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J. Phys. Chem. 1993.97, 4247
Reply to Comment on “Does the DLVO Account for Interactions between Charged Spheric Vesicles?” A. M. Carmona-Ribeiro Departamento de Bioquimica, Instituto de Quimica. Uniuersidade de Sao Paulo, CP 20780 Sao Paulo, Brazil Received: December 28, 1992
Dr. Washington suggests an alternative explanation for the rapid flocculation observed in DODAC and DHP vesicle systems upon NaCl addition.’+ Flocculation would be due to a reversible aggregation at the secondary minimum instead of irreversible aggregation at the primary minimum. From this, he claims that the Verwey-Overbeek equation would be inappropriate for calculating flocculation rates into this energy minimum. He indicates as suitable a kinetic model by Marmur5which describes a relationship between the depth of the secondary minimum and the stability ratio for flocculation into this minimum. Calculations previously presented in ref 1 as Figure 2 are presented again with an amplified vertical scale (Figure 1). From this, it becomes evident how shallow secondary minima for DODAC and DHP vesicles are. For DODAC vesicles, NaCl addition to 40 or 63 mM final concentration (Figure 1, curves a and b, respectively)causesve-sicle fusion: a phenomenon scarcely understandable from aggregation at a very shallow secondary minimum which is more than 100 A further apart (Figure la,b). On the other hand, upon 100 mM NaCl addition (Figure IC),the secondary minimum deepens to -0.26kT at 75-A separation distance. In this case, visible flocs that can be completely redispersed by hand-shaking were observed.* Also, DHP vesicle fusion upon NaCl addition was never reported: a fact which supports the hypothesis of an aggregation at the secondary
I
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I 50
1 0 0 0
Separation
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loo
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Figure 1. Total energy of interaction (V) as a function of separation distance (in A) as calculated for interacting DODAC (A) and D H P (B) vesicles with 123- and 172-nm mean radius, respectively. Emphasis is placedonthesecondaryminimumregion. Formoredetailsoncalculations, Figure 2 in ref 1 should be consulted.
minimum. Nevertheless, in curves e-h (Figure l), as NaCl concentration increases from 0.1 to 0.2 M, the secondary minima deepens from -0.29kT to -0.70kT. These figures still allow reversible flocculation by hand-shaking, a phenomenon we have not observed. Overall, a systematic study on reversibility of salt-induced aggregation of charged vesicles is a valuable suggestion.
References and Notes (1) Carmona-Ribeiro, A. M. J. Phys. Chem. 1992, 96, 9555. (2) Carmona-Ribeiro, A. M.; Yoshida, L. S.;Chaimovich, H.J . Phys. Chem. 1985,89, 2928. (3) Carmona-Ribeiro, A. M. J . Phys. Chem. 1989,93, 2630. (4) Carmona-Ribeiro, A. M.; Chaimovich, H. Biophys. J. 1986,50,621. ( 5 ) Marmur, A. J. Colloid Interface Sci. 1979, 72, 41.
0022-3654/93/2097-4241$04.00/00 1993 American Chemical Society
