Incorporation of Micelle-Forming Local Anesthetics into Surface

The incorporation of two micelle-forming local anesthetics, dibucaine hydrochloride (DC·HCl) and tetracaine hydrochloride (TC·HCl), into surface-ads...
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Langmuir 1997, 13, 2687-2693

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Incorporation of Micelle-Forming Local Anesthetics into Surface-Adsorbed Films and Micelles of Decylammonium Chloride Hitoshi Matsuki,* Shinichiro Hashimoto, and Shoji Kaneshina Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770, Japan

Michio Yamanaka Department of Chemistry, Faculty of Science, Kyushu University Ropponmatsu, Ropponmatsu, Fukuoka 810, Japan Received September 18, 1996. In Final Form: March 10, 1997X The incorporation of two micelle-forming local anesthetics, dibucaine hydrochloride (DC‚HCl) and tetracaine hydrochloride (TC‚HCl), into surface-adsorbed films and micelles formed by decylammonium chloride (DeAC) was studied by measuring the surface tension of aqueous solutions of DeAC-DC‚HCl and DeAC-TC‚HCl binary mixtures. The quantities of the anesthetics incorporated into the adsorbed film and micelle of DeAC were estimated from the compositions of the anesthetics in the adsorbed film and micelle, which were obtained by thermodynamic analysis of surface tension data. The phase diagrams of surface adsorption and micelle formation constructed on the basis of composition in the molecular aggregates indicated that although significant amounts of both anesthetics were incorporated into the adsorbed film and micelle, larger quantities of DC‚HCl than of TC‚HCl were present in the hydrophobic environment of the aggregates. The relative ease with which these two anesthetics were incorporated into the model membranes correlates with their observed clinical potency. Interactions between DeAC and the anesthetic molecules in the adsorbed and micellar states were also analyzed by calculating the ideal mixing lines in both states. It was shown that the bulky polar head groups of the anesthetics were more favorable for micelle formation than for surface adsorption by comparing the incorporation of the anesthetics into the adsorbed film with that into the micelle at the critical micelle concentration.

Introduction Local anesthesia results from nerve blockage in the cellular membrane of the neuron as the ion permeability of the sodium channel in the membrane changes. A detailed molecular mechanism for local anesthesia has not yet been clarified, but it has been proposed that local anesthetic molecules either interact with membrane proteins directly or disturb the phospholipid matrix in the membrane and subsequently influence the proteins indirectly. Since local anesthetics are known to expand cellular membranes as part of the action of anesthesia, they must first be incorporated into the hydrophobic environment of the cell membrane. It is especially expected that local anesthetic molecules are easily incorporated into this environment because they are amphiphile molecules acting as a kind of cationic surfactant. It is interesting that the colloidal properties of these anesthetics, e.g. surface adsorption, micelle formation, and Krafft phenomenon, etc., seem to be caused by the same hydrophobic driving force which causes the anesthetics to be incorporated into cell membranes. We have characterized their molecular aggregate formation thermodynamically by measuring the physicochemical properties of aqueous anesthetic solutions.1-5 In the next step, it is important to clarify the mechanism by which anesthetic * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Satake, H.; Matsuki, H.; Kaneshina, S. Colloids Surf., A: Physicochem. Eng. Aspects 1993, 71, 135. (2) Matsuki, H.; Hashimoto, S.; Kaneshina, S.; Yamanaka, M. Langmuir 1994, 10, 1882. (3) Matsuki, H.; Maruyama, S.; Kaneshina, S. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 97, 21. (4) Matsuki, H.; Yamanaka, M.; Kaneshina, S. Bull. Chem. Soc. Jpn. 1995, 68, 1833. (5) Matsuki, H.; Ichikawa, R.; Kaneshina, S.; Kamaya, H.; Ueda, I. J. Colloid Interface Sci. 1996, 181, 362.

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molecules are incorporated into hydrophobic environments, using this as a model for the interaction of the anesthetics with biomembranes. Many studies concerning the incorporation or partition equilibrium of anesthetics into model membranes have been performed using multilamellar vesicles of phospholipid bilayers.6-15 On the other hand, molecular aggregates such as surface adsorbed films and micelles formed by surfactants produce microscopic hydrophobic environments and are also suitable for use as a model system for a biological membrane. Moreover, the latter system may actually be more desirable because experimental values using the surface adsorbed films and micelles can easily be obtained at a thermodynamic equilibrium, whereas the experimental values using a phospholipid bilayer must be obtained in a metastable equilibrium state. However, few studies examining the incorporation of local anesthetics into the molecular aggregates of surfactants have been carried out.16,17 This (6) Papahadjopoulos, D.; Jacobson, K.; Poste, G.; Shepherd, G. Biochim. Biophys. Acta 1975, 394, 504. (7) Kamaya, H.; Ueda, I.; Moore, P. S.; Eyring, H. Biochim. Biophys. Acta 1979, 550, 131. (8) Schlieper, P.; Steiner, R. Chem. Phys. Lipids 1983, 34, 81. (9) Ohki, S. Biochim. Biophys. Acta 1984, 777, 56. (10) Limbacher, H. P., Jr.; Blickenstaff, G. D.; Bowen, J. H.; Wang, H. H. Biochim. Biophys. Acta 1985, 812, 268. (11) Auger, M.; Jarrell, H. C.; Smith, I. C.; Siminovitch, D. J.; Mantsch, H. H.; Wong, P. T. Biochemistry 1988, 27, 6086. (12) Kaminoh, Y.; Tashiro, C.; Kamaya, H.; Ueda, I. Biochim. Biophys. Acta 1988, 946, 215. (13) Bottner, M.; Winter, R. Biophys. J. 1993, 65, 2041. (14) Ueda, I.; Chiou, J. S.; Krishna, P. R.; Kamaya, H. Biochim. Biophys. Acta 1994, 1190, 421. (15) Peng, X.; Jonas, A.; Jonas, J. Chem. Phys. Lipids 1995, 75, 59. (16) Kaneshina, S.; Kamaya, H.; Ueda, I. Biochim. Biophys. Acta 1984, 777, 75. (17) Louro, S. R.; Nascimento, O. R.; Tabak, M. Biochim. Biophys. Acta 1994, 1190, 319.

© 1997 American Chemical Society

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may be due to the fact that biological membranes are composed of two hydrophobic chains of phospholipids, not one chain of surfactants. In the present study, we selected two local anesthetics, dibucaine hydrochloride (DC‚HCl) and tetracaine hydrochloride (TC‚HCl). Both have shown strong anesthetic potency in clinical use18,19 and are known to form micelles in aqueous solution.2,20-22 In order to examine the incorporation of the anesthetics into surface-adsorbed films and micelles of the cationic surfactant, decylammonium chloride (DeAC), we measured the surface tension of aqueous solutions of DeAC-DC‚HCl and DeAC-TC‚HCl mixtures. The level of incorporation for the anesthetics was determined using thermodynamic phase diagrams of surface adsorption and micelle formation. Differences in incorporation levels between DC‚ HCl and TC‚HCl were examined, and the difference between the incorporation of the anesthetics into the surface-adsorbed film and that into the micelle was also analyzed. Experimental Section Materials. Dibucaine (2-butoxy-N-[2-(diethylamino)ethyl]4-quinolinecarboxamide) hydrochloride and tetracaine (2-(dimethylamino)ethyl 4-(butylamino)benzoate) hydrochloride were purchased from Sigma Chemical Company. The anesthetics were purified by repeated recrystallizations from a mixture of ethanol and carbon tetrachloride for dibucaine hydrochloride and from ethanol for tetracaine hydrochloride, respectively. Decylammonium chloride was synthesized by neutralization of decylamine (Tokyo Kasei Kogyo (Japan)), which was fractionally distilled under reduced pressure, with hydrochloric acid. The obtained product was recrystallized four times from ethanol. The purities of the anesthetics and surfactant were confirmed by elemental analysis and by the fact that there was no minimum on their surface tension vs concentration curves in the vicinity of the critical micelle concentration. Water was distilled three times after deionization, where the second and third stages were done from dilute alkaline permanganate solution. Automatic Surface Tension Measurements. An automatic surface tension measurement apparatus based on the principle of the drop volume method23 was employed for measurements of the surface tension of aqueous surfactantanesthetic solutions. The measurements were carried out with the temperature kept constant at 298.15 K within 0.01 K by immersing the measurement cell in a thermostat under atmospheric pressure. The densities of aqueous solutions of anesthetics, surfactant, and their mixtures, which are required to estimate the surface tension values of their aqueous solutions, were measured by a vibrating-tube density meter (Anton Paar DMA60/602) under the same experimental conditions as the surface tension measurements described previously.2 The experimental error for the value of the surface tension was less than 0.05 mN m-1.

Results We chose the total molality of DeAC and local anesthetic m and the mole fraction of anesthetic in the DeACanesthetic mixtures X2 as the independent variables at constant temperature and pressure, since it was shown that both concentration variables are advantageous in examining the miscibility of surfactants in their molecular aggregates such as surface-adsorbed films and mi(18) Covino, B. G.; Vassallo, H. G. Local Anesthetics: Mechanism of Action and Clinical Use, 1st ed.; Grune and Stratton: New York, 1976. (19) Covino, B. G. In NEURAL BLOCKADE in Clinical Anesthesia and Management of Pain, 2nd ed.; Cousins, M. J., Bridenbaugh, P. O., Eds.; J. B. Lippincott Company: Philadelphia, 1988; p 111. (20) Fernandez, M. S. Biochim. Biophys. Acta 1980, 597, 83. (21) Attwood, D.; Florence, A. T. In Surfactant Systems; Chapman and Hall: London, 1983; Chapter 4, p 153. (22) Attwood, D.; Fletcher, P. J. Pharm. Pharmacol. 1986, 38, 494. (23) Matsuki, H.; Kaneshina, S.; Yamashita, Y.; Motomura, K. Langmuir 1994, 10, 4394.

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Figure 1. Surface tension vs total molality curves of the DeACDC‚HCl system at constant composition: (1) X2 ) 0, (2) 0.190, (3) 0.350, (4) 0.500, (5) 0.640, (6) 0.735, (7) 0.828, (8) 0.916, and (9) 1.

Figure 2. Surface tension vs total molality curves of the DeACTC‚HCl system at constant composition: (1) X2 ) 0, (2) 0.288, (3) 0.500, (4) 0.613, (5) 0.702, (6) 0.782, (7) 0.862, (8) 0.932, and (9) 1.

celles.24,25 Here m and X2 are defined respectively by the equations

m ) m1 + m2

(1)

X2 ) m2/m

(2)

and

where m1 and m2 represent the molalities of DeAC and anesthetic, respectively. The surface tension measurements for the aqueous solutions of the mixtures were performed as a function of m and X2. The resulting surface tension γ vs m curves of the DeAC-DC‚HCl system and the DeAC-TC‚HCl system at various compositions are illustrated in Figures 1 and 2, respectively. The γ values of both systems are seen to decrease with increasing m, and the shape of the γ vs m curves changes regularly with X2 from DeAC (X2 ) 0) to anesthetics (X2 ) 1). We observe that the γ vs m curves of pure DC‚HCl and TC‚HCl have break points attribut(24) Motomura, K.; Ando, N.; Matsuki, H.; Aratono, M. J. Colloid Interface Sci. 1990, 139, 188. (25) Motomura, K.; Aratono, M. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; p 99.

Anesthetic Incorporation into Surfactant Aggregates

Figure 3. Total molality vs composition curves of the DeACDC‚HCl system at constant surface tension: (1) γ ) 65, (2) 60, (3) 55, (4) 50, (5) 45, (6) 40, and (7) 35 mN m-1; (8) C vs X2.

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Figure 5. Surface tension at the cmc vs composition curves: (1) DeAC-DC‚HCl system; (2) DeAC-TC‚HCl system.

γC are determined from the break points of the γ vs m curves in Figures 1 and 2 or from those of the γ vs log m curves obtained from the replot of the γ vs m curves. In Figures 3 and 4 are also depicted the C values in the form of the C vs X2 plots, and the γC vs X2 curves of both systems are shown in Figure 5. It is found that the C vs X2 curves and γC vs X2 curves of both systems are similar in shape to each other; the C and γC values increase monotonously with an increase in X2. On the basis of the above surface tension data, the incorporation of the anesthetics into the surface-adsorbed film and micelle formed by DeAC will be considered below. Discussion Because local anesthetics (AHCl) in clinical use are weak electrolytes with relatively high pKa values, they dissociate in aqueous solution as shown by the following two steps:

Figure 4. Total molality vs composition curves of the DeACTC‚HCl system at constant surface tension: (1) γ ) 65, (2) 60, (3) 55, (4) 50, (5) 45, (6) 40, and (7) 35 mN m-1; (8) C vs X2.

able to the micelle formation of the anesthetics in the aqueous solution,2 and all the γ vs m curves of mixtures break at concentrations corresponding to the critical micelle concentration (cmc). It is noted in the DeACDC‚HCl system that the γ vs m curve of pure DeAC intersects that of pure DC‚HCl at 15.76 mmol kg-1 and 57.45 mN m-1, and the composition dependence of γ below the point and that above the point reverse the order. Reading the m values at a given γ value from Figures 1 and 2, the m vs X2 curves at constant γ are obtained. The results are shown for the DeAC-DC‚HCl system in Figure 3 and for the DeAC-TC‚HCl system in Figure 4. The m values of the DeAC-TC‚HCl system increase with increasing X2, and the slope of the curve becomes steeper at a lower γ value. In the case of the DeAC-DC‚HCl system, the values of m at a high γ decrease slightly with increasing X2 while those at a low γ increase with X2, as expected from Figure 1. Further, the values of the total molality at the cmc C and the surface tension at the cmc

AHCl S AH+ + Cl-

(3)

AH+ S A + H+

(4)

where AH+ and A represent the protonated (charged) and free base (uncharged) forms of the anesthetic, respectively. The first step for the dissociation of a hydrochloride salt is usually complete, so the second step becomes the determining step in the dissociation. The acid dissociation constant Ka of a local anesthetic cation is expressed as

Ka ) [A][H+]/[AH+]

(5)

where the square brackets denote concentration. The partition of charged and uncharged local anesthetic molecules in aqueous solution can be described by the Henderson-Hasselbalch equation:

pH ) pKa + log [A]/[AH+]

(6)

We measured the pH value for the aqueous solutions of pure anesthetics and mixtures, and pH values between 5.0 and 5.5 were obtained in the concentration range measured. Taking into consideration literature pKa values

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of 8.73 and 8.46 for DC‚HCl and TC‚HCl, respectively,26 eq 6 shows that the effect of partial dissociation of the anesthetic cation is negligible. In addition, the literature pKa value of the decylammonium ion is 10.64.27 We therefore analyzed the surface tension data by assuming that DeAC and the local anesthetics were essentially uni-univalent electrolytes. The quantities of the anesthetics incorporated into surface-adsorbed films and micelles formed by DeAC can be determined by examining the miscibility of DeAC and the anesthetics in these molecular aggregates. Motomura et al. have shown24,25 that the compositions of surfactants in the adsorbed film and micelle, determined by the application of advanced thermodynamic equations to the surface tension data, using surface excess thermodynamic quantities can provide useful information regarding the miscibility of surfactants in the adsorbed film and micelle. In this study we have adopted these thermodynamics of surface adsorption and micelle formation for uni-univalent ionic surfactants with a common ion25 and have analyzed the experimental data by use of the thermodynamic equations. Incorporation of DC‚HCl and TC‚HCl into the Surface-Adsorbed Film of DeAC. Let us start by examining the incorporation of DC‚HCl and TC‚HCl into surface-adsorbed films of DeAC. The total quantity of DeAC and anesthetic existing in the surface-adsorbed film is represented by an important surface quantity, total surface density ΓH, defined as H ΓH ) ΓH 1 + Γ2

Figure 6. Total surface density vs total molality curves of the DeAC-DC‚HCl system at constant composition: (1) X2 ) 0, (2) 0.190, (3) 0.350, (4) 0.500, (5) 0.640, (6) 0.735, (7) 0.828, (8) 0.916, and (9) 1; (b) total surface density at the cmc ΓH,C.

(7)

H where ΓH 1 and Γ2 are the surface densities of DeAC and anesthetic, which are respectively defined with reference to the two dividing planes, making the excess numbers of moles of water and air zero.25 Assuming an ideal solution and applying the next equation25

ΓH ) -(m/2RT)(∂γ/∂m)T,p,X2

(8)

to the slope of the γ vs m curves at concentrations below the cmc shown in Figures 1 and 2, ΓH values are calculated and demonstrated in the form of ΓH vs m plots in Figures 6 and 7. These values are observed to increase with both increasing m at constant X2 and decreasing X2 at constant m in both systems. The regularity of variation in ΓH vs m curves with X2 suggests that DeAC and the anesthetics are completely miscible in the adsorbed state. The ΓH values at X2 ) 0 (DeAC) and 1 (DC‚HCl or TC‚HCl) change discontinuously in the low-concentration region, corresponding to the phase transition in the surface-adsorbed films28 though we do not focus our attention on this phenomenon in this study. We now proceed to evaluate the composition of the anesthetics in the surface-adsorbed film XH 2 , defined by H H XH 2 ) Γ2 /Γ

(9)

in order to quantify their incorporation into the adsorbed film of DeAC. The XH 2 values are obtained straightaway at constant surface tension by use of the relation25 (26) Kamaya, H.; Hayes, J. J., Jr.; Ueda, I. Anesth. Analg. 1983, 62, 1025. (27) Hoerr, C. W.; McCorkle, M. R.; Ralston, A. W. J. Am. Chem. Soc. 1943, 65, 328. (28) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33.

Figure 7. Total surface density vs total molality curves of the DeAC-TC‚HCl system at constant composition: (1) X2 ) 0, (2) 0.288, (3) 0.500, (4) 0.613, (5) 0.702, (6) 0.782, (7) 0.862, (8) 0.932, and (9) 1; (b) total surface density at the cmc ΓH,C.

XH 2 ) X2 - (2X1X2/m)(∂m/∂X2)T,p,γ

(10)

and the slopes of the m vs X2 curves in Figures 3 and 4. The phase diagram of surface adsorption,25 shown as an m vs XH 2 cuve (broken line), and an m vs X2 curve (full line) are depicted for the DeAC-DC‚HCl system in Figure 8 and for the DeAC-TC‚HCl system in Figure 9, respectively. The XH 2 value is found to be smaller than the X2 value (that is, the adsorbed film is richer in DeAC than the aqueous solution) in the low surface tension range of the DeAC-DC‚HCl system and over the entire surface tension range of the DeAC-TC‚HCl system. In the high surface tension range of the DeAC-DC‚HCl system, XH 2 has a larger value than X2 (that is, the adsorbed film is enriched in DC‚HCl). This behavior may be attributable to the reversal of the surface activities of DeAC and DC‚ HCl at 15.76 mmol kg-1 and 57.45 mN m-1 as seen in Figure 1. The phase diagrams of surface adsorption at 50 mN m-1 for both systems are compared in Figure 10. It can be seen that the diagram for the DeAC-DC‚HCl system has a thin cigar shape, while that of DeAC-TC‚ HCl has a swollen cigar shape. This indicates that the difference in composition between X2 and XH 2 of the DeAC-DC‚HCl system is small compared to that of the

Anesthetic Incorporation into Surfactant Aggregates

Figure 8. Total molality vs composition curves of the DeACDC‚HCl system at constant surface tension: (1) γ ) 60, (2) 55, (3) 50, and (4) 45 mN m-1; (s) m vs X2; (- - -) m vs XH 2.

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Figure 10. Total molality vs composition curves at γ ) 50 mN m-1; (1) DeAC-DC‚HCl system; (2) DeAC-TC‚HCl system; (s) m vs X2; (- - -) m vs XH 2 ; (‚‚‚) ideal mixing line calculated from eq 11.

(m)2 ) (m01)2 - [(m01)2 - (m02)2]XH 2

Figure 9. Total molality vs composition curves of the DeACTC‚HCl system at constant surface tension: (1) γ ) 60, (2) 55, (3) 50, and (4) 45 mN m-1; (b) cmc; (s) m vs X2; (- - -) m vs XH 2.

DeAC-TC‚HCl system. In other words, DC‚HCl is more fully incorporated into the adsorbed film of DeAC than TC‚HCl. We next consider the interaction between DeAC and the anesthetics in the mixed adsorbed film. Motomura has thermodynamically shown that, in the case of a nonionic surfactant mixture in an adsorbed film, a linear relation between m and XH 2 holds at a fixed γ if nonionic surfactants mix ideally in the adsorbed film.29,30 Extending the thermodynamic treatment to the mixture of uniunivalent ionic surfactants with a common ion, the equation describing the ideal mixing of DeAC and anesthetics in the adsorbed film becomes (29) Todoroki, N.; Tanaka, F.; Ikeda, N.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1993, 66, 351. (30) Iyota, H.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1996, 178, 53.

(11)

where m01 and m02 are the molality of pure DeAC and anesthetics at a given γ. It should be noted that a linear relation no longer holds for the mixtures in the present study. The estimated ideal mixing line of the two systems is shown by the dotted line in Figure 10. A negative deviation of the m vs XH 2 curves from the ideal mixing line is observed for the DeAC-DC‚HCl system across the entire range of composition and for the DeAC-TC‚HCl system at a low composition. Thus, we can conclude that the miscibility of DeAC and the anesthetic molecules is nonideal and especially that both the molecules attract one another over this composition range in the adsorbed state. Incorporation of DC‚HCl and TC‚HCl into the Micelle of DeAC. DC‚HCl and TC‚HCl have a strong enough hydrophobicity to form micelles by themselves in aqueous solution, as seen in Figure 1. Therefore it is possible to investigate the incorporation of the anesthetics into micelles of DeAC over the whole range of composition. The composition of anesthetic in the micelle XM 2 is defined by M M M XM 2 ) N2 /(N1 + N2 )

(12)

M where NM 1 and N2 are the aggregation numbers of DeAC and anesthetics in one micelle particle, respectively. Here they are defined by the surface excess quantity using the dividing spherical interface which makes the excess number of moles of water zero in a similar way to the surface density described in the previous section.24,25,31 Unlike surface density, however, the values for the aggregation number cannot be estimated from the thermodynamic equations. The XM 2 value is evaluated from the composition dependence of the cmc in Figures 3 and 4 and the following equation25

(31) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984, 262, 948.

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Figure 11. Critical micelle concentration vs composition curves: (1) DeAC-DC‚HCl system; (2) DeAC-TC‚HCl system; (s) C vs X2; (- - -) C vs XM 2 ; (‚‚‚) ideal mixing line calculated from eq 14.

XM 2 ) X2 - (2X1X2/C)(∂C/∂X2)T,p

(13)

The diagram which is constructed by the calculated C vs XM 2 curve and C vs X2 curve is called the phase diagram of micelle formation,25 and the resulting phase diagrams for both systems are depicted in Figure 11. The relation XM 2 < X2 over the whole range of composition indicates that the micelles exist in greater quantities in DeAC than in aqueous solution. In addition, the thinner shape of the DeAC-DC‚HCl system’s phase diagram suggests that greater quantities of DC‚HCl than of TC‚HCl are incorporated into the micelle of DeAC, as is shown by the phase diagram of surface adsorption in Figure 10. Clinical experiments18,19,32-34 have shown that DC‚HCl has a stronger anesthetic potency than TC‚HCl. Therefore, we can say that the quantity of anesthetic incorporated into the hydrophobic environment of the surface-adsorbed film and micelle seems to correspond to the potency of the anesthetic: the larger the quantity of anesthetic incorporated into the biological membranes, the stronger the clinical effects of the anesthetic. The ideal mixing of both DeAC and the anesthetics in the micellar state was also considered. The ideal mixing line of DeAC and the anesthetics is given by the relation

(C)2 ) (C01)2 - [(C01)2 - (C02)2]XM 2

(14)

where C01 and C02 are the cmc of pure DeAC and anesthetics. The above equation is derived by extending the thermodynamic equations of ideal mixing for a nonionic surfactant mixture to the present surfactant-anesthetic mixture.29,30 The ideal lines calculated are also included in Figure 11. The C vs XM 2 curve of the DeAC-DC‚HCl system deviates from the line negatively, as was observed in the adsorbed film, suggesting an attractive interaction between DeAC and DC‚HCl in the micelle. On the other hand, the C vs XM 2 curve of the DeAC-TC‚HCl system (32) Truant, A. P.; Takman, B. Anesth. Analg. 1959, 38, 478. (33) Strichartz, G. R.; Sanchez, V.; Arthur, G. R.; Chafetz, R.; Martin, D. Anesth. Analg. 1990, 71, 158. (34) Langerman, L.; Bansinath, M.; Grant, G. J. Anesth. Analg. 1994, 79, 490.

Figure 12. Surface tension at the cmc vs composition curves: (1) DeAC-DC‚HCl system; (2) DeAC-TC‚HCl system; (s) γC H,C C vs XM 2 ; (- - -) γ vs X2 .

deviates negatively in the low composition range, whereas in the high composition range we see a positive deviation, indicating repulsive interactions. The reasons for this complicated interaction between DeAC and TC‚HCl in the micellar state have not yet been made clear. Since micelles coexist with adsorbed films at the cmc, it is interesting to explore differences between the incorporation of the anesthetics into the adsorbed film and that into the micelle at the cmc. So we finally evaluate the composition of anesthetics in the adsorbed film at the M cmc XH,C 2 . The X2 value in Figure 10, the total surface density value at the cmc ΓH,C in Figures 6 and 7, the composition dependence of γC in Figure 5, and the following equation24,25 H,C XH,C ) XM )(∂γC/∂X2)T,p 2 2 - (X1X2/RTΓ

(15)

give us the XH,C value. Figure 12 compares the value of 2 C XH,C and XM 2 2 of both systems in the forms of the γ vs M C XH,C and γ vs X plots. The resulting shape of the phase 2 2 diagram for the DeAC-DC‚HCl system resembles that of the DeAC-TC‚HCl system; the XM 2 values of both systems are considerably larger than the XH,C value. It was 2 found that, in the binary mixtures of surfactant, the surfactant with the larger polar head and shorter hydrophobic chain is more abundant in the micelle component than in the adsorbed film.29 In fact, the phase diagrams for both systems are similar to those observed for mixtures of cationic surfactants having polar head groups of different sizes.35-37 Hence, anesthetics with larger polar head groups may be said to be more geometrically favorable for the formation of spherical micelles than for surface adsorption at the plane interface. Here we assume that the influence of the hydrophobic aromatic rings of the anesthetic molecules is roughly similar to that of the methylene groups of the straight-chain surfactant, judging from the regularity of variation in ΓH (35) Ikeda, N.; Shiota, E.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1989, 62, 410. (36) Ikeda, N.; Sanefuji, N.; Abe, K.; Todoroki, N.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1992, 65, 858. (37) Ikeda, N.; Sanefuji, N.; Lu, K.-K.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1994, 164, 439.

Anesthetic Incorporation into Surfactant Aggregates

vs m curves with X2. Furthermore, the phase diagrams for γC vs composition curves form a striking contrast to those of surface adsorption and micelle formation. Because of their hydrophobicity, DC‚HCl and TC‚HCl seem to correspond to straight-chain cationic surfactants having, respectively, decyl and nonyl groups, which have been analyzed in a previous study.2,4 For this reason, quantities of DC‚HCl and TC‚HCl incorporated into the adsorbed film and micelle of DeAC are remarkably different from each other because there is a difference of nearly one methylene group in hydrophobicity between DC‚HCl and TC‚HCl. Although the hydrophobic groups on the anesthetics seem to have a significant impact on their incorporation into the model membrane system, the influence of the polar head groups appears to be minimal. The miscibilities of DC‚HCl, with its diethylammonium group, and TC‚HCl, with its dimethylammonium group, into DeAC proved to be essentially equivalent. The present study has shown that local anesthetics which have a strong enough hydrophobicity to form micelles by themselves in aqueous solution can be incorporated into model membranes of surfactant. Further valuable information regarding the incorporation of anesthetics into model membranes may be obtained by investigating a mixture of DeAC and an anesthetic, which unlike DC‚HCl or TC‚HCl, has weak hydrophobicity. The results of these studies will be reported in the near future.

Langmuir, Vol. 13, No. 10, 1997 2693

Conclusions Local anesthetic molecules must be incorporated into biological membranes before local anesthetic action occurs, although the molecular mechanism of anesthetic action has not yet been discovered. The incorporation of the local anesthetics DC‚HCl and TC‚HCl into model membranes, adsorbed films and micelles of DeAC was investigated using surface tension measurements for the DeAC-anesthetic mixtures. On the basis of the phase diagrams for surface adsorption and micelle formation obtained through application of the thermodynamic equations to the surface tension data, we found that DC‚HCl and TC‚HCl molecules are significantly incorporated into the hydrophobic environments of these molecular aggregates and that larger quantities of DC‚HCl than TC‚ HCl are incorporated into these environments. These findings were consistent with the anesthetic potency of the anesthetics observed in clinical experiments: a good correlation was found between the ease with which the anesthetics were incorporated into the membranes and the magnitude of anesthetic action. It was also shown that the bulky polar head groups on the anesthetics make them more apt to form micelles than to udergo surface adsorption. LA960909O