Surface-adsorbed Water Research Articles

Lateral electrical conductivity of mica-supported lipid bilayer membranes measured by scanning tunneling microscopy

Lateral electric conductivity of mica-supported lipid monolayers and of the corresponding lipid bilayers has been studied by means of scanning tunneling microscopy (STM). The surface of freshly cleaved mica itself was found to be conductive when exposed to humid air. Lipid monolayers were transferred onto such a surface by means of the Langmuir-Blodgett technique, which makes the mica surface hydrophobic and suppresses the electric current along the surface in the experimentally accessible humidity (5-80%) and applied voltage (0-10 V) range. This is true for dipalmitoylphosphatidylethanolamine (DPPE) as well as dipalmitoylphosphatidylcholine (DPPC) monolayers. Repeated deposition of DPPC layers by means of the Langmuir-Blodgett LB technique does not lead to the formation of a stable surface-supported bilayer because of the high hydrophilicity of the phosphatidylcholine headgroups that causes DPPC/DPPC bilayers to peel off the supporting surface during the sample preparation. In contrast to this, a DPPE or a DPPC monolayer on top of a DPPE monolayer gives rise to a rather stable mica-supported bilayer that can be studied by STM. Electric currents between 10 and 100 fA, depending on the ambient humidity, flow along the DPPE bilayer surface, in the humidity range between 35 and 60%. The DPPC surface, which is more hydrophilic, is up to 100 times more conductive under comparable conditions. Anomalous high lateral conductivity thus depends on, and probably proceeds via, the surface-adsorbed water layers. The prominence of ambient humidity and surface hydrophilicity on the measured lateral currents suggests this. The combination of our STM data and previously published water adsorption isotherms as a function of the relative humidity indicate that one layer or less of adsorbed water suffices for mediating the measurable lateral currents. The fact that similar observations are also made for other hydrophilic substrates supports the conclusion that lateral conductivity via surface-adsorbed water is a rather general phenomenon.

Investigation of Amine-Catalyzed Alkoxysilane-Modified Cab-O-Sil by Drift Spectroscopy

Cab-O-Sil fumed silica was modified in nonaqueous solutions with a variety of alkyltrialkoxysilanes under various reaction conditions in the presence and absence of an amine catalyst. Factors taken into consideration include physisorbed water and silane alkyl Chain length. Unassociated silanol loss is discussed as an indication of silane reaction. A decrease in the intensity of the unassociated silanol infrared absorption band should not be used as a measure of silane reaction to the surface. Surface adsorbed water influences the amount of silane coverage both in the presence and absence of an amine catalyst. The advantage of an amine catalyst is lost with increasing surface-adsorbed water resulting in decreased surface silane loading. Cab-O-Sil equilibrated at 50% relative humidity and reacted in the presence of an amine catalyst produced the most silane loading for short chain alkyltrialkoxysilanes, whereas less silica surface moisture is optimal for reaction with longer chain alkyltrialkoxysilanes.

Studies of silylation of Cab-O-Sil with methoxymethylsilanes by diffuse reflectance FTIR spectroscopy

Diffuse reflectance sampling technique and Fourier transform infrared (FTIR) spectroscopy were used to investigate the reaction of methoxymethylsilanes in dry toluene with Cab-O-Sil. Parameters investigated include the effect of surface-adsorbed water and postreaction curing temperature on relative surface silane concentration and extent of methoxy group hydrolysis. A comparison of the reactivities of trimethoxymethylsilane, dimethoxydimethylsilane, and methoxytrimethylsilane was made. In the absence of water, none of the silanes bonded to the silica surface after high-temperature curing. When surface-adsorbed water was present, postreaction curing at 140°C or higher resulted in a decrease in the OCH 3/CH 3 band area ratio suggesting increased hydrolysis and condensation to form siloxane bonds. Results also showed an increase in the relative silane concentration as the number of methoxy groups in the silane increased.

Vibrational studies of metal cation reduction at oxide glass surfaces

Reduction of metal oxide metaphosphate glasses (M 2O · P 2O 5, or MO · P 2O 5, where M is a metal cation) is observed above ca. 100°C when the glass is treated with one several gaseous reducing agents such as H 2, atomic hydrogen, Hg or K. Such amorphous systems are characterized by rapid cation diffusion, with cation reduction occurring principally at the glass surface, which leads to metal cluster formation and eventual continuous metal film growth. Structural changes induced in the glass by cation migration and subsequent reduction are studied by transmission infrared spectroscopy on thin-film samples during the reaction. Scanning electron microscopy and Raman spectroscopy are also employed to study and characterize the glass surfaces. The effect of water vapor on increasing the reaction rate for the mercuric metaphosphate/hydrogen reaction is demonstrated by continuous measurements of vibrational intensity changes. Surface-adsorbed water is seen to reduce the reaction induction period with little effect on the subsequent reaction rate. These results suggest that the rate-determining step of metal ion reduction at glass surface is a diffusion-limited process.