Langmuir 1990,6, 125-132
125
Aging and Stability of Vesicular Dispersions Hassan Madani and Eric W. Kaler**+ Department of Chemical Engineering, BF-10, University of Washington, Seattle, Washington 98195 Received April 18, 1989. I n Final Form: J u n e 9, 1989 Vesicle dispersions formed by sonication of aqueous dispersions of the synthetic surfactant sodium 4 4 1'-heptylnony1)benzenesulfonate (SHBS) have been characterized with quasielastic light scattering
(QLS) and electrical conductivity as a function of time. We present evidence that SHBS vesicles grow by means of size disproportionation, wherein surfactant monomers diffuse from small vesicles to large and the total number of vesicles ones. The average size of the vesicles increases with time t as decays as t-0.6,while the exponents predicted for diffusion-limited growth are respectively 0.33 and -0.67. Size distribution measurements show that the vesicle population is nearly monodisperse immediately after sonication but becomes polydisperse with age. Vesicle stability and size are strong functions of the sonication process, temperature, and electrolyte concentration. For example, vesicles formed by indirect sonication using a cup sonicator are more stable than those formed by sonication with the sonicator probe in direct contact with solution. Liquid crystals are observed in samples formed by direct sonication seconds after sonication, while they appear 24-48 h after sonication in samples made by indirect sonication. SHBS vesicles are more stable in electrolyte solutions due to reduction of surfactant solubility.
Introduction Vesicles are hollow spheres ca. 500 A in diameter consisting of a t least one surfactant bilayer. Vesicles are important biological structures, and their ability to encapsulate an aqueous environment makes them suitable for many practical applications. Spontaneous vesiculation of certain surfactants under favorable conditions has been reported,' but vesicles are usually formed from liquid crystalline dispersions with the input of mechanical energy (sonication)' and are therefore metastable structures. Unfortunately, routine use of vesicles is limited in part by their gradual reversion to the liquid crystals from which they were f ~ r m e d .Vesicles ~ age by aggregation followed by fusion4s5or by size disproportionation, whereby large vesicles grow a t the expense of smaller ones.6 Aging can be extremely slow, and for biological systems it is usually limited by the degradation of the surfactant, so it is not known whether vesicles revert completely to liquid crystals. Sodium 4-(1'-heptylnony1)benzenesulfonate (SHBS) is a double-tailed synthetic surfactant that is chemically and biologically stable and is thus suited for longterm studies of vesicle stability. Unilamellar vesicles are formed upon sonication of liquid crystalline dispersions.' SHBS vesicles revert only partially to liquid crystals, and after several months the dispersion reaches a kinetically stable state that is a mixture of vesicles and liquid crystals. Aging in SHBS dispersions has been observed previously with several methods,*-lo but the mechanism of vest New address: Department of Chemical Engineering, University of Delaware, Newark, D E 19716. (1) Ninham, B. W.; Evans, D. F.; Wei, G. J. J. Phys. Chem. 1983,
87,5020-5025. (2) Huang, C.Biochemistry 1969,8,344-351. (3) Larrabee, A. L.Biochemistry 1979,18,3321-3326.6. (4) Chang, E.L.;Gaber, B. P.; Sheridan, J. P. Biophys. J. 1982,39, 197-201. (5) Lawaczck, R.J. Colloid Interface Sci. 1978,66,247-256. (6) Johnson, N.W.; Kaler, E. W. J . Colloid Interface Sci. 1986,116, 444. (7) Kilpatrick, P. K.; Miller, W. G. J. Phys. Chem. 1984,88, 16491655. (8) Kaler, E. W.; Falls, A,; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Colloid Interface Sci. 1982,90,424-443.
0743-7463/90/2406-0125$02.50/0
sodium 4 41'-heptylnonyl)benzenesutfonale (SHBS)
icle reversion is still unknown. The usual electron microscopy techniques are prone to sample preparation artifacts and image only vesicles larger than 500 A,'' Other techniques including gel permeation chromatography (GPC), ultracentrifugation, and ultrafiltration are ineffective in sizing SHBS vesicles, and sometimes GPC fails even in separating liquid crystals from vesicles.8 Quasielastic light scattering (QLS) is a fast and noninvasive technique for measuring particle size distributions in colloidal system^.'^-'^ QLS has the disadvantage of being sensitive to the presence of small amounts of large particles, which heavily distort the results, while inherent experimental noise limits the resolution of the size distribution.15 Improved methods of polydispersity analysis of QLS datal6 together with measurement at multiple scattering angles make it partly possible to overcome such difficulties. In addition, the presence of dust and other large particles can be accounted for in the data analysis,17 although in the present experiments the effect of large particles is minimized because liquid crystals in SHBS dispersions are easily disrupted by sonication or separated by centrifugation. The proposed mechanism of SHBS vesicle aging involves diffusion of monomers between vesicles. The mecha(9) Kachar, B.;Evans, D. F.; Ninham, B. W. J. Colloid Interface Sei. 1984,100,287-301. (10) Franses, E. I.; Talmon, Y.; Scriven, L. E.; Davis, H. T.; Miller, W. G. J. Colloid Interface Sci. 1982,86,449-467. (11) Talmon, Y. J. Colloid Interface Sci. 1983,93,366-382. (12) Berne, B.J.; Pecora, R. Dynamic Light Scattering; Wiley: New York, 1976. (13) Chu, B. Laser Light Scattering, Academic Press: New York, 1969. (14) Muddle, A. G.;Higgins, J. S.; Cummins, P. G.; Staples, E. J.; Lyle, I. G. Faraday Discuss. Chem. SOC.1983,76,77-92. (15) McWhirter, J. G.; Pike, E. R. J. Phys. A 1978,11, 1729-1745. (16) Ostrowsky, N.; Sornette, D.; Parker, P.; Pike, E. R. Opt. Acta 1981,28,1059-1070. (17) Lincinio, P.; Delaye, M. J. Phys. Chem. 1987,91, 231-235.
0 1990 American Chemical Society
126 Langmuir, Vol. 6 , No. I , 1990
nism is active because the vesicle surface charge of -60 mV eliminates the possibility of significant vesicle aggregation and fusion, while the high monomer concentration allows a diffusion-mediated process of vesicle reversion, similar to Ostwald ripening in classical colloidal systems. The driving force for intervesicle monomer diffusion is the curvature energy of the bilayer. Intervesicle exchange of fluorescence-active compound^'^'^^ and the transfer of radioactively labeled cholesterol from small vesicles to large oneszo,21have been found to be first-order processes and therefore attributed to a size disproportionation mechanism. Recently the transfer of phospholipid molecules between small sonicated vesicles has been doc~mented.~~.~~ Here we report observations of vesicle aging using QLS and electrical conductivity measurements. Size distributions of the aggregates in pure water and in electrolyte solution have been measured as a function of time after sonication. The results indicate growth of large vesicles and dissolution of small ones, while the vesicle population remains unimodal. Measurement of the electrical conductivity of the dispersions, which is sensitive to monomer concentration and substantially less so to changes in vesicle size, provides a record of changes in free monomer concentrations with time.24 Vesicle growth follows a power law function of time as predicted by the LifshitzSlyozovZ5and Wagnerz6 (LSW) model of phase separation in colloidal systems.
Theory Vesicles are unstable with respect to the gradual growth of large vesicles at the expense of smaller ones2’ because of the bending energy associated with bilayer curvature. Sonication forms curved bilayers and increases the standard-state chemical potential of monomers in a vesicle of size R. If the bilayer has a positive curvature (bending) modulus, the standard-state chemical potential of a monomer in a vesicle of size R and aggregation number N is2’
where u is the molar volume of the surfactant monomer, k , is the elastic bending constant (energy per unit area), and 6 is the thickness of the bilayer. pmois the standardstate chemical potential of a flat bilayer. The effect of curvature on chemical potential in turn leads to a sizedependent monomer concentration around a vesicle of size R2’
with c,,(R) being the equilibrium monomer concentra(18) Roseman, M. A,; Thompson, T. E. Biochemistry 1980,19,439444. (19) Nicholes, J. W.; Pagano, R. E. Biochemistry 1981, 20, 27832789. (20) Mclean, L. R.; Philips, M. C. Biochemistry 1981, 20, 28932900. (21) Yeagle, P. L. Biochim. Biophys. Acta 1985,822, 267. (22) Barerl, T. M.; Schmidt, C. F.; Sackmann, E. Biochemistry 1988,27,6078. (23) Jones, J. D.; Thompson, T. E. Biochemistry 1989,28, 129. (24) Johnson, N. W. M.S.Thesis, University of Washington, 1985. (25) Lifshitz, I. M.; Slyozov, V. V. Phys. Chem. Solids 1961,19,35m (26) Wagner, C. 2.Electrochem. 1961, 65,581. (27) Dunning, W. J. Particle Growth in Suspensions; Smith, A. L., Academic Press: New York, 1973. (28) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985. 11.
Madani and Kaler tion around a vesicle of size R and ceq(m) the monomer concentration in equilibrium with an infinite bilayer. k , is Boltzmann’s constant and T the absolute temperature. Vesicles do not obey the Kelvin equation, which would require that their internal pressure be higher than the external pre~sure.~’Because the vesicle wall is permeable to ~ a t e r , the ~ ” chemical ~~ potential of water, and therefore the pressure, must be the same inside and outside of the vesicles. After sonication, vesicles minimize their bending energy by growth and reversion toward flat bilayers of zero curvature, a process which favors large vesicles. New vesicles do not form, thus the number distribution of the vesicles, n(R,t),obeys the continuity equation
an(R,t) a V [ R , d t 1)n(R,t) =O at aR where V[R,c(t)]is the growth rate for a vesicle of size R in a solution with bulk concentration c ( t ) . The total number of vesicles at time t is +
N ( t ) = Jmn(R,t)dR and mass conservation requires
c ( t ) = Cin where ein is the initial surfactant concentration before sonication. The two common rate-limiting steps in growth processes are either the diffusion of monomers from one vesicle to the other or the rate of exchange of monomer between the solvent and donor or acceptor vesicles (i.e., interfacial kinetics). If the process is limited by monomer diffusion, the growth law is
V[R,c(t)l= ( D v / R ) [ c ( t-) c,,(R)l where D is the monomer diffusion coefficient. If the interfacial kinetics control the process V[R,c(t)l= k,[c(t)- ceq(R)l where k , is the mass-transfer coefficient for monomer transfer to the vesicle bilayer. The continuity equation, mass conservation, and the growth law describe vesicle aging by size disproportionation. The first analysis of the time behavior of a particle population undergoing size disproportionation was done by Lifshitz and S l y o ~ o and v ~ ~Wagner.26 Using a transformation of the variables of particle radius, monomer concentration, and time, they obtained a scaled form of the particle size distribution for an infinitely dilute system in the limit of long time (Le., when the supersaturation is very low). For both diffusion-limited and interface kinetic-limited growth, the asymptotic distribution is independent of time and the initial conditions, and the observables (density (N), monomer concentration (c), and average size ( R ) )obey universal scaling laws. This theory has been extended to include the dependence on the volume fraction of the particles32 and the effect of deviation from ideality in the bulk Volume fraction corrections do not affect the exponents in (29) Adam, N. K. The Physics and Chemistry of Surfaces, 3rd ed.; Oxford University Press: New York, 1941. (30) Finkelstein, A. J. Gen. Physiol. 1976,68, 127-135. (31) Tanford, C. Proc. Natl. Acad. Sci. U.S.A. 1979, 76,3318. (32) Marqusee, J. A.; Ross, J. J . Chem. Phys. 1984,80, 536. (33) Chaix, J. M.; Eustathopoulos, N.; Allibert, C. H. Acta Metall. 1986,34, 1589. (34) Chaix, J. M.; Allibert, C. H. Acta Metall. 1986, 34, 1593-1598.
Langmuir, Vol. 6, No. 1, 1990 127
Aging and Stability of Vesicular Dispersions the power law behavior but do alter the amplitude of changes in N , R , and c. Diffusional effects between the particles also broaden the particle size distributions. More complicated growth mechanisms have been examined,35 but the theory has not been well tested experi-
I
I
men tall^.^^"^
Calculation of the particle size distribution and its time scaling can be done economically as follows.3s Reduced variables describing vesicular growth are I I1
11 I I
where for diffusion-limited and interface kinetic-limited growth
T=-
a2
a
andT=Dvc,(m) kIc,( ~ 0 ) respectively, and F ( r , ~ ) d r= n(R,t)dR. The continuity equation, the growth laws, and the mass conservation expression are aF(r,7) a7
1 -[
V[r,u(~)]= r u(7) -
[
+]
f]
(diffusion limited)
(interface kinetic limited)
u(7) = uin- ~ L ~ r ' F ( r , 7d)r
K
47ra26 =-
vc,("")
The distribution function of the vesicles is expanded about its long-time behavior as38
F(r,7) = 7-Y(F0(z) where
+
7-Y1F1(z)
+ T'~F,(Z) + ...)
z = r7-'
Exponents are obtained by substituting the expanded distribution into the mass conservation equation. The distribution function of the vesicles is
u(7)
a 7-'
N(7)a
7-&
P ( 7 ) 0: 7'
where F is the average radius of the vesicles. The unknown exponents and the distribution function are obtained by substituting for the growth law in the continuity equation and balancing powers of t ; the results are x = 1/3 for diffusion-limited growth and x = 1 / 2 for interface kinetic-limited growth. Therefore, the scaling laws are u(7)
a 7-".33
N(7) a
7-".67
~ ( 7a ) 7°.33
for diffusion-limited growth and u(7)
a 7-".5
tion for the lowest order term, Fo(z),is
for diffusion-limited growth and
aF(r,7)V[r,ddI =O ar
+
V[r,a(~)]= u(7) -
Figure 1. Asymptotic vesicle distribution F&) as a function of the scaled radius z for diffusion-limited growth. This curve is obtained by numerical integration of the continuity equation for the lowest order term Fo(z).
N ( 7 ) 0: 7-l
P ( T ) a 7°.5
for interface kinetic-limited growth. The differential equa(35) Jain, S.C.;Huges, A. E. J. Mater. Sci. 1978,13, 1611. (36) Kabalnov, A. S.;Pertzov, A. V.; Shchukin, E. D. J. Colloid. Interface Sci. 1978,118,590-597. (37) Enomoto, Y.; Kawasaki, K.; Tokuyama, M. Acta Metall. 1986, 35,915. (38) Marqusee, J. A.; Ross, J. J. Chem. Phys. 1983,79, 373.
for interface-kinetic limited growth. Similar equations are obtained for the higher order terms. Mass conservation of surfactant requires that Fo(z)go to zero a t a finite value of z and sets u ~ For . comparison ~ ~ with experiment, the vesicle size distribution is obtained by numerical integration of the continuity equation for the lowest order term Fo(z) (Figure 1). Finally, for comparison with measured size distributions, dimensional calculated distributions as a function of aging time are shown in Figure 2. The reduced radius r a n d the parameter a are calculated from z by using the average size of the vesicles measured 8-10 days after sonication. The number distribution n(R,t)is obtained from the scaled solution by using a monomer diffusion coefficient of lo4 cm2/s, a surfactant density of 1.1 g/cm3, and a surfactant solubility of 0.20 wt %. The intensity of light scattered from a vesicle of size R , I(R,q),is given by
I ( R d = R4P(q,6)n(R,t) Here q (= 4 m / h sin (0/2)) is the magnitude of the scattering vector, where X is the wavelength of light in the scattering medium, 0 is the scattering angle, and n is the refractive index of the medium. P(q,6) is the particle form factor for a hollow sphere with bilayer thickness r 39
0.
Materials and Methods Sodium 4-(l'-heptylnonyl)benzenesulfonate was obtained from IRCHA (Vert-Le-Petit, France). The surfactant was purified to greater than 98% by repeated recrystallization from acetone a t 50 "C. Solutions were sonicated in a Heat Systems Ultrasonics Model W-225 either directly with a 0.5-in.-diameter metal tip in the sample vial or indirectly in a cup sonicator with external cooling circulation of ethylene glycol. Continuous sonication a t a power level of 20-30 W was used for both direct and indirect sonication. In each indirect sonication batch, three scintillation vials, each containing 6 mL of 1 w t % SHBS solution, were placed in the sonicator cup. Sonication was continued until the solution was clear blue and the hydrodynamic radius measured by light scattering did not change with further sonication. For samples with (39) Guinier, A.; Fournet, G. Small-Angle Scattering of X-rays; Wiley: New York, 1955.
128 Langmuir, Vol. 6, No. 1, 1990
o I0
I00
1000
Madani and Kaler
I0000
200' 0
20
40
I
60
Time (min)
Figure 3. Electrical conductivity at 25 "C of sonicated dispersions of 1 wt 3'% SHBS in pure water measured after indirect sonication a t 15, 25, and 40 "C for 12-16 h at 30% power in a cup-horn sonicator.
2lXX)
c
Radius
(AJ
Figure 2. Calculated size distributions of the vesicles from the LSW model a t 1 day (A), 3 days (B), and 30 days (C) after sonication. Dimensional size distributions are derived directly from F J z ) in Figure 1 by using a monomer diffusion coefficient of cm2/s, a surfactant density of 1.1g/cm3, and a surfactant solubility of 0.2 wt %. The number distribution of the vesicle was related to the distribution of scattered light after correction for the effect of the scattering form factors, with vesicles modeled as hollow spheres with a wall thickness of 30 A. surfactant concentration less than 2 w t % and electrolyte concentration of 0.1 M or less, 14-20 h of continuous sonication a t 30% power was required. An Orion Model 101 digital conductivity meter was used for electrical conductivity measurements. The cell constant of the electrode was approximately 1, and the electrode was calibrated with standard solutions of KCl. Sample cells were acid washed and rinsed with deionized water. Solutions with low electrolyte concentration were tightly sealed to prevent CO, absorption and solvent evaporation. The measurement temperature was 25.0 f 0.05 "C. The QLS light source was a Spectra-Physics Model 165 Argon ion laser operating at 488 nm. A Langley-Ford correlator (Model 1096) with 80 equally spaced channels and with a software dust discriminator calculated the correlation functions. The dust discriminator functioned by detecting increases in the scattered intensity that indicated a dust particle or large liquid crystalline particle had entered the scattering volume. Typically a threshold intensity of 10% above the average intensity was set; thus when a dust particle entered the scattering volume and increased the intensity above this threshold, data collection was halted. T o further minimize the effect of large particles on the correlation functions, scattering experiments were done after centrifuging the samples a t 50000g for 1 h. Removal of liquid crystals by centrifuging improved the reliability of polydispersity analysis. The scattering cells, 1-in.-diameter scintillation vials, were acid washed and rinsed with DI water. Temperature during measurement was controlled to f0.1 "C by using circulating water through a copper coil in direct contact with the index matching fluid (dodecane). The scattered light was detected with an EM19893B/350 photomultiplier tube. All the measurements were done a t 90" and 120" scattering angles, and
the run time for each correlation function was at least lo7 times the sample time with a t least lo6 counts/channel. The time per channel was such that the scattered intensity was approximately one count per sample time. Percent transmitance through the sample vial was measured with a Newport (Model-820) power meter. The autocorrelation function of the scattered intensity was analyzed with the exponential samplingM method and the cumulant method.41 The calculated intensities were corrected for the particle form factor, with the vesicles modeled as hollow spheres with walls 30 A Details of data analysis are given elsewhere.42 As shown below, the effect of interparticle interactions on the QLS signal was negligible. In this case, the diffusion coefficient of a particle is related to its hydrodynamic radius ( R ) through the Stokes-Einstein relation D = k,T/ ( ~ x w R where ), p is the solvent viscosity.
Results Initial vesicle size distributions are a strong function of sonication power, sonication time, and electrolyte concentration. Vesicle size decreases with longer sonication time and eventually reaches a constant value. Vesicle formation in solutions containing added electrolyte or sonication at low power requires longer sonication times. Upon prolonged sonication (12-16 h) in pure water, SHBS monomers form unilamellar vesicles with a mean radius of 180 f 10 A (UV absorption measurementsz4show that SHBS does not degrade after extended sonication). In 0.1 M NaC1, the average vesicle radius decreases to 120 f 10
A. SHBS vesicles are charged, and therefore determination of size by QLS may be influenced by electrostatic interactions. The interparticle separations in pure water were estimated to be at least four particle diameters, and the Debye screening length was 80 A. In pure water, the vesicle size measured a t several scattering angles changes less than 5% after dilution from 1 to 0.2 wt % surfactant, a result that strongly indicates that intervesicle electrostatic interactions are not important here. Below the solubility limit of 0.2% SHBS, vesicles are not present. The concentration of free monomer in 1 wt 7% dispersions of SHBS, as monitored by electrical conductivity, changes relatively rapidly after sonication. Sonication at 25 "C increases the electrical conductivity of the dispersion and produces a clear blue solution of vesicles. After sonication, and when aged a t 25 "C, the electrical conductivity decreases (Figure 3). The rate of change of (40) McWhirter, J. G. Opt. Acta 1980, 27, 83. (41) Koppel, D. E. J. Chem. Phys. 1972,57,4814. (42) Madani, H. Ph.D. Thesis, University of Washington, 1989.
Langmuir, Vol. 6, No. 1, 1990 129
Aging and Stability of Vesicular Dispersions
I
u'Lu
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(A) Figure 4. The size distributions of 1 wt % SIIBS vesicles in pure water immediately after sonication (A) and 10-15 min of aging (B). Vesicles were prepared by indirect sonication (1216 h) at 30% power in a bath sonicator at 25 "C and aged at Radius
room temperature. The exponential sampling method was used
to determine the size distribution from QLS measurements.
electrical conductivity increases when the sonication temperature increases. After sonication a t 40 "C and cooling to 25 "C, electrical conductivity decreases dramatically with time (Figure 3). On the other hand, after sonication a t 15 OC and warming t o 25 "C, electrical conductivity increases with time (Figure 3). Temperature equilibration during these experiments is rapid; a 0.1 M NaCl solution reaches its equilibrium value of electrical conductivity in less than 5 min after such temperature excursions. In all cases, the electrical conductivity reaches a value of ca. 270 wS/cm after 1-2 h, and that value decreases with time very slowly. The turbidity of the solutions also changes in the first 1-2 h after sonication. During sonication and aging at 25 "C, the turbidity increases slightly. After sonication a t 40 "C and cooling to 25 "C, the sample becomes substantially more turbid; on the other hand, after sonication at 15 "C and warming to 25 "C, turbidity is constant. The longer term stability of sonicated dispersions is a strong function of sonication process. Liquid crystals are observed visually in samples formed by direct sonication minutes after sonication, while no liquid crystals appear for 24-48 h after indirect sonication. The presence of liquid crystals makes it difficult to estimate the size distribution of the vesicles present with them because of the disproportionate contribution of large particles to the scattering signal. Therefore, only indirect sonication was used to produce vesicles for aging experiments. SHBS vesicles in pure water are unstable. The size distribution of vesicles in 1 wt % SHBS dispersions is initially unimodal (Figure 4A). Within 15-30 min after sonication, the distribution becomes bimodal (Figure 4b) and consists of aggregates with radius of 180 A, which are presumed to be vesicles, and 1000-A particles, which are assumed to be nascent liquid crystallites based on
0.05
0.00 10
100 Radius
(A)
Figure 5. Size distributions of 1 w t % SHBS vesicles in pure water at 1day (A), 3 days (B), and 30 days (C) after sonication (conditions as in Figure 4). Vesicle solutions were centrifuged at 50000g for 1 h to remove liquid crystals before QLS measure-
ments.
previous examinations with electron microscopy.' This change in vesicle size and the appearance of larger particles are consistent with the observed increase in turbidity. If such a sample is left undisturbed, the larger particles grow substantially and a precipitate will appear. QLS measurements of such samples are unrewarding because the scattering from the large particles overwhelms the scattering from the smaller vesicles. Therefore, all samples more than 1 h old are centrifuged to remove liquid crystals before QLS measurements. The distribution of the remaining vesicles is unimodal, the vesicles grow with time (Figure 5), and the growth rate eventually slows. Although liquid crystals continue to form, more than 90% of the original surfactant is retained in the supernatant. Most vesicle growth occurs during the first 40 days after sonication, and the average radius increases from 180 to 450-500 A. During this time, the mean radius of the vesicles in the supernatant increases as (Figure 6A) and their number decays as t-0.6 (Figure 6B); the number of vesicles is calculated by using a value for the SHBS head group area of 41 After 40 days, the size increase slows markedly, and after 8 months the vesicle radii are 500-550 A. QLS measurements of vesicles aged for more than 40 days show an increase in apparent size with scattering angle, a result suggestive of the presence of nonspherical structures or interparticle interactions. The behavior of SHBS vesicles formed in the presence of 0.1 M NaCl is substantially different. Vesicles in 0.1 M NaCl are initially monodisperse with an average radius of 120 A (Figure 7). The rate of increase of turbidity and size is much slower than in pure water, and fewer liquid crystallites visible to the naked eye form.
130 Langmuir, Vol. 6, No. 1, 1990
Madani and Kaler
I(XX1
A pure water I
x
Radius
(A)
Figure 8. Size distribution of vesicles in 1 wt % SHBS and 0.1 M NaCl30 days after sonication (conditions as in Figure 4). Solutions were centrifuged at 50000g for 1 h before QLS mea-
0.1 M NaCl
surements.
Q
0'
I
I
I00
IO Time (days)
Figure 6. Average size (A) and total number of the vesicles (B) in 1 wt % SHBS solutions in pure water and in 0.1 M NaCl as a function of time after sonication (conditions as in Figure 4). The total number of vesicles is calculated by using a value
for the SHBS head group area of 41 A2.
Radius
0.15
(A)
Figure 9. Size distribution of vesicles in 1 w t % SHBS and 0.1 M NaCl 1 h after sonication (conditions as in Figure 4 except that the aging temperature was 50 O C ) .
1
bimodal, with peaks a t 140 and 600-700 after heating (Figure 9).
A, within 1 h
Discussion Two distinct stages in the aging process can be iden0.00 10
I 00 Radius
IA)
I000
IO000
Figure 7. Size distribution of vesicles in 1 wt % SHBS and 0.1 M NaCl at 25 "C 1 h after sonication (conditions as in Fig-
ure 4 ) .
After 30 days, the distribution of particles remaining after centrifugation is bimodal and consists of 130- and 500-.& particles (Figure 8), unlike the unimodal distributions observed in pure water. It is likely that the 130-A particles are vesicles that have changed only slightly over this time, while the second peak is due to aggregates of the original small vesicles. (Such aggregates are less dense than liquid crystallites and so do not settle in the ultracentrifuge.) After 8 months, a similar bimodal distribution is observed, and the vesicle radius has increased to 160 A. Increasing temperature hastens the formation of aggregates of vesicles in 0.1 M NaCl. During the first 2 h after sonicated samples are heated to 50 "C, the average size and the turbidity of the vesicular solutions increase, while those held a t room temperature do not change. The vesicle population in pure water remains unimodal even after heating (not shown), while those in 0.1 M NaCl become
tified, namely, an initial stage during the 1-2 h after sonication followed by long-term aging lasting several weeks. The initial process is the rapid growth of liquid crystallites that are responsible for the turbidity increase. Much of this growth is due to relaxation of the monomer concentration in the supersaturated sonicated dispersion, and the change in electrical conductivity with time mirrors the relaxation of supersaturation (Figure 3). Sonication produces supersaturation by increasing the monomer concentration beyond its solubility. Supersaturation is likely due to both heating of the solution during sonication and production of many small vesicles or other aggregates with highly curved surfaces. Initially, however, the initial supersaturation relaxation is due only to readjustment of the monomer concentration in the bulk solution, and conductivity decreases as monomers "condense" and form large particles which do not contribute significantly to the conductivity. With the growth of large aggregates after sonication, the size distribution of the particles in solution becomes bimodal, consisting of vesicles formed during sonication and aggregates formed after sonication (Figure 4B). These large aggregates grow with time, and eventually larger (ca. >1 pm) liquid crystals settle. The rate and extent of early aggregate growth depend upon the degree of supersaturation. Monomer solubility increases with increasing temperature; thus the amount of supersaturation developed during sonication and the
Aging and Stability of Vesicular Dispersions readjustment of monomer concentration after sonication both depend on temperature. When the temperature is changed, the bulk monomer concentration always readjusts itself toward equilibrium. The clearest evidence for this is the increase in electrical conductivity (and so monomer concentration) with time in subsaturated solutions, i.e., the sample sonicated a t 15 OC and warmed to 25 OC (Figure 3), for here vesicles are dissolving to allow the bulk monomer concentration to increase toward equilibrium. On the other hand, increasing the sonication temperature increases the monomer concentration. Except for perhaps growing slightly, vesicles remaining after centrifugation are unaffected by the initial supersaturation relaxation (Figure 4), and when supersaturation becomes small the second stage of aging begins. In pure water, the active mechanism in the second stage is unlikely to be vesicle aggregation for several reasons. First, in the context of DLVO theory, the high surface potential of the vesicles makes them electrostatically stable. Second, added electrolyte has the effect opposite that predicted by DLVO theory if changes in the dispersion in pure water occurred by an aggregation-dependent process, for in that case a greatly increased rate of aggregation would be observed due to the compression of the electric double layer by added electrolyte. On the other hand, SHBS vesicles age more slowly in electrolyte. This is a result of the improved stability of SHBS vesicles with respect to a monomer diffusion mechanism in 0.1 M NaCl because of the reduction of monomer solubility in the electrolyte solution. However, SHBS vesicles do aggregate in 0.1 M NaC1, as can be seen by examining the size distribution of vesicles as they age (Figure 8). The persistence of smaller vesicles and the appearance of larger aggregates as time progresses are consistent with an aggregation-fusion mechanism. The appearance of a bimodal distribution of particles (primary vesicles and their aggregates) is necessarily due to an aggregation-fusion mechanism since a "ripening" or size disproportionation process cannot convert a unimodal distribution into a bimodal di~tribution.'~ For small sonicated DMPC vesicles undergoing aggregationfusion, a bimodal distribution of vesicles is measured during aging,43and the results agree with the calculated distributions obtained by using the generalized Smoluchowski equation. The slight aggregation caused by 0.1 M NaCl can be magnified by increasing temperature. The similarity of the vesicle size distributions measured 30 days after sonication at room temperature (Figure 8) and those measured within an hour a t 50 "C (Figure 9) suggests that the aging mechanism is the same, although the kinetics and the extent of growth are temperature dependent. In contrast, vesicles in pure water remain unimodal even after heating, consistent with the thesis that aggregation is unimportant in pure water at 25 "C. We therefore conclude that during the long-term aging process SHBS vesicles in pure water grow by a size disproportionation mechanism that is the result of a readjustment in the vesicle size distribution. The evolution of vesicle sizes is such that the curvature energy in the vesicle bilayer is minimized and the average size of the vesicles increases (Figure 5). The relatively high monomer solubility of 0.2 wt % increases the amount of available material for exchange and hence promotes ripen(43) Sornette, D.; Hesse-Bezot, C.; Ostrowsky, N. Biochimie 1981, 63, 955.
Langmuir, Vol. 6, No. 1, 1990 131 ing. Addition of electrolyte, in contrast, suppresses size disproportionation by lowering the surfactant solubility. SHBS solubility in 0.1 M NaCl is at least 100 times less than the solubility in pure water.24 At 50 "C, increased monomer solubility and diffusivity enhance aging with respect to size disproportionation. As the LSW model predicts, size disproportionation rates are proportional to monomer solubility and diffusivity and so increase with temperature. There are other features of aging consistent with a diffusion-mediated process. The kinetic stability of the particle size distribution at long times is characteristic of size disproportionation and inconsistent with an aggregation-fusion mechanism. As vesicles grow, disproportionation slows because the bilayer curvature energy, the driving force for growth, becomes smaller. The LSW model accounts for this slowing down. On the other hand, if aggregation occurred, aging would be enhanced as vesicles grow (since large vesicles aggregate more readily than small ones44), and aggregation would continue indefinitely. Vesicle size disproportionation can be affected by monomer solubility, monomer diffusivity, monomer exchange between the two bilayer leaflets (flip-flop), and the rate of monomer association and dissociation from the bilayer. Vesicle reversion in SHBS dispersions appears to be diffusion limited because the exponents in the scaling laws (Figure 6) are close to the ones predicted for diffusionlimited growth (0.33 and -0.67). The kinetics of monomer association and dissociation apparently do not affect aging; otherwise the exponents in the scaling laws would be closer to the interface kinetic limited values of 0.5 and -1.0. Furthermore, the measured size distributions (Figure 6) are in agreement with those calculated by using a diffusion-limited growth law (Figure 2). The increase in aging at 50 "C is also consistent with diffusion-limited growth, since temperature enhances monomer diffusivity and therefore size disproportionation. Although increasing temperature results in a significant increase in monomer diffusivity, the kinetics of monomer association and dissociation, which are strong functions of membrane fluidity, are not affected significantly, because all temperatures are above the SHBS gel transition temperature of -70 "C. A final point is the behavior of the vesicular dispersions after ca. 40 days (Figure 6). The vesicle population reaches an apparently stable state in that the growth in average radius stops. At this point, the QLS measurements suggest that nonspherical structures are present. Such structures, which may be large complex vesicles and/ or long t ~ b u l e s ,are ~ ~of? course ~ ~ not accounted for the LSW theory, nor are current theories of surfactant aggregation able to account quantitatively for such transitions.
Summary Vesicles formed by sonication of an aqueous dispersion of SHBS are unstable. SHBS vesicles in pure water do not aggregate or fuse but instead grow by size disproportionation. The driving force for aging is the bending energy associated with bilayer curvature. SHBS vesicles grow 4-6 weeks after sonication to an average radius (44) Nir, S.; Bentz, J.; Duzgunes, N . J. Colloid Interface Sci. 1981, 84, 266. (45) Talmon, Y. Colloids Surf. 1986, 19, 237. (46) Miller, D. D.; Bellare, J. R.; Evans, D. F.; Talmon, Y.; Ninham, B. W. J . Phys. Chem. 1987,91, 674.
Langmuir 1990, 6, 132-136
132
of 450-500 A before reaching a stable size distribution. Aging appears to occur as a power law of time where the vesicle radius increases as and vesicle number density decays as t-0.6. Size distributions measured with QLS are in excellent agreement with those predicted by the Lifshitz-Slyozov and Wagner model of phase separation in colloidal systems. Vesicle stability is enhanced in NaCl solution due to a lowering of surfactant solubility.
Acknowledgment. This research was supported by the National Science Foundation (PYIA 8351179), the Shell Development Co., the 3M Co., and Johnson and Johnson. We acknowledge very helpful discussions with Professor Jacob Israelachvili. Registry No. SHBS, 67267-95-2.
Dilute Phase Behavior of Cetyl Alcohol, Sodium Lauryl Sulfate, and Water Richard J. Goetz and Mohamed S. El-Aasser* Department of Chemical Engineering, Emulsion Polymers Institute, Center for Polymer Science and Engineering, Lehigh University, 111 Research Drive Bldg. A , Bethlehem, Pennsylvania 18015 Received February 17, 1989. I n Final Form: J u n e 12, 1989 A ternary phase diagram of cetyl alcohol (CA), sodium lauryl sulfate (SLS), and water is constructed at room temperature in the dilute corner with a composition range of 96-100 wt % water, and 4-0 wt % of the surfactant and alcohol. The phase behavior is examined though differential scanning calorimetry and associated with the polymorphism of CA. Three different phases are observed: (i) a gel phase, (ii) a coagel phase, and (iii) a micellar solution of SLS and crystals of CA. With aging, certain regions of the gel transform into the coagel phase. This transformation and the existence of the micelle + /3 crystal region are related to the collapse of the bilayer structure of the gel.
Introduction The phenomenon of the aggregation processes in aqueous surfactant solutions is important in understanding biomembrane dynamics, the properties of drug delivery vehicles, and the phase behavior of microemulsion systems.',2 Model systems typically consist of aqueous surfactant and alcohol mixtures, which aggregate in water to form a variety of liquid crystalline structures. The four common liquid crystalline structures are the lamellar, normal hexagonal, reversed hexagonal, and cubic phases. The physical nature of these phases and the location in the ternary phase diagram are strongly related to the chemical nature of the amphiphile, i.e., the balance between the hydrophilic and lipophilic moieties. These phases have been intensively studied over the past 30 years; however, except for the cubic phase, there is little controversy regarding their molecular arrangement.394 Because of their location in the ternary phase diagram, most of these studies have mainly focused on concentrated systems, using short-chain alcohols. The present study is aimed a t determining the phase behavior of dilute aqueous surfactant systems using a long-chain alcohol, cetyl alcohol (CA), which is a solid at room temperature. Recently, Benton and Miller have reported lyotropic liquid crystalline phases with 90% water in surfactant~
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(1) Bader, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H. Adu. Polym. Sci. 1985, 64, 1.
(2) Bellocq, A,; Roux, D. Microemulsion Structure and Dynamics; Friberg, S., Botherol, P., Eds.; CRC: Boca Raton, FL 1987; pp 33-78. (3) Fontell, K. Mol. Cryst. Liq. Cryst. 1981, 63, 59. (4) Ekwall, P. Advances in Liquid Crystals;Brown, G. H., Ed.; Academic: New York, 1975; Vol. 1. pp 1-142.
0743-7463/90/2406-0132$02.50/0
alcohol-brine systems using various alcohol chain lengths up to decan01.~ These solutions are ideal for enhanced oil recovery due to the low viscosity and o/w interfacial tension, enhancing the ability to displace oiL6 Aqueous surfactant-alcohol systems with alcohol carbon chain lengths greater than 12 are used for the preparation of topical pharmaceutical and cosmetic creams. Cetyl and stearyl alcohols are commonly used since they impart a high consistency to the p r e p a r a t i ~ n .As ~ the concentration of the alcohol and surfactant in the aqueous solution increases, the system progressively changes from a fluid to being solidlike. Barry's738extensive rheological studies showed that the consistency is due to the formation of a viscoelastic network comprised of the mixed surfactants located in the aqueous phase. The surfactant network, observed through various microscopic techniques, changes in structure with increasing alcohol concentration from vesicular in form to a system consisting of lamellar liquid crystals nucleating from alcohol particle^.^,'^ However, the existence of this network depends on the polymorphism of the alcohol." Long-chain alcohols exist in two crystal forms stable over different temperature (5) Benton, W. J.; Miller, C. A. J . Phys. Chem. 1983,87, 4981. ( 6 ) Miller, C. A.; Ghosh, 0.; Benton, W. J. Colloids Surf. 1986, 19, 197. (7) Barry, B. W. Adu. Colloid Interface Sci. 1975,5, 37. (8) Barry, B. W.; Shotton, E. J. Pharm. Pharmacol. 1967,19, 1105, 1215. (9) Patel, H. K.; Rowe, R. C.; McMahon, J.; Stewart, R. F. Int. J . Pharm. 1985,37,899; 1985, 37, 564; 1985,25, 13. (10) Rowe, R. C.; McMahon, J. Colloids Surf. 1987, 27, 367. (11) Fukushima, S.; Takahashi, M. J . Colloid Interface Sci. 1976, 57, 201; 1977, 59, 159.
0 1990 American Chemical Society
