44979-G5
Studying Monolayer Formation on Alloys
In this 2008 reporting period we have 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.
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 amines, alcohols and alkanes. In this reporting period we have attempted monolayer formation with hydroxamic (pKa = 8) and sulfonic acids (pKa = 2). 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 AFM and MALDI-TOF MS.
Octadecylhydroxamic acid formed ordered films upon deposition that were stable to rinsing (νCH2 asymm = 2916 cm-1). By AFM, it was concluded that the films were made up of micelles that were easily removed through sonication. The deposition of sulfonic acids has led to weakly bound multilayers (νCH2 asymm = 2913 cm-1; AFM indicates multilayer). The films were stable to rinsing but not sonciation in THF (Table 1). The reactivity of the organics to SS316L substrate is, thus far, found to follow the order: phosphonic acid > carboxylic acid > hydroxamic acid = sulfonic acid> amine > alcohol > alkane. This is not consistent with increasing reactivity with increasing pKa due to the low stability of the sulfonic acid system. The sulfonic acid head group is larger than the other head groups which may hinder packing of the film and therefore stability of the film. Additionally, the deprotonated form of sulfonic acid, which is thought to be the reactive species in monolayer formation, is stabilized due to the delocalization of the charge by resonance, potentially rendering the acid less reactive. Additionally, both the phosphonic and carboxylic acids are bound to the surface in a bidentate manner, while the hydroxamic and sulfonic acids are in a monodentate orientation after rinsing.
Goal 2: 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. The
results are summarized in Table 1. Monolayer formation with octadecylphosphonic
acid was straightforward on all of the substrates, while carboxylic acid only
formed monolayers on iron and molybdenum oxides and
was only stable to rinse on iron.
Interestingly, these unreactive surface species
do not prohibit formation of a complete monolayer of carboxylic acids on
SS316L. The monolayers have been characterized by
infrared spectroscopy, atomic force microscopy, and MALDI-TOF MS. The mass
spectrometry data confirms the IR data presented here. Contact angle measurements
are presented in Table 2.
ACIDS Phosphonic acid Carboxylic acid Hydroxamic acid Sulfonic acid Metals Rinse Sonic Rinse Sonic Rinse Sonic Rinse Sonic SS 316L 2912 2911 2915 2917 2916 ----- 2913 ---- Nickel 2914 2915 ---- ---- 2916 ----- 2918 ---- Iron 2915 2916 2915 2915 ---- ----- 2916 ----- Chromium 2914 2914 ----- ----- ----- ----- 2915 ----- Molybdenum 2914 2914 ----- ----- 2913 2913 2914 ----- Manganese 2914 2914 ---- ----- 2914 ----- 2914 ---- Table 1.
ACIDS
| Phosphonic acid
| Carboxylic acid
| Hydroxamic acid
| Sulfonic acid
| ||||
Metals
| θ °
| + Stdev | θ °
| + Stdev | θ °
| + Stdev | θ °
| + Stdev |
SS 316L
| 108
| 3
| 104
| 1
| 76
| 9
| 90
| 4
|
Nickel
| 109
| 5
| ----
| ----
| 100
| 4
| 93
| 8
|
Iron
| 98
| 4
| 87
| 5
| ----
| ----
| 83
| 10
|
Chromium
| 99
| 8
| ----
| ----
| ----
| ----
| 17
| 6
|
Molybdenum
| 80
| 8
| ----
| ----
| 22
| 9
| 31
| 7
|
Manganese
| 58
| 7
| ----
| ----
| 68
| 13
| 101
| 6
|
Table 2. Static water contact angles on modified substrates with standard deviations.
In the extension reporting period, the group plans to explore fundamental surface reasons (such as isoelectric point, grain boundaries, etc) for the differences in reactivity between the alloy and its substituents and explore experimental changes to enhance the stability of sulfonic acid films on the surface. Additionally, chain length effects of the acids on monolayer formation and stability.
