Sum-Frequency Spectroscopy Analysis of Two-Component Langmuir Monolayers and the Associated Interfacial Water Structure

Abstract

Sum-frequency spectroscopy (SFS) in the CH and OH stretching regions was employed to obtain structural information about Langmuir monolayers on the H(2)O subphase of the model lipid dioctadecyldimethylammonium bromide (DOMA) and of the neutral surfactant methyl stearate (SME) and their mixtures and about the interfacial water structure underneath the films. These results were compared with the sum-frequency spectra of the interface between Langmuir monolayers of stearic acid and stearic acid-DOMA monolayers and water to prove that the uncompensated headgroup charge of DOMA at the interface is the reason for structuring of interfacial water close to the studied monomolecular films. Sum-frequency spectra on D(2)O subphase were also studied to account for the interference between the CH and OH spectral signatures because of the coherent nature of the SFS signals. Interfacial water structure proved to be a determining factor in the behavior of the mixed lipid monolayers. A mixing induced amplification in the surface potential DeltaV observed in our previous work was explained with total increase of the dipole moment for the mixed films, bigger than the arithmetic average for DOMA and SME monolayers alone. The increase is due to the better packing of the molecules in the mixed films and to the decrease in the interfacial water dipole moment arising from a more disordered water structure underneath the mixed monolayers.

Shedding light on water structure at air–aqueous interfaces: ions, lipids, and hydration

“…We have recently delved into the area of fatty acid and lipid films on purely aqueous and on aqueous electrolyte solutions. 87,108,109 There has been a small body of work on understanding the underlying OH hydrogen bonding region of lipid films; 56,110,111 however, the free OH of surface water had not been of interest previously. This may have been from the assumption that the free OH was not present in the presence of a lipid monolayer or fatty acid film.…”

Environmental Chemistry at Vapor/Water Interfaces: Insights from Vibrational Sum Frequency Generation Spectroscopy

“…Beyond examining the water structure at the vapor/neat water interface, there is much interest in elucidating molecular behavior via VSFG at the vapor/water interface for a wide variety of chemical systems. With perspective toward understanding the role that tropospheric aqueous aerosols play in atmospheric chemistry, among other applications, VSFG has been applied to study the behavior that inorganic ions (25,(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57), small molecular solutes (25,(58)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69), and lipids (32, 35,[70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88]…”

In situ molecular level studies on membrane related peptides and proteins in real time using sum frequency generation vibrational spectroscopy

“…SFG permits the identification of interfacial molecular species (or chemical groups), and also provides information about the interfacial structure, such as the orientation and the orientation distribution of functional groups on the surface. SFG has been applied to study the structure and orientation of biomolecules, such as lipids (Anderson et al, 2006; Anglin et al, 2007; Anglin et al, 2008; Chen and Wang, 2007a; Doyle et al, 2004; Harper et al, 2007; Kim and Kim, 2001; Levy et al, 2007; Liu et al, 2004a; Liu et al, 2004b; Liu et al, 2005a; Liu et al, 2005b; Liu et al, 2007; Lobau et al, 1999; Ma et al, 2006; Ma et al, 2007; Nickolov et al, 2006; Ohe et al, 2004; Petralli-Mallow et al, 1999; Sovago et al, 2007; Watry et al, 2003; White et al, 2006), and peptides/proteins (including membrane-related proteins/peptides) (Chen and Chen; 2006; Chen and Clarke, 2005; Chen and Wang, 2005; Clarke et al, 2005; Dreesen et al, 2004; Evans-Nguyen et al, 2006; Humbert et al 2006; Kim and Cremer, 2001; Kim and Gurau, 2002; Kim and Gurau, 2003; Kim and Somorjai, 2003; Knoesen et al, 2004; Mermut et al, 2006; Rocha-Mendoza et al, 2007; Sartenaer et al, 2007; Wang and Buck, 2003; Wang and Chen, 2005; Wang and Chen, 2006; Wang and Clarke, 2003; Wang and Even, 2003; York et al, 2008). …”