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In this 2009 reporting period of our grant extension we made progress on investigating the metal oxide-organic interaction on stainless steel 316L and its component oxides by working on the two goals of the proposal:  1. forming organic monolayers on the surface and varying the organic head group pKa from acidic to basic to determine if acidity of the head group is the controlling factor in reactivity; 2. studying the surface metal role in monolayer formation and stability by using the alloy and its individual components. The summary of the work and been submitted to Langmuir. The results obtained were utilized as preliminary data for an NIH R15 (AREA) grant application that was funded in January of 2009.

Goal 1: Prior to this reporting period we had successfully formed monolayers on the oxide surface of SS316L using octadecylcarboxylic acid and octadecylphosphonic acid and attempted to form monolayers using hydroxamic acids, amines, alcohols and alkanes.  In this granting period we successfully formed stable octadecylsulfonic acid monolayers. All attempted surface modifications were analyzed by diffuse reflectance infrared spectroscopy. If modification was successful, the substrates were rinsed in the deposition solvent and sonicated to test the chemical and mechanical stability of the monolayer. Films stable to these tests were then analyzed by atomic force microscopy and MALDI-TOF MS. Through this testing, we accomplished our stated goal of attempting film formation on SS316L organic molecules with pKas ranging from 2 to 15, thus testing the reactivity of the surface. While the films formed according to pKa, the stability of the film to rinsing and sonication was not based on pKa (Table 1). During this process we noted significant disparities in the conditions required for film formation. For example, some organic molecules formed monolayers with only a short deposition time, while others required significant deposition time or cooling of the substrates. We have recently undertaken significant AFM studies to better understand these disparities. The first portion of the study, examining the need for post-deposition heating, was recently submitted to Thin Solid Films. Finally, we studied chain length variations of the organic acids that did form monolayers on SS316L. For phosphonic acid, eight and eleven carbon chains can be utilized to form SAMs, while eighteen carbons are required for sulfonic and carboxylic acids.

Table 1. Infrared stretching frequency of CH_2asymm

Goal 2: In this reporting period, undergraduates confirmed results from previous reporting periods and attempted to improve film formation on several substrates. The surface of SS316L is composed of oxides of the following metals: Fe 66.01%, Cr 19.19 %, Ni 9.17%, Mn 3.22% and Mo 2.42%. To determine the role of the surface components in monolayer formation we used each metal oxide as a substrate for monolayer formation. Interestingly, unreactive surface species do not prohibit formation of a complete monolayer of carboxylic acids, phosphonic acids sulfonic acids on SS316L. The monolayers have been characterized by infrared spectroscopy, AFM, contact angle measurements and MALDI-TOF MS. The mass spectrometry data confirms the IR data presented in Table 1.

In an attempt to understand the fundamental chemistry of film formation of monolayers on alloys we delved into popular theories behind film formation on metal oxides. In Goal 1, we examined the acid-base theory, in which organic acidity controls the surface reaction. In this theory, the more acidic molecules would be more reactive. While we found them to be reactive in the order of pKa, the films were not stable according to their pKa. The isoelectric point of the surface, based on literature values, was also not consistent with our results. Finally, we studied the hydroxyl content of the these surfaces in the literature and found that increasing hydroxyl content (i.e. Nickel) did not improve film formation or stability on the surfaces, nor did low hydroxyl content (Molybdenum) prevent film formation or stability.