample was measured at least three times until a reproducible value was obtained. 2.3.3. Surface Adsorption and Thermodynamic Parameters. The adsorption amount of surfactant Γ and occupied area per molecule Acmc were evaluated according to the Gibbs adsorption isotherm equation19 Γ=−

ij dγ yz 1 jj zz 2.303nRT jjk d log C zz{

Acmc =

1 N Γcmc

(4)

where T is the absolute temperature, Xcmc is the cmc as a molar fraction (Xcmc = cmc/55.4), and γ0 and γcmc are the surface tensions of water and surfactant solution at the cmc, respectively. 2.3.4. Steady-State Fluorescence Measurements. The fluorescence properties of the synthesized sulfobetaine surfactant were measured by a RF5301 fluorescence spectrophotometer. The emission spectra were scanned in the range of 350−500 nm, and the excitation wavelength was set as 335 nm. The slit widths of excitation and emission were 3.0 nm. The pyrene concentration was 1 × 10−6 mol/L in all surfactant solution. The fluorescence intensity ratio of the first to the third vibronic peaks, I1/I3, is sensitive to the variation of micropolarity in solubilization site of pyrene molecules: the decrease of I1/I3 ratio indicates the enhancement of hydrophobicity of the pyrene environment.22 2.3.5. Dynamic Light Scattering. Dynamic light scattering (DLS) measurements were carried out using a Nano ZS Zetasizer with a Helium−Neon laser (λo = 632.8 nm). The scattering angle was 173°. The distribution of the aggregates size was obtained using Stokes−Einstein equation.23 All prepared solutions were filtered through a 0.45 μm membrane filter before measurements. 2.3.6. Cryogenic Transmission Electron Microscopy. Cryogenic transmission electron microscopy (Cryo-TEM) measurement was performed with a FEI Tecnai F20 TEM D545. The operational procedure was according to one literature.24 2.3.7. IFT Measurements. The IFTs of CnOBSb solution against Xinjiang crude oil were measured with a TX-500C spinning drop IFT meter at 40 ± 0.1 °C. The rotation speed was 5000 rpm. The viscosity and density of Xinjiang crude oil were 15.8 mPa·s and 0.86 g/cm3, respectively. 2.3.8. Wetting Ability. The contact angle of surfactant solution was determined by a sessile drop technique using a JC2000C contact angle meter at 25 ± 0.1 °C. The paraffin film was used as base plates to determine the contact angle of the CnOBSb solution. The results were recorded as the means of five repeated measurements. 2.3.9. Emulsion Stability and Foaming Properties. The emulsion stability of CnOBSb was measured by vigorously stirring a mixture of 40 mL surfactant solution (0.1% w/v) and 40 mL light paraffin oil at 25 °C. The time required for separating 10 and 20 mL of the aqueous phase solution from the emulsion was determined.25 Foaming properties were determined in terms of a method reported in one literature.26

(1)

3. RESULTS AND DISCUSSION 3.1. Thermogravimetric Analysis. To evaluate temperature scope of application, the thermal stability of CnOBSb (n = 12, 14, and 16) was determined through thermogravimetric analysis (TGA). The TGA curves of CnOBSb are shown in Figure 1. The results depict that the decomposition of CnOBSb begin at the temperature range of 243−245 °C. With the increase of temperature, these compounds quickly decompose until the weight remains unchangeable at about 600 °C. These findings indicate that C nOBSb possess relatively high decomposition temperature, and the application temperature of this phenyl-containing surfactant is up to 245 °C. Besides,

(2)

where R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature in K, C is the surfactant concentration in mmol/L, N is Avogadro’s number (6.022 × 1023 mol−1), and the value of n is assumed to be 1 for a zwitterionic surfactant.20 The standard free energy of micellization (ΔG0mic) and 0 adsorption (ΔGads ) can be calculated according to the following equations21 0 ΔGmic = RT ln Xcmc

Article

(3) C

DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Surface Acitivity Parameters of CnOBSb surfactants

cmc (mmol/L)

γcmc (mN/m)

106 Γcmc (mol/m2)

Acmc (nm2)

C12OBSb C14OBSb C16OBSb C12OBCb C14OBCb C16OBCb C12Sb C14Sb

0.0760 0.0161 0.0015 0.1881 0.0229 0.0023 3.389 0.708

30.14 29.78 28.32 27.3 26.7 25.6 32.30 29.89

2.12 2.18 2.30 2.63 2.66 2.76

0.78 0.76 0.72 0.63 0.62 0.60

between adjacent surfactant molecules.27 Furthermore, the cmc values of CnOBSb are slightly lower than those of CnOBCb with the same hydrocarbon chain length, which result from the solvation effects.28 On the other hand, the γcmc values of CnOBSb decrease with the increase of the hydrocarbon chain length. For C12OBSb and C14OBSb, the γcmc value is slightly lower than those of corresponding straight-chain sulfobetaine surfactants. The lowest surface tension obtained by C16OBSb is 28.32 mN/m. It is suggested that the phenyl-containing sulfobetaine surfactants can give greater efficiency in lowering the surface tension of water and orient themselves in a better conformation at the air/water interface. Obviously, the CnOBSb surfactants exhibit remarkable surface activity and may have potential application in some fields. 3.3. Adsorption Properties. The surface excess Γcmc is an effective parameter to measure the adsorption of surfactants at the air/water interface, whereas the Acmc value is an important reflection of packing density of surfactant. The values of Γcmc and Acmc are tabulated in Table 1. With the increase of hydrocarbon chain length, the value of Γcmc decreases, suggesting that CnOBSb with longer hydrophobic tails have greater preference to absorb at the air/water interface. In addition, the Acmc values are 0.78, 0.76, and 0.72 for C12OBSb, C14OBSb, and C16OBSb, respectively. The decrease of Acmc values with the increasing hydrocarbon chain length suggests that C16OBSb has the highest packing density at the air/water interface among these surfactants, which is attributed to the enhancement of hydrophobic interactions between the hydrocarbon chains. This result is also in accordance with conventional betaine amphoteric surfactants.15 Besides, the Acmc values of CnOBSb (n = 12, 14, and 16) are slightly larger in comparison with the CnOBCb surfactants having the same hydrophobic chain length,18 which is caused by the fact that the size of sulfonic group is larger than that of the carboxyl group.29 The pC20 value is an important parameter to illustrate the adsorption efficiency, whereas the cmc/C20 ratio represents the adsorption effectiveness.30 The relatively larger values of the pC20 and cmc/C20 ratio suggest that the surfactant is more prone to adsorb at the air/water interface rather than aggregate into micelles.31 As shown in Table 2, the pC20 values and cmc/ C20 ratio values of the CnOBSb increase as the hydrocarbon chain length increases from 12 to 16. It means that CnOBSb molecules with a longer hydrophobic chain have a higher inclination to adsorb at the air/water interface. The negative values of ΔG0mic and ΔG0ads suggest that the micellization and adsorption of CnOBSb in the bulk solution are spontaneous processes. According to the data shown in Table 2, ΔG0ads is more negative than ΔG0mic for CnOBSb, which

Figure 1. TGA curves of CnOBSb.

the starting temperatures of decompositions of CnOBSb are higher than that of corresponding phenyl-containing carboxybetaine surfactants CnOBCb.18 It reveals that sulfobetainetype surfactants are more stable than carboxybetaine-type surfactants in terms of TGA, and the decomposition of betaine-type surfactants initially occurred at the hydrophilic headgroups because of the same chemical structure of hydrophobic group. 3.2. Equilibrium Surface Tension. The surface tension of CnOBSb (n = 12, 14, and 16) versus surfactant concentration is plotted in Figure 2. The surface tension of CnOBSb initially

Figure 2. Surface tension vs surfactant concentration for CnOBSb at 25 °C.

decreases upon increasing surfactant concentration until an equilibrium value is obtained. The cmc values are regarded as the breakpoints of the curves. The cmc, γcmc, Γcmc, and Acmc values of CnOBSb are shown in Table 1 in comparison with those of straight-chain sulfobetaine surfactants C n Sb (CnH2n+1N+(CH3)2CH2CH(OH)CH2SO3−, n = 12 and 14)13 and CnOBCb (n = 12, 14, and 16).18 A conspicuous result from Table 1 illustrates that the cmc value of phenylcontaining sulfobetaine surfactants is in the range of 0.0015− 0.076 mmol/L and decreases with increasing the hydrocarbon chain length. In addition, the cmc value of CnOBSb can be 2 orders of magnitude lower than that of CnSb with the same hydrocarbon chain length. This means that the introduction of phenyl group in hydrophobic chain is beneficial to forming micelles in the solution. This phenomenon may result from the enhancement of hydrophobic interactions by phenyl groups D

DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

formation of micelles at the concentration of cmc, making pyrene molecules transfer into the hydrophobic region of micelles from the water. The determined values of cmc are 0.07, 0.01, and 0.001 for C12OBSb, C14OBSb, and C16OBSb, respectively, which are in corroboration with the surface tension results. Thereafter, the I1/I3 ratios almost reach constant values. Furthermore, the I1/I3 ratio can also reflect the compactness of aggregates.33 The relatively high values of I1/I3 ratios indicate that CnOBSb surfactants form loose aggregates in aqueous solution. The loose aggregate structures of CnOBSb may be associated with the introduction of bulky phenyl group. The large phenyl group may lead to the increase of steric hindrance on the process of aggregation and loose packing arrangement. 3.5. Aggregate Morphology. DLS measurement was performed to determine the size of aggregates for CnOBSb solution at 25 °C. Figure 4 shows the apparent hydrodynamic diameters (Dh) for CnOBSb (n = 12, 14, and 16) at the concentration of 2× cmc and 5× cmc. Only one major size distribution (100−400 nm) was observed for CnOBSb at each concentration, suggesting that CnOBSb molecules can spontaneously aggregate to form large vesicles. This presence of vesicles can be interpreted by the theory of the surfactant packing parameter, P = V/(a0lc), where V is the volume occupied by the hydrocarbon tail, a0 is the optimal headgroup area, and lc is the critical chain length of the tail.34,35 A reasonable explanation is that the presence of a bulky phenyl group in the hydrophobic chain may dramatically increase the hydrophobic volume (V). It is more favorable for CnOBSb to form vesicles by means of increasing packing parameter. Furthermore, The Dh values of vesicles of CnOBSb (n = 12, 14,

Table 2. Micellization and Adsorption Properties of CnOBSb surfactants

pC20

cmc/C20

ΔG0mic (kJ mol−1)

ΔG0ads (kJ mol−1)

C12OBSb C14OBSb C16OBSb C12OBCb C14OBCb C16OBCb

5.62 6.33 7.38 5.13 6.06 7.07

32.9 34.8 36.2 25.4 26.3 27.1

−33.45 −37.29 −43.17 −31.20 −36.42 −42.12

−51.44 −55.26 −60.58 −46.82 −52.03 −57.60

indicates that surfactant molecules are more inclined to adsorb at the interface in comparison with its tendency to form micelles. This result is supported by the relatively large values of the pC20 and cmc/C20 ratio. In addition, the values of both ΔG0ads and ΔG0mic increase with the increase of the hydrocarbon chain length; it is suggested that the longer hydrophobic chain length is beneficial to the enhancement of driving forces for micellization and adsorption. 3.4. Micropolarity. The micellization properties of CnOBSb (n = 12, 14, and 16) were further studied by steady-state fluorescence measurements. The I1/I3 ratio is sensitive to the change in micropolarity of environment where pyrene molecule is immersed.32 The variations of the I1/I3 ratio with the CnOBSb concentration are presented in Figure 3. First, the values of I1/I3 ratios of these three surfactants changed slightly around 1.8 at concentrations below the cmc, reflecting that the surfactant molecule exists in aqueous solution as a monomer. With further increase of the surfactant concentration up to the cmc, the I1/I3 ratios begin to drop sharply for all surfactants. This phenomenon results from the

Figure 3. Fluorescence spectrum of C12OBSb (a); C14OBSb (b); C16OBSb (c), and effects of surfactant concentration on I1/I3 ratio for CnOBSb at 25 °C (d). E

DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Left: Effects of surfactant concentration on size distribution by DLS for (a) C12OBSb, (b) C14OBSb, and (c) C16OBSb. Right: Cryo-TEM images of vesicles formed at 5× cmc for (A) C12OBSb, (B) C14OBSb, and (C) C16OBSb.

to investigate the interfacial behavior of CnOBSb at the oil/ water interface, sets of IFT measurements were carried out to study the influencing factors including the surfactant structure, surfactant concentration, salinity, and temperature. The IFT as a function of the CnOBSb concentration against Xinjiang crude oil is illustrated in Figure 5a−c. The CnOBSb surfactants exhibit similar trends against Xinjiang crude oil. Namely, the dynamic IFT reduction was faster and greater with increasing surfactant concentration ranging from 0.01 to 0.2 mmol/L. This is due to the fact that the elevated concentration is helpful to increase the surfactants’ adsorption and decrease the adsorption time at the oil/water interface. Additionally, as the testing time goes on, the IFT almost initially decreases and thereafter increases until reaching a stable value for each sulfobetaine surfactant. This phenomenon results from the surfactant molecules’ dynamic equilibrium between adsorption and desorption at the oil/water interface.36 Furthermore, it is noteworthy that the IFT of C12OBSb against Xinjiang crude oil can reach an ultralow value at the surfactant concentration of 0.1−0.2 mmol/L, which means that C12OBSb may be a good candidate for application in EOR.

and 16) slightly increase with increasing surfactant concentrations, which indicates that the aggregation ability of CnOBSb is gradually enhanced at a higher surfactant concentration. This phenomenon may be attributed to the enhancement of hydrophobic interaction imparted by phenyl groups in adjacent surfactant molecules. The morphology of the self-assemblies for CnOBSb was observed at the concentration of 5× cmc. As shown in Figure 4, spherical vesicles with the diameter ranging from 100 to 400 nm are observed in solution for these three surfactants. The diameters of spherical vesicles are broadly consistent with the results of DLS measurements. In terms of DLS and cryo-TEM results, they suggest that the formation of vesicles occurs at extremely low concentrations above the cmc. Such vesicles spontaneously formed in aqueous solution may have potential application in model cell membranes and drug delivery systems. 3.6. IFT Measurements. The IFT value between surfactant solution and crude oil is an essential parameter for application of the surfactant in enhanced oil recovery (EOR). It is well known that an ultralow value of IFT ( C14OBSb > C12OBSb, suggesting that the emulsion stability of CnOBSb increases with the increase of hydrocarbon chain length. This is due to the enhancement of the hydrophobicity of surfactant, which leads to more solubilization of hydrophobic tail in the oleic phase.25 With regard to foaming properties, C12OBSb exhibits better foaming ability than C14OBSb and C16OBSb. It can be inferred that the relatively long hydrocarbon chain makes a slower migration of surfactant molecules to the air/water interface, leading to lower foaming ability with increasing hydrocarbon chain length. These results indicate that CnOBSb may be suitable to be used as a foaming agent and emulsifier in different industrial processes.

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4. CONCLUSIONS In summary, three kinds of phenyl-containing sulfobetaine surfactants 3-((4-(alkoxy)-3,5-dimethylbenzyl)dimethylammonio)-2-hydroxy-propane-1-sulfonate were synthesized and characterized by ESI−HRMS, 1H NMR, and 13C NMR spectrometry. Their thermal properties showed that all these three surfactants exhibit high thermal stability and are difficult to decompose below 245 °C. The results of surface tension indicated that the cmc and γcmc of CnOBSb (n = 12, 14, I

DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.9b01907 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX