J . Phys. Chem. 1989, 93, 2630-2634
2630
Theoretical and Experimental Stabilities of Amphiphile Vesicles in the Presence of NaCl A. M. Carmona-Ribeiro Departamento de Bioquimica, Instituto de Quimica. Universidade de Suo Paulo, CP20780 Sao Paulo, SP, Brazil (Received: February 25, 1988; I n Final Form: September 7 , 1988)
Theoretical and experimental stability values (Wj for dioctadecyldimethylammonium chloride (DODAC) or sodium dihexadecyl phosphate (DHP) vesicles in the presence of NaCl salt are compared. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for the stability of charged spherical colloidal particles is used to calculate some theoretical stabilities for small or large synthetic amphiphile vesicles in the presence of NaCI. The agreement between theoretical and experimental Wvalues is interpreted as merely fortuitous since both inherent problems with the model and other factors operating in real systems can influence the vesicle stability. By improving the model and the inputs, the theoretical stability is found to be much higher than the experimental one for small DHP vesicles.
Introduction Interactions between phospholipid model membranes have been extensively studied.I-l3 At long ranges, the main interactions are electrostatic (for charged lipids) and electrodynamical (van der Waals). The pioneer techniques on direct measurements of forces between phospholipid bilayers in aqueous solutions have shown the existence of very strong repulsive forces at short ranges probably due to the structural ordering of water molecules induced by the phospholipidic headgroups.1*12At low ionic strength and neutral pH, hydration forces were absent for interacting bilayers of dihexadecyldimethylammonium acetate (DHDAA) or bromide (DHDAB) adsorbed on the anionic mica surface^'^*'^ or for interacting monolayers of sodium dihexadecyl phosphate (DHP) adsorbed on DHDAB-coated mica surfaces.I5 Vesicles composed of dioctadecyldimethylammonium chloride (DODAC) or sodium dihexadecyl phosphate (DPH)I6,l7interact at low monovalent salt c o n ~ e n t r a t i o n . ’ ~This ~ ~ ~contrasts with the high stability of phospholipid vesicles at comparatively higher monovalent salt.” Qualitatively, DODAC vesicles fuse upon addition of NaCl whereas D H P vesicles mainly aggregate,{’ a behavior consistent with differences in packing of DHDAA or DHP molecules in monolayer films and with differences in directly measured interaction forces.15 Since hydration forces are absent
(1) Le Neveu, D. M.; Rand, R. P.; Parsegian, V. A. Nature 1976,259,601. (2) Le Neveu, D. M.; Rand, R. P.; Parsegian, V . A,; Gingell, D. Biophysical J . 1977, 18, 209. (3) Cowley, A. C.; Fuller, N. L.; Rand, R. P.; Parsegian, V . A. Biochemistry 1978, 17, 3163. (4) Nir, S.; Bentz, J . J . Colloid Interface Sci. 1978, 65, 399. ( 5 ) Lis, L. J.; Parsegian, V . A.; Rand, R. P. Biochemistry 1981, 20, 1761. (6) Lis, L. J.; Lis, W. T.; Parsegian, V. A,; Rand, R. P. Biochemistry 1981, 20, 1771. (7) Rand, R. P. Annu. Rev. Biophys. Bioeng. 1981, IO, 211. ( 8 ) Nir, S.; Bentz, J.; Duezguenes, N. J . Colloid Interface Sci. 1981,84, 266. (9) Lis, L. J.; McAlister, M.; Fuller, N.; Rand, R. P.; Parsegian, V. A. Biophysical J . 1982, 37, 651. (IO) Afzal, S.; Tesler, W. J.; Blessing, S. K.; Collins, J. M.; Lis, L. J. J . Colloid Interface Sci. 1984, 97, 303. ( 1 I ) Ohki, S.; Roy, S.; Ohshima, H.; Leonards, K. Biochemistry 1984, 23, 6126. (12) Marra. J.; Israelachvili, J. N. Biochemistry 1985, 24, 4608. (13) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D.F. J . Phys. Chem. 1986, 90, 1637. (14) Marra, J . J . Phys. Chem. 1986, 90, 2145. ( I 5) Claesson, P. M.; Carmona-Ribeiro, A. M.; Kurihara, K. J . Phys. Chem., in press. (16) Carmona-Ribeiro, A. M.; Chaimovich, H. Biochim. Biophys. Acta 1983, 733, 172. (17) Carmona-Ribeiro, A. M.; Yoshida, L. S.; Sesso, A,; Chaimovich, H. J . Colloid Interface Sci. 1984. 100. 433. (18) Carmona-Ribeiro, A. M.;Yoshida, L. S ; Chaimovich, H J . Phys. Chem. 1985, 89, 2328. (19) Carmona-Ribeiro, A. M.; Chaimovich, H. Biophys. J . 1986, 50, 621.
0022-3654/89/2093-2630$01 S O / O
when amphiphile surfaces interact at 10w13-15ionic strength, this work explores the possibilities of the kinetical DLVO in explaining the experimental stability of DODAC or DHP vesicles for a range of NaCl concentration. The calculations demonstrate that additional destabilizing factors and/or improvements in the model are needed to explain the experimental stability of small DHP vesicles obtained by sonication.
The Theory for Colloids The total potential energy (V) for the interaction between two colloidal particles is defined as
v = VR + v A
(1) VR is the term for the repulsive interaction between the electrical double layers associated with the particles explicitly given by Reerink and Overbeek2*(25 OC, in water) VR = 4.62 X 10”(ay2/vZ) exp(-m) (2) where
Y = (exp(z/2) - l)/(exp(z/2) + 1); v
z =
qevh/kT
(3)
is the valency of the gegenions
u = Ho/a (4) where Ho is the shortest distance between particle surfaces (in A) and a, the particle radius (in A): 7
= Ka
(5)
K = ((8n~~q,2)/(&7‘))’/~ or K = C , 1 / 2X 107/(0.31) cm-I
with C, as the salt concentration (in M). A layer of gegenions adheres to the particle surface forming an ionic condenser, the Stern layer,22the diffuse part of the double layer extending into the solution. This layer of gegenions adhering to the particle surfaces accounts for Stern potentials smaller than the corresponding surface potentials. VA is the van der Waals term for the nonretarded attraction potential between two equal spheres for short distances ( u
