Geetha Baskar

Molecular dynamics of methoxy polyoxyethylene macromonomer micelles in the absence and presence of SDS micelles using 1H NMR spin-lattice relaxation time measurements

Abstract Molecular dynamics studies performed by 1 H NMR spin-lattice relaxation time measurements suggest the occurrence of a mutual interaction between aqueous micelles of a macromonomer (acrylic ester of ω-methoxypolyethyleneglycol, Mol. wt. 460) and sodium dodecylsulfate (SDS). The α-CH 2 of SDS and ω-OCH 3 of the macromonomer seem to contribute to the maximum electrostatic interaction. The counterion association for SDS micelles is also hindered in the presence of larger macromonomer micelles as observed by conductivity measurements.

A facile methodology for the design of functionalized hollow silica spheres

A new amino acid derived amphiphile, lauryl ester of tyrosine (LET) is shown to provide a facile methodology for the preparation of hollow silica spheres. In a previous study on the interface adsorption, it was shown that phenolic OH group in LET plays a key role in the formation and stabilization of close packed structures, typically at the oil/water interface. Drawing an analogy between the air/water and the oil/water interface, we detail here a procedure where air droplets are capped with LET aggregated structures, and in turn they are utilized as viable templates in the production of hollow silica spheres. We demonstrate that hollow silica spheres are formed at pH 4.0 specifically under conditions of vortexing within a short period of time (ca. 15 min). The dimensions of the structures are 0.43±0.15 μm in diameter and they have then subsequently been used as templates for directing the synthesis of silica-silver and silica-polyanthranilic composite hollow spheres.

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.

Amphiphilic lauryl ester derivatives from aromatic amino acids: significance of chemical architecture in aqueous aggregation properties.

Lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) were, in a previous interface adsorption study, found to adopt very different interfacial conformations. The present study is an investigation of their aqueous aggregation properties with the goal of elucidating the effects of the presence in LET and absence in LEP of the phenolic OH group on their aqueous aggregate structures and micellar conformations of the surfactant monomers. The measured properties included aggregation numbers from time-resolved fluorescence quenching (TRFQ), interface hydration index and microviscosity by electron spin resonance (ESR), chemical shifts of (1)H resonance lines by NMR, and Krafft temperatures and enthalpies of structural transitions by differential scanning calorimetry (DSC). The TRFQ, ESR, and NMR experiments were conducted at various temperatures from 23 to 70 degrees C for various surfactant concentrations from 0.050 to 0.200 M. Markedly different temperature dependences of aggregation number and (1)H NMR chemical shifts are exhibited by LET and LEP micelles. LET and LEP form ionic micelles. The aggregation number of LEP decreases as is characteristic of ionic micelles, but that of LET increases slightly with temperature. The changes with temperature in the NMR chemical shifts and width of the resonance lines are significantly greater for the various LEP protons than for those of LET. The differences in these properties and other fluorescence decay characteristics of fluorophores incorporated into the micelles could be attributed to the difference in the micellar conformations of LET and LEP which are postulated to be similar to that at oil-water interfaces. The phenolic group is hypothesized to be in the micelle-water interface as part of the headgroup in LET micelles, and its location does not change with temperature. On the other hand, in LEP micelles, the phenyl ring is folded into the core overlapping with the flexible hydrophobic chains. The resulting closer proximity between the phenyl ring and the flexible hydrocarbon chain causes interdependence of the phenyl ring and chain proton resonances, leading to the observed temperature dependence of the chemical shifts in LEP. The TRFQ and ESR data are combined together in a molecular space-filling model, referred to as the polar shell model, to derive the geometrical properties of the micelle. The DSC scans in the temperature range 10-55 degrees C showed the presence of distinctly different endotherms for LET and LEP. The Krafft temperatures, K(T), and the enthalpies were determined. The higher K(T) and broader peak of the DSC endotherm of LET as compared to LEP are attributed to the stabilization of fiberlike structures below the Krafft temperature due to its chirality and the hydrogen bonding capability of the phenolic OH and also to the ion-dipole interactions. Thus, all of the observed differences between LET and LEP could be attributed to the difference in their chemical architecture.

Formation of self-aggregated structures of different types in water of chiral polymerizable amphiphiles from L-tyrosine and L-phenylalanine.

Sodium salts of maleamic acid derivatives from lauryl esters of L-tyrosine (MTNa) and L-phenylalanine (MPNa) were synthesized and characterized. The aggregated structures of MTNa and MPNa in water were investigated, employing several independent methods. MPNa showed secondary aggregated structures in contrast to MTNa at concentrations of >1 × 10(-3) M. The results from dynamic light scattering, transmittance, conductivity, and viscosity measurements suggested the formation of aggregated structures of different types in MTNa and MPNa solutions. The measured fluorescence anisotropy (r) at 0.180 of the fluoroprobe, 1,6-diphenyl-1,3,5-hexatriene (DPH), and the d spacing of 38 Å from small-angle X-ray diffraction (SAXD) experiments confirmed the bilayer structures in MPNa. Scanning electron microscope (SEM) images provided the morphological features. The emulsion produced using MPNa solution was more stable. The confocal fluorescence microscopy image of the emulsion from MPNa confirmed the entrapment of water-soluble dye, rhodamine. The models of MTNa and MPNa molecules and the aggregated structures are presented.

Adsorption and emulsification properties of amphiphilic poly(styrene-co-octadecyl maleamic acid salt) with comb-like architecture.

Amphiphilic poly(styrene-co-octadecyl maleamic acid salt) (PS-co-ODMAS) with a comb-like architecture was synthesized employing a heterophase aqueous polymerization reaction. The side-chain comonomer from octadecyl maleamic acid salt exhibited hydrogelation characteristics and provided conditions for a controlled polymerization reaction. PS-co-ODMAS polymers consisting of 5 and 10 mol% side-chain monomer showed a high molecular weight on the order of 10(6) and narrow polydispersity index at 1.33+0.02. The polymer consisting of 10 mol% side-chain monomer was shown to exhibit properties that are significantly different from those with 5 mol%, and the control poly(styrene) synthesized using sodium dodecyl sulfate surfactant. The polymer consisting of 10 mol% side-chain monomer formed hollow spherical spheres of 30 nm size. It also showed close-packed structures of the side-chain monomer on the polymer surface. Detailed adsorption studies at the liquid/liquid interface were carried out to evaluate and understand the scope for modification in surface energy characteristics of polymers in the presence of additives drawn from simple surfactants. Triton X-100 was shown to undergo effective adsorption on the polymer consisting of 10 mol% side-chain monomer and thus could generate stable emulsions with different volume fractions of heptadecane. The results from electron microscopy studies and viscosity of emulsions are discussed.

Solution structure of a modified comb-like polymer from octadecyl methacrylate and acrylic acid

Abstract A water soluble comb-like polymer from the statistical copolymer of acrylic acid and octadecyl methacrylate has been accomplished through transesterification reaction with methoxy poly(ethylene glycol) of molecular weight 5000. The high non-polar characteristics of microdomains as suggested from an higher I 3 / I 1 emission intensity ratio of pyrene, and a very high aggregation number of 821±4, demonstrate that PEO grafts play a significant role in promoting the hydrophobic association process in aqueous medium. A very low intrinsic viscosity (29 ml/g) and the linear variation of η sp / c with c demonstrate formation of aggregated structures in compact coil conformation.

Hydrogelation characteristics of amphiphilic N-octadecyl maleamic acid derivative

N-Octadecyl maleamic acid (ODMA), on conversion to sodium salt (ODMG) is shown to form gel-like structures in water. Specific conductivity and viscosity estimations at different concentrations in the temperature range of 25–70 °C indicated that ODMG is capable of gelling water at a concentration as low as 0.75% at 25 °C. The hydrogel tends to undergo reorganization on increasing the temperature. Thus, 1% gel was observed to transform into viscous solution on increasing the temperature, the onset temperature of transformation occurring at 52 °C. This is established from 1H NMR spectral measurements in D2O which showed sharp increase in intensity of methyl (δ = 0.86 ppm) and methylene (δ = 1.25 ppm) protons of ODMG gel on raising the temperature from 50–70 °C. The small angle X-ray diffraction studies performed on ODMG gel showed d-spacing in ratio of 1 : 0.5 : 0.3 indicative of lamellar structures. Molecular modelling calculations performed using the Cerius2 package further supported the lamellar structures of ODMG. The scanning electron micrograph of ODMG gel showed uniform fibrillar-aggregated structures with a size of about 65 nm. The viscosity estimations on a 1.5% aqueous solution as a function of shear rate in range of 10–100 s−1 indicated non-Newtonian behaviour. ODMG gel showed birefringence when viewed with an optical microscope under cross polarisers. This is suggestive of liquid crystalline behaviour typical of viscoelastic materials. The results from Langmuir film balance (LFB) measurements on ODMA indicated almost similar packing characteristics at the air water interface at pH 6.5 and 10.2. ODMA was found to exhibit A0 of 30.89 and 33.60 A2 per molecule at pH 6.5 and 10.2 respectively. However, at pH 10.2, ODMG exhibited more expanded packing structures in pressure region of 5–15 mN m−1.

High performance controlled reactors from micellar assemblies of aromatic amino acid amphiphiles for nanoparticle synthesis

The micellar assemblies of lauryl esters of tyrosine (LET) and phenylalanine (LEP) show extraordinary emulsification properties. The structural similarity in respect of the aromatic ring between the dispersed phase, styrene, and the surfactants facilitates solubilization of styrene up to four times the weight of LEP and 11 times that of LET. We propose that the solubilization site varies between core in the LEP and shell in the LET micelles. LET affords double emulsions, water in oil in water (w/o/w) over a narrow phase composition, for example at R=7.15, where R represents ratio of weight of styrene to LET. A schematic model depicting the solubilization site of styrene at different regions in LET and LEP micelles has been proposed. On polymerization, the emulsions with LET and LEP micelles generated high molecular weight polymer nanoparticles of size 12-49 nm with low polydispersity index (P(d)). This demonstrates that micellar assemblies act as templating controlled reactors for the polymerization reaction. We demonstrate that PS nanoparticle dispersion of size 49 nm with polymer weight fraction as high as 42.80% could be produced from emulsions with the LET surfactant, in contrast to LEP that is 50% less efficient. Polymerization of these emulsions occurs predominantly through a micellar nucleation mechanism. It is significant to note that under the same conditions, the control experiments with sodium dodecylsulfate (SDS) micelles resulted in polymers with broad distribution, P(d)>3.0 in molecular weight.

Ionic polymeric amphiphiles with cholesterol mesogen: adsorption and organization characteristics at the air/water interface from Langmuir film balance studies.

Ionic polymeric amphiphiles consisting of cholesterol mesogen were investigated for the interfacial adsorption characteristics at the air/water interface using a Langmuir film balance with an aim to understand the influence of ionic segment from 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) on the packing behavior of cholesterol at the interface. From surface pressure (pi)-area (A) isotherm characteristics, it is demonstrated that the homopolymer and the copolymer C consisting of 0.15 mol fraction CAB segments exhibit the most expanded structures contributing to surface area of about 84 A(2)/molecule. It is shown that the copolymer B with 0.1 mol fraction CAB provides optimum hydrophilic liphophilic balance to form the most compact structures contributing to a surface area of 35.75 A(2)/molecule. The high surface pressure, >40 mN/m, in contrast to that of PAMPS demonstrates significant adsorption of the copolymers at the interface. An interesting correlation among interfacial packing characteristics, thermal behavior, and solution structures is demonstrated. From molecular models developed for CAB, it is shown that the horizontal orientation of the linker group with respect to cholesterol chain in CAB underlies the expanded structures observed in PCAB and copolymer C.