Estimation of degree of counterion binding and related parameters of monomeric and dimeric cationic surfactants from cloud point measurements by using triblock polymer as probe

The UV Spectroscopic Determination of Two Critical Micelle Concentrations of Domiphen Bromide in Solutions with KBr and Calculations of the Packing Parameter

Three different forms of the surface-active domiphen cations were detected by the UV spectroscopy in aqueous solutions of domiphen bromide, eventually with KBr additive. In sufficiently dilute solutions there are free domiphen cations, which aggregate into two types of micelles at two distinct concentrations, interpreted as the first and second critical micelle concentration, cmc and cmc2. From the logarithmic dependences of cmc and cmc2 on the bromide concentrations the corresponding degrees of counterion binding were estimated. Conformation analysis of the domiphen cation was performed and the packing parameters of the lowest energy conformers were calculated. The observed spectral shifts and the calculated packing parameters suggest that two conformers of domiphen cation with different arrangement of the phenoxyethyl group are responsible for the respective formation of two types of micelles.

1H NMR spectroscopic investigations on the conformation of amphiphilic aromatic amino acid derivatives in solution: effect of chemical architecture of amphiphiles and polarity of solvent medium

In this study, the conformation of the amphiphilic lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) in water and dimethyl sulfoxide is established. The alkyl chain protons of LEP in D(2)O appear at δ 1.010-1.398 and show an upfield shift and large line width, suggesting the proximity of the phenyl ring to the alkyl chain in contrast to that of LET. Quite interestingly, in DMSO-d(6), the (1)H NMR spectra of LET and LEP show a strong similarity that is suggestive of an orientation that positions the aromatic ring and aliphatic chain away from each other. These results are substantiated with two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOSEY). Theoretical molecular models of the conformation at the interface corroborate the experimental findings. Investigations of the solvent polarity and chemical structure-dependent conformation are discussed.

How PEO-PPO-PEO triblock polymer micelles control the synthesis of gold nanoparticles: temperature and hydrophobic effects

Aqueous micellar solutions of F68 (PEO(78)-PPO(30)-PEO(78)) and P103 (PEO(17)-PPO(60)-PEO(17)) triblock polymers were used to synthesize gold (Au) nanoparticles (NPs) at different temperatures. All reactions were monitored with respect to reaction time and temperature by using UV-visible studies to understand the growth kinetics of NPs and the influence of different micellar states on the synthesis of NPs. The shape, size, and locations of NPs in the micellar assemblies were determined with the help of TEM, SEM, and EDS analyses. The results explained that all reactions were carried out with the PEO-PPO-PEO micellar surface cavities present at the micelle-solution interface and were precisely controlled by the micellar assemblies. Marked differences were detected when predominantly hydrophilic F68 and hydrophobic P103 micelles were employed to conduct the reactions. The UV-visible results demonstrated that the reduction of gold ions into nucleating centers was channeled through the ligand-metal charge-transfer complex (LMCT) and carried out by the surface cavities. Excessive hydration of the surface cavities in the case of F68 micelles produced a few small NPs, but their yield and size increased as the micelles were dehydrated under the effect of increasing temperature. The results concluded that the presence of well-defined predominantly hydrophobic micelles with a compact micelle-solution interfacial arrangement of surface cavities ultimately controlled the reaction.

Effects of organic salt additives on the behavior of dimeric ("gemini") surfactants in aqueous solution

The effects of a series of aromatic anions, so-called hydrotropes, on characteristic solution properties of a family of ammonium gemini surfactants with dodecyl chains were explored. The stoichiometric addition of the organic salts to the geminis can result in clear solutions or in phase separation/precipitation, depending on the detailed nature of the added counterions and on the spacer group of the gemini surfactant. Many organic anions induce synergistic effects, strongly reducing the critical micellization concentration (cmc) and the surface tension at the cmc. Furthermore, a number of combinations of organic anions and geminis exhibit thickening of their aqueous solutions. The effects of the added salts are strongly enhanced for the gemini surfactants compared to the monomeric analogue N-dodecyl-N,N,N-trimethylammonium chloride. Even anions such as benzoate may be effective for thickening, and viscoelastic solutions can be obtained with salicylate despite the relatively short alkyl chains.