Correlation of Dynamic Surface Tension with Sedimentation of PTFE Particles and Water Penetration in Powders

The dynamic surface tension of aqueous poly­(ethylene oxide)-poly­(propylene oxide)-poly­(ethylene oxide) [(PEO-PPO-PEO)]-type polymeric surfactant (P103, P105, F108, P123, and F127) solutions were correlated with water penetration in packed Teflon powders, the sedimentation of Teflon suspensions in these solutions, foamability, and contact angle measurements on a Teflon surface. The DST trend with bubble lifetime indicated that the overall slowdown in the diffusion process in aqueous solutions is a function of a higher poly­(ethylene oxide) (PEO) molecular weight for a given series of block copolymers containing equal PPO molecular weights, favoring slower diffusion kinetics to the air–water interface caused by preferential partitioning in bulk water. The wettability of poly­(tetrafluoroethylene) (PTFE) powder illustrates better water penetration for polymers with low molecular weight and lower HLB values. The wettability of F127 solutions decreases with corresponding increases in concentration resulting from higher viscosity, which restrains the diffusion kinetics at the PTFE–water interface. The foamability decreases drastically with higher PEO molecular weight as attributed by slower diffusion kinetics, leading to a decrease in the effective concentration of molecules at the foam interface. The contact angle on glass and the PTFE surface are in good agreement with assumptions made by other analytical techniques showing a lower value of the contact angle with a lower HLB of the Pluronic, which relates to the higher adsorption of molecules at the interface. It is concluded that the adsorption of molecules at the PTFE–water interface decreases in aqueous Pluronic solutions with corresponding increases in the hydrophilic lipophilic balance (HLB), which is consistent with foaming, water penetration in a packed powder of PTFE, the rate of sedimentation, and DST data. A PTFE dispersion containing P123 showed the maximum wettability and lowest sedimentation among the series of block copolymers introduced, which is attributed to faster diffusion kinetics and a higher PPO contribution fostering faster adsorption at the PTFE surface. The dynamic surface tension of aqueous Pluronic solutions seems to correlate well with the adsorption characteristics at the air–water and PTFE–water interfaces

Role of stabilizing agents in the formation of stable silver nanoparticles in aqueous solution: Characterization and stability study

<p>The stability of silver nanoparticles is controlled mainly by two major factors, namely, aggregation and oxidation. In the present study, silver nanoparticles were synthesized by using different series of reducing agents like a strong reducing agent (sodium borohydride), a mild reducing agent (tri-sodium citrate), and a weak reducing agent (glucose) with different capping agents, namely, polyvinyl pyrrolidone (PVP K 30), starch, and sodium carboxyl methyl cellulose (NaCMC). The synthesized silver nanoparticles were characterized by UV-Visible absorption spectroscopy, dynamic light scattering (DLS), atomic force microscopy (AFM), and anti-microbial activity. The particle size of silver nanoparticles varies in the following order: sodium borohydride < tri-sodium citrate < glucose. Combination of sodium borohydride–polyvinyl pyrrolidone and tri-sodium citrate-polyvinyl pyrrolidone yields stable silver nanoparticles compared to other combinations of reducing agents and capping agents. The stability results confirmed that a refrigerated condition (8°C) was more suitable for storage of silver nanoparticles. Anti-microbial activity of silver nanoparticles synthesized in a sodium borohydride–polyvinyl pyrrolidone mixture shows a larger zone of inhibition compared to other silver nanoparticles. Anti-microbial results confirmed that the anti-microbial activity is better with smaller particle size. The size and stability of silver nanoparticles in the presence of different combinations of stabilizing and capping agents are reported.</p