Lecture 3: Biomaterials Surfaces: Chemistry

Transcription

1 1 Lecture 3: Biomaterials Surfaces: Chemistry Surfaces are high-energy regions of materials and thereby facilitate chemical reactions that influence performance of biomaterials. We will focus on 3 classes of surface chemistry relevant to biomaterials: Chemisorption on metals and oxides Aqueous corrosion of metals Hydrolysis of polymers 1. Chemisorption Strong modifications to electronic structure/ electron density of adsorbate molecule (> 0.5 ev/surface site) Important Examples: a) etal xide Formation on etals metals just wanna be oxides x + ½y 2 x y G 0 of oxide formation is negative for all but a few metals (e.g., Au) Reaction G 0 (joules) T range (K) 2Cr + 3/2 2 = Cr 2 3-1,120, T Fe + ½ 2 = Fe -259, T Fe + 3/2 2 = Fe , T Ti + 2 = Ti 2-910, T from D.R. Gaskell, Intro. To etallurgical Thermodynamics, cgraw-hill, 1981

2 2 How does metal oxidation happen? ne scenario is step 1: physisorption of 2 ; ~20-25 kj/mol 1 ev/molec = 96.5 kj/mol kt ev step 2: molecular oxygen dissociates and reduces by chemisorption; ~600 kj/mol step 3: bond rearrangement; crystallization of oxide layer Resultant reduction in surface energy Compare: at 1400 C: γ δ-fe = 1900 dyn/cm γfe = 580 dyn/cm Consider metal oxidation as 2 half reactions: 2 + ze - ½z 2- Electrons and ions must traverse the oxide layer for rxn to proceed. z+ z+ + ze - Across the oxide film, an oxidation potential, E 0 ~ 1V generates an electric field: x~1 nm G = E 0 0 z I E-Field 1 V/nm = 10 V/cm Ionic species are pulled through oxide film! I= 96,480 C/mol e - 1 J = 1 V-C

3 3 WHAT HAPPENS AS THE XIDE CNTINUES T GRW? The E-field decreases. Subsequent oxide growth occurs by thermal diffusion of z+ to oxide surface or 2- to metal/oxide interface under the concentration gradient c: dm dt dc = D A dl m = transferred mass of metal A= area 2 l = k p t l oxide thickness k = p D c xρ x y time Requirements for Passivation: i) small k p (rate const) Ex: Cr incorporation into Fe from D.A. Jones, Corrosion, acmillan: 1992, p. 430 k p at 1000 C wt% Cr (g 2 cm 4 /s) ~ ~ ~ ~

4 4 ii) adherent oxide xide layer must not scale or spall minimize V molar & stress build-up x + ½y 2 x y ex., Ti (Ti 2 ), Cr (Cr 2 3 ), Al (Al 2 3 ) (Al metal not used in biomaterials applications due to toxicity) Pilling-Bedworth ratio: PB = V oxide ( formed ) V = ρ x y x y metal (consumed ) x ρ Theoretically, want PB ~ 1 (PB > 1) for adherance of oxide to underlying metal in practice, this rule is marginally predictive, however. ther etal xidation Rxns by Chemisorption: Reaction with water: x + yh 2 x y + yh 2 Reaction with C 2 : x + yc 2 x y + yc After oxide formation, surface remains reactive Why?

5 5 xides are still relatively high γ surfaces! b) Acid/Base (Acceptor/Donor) Rxns on xides i) **H 2 adsorbed + 2- lattice H - lattice + H - surface Ubiquitous! e.g., oxides of Co, Ti, Cr, Fe, etc. H 2 cleavage with H + transfer to surface basic 2- site & H - coordination with 2+ H H H - H acts as Lewis acid (e - pair acceptor) for oxygen lone pairs 2 ii) C 2,adsorbed + 2- lattice C 3 (carbonate formation) Experimentally seen, e.g., on Ti 2 (110) iii) w/ Hydrocarbons: Alcohols (similar to HH): RH adsorbed + 2- lattice R - + H - Carboxylic Acids: RCH adsorbed + 2 lattice RC - + H

6 6 c) Redox (xidation/reduction) Reactions on xides Examples i) Alcohol dehydrogenation to aldehyde: RCH 2 H RC H + 2H - + 2e - alcohol is oxidized electrons reduce z+ at surface or RCH 2 H + 2 R H + H 2 + 2e - C loss of water ii) Aldehyde oxidation to carboxylate: RC H RC - + H - + 2e - See V.E. Henrich and P.A. Cox, The Surface Science of etal xides, Cambridge Univ. Press: 1994 for more details on chemisorption on metal oxides

7 7 2. Aqueous Corrosion of etals In water or in vivo, even a passive oxide layer (terminated by bound water) becomes susceptible to corrosion. Why? z+ diffusion will always occur xide may dissolve Corrosion: the destructive result of chemical rxn between a metal or metal alloy and its environment. Aqueous corrosion: involves electronic charge transfer i.e., an electrochemical rxn Typically, metal surface acts as both anode (oxidation=loss of e - ) & cathode (reduction) in different regions anodic rxn: z+ + ze - cathodic rxns: 2 (dissolved) + 4H + + 4e - 2H 2 2H + + 2e - H 2 in acidic 2 (dissolved) + 2H 2 + 4e - 4H - in neutral or basic

8 8 Locally, a biological environment can be either acidic or basic. Basic steps of corrosion reaction: etal z+ 1. etal ions leave surface; surface becomes negatively charged. 2. n+ are attracted back toward surface, establishing a dynamic equilibrium. H + 2e - H + H 2 3. This charged double layer exhibits a characteristic V. e.g., 2H + + 2e H 2 (acidic condns) mechanism: H + + H 2 H 3 + (adsorbed H 2 layer) H e - H ads + H 2 (H stays adsorbed) H ads + H ads H 2 (H ads migrates, combines) Values of V measured relative to a reference electrode (e.g., std. H 2 electrode = SHE), give metric of reactivity in aqueous soln. etal Potential (V) Au 1.43 Pt 1.2 Ag 0.79 H 0.0 Sn Co Fe Cr Al Ti Li anodic emf series (Table 1, p. 261 of text)

9 9 V on Co electrode: Co Co e - a Co =1 Inert reference electrode on Pt electrode: H 2 2H + + 2e - Co a Co 2+ =1 a H + =1 Pt P H2 1atm Semi-permeable membrane A Simple Corrosion Rule: anything that upsets the dynamic equilibrium of the charged double layer can accelerate corrosion. Galvanic Corrosion: if 2 metals in contact where V A > V B (i.e., A more neg./anodic), B becomes an e - sink accelerating corrosion of A B e - A The in e - from A surface allows release of A z+ Can be macroscopic scale or microscopic scale effect! Examples: Plate & screw of different alloys Cr-depleted region at grain boundary (due to carbide formation) Formation of surface oxides or sulfides that conduct e Alloys exhibiting 2 phases