Surface and volume properties of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide

Analysis of Adsorbed Layers of Benzyldimethyldodecylammonium Bromide on Silica Particles in Water Using the Sorbent Mass Variation Method

A "sorbent mass variation" (SMV) method has been suggested to investigate the adsorption at solid-liquid interfaces, which can provide information on the adsorbed layer structure including its thickness and composition. However, there has been little research focused on the method, and therefore, it is essential to examine its general applicability. Herein, the adsorption of benzyldimethyldodecylammonium bromide (BDDABr), a cationic surfactant, on silica (SiO₂) nanoparticles (with ∼12 and 24 nm in size, denoted as S-SiO₂ and L-SiO₂, respectively) in water was investigated using the SMV method. The adsorption isotherms all show a linearly declining tendency in the saturated adsorption regime, consistent with the prediction of the SMV model. The adsorption is interpreted to form noncomplete bilayers (or isolated admicelles). The thicknesses of the adsorbed bilayers on S-SiO₂ and L-SiO₂ are estimated to be ∼2.9 and 2.7 nm, respectively, and the volume fractions of BDDABr in the saturated adsorbed layers are 0.63 and 0.68, respectively. In addition, the change in the Gibbs free energy of the adsorption process is also analyzed, showing its spontaneous nature. This work demonstrates that the SMV method is available for investigation on the adsorption of surfactants at solid-liquid interfaces, which can provide information on the structure and formation thermodynamics of adsorbed layers.

Self-assembling systems based on amphiphilic alkyltriphenylphosphonium bromides: elucidation of the role of head group

A systematic study of the aggregation behavior of alkyltriphenylphosphonium bromides (TPPB-n; n=8, 10, 12, 14, 16, 18; here n is the number of carbon atoms in alkyl groups) in aqueous solutions has been carried out and compared with trimethyl ammonium bromides (TMAB-n). Critical micelle concentrations (cmcs) of TPPB-n and TMAB-n decrease with the number of carbon atoms with the slope parameter of ca.0.3. The low cmcs and effective solubilization power toward Orange OT indicate high micellization capacity of phosphonium surfactants. The low counterion binding parameter β is revealed for TPPB-10 and TPPB-12, while high counterion binding of ≥80% is observed for high TPPB-n homologs. Values of the surface potential ψ calculated on the basis of pK(a) shifts of p-nitrophenols is similar for both series and monotonously increase with alkyl chain length. Several points indicate non-monotonic changes within TPPB-n series. There are peculiarities of the tensiometry and solubilization plots for high homologs and above mentioned increases in counterion binding on transiting from low to high molecular weight surfactants. Differences in aggregation behavior between TPPB and TMAB series and between low and high homologs can be due to the specific structural character of the TPP(+) cation, which is supported by X-ray data.