A new class of gemini surfactants, bis(Args) (Cn(LA)2, where n ) 3, 6, and 9), has been synthesized. Their solution and tension behaviors were studied and compared to those of the corresponding monomeric surfactant, LAM (N R -lauroylarginine methyl ester) and of a common cationic surfactant, CPC (cetylpyridinium chloride). Bis(Args) are made up of two symmetrical long chain N R -acyl-L-arginine residues of 12 carbon atoms linked by amide covalent bonds to an R,ω-alkylidenediamine spacer chain of varying length (n). By being produced from amino acid sources (arginine), these surfactants are biocompatible and less toxic to the environment. The solution behavior is also important for potential applications in foaming, agrichemical spreading aids, and cleaning processes, and in understanding the interfacial behavior. Strong evidence of two cmc's with different characters of aggregates was obtained from different techniques for the gemini surfactants but not for the monomeric surfactants. Surface tensiometry indicates that the geminis form aggregates of substantial size at 0.001-0.01 mM (at 25 °C) or at concentrations about 3 orders of magnitude lower than that of LAM. Fluorescence results and lower chloride counterion binding than that for LAM suggest that the aggregates are nonglobular. These methods reveal also a second cmc for larger globular aggregates at 0.09-0.5 mM. Conductivity measurements and calculations are consistent with the above inferences and were used to estimate the aggregation number N and the counterion binding parameter β. The nonglobular aggregates have lower β and smaller N values than the globular aggregates (micelles), and unlike conventional micelles, they tend to increase the molar conductivity compared to that of the pre-cmc solution.

The adsorption of dilauroylphosphatidylcholine (DLPC) at the air/ water interface was investigated with tensiometry, infrared reflection-absorption spectroscopy (IRRAS), and ellipsometry. The tension dynamics at 25 °C at constant and pulsating area depends strongly on the concentration (10-1000 ppm) and sizes of dispersed DLPC particles, which are liposomes or vesicles. Dynamic surface tensions as low as 1-5 mN/m are observed for DLPC. For dipalmitoylphosphatidylcholine (DPPC), such tensions are also observed, but only for certain special preparation procedures. Direct probing of the surface by IRRAS and ellipsometry indicates that DLPC adsorbs by a molecular adsorption mechanism, controlled largely by the rate of dissolution of the dispersed particles. The surface layer is a monolayer with no particles attached to it, unlike DPPC in which the surface monolayer forms by particles which reach the interface and remained attached to it (Wen, X.; Franses, E. I. Langmuir 2001, 17, 3194). Spectroturbidimetry and dialysis experiments, used to determine the DLPC solubility in water as 4 ( 1 ppm and the DPPC solubility as essentially zero, support the above mechanisms. For DLPC, a simple diffusion/adsorption model with molecular diffusion and variable effective diffusion length can account for the dynamic surface tension data.

Infrared reflection absorption spectroscopy (IRRAS) has been used for probing the surface densities and compositions of adsorbed layers of aqueous sodium tetradecanoate, or myristate, at 25 °C. Aqueous sodium myristate normally becomes protonated (myristic acid) to an extent of about 0.5-1%, yielding a natural pH from 8 to 9, depending on concentration. The myristic acid, and possibly an acid-soap complex, are quite surface active compared to myristate, making impractical the application of the Gibbs adsorption isotherm for determining surface densities from tension data. At pH ) 12 (in 10 mM NaOH), only myristate is expected in the bulk, and the tension is higher; for 2 mM total surfactant concentration, the tension is ∼ 43 mN/m vs 23 mN/m. IRRAS spectra confirm that only myristate is present in the monolayer at pH ) 12. At natural pH (8-9), in addition to the band due to the myristate group, a significant band due to myristic acid is observed. Solutions in D2O were used for observing the carbonyl and carboxylate bands, after eliminating the H2O vapor noise in the polar group region (1800-1300 cm -1 ), and for having larger reflectance-absorbance intensities due to the smaller complex refractive index of D2O than that of H2O in the hydrocarbon stretching region (2950-2850 cm -1 ). The surface densities of adsorbed sodium myristate layers at pH ) 12 as determined from tension data by using the Gibbs adsorption isotherm agree to better than 10% to those determined from IRRAS data by using the model of either an isotropic film or an anisotropic film on the surface. The surface densities at pH ) 12 range from 1 × 10 -6 to 4 × 10 -6 mol/m 2 as the concentration increases from 0.05 to 4 mM. At pH ≈ 8-9, the surface density is 8 × 10 -6 mol/m 2 at 4 mM, as explained by the lower tension. The frequencies of both antisymmetric and symmetric methylene stretching vibration bands are lower at natural pH, indicating more ordered and almost all-trans conformations at the higher surface densities.

No abstract