Langmuir 1995,11, 2938-2943
2938
Interactions between Dioctadecyldimethylammonium Chloride or Bromide Bilayers in Water L. R. Tsuruta, M. M. Lessa, and A. M. Carmona-Ribeiro" Departamento de Bioquimica, Instituto de Quimica, Universidade de S. Paulo, CP 26077, S. Paulo, Brazil Received February 17, 1995. In Final Form: May 17, 1995@ Interactions between dioctadecyldimethylammonium bromide (DODAB) or chloride (DODAC) closed bilayers in water are investigated. First, bilayers from vesicles are deposited onto oppositely charged polystyrene microspheres. Thereafter, a further increase in amphiphile concentration leads to vesicle adhesion to the bilayer-coveredlatex. For DODAC vesicles, the extent of vesicle adhesion to the covered latex is much higher than for DODAB vesicles. There is no rupture of adhered DODAC vesicles. In contrast, adhesion of DODAB vesicles is followed by vesicle rupture. Rupture for DODAB is associated with the absence of hydration repulsion whereas the absence of rupture for DODAC is related to its occurrence. A high extent of interdigitation in the DODAC bilayer causes the long-ranged hydrophobic attraction responsiblefor adhesion and can be associatedwith a long-rangedhydrophobicattraction between bilayers. The possibility of adhesion at a secondary minimum is ruled out for interactions in water.
Introduction Intra- and interlayer interactions between bilayers of quaternary ammonium surfactants exhibit a pronounced counterion ~pecificity.'-~The smaller the hydrated anion, the more contracted the monolayer compression isotherm becomes.2 Expanded phases for monolayers2 and interdigitated and thin bilayers of dihexadecyldimethylammonium acetate (DODA)3,4have been described. In accordance with this strong repulsive intralayer force that causes expansion in the DODA monolayer and interdigitation in the bilayer, a short-ranged hydration repulsion between layers develops, as depicted from measurements using the osmotic stress t e ~ h n i q u e . ~Unfortunately, ,~ direct measurements of interaction forces between dioctadecyldimethylammonium chloride (DODAC)are not available in the literature, whereas those for the bromide surfactant (DODAB)3were deemed unreliable a t short ranges .5,6 DODAC vesicles had their DLVO stability ratios calculated and compared with stability ratios obtained experimentally from NaC1-inducedf l o c c ~ l a t i o n . ~From -~ the calculations, the stability of such vesicles in electrolyte solution should be very high. However, experimental stability ratios were much lower than those predicted from t h e ~ r y .Hydrophobic ~ defects in the DODAC bilayer due to thinning and interdigitation are more frequent a t low monovalent salt concentration, whereas aggregation a t the secondary minimum dominates interactions over a high NaCl concentration range.1° * Author to whom correspondence should be addressed. (Telephone: 55 11 8187964. Fax: 55 11 8155579.) Abstract published in Advance ACS Abstracts, July 1, 1995. (1)Ninham, B. W.; Evans, D. F.; Wei, G. J. J.Phys. Chem. 1983,87, 5020. (2)Marra, J. J . Phys. Chem. 1986,90,2145. (3)Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D. F. J. Phys. Chem. 1986,90,1637. (4) Claesson, P. M.; Carmona-Ribeiro, A. M.; Kurihara, K. J. Phys. Chem. 1989.93. , - - .917. (5)Tsao,Y.; Evans, D.F.; Rand, R. P.; Parsegian, V. A. Langmuir 1993,9,233. (6)Parsegian, V .A.;Rand, R. P.; Fuller, N. L. J. Phys. Chem. 1991, 95.4777. -(7) Carmona-Ribeiro, A. M.; Yoshida, L. S.; Chaimovich, H. J.Phys. Chem. 1985,89,2928. ( 8 ) Carmona-Ribeiro, A. M. J. Phys. Chen. 1989,93,2630. (9)Carmona-Ribeiro, A. M. J. Phys. Chem. 1992,96, 9555. @
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Table 1. Mean Diameter (D),Area per Charge Group (A), and Specific Surface Area (SSA)for Sulfate Polystyrene Microspheres (SP) in Water
latex
Dlnm
A/&
SSA/(cm29-1)
SPlOO
100 249
1766 772
571,048 228,401
SP249
In this work interactions between adjacent DODAB or DODAC closed bilayers in pure water, where the secondary minimum does not occur,ll are investigated. First, one bilayer is deposited onto oppositely charged polystyrene microspheres.12 Therafter, as surfactant concentration increases, vesicles adhere to the covered latex with or without disruption, depending on the surfactant counterion. Thereby, vesicles are brought into close approach in pure water. By measurement of vesicle adhesion and rupture, the nature of interaction forces a t short-ranges is investigated for both surfactants.
Material and Methods Dioctadecyldimethylammonium chloride (DODAC)or bromide (DODAB) were obtained from Herga Industrias Quimicas do Brasil (Rio de Janeiro, RJ) and Sigma Chemical Co. (St. Louis, MO), respectively. DODAC was routinely purified and analyzed by mass spectrometry. The dialkyl chain distribution was 60% C18:C18, 38% C18:C16, and 2% C16:C16. DODAB was used without furtherpurification. [14C]sucrose (400-700 mCi/mmol) was obtained from New England Nuclear Research Products Co. (Seattle,WA).
Charged polystyrene microspheres stabilized by negative sulfate charges (SP)and described as ultraclean by the supplier were obtained from InterfacialDynamics Co. (Portland,OR) and used as supplied. The main characteristicsof the microspheres as specified from the supplier are given in Table 1. Mean diameters were obtained by the supplier using an electron microscope. All other reagents were analyticalgrade and used without further purification. Water was purified by a Milli-Q system. Small DODAB vesicles (SV)with 85-nm mean
