5366 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
ammonium chloride S V and DHP SV;IZI3 (4) the huge theoretical DLVO stability of SV in comparison with the low experimental one obtained from flocculation data;=*%(5) the higher T, values obtained for SV' than for LV6 2 h after dispersion. Concerning points 1 and 5, gel LV and SV of phospholipids behave differently. For phospholipids, not only the fluidity of SV is higher than that of LV2' but also the T, for LV is always higher than the T,for SV.2s In fact, the planar structure of a DHP bilayer fragment could be more tightly packed than the bilayer structure in a large vesicle. Also, the hydrophobic interaction between edges of bilayer fragments could well account for the extra attractive interaction that was demonstrated between SV at different salt concentration~.~~*~' A pH change in a DHP bilayer dispersion in the gel state does not seem to change the surface hydration and thereby the vesicle (23) Carmona-Ribciro, A. M.;Yoshida, L. S.; Chaimovicb, H. J . Phys. Chem. 1985,89, 2328. (24) Carmona-Ribciro, A. M. J. Phys. Chem. 198!3,93,2630. (25) Takemoto, H.; Inoue, S.; Yasunaga, T.; Sukigara, M.; Toyoshima, Y. J . Phys. Chem. 1981.85. 1032.
Additions and Corrections size. For the gross structural changes to occur, the bilayer for which the surface charge has changed has to undergo a phase transition. Further investigations about the effect of the bilayer state on the shape of the titration curves are desirable.
Conclusions Including a charge in a bilayer structure by titration initializes the gross structural changes that will take place to produce a stationary bilayer structure at a given pH and ionic strength. The attainment of the stationary structure involves changes in the surface hydration able to lead to changes in vesicle size. The DHP bilayer can exist as a stationary structure only up to pH 6. The stationary character of a bilayer dispersion can be checked by monitoring a structural property before and after an annealing procedure. The DHP dispersion obtained by sonication is not stationary at any pH value. Acknowledgment. Dr. Thelma M. Hardman, Dr. Brian R. Midmore, and the referees are gratefully acknowledged for valuable criticisms. This work was financially supported by CNPq, FAPESP, FINEP, TWAS, CONICET, and BID.
ADDITIONS AND CORRECTIONS 1991, Volume 95
Y.M. Hamrick, R. J. Van Zee, J. T. Godbout, W. Weltner, Jr.,* W. J. Lauderdale, J. F. Stanton, and R. J. Bartlett: The BCO Molecule. Page 2843. The vibrational assignment in Figure 6 is incorrect. A thorough study by T. R. Burkholder and L. Andrews (to be published) finds the C-O stretching frequency in BCO to be 2002.3 cm-I in solid argon rather than the value of 2091 cm-' reported by us. (Our assigned bands are due to natural abundance 13C0 and C1*0,observed because of the high concentration of CO in the matrix.) This revised assignment is actually in better accord with the calculated value (2101 cm-l) since DZP-MBFT(2) harmonic frequencies obtained from ab initio theory are usually about 5% higher than the experimental values. Burkholder and Andrews corroborate the exceptionally small IlB to O ' B isotope shift calculated for this stretching mode.
