An infrared spectroscopy study of the hydrogen bonding and water restructuring as a trisiloxane superspreading surfactant adsorbs onto an aqueous-hydrophobic surface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 277 (1-3):
98--106 (Apr 5, 2006)DOI: 10.1016/j.colsurfa.2005.11.040
Abstract
Trisiloxanes are surfactants with a compact trisiloxane group ((CH3)3Si-O-Si(CH3)(C3H6-Ej)-O-Si(CH3)3) as the nonpolar moiety and an oligomeric polyethylene oxide chain, Ej ((OCH2CH2)j-R) as the polar group. ("R" is the cap of the polar group.) Trisiloxane surfactants with four to eight ethoxylates facilitate the rapid and complete spreading of drops of their aqueous solutions on very hydrophobic surfaces not otherwise wet by water. This superspreading ability, which is not completely understood, is unique to these amphiphiles in that solutions of conventional alkyl chain surfactants do not wet out on very hydrophobic surfaces. In this study, in an effort to understand the origin of superspreading, we use Fourier transform infrared spectroscopy in total attenuated internal reflection mode (FTIR-ATR) to gain a molecular picture of the adsorption of a trisiloxane surfactant onto an aqueous/hydrophobic-solid interface, and for comparison a nonsuperspreading alkyl chain surfactant. We focus on the OH stretch of water molecules to detail the changes in the hydrogen bonding structure of water at the interface, and methyl and methylene stretches to measure the adsorption densities. We compare the trisiloxane superspreader TE8 ((CH3)3Si-O-Si(CH3)((C3H6)(OCH2CH2)8OH)-O-Si(CH3)3) with a conventional non-superspreading n-dodecyl polyoxyethylene glycol surfactant with the same ethoxylate chain length C12E8 (CH3(CH2)11(OCH2CH2)8OH), at concentrations below and above their critical aggregate concentrations. The spectra of C12E8 indicates that the adsorption removes the water molecules in the molecular layers in the vicinity the water/hydrophobic interface which have been shown to have a hydrogen bond coordination greater than those of bulk water. The trisiloxane spectra initially show this same behavior, but for longer times demonstrate the ability to remove water molecules which immediately straddle the surface with a dangling OH bond, and reconfigure them into a tightly hydrogen bonded network. This water restructuring is unobserved in the adsorption of the conventional surfactant, and, as it maximizes hydrogen bonding, is energetically favorable and may provide the enhanced driving force for adsorption and superspreading. Measurements of the adsorption densities provide additional information on how the architectures of the trisiloxane and polyethoxylate molecules determine the adsorption geometry and hydrogen bonding with water. From the measured surface densities at maximum adsorption we infer that the compact hydrophobic trisiloxane groups of the TE8 molecules pack tightly along the hydrophobic surface where they exclude water molecules. This close packing is unimpeded by the oligomeric ethoxylate chain which is of equal cross-section as the hydrophobe, and extends into the aqueous phase within the hydrophobic footprint. In the case of C12E8, the measured densities at maximum packing indicate that the slender alkyl chain, while extending down to the hydrophobic surface, is spaced apart by the larger ethoxylate chain, thereby leaving water molecules immediately adjacent to the hydrophobic surface.