Surface Properties And Phase Behavior Of 1:1 Salt-free Catanionic Surfactants

Cationic-anionic surfactants possess many salient properties, such as dramatically low surface (interface) tension and critical aggregation concentrations, very high surface activity and rich amphiphilic molecules ordered aggregates. These unique properties can be exploited in many applications, such as detergents, drug-reduction agents, catalyst templates, microreactors and model membranes. Most current researches focused on the catanionic surfactants containing salts. However, most of the equimolar mixtures form precipitates or become turbid at very low concentration, owing to the shielding effect of the inorganic salts.In recent years, a renewed interest has been devoted to the study of salt-free catanionic systems, obtained by mixing cationic with equimolar anionic surfactant, of which OH- and H+ as counterions, respectively. However, cationic surfactants with OH" as counterions were prepared from alkyltrimethylammonium bromide aqueous solution treated by a strong base anion exchange resin. This method not only gave a low synthesis rate of alkyltrimethylammonium hydroxide, but also generated secondary wastewater due to the alkali-washing of the anion exchange resin. Thus, salt-free catanionic surfactants obtained by mixing cationic and anionic surfactants with OH" and H+ as counterions were limited. Most researches on catanionic surfactants limited in regions far from equimolar composition or within very low concentration, which limited their research and application.In this paper, a novel method was reported to synthesize salt-free catanionic surfactant directly. Green reagent dimethyl carbonate reacted with tertiary amine to obtain alkyltrimethylammonium methylcarbonate, followed by the reaction of alkyltrimethylammonium methylcarbonate with alkyl carboxylic acids to synthesize corresponding salt-free catanionic surfactants. The surface properties, self-assembly structure and phase behavior of salt-free catanionics were studied, and the effects of temperature, salinity, pH and hydrocarbon chain length were also investigated.The 1:1 salt-free catanionic surfactant dodecyltrimethylammonium decanoate (C+12-C-10) was synthesized directly by a novel method, and the product was recrystallized to come up to the purity received.Surface activity and self-assembly of 1:1 salt-free catanionic surfactant C+12-C-10 and the influences of temperature, pH and salinity on them have been studied in the present work. The critical aggregation concentration (cac), surface tension reduction (γcac), surface excess (Γmax), and mean molecular surface area (Amin) were determined from plots of surface tension (y) versus logarithm of surfactant concentration. The results showed that the salt-free catanionic surfactant had much lower cac, γcac and Amin values than their corresponding single cationic and anionic surfactants. The tested temperature and salinity had no significant effect on the surface properties of C+12-C-10. The surface activity enhanced in acidic pH and decreased slightly in alkaline pH. Aggregates structure almost had no change with temperature, but transformed into spheres with addition of inorganic salts. The pH effect was complex, the ability of vesicles formation was improved in strong alkaline condition; vesicles transformed into large disks or large cubes in acidic condition.Phase behavior and phase structure of C+12-C-10 with water at full concentration range were studied at 20-45℃, using Fourier transform infrared (FT-IR), Differential scanning calorimetry (DSC), freeze-fracture transmission electron microscopy (FF-TEM), polarization microscopy, and small-angle X-Ray diffraction (XRD) techniques. The C+12-C-10-water system formed a large aqueous multi-phase region, a large lamellar liquid crystal (La phase) region and a hydrated crystal phase. The densely packed vesicles and the lamellar structure induced the formation of aqueous multi-phase region. With the temperature increased, the multi-phase region and the La phase region reduced, the isotropic solution phase (L1 phase) appeared.The equilibrium and dynamic surface tension properties of a series of 1:1 salt-free catanionic surfactants (alkyltrimethylammonium decanoate) were investigated; and, the influence of the length of hydrocarbon chain of cationic part (carbon number of 10,12, 14,16,18, respectively) on the surface properties was discussed. The plot of the logarithm of the critical aggregation concentration (cac) against the carbon number showed a linear decrease with an increase in chain length. On the basis of the slope of this plot, it was found that the variation of cac with the chain length is smaller for salt-free catanionic surfactants (C+n-C-10) than that for single anionic or cationic surfactants. The surface tension at the cac was quite low, much lower than that of single cationic or anionic surfactants at cmc. At dilute surfactant concentration, the adsorption process was diffusion-controlled adsorption; at higher concentration (above cac), it became the mixed diffusion-kinetic mechanism. The diffusion rate of the monomer for the salt-free catanionic surfactants was lower than that of conventional cationic or anionic surfactants, due to a larger molecular size. Furthermore, it was confirmed that the kinetic energy of adsorption at the gas/solution interface decreased with an increase in the hydrocarbon chain length.Phase behavior and applied performances (foaming, emulsification and wetting power) of a series of 1:1 salt-free catanionic surfactants (C+n-C-10,n=10,12,14,16,18) were investigated; and the influence of the length of hydrocarbon chain of cationic part on the phase behavior and applied performances was discussed. Phase behavior and phase structure of C+10-C-12 with water at full concentration range were studied at 20-45℃, using negative-transmission electron microscopy (negative-TEM), polarization microscopy, and small-angle X-Ray diffraction (XRD) techniques. The C+n-C-10-water system formed a large lamellar liquid crystal phase (La) region; and with increasing of the length of hydrocarbon chain, the La phase region narrowed down. Compared with C+12-C-10, other C+n-C-10-water systems can not form aqueous multi-phase region. C+14-C-10, C+16-C-10 and C+18-C-10-water systems formed vesicle phase region at low concentration. The performance of foaming property, emulsification and wetting power were studied. The C+12-C-10 and C+14-C-10 had excellent foaming power and foam stability, and had better emulsifying ability to liquid paraffin than nonionic surfactants NP-10. In addition to C+18-C-10, the other several C+n-C-10 had good wetting power on paraffin membrane surface.The surface properties and phase behavior of C+12-C-10 and C+10-C-12 with the same carbon number were studied. The equilibrium and dynamic surface tension were measured, and the results showed:C+12-C-10 and C+10-C-12 had the same Γmax, Amin and γcac; the cac of C+10-C-12 was the half of C+12-C-10.The plots of the dynamic surface tension against t had the same trend. At the initial stage of adsorption, the adsorption process was diffusion-controlled adsorption; and with the progress of adsorption, it became the mixed diffusion-kinetic mechanism. At 20℃, the aqueous two-phase region in the phase diagram of C+10-C-12 was smaller than that of C+12-C-10, and the formation concentration of aqueous two-phase increased to 20 wt%. With the temperature rising, the aqueous two-phase region disappeared and transformed into L1 phase.