Langmuir 1995,11, 2860-2865
2860
Articles Interactions between Sodium Taurodeoxycholate Micelles and Cryptands Bianca Sesta,” Alessandro D’Aprano, Germana Maddalena, and Noemi Proietti Department of Chemistry, University of Roma “La Sapienza”, Piazzale Aldo Moro, 5, Roma 00185, Italy Received January 17, 1995. I n Final Form: April 10, 1995@ The influence of the synthetic macrocycle 4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexacosane on the intermolecular interactions of sodium taurodeoxycholate aqueous solutions has been investigated by cryoscopic, conductometric, and surface tension methods. The results show that the cryptand causes an appreciable shift of the critical micellar concentrations, toward the lower bile salt concentration. In addition, the complexing agent strongly affects the conductance and enhances the surfactant properties of the taurodeoxycholate. These features have been interpreted in terms of hydrophobic interactions between complexed counterion and amphipatic anion.
Introduction The physicochemical properties of bile salts have been largely investigated in the last y e a r ~ . l -The ~ interest for such compounds is justified by their important role in the metabolic adsorption processes a t the intestinal level. Furthermore, they are attractive subjects of investigation for the peculiarity of molecular structure and R e ~ e n t l y ,the ~ effects of sodium glycodeoxycholate (NaGDO) and taurodeoxycholate (NaTDO) on the transmembrane transport activated by natural or synthetic macrocycles were investigated in our laboratory. A synergism between the surfactants and the macrocycles was found, and some differences between the two biological surfactants were pointed out. The results were rationalized assuming that the penetration of the bile anions into the lipid bilayers1° causes a weakening of the black films. In such less rigid structures the cationic transmembrane flux,mediated by the macrocyclic carrier, is facilitated by hydrophobic interactions between the oxyethylenic rings, hosting the cations, and the bile salt counterions. To establish the existence of hydrophobic interactions between the cation inclusion complexes and the phenantrenic anions, both in the molecular state as well as in structured aggregates, the conductometric, viscosimetric, volumetric, and surface tension properties of sodium glycodeoxycholate in water and in water 4,7,13,16,2 1,24-hexaoxa-l,l0-diazabicyclo[8.8.8] hexacosane (C222)
+
* Author to whom correspondence should be addressed.
Abstract published in Advance A C S Abstracts, June 15,1995. (1)Small, D. M. In The Bile Acids;Nair, P. P.; Kritchevsky, D. Eds.; Plenum Press: New York, 1971;Vol. I, Chapter 8. (2)Carey, M. C.; Small, D. M. Arch. Intern. Med. 1972,130, 506. (3)Carey, M.C. In Bile Acids in Gastroenterology; Dowling, R. H., Hofmann, A. F. et al. Eds.; MTP Press, Lancaster, England, 1983. (4) Kratohvil, J. P. Hepatology 1984,4 , 856. (5)Ekwall, P. J. Colloid Sci. Suppl. 1964,1, 66. (6)Mazer, N. A.; Carey, M. C.; Kwasnick, R. F.; Benedek, G. B. Biochemistry 1979,18, 3064. (7)DArrigo, G.; La Mesa, C.; Sesta, B. J. Chem. Phys. 1980,73, 4562. (8)Sesta, B.; La Mesa, C.; Bonincontro, A,; Cametti, C.; Di Biasio, A. D. Ber. Bunsenges. Phys. Chem. 1981,85,803. (9)DAprano, A,; Sesta, B.; Filippi, C. J.Biol. Chem. In press. (10)Mazer, N. A.; Carey, M. C.; Kwasnick, R. F.; Benedek, G. B. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977;Vol. I. @
sodium taurodeoxycholate
cryptand 222
Figure 1. Structure formula of NaTDO and C222.
have been investigated by some of us.11J2The results have shown that hydrophobic interactions between the complexed cation and glycodeoxycholate anion enforces the Coulombic ion-ion interactions (i.e., the ion pairs formation) and enhances the micellization processes. In order to find out if such effects were peculiar to NaGDO or common to other bile salts derivatives, the research has been extended to include sodium taurodeoxycholate, for which a micellar assembly different from sodium glycodeoxycholate has been postulated.13J4 The osmotic, conductometric, and surface tension properties of NaTDO in water and in water C222 are presented and discussed in the present paper.
+
Experimental Section Materials. Sodium taurodeoxycholate (NaTDO),a Sigma product,was further purified by crystallization. It was solubilized in a minimum amount of bidistilled water and the solution filtered;reagent grade acetone was then added, drop by drop, to the concentrated aqueous solution of NaTDO until a white, soft precipitate was obtained. The solvent was finally removed. The crystallization was twice repeated and traces of solvent were stripped under vacuum, at 50 “C. 4,7,13,16,21,24-Hexaoxa-l,lO-diazabicyclo[S.S.S]hexacosane (C222) was a Merck-Schuchardt product, used
without furtherpurification. The structures ofboth NaTDO and C222 are given in Figure 1. (11)Sesta, B.; D’Aprano, A.; Princi, A,; Filippi, C.; Iammarino, M. J. Phys. Chem. 1992,96,9545. (12)DAprano, A,; Sesta, B.; Iammarino, M.; Filippi, C..; Princi, A,; Proietti, N. Langmuir 1994,10,2100. (13)Campanelli,A. R.;Candeloro De Sanctis,S.;Chiessi, E.; D’Alagni, M.;Giglio, E. J.Phys. Chem. 1989,93,1536. (14)Giglio, E.; Loreti, S.;Pavel, N. V. J.Phys. Chem. 1989,92,2858.
0743-7463/95/2411-2860$09.00/00 1995 American Chemical Society
Langmuir, Vol. 11, No. 8, 1995 2861
Interactions with Taurodeoxycholate Micelles
Table a. Conductance Data for NaTDO in Water and in Water C222 (0.04 m)at 26 "C
+
7s
water
water
water f C222
65
A R-1 1 0 4 ~ cm2 ~ mol-' 73.50 2.7891 72.92 6.8641 72.54 10.758 72.24 14.746 72.02 18.749 71.82 22.562 71.70 26.420 71.63 30.140 71.55 33.989 71.47 37.210 71.46 40.322 71.61 44.890 71.79 48.581 72.05 52.273 72.45 59.753 72.77 67.076 72.95 74.477 73.05 81.722
104CM c"2 mol-' 88.925 73.08 96.040 73.05 102.82 73.02 109.83 72.94 116.64 72.83 126.56 72.65 136.66 72.44 149.82 72.13 162.56 71.83 174.77 71.50 186.87 71.24 198.81 70.95 213.74 70.57 227.71 70.24 241.49 69.85 257.60 69.48 269.29 69.21
1 0 4 ~ cmz ~ mol-1 4.9280 44.98 10.176 44.22 15.445 43.73 20.749 43.28 28.516 43.02 36.001 43.02 47.197 43.21 57.919 43.08 68.393 42.75 78.677 42.35 88.925 41.81 101.81 41.25 109.62 40.67 119.68 40.13 129.50 39.57 138.77 39.07 151.04 38.44 163.33 37.86 178.49 37.06 196.01 36.26 215.80 35.39 237.47 34.46 261.47 33.42
'\
\
T \
1.
55
45
-9
a
i;
-7
-5
In e,
Figure 2. y us In c, for NaTDO in water at 15 "C (U) and at 25 "C (m) and in water C222 at 15 "C (0)and at 25 "C (0).
+
Table 1. Cryoscopic Data for NaTDO in Water and in Water C222 (0.04 m) at 2 1 "C
+
0.0
6.08 7.10 7.83 10.00
11.77 13.29 23.47 25.70 49.10 78.00
water 1 0 3 ~cp~ 0.0 1.0 2.2 0.98 2.6 0.98 2.8 0.98 3.6 0.97 4.2 0.96 4.8 0.96 8.4 0.96 9.0 0.94 15.9 0.87 22.0 0.76
water 104c, 1 0 3 78.90 22.3 103.0 26.7 137.0 32.6 170.0 37.3
~ D~ 0.76 0.72 0.64 0.59
183.0 227.0 291.0 323.0 383.0 419.0
0.57 0.54 0.49 0.48 0.48 0.46
-
39.0 45.8 53.6 58.2 65.2 71.4
water .t C222 1 0 3 ~a~ 0.0 0.0 1.0 13.40 3.9 0.91 23.90 6.6 0.87 33.70 8.6 0.80 46.30 10.3 0.70 104.0 16.5 0.50 142.0 20.0 0.44 190.0 23.6 0.39 253.0 28.9 0.36 398.0 46.1 0.36
ioc,
High purity water with specific conductivity xo = 1-2 x
(n-1cm-11, at 25 "C was obtained by distillation over KMn04 and NaOH.
Methods. All solutions were prepared by weight. Experimental runs were carried out by adding a concentrated stock solution of NaTDO to the solvent. For the ternary systems, a mixture of water C222 (0.04 m )was used as solvent and to prepare the master solution of NaTDO.
+
The freezing point depression, AT, of the binary and ternary systems was measured by a Knauer cryogenic unity provided with a resistance thermometer and an electronic display. About 1.5 cm3 of sample was supercooled by the Peltier effect. The transition temperature, T, from the metastable liquid to solid state, occurring by shaking the sample, was measured with an accuracy of i ~ 0 . 0 0 0 1"C. The calibration of the cryogenic
apparatus was made with NaCl solutions of known concentration, using Scatchard references.l5 Conductance measurements have been performed, with an accuracy of f0.01%, using a modified Shedlovsky resistance bridge already described.16 The electrical resistance of solutions, measured at 1, 2, 5, 10 kHz, were extrapolated to infinite frequency for the usual correction. A Fuoss-Chiu type conductance celP7 of about 100 mL was used. The cell constant, K, = 2.72431 h 0.00002 cm-l was determined with KCl aqueous solutions according to the method described by Lind, Zwolenik, and Fuoss.l8 The temperature of the cell was maintained at 25 f0.002 "C by a Leeds and Northrup 120 L volume oil thermostat. The conductance runs were made by the concentration method using the following procedure: a master solution of NaTDO in water or in water C222 (0.04 m ) was made up by using a weight buret. After determination of the solvent conductance (water or water C222) a weighed portion ofthe master solution was added into a known amount of solvent in the conductance cell to give the starting solution for the conductance run. Further points were obtained by adding successive weighed portion of the master solution to the cell. The molal (mol kg-l) concentrations of the solutions, c, were transformed to molar (mol L-l) concentrations,CM,by the equation
+
+
c&,
= do - Ac,
where do is the solvent density and A an experimentally determined constant. The measured resistance of each concentration of the conductivity run was converted in the corresponding specific conductance (x = KdR), corrected for the solvent contribute, xo, to obtain the equivalent conductance, A = (2 - &)1ooo/cM. The surface tension measurements were performed with the ring method, with an accuracy of hO.l dyn cm-l, by a Lauda digital tensiometer provided with an automatic device to select the rising velocity of the platinum ring and the time occurring between two measurements. The measures were carried out on about 20 cm3 of solution, contained in a glass vessel surrounded with a metallic cup through which thermostated water from an (15) Scatchard,G.;Prentiss, S. S. J.Am. Chem. SOC.1933,55,4355. (16)DAprano, A.; Fuoss, R. M. J . Solution Chem. 1974, 4 , 91. (17) Chiu, Y. C.; Fuoss, R. M. J.Phys. Chem. 1988,72,4123. (18)Lind, J. E., Jr.; Zwolenick, J. J.;Fuoss, R. M. J . Am. Chem. SOC. 1969,81, 1557.
Sesta et al.
2862 Langmuir, Vol. 11, No. 8, 1995 Table 3. Surface Tension of NaTDO in Water and in Water + C222 (0.04 m)at 16 and 25 "C 15 "C
water + C222
water 1 0 4 ~ ~
y, dyn cm-*
1 0 4 ~ ~
73.5 66.0 62.0 59.3 57.7 55.0 53.5 52.0 51.1 50.4 49.9 49.2 48.8 48.2 48.2 48.2 48.2
0.0 4.705 7.286 10.01 11.60 14.47 17.30 20.01 22.71 25.22 27.59 30.00 34.66 38.98 45.02 52.34 60.65
25 "C
0.0 1.753 5.167 7.099 9.235 11.52 13.25 15.03 16.57 19.06 21.83 25.33 30.52 35.66 40.53 47.41 59.38
water + C222
water
y dyn cm-I
62.2 58.8 56.8 55.6 54.1 53.2 52.2 51.5 51.0 50.5 50.0 49.5 49.2 49.0 49.0 49.0 49.0
1 0 4 ~ ~
0.0 0.5451 1.227 1.823 2.493 3.128 4.069 4.965 5.741 6.902 8.209 9.602 11.33 12.89 14.76 16.87 19.02 21.34 23.77 27.25 30.95 36.32 44.11 55.57
y dyn cm-I
104c, 0.0 1.839 4.009 6.032 8.002 10.13 12.08 13.89 18.12 21.69 27.36 34.04 41.36 50.76 62.84 67.95
72.0 63.8 62.0 60.4 59.2 58.4 57.1 56.1 55.0 54.0 52.9 52.3 51.3 50.7 50.0 49.0 48.3 48.0 47.9 48.0 48.0 48.0 48.0 48.1
y, dyn cm-I
58.6 53.0 52.0 51.1 50.3 49.6 48.8 48.0 47.0 46.5 46.5 46.5 46.5 46.6 47.0 47.0
Table 4. Surface Tension Results for NaTDO in Water and in Water + C222 (0.04 m)at 15 and 25 "C solvent T,"C 10Wm,, mol cm-2 102Amh,nm2 nm,, dyn cm-I 103cmc,mol kg-' AGomie,k J mol-' 25.3 3.5 -23 15 1.89 88 f 2 water 148 f 7 23.8 2.0 -25 water 25 1.13
water + C222 water
15 25
+ C222
1.04 0.64
160 f 10 260 f 10
Heto cryostatic bath (f0.02"C) was pumped. The apparatus was calibrated with acetone, methanol, and water.lg
Results and Analysis The freezing point depressions, AT = T - To(Tois the transition temperature of solvent), for the NaTDO solutions in water and in water C222 in the concentration range covering the primary micelles region20-22are listed in Table 1together with the BjerrumZ3osmotic function, @, calculated asz4
+
Cp = Cpf[l
+ O.OOllC,Of
- O.O00026(~,Cpf)~]
(1)
where @f = ATJ2K" (Kb is a constant depending on the fusion enthalpy of the solvent). @ gives a sensitive measure of the nonideality of a solution. A perusal of Table 1shows that the @valuesobtained in both solvents are near unity a t low bile salt concentration (0.002-0.003 m )and thereafter smoothly decrease. The result, expected for NaTDO in water where the transition between monomers and primary micelles has been observed in such concentration range,20-22indicates that a similar process occurs in water C222 mixtures. The conductometric results of NaDTO in water and in water C222 (0.04 m )at 25 "C are summarized in Table 2, where A (W1cm2mol-l) is the molar conductance and
+
+
(19) Weast, R. C.; Astle, M. J. Handbook ofchemistry and Physics, 61th ed.; CRC Press: Boca Raton, FL, 1980-81; pF-46. (20) Krahtovil, J. P.; Delli Colli, H. T. Can. J. Biochem. 1968,46, 3945. .. ~.
(21)Berchiesi, G.; Berchiesi, M. A,; La Mesa, C.; Sesta, B. J.Phys. Chem. 1984,88,3665. (22) Berchiesi, G.; La Mesa, C.; Sesta, B. Fluid Phase Equilibria 1985,20,241. (23) B j e m m , N. 2.Elektrochem. 1907,24, 259.
(24) Harned, H. S.;Owen, B. B. Thephysical ChemistryofElectrolytic Solutions Reinhold Publishing Corp.: New York, 1943.
13.2 11.8
3.0 1.7
-24 -26
CM the NaTDO concentration. An examination ofthe table reveals a n anomalous dependence of the molar conductance with NaTDO concentration. The results of the surface tension measurements a t 15 and 25 "C are reported in Table 3. Figure 2 represents the trend of the surface tension results ( y ) as a function of In cm for NaTDO in water and in water C222 mixtures at 15 and 25 "C. In the figure, the y values for the various systems have been added by a suitable constant to avoid the superimposition of the curves. According to the Gibbs equation,25the surface tension of 1:1electrolytes solutions decreases with the increasing of the salt concentration.
+
dy = -2I'PT d In a In the above equation T2 is the surface excess per unit area, R the gas constant, and Tthe absolute temperature. The parameter a refers to the activity of the electrolyte which, for very diluted solutions, may be approximated to the solute molal concentration. As can be observed from Figure 2, the surface tension of NaTDO in water, in agreement with eq 2, exhibits a regular decrease with increasing concentration up to about 3.5 x m at 15 "C and up to about 2.0 x m a t 25 "C. Above such concentrations, the surface tension remains practically constant. Such a behavior, typical of surfactant compounds, indicates the occurrence of the micellization processes. A similar trend may be observed for the ternary system NaTDO water C222. In this case, the surface tension changes its regular decrease a t and 1.7 x m a t 15 and 25 "C, about 3.0 x respectively.
+
+
(25) Gibbs, J. W. Collected Work; Longman Green Ed.: New York, 1928; Vol. 1.
Langmuir, Vol. 11, No. 8, 1995 2863
Interactions with Taurodeoxycholate Micelles The parameter Tz assumes its maximum value, rmsx, in proximity to the critical micellar concentration of surfactant. From this parameter the minimum area for the head group, Ami,,, may be calculated asz6
Amin= 1014/NTm,
100
(3) 80
where N is Avogadro's number. Table 4 summarizes the calculated values of rmax and the correspondingA,i,. Values of the maximum surface pressure (4)
around the critical range of concentrations are also reported in the table. In the above expression yo is the surface tension of the solvent. The measured values for water are yo = 73.5 dyn cm-l at 15 "C and yo = 72.0 dyn cm-' at 25 "C and for water C222 (0.04 m )yo = 62.2 dyn cm-' at 15 "C and yo = 58.6 dyn cm-l a t 25 "C. The values obtained in water are in good agreement with literature data.llJg Given the difficulties in the determination of the transition from molecular to aggregate systems (different from a typical detergent, the micellization process of bile salts occurs over a relative broad range of concentration), the data of Table 4 suffer some uncertainty, in particular for the systems containing C222 where the constancy in tensiometric curves appears after a broad decrease of y . Table 4 summarizes also the standard free energy of micellization, AGmio calculated by the equationz7
+
AGOmic= RT In X
(5)
where X is the molar fraction of solute a t the cmc.
rz
2(R(T*)2/hHe,,,)cmM= 2Kbc,
(6)
where r" is the boiling point of solvent, AHevapis the enthalpy of evaporation, M is the molecular mass of solvent, and the other symbols have the usual meaning. A value of 1.858 was used for the freezing point constant, Kb,in water. An inspection of Figure 3 shows that at very low NaTDO concentrations, the experimental curves follow the theoretical trend expected for the nonassociated 1 : l electrolytes, whereas a t the higher concentrations significant differences, between the experimental and theoretical values appear. The differences are, in addition, very much higher for the system containing C222 than for NaTDO in water. Since, as is well known, the freezing point depression depends on the population of the species in solutions, such differences may be assigned to the micellization process (Le. the decreases of the monomers in solutions). As far as the greater deviation observed for NaTDO in water C222 is concerned, we must assume that the presence of a further phenomenon concurs to the decrease of the number of species in solution. Such a phenomenon has, in addition, a favorable effect on the molecular aggregation of the sodium taurodeoxycholate, as shown by the starting point of the deviations from the
+
~~~~~~
(26)Rosen, M. J.;Aranson, S.Colloid Surf. 1981,3 , 201. (27)Mukejee, P.Adu. Colloid Interface Sci. 1967,1 , 241.
I 40
20
0
-
.
100
, 200
~
-~~ ,
300
400
10000 Cm
Discussion Figure 3 represents the freezing point depressions of NaTDO in water with and without C222 (curves a and b, respectively) as a function of bile salt concentration. Curve c represents the theoretical trend calculated as
AT
60
Figure 3. Freezing point depression of NaTDO in water + C222 (curve a) and in water (curve b) as a function of bile salt concentration. Curve c represents the theoretical trend.
theoretical trend occurring at about 0.002 m for NaTDO in water and a t about 0.0015 m in water C222 mixtures (see Figure 3). Another feature of Figure 3 is that instead of the sharp decrease of freezing points, shown by most of the s ~ r f a c t a n t sabove ~ ~ - ~the ~ critical micellar concentration, a smooth decrease of AT is observed for our systems. Such trend provides us a further experimental evidence of the progressive molecular aggregation occurring in the micellization process of the bile salts. Such peculiarity of the bile salts micellization is also evident in Figure 4, where the conductometric results are reported in the typical A versus 1 / c plot. ~ Both curves display, in fact, a dependence of conductance with concentration much more complex than that usually obtained for 1:1electrolytes. Analogous behavior obtained for sodium glycodeoxycholate in water and in water C222 has been previouslyll discussed in terms of transient structures (i.e., monomers, dimers, tetramers, and higher oligomers) in dynamical equilibrium. Some differences characterize the two conductometric curves. Among them we observe that the minimum and the maximum in the A functions in the water C222 system occur a t lower NaTDO concentrations with respect to water. Considering that the minimum is representative of.the point where the self-association of monomers in micelles modifies the Debye-Huckel ionic cloud respon-
+
+
+
(28)Burchfield,T.E.;Woolley, E.M. J . Phys. Chem. 1984,88,2149. (29)Desnoyers, J.E.;Caron, G.;De Lisi, R.; Roberts, D.; Rouse, A,; Perron, G. J . Phys. Chem. 1983,87,1397. (30)Sesta, B. J.Phys. Chem. 1989,93, 7677.
Sesta et al.
2864 Langmuir, Vol. 11, No. 8, 1995 7r
2!
74
2(
7c
15
ba
a
C 66
1c
62
5
b
sa 0
10
5
0
15
-1 0
1OOv/CM
-a
-7
-6
-5
lncM
Figure 4. Experimental trend ofA us JCM for NaTDO in water (0) and in water C222 (U) compared with Onsager's limiting
+
law.
-9
Figure 5. Il us In cm for NaTDO in water at 15 "C (0) and at 25 "C (U) and in water C222 at 15 "C (0)and at 25 "C (a).
+
where A, is the molar conductance a t infinite dilution and S = aA, /3 is a constant depending on temperature and on the bulk macroscopic dielectric constant and viscosity of solvent (a and /3 are the terms accounting for the relaxation and electrophoretic effect, respectively). As can be seen, while for NaTDO in water a t high dilution the experimental conductance is very close to the theoretical trend of eq 7, the experimental curve for C222 is far below the Onsager NaTDO in water
theoretical plot because of the ion pairing deriving from the Coulombic and hydrophobic interactions between the Na+-C222 complex and the amphipatic anion. The presence of ion pairs accounts for the reduction of ionic species postulated to explain the freezing point depression experiments in the water C222 mixture (see above). We discuss next the surface tension results. Shown in Figure 5 are the trends of the surface pressure, II = y yo as a function of NaTDO concentration and temperature. As can be seen, all the functions exhibit the typical break point observed for the surfactants a t the beginning of the micellization process; thereafter the surface pressure remains practically constant. As for the osmotic properties and the conductometric behavior, the transition from the monodispersed to the micellar state is not abrupt but occurs in relatively narrow range of concentrations. In presence of C222, the surface pressures slowly increase upon increasing the concentration, and the saturation of the interface by the surfactant molecules begins a t lower concentrations of NaTDO than in water. In Figure 5 we observe that, as found for other anionic surfactant^,^^,^^ the decrease of temperature significantly shifts the break point of the tensiometric curve of NaTDO in water toward higher bile salt concentrations. Such a result can be explained by considering that the micellization process may be regarded as an example of hydro-
(31)Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J.; Sen, D. Chem. Rev. 1986, 85, 271. (32) Onsager, L. 2.Phys. 1926,27, 338.
(33)Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. In Colloidal Surfactants;Academic Press: New York, 1963. (34) Flockhart, B. D. J . Colloid Sci. 1961,16, 484.
sible for the relaxation and electrophoretic effects (ie., the dependence of conductance with concentration), such features are in full agreement with the results obtained for the freezing point depression (see above). The increase in conductance afker the minimum is much more pronounced for NaTDO in water than in presence of C222. In our opinion such differences can be related to the shielding of the electric charge of sodium ion caused by the inclusion complex formation with C222 ligand3] a n d o r to some differences in the dynamics of the aggregation process discussed below. Significant differences appear in the conductance of NaTDO in water and in water C222 when compared (Figure 4)with Onsager's limiting law:32
+
A = A, - S ~ C ,
(7)
+
+
+
Interactions with Taurodeoxycholate Micelles
Langmuir, Vol. 11, No. 8, 1995 2865
phobic interactions, thus it is necessarily favored by an increase in temperature. The micellization free energy changes, listed in Table 4,give an estimate of the effect of temperature on the micellar aggregation of NaTDO in water and in water C222. As far as the enthalpic and entropic contributes to the total AGomic are concerned, it is worth pointing out that, according to Carey and the increase of the repulsive forces between the charged head groups promoted by the temperature, in the range between 10 and 30 "C, shows itself in an increase in the enthalpy of micellization; thus, we can assume that the NaTDO micelles in water, at least in the temperature range examined, are mainly stabilized by entropic effects. The dependence of ll on temperature for NaTDO in water C222 is quite different from that discussed for water. The surface pressure plots for the ternary systems are, in fact, less influenced by the temperature and lie close to one another in the premicellar region. This phenomenon may be due to the bulkier complexed cations which offer a high resistance to the dynamical motion occurring a t the airlwater interface. As shown in Table 4 the temperature and the presence of cryptand also influenced theA,i,. The values at 25 "C are, in fact, higher than that at 15 "C for both binary and ternary systems, whereas at each temperature the Ad,, in water C222 are about twice those in water. The
+
+
+
(35) Carey, M. C.; Small,
382.
D.M. J. Colloid Interface Sci.
1969, 31,
effect of the temperature can be attributed to the enforcement of the thermal motion a t the airlwater interface, whereas the increased size of the complexed cation that take part in the monofilm can be invoked to explain the effect of C222 on Am,,.
Conclusion Most of the experimental evidences discussed in the present paper indicate that the interactions between the bile salts and C222 are not limited to the complexation phenomenon of the cation but also impact the anionic organized structures in the micellar assembly as well as the ion pairing process at very low concentration. In the micellar systems, such interactions could be limited to ionic binding of the complexed cations a t the micellar surface or led to mixed micelles favored from the hydrophobic afinity between the phenantrenic group and the oxyethylenic chains. The trend of the surface pressure in the transition region, similar to that observed for the surfactants giving mixed micelles, appears in line with this hypothesis.
Acknowledgment. Financial support by MURST (Minister0 dell'Universitb e della Ricerca Scientifica e Tecnologica) is gratefully acknowledged. LA950032D
