Journal of Surface Science and Technology

H. B. Bohidar ¹

Affiliations
1 Polymer and Biophysics Laboratory, Nanomaterials and Nanocomposites Laboratory, School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110 067, IN

Abstract

Coacervation is usually defined as the spontaneous formation of a dense liquid phase from a macromolecular solution of poor solvent affinity. In "coacervation", the loss of solvation arises from the interaction of complementary macromolecular species. The formation of such macromolecule-rich fluids is well-known in mixtures of complementary polyelectrolytes; it can also occur from mixtures of polyelectrolytes with colloidal particles, leading to condensed soft matter phases with interesting properties. Though the polymer solution and gel states are adequately studied, characterized and mostly understood, the same for the coacervate phase is not true. Coacervates are macro-ionic hydrated complexes of a pair (self or complementary) of charge-neutralized polymers. Such a condensed phase remains in thermodynamic equilibrium with its supernatant that mostly contains a dilute dispersion of smaller intermolecular aggregates of the constituent polymers. This article intends to elucidate the salient features of this novel soft matter with some specific examples.

Keywords

Polyelectrolytes, Intermolecular Complexation, Phase Transition, Coacervation.

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Biswaranjan Mohanty ¹, V. K. Aswal ², J. Kohlbrecher ³, H. B. Bohidar ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, IN
2 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, IN
3 Paul Scherrer Institute, CH-5232 Villigen, CH

Abstract

Dynamic light scattering (DLS), Transmission electron microscopy (TEM) and Small Angle Neutron Scattering (SANS) experiments are performed on biodegradable gelatin nanoparticles for size measurements and stability analysis. Though gelatin nanoparticles were previously prepared by the desolvation method [1], the simple coacervation [2] process is being proposed as a new and simple method to prepare very small and stable nanoparticles. Gelatin nanoparticles were found to have spherical conformation by transmission electron microscopy having a typical diameter 45±5nm, which was supported by dynamic light scattering data. This is very small compared to the same reported earlier for this polypeptide (∼200nm). Electrophoresis measurement showed that the nanoparticles present in the supernatant are negatively charged.

Keywords

Gelatin, Nanoparticle, Light and Neutron Scattering, Transmission Electron Microscopy and Electrophoresis.

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S. Chatterjee ¹, H. B. Bohidar ¹

Affiliations
1 Polymer and Biophysics Laboratory, School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, IN

Abstract

Rheology studies, on aqueous gelatin gels, were performed below gelation temperature (T_g ≈ 28°C) in the temperature range, T= 5-25°C as a function of NaCl concentration (0.01-0.1M). In the low frequency domain and at a given temperature, both the storage (G') and loss (G") modulii exhibited power-law frequency dependence, G' ∼ ω^0.02±0.005 and G" ∼ ω^0.8±0.2. The network size, ξ estimated from low-frequency relaxation modulus, G₀ ≈ K_BT/ξ³ increased three fold as the temperature was increased from 5 to 25°C indicating thermal swelling of the network. The strain peaks were observed to shift to higher frequency as the temperature was lowered. Higher salt concentration had the same effect on the gel as higher temperature.

Keywords

Rheology, Gelatin Gels, Subgel Temperature, Viscoelastic Properties.

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H. B. Bohidar ¹, M. Behboudnia ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110 067, IN

Abstract

Reverse micelles of Water-AOT-Isooctane have been studied for the Water/AOT concentration ratio, W₀ varying from 1 to 50, by dynamic laser light scattering and capillary viscometry techniques in the AOT concentration range, 100, to 500 mM. The ratio of the rigidity of the interface, K-to that of effective surface tension, γ; (K/γ) defines a new length scale, ζ, (K/2γ) = ζ,² = R₀R where a reverse micelle of radius R is formed of surfactants having a spontaneous curvature 1/R₀ We show that for W₀ ≤ 20, the reverse micelles can be modelled as spheres purely from geometrical considerations and for W₀ > 20, these conformationally resemble deformed spheres. Correspondingly, the relative viscosity of the solution, n_r shows a micellar volume fraction dependence, , n_r(φ) = (1-φ_m-λφ²)^δ. For low AOT concentration (100 mM), δ = 2.52 5 ± 0.06 implying a near spherical shape for the micelles. This increased to δ = 2.91 ± 0.06 at the AOT concentration, 500 mM which we envisage as micelles having deformed spherical conformation. The micellar stability has been deduced from free energy calculations which yields an explicit expression for the pressure, P(R) inside the water core of a reverse micelle. The intermicellar interaction has been discussed within the frame work of short-range interaction potential and perturbation potential ∼r^-6.

Keywords

Reverse Micelzes, DLS Stability, Conformation.

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Anita Saxena ¹, H. B. Bohidar ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, IN

Abstract

Binding of the anionic surfactant, sodium dodecyl sulfate, SDS to gelatin chains in aqueous buffer (pH = 7.0) medium has been studied by dynamic light scattering technique at 30°C. In the surfactant concentration range varying from 0 to 100 mM, SDS exhibited electrostatic binding to the charged groups of the polypeptide chain resulting in considerable reduction in the hydrodynamic radius (R_h) of gelatin upto the critical aggregation concentration (CAC); at higher concentrations both the SDS micelles and gelatin-SDS complexes were found to coexist in equilibrium. The micellar shapes were found to be near-spherical for SDS. Results have been interpreted through the necklace-bead model of polymer-surfactant interactions.

Keywords

Gelatin-SDS Binding, Necklace-Bead Model, Micellisation.

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Saroj Maity ¹, H. B. Bohidar ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110 067, IN

Abstract

Photon Correlation Spectroscopy (PCS) study carried out on semi-dilute, 4%(w/v) aqueous gelatin sols with the anionic surfactant, sodium dodecyl sulphate (SDS) showed three relaxation processes in the dynamic structure factor data; first an exponential process S (q,t) ~ exp (-D_f(q²t) at t ≤ 20 μs called the gel mode, was observed followed by an intermediate power law regime S(q,t) ~ t^-α(q) at 20 μS ≤ t ≤ 1 ms and finally a stretched exponential decay S(q,t) ~ exp -(t/τ_c)^β observed for t ≥ 1 ms. The power-law exponent (α) and characteristic time τ_c in stretched part were found to be q-dependent, α ~ q^X with x = 1.80 ± 0.09 in the sol state; and τ_c~q^-y, y = 2.05 ± 0.08 also in the sol state but independent of SDS concentration. The temperature dependence of α and τ_c was measured. With SDS the different characteristic length scales alter the conformation of gelatin. Our results agree with the predictions of anomalous Gaussian diffusion model in the sol state.

Keywords

Photon Correlation Spectroscopy, Relaxation, Sol-Gel Transition, Diffusion.

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Nidhi Joshi ¹, Kamla Rawat ², Pratima R. Solanki ³, H. B. Bohidar ³

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, IN
2 Inter University Accelerator Centre (IUAC), New Delhi 110067, IN
3 Special Center for Nanosciences, Jawaharlal Nehru University, New Delhi 110067, IN

Abstract

Pectin is a branched polysaccharides found in the cell wall of the plants and commonly used in food industry as a gelling agent, emulsifier or stabilizer. The effect of calcium chloride on the gelation of pectin dispersions was studied using rheology and light scattering measurements. Addition of calcium induced the gel formation in pectin dispersions follows egg-box crosslink mechanism. Zeta potential measurements revealed the formation of cationic and anionic pectin gels on concentration of calcium. The cationic gels had higher rigidity compared to anionic gels. The sol-gel transition has been investigated for pectin-calcium system from the structure factor data which indicated cationic gels undergo transition earlier compared to anionic ones. The gelation time was determined from rheology and viscosity experiments and found to be less for cationic gels.

Keywords

Ca2+-pectin Gels, Egg-box Model, Gel Elastic Behaviour, Gel Stiffness, Gel Structure Development, Pectin, Rheology of Gels

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Jyotsana Pathak ¹, Kamla Rawat ², H. B. Bohidar ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110067, IN
2 Special Center for Nanosciences, Jawaharlal Nehru University, New Delhi - 110067, IN

Abstract

Herein, we report interaction between Gelatin B (GB) and Bovine Serum Albumin (BSA) which is mainly arising from surface selective patch binding occurring at their common pI (≈5.0 ± 0.5). The dispersion remained homogeneous with no indication of either complex formation or phase separation, but the dispersion turns gel-like phase over time. It was found that the initial tenuous network of GB provided a scaffold for network organization, the BSA molecules aggregated along the GB chains, and functioned as cross-linking agent, and more elastic gels were obtained for samples with higher GB content. We probe this dynamics in finer details and the gel phase displayed considerable change with aging time as the system moved from ergodic to non-ergodic state. GB-BSA based system exhibited anomalous relaxation in gels changing with aging time t_w at their common pI. At t_w = 0, the correlation function exhibited one relaxation mode due to the system residing deeply inside the ergodic phase and purely mirroring Brownian dynamics. After a characteristic waiting time, an additional mode (slow relaxation) appeared which was attributed to inter-chain interaction induced reorganization of entanglements.

Keywords

Dynamic Light Scattering, Ergodic–Non Ergodic Phase, Gel Structure Development, Mixed Protein Dispersion, Relaxation Dynamics in Gels.

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Pankaj Kumar Pandey ¹, Kamla Rawat ², H. B. Bohidar ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110067, IN
2 Department of Chemistry, School of Chemical and Life Sciences, Jamia Hamdard, Delhi - 110062, IN

Abstract

DNA ionogels prepared by adding 1-ethyl-3-methylimidazolium chloride on low energy gamma irradiated DNA solution samples reveal non-trivial self-assembly. Variations in secondary structure and low-frequency gel rigidity modulus G0 captured this unique hitherto unexplored features of these gels. Interestingly, at higher radiation dose (0 to 100 Gy) samples could partially lose their initial rigidity. Dynamic light scattering revels dose dependent relaxation dynamics corresponding to ergodicity breaking time. In particular, viscosity and rheology showed that the time of gelation t_gel, temperature of gelation T_gel and strength of gelation G₀ are gamma ray dose dependent. DNA Ionogel melting with temperature shows self-assembled characteristics of this biomaterial. Gelation kinetics of ionizing radiation treated DNA strands have been studied in literature.

Keywords

DNA Ionogel, Gelation, γ-Radiation, Ionizing Radiation.

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Rahul Ranjan ¹, Kamla Rawat ², H. B. Bohidar ¹

Affiliations
1 School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110067, IN
2 Department of Chemistry, School of Chemical and Life Sciences, Jamia Hamdard, New Delhi - 110062, IN

Abstract

TX-100 molecular hydrogels exhibited re-entrant melt-gel-sol phase transition driven solely by dilution, which sequentially altered the self-assembly of the micellar formations and their population was investigated through the monitoring of the physical parameters namely, solution viscosity, particle size histogram, ergodicity, and gel rigidity modulus. This phenomenon was noticed at 20° C in the TX-100 concentration region of 0.2 to 1.2 M much above the critical micellar concentration of 0.22 mM. The particle size histograms revealed the presence of spherical micelles (size »3 nm) in the solution ([TX-100] < 0.5 M) which formed entangled wormlike cylindrical micelles (apparent hydrodynamic radius » 50 nm) when (0.5 M< [TX-100] < 0.9 M) giving rise to a gel-like structure. Further increase in the TX-100 concentration increased the propensity of these wormlike cylindrical micelles that got randomly distributed creating a dense melt phase. Interestingly, we observed transition solely driven by dilution which defined complete re-entrant behavior at room temperature. These molecular gels could be created by dilution of the melt or concentration of the sol unlike in the polymer gels. Remarkably, this hitherto little known unique phenomenon was exhibited by a simple system of non-ionic surfactant solution. Thus, we have a hydration reversible gel at our disposal which has a special place in soft matter arena.

Keywords

Bulk Phenomena, Gelation Mechanism, Re-Entrant Phase Transition, Surfactant Gels, TX-100 Gel.

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