Article pubs.acs.org/Langmuir
Solubilization of 1‑Hexanol in Aqueous Solutions of Sodium Dodecyl Sulfate at Pressures up to 140 MPa: Partial Molar Volumes, Compressibilities, and Partition Coefficients Harald Høiland,*,† Edin Alagic,†,‡ and Geir Martin Førland§ †
Department of Chemistry, University of Bergen, Allegt. 41, 5020 Bergen, Norway Bergen University College, Nygårdgaten 112, 5020 Bergen, Norway
§
ABSTRACT: Partial molar volumes and partial molar compressibilities of aqueous solutions of sodium dodecyl sulfate (SDS) and 1-hexanol at pressure up to 140 MPa have been determined. For aqueous SDS solutions the partial molar compressibility increases with pressure below the cmc and decreases with pressure above the cmc. The partial molar compressibility of aqueous 1-hexanol increases with pressure. The increased partial molar compressibility reflects that the structure of water is gradually broken down by increased pressure. Thus, the negative effect of electrostriction around the charged parts of SDS decreases as do the effects of hydrophobic hydration around the CH2 groups. In the micellar state the compressibility of the aggregate is the main factor, becoming less compressible as pressure increases. The cmc of SDS, as determined by speed of sound measurements, increases with pressure and goes through a shallow maximum at about 110 MPa. When 1-hexanol is added to SDS solutions, it will be partitioned between the aqueous and micellar (pseudo) phases. The partition coefficient has been determined from partial molar compressibilites, and it appears to decrease with pressure, reach a minimum around 80 MPa, and then increase, though the change with pressure is small.
1. INTRODUCTION An important property of aqueous micellar solutions is their ability to enhance the solubility of organic solutes that are otherwise sparingly soluble. This is normally called solubilization. The basic parameter of solubilization is the partition coefficient, i.e., how the solute is partitioned between the micelles and the aqueous surroundings. The simplest and most used model for determining partition coefficients is the pseudophase or phase separation model where the micelles are treated as a separate (pseudo) phase. The distribution coefficient in a micellar solution will thus be defined and calculated in the same way as between two real phases. Numerous experimental techniques have been used for this purpose in numerous aqueous surfactant systems, see, for instance, reviews by Marangoni and Kwak1 and Høiland and Blokhus.2 Most of the data are reported at atmospheric pressure and 298.15 K. As far as pressure effects on solubilization equilibria are concerned, two papers have used the cmc-decreasing power of the solubilizate to calculate the partition coefficient. The first deals with p-nitrophenyl esters in cetyltrimethylammonium bromide and the other 1-heptanol in SDS.3,4 From the data it seems that the partition coefficient first decreases with pressure and then increases; the minimum is found around 120 MPa. However, in all cases the effect of pressure is small and the corresponding error in the partition coefficients relatively large. We previously used partial molar volumes from density measurements and partial molar compressibilities from speed of ultrasound measurements at atmospheric pressure to calculate volumetric properties of solubilizates and consequently the partition coefficients.5 Since we can measure the speed of © XXXX American Chemical Society
ultrasound very precisely up to 140 MPa, it is possible to determine partial molar volumes and compressibilities with good accuracy at these elevated pressures and then use this information to calculate partition coefficients. In order to do this we need data on surfactants in aqueous solution as well as data for the surfactant−solubilizate mixtures. Aqueous solutions of ionic surfactants at high pressures have received relatively little attention even though pressure is one of the fundamental thermodynamic variables. A few investigations of the cmc variation with pressure have been carried out.6−9 It appears that the cmc of surfactants first increases with pressure and then reaches a maximum at about 120 MPa. The partial molar volumes of alkylsulfates10 and partial molar volumes and compressibilities of sodium decanoate11 and sodium perfluorheptanoate12 have been subject to investigation. Since the partial molar compressibility of monomeric surfactant molecules (below the cmc) at atmospheric pressure is negative for ionic surfactants, the partial molar volume of aqueous monomers should increase with increasing pressure, as is indeed found. The opposite is true for surfactants in the micellar state, and the partial molar volume in this case decreases with pressure. The partial molar compressibilites also seem to increase with pressure for the monomers and decrease with pressure for surfactants in the micellar state. In this paper we investigate the solubilization of 1-hexanol in sodium dodecyl sulfate at high pressures, up to 140 MPa. We report partial molar volumes and compressibilities of sodium Received: March 19, 2014 Revised: June 12, 2014
A
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dodecyl sulfate (SDS) in water and 1-hexanol in water and for 1-hexanol solubilized in aqueous sodium dodecyl sulfate in order to calculate the partition coefficient of 1-hexanol between water and SDS micelles.
2. EXPERIMENTAL SECTION Sodium dodecyl sulfate was from Merck (>99%) and 1-hexanol from Fluka, Puriss. The SDS was dried in an evacuated desiccator for 24 h, and 1-hexanol was used as received. Densities were measured by a Paar vibrating tube densitometer, DMA 602, and the speed of sound by a Rubidium clock sound velocity meter.13 Temperature was controlled to better than ±0.02 K, and the errors in density and speed of sound at these conditions were estimated to ±3 × 10−6 g cm−3 and ±0.03 m s−1. The speed of sound was measured at various pressures up to 140 MPa as measured by a Hottinger Baldwin pressure transducer, model P3M, with a resolution of 0.01 MPa. A detailed account of the high-pressure apparatus has been provided elsewhere.14
Figure 2. cmc of SDS as a function of pressure as measured by the speed of sound at 298.15 K.
3. RESULTS AND DISCUSSION The speed of sound is normally a sensitive method for determining the cmc.15,16 In Figure 1 the relative speed of
above the cmc decreases with increasing concentration. This is surprising since the speed of sound normally increases with increased density, as observed for other surfactants like sodium carboxylates.15 Junquera et al.16 measured at several temperatures up to 45 °C, and it seems that this decrease with concentration becomes more pronounced with temperature. At high pressures the speed of sound versus concentration curves become normal, in the sense that the speed increases with increasing density as seen from Figure 1. The speed of sound versus concentration curve at atmospheric pressure also suggests a second break just above the cmc. Junquera et al.16 observed this second break at a concentration of around 0.015−0.02 m. The present data suggest a second break around 0.025 m. We thus have a small yet significant effect on the speed of sound of SDS solutions that is only observed at atmospheric pressure. This is probably related to the aggregation process of SDS that may be more complex than that of carboxylates. Since it is appears to be temperature and pressure dependent, it could be related to specific surfactant− water interactions. However, this effect has so far only been observed by extremely precise speed of sound measurements, and it is, as pointed out by Junquera et al.,16 premature to make any firm conclusions. The densities at high pressures were calculated using the equation
Figure 1. Relative speed of sound of aqueous solutions of SDS as a function of concentration at various pressures and 298.15 K. Δu = u − uo, where uo is the speed of sound of water.
⎛ ∂ρ ⎞ γ ⎜ ⎟ = ⎝ ∂P ⎠T u2
sound, i.e., relative to water, versus SDS concentration has been plotted at various pressures, and a pronounced break in the slope identifies the cmc. The speed of sound data at atmospheric pressure are in good agreement with the data of Junquera et al.,16 though at concentrations above the cmc our values seem to be systematically lower by about 0.15 ms−1. The present data suggest a cmc at 0.0083 m at atmospheric pressure, in good agreement with the literature. Reported cmc values are mostly in the range from 0.0079 to 0.0085 m,17,18 though some have reported a cmc around 0.005 m.9 At higher pressures the cmc of SDS goes through a shallow maximum at around 110 MPa, as shown in Figure 2, in good agreement with the data of Kaneshina et al.6 Qualitatively it is also in agreement with the data of Kato et al.,9 but they find the cmc at atmospheric pressure to be 0.0046 m, and values at higher pressures seem to be systematically lower by approximately the same factor. The speed of sound increases with concentration (and density) below the cmc, as is normal for aqueous solutions. At atmospheric pressure the speed of sound of SDS solutions
(1)
Here γ is the adiabatic constant, γ = κT/κs, where κT is the isothermal compressibility and κs the isentropic compressibility. The latter can be determined from speed of sound and density measurements since κs = 1/(u2ρ). The relation between κT and κs is (α 2 T) (2) σ Here α is the expansibility and σ the volumetric heat capacity. For sodium dodecyl sulfate solutions we used the data of Sadeghi and Shahabi17 plus some of our own unpublished data measured at 35 and 50 °C to determine the expansibility at 25oC. This provides data with an uncertainty of about 2−3%. Heat capacities have been calculated from the data of Musbally et al.19 Conversion from isentropic to isothermal compressibilities for 1-hexanol has been carried out by calculating κT = κs +
B
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expansibilities from the data by Nakajima et al.20 and using the heat capacity data for pure water. For aqueous solutions containing both sodium dodecyl sulfate and 1-hexanol the adiabatic constant, γ = κT/κs, has been estimated by combining data for aqueous solutions of 1-hexanol and sodium dodecyl sulfate. Since the correction term (α2T/σ) of eq 2 is about 1% of the κT or κs values, an error of a few percent in this term is acceptable, and this can be achieved by the methods suggested above. For details concerning the calculations of densities and compressibilities at high pressures see Hedwig et al.14 The partial molar volume and the partial molar compressibility (of a solute, component 2) are defined as ⎛ ∂V ⎞ (1000 + M 2m) ⎛ ∂ρ ⎞ M2 ⎜ ⎟ + V2 = ⎜ ⎟ =− 2 ⎝ ⎠ ∂m ρ ρ ⎝ ∂n2 ⎠ p , T
(3)
⎛ ∂V ⎞ (1000 + M 2m) ⎛ ∂κ ⎞ ⎜ ⎟ + κV K 2 = −⎜ ⎟ = 2 ⎝ ∂P ⎠ ⎝ ∂m ⎠ ρ
(4)
since the same type of equipment has been used. Thus, the uncertainty is probably about ±1 cm3 mol−1. With this in mind the two sets of data are in reasonable agreement. Data above the cmc should be more precise for both sets of data, and indeed, the agreement here is good. Sadeghi and Shahabi17 also measured compressibilities, but have not calculated partial molar compressibilities, and it is difficult to compare the two sets of data The partial molar volumes and compressibilities of SDS are qualitatively similar to other surfactants. One observes an increase in the partial molar volume of 11.6 cm3 mol−1 for the micellization process and a relatively large increase in the partial molar compressibility, 145.3 × 10−9 cm3 mol−1 Pa−1. Below the cmc the surfactant molecules behave mainly like normal 1:1 electrolytes where a dense, less compressible hydration layer is formed around the charged parts (here Na+ and SO4−). This is normally referred to as electrostriction and results in negative partial molar compressibilites.21 In the micellar state, the electrostrictive effect is less pronounced since a large portion of the counterions is associated with the micellar surface, probably in a way similar to ion pairing. The interior of the micelle is reminiscent of liquid hydrocarbon, which is significantly more compressible than water, and it is thus not surprising that the overall partial molar compressibility of the aggregated SDS becomes positive. It seems reasonable to compare sodium dodecyl sulfate to sodium tridecanoate. The only difference between these two molecules is the headgroup, SO4− compared to CO2−. Unfortunately no data for tridecanoate could be found, but it is relatively easy to interpolate from data on dodecanoate and tetradecanoate in order to obtain values for tridecanoate.15 The partial molar compressibility, in this case the isentropic value, of the tridecanoate monomer at infinite dilution is −70 × 10−9 cm3 mol−1 Pa−1, compared to −52.7 × 10−9 for SDS. The main difference is probably related to the size of the charged group, SO4− being larger than CO2−. The negative contribution due to elctrostriction is thus less pronounced in the SO4− group. Table 2 shows data at high pressures. The partial molar volume of SDS below the cmc increases with pressure, and
In these equations the factor 1000 appears when the densities, ρ, are given in g cm−3. M2 is the molar mass of the solute, m the molality, and κ the compressibility of the solution. In order to calculate partial molar volumes and compressibilities one needs to know how the density and compressibility varies with molality, i.e., (∂ρ/∂m) and (∂κ/∂m). This can be achieved by a least-squares fitting procedure. This fit, however, must be precise. For instance, the difference between measured and fitted densities must be better than ±10−5 g cm−3 for densities and better than ±3 × 10‑14 Pa−1 for compressibilities, in both cases just marginally greater than the experimental error. With this in mind eqs 3 and 4 can be used to calculate partial molar volumes and compressibilities. The data at infinite dilution for sodium dodecyl sulfate in water are given in Table 1. For SDS the partial molar volumes Table 1. Partial Molar Volume, V2°, Isentropic Partial Molar Compressibility, K2,s°, Isothermal Partial Molar Compressibility, K2,T°, and partial Molar Expansibility, E2°, All Extrapolated to Infinite Dilution at 298.15 Ka −1
V2° (cm mol ) K2,s° × 109 (cm3 mol−1 Pa−1) K2,T° × 109 (cm3 mol−1 Pa−1) E2° (cm3 mol−1 K−1) 3
Δ(micellization)
below the cmc
above the cmc
238.5 ± 1 −52.7 ± 1
250.13 ± 0.2 97.0 ± 0.2
11.6 ± 1 149.7 ± 1
−37.8 ± 1
107.5 ± 0.5
145.3 ± 1
Table 2. SDS Solutions at Various Pressures and at 298.15 Ka below the cmc
0.50 ± 0.05
0.34 ± 0.05
−0.14 ± 0.07
pressure (MPa)
109 × K2,s° (cm3 mol−1)
109 × K2,T° (Pa−1)
V2° (cm3 mol−1)
109 × K2,s° (cm3 mol−1)
109 × K2,T° (Pa−1)
0.1 20 40 60 80 100 120 140
238.49 239.16 239.67 240.04 240.32 240.54 240.72 240.88
−52.7 −43.1 −35.0 −28.6 −23.8 −20.5 −18.7 −18.6
−37.8 −29.1 −21.9 −16.2 −12.1 −9.4 −8.3 −8.3
249.77 248.10 246.50 245.01 243.61 242.29 240.71 239.90
97.0 83.9 72.1 63.8 58.0 53.5 50.1 48.7
107.5 93.9 81.7 72.9 66.7 61.9 58.2 54.1
a
The difference between SDS in the micellar state and in the aqueous solution is also given.
and compressibilities are based on measurements of 8 concentrations below the cmc and more than 20 above. A linear fit is satisfactory for determining (∂ρ/∂m) and (∂κ/∂m) with relevant accuracy below the cmc, but due to the low concentrations the error in these quantities will still be large. For instance, errors of 10−5 g cm−3 in density at 0.001 m could result in an error of ±1 cm3 mol−1 in the partial molar volume. The partial molar volumes of SDS in water can be compared to the data of Sadeghi and Shahabi.17 They find that the partial molar volume of monomers at infinite dilution is 236.6703 cm3 mol−1. On the basis of the number of digits provided, it appears that they have exceptional accuracy. However, it seems unlikely that their data are significantly more precise than the present
above the cmc
V2° (cm3 mol−1)
a
cmc, partial molar volume, and partial molar compressibilities at infinite dilution.
above the cmc it decreases. This could be expected since the partial molar compressibilities at atmospheric pressure are negative below and positive above the cmc, respectively. The partial molar compressibility of aqueous SDS below the cmc increases significantly with pressure, though it remains negative. C
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significantly with pressure above 140 MPa but remain constant at about 62 × 10−9 cm3 mol−1 MPa−1. The partial molar compressibility of 1-hexanol in water at infinite dilution is positive and increases with pressure, as shown in Table 3. As pressure increases the structure of the hydration layer around the hydrophobic groups seems to be gradually weakened and more easily compressed. The present data allows calculation of (∂K∂P)T with reasonable accuracy. At low pressures significant differences are observed. At atmospheric pressure (∂K∂P)T equals 2.1 for 1-hexanol, 4.7 for SDS monomers, and 8.2 for SDS in the micellar state, all in units of 10‑11 cm3 mol−1 Pa−2. For 2-hexanol a value of 4.1 × 10‑11 cm3 mol−1 Pa−2 can be calculated,23 almost twice the value of 1-hexanol. It suggests that the water around the terminal CH3 groups is more affected by pressure than the CH2 groups or the OH group. The large (∂K∂P)T value for SDS monomers compared to 1-hexanol is probably due to the effect of pressure on electrostriction. For all three solutes mentioned here, (∂K∂P)T decreases with pressure and the significant differences observed at low pressures seem to disappear around 140 MPa, where (∂K∂P)T comes close to zero for all these solutes in water. When 1-hexanol is added to an SDS solution above the cmc, a certain number of hexanol molecules will be solubilized and reside in the micellar (pseudo)phase. The partition coefficient can then be defined as
The increase seem to reflect that the open structure of bulk water is gradually broken down with pressure, becoming denser and thus differ less from the water of the hydration layer around the ionic groups.15,22 In addition, the compressibility of the hydrophobic part is also likely to increase with pressure, as is observed for 1-hexanol and other alcohols.23 This is presumably due to the same effect; the structure of water is gradually broken down by pressure, and the hydrophobic hydration around the CH2 groups becomes less rigid. Above the cmc the partial molar compressibility decreases with pressure. It suggests that for SDS in the micellar state it is the interior of the micelle that is the determining factor. It can safely be assumed that the interior of the micelle is like an unstructured hydrocarbon-like liquid that is relatively easy to compress, but the more it is compressed, the more it will resist compression. This effect clearly outweighs any effect of the charged surface of the micelles and changes in the electrostriction. ΔV° and ΔK° values are plotted in Figures 3 and 4. It can be seen that the volume change goes from positive to negative
Kx = x H,mic/x H,aq
(5)
where xH,mic is the mole fraction of hexanol in the micellar phase and xH,aq the mole fraction of hexanol in the aqueous phase. It should be mentioned that the pseudophase model has been criticized for being inconsistent with the phase rule.24 If α is the fraction of hexanol in the micellar phase, eq 5 can be transformed to5 Kx = 55.5α /[(1 − α)(αmt + ms)
Figure 3. ΔV° of micelle formation of SDS as a function of pressure at 298.15 K.
(6)
Here mt is the total molality of hexanol in the solution and ms is the molality of SDS in the micelles. Any thermodynamic value can now be written as a combination of values in the aqueous and micellar phases. For partial molar volumes and compressibilities of 1-hexanol solubilized in a surfactant solution this means V2 = αV2,mic + (1 − α)V2,aq ↓ V2 = (V2,mic − V2,aq)α + V2,aq
(7)
K 2 = αK 2,mic + (1 − α)K 2,aq Figure 4. ΔK° of micelle formation of SDS as a function of pressure at 298.15 K.
↓ K 2 = (K 2,mic − K 2,aq)α + K 2,aq
values as the pressure increases. It is zero at approximately 125 MPa. The pseudophase model gives a relation between ΔV° and the cmc, ΔV° = (1 + β)RT(∂ln cmc/∂P), where β is the fraction of counterions associated with the micelle. However, the cmc values are not sufficiently accurate to do this properly, but the order of magnitude is correct. Notably, the maximum in the cmc values should correspond to ΔV° = 0, and this appears to be in good agreement with the experimental data. ΔK° decreases significantly with pressure but remains positive even at 140 MPa. In fact, the plot suggests that ΔK° will not change
(8)
Here the subscript mic means in the micellar phase and aq in the aqueous phase. It should be noted that in going from eq 7 to eq 8 we left out a relaxation term (∂α/∂P) as shown by De Lisi et al.25 We see that the model predicts linearity between the measured partial molar volume or compressibility and the fraction of hexanol in the micellar phase, α. The partial molar volume and compressibility of 1-hexanol has been determined at four SDS concentrations between 0.05 and 0.15 m. The partial molar volumes and compressibilities at each SDS concentration has been extrapolated to infinite dilution (with D
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Table 3. Data on 1-Hexanol at 298.15 K; Partition Function, Partial Molar Volumes, Isentropic and Isothermal Partial Molar Compressibilities in Water and in the Micellar State of Water/SDS Solutions in water
in the micellar state
pressure (MPa)
Kx
V2° (cm3 mol−1)
109 × K2,s° (cm3 mol−1)
109 × K2,T° (Pa−1)
V2° (cm3 mol−1)
109 × K2,s° (cm3 mol−1)
109 × K2,T° (Pa−1)
0.1 20 40 60 80 100 120 140
2225 2170 2140 2120 2120 2140 2150 2200
118.65 118.53 118.34 118.09 117.78 117.43 117.09 116.63
0.5 4.9 8.0 11.1 13.7 15.9 17.5 18.8
4.5 8.3 11.6 14.5 16.9 18.8 20.7 21.7
122.60 120.94 119.55 118.47 117.45 116.51 115.65 114.60
83.9 69.0 57.6 49.2 43.5 39.3 36.4 34.4
89.1 73.1 62.3 53.7 48.0 43.3 40.8 38.6
respect to 1-hexanol). The fraction of hexanol in the micellar phase, α, can be calculated for each SDS solution from eq 6 by guessing a value of Kx. These α values are used in eq 8. A linear fit is expected, and the intercept, α = 0, should equal the partial molar compressibility in water. If this is not the case we have to adjust Kx to obtain a new set of α values, recalculate eq 8, and check. This process is carried out until the requirements of eq 8 are fulfilled. In this way we can determine the distribution coefficient of 1-hexanol between the micelles and water and determine K2,mic°, the partial molar compressibility of 1-hexanol in the micellar state at infinite dilution. Equation 7 could also be used to determine Kx values from volumes in the same way. However, the difference (V2,mic − V2,aq) is too small to make this a reliable method. Instead, we have taken the Kx values obtained from partial molar compressibilities and used these data to calculate V2,mic°, the partial molar volume of 1-hexanol in the micellar state at infinite dilution. The relevant data are given in Table 3. Figure 5 shows a plot of the partial molar compressibility of 1-hexanol versus α for the best fit, the best Kx value. Only three
Figure 6. Plot according to eq 7; the partial molar volume of 1-hexanol in aqueous solutions of SDS versus the fraction of 1-hexanol in the micellar phase. SDS concentrations are 0.05, 0.1, 0.15, and 0.2 m.
micelle is higher than in water at low pressures but that this reverses around 60 MPa. From Table 3 it is evident that the partition coefficient does not change much with pressure. This is expected if the solubilization process can be treated as a regular equilibrium between two phases since thermodynamically ⎛ ∂lnK ⎞ ΔV ° ⎜ ⎟ = − ⎝ ∂P ⎠T RT
(9)
Since ΔV° is small and changes sign just above 60 MPa, the partition coefficient should not change much with pressure and will go through a minimum around 60 MPa as observed. It is apparent from the data that the pseudophase model is adequate for describing the solubilization process. There may be several reasons for this. One is related to data being at infinite dilution with respect to 1-hexanol, and thus, the hexanol will not change the SDS micelles significantly. A second one is that as far as 1-hexanol is concerned it is just a two-state model. Alcohol can either be in water or in the micelle, and there should be an equilibrium between the two states as suggested in eq 5. The use of eq 8 allows the calculation of the partial molar compressibility of 1-hexanol solubilized in the micelles. The isentropic value at atmospheric pressure is in fair agreement with the previously published value of 81.4.5 If we compare the partial molar compressibilities of 1-hexanol in water and in the micelles, the pressure dependence is opposite. In water the partial molar compressibility increases with pressure; in the
Figure 5. Plot according to eq 8; the partial molar compressibility of 1hexanol in aqueous solutions of SDS versus the fraction of 1-hexanol in the micellar phase. SDS concentrations are 0.05, 0.1, 0.15, and 0.2 m.
different pressures, 0.1, 20, and 140 MPa, are shown. The slope of the curves decreases significantly with pressure, illustrating the point that the difference between the partial molar compressibilities of 1-hexanol in water, and in the micellar state decreases with pressure. A similar plot for the partial molar volumes is shown in Figure 6. Here the slope changes from positive at low pressures to negative at high pressures, reflecting that the volume requirements of 1-hexanol in the E
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(10) Tanaka, M.; Kaneshina, S.; Shin-No, K.; Okajima, T.; Tomida, T. Partial Molar Volumes of Surfactant and its Homologous Salts under high Pressures. J. Colloid Interface Sci. 1974, 46, 132−138. (11) Vikingstad, E.; Skauge, A.; Høiland, H. The Effect of Pressure Temperature on the Partial Molal Volume and Compressibility of Sodium Decanoate Micelles. J. Colloid Interface Sci. 1979, 66, 59. (12) Fukada, K.; Kobayashi, Y.; Ota, Y.; Fujii, M.; Kato, T.; Seimiya, T. Effect of Pressure and Temperature on Adiabatic Compressibility of Aqueous Solutions of Amphiphiles with a Perfluorocarbon Chain. Thermochim. Acta 2000, 352-353, 189−197. (13) Høgseth, E.; Hedwig, G. R.; Høiland, H. Rubidium Clock Sound Velocity Meter. Rev. Sci. Instrum. 2000, 71, 4679−4680. (14) Hedwig, G. R.; Høgseth, E.; Høiland, H. Volumetric Properties of the Glycyl Group of proteins in Aqueous Solution at High Pressures. Phys. Chem. Chem. Phys. 2007, 9, 1−16. (15) Vikingstad, E.; Skauge, A.; Høiland, H. Partial Molal Volumes and Compressibilities of Sodium Alkylcarboxylates R6COONa− R13COONa in Aqueous Solution. J. Colloid Interface Sci. 1978, 66, 240−246. (16) Junquera, E.; Pena, L.; Aicart, E. Influence of Temperature on the Micellization of Sodium Dodecylsulfate in Water from Speed of Sound Measurements. J. Solution Chem. 1994, 23, 421−430. (17) Sadeghi, R.; Shahabi, S. A Comparison Study between Sodium Dodecyl Sulfate and Sodium dodecyl Sulfonate with Respect to the Thermodynamic properties, Micellization, and interaction with Poly(ethylene glycol) in Aqueous Solutions. J. Chem. Thermodyn. 2011, 43, 1361−1370. (18) Tan, C. H.; Huang, Z. J.; Huang, X. G. Rapid Determination of Surfactant Critical Micelle Concentration in Aqueous Solutions using Fiber-optic Refractive Index Sensing. Anal. Biochem. 2010, 401, 144− 147. (19) Musbally, G. M.; Perron, G.; Desnoyers, J. E. Apparent Molal Volumes and Heat capacities of Ionic Surfactants in Water at 25oC. J. Colloid Interface Sci. 1974, 48, 494−501. (20) Nakajima, T.; Komatsu, T.; Nakagawa, T. Apparent Molal Volumes and Adiabatic Compressibilities of n-.Alkanols and α,ωAlkane Diols in Dilute Aqueous Solutions at 5, 25, and 45 °C. I. Apparent Molal Volumes. Bull. Chem. Soc. Jpn. 1975, 48, 783−787. (21) Millero, F. J.; Ward, G. K.; Chetirkin, V. Relative Sound velocities of Sea Salts at 25 °C. J. Acoust. Soc. Am. 1977, 61, 1492− 1498. (22) Chalikian, T.V.; Sarvazyan, A. P.; Funck, T.; Cain, C. A.; Breslauer, K. J. Partial Molar Characteristics of glycine and Alanine solutions at high pressures calculated from Ultrasonic velocity Data. J. Phys. Chem. 1994, 98, 321−328. (23) Hakin, A. W.; Høiland, H. Speed of Sound Measurements Conducted at High pressures on Aqueous Alcohol and Aqueous Diol Systems at T = 298.15K. Phys. Chem. Chem. Phys. 2005, 7, 1−9. (24) Moroi, Y. Distribution of Solubilizates among Micelles and Kinetics of Micelle-catalyzed Reactions. J. Phys. Chem. 1980, 84, 2186−2190. (25) DeLisi, R.; Millioto, S.; Verrall, R.E. Partial Molar Volumes and Compressibilities of Pentanol in Aqueous Dodecyltrimethylammonium Bromide Solutions at 15, 25, and 35 °C. J. Solution Chem. 1990, 19, 97−128.
micellar state it decreases. However, it remains higher in the micellar state at all pressures (here up to 140 MPa). It is exactly the same trend as for SDS molecules in water and in the micellar state and just shows that they are part of the same aggregate. The fact that the partial molar compressibility of 1hexanol is lower than SDS at all pressures is probably just an effect of size, the larger SDS molecules exhibiting the largest partial molar compressibility.
4. CONCLUSION The data shows that the volume change of micelle formation for SDS is positive at low pressures, becoming negative around 110 MPa. The cmc increases with pressure and reaches a shallow maximum at about the same pressure as expected from the relationship between these two quatities. The partial molar compressibility of SDS in the micellar state decreases, and in the monomer state it increases with pressure, reflecting the different environments of the molecules. When 1-hexanol is added, it is partitioned between the aqueous and micellar (pseudo) phases. The partition coefficient decreases with pressure up to about 80 MPa and then starts increasing. This is in accordance with the volume change of solubilization for 1hexanol which is positive at low pressures reaching zero at about 80 MPa and then becomes negative. The data also shows that despite criticism the pseudophase model appears to function thermodynamically.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ‡
CIPR, Allegt. 41, 5020 Bergen, Norway.
Notes
The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/la501029b | Langmuir XXXX, XXX, XXX−XXX
