Thermodynamic Study of Bile Salts Micellization - Journal of Chemical

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Thermodynamic Study of Bile Salts Micellization Alfredo Maestre, Pilar Guardado, and María Luisa Moyá* Department of Physical Chemistry, University of Seville, Profesor García González 1, 41012 Seville, Spain S Supporting Information *

ABSTRACT: The aggregation process of sodium cholate, NaC, sodium deoxycholate, NaDC, sodium glycocholate, NaG, sodium glycodeoxycholate, NaDG, sodium taurocholate, NaT, and sodium taurodeoxycholate, NaDT in aqueous solution has been investigated at several temperatures, in the absence and in the presence of NaCl 0.15 mol kg−1. Results show that both a decrease in the number of ring hydroxyls and an increase in the length of the side chain favor micellization, in the absence as well as in the presence of salt. These observations can be explained by considering that the hydrophobic effect is the driving force for the self-association process. This is in agreement with the ΔmicCp° values, which point out that the micellization process leads to a diminution of the hydrophobic surface of the bile salt molecules exposed to water. The presence of NaCl 0.15 mol kg−1 in the aqueous phase favors micellization by decreasing the CMC due to a diminution in the electrostatic repulsions within the micelles. However the presence of the background electrolyte has no substantial effect on either the micellar ionization degree or the enthalpy of micellization. The thermodynamic magnitudes indicate that the bile salts micellization is entropy driven.

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surfactants, in the region to 2−9 molecules,4 because it is difficult to form large aggregates and maintain contact between water and all the hydrophilic faces. The number and orientation of the ring hydroxyls and the length and polarity of the side chain play an important role in the micellization process of BS since they influence the hydrophilic−hydrophobic balance. In this work the aggregation process of sodium cholate, NaC, sodium deoxycholate, NaDC, sodium glycocholate, NaG, sodium glycodeoxycholate, NaDG, sodium taurocholate, NaT, and sodium taurodeoxycholate, NaDT, in aqueous solution was investigated at several temperatures, in the absence and in the presence of NaCl 0.15 mol kg−1. Both the critical micelle concentration and the enthalpy of micellization were estimated by isothermal titration calorimetry, ITC. Conductivity and steady-state fluorescence measurements were also carried out in order to determine the micellar ionization degree of the bile salt aggregates. The data reported in this work provides relevant information on the effect of the number of ring hydroxyls and the nature of the side chain on the micellization of bile salts in the presence and in the absence of NaCl 0.15 mol kg−1.

INTRODUCTION Bile salts, BS, are biosurfactants important in the digestion process by humans.1,2 They are produced by the liver and stored in the gallbladder. Bile salts solubilize apolar material such as cholesterol, lipids, fatty acids, monoglycerides, and fat soluble vitamins. BS are surface-active molecules with nonconventional surfactant-like properties. Conventional surfactants consist of a hydrophobic tail and a polar headgroup which spontaneously self-aggregate into micelles above a certain surfactant concentration, called the critical micelle concentration, CMC. Bile salts do not have well-defined tail and head groups. The basic structure of bile salts consists of a rigid steroid backbone with a hydrophobic and a hydrophilic face to which a short and flexible tail is attached.3 The hydroxyl groups are generally located on one face and the methyl groups on the opposite face (see Scheme 1). As a consequence of this planar polarity bile salts form smaller micelles than conventional Scheme 1. Chemical Structure of Sodium Bile Salts

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EXPERIMENTAL SECTION Materials. Sodium cholate (NaC, mass fraction purity ≤ 0.99), sodium glycocholate (NaGC, mass fraction purity ≤ 0.97), sodium taurocholate (NaTC, mass fraction purity ≤ 0.95), sodium glycodeoxycholate (NaGDC, mass fraction purity ≤ 0.97) and sodium taurodeoxycholate (NaTDC, mass fraction purity ≤ 0.97) were purchased from Sigma. They were of the highest quality available (UltraSigma) and were purified

Received: October 10, 2013 Accepted: January 2, 2014

© XXXX American Chemical Society

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dx.doi.org/10.1021/je400903n | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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fluorescence spectrophotometer. The temperature was kept constant by a water flow cryostat with a temperature fluctuation within ± 0.1 K, connected to the cell compartment. CMCs Determination by Using Pyrene as Probe. The 1· 10−6 M pyrene surfactants solution was prepared in double distilled water. The excitation wavelength was 335 nm and the fluorescence intensities were measured at 373 nm (band 1) and 384 nm (band 3). Excitation and emission slits were 2.5 nm and a scan speed of 60 nm/min was used. The intensity ratio of the vibronic bands (1:3) is called the pyrene 1:3 ratio. The introduction of pyrene in the aqueous BS solutions was done following the same procedure described in ref 8.

by several recrystallizations before use. Thin-layer chromatography showed a single spot for each bile salt. Sodium dodecylsulfate (SDS, mass fraction purity > 0.98) was purchased from Aldrich and was purified by repeated recrystallizations from ethanol. Pyrene (mass fraction purity 0.99) was from Aldrich, and it was purified before use by methods reported in the literature.5 All products were stored over P2O5. Solutions were made up by weight using degassed doubledistilled water produced by a Milli-Q system (Millipore, resistivity 18 MΩ cm). An analytical balance (AB 204-S; Metter Toledo, Switzerland) with an uncertainty of ± 1·10−4 g was used. Isothermal Titration Calorimetry (ITC). A multichannel thermal activity monitor (TAM) isothermal heat conduction microcalorimeter (Thermometric AB 2277/201, Järfälla, Sweden), with an uncertainty of ± 0.5 μJ s−1, with a 1 or 2 mL sample cell equipped with a stirring device, was used for measuring directly the critical micellar concentration, CMC, and the enthalpy of micelle formation of the surfactant solutions. The calorimeter was connected to an external water circulator (Heto), and the whole system was placed in a room in which the temperature was kept constant within ± 0.5 K. The sample cell of the calorimeter was initially loaded with 0.9 mL of double-distilled water, at the desired temperature. A surfactant solution of a concentration ∼15fold the CMC was injected in small aliquots to the stirred sample (60 rpm) cell using a 250 μL Hamilton syringe, which was positioned in a computer controlled syringe pump (Hamilton Microlab M). Each aliquot was 10 μL, with a 240 s interval between injections, and addition of the concentrated solution continued until the desired range of BS concentration had been covered. The experiments were computer controlled using Digitam 4.1 software (Thermometric); the same program was used for data analysis. The calorimeter was electrically calibrated before each experiment. A chemical calibration was made by means of the binding of Ba2+ to 18-crown-6 in water, method proposed by L.E. Briggner and I. Wadsö.6 Since the substances used in this study are sodium salts of anionic surfactants, we have also carried out measurements of the micellization enthalpy of SDS, a conventional anionic surfactant, as an additional check. The results obtained at 308.15 K were ΔmicH° = (−4.3 ± 0.3) kJ mol−1 and CMC = (8.8 ± 0.4)·10−3 mol kg−1, in agreement with the literature values ΔmicH° = (−4.6 ± 0.5) kJ mol−1 and CMC = (8.6 ± 0.4)·10−3 mol kg−1.7 Each experiment was performed in duplicate. Conductivity Measurements. Conductivity was measured with a Crison GLP31 conductimeter, connected to a water-flow cryostat of accuracy ± 0.1 K. The conductimeter was calibrated with KCl solutions of the appropriate concentration range. A dispenser Crison Buret 1S, with the uncertainty of ± 0.1 μL, was programmed to add the adequate quantities of a concentrated surfactant solution in order to change the surfactant concentration. The specific conductivity, κ, was measured after each addition followed by thorough mixing. The specific conductivity was measured six times at each concentration, and the average value was obtained with an uncertainty within ±1 μS cm−1. This method allows one to obtain a large number of experimental conductivity data, the estimation of the CMC being more accurate. Steady-State Fluorescence Measurements. Fluorescence measurements were made using a Hitachi F-2500

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RESULTS AND DISCUSSION The differential molar enthalpies of dilution were determined by isothermal titration calorimetry. Figure 1a shows the

Figure 1. Typical calorimetric curves for sodium taurodeoxycholate in NaCl 0.15 mol kg−1 at (323.15 ± 0.01) K. (a) Reaction heat per mole vs the total bile salt concentration. (b) First derivative of curve a calculated numerically from interpolated values.

reaction heat per mole as a function of the total bile salt concentration for NaDT at 323.15 K in the presence of NaCl 0.15 mol kg−1. As has been previously discussed,9,10 when the final concentration is in the premicellar region, the added micellar aggregates dissociate into monomers and the monomers are then diluted. When the final concentration is above the CMC, the added micellar aggregates are only diluted. Therefore ΔmicH° was obtained from the difference between the observed enthalpies of the two linear segments of the plots (see Figure 1a). Figure 1a shows that the concentration region for the aggregation of BS is broad, which is caused by the small aggregation numbers of BS micelles. B



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Table 1. Thermodynamic Parameters for the Micellization of Bile Salts in Water at Several Temperatures 103·CMCa

T bile salt NaC

NaDC

NaG

NaDG

NaT

NaDT

K 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

mol kg 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

8.8 15.0 10.2 19.4 19.1 6.6 5.8 4.5 8.2 10.2 13.6 12.5 6.8 14.7 16.0 5.8 4.5 3.43 6.0 6.6 8.8 8.1 5.6 13.7 15.0 4.56 3.11 2.44 4.53 5.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

ΔmicH°a

−1

0.8 0.6 0.4 0.8 0.8 0.3 0.2 0.2 0.3 04 0.5 0.5 0.3 0.6 0.6 0.2 0.2 0.13 0.2 0.3 0.3 0.3 0.2 0.5 0.6 0.18 0.12 0.10 0.18 0.2

−1

α 0.92

kJ mol

1

0.85 ± 0.04b;0.763

0.701 0.78 ± 0.04b;0.792

0.80 ± 0.04b;0.82 ± 0.044

0.79 ± 0.04b;0.78 ± 0.04c

0.79 ± 0.04b;0.74 ± 0.033

0.703 0.74b

3.72 2.29 0.97 −1.62 −4.8 3.78 1.73 0.57 −2.30 −4.7 3.41 1.67 0.64 −1.70 −3.29 2.62 0.50 −1.62 −4.4 −6.7 1.80 0.90 0.103 −1.29 −2.83 −2.21 0.76 −1.62 −3.8 −7.1

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

018 0.11 0.08 0.10 0.2 0.19 0.11 0.06 0.11 0.2 0.17 0.11 0.07 0.09 0.17 0.13 0.06 0.09 0.2 0.3 0.10 0.07 0.010 0.10 0.14 0.12 0.08 0.08 0.2 0.4

ΔmicG° −1

kJ mol

TΔmicS° kJ mol

−1

ΔmicCpa kJ mol−1 K−1

−24.6 ± 1.3

25.6 ± 1.3

−0.233 ± 0.013

−28.5 ± 1.7

29.1 ± 1.5

−0.212 ± 0.013

−26.8 ± 1.6

27.4 ± 1.7

−0.168 ± 0.011

−29.1 ± 1.8

27.5 ± 1.6

−0.233 ± 0.015

−20.7 ± 1.4

21 ± 2

−0.116 ± 0.012

−31.3 ± 1.6

29.7 ± 1.5

−0.233 ± 0.013

a

Estimated by ITC; ITC experiments were performed in duplicate and the mean values were reported; 1ref 17.; 2ref 19. 3ref 20. bThis work, conductivity measurements. cThis work, using the Corrin-Harkins equation.

Tables 1 and 2 summarize the ΔmicH° values for the different aqueous bile salt solutions at several temperatures, in the absence and in the presence of NaCl 0.15 mol kg−1, respectively. The experimental calorimetric data from which the ΔmicH° values were derived are listed in Tables S1 to S60 in Supporting Information. The CMC was estimated by taking into account that the CMC corresponds to the concentration at which the first derivative of the reaction heat with respect to the total bile concentration displays an extreme value.11 Therefore, the CMC values were obtained from the first derivative of the curve in Figure 1a, calculated numerically from interpolated values (see Figure 1b). The CMCs are also listed in Tables 1 and 2. The change in the heat capacity of micellization, ΔmicCp, was calculated from the slope of the ΔmicH° versus temperature plots. These plots are straight lines (0.991 < R < 0.999, R being the correlation coefficient) and for this reason only ΔmicCp values corresponding to 298.15 K are included in Tables 1 and 2. To calculate TΔmicS°, the Gibbs energy of micellization was estimated using eq 1:12 ΔmicG° = RT(2 − α) ln(CMC)

equation for BS is an approximation because BS micelles have quite low aggregation numbers. The micellar ionization degree was estimated, in the absence of NaCl, by conductivity measurements. Figure 2 shows the experimental dependence of the specific conductivity on BS concentration for sodium taurocholate aqueous solutions. The Carpena method13 was used in order to obtain the CMC and the micellar ionization degree. The conductivity experimental data are summarized in Tables S61 to S65 in Supporting Information. The CMC values were in good agreement with those determined by ITC. The micellar ionization degree was also determined by a second method, based on the dependence of the CMC on the counterion concentration. The CMC in the presence of several NaCl concentrations was estimated by a fluorescence method, based on the variations of the pyrene intensity ratio II/IIII following the micellization process. The fluorescence experimental data are summarized in Tables S66 to S75 in Supporting Information. All II/IIII plots show a decrease as the total BS concentration increases, associated with the formation of micelles (see Figure 3). To calculate the CMC values, the procedure proposed by Zana et al. was used.14,15 II/ IIII experimental data were fitted to a sigmoid (Boltzmann type) curve, and the center of the sigmoid was identified as the CMC. The effect of the counterion concentration on the CMC is illustrated in Figure 4. This figure shows the dependence of ln(CMC/mol dm−3) on ln((CMC+[Na+])/mol dm−3) for

(1)

where R and T have their usual meaning, CMC is the critical micelle concentration, expressed as mole fraction, and α is the micellar ionization degree. It is worth noting that the use of this C



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Table 2. Thermodynamic Parameters for the Micellization of Bile Salts in Aqueous NaCl 0.15 mol kg−1 Solutions at Several Temperatures bile salt NaC

NaDC

NaG

NaDG

NaT

NaDT

a

T

103·CMCa

K

mol kg−1

283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15 283.15 291.15 298.15 310.15 323.15

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

10.1 8.7 6.7 9.2 10.4 2.76 2.54 3.54 3.01 3.94 6.5 6.3 4.86 8.1 9.9 1.91 1.74 1.30 2.15 2.80 5.2 4.9 3.41 7.6 9.4 1.46 1.31 0.94 1.60 2.08

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.4 0.4 0.3 0.4 0.4 0.11 0.10 0.14 0.12 0.16 0.3 0.3 0.19 0.3 0.4 0.07 0.07 0.07 0.09 0.11 0.2 0.2 0.14 0.3 0.4 0.06 0.05 0.04 0.06 0.08

α

0.851

0.701

0.752

0.712

0.792

0.782

ΔmicH°a

ΔmicG°

TΔmicS°

ΔmicCpa

kJ mol−1

kJ mol−1

kJ mol−1

kJ mol−1 K−1

−25.7 ± 1.3

26 ± 2

−0.228 ± 0.018

−31.2 ± 1.6

31 ± 2

−0.254 ± 0.019

−28.9 ± 1.7

28.7 ± 1.9

−0.170 ± 0.011

−35.8 ± 1.8

33 ± 2

−0.276 ± 0.017

−29.1 ± 1.5

29 ± 17

−0.13 ± 0.08

−35.5 ± 1.8

32.7 ± 1.8

−0.228 ± 0.013

3.4 1.92 0.66 −1.89 −4.6 3.22 1.10 −0.42 −3.4 −7.1 2.43 1.32 −0.20 −2.18 −4.3 1.90 −0.58 −2.53 −5.8 −9.2 1.61 0.71 −0.15 −1.90 −3.4 0.50 −1.22 −2.78 −6.5 −9.1

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 0.11 0.07 0.10 0.3 0.16 0.10 0.06 0.2 0.4 0.12 0.09 0.08 0.13 0.02 0.12 0.10 0.15 0.3 0.5 0.11 0.11 0.09 0.11 0.2 0.11 0.07 0.15 0.3 0.4

Estimated by ITC; ITC experiments were performed in duplicate and the mean values were reported; 1ref 17. in NaCl0.1 M. 2ref 18.

Figure 2. Dependence of the specific conductivity, κ/μS cm−1, on BS concentration for NaT aqueous solutions, at (298.15 ± 0.10) K. The solid line corresponds to the Carpena fitting.

Figure 3. Effect of BS concentration on pyrene II/IIII ratio in aqueous NaDG solutions at (298.15 ± 0.10) K. The solid line corresponds to the fitting of the experimental data to a sigmoid (Boltzmann type) curve.

sodium glycholate, at 298.15 K. Here CMC is the critical micelle concentration in the presence of the different NaCl concentrations and [Na+] is the sodium ion concentration coming from the background electrolyte, NaCl. This relation is the well-known Corrin−Harkins plot, and the degree of counterion binding, β, where β = 1 − α, can be estimated from the absolute value of the slope.16 The α values obtained

by the two methods are listed in Table 1, together with some literature values. α cannot be obtained by any of the two methods commented above in the presence of NaCl 0.15 mol kg−1. The micellar ionization degrees summarized in Table 2 were taken from the literature. D



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The increase in the side chain length as well as the diminution of hydroxyls result in an increment of the hydrophobic/ hydrophilic balance of the BS molecules, which favors the transfer of the hydrophobic parts of the bile salt molecules from the aqueous phase into the micellar interior. The Gibbs energy change accompanying this transfer is the driving force for the self-association process of amphiphiles.23 It is known that hydrogen bonds are present in BS micelles and are likely to stabilize certain micelle configurations. However, the observed increase in ΔmicG° upon augmenting the number of hydroxyls was recently rationalized by theoretical calculations considering that hydrogen bonds do not drive aggregation.3 Changing the temperature leads to a change in sign of the micellization enthalpy. ΔmicH° becomes negative above room temperature and decreases further with increasing temperature, as was previously observed.17 This dependence shows that variations in hydrophobic hydration are involved in the micellization process. The contribution corresponding to the changes in hydration of the polar parts of the molecules is expected to be small because the environment of the polar groups does not substantially vary when aggregation of BS occurs.17 Information about the extent of hydrophobic hydration can be obtained from the change in heat capacity of micellization, ΔmicCp°, estimated from the slope of the ΔmicH° versus temperature plots, since ΔmicCp° is proportional to the hydrophobic surface area that gets exposed to water during the micellization process.24 Table 1 shows that the ΔmicCp° values are all negative, which indicates that the aggregation process leads to a diminution in the hydrophobic surface area exposed to water. This is in agreement with the formation of micelles being driven by the hydrophobic effect. It is possible to infer from the comparison of the ΔmicG°, ΔmicH°, and TΔmicS° values summarized in Table 1 that the BS aggregation is entropy driven. CMC values in Table 2 show that the presence of the background electrolyte makes the aggregation of BS easier. This is mainly due to a decrease in the electrostatic repulsions between the charged hydrophilic groups within the micelles, which stabilizes the aggregates. Besides, the NaCl 0.15 mol kg−1 aqueous solution is a worse solvent for the hydrophobic parts of the bile salt molecules than pure water, this favoring the transfer of the BS molecules from the bulk phase into the aggregates. However, the dependence of ln(CMC) on 1/T is similar to that shown in Figure 5, and the presence of NaCl 0.15 mol kg−1 does not cause a substantial effect on either the micellar ionization degree, or the ΔmicH° values. Consequently, ΔmicCp° is not practically influenced by the presence of NaCl 0.15 mol kg−1. As in pure water, when ΔmicG°, ΔmicH° and TΔmicS° values summarized in Table 2 are compared, it is possible to conclude that the bile salt aggregation in aqueous NaCl 0.15 mol kg−1 solution is entropy driven.

Figure 4. The Corrin−Harkins plot for the determination of the micellar ionization degree of NaG micelles in aqueous solution at (298.15 ± 0.10) K.

The ΔmicG° values were calculated using eq 1 and, once ΔmicG° and ΔmicH° were known, TΔmicS° could be estimated. ΔmicG° and TΔmicS° for the aggregation of the bile salts investigated at 298.15 K are summarized in Tables 1 and 2. The structure of bile salt micelles has been the subject of discussion since the 1960s, and different models have been proposed in the literature.17 Recent theoretical studies3,17 suggest that micelles formed are on average oblate. Contact between the steroidal backbones is mediated by hydrophobic interactions and through hydrogen bonding of hydroxyl groups and, as expected, charge groups remain preferentially on the outside of the micelles. However, the hydrophobic parts of the molecules are not fully shielded from water and they are often found at the surface of the micelles. Several molecular orientations are possible, and this ability of bile salts to pack in many different orientations is thought to be of biological significance.21 Figure 5 shows the dependence of ln(CMC) on

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Figure 5. Dependence of ln(CMC), expressed as mole fraction, on 1/ T for the bile salts investigated in this work at (298.15 ± 0.01) K.

CONCLUSIONS The aggregation of several bile salts in aqueous solution has been studied at different temperatures in the presence and in the absence of NaCl 0.15 mol kg−1. Results show that both a decrease in the number of ring hydroxyls and an increase in the length of the side chain favor micellization, in the absence as well as in the presence of salt. This is due to an increment of the hydrophobic/hydrophilic balance of the BS molecules, which favors the transfer of the hydrophobic parts of the bile salt molecules from the aqueous phase into the micellar interior. This transfer is the driving force for the self-association

1/T, in water, for all the BS studied, where the CMC is expressed as mole fraction. A minimum was observed at around 296 K. A similar result was obtained in previous works,22 where a maximum in the aggregation number was found at the same temperature. One can see in Figure 5 that both a decrease in the number of hydroxyl groups and an increase in the length of the side chain favor micellization. These observations are in agreement with previous experimental and theoretical results.3,4 E



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process of BS as is shown by the ΔmicCp° values, which indicate that the self-aggregation process leads to a diminution of the hydrophobic surface of the BS molecules exposed to water. Consideration of the ΔmicHo, ΔmicG°, and TΔmicS° values indicates that BS micellization is entropy driven. The presence of 0.15 mol kg−1 NaCl in the aqueous phase favors micellization mainly by decreasing the CMC due to the decrease of the electrostatic repulsions between the negatively charged groups of the BS molecules forming the micelles.

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(12) Desnoyers, J. E.; Perron, G. Temperature dependence of the free energy of micellization from calorimetric data. Langmuir 1996, 12, 4044−4045. (13) Carpena, P.; Aguiar, J.; Bernaola-Galván, P.; Carnero Ruiz, C. Problems associated with the treatment of conductivity−concentration data in surfactant solutions: Simulation and experiments. Langmuir 2002, 18, 6054−6058. (14) Zana, R.; Lévy, H. Alkanedyil-α,ω-bis(dimethylalkylammonium bromide) surfactants (dimeric surfactants). Part 6. CMC of the ethanedyil 1,2-bis(dimethylalkylammonium bromide) series. Colloids Surf. A 1997, 127, 229−232. (15) Paddon-Jones, G.; Regismonde, S.; Kwatkat, K.; Zana, R. Micellization of non ionic surfactant dimers and the corresponding surfactant monomers in aqueous solution. J. Colloid Interface Sci. 2001, 243, 496−502. (16) Moroi, A. Micelles. Theoretical and Applied Aspects; Plenum Press: New York, 1992; Chapters 4, 12. (17) Gardiel, P.; Hildebrand, A.; Neubert, R.; Blume, A. Thermodynamic characterization of bile salt aggregation as a function of temperature and ionic strength using isothermal titration calorimetry. Langmuir 2000, 16, 5267−5275. (18) Matsuoka, K.; Suzuki, M.; Honda, Ch.; Endo, K.; Moroi, Y. Micellization of conjugated chenodeoxy- and ursodeoxycholates and solubilization of cholesterol into their micelles: comparison with other four conjugated bile salts species. Chem. Phys. Lipids 2006, 139, 1−10. (19) Matsuoka, K.; Moroi, Y. Micelle formation of sodim deoxycholate and sodium ursodeoxycholate. Biochim. Biophys. Acta 2002, 1580, 189−199. (20) Coello, A.; Meijide, F.; Rodríguez, Núñez; Vázquez tato, J. Aggregation behavior of bile salts in aqueous solution. J. Pharm. Sci. 1996, 85, 9−15. (21) Warren, D. B.; Chalmers, D. K.; Hutchinson, K.; Dang, W.; Pouton, C. W. Molecular dynamics simulations of spontaneous bile salt aggregation. Colloids Surf. A 2006, 280, 182−193 and references therein. (22) Sugioka, H.; Matsuoka, K.; Moroi, Y. Temperature effect on formation of sodium cholate micelles. J. Colloid Interface Sci. 2003, 259, 156−162. (23) Nagarajan, R.; Wang, Ch-Ch. Theory of surfactant aggregation in water−ethylene glycol mixtures. Langmuir 2000, 16, 5242. (24) Costas, M.; Kronberg, B.; Silveston, R. General thermodynamic analysis of the dissolution of non polar molecules into water. Origin of hydrophobicity. J. Chem. Soc., Faraday Trans. 1994, 90, 1513−1522.

ASSOCIATED CONTENT

S Supporting Information *

The experimental calorimetric data from which the ΔmicH° values were derived are listed in Tables S1 to S60. The conductivity experimental data are summarized in Tables S61 to S65. The fluorescence experimental data are summarized in Tables S66 to S75. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. This work was financed by the DGCYT (grant CTQ200907478), the European Union and Consejeriá de Innovación, Ciencia y Empresa de la Junta de Andaluciá (FQM-274 and P12-FQM-1105).

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REFERENCES

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Thermodynamic Study of Bile Salts Micellization - Journal of Chemical

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