Effect of NaCl, NaOH, and Poly(ethylene oxide) on Methane

1640

Langmuir 1999, 15, 1640-1643

Effect of NaCl, NaOH, and Poly(ethylene oxide) on Methane Solubilization in Sodium Dodecyl Sulfate Solutions Mingtan Hai,† Buxing Han,‡,* Guanying Yang,‡ Haike Yan,‡ and Qiyong Han† Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Department of Physicochemistry, University of Science and Technology of Beijing, Beijing 100083, People’s Republic of China Received May 28, 1998. In Final Form: October 28, 1998 Solubility of methane in aqueous solutions of sodium dodecyl sulfate (SDS), SDS + 0.1 M NaCl, SDS + 0.1 M NaOH, SDS + 0.1 wt % poly(ethylene oxide) (PEO), SDS + 0.1 wt % PEO + 0.1 M NaCl, and SDS + 0.1 wt % PEO + 0.1 M NaOH have been determined at 298.15, 308.15, and 318.15 K. The molality of SDS (mSDS) is up to 0.050 mol/kg. The solubility decreases with increasing temperature and increases linearly with concentrations of SDS above the critical micelle concentration (CMC) or critical aggregation concentration(CAC) of the surfactant, indicating that micelles in the solutions solubilize the gas molecules. It was found that the solubilization ability of the micelles bound to PEO and the free micelles is the same. The solubilization property of SDS is changed by the addition of PEO, although the solubilizing effect of the polymer alone is not considerable. NaCl and NaOH also affect the solubilization noticeably. The standard Gibbs energies for the transfer of methane from bulk solutions to the micelles are large negative values, indicating that the hydrophobic gas prefers to exist in the hydrophobic interior of the micelles.

Introduction Studies of surfactant/polymer solutions are of great importance to both pure and applied sciences, and the properties of this kind of systems have been studied extensively using different techniques.1-15 One of the features of surfactant solutions is that organic substances normally insoluble in water show enhanced solubility in aqueous solutions of surfactants. This is commonly known as solubilization. Solubilization of organic solids and liquids in surfactant solutions plays an important role in many industrial processes, and a lot of work has been done on this subject. The study of the solubilization of nonpolar gases in surfactant solutions is not as important as those of solids and liquids for industrial processes. However, using this tool to study the properties of micelle solutions has two advantages. First, because a nonpolar gas has low solubility in both water and micelles and is relatively small, the effect of the dissolved gas on the properties of the micelles and the bulk solution is not as great as that of large molecules. Second, a liquid or solid can be dispersed * To whom the correspondence should be addressed. † University of Science and Technology of Beijing. ‡ Chinese Academy of Sciences. (1) Minatti, E.; Zanette, D. Colloids Surf., A 1996, 113, 237. (2) Goddard, E. D. Colloids Surf. 1986, 19, 25. (3) Thuresson, K.; Nilsson, S.; Lindman, B. Langmuir 1996, 12, 530. (4) Shimizu, T. Colloids Surf. 1995, 94, 115. (5) Lissi, E. A.; Abuin, E. J. J. Colloid Interface Sci. 1985, 105, 1. (6) Dubin, P. L.; Gruber, J. H.; Xia, J.; Zhang, H. J. Colloid Interface Sci. 1992, 148, 35. (7) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (8) Xia, J.; Dubin, P. L.; Kim, Y. J. Phys. Chem. 1992, 96, 6805. (9) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985, 89, 41. (10) Murata, M.; Arai, H.; J. Colloid Interface Sci. 1973, 44, 475. (11) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J. V. Langmuir 1992, 8, 2405. (12) Brown, W.; Fundin, J.; Mgnel, M. G. Macromolecules 1992, 25, 7192. (13) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996, 12, 2446. (14) Olofesson, G.; Wang, G. Pure Appl. Chem. 1994, 98, 603. (15) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967.

by the formation of an emulsion. Because the solutes in a system can exist in three formsssolubilized, emulsified, and dissolvedsthis makes determining the solubilization properties more difficult as the amount of solute emulsified cannot be measured accurately. Because a gas cannot be emulsified and can only be dissolved in bulk solutions and in micelles, the advantage of our system is clear. The properties of different surfactants for solubilizing nonpolar gases in the absence of polymers16-29 have been studied. Recently, King30 gave a review on this topic. In this work, the solubility of methane in aqueous solutions of SDS, SDS + 0.1 M NaCl, SDS + 0.1 M NaOH, SDS + 0.1 wt % PEO, SDS + 0.1 wt % PEO + 0.1 M NaCl, and SDS + 0.1 wt % PEO + 0.1 M NaOH have been determined at 298.15, 308.15, and 318.15 K. The solubilization properties and the related thermodynamic properties are studied. (16) Matheson, I. B. C.; King, A. D., Jr. J. Colloid Interface Sci. 1978, 66, 464. (17) Wishnia, A. J. Phys. Chem. 1963, 67, 2079. (18) Li, P. X.; Han, B. X.; Yan, H. K.; Liu, R. L. J.Colloid Interface Sci. 1995, 175, 57. (19) Ben-Naim, A.; Battino, R. J. Solution Chem. 1985, 14, 245. (20) Ownby, D. W.; King, A. D., Jr. J. Colloid Interface Sci. 1984, 101, 271. (21) Bolden, P. L.; Hoskins, J. C.; King, A. D., Jr. J. Colloid Interface Sci. 1983, 91, 454. (22) Ben-Naim, A.; Wilf, J. J. Solution Chem. 1983, 12, 267. (23) Ben-Naim, A.; Wilf, J. J. Solution Chem. 1983, 12, 861. (24) Prapaitrakul, W.; King, A. D., Jr. J. Colloid Interface Sci. 1985, 106, 186. (25) Prapaitrakul, W.; King, A. D., Jr. J. Colloid Interface Sci. 1986, 112, 387. (26) Prapaitrakul, W.; Shwikhat, A.; King, A. D., Jr. J. Colloid Interface Sci. 1987, 115, 443. (27) Prapaitrakul, W.; King, A. D., Jr. J. Colloid Interface Sci. 1987, 118, 224. (28) Nugara, N.; Prapaitrakul, W.; King, A. D., Jr. J. Colloid Interface Sci. 1987, 120, 118. (29) Ownby, D. W.; Prapaitrakul, W.; King, A. D., Jr. J. Colloid Interface Sci. 1988, 125, 526. (30) King, A. D., Jr. In Surfactant Science Series; Dekker: New York, 1995; Vol. 55, Chapter 2.

10.1021/la980626r CCC: $18.00 © 1999 American Chemical Society Published on Web 01/30/1999

Methane Solubilization in SDS Solutions

Figure 1. Schematic diagram of the apparatus. Key: (1) equilibrium cell, (2) thermostat, (3) magnetic stirrer, (4) pressure transducer, (5) pressure indicator, (6) plug, (7) gas cylinder, (8) vacuum system, (9-11) valves.

Langmuir, Vol. 15, No. 5, 1999 1641

Figure 2. Solubility of methane at 298.15 K in (9) SDS, (b) SDS + NaCl (0.1 M), (2) SDS + NaOH (0.1 M), (1) SDS + 0.1 wt % PEO, ([) SDS + 0.1 wt % PEO + NaCl (0.1 M), and (+) SDS + 0.1 wt % PEO + NaOH (0.1 M). Table 1. Solubility of Methane (X2 × 105 ) in Pure Water at 1.013 bar

Experimental Section Materials. The methane was supplied by the Beijing Analytical Instrument Factory, and the purity was 99.995%. The ultrapure SDS was obtained from Bethesda Research Laboratories and contained more than 99.5% SDS. NaCl and NaOH were the analytical reagent grade produced by the Beijing Chemical Plant. PEO was supplied by Aldrich with a molecular weight of 100 000. Principle of the Apparatus. The apparatus was based on the principle that a gas will be dissolved in a degassed solvent when the gas contact with solvent and vapor-liquid equilibrium is reached. The solubility of the gas in the solvent can easily be calculated on the basis of temperature, the volume of the system, the quantity of the solvent, the density of the liquid phase, the original and equilibrium partial pressures of the gas, and a suitable equation of state. Apparatus. The schematic diagram of the apparatus is shown in Figure 1. It consisted mainly of an equilibrium cell (1), a constant temperature water bath (2), a magnetic stirrer (3), a pressure transducer (4), a pressure indicator (5), a stainless steel plug (6), a gas cylinder (7), a vacuum pump (8), and valves and fittings of different kinds (9-10). All of the metallic parts contacting the chemicals were made of stainless steel. The temperature of the water bath was maintained within (0.03 K of the desired temperature by Haake D3 controller. The stirrer was used to facilitate dissolution of gas in the solvent. The accuracy of the pressure transducer (IC Sensor Co., model 93) was (0.005 bar in the pressure range of 0-2 bar, and the resolution of the indicator connecting to the transducer was 0.001 bar. A Mettler MP1200 and a DT-100 balance were used for the weight determination, and their sensitivities were 0.001 and 0.000 05 g, respectively. Two steps were required for calibrating the volume of the system. First, the volume of the equilibrium cell was calibrated by a gravimetric method, and then the total volume of the system was obtained by a gas-expansion technique. It was estimated that the accuracy of the volume calibration was better than (0.1%. The stainless steel plug can be lifted by a magnet (not shown) outside of the cell. Procedures. After the system was cleaned thoroughly using different solvents. A suitable amount of water was first charged into the equilibrium cell, and then was degassed under vacuum until the vapor pressure of the solvent did not changed with degassing time. The cell was disassembled at position A, and the water in the cell was determined by weight. The desired amount of SDS was put into the cell. The cell was then reassembled as shown in Figure 1. The air dissolved into the degassed water in the above process was negligible because the area of the water surface contacting with air was very small, as shown in Figure 1. The air in the system was removed by vacuum. The stainless steel plug was lifted and lowered repeatedly, and the stirrer was started until the SDS was dissolved into the water. Then the plug was lowered. The system was charged with methane until

this work literature

298.15 K

308.15 K

318.15 K

2.50 2.52

2.13 2.16

1.94 1.93

the desired pressure (P0) was reached. Experiments showed that the gas dissolved was negligible at this time because the time to charge the gas was very short and the area of the liquid contacting with the gas was very small. The plug was raised and the level of the liquid surface fell, so the gas and the liquid in the system contacted sufficiently. The stirrer was started to facilitate dissolution of the gas in the liquid. After the pressure of the system had been constant for at least 5 h, equilibrium was assumed to have been reached and the pressure was P1. The solubility of the gas in the liquid was calculated according to the temperature, pressures (P0, P1), total volume of the system, density and weight of the liquid phase, and Virial equation and Virial coefficient in the literature.31

Results and Discussions Solubility Data. The solubility of methane in water and in the aqueous solutions of SDS, SDS + 0.1 M NaCl, SDS + 0.1 M NaOH, SDS + 0.1 wt % PEO, SDS + 0.1 wt % PEO + 0.1 M NaCl, and SDS + 0.1 wt % PEO + 0.1 M NaOH were determined at 298.15, 308.15, and 318.15 K and at pressures close to 1.013 bar. The solubility data are corrected to 1.013 bar by assuming that Henry’s law is obeyed. Table 1 lists the solubility data of methane in pure water determined in this work and those reported in the literature.32 The results of this work agree well with those in the literature, which verifies the reliability of the apparatus. Tables 2-4 give all the solubility data of methane in different aqueous solutions determined at different conditions. In the tables, X2 and mSDS stand for the mole fraction of methane in the liquid phase and the molality of SDS in the solvent, respectively. Solubilization without PEO. From the results in Tables 2-4 it can be seen that the solubility in the solutions increases linearly with the mSDS when the mSDS is higher than the critical micelle concentration (CMC) of SDS, which is about 8 mM in the salt-free solution and decreases with increasing ionic strength.12 As an example, Figure 2 shows the dependence of the solubility of methane in different solutions at 298.15 K. According to phase separation theory, the concentration of the monomer of SDS is nearly constant when the mSDS (31) Dymond, J. H.; Smith, E. B. The Virial Coefficients of Pure Gases and Mixtures; Clarendon: Oxford, 1980. (32) Rettich, T. R.; Handa, Y. P.; Battino, R.; Wilhelm, E. J. Phys. Chem. 1981, 85, 3230.

1642 Langmuir, Vol. 15, No. 5, 1999

Hai et al.

Table 2. Solubility of Methane in Different Solvents at 298.15 K and 1.013 bar (mSDS/mmol‚kg-1) SDS + 0.1 M NaCl

SDS X2 ×

mSDS 0.000 8.700 17.36 26.00 34.66 43.16

105

2.50 2.52 2.56 2.60 2.63 2.66

SDS + 0.1 wt % PEO

X2 ×

mSDS 0.000 8.730 17.47 26.06 34.72 43.26

SDS + 0.1 M NaOH 105

2.40 2.48 2.53 2.58 2.61 2.65

SDS + 0.1 wt % PEO + 0.1 M NaCl

mSDS

X2 × 105

0.000 8.730 17.42 26.06 34.79 43.30

2.37 2.48 2.52 2.58 2.64 2.71

SDS + 0.1 wt % PEO + 0.1 M NaOH

mSDS

X2 × 105

mSDS

X2 × 105

mSDS

X2 × 105

0.000 8.739 17.40 25.97 34.65 38.92 47.37

2.42 2.45 2.49 2.53 2.56 2.59 2.62

0.000 8.732 17.42 26.08 34.75 43.43 47.60

2.31 2.37 2.40 2.44 2.48 2.50 2.54

0.000 8.684 17.42 26.13 34.74 39.00 43.36

2.25 2.28 2.32 2.37 2.41 2.43 2.47


SDS

SDS + 0.1 M NaOH

mSDS

X2 × 105

mSDS

X2 × 105

mSDS

X2 × 105

0.000 8.700 17.36 26.00 34.66 43.16

2.13 2.20 2.26 2.32 2.37 2.41

0.000 8.730 17.47 26.06 34.72 43.26

2.08 2.17 2.22 2.27 2.31 2.36

0.000 8.730 17.42 26.06 34.79 43.30

2.14 2.21 2.27 2.32 2.38 2.44

SDS + 0.1 wt % PEO



mSDS

X2 × 105

mSDS

X2 × 105

mSDS

X2 × 105

0.000 8.739 17.40 25.97 34.65 38.92 47.37

2.27 2.32 2.36 2.40 2.43 2.45 2.49

0.000 8.732 17.42 26.08 34.75 43.43 47.60

2.21 2.28 2.30 2.33 2.35 2.39 2.41

0.000 8.684 17.42 26.13 34.74 39.00 43.36

2.11 2.17 2.20 2.23 2.26 2.29 2.32


SDS

SDS + 0.1 M NaOH

mSDS

X2 × 105

mSDS

X2 × 105

mSDS

X2 × 105

0.000 8.700 17.36 26.00 34.66 43.16

1.94 2.04 2.11 2.17 2.21 2.26

0.000 8.730 17.47 26.06 34.72 43.26

1.88 2.12 2.18 2.23 2.28 2.32

0.000 8.730 17.42 26.06 34.79 43.30

1.91 2.13 2.19 2.23 2.29 2.33

SDS + 0.1 wt % PEO



mSDS

X2 × 105

mSDS

X2 × 105

mSDS

X2 × 105

0.000 8.739 17.40 25.97 34.65 38.92 47.37

2.14 2.22 2.26 2.30 2.32 2.35 2.38

0.000 8.732 17.42 26.08 34.75 43.43 47.60

2.17 2.22 2.24 2.26 2.29 2.31 2.33

0.000 8.684 17.42 26.13 34.74 39.00 43.36

2.08 2.11 2.14 2.18 2.20 2.22 2.24

is higher than the CMC of the surfactant. Thus, the concentration of micelles increases linearly with mSDS, but the increment should be very limited because the concentration range of SDS is narrow and the aggregation number of the micelle is relatively large.33,34 Then it can be approximately assumed that the effect of SDS on the solubility of the gas in the bulk solutions remains unchanged with the mSDS in this narrow concentration (33) Mysels, K. J.; Princen, L. H. J. Phys. Chem. 1959, 63, 1696. (34) Llanos, P.; Zana, R. J. Phys. Chem. 1980, 84, 3339.

range of SDS. It can be concluded that the concentration of the gas in the micelle phase is independent of the mSDS because X2 increases linearly with the mSDS. The mole fraction of methane in the micelle phase X2m can be calculated using the following equation16,18

X2m )

1000(X2 - X2′) MH2O (mSDS - mSDS′)

(1)

where X2 is the solubility when the concentration of SDS

Methane Solubilization in SDS Solutions

Langmuir, Vol. 15, No. 5, 1999 1643

Table 5. Solubility of Methane in Micelle Phase (X2m × 103 ) and ∆Gt0 (kJ‚mol-1) of Methane in Different Solvents at 1.013 bar 298.15 K

308.15 K

318.15 K

solution

X2m

∆Gt0

X2m

∆Gt0

X2m

∆Gt0

SDS SDS + 0.1 M NaCl SDS + 0.1 M NaOH SDS + 0.1 wt % PEO SDS + 0.1 wt % PEO + 0.1 M NaOH SDS + 0.1 wt % PEO + 0.1 M NaCl

2.26 2.71 3.72 2.41 2.04 2.93

-11.16 -11.71 -12.53 -11.41 -11.11 -12.07

3.42 3.03 3.56 2.34 1.83 2.35

-13.01 -12.76 -13.10 -11.88 -11.31 -12.08

3.48 3.22 3.21 2.32 1.62 2.06

-13.73 -13.60 -13.56 -12.40 -11.42 -12.15

is the mSDS, X2′ is the solubility corresponding to a lower SDS concentration mSDS′(mSDS′ > CMC), and MH2O denotes the molecular weight (gram) of water. The values of X2m are listed in Table 5. The data in Tables 2-5 indicate that the solubility of methane in the micelle phase is much higher than those in the bulk solutions because of the hydrocarbon-like interior of the micelles. In other words, the gas is solubilized by the micelles. In the absence of polymer and electrolyte, the X2m determined in this work agrees reasonably with those reported by other authors.16,19 The data in Table 5 illustrate that at 298.15 K addition of NaCl and NaOH to a SDS solution results in an increase in X2m. However, the effect of the electrolytes on X2m is not significant at higher temperatures. In SDS and SDS + 0.1 M NaCl solutions, X2m increases with temperature, but it decreases with temperature in a SDS + 0.1 M NaOH solution. Solubility of a gas in the micelles depends on both the fugacity of the gas in vapor phase and the properties of the micelles. At a fixed temperature and pressure, the fugacity of the gas in the vapor phase is fixed. It can be deduced that the effect of the electrolytes on the solubilization results from the variation of the properties of the micelles. At 298.15 K, X2m is increased by the addition of the electrolytes. This may result mainly from the increase of the aggregation number of the micelles caused by the addition of the electrolytes. Solubilization with PEO. In the presence of PEO, the aggregation of SDS is not as simple as the case without PEO because SDS can form complexes with the polymer, i.e., SDS micelles are bound to PEO. Some free SDS micelles exist in the solution when the concentration of SDS is high enough. Surface tension measurements are a useful tool to study the aggregation in surfactant/polymer solutions. Many systems have been studied by other authors,1,35,36 and each surface tension vs surfactant concentration curve shows two breakpoints. The authors pointed out that the first breakpoint corresponds to the critical aggregation concentration (CAC), where the micelles bound to polymer begin to form. The second breakpoint occurs at the polymer saturation point (PSP), where the polymer is saturated with the surfactant. In this work, surface tensions of SDS + 0.1 wt % PEO, SDS + 0.1 wt % PEO + 0.1 M NaCl, and SDS + 0.1 wt % PEO + 0.1 M NaOH solutions were also determined by the drop volume method.37 The CAC and PSP (4 nmol/L and 17 mmol/L, respectively) were obtained from the breakpoints in surface tension vs mSDS curves. The effects of electrolytes and temperature on the CAC and the PSP are not noticeable. As discussed above, micelles bound to PEO are dominant when the mSDS is lower than the PSP. PEO is saturated by SDS when (i) mSDS > PSP, (ii) PEO-bound and free micelles exist in the system, and (iii) increasing mSDS (35) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (36) Bahadur, P.; Bubin, P.; Rao, Y. K. Langmuir 1995, 11, 1952. (37) Tornberg, E. J. Colloid Interface Sci. 1977, 60, 50.

further mainly increases the free micelles. Methane can be dissolved in both PEO-bound micelles and the free micelles of SDS. Figure 2 and Tables 2-4 show that the solubility of methane in SDS solution with PEO also increases linearly with mSDS, indicating that the ability of PEO-bound micelles to solubilize methane is the same as that of free micelles. Breakpoints should be observed in the solubility curves at PSP if the solubilization abilities of the two kinds of micelles were considerably different. The solubility of the gas in the bound and the free micelles can also be calculated from eq 1, which is also represented by X2m . As shown in Table 5, X2m decreases with increasing temperature in the SDS + PEO solution. The addition of PEO to SDS and SDS + NaCl solutions changes the temperature coefficient of X2m from positive to negative. There may be two possible reasons for this. The first is that the interaction between PEO and the micelles changes considerably with temperature, which affects the interaction between methane and SDS in the micelles. The second is that PEO varies the temperature dependence of the aggregation number of the micelles. At 298.15 K, the effect of PEO on the solubilization ability of SDS is not significant, as shown in Table 5. At higher temperatures, however, PEO reduces the ability of SDS to solubilize methane. The effect of NaOH on X2m in SDS + PEO solution is much stronger than that of NaCl; that is, OH- has a much stronger effect on the properties of SDS + PEO solution than Cl-. Thermodynamics of the Solubilization. The standard Gibbs energy change for the transfer ∆Gt0 of a gas molecule from a solution without SDS to the micelles can be calculated by the following equation.16

∆Gt0 ) - RTln(X2m/X2°)

(2)

where X2° is the solubility of the gas in the solution in which mSDS ) 0 mol/kg. ∆Gt0 reflects the solubilization of a surfactant solution. Table 5 lists the ∆Gt0 values of different systems at different temperatures. The large negative values of ∆Gt0 indicate that methane prefers to exist in the hydrophobic interior formed by SDS. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 29633020) and the State Science and Technology Commission of China for financial support. LA980626R

Effect of NaCl, NaOH, and Poly(ethylene oxide) on Methane

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