Solubility of Nonionic Hydrocarbon Surfactants ... - ACS Publications

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Solubility of Nonionic Hydrocarbon Surfactants with Different Hydrophobic Tails in Supercritical CO2 Qingzhao Shi,† Lishuai Jing,‡ Chongqiao Xiong,§ Chenyu Liu,† and Weihong Qiao*,† †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Tianjin Fire Research Institute of MPS, Tianjin 300000, People’s Republic of China § College of Letters and Science, University of California, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The experiment utilizes a high-pressure view chamber that mainly aims at identifying the effect of surfactant architecture on solubility in CO2. First, when solubility pressures of three series of nonionic hydrocarbon surfactants are measured, results reveal that surfactants with a methylated tail exhibit the best solubility. Unexpectedly, surfactants with a branched tail show worse solubility than linear surfactants. This is conjectured to be caused by attractive molecule−molecule interactions. TX45 that possesses the shortest ethylene oxide chain and a highly methylated tail performs the best in CO2 solubility. Additionally, the experimental study on the solubility of TX45 in CO2 at T = (318 to 343) K demonstrates that solubility has a direct correlation with pressure and CO2 density, while it is inversely related to temperature. Finally, when we look at effects of the added alcohols on surfactant TX45 solubility, the results indicate that hexanol is the most effective alcohol in enhancing solubility. The reason is assumed to be that hexanol molecules could insert themselves between surfactant tails and hinder molecule−molecule interactions.

1. INTRODUCTION CO2 becomes more and more noteworthy as an environmentally benign solvent for its numerous attractive characteristics: nontoxic, nonflammable, low viscosity, easily removed, and recyclable. It also has moderate critical constants (Tc = 31.1 °C; Pc = 7.38 MPa). Therefore, it is widely used in a variety of fields, including extraction,1 nanotechnology,2 enhanced oil recovery technologies,3 and polymer processing,4 etc. However, not being able to dissolve compounds with high molecular weight or hydrophilic molecules, such as proteins, asphaltene, colloids, and many other polymers, due to its extremely low dielectric constant and polarizability, limits its applications.5 One potential way to overcome the limitation is to design CO2-philic surfactants.6−8 These CO2-philic surfactants have two mutually incompatible components: a CO2-philic tail with an affinity for CO2 molecules and a CO2-phobic head with repulsion for CO2 molecules.9 It is worthwhile noting that the excellent solubility of surfactants in CO2 is significantly important. However, previous studies indicate that dissolution of surfactants in CO2 is much more limited than in compressed alkanes, such as ethane and propane.10−12 Lacking solvent strength prevents direct solubilization of hydrophile compounds which is unfavorable to solubility of conventional surfactants. To solve this problem, new kinds of surfactants endowed with enhanced solubility are designed. Most CO2-philic surfactants tend to be silicone13,14 and fluorocarbon surfactants.15−20 Considering the surfactants’ high cost, complex production process, and environmental issues they bring about (they are difficult to degrade), researchers are paying more and more © XXXX American Chemical Society

attentions to the economically viable hydrocarbon surfactants.21−28 Consani and Smith performed lots of experiments where solubilities of over 130 commercially available surfactants and related molecules in carbon dioxide are presented.12 All of them turn out to be insoluble or slightly soluble in CO2. Surprisingly, Eastoe et al. find that anionic hydrocarbon surfactant with a high degree of methylated chain boosts its solubility in CO2.29 Moreover, Eastoe et al. synthesized bichain and trichain hydrocarbon surfactants loaded with terminal methyl groups. Surfactants’ compatibility with scCO2 is enhanced dramatically.30−32 Studies find that, although regular surfactants show poor solubility in scCO2, addition of a cosolvent or cosurfactants could boost their solubility in scCO2.22,25,33,34 However, debates in the literature about the exact reason for the effect of alcohols on solubility enhancement have never stopped. Most authors regard alcohol as a cosurfactant that concentrates in the surfactant layer. Some studies suggest that alcohol molecules function by placing themselves between surfactant tails, reducing tail−tail and micelle−micelle interactions, while others argue that alcohols act on the interface and reduce the interfacial tension, hence promoting the formation of stable reverse micelles.35 One of our objectives is to compare the solubility of the nonionic hydrocarbon surfactants with various tail architectures and head groups in CO2, thus to investigate effects of tail and Received: April 15, 2015 Accepted: July 10, 2015

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DOI: 10.1021/acs.jced.5b00345 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Scheme 1. Molecular Structures and Abbreviations of the Studied Surfactants

Figure 1. Schematic diagram of experimental apparatus for solubility pressure measurement: 1, carbon dioxide cylinder; 2, cold trap; 3, highpressure syringe pump; 4, stirrer and temperature controller; 5, highpressure view cell with sapphire windows; 6, magnetic stirrer; 7, outlet valve.

are described later. Before each solubility test, acetone cleans the cell and then it is dried. The desired amount (about 0.06 g, weighed by an analytical balance which has an accuracy of 0.001 g) of surfactant is first added to the cell. To discharge air in the system, we open the CO2 entrance gently to let in some CO2 (gas) first and then close the entrance and open the relief port. The preceding procedure is repeated three times to make sure air is completely out of the system. The temperature is set at 343 K. When the temperature is stable, liquid CO2 is slowly injected into the cell. The pumping of CO2 is stopped, and observation is made to determine whether the surfactant is completely dissolved for every 1 MPa increase in pressure, until the system becomes homogeneous and transparent. During the pumping process, the stirrer is turned on to ensure good miscibility. Stirring is stopped; after about 10 min, the temperature and pressure of the system are stable. The pressure at this point is solubility pressure. The temperature is set to 338 K; no more CO2 is pumped into the system during the temperature decreasing process, and in these conditions, some surfactant would dissolve out of the system. The preceding CO2 pumping procedure is repeated until the solubility pressure at 338 K is stabilized. Solubility pressures at other temperatures are tested in the same way. The solubility of every surfactant is tested three times in each condition. The average solubility pressure value is taken as the final solubility pressure. This procedure for solubility test avoids the CO2 releasing process, which makes the result more accurate. The procedures for measuring the solubility pressure of surfactants/alcohol in CO2 are similar. We refer to our previous works for accuracy and repeatability of apparatuses and the solubility pressure test.37 The density of CO2 is obtained from the NIST (National Institute of Standards and Technology) fluid property database. Here, the solubility refers to the maximum mass percentage of the dissolved surfactant in CO2 at a certain temperature. It is calculated by the following equation:

head groups on the solubility. These hydrocarbon surfactants, shown in Scheme 1, are selected because of their low cost (about RMB15000/t (RMB = renmimbi); Haisen Chemical Co., Ltd., Shijiazhuang, China; year, 2014) and differences in tails, which are highly methylated, bichain, and linear, respectively. This work also explores the effect of temperature, pressure, CO2 density, and addition of different alcohols (ethanol, butanol, hexanol, octanol, and decanol) on the solubility of surfactant in scCO2.

2. EXPERIMENTAL SECTION 2.1. Materials. Carbon dioxide with a mass purity greater than 99.5% is purchased from Dalian Gas Co. Ltd. TX45 and TX100 (mass fraction purity > 99%) are purchased from SigmaAldrich. Guerbet alkyl polyoxyethylene ethers (mass fraction purity > 99%) and linear alkyl polyoxyethylene ethers (mass fraction purity > 99%) are prepared from Guerbet alkyl alcohol (Jarchem Industries, Inc., Newark, NJ, USA) at Liaoning Oxiranchem Co., Ltd., Liaoyang, China.36 Mass spectra of the surfactants are listed in our previous work.36 A series of Guerbet alkyl polyoxyethylene ethers use the abbreviation GCm(n), where m represents hydrocarbon chain length (Scheme 1). Cm(n) denotes linear alkyl polyoxyethylene ethers. In Scheme 1, the subscripts n represent the average number of the repeat units (EO) per molecule. The nonionic surfactant is placed in a vacuum drier at 60 °C for 24 h to remove moisture before each test. Ethanol, butanol, hexanol, octanol, and decanol all are AR grade (mass fraction purity > 99.5%) and obtained from Tianjin Fuyu Fine Chemical Co., Ltd., Tianjin, China. Refractive indices and densities of the alcohols are tested; results are listed in the Supporting Information. All of the alcohols are dehydrated by molecular sieves before use. 2.2. Apparatus. A high-pressure stainless steel view cell consisting of two in-line sapphire windows is utilized to investigate the solubility of surfactants in CO2. The volume of the cell is 55 cm3. The apparatus shown in Figure 1 mainly consists of a high-pressure view cell, a heating system, a magnetic stirrer, a high-pressure syringe pump (2J-X6.4/32, Hangzhou Zhijiang Petrochemical Equipment Co., Hangzhou, China), and a gas cylinder. Temperature of the high-pressure cell is controlled by a temperature controller and measured by a thermocouple with ± 1 K accuracy. Pressure of the system is measured by a pressure gauge with ± 0.001 MPa accuracy. A rotation controller with maximum 1200 rpm speed is installed under the cell. 2.3. Procedures for the Solubility Pressure Test. The procedures for measuring solubility pressure of surfactant in CO2

x = [m /(ρV + m)] × 100%

(1)

−3

where ρ (g·cm ) is the density of CO2, m (g) is the added weight of the surfactant, and V (mL) is the volume of the cell.

3. RESULTS AND DISCUSSION 3.1. Solubility of Nonionic Surfactants in scCO2. At constant temperature, the solution becomes turbid with decreasing pressure (Figure 2). During the depressurization process, CO2−tail interactions become significantly weaker than tail−tail interactions of the surfactant, leading to intensified attraction among molecules. Approaching molecules generate tail−tail overlaps and an enlarged area of contact, thus increasing the opportunity of precipitation.38 B



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surfactant. For surfactants having the same EO number, their solubility follows the law that an increase in hydrocarbon chain length decreases the solubility of the surfactants. This results from the higher molecular weight and the increased tail−tail interactions between surfactants. Surfactants dissolve in CO2 in the form of free monomers at low concentration (Figure 4).When concentration of the surfactant is high, molecular distance becomes small and tails of the surfactants have a tendency to approach each other because of a similar polarity (Figure 5). Therefore, tail solvation of monomer and interactions between molecular tails greatly influence the solubility. According to our experiment, cloud point pressures of 0.1 mol of benzene and 0.1 mol of n-hexane at 333 K are (10.183 and 9.355) MPa, respectively, which means that benzene ring compared with alkane chain would affect negatively the solubility. Despite the fact that benzene ring exists in the molecule through tert-butyl surfactant TX45 which possesses highly methyl-branched tails, shorter ethylene oxide chain and lower molecular weight exhibit the best affinity for CO2 with 12.394 MPa solubility pressure at 318 K. Methylated tails (tertbutyl) are much bulkier than tails of linear and bichain surfactants. Thus, this kind of CO2-philic molecules arranges at the interface closely, shown in Figure 4a. The tight and stubby nature of the tails minimizes the contact area between molecules, shown in Figure 5a, resulting in weaker intermolecular interactions. Furthermore, the high density of cohesive energy methyl greatly raises the solvation on tails of Triton surfactant molecules. Therefore, triton surfactants induce excellent solubility. Interestingly, Guerbet alkyl polyoxyethylene ether surfactants with two asymmetric hydrocarbon chains show inferior solubility. On one hand, the solvation degree of different tail structures is methylated tail (tert-butyl) > branched tail > linear tail, shown in Figure 4. Compared with the linear tail, a branching tail has greater hydrophobicity and compatibility with CO2. On the other hand, when CO2 solution is nearly saturated by the three series of surfactants and their concentration is the same, a decrease in the solvation of CO2 by changing the external conditions (P, T, and so on) of the system implies that the dissolved molecules of surfactant tend to come closer to each other and dissolve out of the CO2 solution. Branched surfactant (GC series) molecules pack loosely when approaching, as shown in Figure 5b, resulting in greater penetration of the tails between molecules. Bigger overlapping space between surfactant tails produces more attractive interaction, which makes solubility of the branched surfactant the worst. The attractive interactions between molecules follow the order of branched tail > linear tail > methylated tail (tert-butyl), as shown in Figure 5. Here, interactions between molecular tails are the leading factor in comparison with the solvation; that is the reason why Guerbet surfactants generate phase separation easily. Surface tensions of aqueous solutions of GC16(5) and C16(5) at 298 K are tested to investigate the difference in space occupied by their surfactant tails and support the solubility results. Surface tension results of GC16(5) and C16(5) at 298 K are listed in the Supporting Information. Absorbing capacity (Γm) in the air/ water interface and area occupied by one surfactant molecule (Amin) are calculated with the surface tension data by the following equations:

Figure 2. Images of dissolving behavior of TX45 in CO2 at 318 K: a, TX45 not completely dissolved; b, TX45 completely dissolved at cloud point pressure.

Figure 3. Solubility pressure as a function of temperature for different nonionic surfactants in CO2: (a) ■, GC16(3); ●, GC16(5); ▲, GC16(7); ▼, GC18(5); ◀, GC20(5). (b) ■, C14(5); ●, C16(3); ▲, C16(5); ▼, C18(3); ◀, C18(5). (c) ■, TX 45; ●, TX 100.

Solubility pressures for different nonionic surfactants as a function of temperature are shown in Figure 3 and Table 1. From Table 1, we can see that, for the same series of surfactants with equal hydrophobic chain length, an increase in the number of ethylene oxide groups greatly increases the solubility pressure in CO2, which means a reduction in the solubility of

Γm = − C

1 ⎛ dγ ⎞ ⎜ ⎟ RT ⎝ d log C ⎠

(2) DOI: 10.1021/acs.jced.5b00345 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 1. Experimental Solubility Pressures and Mass Fraction Solubilities of Surfactants in CO2 at Temperature Ta solute (liquid)

T/K

P/MPa

x/(wt %)

GC16(3)

318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343

13.966 14.792 15.800 16.882 17.872 18.516 15.036 15.644 16.568 17.582 18.387 19.349 15.682 16.659 17.840 19.203 20.047 21.140 14.735 15.599 16.612 17.669 18.408 19.369 15.173 15.997 16.938 17.784 18.640 19.845 13.092 14.449 15.604 16.390 17.101 18.010

0.154 0.159 0.164 0.167 0.171 0.177 0.151 0.157 0.163 0.165 0.170 0.174 0.147 0.150 0.154 0.156 0.160 0.162 0.152 0.157 0.162 0.165 0.170 0.174 0.148 0.153 0.157 0.162 0.166 0.168 0.160 0.163 0.167 0.172 0.178 0.182

GC16(5)

GC16(7)

GC18(5)

GC20(5)

C14(5)

a

solute (liquid) C16(3)

C16(5)

C18(3)

C18(5)

TX45

TX100

T/K

P/MPa

x/(wt %)

318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343 318 323 328 333 338 343

11.872 13.171 14.290 15.186 16.200 16.870 13.322 14.701 15.933 16.748 17.554 18.432 12.532 13.658 14.833 16.140 17.086 18.163 13.773 14.910 15.830 16.816 17.732 18.507 12.394 13.374 14.366 15.129 16.242 17.009 18.395 19.781 21.022 22.217 23.229 24.311

0.170 0.172 0.176 0.182 0.185 0.192 0.158 0.160 0.164 0.169 0.174 0.178 0.170 0.174 0.177 0.179 0.184 0.187 0.154 0.158 0.162 0.167 0.171 0.176 0.163 0.169 0.174 0.180 0.183 0.188 0.138 0.140 0.143 0.145 0.148 0.150

Standard uncertainties u are u(T)=1K,u(P)=0.170 MPa,ur(x)=0.003.

Figure 4. Solvation of the surfactant molecules in scCO2: a, TX45; b, GC16(3); c, C16(3). Degree of solvation: a (methylated tail) > b (branched tail) > c (linear tail).

A min =

1014 N Γm

The surface tension results are listed in the Supporting Information, and the corresponding critical micelle concentration (cmc), γcmc, Γm, and Amin values are shown in Table 2. From the results, Amin of GC16(5) is two times that of C16(5); this means branched-tail molecule GC16(5) takes up a larger space than linear-tail molecule C16(5). Large taken-up space

(3)

where R is 8.314 J·mol−1·K−1, T (K) stands for absolute temperature, γ (mN·m−1) is the surface tension, C (mol·L−1) is the concentration of the solution, and N = 6.02 × 1023. D



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Table 3. Mass Fraction Solubility of TX45 and Apparent Density of CO2 at Different Pressures and Temperaturesa

Figure 5. Interactions among molecules in scCO2: a, TX45; b, GC16(3); c, C16(3). Attractive interactions: b (branched tail) > c (linear tail) > a (methylated tail).

T

P/MPa

density/(g·cm−3)

x/(wt %)

318 K

14.093 14.404 14.756 15.732 16.372 15.210 15.612 15.925 17.736 18.312 16.001 16.570 17.400 18.379 19.564 17.117 17.741 18.301 19.111 20.204 18.155 18.608 19.277 20.136 21.372 19.170 19.710 20.312 21.209 22.400

0.724 0.731 0.738 0.756 0.767 0.706 0.715 0.721 0.754 0.762 0.682 0.696 0.713 0.731 0.749 0.669 0.683 0.694 0.710 0.728 0.655 0.665 0.679 0.695 0.716 0.642 0.654 0.666 0.683 0.702

0.393 0.454 0.517 0.673 0.846 0.403 0.464 0.528 0.676 0.851 0.416 0.477 0.535 0.697 0.866 0.425 0.486 0.549 0.718 0.892 0.434 0.499 0.562 0.732 0.907 0.442 0.507 0.572 0.746 0.924

323 K

Table 2. Critical Micelle Concentration (cmc), γcmc, Γm, and Amin of GC16(5) and C16(5) at 298 K γcmc

cmc −1

Γm −1

surfactant

(mol·L )

(mN·m )

GC16(5) C16(5)

8.31 × 10−6 3.25 × 10−4

29.65 32.70

(10

−10

Amin −2

mol·cm )

nm2

7.54 3.80

0.44 0.22

328 K

333 K

would benefit the packing between molecules which would reduce the solubility. The results of Amin and those of solubility pressure agreed. 3.2. Effect of Temperature, Pressure, and Density of CO2 on the Solubility. To investigate the role of temperature, pressure, and density of CO2 on solubility, the solubility pressures of different amounts of TX45 are measured at T= (318 to 343) K. The solubility of TX45 in scCO2, as well as the corresponding apparent density of CO2 (grams of CO2 per milliliter of fluid) at various temperature and pressure conditions, is shown in Table 3. In Figure 6, the solubility of TX45 is plotted as a function of pressure at different temperatures. As expected, the solubility of TX45 increases linearly with pressure at every temperature point. This can be explained by the fact that the force generated by pressure squeezes the molecules of the surfactants into the CO2 phase, which contributes to the solubility. This figure also shows that temperature affects the solubility negatively when pressure remains unchanged. This is because higher temperature leads to the decrease in density as well as the solvent strength of CO2. At constant pressure, density and solvent power of CO2 decrease with increasing temperature. These decreases are unfavorable for CO2 to dissolve surfactants. Figure 7 illustrates the dependency of solubility on the apparent density of CO2. Owing to the fact that solvent power increases with CO2 density at constant temperature, the higher the solvent power, the easier it is for the surfactants to dissolve in CO2. When the apparent density of CO2 remains unchanged, the solubility of TX45 and temperature have a positive correlation. Chrastil first developed a model to correlate the solute solubility in scCO2 with the density of CO2, and this model is now commonly used by other researchers.39 The equation is as follows: ln S = k ln ρ + m /T + n

338 K

343 K

a

Standard uncertainties u are u(T)=1K,u(P)=0.150 MPa,ur(x)=0.002.

Figure 6. Effect of pressure on the solubility of TX45 in scCO2 at various temperatures: ■, 318 K; ●, 323 K; ▲, 328 K; ▼, 333 K; ◀, 338 K; ▶, 343 K.

(4)

data-fitting line (shown in Figure 8) from eq 4 gives the corresponding k, m, and n values which are 11.30, −179732, and 475.57, respectively. 3.3. Effect of Alcohols Addition on the Solubility of Surfactants. Constant concentrations (0.1557 mol/L) of different alcohols, including ethanol, butanol, hexanol, octanol, and decanol, are added and mixed with a constant mass of TX45 (0.1570 g) to investigate the effects of alcohol addition on the solubility of surfactants. The concentration of the added alcohol

−3

where ρ (kg·m ) is the density of CO2, and T (K) is the system temperature; k, m, and n are characteristic constants; S (kg·m−3) is the solubility of the solute, which can be calculated with the weight of surfactant and volume of the cell. According to Chrastil’s semiempirical equation, there is a linear relationship between solvent density and the solute’s solubility. Solubility results of TX45 are correlated with the Chrastil model. The corresponding ln ρ−ln S are shown in Figure 8, and a E



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Figure 9. Effect of alcohols on the solubility pressure of surfactants in CO2: ■, CO2 only; ●, ethanol + CO2; ▲, butanol + CO2; ▼, hexanol + CO2; ◀, octanol + CO2; ▶, decanol + CO2.

Figure 7. Effect of apparent density of CO2 on the solubility of TX45 in scCO2 at various temperatures: ■, 318 K; ●, 323 K; ▲, 328 K; ▼, 333 K ◀, 338 K; ▶, 343 K.

Table 4. Effect of Alcohols on the Solubility Pressure of Surfactants in CO2 at Temperature Ta P/MPa

a

alcohols

318 K

323 K

328 K

333 K

338 K

343 K

blank ethanol butanol hexanol octanol decanol

14.093 13.880 13.081 12.431 12.596 13.454

15.210 14.822 14.483 14.152 14.295 14.563

16.001 15.882 15.756 14.984 15.168 15.585

17.117 16.954 16.723 16.116 16.162 16.371

18.155 17.829 17.657 17.152 17.238 17.526

19.170 18.712 18.566 18.205 18.385 18.507

Standard uncertainties u are u(T)=1K,u(P)=0.050 MPa.

Figure 8. Logarithmic relationships between S and the density of pure CO2 (the points are data from experiment, and the lines are data fitted by Chrastil model): ■, 318 K; ●, 323 K; ▲, 328 K; ▼, 333 K.

interact with surfactant tails closely and arrange steadily between tails. It seems that longer and shorter alcohols both produce inferior effect compared with a middle-length alcohol. Results indicate that middle alcohols are the best solubility enhancer in our experiment.

is varied in a series of experiments, which exhibit that too much added alcohol would not dissolve well in the CO2. If alcohol is added in small proportion, the effect on solubility will not be obvious. Figure 9 and Table 4 show the solubility pressures of TX45 in CO2 at varying temperatures with different alcohols added. As shown in Figure 9, with the addition of alcohol, the pressure still shows a growing tendency with increasing temperature. The addition of alcohols appears to have a positive effect on the solubility. At every constant temperature, the efficiency to decrease the solubility pressures follows the order hexanol > octanol > butanol > decanol > ethanol. The results reveal an optimum performance of the alcohol with a chain containing six carbon atoms, i.e., hexanol. In terms of solvation, it is noted that short alcohols have stronger polarity for improving the solvent quality of CO2 compared with middle and long alcohol molecules. In contrast, when alcohol acts as a cosurfactant, they insert themselves between surfactant tails and interact with tails of surfactant tightly due to a similar middle chain length. Consequently, the middle alcohol molecules show better ability to reduce the tail−tail and molecule−molecule interactions and may alter the properties of molecules directly.33,35 The longer alcohol (decanol) molecules have minor effects on the properties of the molecule, because the longer alcohol cannot effectively be randomly between surfactant tails. Usually, long-chain alcohol is considered as a cosolvent. Shorter alcohols, such as ethanol, may not

4. CONCLUSION In this work, solubilities of three series of economically viable nonionic hydrocarbon surfactants (which possessed tert-butyl, bichain, or linear tails) in scCO2 are systematically investigated. The results surprisingly show that Guerbet surfactants with branched chains provide the worst solubility due to the largest attractive molecule−molecule interactions. The solubility of the three series surfactant follows the order methylated tert-butyl tail > linear tail > branched bichain tail. An increase in length of ethylene oxide chains and an increase in length of alkyl chains both have negative effects on solubility. TX45 possessing the shortest ethylene oxide and alkyl chains, as well as a tert-butyl tail, performs the best in solubility tests. Experimental study on solubility of TX45 in CO2 at T = (318 to 343) K demonstrates that its solubility has a direct relation with the pressure and CO2 density, while it has an inverse relation with temperature. Finally, solubility tests of TX45 at 318 K with different alcohols indicate that alcohol with middle chain length (hexanol) is the most effective one to enhance the solubility. We speculate that the reason is hexanol molecules could insert themselves between surfactant tails and hinder molecule−molecule interactions more effectively than the other alcohols. We expect that these principles will be helpful, whether in designing economically viable hydrocarbon surfactants or in choosing cosolvent or cosurfactant to enhance solubility of surfactants in CO2. F



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ASSOCIATED CONTENT

S Supporting Information *

Refractive indices and densities of used alcohols and surface tensions of GC16(5) and C16(5) solutions. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00345.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 41184986232. Author Contributions

The manuscript was written through contributions of all authors. Initial conception, design, provision of resources, collection of data, analysis, interpretation of data and manuscript writing were completed by Q.S., L.J., C.L. Prof. W.Q. provided theoretical guidance; C.X. made substantial contributions to writing the paper. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Funding

The authors greatly appreciate the support (Grant DQYT0508003-2012-JS-362) by Petro China Daqing Oilfield Co., Ltd.

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REFERENCES

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