Sugam Kumar

Size-Dependent Interaction of Silica Nanoparticles with Different Surfactants in Aqueous Solution

The size-dependent interaction of anionic silica nanoparticles with ionic (anionic and cationic) and nonionic surfactants has been studied using small-angle neutron scattering (SANS). The surfactants used are anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethyl ammonium bromide (DTAB), and nonionic decaoxyethylene n-dodecylether (C(12)E(10)). The measurements have been carried out for three different sizes of silica nanoparticles (8, 16, and 26 nm) at fixed concentrations (1 wt % each) of nanoparticles and surfactants. It is found that irrespective of the size of the nanoparticles there is no significant interaction evolved between like-charged nanoparticles and the SDS micelles leading to any structural changes. However, the strong attraction of oppositely charged DTAB micelles with silica nanoparticles results in the aggregation of nanoparticles. The number of micelles mediating the nanoparticle aggregation increases with the size of the nanoparticle. The aggregates are characterized by fractal structure where the fractal dimension is found to be constant (D ≈ 2.3) independent of the size of the nanoparticles and consistent with diffusion-limited-aggregation-type fractal morphology in these systems. In the case of nonionic surfactant C(12)E(10), micelles interact with the individual silica nanoparticles. The number of adsorbed micelles per nanoparticle increases drastically whereas the percentage of adsorbed micelles on nanoparticles decreases with the increase in the size of the nanoparticles.

pH-dependent interaction and resultant structures of silica nanoparticles and lysozyme protein.

Small-angle neutron scattering (SANS) and UV-visible spectroscopy studies have been carried out to examine pH-dependent interactions and resultant structures of oppositely charged silica nanoparticles and lysozyme protein in aqueous solution. The measurements were carried out at fixed concentration (1 wt %) of three differently sized silica nanoparticles (8, 16, and 26 nm) over a wide concentration range of protein (0-10 wt %) at three different pH values (5, 7, and 9). The adsorption curve as obtained by UV-visible spectroscopy shows exponential behavior of protein adsorption on nanoparticles. The electrostatic interaction enhanced by the decrease in the pH between the nanoparticle and protein (isoelectric point ∼11.4) increases the adsorption coefficient on nanoparticles but decreases the overall amount protein adsorbed whereas the opposite behavior is observed with increasing nanoparticle size. The adsorption of protein leads to the protein-mediated aggregation of nanoparticles. These aggregates are found to be surface fractals at pH 5 and change to mass fractals with increasing pH and/or decreasing nanoparticle size. Two different concentration regimes of interaction of nanoparticles with protein have been observed: (i) unaggregated nanoparticles coexisting with aggregated nanoparticles at low protein concentrations and (ii) free protein coexisting with aggregated nanoparticles at higher protein concentrations. These concentration regimes are found to be strongly dependent on both the pH and nanoparticle size.

Tuning of nanoparticle–surfactant interactions in aqueous system

The interaction of charged (anionic) silica nanoparticles with ionic and nonionic surfactants has been studied using small-angle neutron scattering (SANS). The surfactants used are anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethyl ammonium bromide (DTAB) and nonionic decaoxyethylene n-dodecylether (C(12)E(10)). The measurements are carried out at fixed concentration (1 wt%) of silica nanoparticles and with surfactant concentration varied in the range 0-2 wt%. It is found that there is no direct interaction between the nanoparticles and the surfactant (SDS) when they both are similarly charged. Both the silica nanoparticles and micelles coexist individually with no significant change in the structure of the micelles with respect to that in the pure surfactant solution. On the other hand, the presence of oppositely charged surfactant (DTAB) leads to the aggregation of silica nanoparticles even with very low surfactant concentration. The aggregation of silica nanoparticles is characterized by fractal structure and its fractal dimension remains constant with the increase in the surfactant concentration. In the case of nonionic surfactant, it interacts with the individual silica nanoparticles. The interaction is examined using two models: one that considers the surfactant layer coating on silica nanoparticles and a second one where the surface of the nanoparticles is decorated by the micelles. Contrast variation SANS measurements confirm the uniform decoration of nonionic micelles on the nanoparticles.

Solubilization of hydrophobic alcohols in aqueous Pluronic solutions: investigating the role of dehydration of the micellar core in tuning the restructuring and growth of Pluronic micelles

Pluronics® are considered to be good carriers for poorly water soluble substances on account of their superior solubilization capacity over other ionic and nonionic surfactants. Understanding the influence of these substances on the aggregation characteristics of Pluronics® is therefore of overriding importance. In this manuscript we report dynamic light scattering, small angle neutron scattering, fluorescence and viscometry studies on the effect of adding hydrophobic alcohols viz. hexan-1-ol, octan-1-ol and decan-1-ol to the aqueous solutions of two Pluronics® (P85 and P123) with different hydrophilic–lipophilic balances. Both these Pluronics® exhibit a sphere to rod micellar shape transition with increasing temperature, the dynamics of the transition being significantly slower in the case of P123 because of its higher hydrophobicity and molecular weight. The aim of our studies was to investigate the restructuring and growth of Pluronic® micelles upon addition of hydrophobic alcohols that have applications in fields ranging from personal care/food products to pharmaceutical formulations. The studies show that alcohol induced micellar restructuring and growth for the two Pluronics® slow down as the concentration of alcohol increases and in addition their aqueous solubility decreases progressively from hexan-1-ol to decan-1-ol. These observations, which were manifested by a decreasing rate of sphere to rod micellar growth, have been attributed to more effective dehydration of the micellar core by alcohols with higher levels of solubilization and hydrophobicity. The results thus shed light on the specific role of additive induced dehydration of micellar cores in the restructuring and growth characteristics of micelles, for both hydrophilic (P85) and hydrophobic (P123) Pluronics®.

Tetraalkylammonium ion induced micelle-to-vesicle transition in aqueous sodium dioctylsulfosuccinate solutions

The aggregation behavior of sodium dioctylsulfosuccinate (AOT) in aqueous media containing tetraalkylammonium bromide (TAAB, where alkyl = ethyl (TEAB), propyl (TPAB) and butyl (TBAB)) was studied by surface tension, fluorescence (with pyrene as the probe), small-angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements. A comparison of the critical micelle concentration (cmc) values of AOT in the presence of the salts showed the order TBAB < TPAB < TEAB < NH4Cl < NaCl. Synergism in the cmc occurs when the solution contains a mixture of sodium and tetraalkylammonium counterions. The counterion binding behavior was examined by applying the modified Corrin–Harkins (CH) equation which revealed that a special counterion binding behavior of AOT exists in aqueous solutions with tetraalkylammonium salts. The modified CH equation and DLS data indicate a change in the shape of the surfactant aggregates, which was confirmed by the SANS data. Dehydration of the head group and the counterion during their interaction appears to induce a micelle-to-vesicle transition in the aggregates.

Interactional and aggregation behavior of twin tail cationic surfactants with pluronic L64 in aqueous solution

The aqueous mixed systems of twin tail cationic surfactants didodecyldimethylammonium bromide, ditetradecyldimethylammonium bromide, and dihexadecyldimethylammonium bromide with pluronic L64 have been studied to determine the bulk aggregation and interactional behavior. Various experimental techniques, namely small-angle neutron scattering (SANS), fluorescence, conductivity, and surface tension, have been employed to investigate the mixed micellization. The SANS data analysis has been employed to determine the shapes of different aggregates formed. Pure twin tail cationic surfactants form vesicles whereas the micelles of pure pluronic L64 are spherical. The mixed systems (surfactant + L64) also form spherical micelles, and the spherical shape of mixed micelles is predominantly controlled by pluronic L64. Various interfacial parameters such as surface excess (Γmax), minimum area per molecule (Amin), and thermodynamic parameters such as the standard Gibbs free energy of micellization (

Influence of Headgroup on the Aggregation and Interactional Behavior of Twin-Tailed Cationic Surfactants with Pluronics

\Delta G_{{mic}}^{{0}}

Transition from long micelles to flat bilayers driven by release of hydrotropes in mixed micelles

), Gibbs free energy of adsorption (