Mixed Micelles of Sodium Deoxycholate and Polyoxyethylene Sorbitan Monooleate (Tween 80)

Journal of Colloid and Interface Science 217, 1–7 (1999) Article ID jcis.1999.6267, available online at http://www.idealibrary.com on

Mixed Micelles of Sodium Deoxycholate and Polyoxyethylene Sorbitan Monooleate (Tween 80) Md. Emdadul Haque, Akhil Ranjan Das, 1 and Satya Priya Moulik Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India; and Centre for Surface Science, Department of Chemistry, Jadavpur University, Jadavpur, Calcutta 700 032, India Received February 5, 1998; accepted April 14, 1999

species involving bile salts and biological lipids (6). Tween 80 is often used as a solubilizing and stabilizing agent in medicinal and pharmaceutical preparations (7– 8). In recent years a number of studies have been published on bile salt– ordinary surfactant systems (9 –14). However, systematic studies dealing with physicochemical behavior are rare. In this paper we report the results of our detailed investigations on the properties of sodium deoxycholate (NaDC), polyoxyethylene sorbitan monooleate (Tween 80), and their mixtures in bulk and at the air/water interface by tensiometric, conductometric, and fluorimetric methods. The composition of the mixed micelles and intermicellar interactions have also been estimated on the basis of recent theoretical models (15, 16).

The results of studies on the interaction of binary mixtures of sodium deoxycholate (NaDC) and polyoxyethylene sorbitan monooleate (Tween 80) in bulk and at the air/water interface obtained from conductance, surface tension, and fluorescence measurements are described. The critical micelle concentration (CMC), thermodynamics of micellization, free energy of interfacial adsorption, minimum average area occupied by the surfactant species at the interface, micellar polarity, and aggregation number of the mixed aggregates have been determined. The mixed micellar composition and the estimation of the interacting forces involved are evaluated on the basis of recent theoretical models. The estimated interaction parameter indicates an overall attractive force in the mixed state, and the proportion of NaDC in the mixed micelle is found to be lower compared to the stoichiometric compositions. The mixed aggregates with higher mole fractions of NaDC show less stability in comparison with those having higher proportions of the nonionic component. © 1999 Academic Press Key Words: sodium deoxycholate; polyoxyethylene sorbitan monooleate; mixed micelles; adsorption; micellization; energetics; microenvironment; composition.

MATERIALS AND METHODS

Sodium deoxycholate (NaDC), Sigma Chemicals, U.S.A., was 99% pure. It was further purified by recrystallization from mixed solvents of acetone and water and dried in a vacuum oven. Polyoxyethylene sorbitan monooleate (Tween 80) of Aldrich, U.S.A., was used as received. Pyrene (the fluorescence probe) from Aldrich, U.S.A., was purified by gel chromatography with cyclohexane. Cetyl pyridinium chloride (CPC), Aldrich, U.S.A., the quencher, was purified by repeated (three times) recrystallization from mixed solvents of acetone and isopropanol after decolorization with activated carbon. Aqueous solutions of surfactants were prepared in phosphate buffer (NaH 2PO 4, Na 2HPO 4 of BDH, U.K.), adjusting the pH in the range of 8.00 to 8.10, and maintaining 0.1 (M) ionic strength by NaCl (BDH, AR). All measurements were taken at 301 6 0.10 K in a thermostated water bath.

INTRODUCTION

Mixtures of surfactant solutions usually form mixed micellar aggregates that frequently exhibit characteristic properties that are remarkably different from those of the individual components (1). The properties of the solutions can be tuned to the desired requirements for various commercial applications, and thus combinations of surfactant solutions are extensively used in various industrial formulations. Mixed micelles are also very important in biology and biochemistry. Bile salts are biologically important molecules and are considered a special class of detergent because of their characteristic structural features composed of hydrophobic and hydrophilic moieties and surfactant-like properties (2). They play vital roles in a number of physiological processes such as lipid digestion, drug absorption, cholesterol solubilization, etc. (3– 6); the physiological functions of the bile salts depend on the formation of the mixed 1

Surface tension measurements. The surface tension of the surfactant solutions was measured with a du Nouy tensiometer Model K8 of KRUSS Germany using a platinum ring. The measured values were subjected to corrections following the procedure of Harkins and Jordan (17). The surfactant solution was added in installments by a microsyringe to water in a glass vessel at a constant temperature, and the surface tension was

To whom correspondence should be addressed.

1

0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.

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HAQUE, DAS, AND MOULIK

measured after thorough mixing with a Teflon coated magnetic stirrer. The accuracy of the instrument was 60.1 dyn/cm. Conductance. Conductance measurements were done in a Jenway (U.K.) conductometer at 10 mHz using a temperaturecompensated, dip-type cell whose cell constant was 1.39 cm 21. In an experimental run, the surfactant solution was added gradually with a microsyringe to water in a glass vessel, and the conductance was measured after attaining temperature equilibrium and thorough mixing. The error limits in measurements were within 60.5%. Fluorescence measurements. Steady-state pyrene fluorescence emission spectra were taken with a Perkin Elmer Model MPF 44B spectrofluorimeter. The excitation wave length was 335 nm, and the pyrene concentration was around 10 25 mol dm 23. The ratio of the intensity of the pyrene emission at 373 and 383 nm, respectively, denoted by I 1 /I 3 provides an estimate of the polarity sensed by pyrene in its micellar solubilization site (18 –20). The aggregation numbers were obtained from quenching experiments using pyrene as a probe and cetylpyridinium chloride as a quencher following a procedure described in the literature (21–24). The probe and the quencher are considered to be located in the same micellar environment and are distributed between the aqueous and micellar pseudophases according to Poisson statistics. The relationship of the pyrene fluorescence intensity to the average aggregation number in the absence and presence of a quencher is given by Ln~I 0 /I!@Q#N# $@S# 2 CMC%, where N# denotes the aggregation number, [Q] is the quencher concentration, I 0 and I represent pyrene emission intensity in the absence and presence of the quencher respectively, [S] is the total surfactant concentration, and CMC is the critical micelle concentration. The values of the aggregation numbers and I 1 /I 3 ratios were checked by time-resolved fluorescence quenching experiments. Prior to measurements, the solutions were degassed to remove oxygen. The apparatus and the procedure were reported elsewhere (25a).

TABLE 1 Values of CMC of Pure and Mixed Systems of NaDC–Tween 80 at 301 K by Different Methods (Ionic Strength 0.1 M) CMC (10 4 mol dm 23)

a NaDC

Surface tension

Fluorescence

Conductance

1.00 0.90 0.75 0.50 0.25 0.00

22 0.76 0.35 0.19 0.12 0.11

22 0.73 0.35 0.18 0.12 0.10

25 0.80 0.37 0.20 0.13 —

show close agreement; the conductance values are found to be slightly higher. The experimental CMCs are observed to be lower to a reasonable extent compared to the theoretical values (Table 5). The CMC values realized in this study of mixed systems show trends comparable with other bile salt–nonionic combinations recently studied (26, 27). The variations in CMC values depending on the method of determination have been reported in literature (26, 27). We used the CMC values obtained from surface tension measurements (CMC) ST for other calculations, comparison, etc. Counterion Binding (g) Ionic as well as ionic–nonionic mixed micelles bind a considerable amount of counterions that can be estimated by electrochemical measurements. The counterion binding of the pure and mixed micelles in this study was determined from the ratio of the postmicellar and premicellar slopes corresponding to the linear plots of specific conductance vs concentration of the surfactants following the procedure of Evans (30) (Fig. 2). The results are presented in Table 2. The extent of counterion binding g (fraction bound) of pure NaDC micelles is observed to be lower than the mixed aggregates of NaDC–Tween 80. The values of g of pure NaDC micelles change from 0.30 to 0.56 on addition of 25% Tween 80. Thereafter, the value decreases with increasing proportion of the latter. This point will be further discussed in a subsequent section.

RESULTS AND DISCUSSION

Interfacial Adsorption Critical Micelle Concentration Critical micelle concentrations of the pure and mixed surfactant systems obtained by different experimental methods are given in Table 1. The points of intersection in any physical property vs concentration plot indicate the CMC values. In Figs. 1 and 2 the results of surface tension and conductance measurements, respectively, are presented graphically. Fluorescence results are not shown to save space. The CMCs determined from surface tension and fluorescence methods

Amphiphiles orient at the air water interface and decrease surface tension. The concentration-dependent adsorption per unit area of the interface follows the Gibbs adsorption equation. For the surfactant mixture in water, the Gibbs surface excess is related to the surface pressure (surface tension of pure water 2 surface tension of solution) by the equation (29) dp 5

OG RT ln C , i

i

[1]

3

MIXED MICELLES OF NaDC AND TWEEN 80

FIG. 1. Surface tension vs log C plot for pure and mixed systems of NaDC and Tween 80 at 301 K. The type of system and the ordinate scale are indicated in each plot. The first component in the ratio is Tween 80 (ionic strength 0.1 M).

where G i is the surface excess, C i is the concentration of the ith component, and T is the Kelvin temperature. The maximum surface excess G max can be written as G max 5 1/4 z 61 RT lim ~d p /d log C !. C3CMC

The minimum area ( A min) per surface active molecule can be calculated from the expression A min 5 10 18 /NG max,

[3]

[2] where N is the Avogrado’s number.

FIG. 2. Specific conductance k vs C plots for pure and mixed systems of NaDC and Tween 80 at 301 K. The surfactant system and coordinate scale are indicated in each plot (ionic strength 0.1 M).

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HAQUE, DAS, AND MOULIK

TABLE 2 Values of G max, A min, and g (Counterion Binding) at 301 K (Ionic Strength 0.1 M) a NaDC

G max 3 10 6/mol m 22

A min /nm 2

g

1.00 0.90 0.75 0.50 0.25 0.00

0.93 0.71 0.72 0.73 0.81 0.67

1.79 2.35 (1.82) 2.31 (1.97) 2.27 (2.14) 2.05 (2.32) 2.48

0.30 0.54 0.56 0.44 0.32

Note. The values given in the parentheses follow from Eq. [4].

G max and A min have been evaluated for the present systems from the p vs log C plot (not shown to save space). The results are given in Table 2. Assuming ideal mixing, the minimum area per amphiphile molecule at the interface has been calculated from the relationship A min 5 a A min 1 ~1 2 a ! A min,

[4]

where a is the stoichiometric mole fraction of NaDC. The values presented in Table 2 show an opposite trend with respect to the experimental results; they decrease with an increasing molar fraction of Tween 80. Thermodynamics of Interfacial Adsorption and Micelle Formation The standard free energy of adsorption (DG ad) of the amphiphiles at the air/water interface is related to the free energy of micellization per mole of monomer unit (DG m), the surface pressure at CMC (p CMC), and the surface excess (at maximum adsorption G max) by the equation (30, 31) DG ad 5 DG m 2 ~ p CMC/G max!.

[5]

TABLE 3 (DG ) , DH , DS , D , and P CMC Values for Pure and Mixed Systems of NaDC–Tween 80 at 301 K (Ionic Strength 0.1 M) 0 m A

0 m

0 m

0 ad

a NaDC

(DG m0) Aa (kJ/mol)

DH m0 (kJ/mol)

DS m0 (kJ mol 21)

0 (DG ad ) (kJ/mol)

P CMC (dyn/cm)

1.00 0.90 0.75 0.50 0.25 0.00

215 (219) 223 (236) 225 (240) 227 (239) 228 (237) 228

20.70

48 (63) 78 (121) 85 (132) 90 (130) 94 (124) 95

244 (248) 255 (267) 258 (275) 256 (268) 252 (261) 258

26 22 23 21 19 20

21

Note. (DG m0) Aa 5 RT ln CMC. Values in parentheses are based on Eq. [6] (considering counterion binding).

FIG. 3. Plot of I 1 /I 3 vs surfactant concentration C of pure and mixed systems of NaDC and Tween 80 at 301 K. The first component of the ratio is Tween 80. Inset shows the plot of I 1 /I 3 vs mole fraction of NaDC at 301 K (at a total surfactant concentration of 100 mmol dm 23; ionic strength 0.1 M).

The p CMC and DG ad values that are given in Table 3 indicate fairly spontaneous adsorption comparable to the recently reported results on mixtures of bile salts with synthetic surfactants (25). The free energies of micellization, DG m, calculated (36) from the equation DG m 5 ~1 1 g! RT ln CMC, using the (CMC) ST and g values (given in Tables 1 and 2, respectively), are presented in Table 3. We attempted to determine the enthalpy of micellization (DH m) by calorimetry. Except for pure NaDC, the enthalpy of micellization has been found to be very low, falling in the uncertainty limit of the instrument and, therefore, taken to be zero. The entropy of micellization (DS m) is obtained from Gibbs equation (DS m 5 (DH m 2 (DG m)/T). The process of micelle formation is found to be entropy-controlled. Randomness arising out of the melting of “icebergs” around the nonpolar tail (34) during micellization, along with the increased disorder of the tails in the interior, are responsible for the observed increase in entropy. Similar behavior has been reported in the micellization process of other bile salt– ordinary surfactant systems (25a, b).

5

MIXED MICELLES OF NaDC AND TWEEN 80

TABLE 5 x, b, and CMC (Theoretical) Values for the Mixed Combinations at 301 K (Ionic Strength 0.1 M)

Microenvironment of the Micelles Information about the solubilization site sensed by pyrene in the surfactant micelles can be obtained from the measurements of the first and third vibronic peaks, I 1 /I 3 , in a monomeric pyrene fluorescence emission spectrum. The ratio is known to be a sensitive index of the polarity in the probe microenvironment (18 –20). Low values of I 1 /I 3 indicate that the environment of solubilized pyrene is nonpolar as in hydrocarbon solvents, 0.6 for cyclohexane and n-hexane, etc. The I 1 /I 3 values in polar solvents are generally high at 1.23. In Fig. 2 the dependence on surfactant concentration of I 1 /I 3 values for the pure and mixed systems are given. The inset of Fig. 3 shows the profile of I 1 /I 3 as a function of the NaDC mole fraction (a NaDC ). The ratio decreases with concentration for all of the surfactant combinations. It is observed that the environment of pure Tween 80 is more polar compared to that of pure NaDC; the polarity of the mixed species lies in the intermediate range. Table 4 contains the I 1 /I 3 and N# values for the pure and mixed systems at the total surfactant concentration of 100 m mol dm 23. Intersurfactant Interaction in Micelles The mixed CMC of a binary surfactant mixture can be estimated by Clint’s relationship (15a) 1/CMCmix 2 a /CMCNaDC 1 ~1 2 a !/CMCTween 80,

[7]

where a is the mole fraction of NaDC in the surfactant mixture. The values at different ratios shown in Table 1 are found to be higher than those obtained from the experimental results. The mixed solutions of NaDC and NaC with alkylsulfates studied by surface tension and fluorescence methods have shown similar negative deviation from ideal behavior (35). The interaction parameter b R for the mixed micelle forma-

TABLE 4 # ) of Pure Values of Polarity (I 1/I 3) and Aggregation Number (N and Mixed Systems of NaDC–Tween-80 at 301 K (Ionic Strength 0.1 M)

a

a NaDC

I 1 /I 3

N#

1.00 0.90 0.75 0.50 0.25 0.00

0.73 0.83 (0.77) 0.92 (0.84) 1.02 (0.94) 1.07 (1.05) 1.15

12 31 53 74 91 124

Values in the parentheses are calculated from an equation (16).

a NaDC

xR

xM

CMC (ideal) a

CMC (calculated) b

0.90 0.75 0.50 0.25

0.21 0.15 0.10 0.12

0.29 0.18 0.06 0.14

1.05 0.43 0.22 0.15

0.79 0.32 0.18 0.13

b av 24.30

a

Ideal CMCs obtained from Clint’s equation. CMC (calculated) values were obtained from the following expression 1/CMC mix 5 a NaDC/( f NaDC CMC NaDC) 1 (1 2 a NaDC)/( f Tween 80 CMC Tween 80) using the average b 5 24.30, Eqs. [10] and [11], respectively. b

tion has been calculated from Rubingh’s (15b) equation based on regular solution theory,

b R 5 ln~CMCmixa/CMCNaDCx R!/~1 2 x R! 2,

[8]

where x R is the mole fraction of NaDC in a mixed micelle; x R is obtained from the equation x 2Rln~CMCmixa/CMCNaDCx R! 5 1. [9] ~1 2 x R! 2 ln~CMCmix~1 2 a !/CMCTween 80~1 2 x R!! The activity coefficients f RNaDC and f RTween 80 in the mixed micelle are evaluated by the following equations: f NaDC 5 exp@ b R~1 2 x R! 2 # R 80 f Tween 5 exp@ b Rx 2R#. R

[10] [11]

Motomura et al. (5) have formulated an analytical model for the estimation of micellar composition in g terms of excess thermodynamic functions. The relationships and procedure have been discussed in recent publications (25a, 32) and so are not presented here. The micellar mole fraction of NaDC by the methods of Rubingh ( x R), Motomura et al. ( x M), and b by Rubingh are given in Table 5. The free energy of micellization without considering counterion binding (DG m0) A in Table 3 has shown a regular spontaneous trend with a NaDC yielding corresponding positive entropy changes. The free energy of interfacial adsorption 0 (DG ad ) A calculated using Eq. [5] also follows the above behavior. The adsorption process is found to be more favorable compared to micellization observed in other mixed surfactant solutions (25a, b). The corrected free energy of micellization (DG m0), entropy of micellization (DS m0), and free energy of 0 adsorption (DG ad ) are shown in parentheses in columns 2, 4, and 5, respectively, in Table 3. The thermodynamic parameters

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HAQUE, DAS, AND MOULIK

show maximum values corresponding to a NaDC 5 0.75. This may be basically due to the high value of g at the same composition of the mixed aggregate. The counterion binding pattern exhibits conspicuous deviation from the normal behavior found in usual micellar assemblies. The mixed species of NaDC and Tween 80 in the presence of NaCl (0.1 M) show the usual trend of increasing counterion condensation with increasing and decreasing proportions of the nonionic and ionic components, respectively. The highest value of counterion association is noted to correspond to an intermediate composition of the mixed micellar aggregates in the presence of NaCl (0.1 M). We also observed unusual behavior of the counterion binding profile in mixed micelles of NaDC and TX-100 in a previous study (25a). The counterion binding phenomenon depends on the micellar surface charge density and aggregation characteristics. The unusual trend observed may be due to the various structural rearrangements and the associated modifications occurring in the process of mixed micelle formation. We can offer no explanation for this unusual behavior. However, we hope to obtain more information on this point when the results of our ongoing investigations involving other bile salt–nonionic mixed micellar combinations are available. The CMCs of the mixed systems evaluated on the basis of ideal mixing of binary surfactant mixtures following Clint’s equation are found to be lower compared to those from experimental measurements (Table 5). The nonideal pseudophase model was applied to the experimental CMC data for the mixed surfactant solutions to obtain the interaction parameter according to Rubingh’s treatment. An average of b 5 24.30 was obtained. The negative value of b indicates synergistic behavior of the components of NaDC and Tween 80 in the mixed micelles. Comparable negative b values were also obtained in some mixed systems of bile salt–nonionic, anionic–nonionic and nonionic–nonionic combinations (32). Theoretical CMC values calculated using the value are given in Table 5 for comparison with those from experimental measurements and Clint’s equation. The compositions of the mixed micelles were obtained following the methods of Rubingh’s and Motomura (Table 5). The mole fraction of NaDC in the mixed species is found to be lower compared to the stoichiometric composition. The molar composition of NaDC in the mixed micelles evaluated by Motomura’s method are found to be higher than those by Rubingh’s approach. Figure 4 depicts the profile of x R and x M as a function of a NaDC. The values of x R and x M show deviation from a regular trend at a NaDC 5 20.25. This may be a reflection of the unstable nature of mixed species at lower mole fractions of the bile salt component. The I/I 3 values decrease steadily with increasing mole fraction of NaDC, show a break point at 0.75 mole fraction, and then decrease sharply above the point (Fig. 2). An abrupt

FIG. 4. Plot of mole fraction of NaDC in the mixed micelle from Rubingh’s models vs stoichiometric mole fraction of NaDC. The dotted line b 5 0; symbol 0, b 5 24.30; ionic strength 0.1 M.

decrease of I 1 /I 3 indicates a considerable increase of hydrophobicity characters in the micelle and consequent change of the solubilization of pyrene. The results suggest a transition of the solubilization site of pyrene from the palisade layer into the micelle interior. The observed sudden decrease of N# may be attributed to the wide difference in the cohesion or shapes of the two structurally different amphiphiles forming mixed micelles. The aggregation numbers of the mixed aggregates are found to be much lower than that of Tween 80 due to the presence of NaDC with small values of N# . Similar behavior of the mixed micelles containing bile salts was observed earlier (10, 25). The polarity of the interior of the mixed micelles shows variation with the size, i.e., aggregation number (Fig. 5). A similar trend is also noticed with the mole fraction ( x R). Both the lines representing the variations when extrapolated correspond to a polarity value of pure NaDC micelles. The decrease of polarity of the mixed species corresponds to the concentration of NaDC in the aggregates. The polarity of the mixed micelle estimated on the basis of the relationship ~I/I 3 ! obs 5 a ~I 1 /I 3 ! NaDC 1 ~1 2 a !~I 1 /I 3 ! Tween 80

[12]

yields a value lower than that observed (shown in the parentheses of column 2, Table 4). As mentioned earlier we also found deviations in the mixed CMCs obtained experimentally compared to the theoretical values (Table 5). The results indicate nonideal behavior of the mixed micellar solutions.

MIXED MICELLES OF NaDC AND TWEEN 80

FIG. 5. Plot of polarity index (I 1 /I 3 ) vs N and x R for pure and mixed systems at 301 K (ionic strength 0.1 M).

CONCLUSIONS

The CMCs of the mixed micelles obtained by surface tension and fluorescence methods show close agreement, whereas slightly higher values are found from conductance measurements. The experimental mixed CMCs are lower than those mix calculated theoretically. The minimum area A min of the mixed surfactants at the air/water interface lies between the values of the pure components. The A min values from experimental and stoichiometric considerations show an opposite trend. Mixed micelle formation involving NaDC and Tween 80 is entropycontrolled. The polarity index and aggregation numbers of the mixed micelles vary with composition, and the values decrease with an increasing proportion of NaDC in the mixed system. The mole fraction of NaDC in the mixed micelles is much lower compared to the stoichiometric concentration. ACKNOWLEDGMENTS We thank Professor M. Chowdhury and Dr. K. Bhattacharya, Department of Physical Chemistry, IACS for use of the fluorescence measurement facility.

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