Surfactant Mobility in Nanoporous Glass Films

Abstract

Polymer molecules when physically confined at nanometer length scales diffuse nonclassically and very differently depending on their molecular weight and the nature of the confinement. Long polymers that exhibit "snakelike" reptation based mobility in melts may diffuse faster in confined nanometer sized cylinders with pore diameter d approximately 15 nm, and short polymers subject to Rouse dynamics have shown signatures of reptation and slower diffusion when confined in nanoporous glass with d approximately 4 nm. However, the mobility of short polymers with radii of gyration similar to a smaller pore diameter (d < or = 2.1 nm) but with extended lengths well larger than the pore diameter has not as yet been studied. In this work, we demonstrate that those short molecules including nonionic surfactants can readily diffuse in strongly hydrophobic nanoporous glasses film with d < or = 2.1 nm. The diffusivity was found sensitive to molecular weight, hydrophilic-lipophilic balance, and molecular structure of surfactants. Remarkably, analysis of the measured diffusion coefficients reveals that short-chain surfactants exhibit signature of reptation based diffusion in the nanoscopic pore confinements. Such reptation mobility in agreement with theoretical predictions is not even observed in reptating polymer melts due to fluctuations of the entanglement pathway. The fixed pathways in the interconnected nanoporous films provide ideal nanoscale environments to explore mobility of confined molecules, and the results have implications for a number of technologies where nanoporous materials are in contact with surfactant molecules.

Molecular Mobility under Nanometer Scale Confinement

“…We recently showed that unentangled surfactant molecules in the bulk become entangled with adjacent molecules when diffusing through interconnected nanopores (d ∼ 2.1 nm) and exhibit signatures of reptation. 9 We also discussed the importance of molecular interactions with pore surfaces. Other studies have shown that long polystyrene molecules with molecular weight above that needed for entanglement diffuse faster in cylindrical alumina nanopores (pore diameter d ∼ 15 nm) due to lower molecular entanglement associated with fewer chains present in the confined region.…”

“…The final film thickness, h ∼ 500 nm, density, F ∼ 1.06 g/cm 3 , pore diameter, d ∼ 2.1 nm, and dielectric constant, k ∼ 2.3, were measured. 9 The MSSQ glass matrix is a structurally modified form of silica produced by replacing one bridging oxygen atom with a terminal methyl group yielding a film with reduced k and strong hydrophobicity (contact angle with water >100°). Organosilicate glass (OSG) and porogen-containing organosilicate glass (pOSG) films with similar composition but more disordered molecular structure were deposited on Si substrates by plasma-enhanced chemical vapor deposition (PECVD) and cured with broad-band UV radiation.…”

“…11 Diffusion coefficients of linear alkanes with various chain lengths from hexane (C 6 H 14 ) to hexadecane (C 16 H 34 ), and linear fatty alcohols from hexanol (C 6 H 13 OH) to undecanol (C 11 H 23 OH), were measured using the lateral solvent diffusion technique. 9,12 To directly observe liquid penetration in the plane of the nanoporous film, the top surface of the film was first hermetically sealed with an optically transparent SiN layer. The silicon substrates containing the films were cleaved to expose a fresh through-thickness surface of the film, and immersed in the liquid of interest.…”

“…The other is a diffusion model 9,12 where D is the Fickian collective diffusion coefficient. Both models exhibit the same square root of time dependence and have been shown to yield equivalent results for fluid penetration in porous media.…”

A gemini-type superspreader: Synthesis, spreading behavior and superspreading mechanism

“…Hence, the HLB of the surfactants plays a very crucial role in the spreading behavior. In our experiments, the HLB of the surfactants was calculated by Griffin's method [42,43]: HLB = 20*M h /M, where M h is the molecular mass of the hydrophilic portion of the molecule and M is the molecular mass of the whole molecule. The HLB values of S m (EO) n are listed in Table 1.…”