Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant (original) (raw)

Characterization of starburst dendrimers by the EPR technique. 1. Copper complexes in water solution

Journal of the American Chemical Society, 1994

The aggregation characteristics of aqueous solutions of a positively charged nitroxide surfactant (CAT16) in the presence and absence of half-generation polyamidoamine starburst dendrimers (n.5-SBDs) have been investigated by electron paramagnetic resonance (EPR). Computer simulation of the EPR spectra allowed the convenient extraction of several parameters that were related to the supramolecular structure of the aggregates formed by CAT16 and SBDs. From examination of the EPR spectra as a function of variation of the concentration of CAT16, the concentration of SBDs, and the ionic strength and application of the EPR parameters available from simulation of the spectra, a paradigm for the structure and dynamics of the aggregates formed by CAT16 in the presence and absence of SBDs under various conditions is deduced. A study of the fluorescence quenching of pyrene in the presence of CAT16 is compared to a previous investigation of the interaction of SBDs with cationic surfactants. It is concluded that at low SBD concentration, for earlier generation SBDs (G < 3.5), whose size is smaller than or comparable to the size of the CAT16 micellar aggregates, the SBDs act as "guests" that bind to the micelles that serve as "hosts". In contrast, at low SBD concentration of the later generation SBD (G > 3.5), the size of the SBD is now larger than that of the micelles so that the latter can serve as "guests" for the former. A bilayer aggregate of the surfactant on the SBD is proposed. Finally, at high concentration of the later generation SBD, it is proposed that because of the large number of sites compared to the number of surfactants, an aggregate in which two or more SBDs are bridged by bilayers is formed by the surfactant and coexists with CAT16 micelles.

Physicochemical characterization of bromide mono- and dimeric surfactants with phenyl and cyclohexyl rings in the head group

Journal of Colloid and Interface Science, 2011

In this work the two monomeric surfactants N-benzyl-N,N-dimethyl-N-(1-dodecyl)ammonium bromide (PH12,Br À) and N-cyclohexylmethyl-N,N-dimethyl-N-(1-dodecyl)ammonium bromide (CH12,Br À) and their two dimeric counterparts N,N 0-(1,3-phenylenebis(methylene))bis(N,N-dimethyl-N-(1-dodecyl) ammonium dibromide (12PH12,2Br À) and N,N 0-(cyclohexane-1,3-diylbis(methylene))bis(N,N-dimethyl-N-(1-dodecyl)ammonium dibromide (12CH12,2Br À) were prepared and characterized. The critical micelle concentration, the micellar ionization degree and the average aggregation number were obtained by using different techniques. The results are discussed with the help of 1 H NMR two-dimensional, 2D, rotating frame nuclear Overhauser effect spectroscopy measurements, ROESY, which seem to indicate that the phenyl and cyclohexyl rings present in the head groups of the surfactants are bent towards the micellar interior in order to avoid contact with water. The surface activity of the surfactants was studied by means of surface tension measurements. The occurrence of morphological transitions upon increasing surfactant concentration and the variations in the polarity and in the microviscosity of the interfacial region accompanying this sphere-to-rod transition were investigated. It was shown that the formation of spherocylindrical micelles also causes changes in the 1 H NMR spectra of the surfactant solutions.

Micellar formation of cationic surfactants

Heliyon

The micellar structure of six alkyl trimethylammonium halides was studied via conductivity. It was found that the aggregation number increased with the decreasing carbon chain length. Furthermore, Br À significantly enhanced the micellar formation over Cl À. However, the aggregation number and ionization degree remain similar for both anions. The modelling results validate that the counter-anions affect micellar formation via equilibrium constants, instead of their hydration size. In particular, the association constants between surfactant (both monomer and micelle) and Br À are significantly higher than Cl À. This is consistent with the qualitative description of hydrated Br À in the literature. The experimental and modelling results confirm that micelles are formed via "ion-paring/ hydration" structure, instead of the conventional "packing" concept.

Micellar aggregation of sulfonate surfactants studied by electron paramagnetic resonance of a cationic nitroxide: an experimental and computational approach

Physical Chemistry Chemical Physics, 2002

The micellization process of three sulfonate surfactants [CH 3 (CH 2 ) nÀ1 SO 3 Na (n ¼ 6,8,10), C n SO 3 Na] has been studied by electron paramagnetic resonance (EPR) spectroscopy by employing )ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl chloride, TC) as a spin label. The dependence of both the nitrogen isotropic hyperfine coupling constant (hA N i) and the correlation time (t C ) of the label on the surfactant molality have been analysed. In order to allow a correct interpretation of the experimental evidence a preliminary study on the factors influencing the EPR spectrum of TC in solution has been performed. EPR spectra of TC in various solvents show that the hA N i value increases with increasing the solvent polarity and, especially, H-bonding ability. The experimental values have been compared with those obtained by a composite ab initio computational approach, in which hA N i is determined by a suitable combination of post-Hartree-Fock and density functional calculations. Solvent effects are modelled by using the polarizable continuum model (PCM) and, for solvents with H-bonding ability, by including a few explicit solvent molecules. The experimental and computed values are in good agreement, confirming the reliability of the adopted computational strategy. The effect of the ionic strength on the EPR spectrum of TC in NaCl and Na 2 SO 4 aqueous solution has been also investigated, finding that the hA N i value is almost constant, whereas t C increases with the electrolyte molality. In surfactants' aqueous solution, both hA N i and t C of TC, plotted as a function of the surfactant molality, show a slope change, corresponding to the critical micellar composition (c.m.c.). The t C increase can be interpreted in terms of a reduction of the label mobility determined by the strong electrostatic interaction between the TC positive charge and the anionic micelles' surface. The hA N i decrease can be ascribed to the embedding of the NO moiety of TC in the outer part of the micellar hydrophobic core. By comparing the data collected for the different surfactants, it can be seen that the variation of both t C and hA N i upon micellization increases with the surfactant chain length. This evidence can be interpreted in terms of an increasing strength of the TC-micelle surface interaction, and of an increasing hydrophobic behaviour of the outer part of the micellar core in which the NO moiety of TC is solubilized. The TC affinity for the micellar pseudo-phase has been estimated by evaluating the distribution coefficient, K d , of the spin label between the micelles and the aqueous medium. The K d value increases with the length of the surfactant hydrophobic chain.

Photophysical investigation of starburst dendrimers and their interactions with anionic and cationic surfactants

Journal of The American Chemical Society, 1990

Fluorescence spectroscopy has been used to characterize the structure of a unique class of anionic macromolecules: starburst dendrimers (SBDs) possessing an external anionic surface. Pyrene was used as a photoluminescence probe to sense various hydrophobic sites in the microheterogeneous architecture offered by poly(amidoamine) starburst dendrimers possessing sodium carboxylated surfaces. A series of IO different "half-generations, n.5" starburst dendrimers, which differ systematically in molecular weight, size, and surface charge density, have been studied. The probe method provides experimental evidence for a structural surface transition between generations 3.5 and 4.5. The same probe method was applied to study how the structural properties of the starburst dendrimers determine their interactions with small ionic amphiphilic molecules. Whereas starburst dendrimers do not noticeably affect the micellization process of an anionic surfactant (sodium dodecyl sulfate, SDS), the association process with a cationic surfactant (dodecyltetrammonium bromide, DTAB) leads to the formation of two different types of SBD-templated surfactant aggregates. Addition of DTAB to aqueous solutions containing the earlier generation (0.5-3.5) SBD leads to the formation of SBD-templated surfactant aggregates that result from noncooperative, random condensation of surfactant molecules on the anionic dendrimer surface. Addition of DTAB to aqueous solutions containing later generation (4.5-9.5) SBDs leads to the formation of SBD-templated surfactant aggregates resulting from initial, noncooperative, random condensation, followed by cooperative condensation of surfactant molecules on the anionic dendrimer surface. The results are shown to be consistent with a change in the morphology of SBDs from an open, branched structure for generations 0.5-3.5 to a closed, increasingly compact surface for generations 4.5-9.5. The dendrimer generation system used in earlier work (see refs la-i) designated the first star-branched species derived from the initiator core as generation 1.0. The present system is preferred wherein that star-branched intermediate is designated generation 0, thus making it consistent with the geometric progression Z = for the number of terminal groups (see .

Surfactant/Nonionic Copolymer Interaction: A SLS, DLS, ITC, and NMR Investigation

The interactions between an oxyphenylethylene-oxyethylene nonionic diblock copolymer with the anionic surfactant sodium dodecyl sulfate (SDS) have been studied in dilute aqueous solutions by static and dynamic light scattering (SLS and DLS, respectively), isothermal titration calorimetry (ITC), and 13 C and self-diffusion nuclear magnetic resonance techniques. The studied copolymer, S 20 E 67 , where S denotes the hydrophobic styrene oxide unit and E the hydrophilic oxyethylene unit, forms micelles of 15.6 nm at 25 °C, whose core is formed by the styrene oxide chains surrounded by a water swollen polyoxyethylene corona. The S 20 E 67 / SDS system has been investigated at a copolymer concentration of 2.5 g dm-3 , for which the copolymer is fully micellized, and with varying surfactant concentration up to approximately 0.15 M. When SDS is added to the solution, two different types of complexes are observed at various surfactant concentrations. From SLS and DLS it can be seen that, at low SDS concentrations, a copolymer-rich surfactant mixed micelle or complex is formed after association of SDS molecules to block copolymer micelles. These interactions give rise to a strong decrease in both light scattering intensity and hydrodynamic radius of the mixed micelles, which has been ascribed to an effective reduction of the complex size, and also an effect arising from the increasing electrostatic repulsion of charged surfactant-copolymer micelles. At higher surfactant concentrations, the copolymer-rich surfactant micelles progressively are destroyed to give surfactant-rich-copolymer micelles, which would be formed by a surfactant micelle bound to one or very few copolymer unimers. ITC data seem to confirm the results of light scattering, showing the dehydration and rehydration processes accompanying the formation and subsequent destruction of the copolymer-rich surfactant mixed micelles. The extent of interaction between the copolymer and the surfactant is seen to involve as much as carbon 3 (C3) of the SDS molecule. Self-diffusion coefficients corroborated light scattering data.

The role of surfactants head group in the formation of self-assembled supramolecular aggregate

Rabah A. Khalil*, Liqaa H. Alsamarrai, 2015

Thermodynamically controlled supramolecular gel or aggregate is commonly resulted from the formation of one-dimensional micelles through specific self-assembling of amphiphilic molecules. Up to present date, the role of surfactant head group in the formation of such a condensed matter is still obscure. Consequently, this work presents a study of the effect of changing surfactant head group on the transformation phenomenon towards one dimensional shape of aggregate. Binary mixture of anionic sodium dodecylsulfate (SDS) and cationic cetylpyridinium chloride (CPC) has been examined for comparison with previously published SDS with cetyltrimethylammonium bromide (CTAB). The work has been extended through using ternary mixtures of SDS, CPC and CTAB at 1:1, 3:1 and 1:3 CTAB:CPC ratios. The thermodynamic properties have been determined for all of these mixtures. The results indicate that the presence of pyridinum cation in CPC surfactant causing a bad effect on the formation of wormlike micelle in contrast to that of trimethylammonium in CTAB. It has been suggested that the formation of supramolecular gel depends on ionization potential of the surfactant head group as needed for head to head electrostatic interaction through the construction of one dimensional shape of aggregate. It has been generally concluded that hydrophilic effect plays a sensible part in the formation of wormlike micelles in contrast to that of hydrophobic which plays the major role and hence supporting the critical intermolecular forces theory.

Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution

Journal of Colloid and Interface Science, 2005

Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C16TAX) with different counterions, F−, Cl−, Br−, NO−3, and ½SO2−4, have been studied. Critical micelle concentration (CMC), degree of micelle ionization (α)(α), and enthalpy of micellization (ΔHmic)(ΔHmic) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the α increase in the order SO2−4 < NO−3 < Br− < Cl− < F−, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. ΔHmicΔHmic becomes less negative through the sequence NO−3 < Br− < Cl− < F− < SO2−4, and even becomes positive for the divalent sulfate. The special behavior of sulfate is associated with both its divalency and its degree of dehydration. Gibbs free energies of micellization (ΔGmic)(ΔGmic) and entropies of micellization (ΔSmic)(ΔSmic) have been calculated from the values of ΔHmicΔHmic, CMC, and α and can be rationalized in terms of the Hofmeister series. The variations in ΔHmicΔHmic and ΔSmicΔSmic have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C16TAX surfactant micelles depends strongly on the binding of the counterion.

SANS study of micellar aggregation of multi-headed surfactants

Applied Physics A: Materials Science & Processing, 2002

We report the aggregation properties of recently synthesised novel single-chain surfactants bearing one, two and three head groups. Small-angle neutron-scattering studies in aqueous solutions of these surfactants indicate that the extent of aggregate growth of these micelles dramatically decreases with the increase in the number of head groups. It is also seen that, unlike single-headed surfactants where the micelles grow on addition of salts (KBr and sodium salicylate), the sizes of the micelles of multi-head-group surfactants are independent of these additives. These studies show that the micellar properties of surfactants could be remarkably influenced by manipulation of the charge densities at the head groups.