Langmuir 1995,11,2380-2384
Spontaneous Vesicle Formation from Aqueous Solutions of Didodecyldimethylammonium Bromide and Sodium Dodecyl Sulfate Mixtures Yukishige Kondo,? Hirotaka Uchiyama,tJl Norio Yoshino,t Katsuhiro Nishiyama,? and Masahiko A b e * ~ t ~ ~ Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan, and Faculty of Engineering and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3,Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received September 7, 1994. I n Final Form: January 23, 1995@ Spontaneous vesicle formation in the aqueous mixture of didodecyldimethylammonium bromide (DDAB) and sodium dodecyl sulfate (SDS)has been investigated with differential interference microscopy, transmission electron microscopy, glucose trapping experiments, 5 potential measurements, and surface tension measurements. The micrographs of the DDAB-SDS mixtures confirm the spontaneous formation of polydispersed vesicles including "giant vesicle" with a diameter less 40 pm. The captured volume of of 0.64and decreases with increasing the DDAB-SDS mixtures is 2.2 Umol at the SDS mole fraction (XSDS) X ~ D SVesicle . formation in DDAB-SDS mixtures will occur in theXsDs range of 0.64-0.75,below the total surfactant concentration of 2.2 wt %. Surface tension measurements for the mixtures exhibit very low values, 23 mN/m, indicating the ion-pairing of DDAB and SDS, a pseudo-zwitterionic surfactant. This surfactant favors the morphology of a vesicle. The mechanism of vesicle self-formation is the penetration of SDS into the DDAB molecules. In addition, spontaneous vesicles are also formed in the mixture of DDAB and sodium decyl sulfate.
Introduction Vesicles consisting of phospholipids, in other words liposomes, have been studied as biomimetic cell models' and carriers in drug delivery systems (DDS).2*3Kunitake et al. showed that a synthetic surfactant, didodecyldimethylammonium bromide (DDAB), can also form vesicles with ultras~nication.~ Since the epoch report, vesicles have been used both biologically and industrially; vesicles are of great interest in organic reaction ~ h e m i s t r ysensor ,~~~ te~hnology,~ and wastewater treatment.* However, the troublesome methods of vesicle preparation, such as ultrasonication and french-press, have restricted the practical use of vesicles. At this point, self-formation of vesicles is very interesting. To date, spontaneous vesicles formed from didodecyldimethylammonium hydroxide (DDAOHIg and the mixtures of single-chained cationic and anionic surfactant,1° and the mixtures of lysolecithin and lecithin" have been reported. Recently, the possibility of vesicle formation in the mixture of DDAB and an anionic surfactant, sodium dodecyl sulfate (SDS),has been realized
* Author to whom all correspondence
should be addressed.
+ Faculty of Science and
Technology. I/ Current address: Research and Development Department, Proctor & Gamble Far East, Inc., 17, Koyo-cho Naka 1-chome, Higashinada-ku,Kobe, 658. t Faculty of Engineering. Institute of Colloid and Interface Science. Abstract published in Advance ACS Abstracts, June 15,1995. (1)Singer, S. J.; Nicolson, G. L. Science 1972, 175, 720. (2) Ostro, M. J.; Cullis, P. R. Am. J. Hosp. Pharm. 1989,46, 1576. (3) Yamauchi, H.; Kikuchi, H. Fragrance J. 1987, 87, 68. (4) Kunitake, T.; Okahata, Y. J. Am. Chem. SOC. 1977, 9, 3860. 1982,104,1757. (5) Shimomura, M.; Kunitake, T. J.Am. Chem. SOC. (6) Moss, R. A.; Bizzigott, G. 0. J.Am. Chem. SOC.1981,103,6512. (7) Sunamoto, J.;Akiyoshi, K.; Koji, K.; Okahata, Y. In Liposome; Nojima, S.; Sunamoto, J., Inoue, K., Eds.; Nankohdoh: Tokyo, 1988; Chapter 8. ( 8 ) Kondo, Y.; Abe, M.; Ogino, K.; Uchiyama, H.; Scamehorn, J. F.; Tucker, E. E.; Christian, S. D. Langmuir 1993,9, 899. (9) Brady, J. E.; Evans, D. F.; Kacharr, R.; Ninham, B. W. J.Am. Chem. Soc. 1984,106,4279. (10)Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadozinski, J. A. N. Science 1989,245, 1371. (11)Hauser, H. Chem. Phys. Lipids 1987,43, 283. 8
@
based upon particle size measurement. l2 However, the formation of the DDAB-SDS vesicle has not been studied in detail. In this study, we discuss the results from transmission electron microscopy (TEM), 5 potential measurement, glucose trapping experiments, and surface tension measurements for the DDAB-SDS mixtures. These results will be related to spontaneous formation of the DDABSDS vesicle. In addition, the spontaneous formation of vesicles in the mixture of DDAB and sodium decyl sulfate (SDeS) will also be discussed.
Experimental Section Materials and Preparation of DDAB-SDS Mixtures.
DDAB was purchased from Eastman Kodak Co., Rochester, NY,
and recrystallized several times with acetone. SDS from Tokyo Kasei Kogyo Co., Tokyo, Japan (reagent grade), was used as received. DDAB-SDS mixtures were prepared by adding SDS crystals to water (a distilled water used for injection, Japanese Pharmacopoeia,Ohtsuka Pharmacy Co., Tokyo, Japan) containing a given amount of DDAB without any other treatment at 30 "C. This solutions were stirred for 3 min. Observation of Vesicles and Liquid Crystals. Vesicles were obsellredwith a differential interferenceoptical microscope (Olympus Optical Co., Tokyo, Japan, model IMT-2) and a transmission electron microscope (JEOL, Tokyo, Japan, model JEM-100SX). Observation by TEM was performed with a negati~e-stainingmethod.~J~J~ As soon as the surfactantmixture solution and an aqueous solution of 2%phosphotungstic acid (PTA, pH = 7) were mixed volumetrically at a ratio of 2:1, the resultant solution was added dropwise to a 150-mesh copper grid coated with collodion sprayed with a carbon film. Excess droplet was instantly removed using a filter paper, and then the grid was dried in a vacuum desiccator for 6 h as a TEM sample. The liquid crystallinephase was observedusing the mentioned optical microscope under optically crossed nicol conditions. Glucose-TrappingExperiment. The glucose-trapping experiments were also employed to determine the existence of (12) Marques, E.; Khan, A.; Da G. Miguel, M.; Lindman, B. J.Phys. Chem. 1993,97,4729. (13) Rupert, L. A. M.; Engberts, J. B. F. N.; Hoekstra, D. J. Am. Chem. SOC. 1986, 108, 3920. (14) Schullery, S. E.; Schmidt, C. F.; Felgner, P.; Tillack, T. W.; Thompson, T. E. Biochemistry 1980, 19, 3919.
0743-7463/95/2411-2380$09.00/00 1995 American Chemical Society
Spontaneous Vesicle Formation /?om DDAB-SDS
equimolar
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Langmuir, Vol. 11,No. 7,1995 2381
\
1:3 DDAB:SDS mixing (XsDs=0.75) line
Figure 1. Ternary phase diagram for the DDAB-SDS-water system: (a)phase diagram at 30 "C determined in the present work, where L is the single liquid phase, M is the isotropic solution, and V is the vesicular solution; (b)from ref 12. The SDS mole fraction, XSDS,is defined by X s ~ = s CSDS/(CDDAB + CSDS)where C s ~ and s CDDAB are the molarities of SDS and DDAB in the mixtures, respectively.
vesiclesin the mixture. DDAB-SDS mixtures, prepared by using 0.28 m o m glucose aqueous solutions instead of solvent water, were put into a regenerated cellulose tube for dialysis (Viskase Sales Co., Chicago, IL). The unencapsulated glucose was transferred into 4 L of the isotonic NaCl aqueous solution over a 10 h period (using 1L of NaCl solution a t 2.5 h intervals) at 0 "C. After dialysis, the glucose encapsulated in vesicles was detected by using a color producing agent for glucose (glucose CII-test Wako, Wako Pure Chemical Industry Co., Tokyo, Japan). The color-producing process is as follows. The glucose is oxidized by glucose oxidase in the CII-test solution and then forms hydrogen peroxide. The hydrogen peroxide works quantitatively as a catalyst to the reaction of 4-aminoantipyrine and phenol in the test solution and consequently produces an absorbance a t 505 nm. The glucose concentration of the reactant is measured by W spectroscopy (Shimadzu Co., Kyoto, Japan, model MPS-2000). Measurement of the Vesicular t Potential, Diameter, and the Surface Tension of Mixtures. The I; (zeta)potential of the vesicleswas measured by the laser-Dopplerelectrophoretic light scattering method15using a zetasizer (Malvern Instrument, model 2c) employing a 632.8 nm He-Ne laser a t 30 "C. The diameter of vesicles was determined by the dynamic lightscattering method16 using a submicrometer. particle analyzer (Malvern Instrument, Worcestershire, U.K., model 4700) with an argon ion laser operating at 488 nm (Coherent Co., Palo Alto, CA, model Innova 90). Measurements were performed at the scattering angle of 90" at a temperature of 30 "C, and the data of the photon correlation spectroscopy was analyzed on the basis of the cumulants method. The surface tension of the mixtures was determined with a surface tensiometer (Kyowa Interface Science Co., Tokyo, Japan, model CBVP-A3) using a platinum plate at 30 "C.
Results and Discussion Stable DDAB-SDS Vesicle Formation. Figure l a is a triangular phase diagram showing the concentration range of the surfactants for vesicle formation in DDABSDS mixtures a t 30 "C. The three apexes of the triangle correspond to water, 3 wt % DDAB, and 3 wt % SDS. The symbol L indicates single liquid phase. L on the DDABrich side is a lamellar phase and on the other side Lis also liquid crystalline phase, the type of which could not be identified. The symbol M denotes an isotropic solution and will be micellar solution because the diameter of the dispersed particles in the solution was ca. 10 nm. The (15) Ozaki,M.Hyohmen 1980,18,612. (16) Finsy, R.; De Jagar, N.; Sneyers,R.; Gelade, E. Part. Part. Syst. Churact. 1992,9, 125.
vesicle region was estimated by optical microscopy, TEM, and glucose-trapping experiments as mentioned above. As can be seen from the figure, the addition of SDS to the DDAB-water mixture initially induces surfactant precipitation. The greatest amount of precipitation is observed a t an equimolar mixture region. It should be noted that further addition of SDS causes the DDABBDS precipitate to disperse and results in the vesicle formation (shown with the symbol V). The DDAB and SDS mixtures yield spontaneous vesicles in the SDS mole fraction (X~DS) range 0.64-0.75, below the total surfactant (DDAB SDS) concentration of 2.2 wt %. The mixtures containing the vesicles are homogeneous, although slightlyturbid, and are stable without precipitation and surfactant phase separation for more than 6 months. Marques et a1.12 reported that SDS-rich vesicles spontaneously formed in DDAB-SDS mixtures a t theXsDs > 0.5 below the total Surfactant concentration of 0.1% a t 40 "C (Figure lb). It is clear that the spontaneous vesicle formation a t 30 "C occurs a t the narrower surfactant composition ratio and even in 20 times concentrated mixtures, compared to the report proposed before. We previously showed that the sonicated DDAB vesicle with the high 5 potential of 78 mV was unstable, resulting in phase separation." The electrostatic repulsive forces between particles undoubtedly contribute to the stability of particle dispersion in the solutions. Figure 2 shows the relationship between the z-averaged 5 potential of dispersed particles in homogeneousmixed solution andXsDs. BelOWXsDs = 0.64, the phase separation occurred and the mixture was heterogeneous(see Figure 1).The 5 potential values are negative and small, -1.8 mV, and are independent of XSDS. Although DDAB-SDS vesicles are slightly negative, the DDAB-SDS vesicle is much more stable than the highly charged DDAB vesicle. Vesicles prepared with mechanical forces like ultrasonication are metastable and their own membranes are under stress.l8Jg The stress will be eliminated sooner or later as a result ofloss of the membrane curvature, leading
+
(17)Kondo, Y.; A b , M.; Ogino, IC; Uchiyama, H.; Tucker E. E.; Scamehorn,J. F.; Christian, S. D.Colloids Su$. B: Biointerfaces1993, 1 , 51. (18) Ohki, IC; Nozawa, Y. In Liposome; Nojima, S.; Sunamoto, J., Inoue, IC, Eds.; Nankohdoh: Tokyo, 1988; Chapter 3. (19) Van Dijk, P. W. M.; De Kruijiff, B.; Aarts, P. A. M. M.; Verkleij, A. J.; De Gier, J. Biochim. Biophys. Acta 1978,506, 183.
Kondo et al. - ..
2382 Langmuir, Vol. 11, No. 7, 1995
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Figure 2. 5 potential measured for DDAB-SDS mixtures. XSDSis the mole fiaction of SDS in the mixtures. to the vesicle morphology. Thus, the high stability of DDAB-SDS vesicles is based on the vesicular curvature which forms spontaneously. Microscopy of DDAB-SDS Vesicles. Figure 3a is the differential interference picture for the mixture in the vicinity of the boundary between the precipitate region and the vesicle region in Figure la (surfactant composition of 10 mM DDAB-18 mM SDS; XSDS= 0.64), and Figure 3b is for the 10 mM DDAB-22 mM SDS (XSDS= 0.69) mixture. In these pictures are seen some donutlike images, pointed out with the filled arrow. It is well-known that the differential interferenceand/or polarizing micrographs of vesicles form ringlike images.20t21One can confirm the self-formation of vesicles. It is found that the vesicles are spherical or orbital-like myeline (shown with the open arrow). Furthermore, a surprising finding is that the vesicles are considerably large (diameter 40 pm). In general, the vesicular size is less 1pm in diameter. There have been few reports on “giant vesicles”more than 1pm in diameter.22s23The DDAB-SDS vesicle will be one of the largest vesicles. The number of “giant vesicle” apparently become fewer in the more SDS-concentrated mixture (Figure 3b)than in the less concentrated solution (Figure 3a). Parts c and d of Figure 3 are transmission electron micrographs of the 10 mM DDAB-22 mM SDS (XSM= 0.69) and 20 mM DDAB-43 mM SDS (XSDS= 0.68) mixtures, respectively. These pictures reveal vesicles with a diameter of 300-400 nm containing PTA in the inner water core. From the optical and electron microscopy, the size of DDAB-SDS vesicles is thought to be polydispersed. Table 1 shows the z-averaged diameter and polydispersity of vesicles obtained from the light-scattering experiments. The diameter decreases with XSDS,which agrees with the optical microscopy of the vesicles. The polydispersity indicates the heterogeneous size distribution of the vesicles, although the value decreases with an increase in XSDSvalue, in other words as the value approaches the micelle region.
-
.
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Glucose Encapsulation of DDAB-SDS Vesicles. The glucose dialysis experiment is one of indirect method (20)Ushio, N.;Solans,C.; Azemar, N.;Kunieda, H. J . Jpn. Oil Chem. SOC.1993,42,915. (21)Bangham, A. D.; Standish, M. M.;Watkins, J. C. J . Mol. Bid. 1965,13,238. (22)Kim, S.; Martin, G. M . Biochem. Biophys. Actu 1981, H6, 1. (23) Papahadjopoulos, D.; Miller, N. Biochim. Biophys. Actu 1967, 135, 624.
Figure 3. Micrographs of DDAB-SDS mixtures: (a) 10 mM DDAB-18 mM SDS (XSDS= 0.64) system, (b) 10 mM DDAB22 mM SDS (XSDS= 0.69) system, (Bar length is 40 pm). (c) 10 mM DDAB-22 mM SDS (XSDS= 0.69) system, and (d) 20 mM DDAB-43 mM SDS (XSDS= 0.68) system.
Spontaneous Vesicle Formation from DDAB-SDS Table 1. %-Averaged Diameter and Polydispersity of the Vesicles in the DDAB 10 mM-SDS Mixtures &DS"
diameter (2-averagedlhm
polydispersityb
-
155 110
0.58
z'
0.65 0.70 0.72 0.75
0.50 0.47 0.43
89 78
Xs~sis the ratio of the SDS concentration to the total surfactant concentration. The monodispersed system, like the micellar solution, shows a value less than 0.1. a
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Langmuir, Vol. 11, No. 7, 1995 2383
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Figure 5. Surface tension for the homogeneous DDAB-SDS
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Figure 4. Change in the captured volume for DDAB-SDS mixtures plotted versus XSDS.Open circles are data points for the 10 mM DDAB system, and filled circle are for the 16 mM DDAB system.
to recognize the vesicle formation. Although the adsorption of glucose to surfactants has been reported to cause the invalid estimation of glucose-trapping ability of vesicles,z4it is confirmed that the DDAB-SDS mixtures as well as the DDAB lamellar bilayer did not interacted with glucose molecules in the aqueous solution. Figure 4 shows the change in the captured volume of spontaneous vesicles as a function of &Ds, in the homogeneous mixtures. Open circles are data points for the 10 mM DDAB system and filled circles for the 16 mM DDAB system. The captured volume per mole of surfactants is determined through the glucose-trapping experiments with the equationz5
C'PE captured volume = CgluCsuIf
where is the glucose concentration retained inside the vesicles and Ctotand CEfiare the total concentration of glucose and surfktnats in the mixtures, respectively. The captured volume decreases with an increase in the SDS mole fraction, reaching 0%atXSDs = 0.75, as a result of the disappearance of the vesicles at any DDAB concentration in the mixtures. The maximum volume, 2.2 IJmol, is obtained at XSDS= 0.64. After the dialysis experiments, the vesicles left in dialysis tube were negatively charged. This indicates that the SDS molecule, having a high cmc (critical micelle concentration), is not removed appreciably through the 10 h dialysis. The cmc values for the DDAB-SDS mixtures by surface tension measurements are 1.5 x lov5 ____
(24) Kihara, K.; Tamori, K.; Esumi, K.; Meguro, K. J . Jpn. Oil Chem. SOC.1993.42. 140. (25) Szoka,'F., Jr.; Papahadjopoulos, D. Proc. Natl. Acad. Sci. U S A . 1978,75, 4194.
M for XsDs = 0.67, 2.2 X M for X s ~ = s 0.75, and 2.4 x M forXsDs = 0.83. The monomer concentration of the DDAB-SDS mixture is lower than that of a SDS solution by a factor of about 1/500. This may be the reason why the dissolution of SDS molecules from vesicles to the bulk phase is decreased. The size ofthe inner water phase in DDAB/SDS vesicles will be reflected in the captured volume. We previously reported that the captured volume for DDAB vesicular solutions prepared by ultrasonication was 0.9 L/mol." DDAB-SDS vesicles seem to have a great trapping ability for water-soluble compounds compared to pure DDAB vesicles. The addition of SDS into DDAB solution will lead to the enhancement of the capturing ability of DDAB vesicles. Mechanism of Spontaneous Vesicle Formation. The surface tension of the filtrate of the DDAB-SDS equimolar mixture, obtained by the filtration of the precipitate (ca. 63 mN/m), was slightly lower value than that of pure water. The scattered-light intensity against a 488 nm laser almost agreed with that ofpure water. The precipitate may consist of a 1:l DDA+:DS- complex which is highly insoluble in water. The adsorption of uncomplexed SDS molecules to the precipitate, neutralized electrically, will give the negative charge to the complex and result in the ionic vesicles. The geometries of surfactnats are important factors dominating the geometries of aggregates formed in aqueous solutions. The double-chained DDAB molecule is cylindrical and accordingly forms lamellar bilayers, while the conical SDS molecules form spherical micelles. Brady and Ninham et al.93263z7 have obtained spontaneous vesicles by exchanging the DDAB counterion with the more hydrophilic OH- that enhances the cross-sectional area of the head group compared to Br-. This suggests that cylindrical surfactants have a cuplike structure, resulting in larger head groups than the original surfactant, which are necessary for the self-formation of vesicles. The relationship between the surface tension for homogeneous mixtures containing 10 mM DDAB and various amounts of added SDS is shown in Figure 5. It is found that the surface tension value is a constant, 23 mN/ m, although the SDS mole fraction increases. The lowest surface tension value for the DDAB-water system is 24.1 mN/m, while that for SDS-water is 35 mN/m. Consider(26) Ninham, B. W.; Evans, D. F.; Wei, G. J . J . Phys. Chem. 1983, 87, 5020. (27) Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1986,90,226.
Kondo et al.
2384 Langmuir, Vol. 11, No. 7, 1995 Scheme 1. Geometric structure of surfactants
DDAB
SDS
cup like structure
4.0
DDAB 10 mM
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2
3
L
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d ing the surface activities of these single-surfactant systems, the very low surface tension (almost the same as an alkane) of the DDAB-SDS mixture indicates the ion-pairingof the DDAB and SDS molecule, which results in the enhancement of the adsorbed amount of the surfactants a t the aidwater interface. As may be expected for the precipitation of almost all surfactants a t equimolar mixing, the 1:l ion pair of the surfactants will be highly hydrophobic. In the SDS-rich mixtures, the head group charge of the ion pair should not completely disappear, because excess (uncomplexed) SDS molecules adsorb to the ion pair. This imperfect charge neutralization will prevent the ion pair from precipitating in the mixtures. The ion-pairing, in other words, the formation of a pseudo-triple-chained zwitterionic surfactant, should result in the cuplike structure shown in Scheme 1. This structure is expected to generate geometrically curved aggregates. The penetration of SDS between DDAB molecules will help in the organization of spontaneous vesicles. On the other hand, as the penetration progresses, this curvature should be enhanced, with the geometry of surfactant approaching the conical form by way of a cuplike form from the cylindrical form. The DDAB-SDS aggregate will accordingly change from a giant vesicle, via the small vesicle, to the micelle, while decreasing the diameter, as can be seen in Table 1. We could not identify the type of the liquid crystal (symbol L)on the SDS-rich side in Figure la. However, the critical packing consideration implies that surfactant aggregate changes from lamellar bilayer to curved bilayer to rod-shaped aggregate to spherical micelle, as the geometry of the surfactant turns from cylindrical to cuplike to conical form.28 The L phase on the SDS-rich side could be the liquid crystalline phase made up of rod-shaped aggregates,namely the hexagonal liquid crystalline phase. Considering the mechanism of vesicle formation mentioned above, it is expected that the addition of SDS increases the charge density on the vesicle surfaces. However, the 5 potentialvalues of the DDAB-SDS vesicles are independent of XSDS,as can be seen in Figure 2. It is well-known that the 5 potential of particles depends on the ionic strength of the solution as well as the surface charge density.29 The ionic strength in the mixture should (28) Israelachvile,J. N. Intermolecularand SurfaceForces;Academic Press: New York, 1985. (29) Ohshima, H.; Healy, T. W.; White, L.R.J. Colloid InterfaceSci. 1982,90, 17.
1.0
0 0.65
0.70
0.75
0.80
0.85
0.90
xSDeS
Figure 6. Change in the captured volume for DDAB-SDeS plotted versus XSM. increase with XSDSbecause the mixtures for 5 potential measurements were prepared by varying the SDS concentration a t the DDAB constant concentrationof 10mM. The increase in the ionic strength will decrease the thickness of the ionic atmosphere, which is the reciprocal of the Debye-Huckelparameter ( K ) , around the SDS head group, and lead to the shielding of the head group charge. Accordingly, the constant 5 potential will indicate that the charge density on the vesicle surfaces increases with XSDS.In contrast, the shielding effect of the head group charge will compensate for the enhancement of the 5 potential by an increase in the surface charge density of vesicles. Spontaneous Vesicle Formation in the Mixture of DDAB and SDeS. The above discussion seems to imply the general interpretation that lamellar bilayers always carry the optimal concentration for the spontaneous vesicle formation with an added conical surfactant like SDS. We attempted vesicle formation using another anionic surfactant, sodium decyl sulfate (SDeS). (SDeS was suppliedfrom Nihon Surfactant Industries Co., Tokyo, Japan. It was recrystallizedfrom ethanol, extracted with diethyl ether in a Soxhlet extractor, and dried in a vacuum oven.) Figure 6 shows that the DDAB-SDeS mixtures have glucose-trapping ability. Below the SDeS mole fraction &Des = 0.75,the mixture is heterogeneous due to precipitation. The captured volume for the homogeneous mixtures decreases with increasing X S ~ S Spon. taneous vesicles will form in the range 0.75 < -= 0.81. SDeS is a more hydrophilic surfactant than SDS, which is based upon a comparison of the cmcs values of the surfactants; SDS cmc = 8.1 mM and SDeS cmc = 35 mM. The larger the hydrophilicity of the anionic surfactant, the weaker the distribution ability of the surfactant into the precipitate. This may be the reason why the mole fraction of anionic surfactant for spontaneous vesicle formation is greater for SDeS than for SDS. LA9407222
