Mixed micelle-to-vesicle transition in aqueous nonionic phospholipid systems

Mixed micelle-to-vesicle transition in aqueous nonionic phospholipid systems

Mixed Micelle-to-Vesicle Transition in Aqueous Nonionic Phospholipid Systems N I C O L E K A M E N K A , * M A H A C I N E EL A M R A N I , * J A C Q ...

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Mixed Micelle-to-Vesicle Transition in Aqueous Nonionic Phospholipid Systems N I C O L E K A M E N K A , * M A H A C I N E EL A M R A N I , * J A C Q U E L I N E APPELL, t AND M A R C L I N D H E 1 M E R * * Laboratoire de Physicochimie des SystOmes Polyphasds, CNRS URA 330, GR "Nouveaux ~VIatdriaux Tensioactifs"and tGroupe de Dynamique des Phases Condensdes, CNRS UA 233, USTL, Place EugOneBataillon, 34095 Montpellier Cddex 5, France

Received July 23, 1990; accepted October 19, 1990 We have investigated the aggregation behavior in aqueous solutions of mixed systemsformed by one nonionic surfactant (Triton X-100 or C12Es) and one phospholipid (egg lecithin or dipalmitoylphosphatidylcholine) by means of quasielastic light scatteringand tracer self-diffusionusingradioactivelabeling. The size of the mixed micellar aggregatesdepends on the surfactant/lipid ratio and increases with phospholipid content. Dilution of the solutions with the highest phospholipid/nonionic surfactantratio induces a transition from mixed micelles to vesicles. © 1991AcademicPress,Inc. INTRODUCTION Mixed aqueous-amphiphile systems where the two amphiphiles are a biological lipid and a small surface active agent are intensively studied in the literature. In particular, as the bile salts play an important biological role during lipid digestion, the solubilization of phospholipids by these cholate derivatives is the subject of m a n y theoretical and experimental studies related to the establishment of phase diagrams ( 1, 4), the structure of mixed miceIles ( 1 - 4 ) , and the transition between mixed micelles and vesicles ( 5 - 7 ) . On the other hand, nonionic surfactants are widely used in the solubilization of the protein and phospholipid components of biological membranes. M a n y studies concern the solubilization ofphospholipids by converting them into mixed micelles (8 and references therein ). Much less is known about the transition between mixed nonionic amphiphile-phospholipid micelles and vesicles. Just recently, a descriptive study appeared of the interaction of Triton X-100 with lecithin vesicles obtained from sonication of pure lecithin in which the

nonionic surfactant is incorporated after their formation (9). It is shown that addition of surfactant results in the formation of large vesicles and mixed micelles depending on the amount of Triton X-100 and that large vesicles are also obtained by diluting mixed micelles having a certain ratio ofsurfactant to lecithin. We report here a study of the structure of mixed micelles formed respectively by one nonionic surfactant, Triton X- 100 ( T X 100) or octaethylene glycol dodecyl ether C~2E8, and one phospholipid, egg lecithin (LEC) or dipalmitoylphosphatidylcholine (DPPC), as a function of the ratio of surfactant to lipid. The nonionic surfactant T X 100 is a polydisperse c o m p o u n d with an average polyoxyethylene n u m b e r equal to 9.5 and C12E8 is a well-defined surfactant. These two nonionic surfactants have been chosen as T X 100 is usually employed to solubilize m e m b r a n e components and it seems interesting to compare the structures of the mixed aggregates obtained with this polydisperse surfactant with those formed with a monodisperse surfactant. For the mixed micelles with the highest phospholipid content the evolution of their

463 0021-9797/91 $3.00 Journal of Colloid and Interface Science, Vol. 143, No. 2, May 1991

Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

464

K A M E N K A ET AL.

size has been studied as a function of dilution to determine whether spontaneous formation of vesicles is induced. The size of all the aggregates has been followed by the tracer self-diffusion method and quasielastic light scattering. MATERIALS A N D M E T H O D S

Egg lecithin was purchased from Lipid Product (U. K.) and dipalmitoylphosphatidylcholine from Sigma. The nonionic surfactants were employed as received: octaethylene glycol dodecyl ether C t 2 E 8 from Nikko (Japan) and Triton X-100 from Rhom and Hass (France). 14C-Enriched C12E8, LEC, and DPPC were obtained from CEA (Departement des Radiorlrments, Gif-sur-Yvette, France) and Amersham (Radiochemical Center, Buckinghamshire, England). The solutions were prepared by mixing different appropriate quantities of nonionic surfactant and lipid in a 1:1 chloroform:methanol mixture. The organic solvent is then removed by evaporation (rotavapor) and the residue is dried under vacuum in the presence of P205 for at least 24 h. It is then dispersed in a 1% N3Na solution to avoid microorganism development. Density and viscosity of solutions were measured respectively with a DMA 02C digital densimeter A Paar and a viscomatic MS. The tracer self-diffusion coefficients were obtained with the open-ended capillary tube method (7). Light scattering experiments were performed on a standard setup (Amtec M M 1000 commercial spectrophotometer and Brookhaven autocorrelator BI-2030). The samples are filtered through a 0.22-#m Millipore membrane directly into a cylindrical cell. The cell is placed in a thermostated bath. The incoming light from an argon ion laser (X = 4880 A) is focused onto the sample. The scattered photons are received on a photomultiplier and counted. The angular distribution of the intensity of scattered light and the autocorrelation function of the scattered intensity can be succesJournal of Colloid and lnterface Science, Vol. 143,No. 2, May 1991

sively measured on a sample. The range of accessible angles (10 ° to 150 °) corresponds to scattering vectors q from 3 × 10 -4 to 3 × 10 -3 A-1. The relaxation rate r is deduced from the autocorrelation function by a cumulant analysis. F is found to follow the expected q2 linear dependence (between 30 ° and 150 ° ) from which the mean m u t u a l diffusion D is measured ( r = Dq2). A fit of the angular distribution of scattered intensity [I(q)] to I(q) = I ( 0 ) ( 1 - ½q2R~) yields the value of the radius of gyration. RESULTS

Simple Micelles of C12Es and Triton X-100 Tracer diffusion. Self-diffusion coefficients of CI2E8 and T X 100 aggregates at different surfactant concentrations between 0.5 and 10% are shown in Fig. 1. For C12E8 solutions the self-diffusion coefficients o f the aggregates are obtained by measuring the self-diffusion coefficient ofdecane or decanol, which are entirely confined in the micelles and have a negligible solubility in the intermicellar solution; so the observed self-diffusion coefficient is the same as the self-diffusion coefficient of the micelles. We must note that it is difficult to obtain accurate measurements of the self-diffusion coefficients of the aggregates at low concentration because the radioactivity o f the commercial decane and decanol labeling the solutions is low and only a very small quantity of the organic compounds is solubilized in the solution. We have also plotted in Fig. 1 the self-diffusion coefficient o f the amphiphile obtained by the Fourier transform pulsed gradient spin-echo technique ( i 1 ). The observed diffusion coefficient of the amphiphile Da is a mean coefficient Da = pao~Dao~ + PmicDmie where paon and Pmic are the fractions of free and aggregated amphiphile and D~o, and Dmic are the self-diffusion coefficients of free

NONIONIC PHOSPHOLIPID SYSTEMS

465

ical over the concentration range studied. The variation of log 7/rel/c as a function of log rel is linear and the shape factor is found to E be 2.6, close to the 2.5 value for spherical par~o ticles. All the results are in agreement with ~s literature data ( 1 l, 13, 15 ). + DQELS As we have previously seen the self-diffusion • Dtraeer coefficient of the aggregates m a y be obtained -/r DRM N (11) from the measurement of the self-diffusion 5 1 ~O % C12E8 coefficient of the amphiphile, so we have chosen to label the T X 100 micelles by *C12Es. The results are plotted in Fig. lb. The apparent hydrodynamic radius at infinite dilution obtained from the Stokes-Einstein equation is equal to 38 A and we obtained b the relation D = 6.5 × 10-11(1 - 3.7~) m 2 s -1 in the concentration range studied. Fur5 ~:~"~÷-~__ + _ + _ _ ~ +~ + e thermore, the intrinsic viscosity (5.2 cm 3 g - i ) is higher than that obtained for globular par+ DQELS ~ • eDc E ticles and the shape factor is equal to 3.2. There 12 8 is a discrepancy between our results and those 10 % T X I O 0 recently published (16) where at infinite diFIG. 1. Surfactant mutual and self-diffusioncoefficients lution the apparent radius obtained from PFGversus surfactant concentration (weight percent) of C12E8 N M R is equal to 31 A. This m a y be due to (a) and TX 100 (b) aqueous solutions, t = 25°C. In (b) the difference in the products as the T X 100 Dc,2E8is obtained by the tracer self-diffusionmethod. molecules are polydisperse with an average n u m b e r of oxyethylene units around 9.5. In m o n o m e r s and micelles. The critical micellar our case the first micelles formed are not concentration of C12E8 is equal to 7 × 10 -5 spherical; an oblate form has been suggested mole kg -~ , making the contribution to Da of by m a n y authors (17). free m o n o m e r s negligible. At infinite dilution In both cases the self-diffusion coefficients the hydrodynamic radius RH is calculated from of the aggregates decrease when the surfactant the Stokes-Einstein equation: RH = kT/ concentration increases. This can be attributed 6~-~D0. RH is equal to 30 A (Do is the self- to repulsive interactions a n d / o r micellar diffusion coefficient of the aggregates at the growth, which both have the same influence cmc, n is the viscosity of the solvent). The on these coefficients. self-diffusion coefficient of the C12E8 aggregates Quasielastic light scattering. The mutual decreases with increasing concentration with diffusion coefficient is expected to increase the following relation: D = 8.1 (1 - 2if) with concentration, D = Do( 1 + k ' f f ) for reX 10 11 m 2 s - 1. This corresponds to aggregates pulsive interactions [k' = 1.45 for hard spheres interaction through a hard spheres potential ( 18 ) ] and is expected to decrease with increaswithout hydrodynamic interactions (12). ~ is ing size of the aggregates. We must note that the volume fraction of the aggregates, and is the temperature of experiments, 25°C, is far calculated from the equation proposed by Zu- from the cloud points of the two surfactants, lauf et al. ( 13 ) assuming a hydration n u m b e r respectively 77 and 64°C for ClzE8 and T X equal to five water molecules per ethylene ox100. As shown in Fig. 1 DQELS has not the ide group (14). Viscosity measurements con- same variation as a function of the concentrafirm that the micelles remain small and spher- tion for the two surfactants. In C12E8 the mu10 ¸

I;

Journal of Colloid and Interface Science, Vol. 143, No. 2, Ma y 1991

466

KAMENKA

tual diffusion coefficient is found to increase steadily with surfactant concentration. Together with the self-diffusion decrease described above this indicates a constant size of the C12E8 micelles and the observed evolutions o f Dt . . . . . and DQELS are indicative of repulsive interactions between aggregates. If we assume, as is reasonable, that the intermicellar interactions are similar in the two systems, the decrease in DQELS with increasing concentration observed in the T X 100 solutions is indicative o f increasing micellar size. At infinite dilution the extrapolated values are respectively 7.5 and 6 × 10 -1' m 2 s -1 for C12E8 and T X 100 solutions; the ratio of the mutual diffusion to self-diffusion coefficients is close to unity, indicating a narrow distribution size (19).

E T AL.

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FIG. 3. C~2E8 and phospholipid tracer self-diffusion coefficients versus T X 100 concentration• (a, b ) T X 100LEC system. (c) T X 1 0 0 - D P P C system, t = 25°C.

C12E 8

Mixed Nonionic SurfactantPhospholipid Systems

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FIG. 2. Surfactant and phospholipid tracer self-diffusion coefficients versus surfactant concentration. (a) C~2Es-LEC system at t = 25°C. ( b ) C~2Es-DPPC system at t = 33°C. Journal of Colloid and Interface Science, Vol. 143, No. 2, May 1991

Tracer diffusion. The results from measurements of surfactant and phospholipid tracer diffusion coefficients at various tensioactive/ phospholipid ratios and total concentrations are plotted in Figs. 2 and 3. Both C12E8 and phospholipid self-diffusion coefficients were measured in the same solutions. Due to the very low solubility of egg lecithin and DPPC in water the self-diffusion coefficients o f phospholipids represent the diffusion of the mixed aggregates. Beside the mixed aggregates, simple micelles o f nonionic surfac-

NONIONIC PHOSPHOLIPID SYSTEMS rants and free amphiphiles may coexist in the solution. If this is the case, the measured selfdiffusion coefficient of the amphiphile is a weight average of the self-diffusion coefficients of free monomers, simple micelles, and mixed aggregates. Let us consider first C12Es-phospholipid systems (Fig. 2). For the C12E8-LEC and C~2Es-DPPC systems the self-diffusion coefficient of the amphiphile is always greater than the self-diffusion coefficient of the mixed aggregates when the solutions are not too concentrated. It seems reasonable to neglect the contribution of free monomers as it has been made in the case of C~2E8 simple micelles and the experimental results suggest the presence of C~2E8 simple micelles coexisting with mixed aggregates. Unfortunately the experimental results are not accurate enough to calculate precisely the amount of amphiphile aggregated in the simple micelle form. We can only say that the a m o u n t of amphiphile not engaged in mixed aggregates is always greater than the cmc. In the case o f T X 100-phospholipid systems, as labeled T X 100 does not exist, we have followed the self-diffusion coefficient of labeled C~2E8. C~2E8 is here a tracer molecule which may be free or solubilized in simple T X 100 micelles or in mixed T X 100-phospholipid aggregates. The C~2E8 self-diffusion coefficients are slightly higher than the self-diffusion coefficients of the mixed aggregates for a T X 100/ LEC ratio of 10/1 (Fig. 3a). At a 5/1 ratio the self-diffusion coefficients of C12E8 and mixed aggregates are practically the same. If we do not take into account, as previously, the diffusion of monomers and if we suppose that the labeled ClzE 8 is solubilized in simple micelles and mixed aggregates, the experimental results suggest that there are no simple T X 100 micelles in the solution or that labeled C~zE8 is preferentially solubilized in the mixed aggregates. Nilsson (10) has calculated the amount of free T X 100, making the hypothesis that only free amphiphile and mixed aggregates coexist, and he found that the free surfactant concentration obtained is larger than the cmc. An explanation of his results and ours

467

is therefore the presence of simple T X 100 micelles. Let us examine the evolution of all the selfdiffusion coefficients of the phospholipids. In the concentration range studied adding lecithin or DPPC to the simple micelles brings about a decrease in the self-diffusion coefficients of the aggregates, depending on the amount of phospholipid solubilized. As a function of concentration the decrease in selfdiffusion coefficients of the mixed aggregates is practically the same as for the simple micelles. The difference between self-diffusion coefficients of simple and mixed aggregates seems to be due to the longer alkyl chains of the phospholipids. We have thus applied the same hard sphere interaction model to estimate the influence ofintermicellar interactions a n d / o r micellar growth on the variation of the self-diffusion coefficient of the mixed micelles. The volume fraction of particles is evaluated assuming a hydration of 23 water molecules per phospholipid (20), and from a plot of D = f ( ~ ) we calculate the k coefficient. As for simple micelles the hydrodynamic radius at infinite dilution is calculated from the Stokes-Einstein equation. RH and calculated k are plotted in Table I. At 25 and 33°C the hydrodynamic radii of C~2E8 simple micelles are respectively 30 and 29 A. Addition of LEC and DPPC leads to an increase in the size of the mixed aggregates which is reasonable considering the longer alkyl chains of phospholipids. The mixed micelle self-diffusion coefficient decreases linearly with the particle volume fraction up to quite high concentrations, k values are consistent with hard sphere interactions; there is no need to invoke additional repulsive forces due to the zwitterionic phospholipid molecules incorporated. The mixed micelles formed by T X 100 and phospholipids present the same evolution as the simple T X 100 miceUes as a function of concentration; likewise for C12Ea-phospholipid systems. The size of mixed micelles increases as the amount of phospholipid increases. For example, when the T X 100/LEC Journal of Colloid and Interface Science, Vo|. 143, No. 2, May 1991

468

KAMENKA

ET AL.

T A B L E Ia

System CI2Es-LEC CI2Es-DPPC TX 100-LEC TX 100-DPPC TX 100-LEC C12Es-LEC C12Es-DPPC TX 100-LEC TX 100-DPPC

t (°C)

R

25 33 25 25 25 25 33 25 25

5/1 5/1 5/1 5/1 10/1 2/1 2/1 2/1 2/1

RH from tracer diffusion (A)

k

RH, QELS (A)

40 42 47 44 40

2.2 2.5 4 3.4 3.2

40 52 51 45 41 114 91 90 65

a For simple micelles C12E8, t = 25°C, RH = 30 A; TX 100, t = 25°C, RH = 38 A; C12E8, t = 33°C, RH = 29 A.

ratio is equal to 10/1 and 5/1, R H increases from 40 A, which is approximately the hydrodynamic radius of T X 100 simple mieelles, to 47 A. For T X 100-LEC and T X 100-DPPC the variation in the self-diffusion coefficients of the aggregates is linear as a function of the volume of the aggregates but the coefficient k is larger than the value 1.5-2 given for hard spheres. F r o m a comparison with QELS measurements (see below), this behavior can be attributed to a change in the mixed aggregate size. Quasielastic light scattering. It is interesting to compare the tracer self-diffusion data with QELS experiments. The experiments were performed on the same solutions and the reSults on series of solutions having various tensioactive/phospholipid molar ratios ( 10/l, 5 / 1, and 2 / 1 ) are shown in Figs. 4 and 5 as a function of surfactant concentration. As will be shown in the following paragraph, dilution of the solutions of mixed aggregates saturated with lecithin, for a ratio equal to 2/1, leads to a drastic decrease in the diffusion coefficients and to the formation of vesicles. Now let us consider the evolution of the size of the mixed micelles existing in the concentration range 0.5 to 10% of surfactant. We have to distinguish the behavior of C12E8 mixed micelles from that of T X 100 mixed aggregates. The evolution of the diffusion coefficients of C12EsLEC and C~zE8-DPPC as a function of surfactant concentration is similar for these two Journal of Colloid and Interface Science, Vol. 143, No. 2, May 1991

phospholipids. We have a linear dependence of mutual diffusion coefficients and the slope of the curves is practically the same for the simple micelles and mixed ones with a 2/1

a

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%C12E8 FIG. 4. M u t u a l diffusion coefficients versus s u r f a c t a n t c o n c e n t r a t i o n for the s y s t e m s C 1 2 E s - L E C at t = 2 5 ° C ( a ) a n d C , 2 E s - D P P C at t = 3 3 ° C ( b ) .

NONIONIC PHOSPHOLIPID SYSTEMS

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FIG. 5. Mutual diffusion coetficients versus surfactant concentration for the systems TX 100-LEC (a) and TX 100-DPPC (b) at t = 25°C. ratio. The diffusion coefficients increase more slowly at a ratio of 5/1. They are practically constant for C~2Es-DPPC solutions. Adding lecithin to the simple micelles m a y involve an increase in the size of the aggregate with the concentration for the ratio 5 / 1, whereas the size of the saturated aggregate (ratio 2/1 ) seems to remain constant, the positive slope of D = f ( C lzE 8 ) expressing the same repulsive interactions as in the case of simple ClzE8 micelles. At infinite dilution the hydrodynamic radius is calculated from the Stokes-Einstein relation. For the ratio 5 / 1 we obtain the same value, 40 A, as calculated from the tracer selfdiffusion coefficient for the C12Es-lecithin system. On the contrary, the hydrodynamic radius of the aggregates formed by C~2EsDPPC is equal to 52 A by QELS. This value is larger than the value measured by the tracer self-diffusion method. This result reflects the

469

polydispersity of aggregate size, as QELS is more sensitive to large particles. Further addition of phospholipids leads to a larger increase in the aggregate size, and for a tensioactive/phospholipid ratio equal to 2, the hydrodynamic radius at infinite dilution attains 114 and 91 A for CI2Es-LEC and CI2Es-DPPC systems, respectively. As for the ClzEs-phospholipid systems, addition of lecithin and DPPC to the T X 100 simple micelles increases the mixed aggregate size and this effect is more pronounced with egg lecithin. T X 100-LEC and T X 100-DPPC systems show the same DQELS evolution. For the two cases and for all the tensioactive/phospholipid ratios studied we observe a decrease in mutual diffusion coefficients at low concentration and an increase at higher concentration, similar to what has been observed for T X 100 solutions. This behavior can be explained assuming that the size of mixed aggregates increases with surfactant concentration. The hydrodynamic radii at infinite dilution for ratios equal to 10/1 and 5/1 correspond to the values obtained from tracer diffusion. We observe a large size increase when the aggregates are saturated with lecithin or DPPC; RH equals 90 and 65 A at infinite dilution for T X 100-LEC and T X 100-DPPC, respectively.

Transition from Mixed Micelles to Vesicles It has been observed (10, 11 ) that the addition of T X 100 to small unilamellar lecithin vesicles gives rise to the formation of mixed micelles at high surfactant concentrations and to the formation of large vesicles at intermediate surfactant concentrations. These large aggregates with a 1000-A radius have been characterized by QELS and cryogenic transmission electron microscopy. The molar ratios at which the various aggregates are found depend strongly on the concentrations of phospholipid and surfactant and vesicles can also be obtained by diluting mixed micellar solutions. Journal of Colloid and Interface Science, Vol. 143, No. 2, May 1991

470

KAMENKA ET AL.

Similarly following the size of aggregates by QELS, we found for example that the radius of the TX 100-LEC system changes from about 60 to 800 A when the amount of lecithin is increased in a sample containing 0.046% surfactant. (For R equal to 5 / 1, 4 / 1, 3 / 1, and 2/1, RH is equal to 65, 75, 520, and 800 A.) We then chose to study the transition from mixed micelles to vesicles for the aggregates when the tensioactive / phospholipid ratio is 2. For all the systems studied (C12E8-LEC, C12Es-DPPC, TX 100-LEC, and TX 100DPPC) we observed a large increase in the hydrodynamic radius when the mixed aggregate solution is diluted while maintaining the tensioactive/phospholipid ratio constant. This change takes place over a short concentration range as shown in Fig. 6 around 0.05% C12E8 and 0.1% TX 100.

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Before this sudden and large increase, the size of the mixed aggregates remains constant. This behavior has previously been observed for TX 100-LEC systems (10). When we further dilute to 0.01% C12E8 and 0.02% TX 100 the hydrodynamic radius decreases again. We have not reported the results for the C~EEs-DPPC system; although we observed a large hydrodynamic radius at 0.03% C]2E8, the experimental results were not well reproducible. The radius of gyration has been measured using a classical static light scattering method on the same solutions previously studied by QELS for C]2Es-LEC, TX 100-LEC, and TX 100-DPPC systems. At 0.03 and 0.01% C12E8 the radii of gyration are equal to 410 and 390 A, close to the values of 430 and 350 A for the hydrodynamic radius obtained from QELS. And at 0.1, 0.05, and 0.02% TX 100, the radii of gyration are respectively 820, 750, and 300 A and are almost identical to the RH's equal to 840, 790, and 295 A. The large increase in size of the mixed aggregates upon dilution indicates a thorough change in their structure. These large aggregates could be vesicles as postulated by others ( 10, 11 ). The observed equality between the radius of gyration and the hydrodynamic radius strongly favors this hypothesis. The transition between mixed micelles and vesicles may be due to the fact that there is not a sufficient quantity of amphiphile to solubilize phospholipid in mixed micelles. At a certain concentration of surfactant the mixed micelles have to change to another aggregate form, vesicles. When the vesicles are diluted the size of the aggregates decreases to approximately the size of pure lecithin vesicles obtained by sonication.

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CONCLUSION

After a comparative study of the diffusion coefficients obtained by the tracer and QELS FIG. 6. Apparentradii deducedfrom mutual diffusion data by Stokes-Einsteinequationas a functionof surfac- methods on nonionic surfactant solutions, alrant concentration,t = 25°C. (a) ClzEs-LECsystem.(b) lowing us to distinguish between the eventual size change of the micelles and the repulsive TX 100-LECand TX 100-DPPCsystems. Journal of Colloid and Interface Science, Vol. 143, No. 2, May 1991

NONIONIC PHOSPHOLIPID SYSTEMS

interactions between the aggregates, we studied mixed aggregates formed by nonionic surfactants and phospholipids. Mixed micelles are obtained at high surfactant concentrations. We find that their size depends on the surfactant/ phospholipid ratio. We describe the transition between small and large mixed aggregates upon dilution of mixed aggregates with a high phospholipid content. Our experimental results strongly favor the formation of large vesicles. ACKNOWLEDGMENTS Professor Bjorn Lindman and Dr. Jean-Claude Montet are gratefully acknowledged for fruitful discussions. REFERENCES 1. Mazer, N. A., Benedek, G. B., and Carey, M. C., Biochemistry 19, 601 (1980). 2. Lindheimer, M., Montet, J. C., Bontemps, R., Rouviere, J., and Brun, B., J. Chim. Phys. 80, 315 (1983). 3. Stard, R. E., Gosselin, G. J., Donavan, J. M., Carey, M. C., and Robert, M. F., Biochemistry 25, 2597 (1986). 4. Schurtenberger, P., and Lindman, B., Biochemistry 24, 7161 (1985). 5. Schurtenberger, P., Mazer, N., and Kfinzig, W., J. Phys. Chem. 89, 1042 (1985).

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Journalof Colloidand lnterJklceScience,Vol.143,No. 2, May 1991