1 3.051J/20.340J Lecture 3: Biomaterials Surfaces: Chemistry Surfaces are high-energy regions of materials and thereby facilitate chemical reactions that influence performance of biomaterials. This lecture will focus on 2 classes of surface chemistry relevant to biomaterials: • Chemisorption on metals and oxides • Aqueous corrosion of metals 1. ChemisorptionStrong modifications to electronic structure/ electron density of adsorbate molecule (> 0.5 eV/surface site) Important Examples: a) Metal Oxide Formation on Metals “metals just wanna be oxides” xM + ½yO2 → MxOy ∆G0 of oxide formation is negative for all but a few metals (e.g., Au) Reaction ∆G0 (joules) T range (K) 2Cr + 3/2 O2 = Cr2O3 -1,120,300 + 260T 298-2100 Fe + ½ O2 = FeO -259,600 + 62.55T 298-1642 2Fe + 3/2 O2 = Fe2O3 -810,520 + 254.0T 298-1460 Ti + O2 = TiO2 -910,000 + 173T 298-2080 from D.R. Gaskell, Intro. To Metallurgical Thermodynamics, McGraw-Hill, 19812 3.051J/20.340J How does metal oxidation happen? One scenario is… step 1: physisorption of O2; ~20-25 kJ/mol 1 eV/molec = 96.5 kJ/mol kT293 ≈ 0.025 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 γCompare: at 1400°C: γδ-Fe = 1900 dyn/cm FeO = 580 dyn/cm Consider metal oxidation as 2 half reactions: O2 + ze- Electrons and ions must traverse the oxide layer for rxn to proceed. Across the oxide film, an oxidation potential, E0 ~ 1V generates an electric field: x~1 nm M Mz+ M → Mz+ + ze -→ ½z O2-∆G0 = − E zF E-Field ≈ 1 V/nm = 10 MV/cm0 Ionic species are “pulled” F= 96,480 C/mol e-through oxide film! 1 J = 1 V-C WHAT HAPPENS AS THE OXIDE CONTINUES TO GROW?3 3.051J/20.340J The E-field decreases. Subsequent oxide growth occurs by thermal diffusion of Mz+ to oxide surface or O2- to metal/oxide interface under the concentration gradient ∆c: 2l = k t p l Requirements for Passivation: i) small kp (rate const) oxide thickness time k = const D ∆cp ii) adherent oxide Oxide layer must not scale or spall ⇒ minimize ∆Vmolar & stress build-up xM + ½yO2 → MxOy ex., Ti (TiO2), Cr (Cr2O3), Al (Al2O3) (Al metal not used in biomaterials applications due to toxicity) MO = xy Pilling-Bedworth ratio: PB = Voxide ( formed ) ρMM Vmetal (consumed ) xM M ρM O xy Want PB ~ 1 (PB > 1) for adherance of oxide to underlying metal—in practice, this rule is marginally predictive, however.• • 4 3.051J/20.340J Other Metal Oxidation Rxns by Chemisorption: Reaction with water: xM + yH2O → MxOy + yH2 Reaction with CO2: xM + yCO2 → MxOy + yCO IS THE FORMED METAL OXIDE SURFACE STABLE? b) Acid/Base (Acceptor/Donor) Rxns on Oxides i) **H2Oadsorbed + O2-lattice→ OH-lattice + OH-surface Ubiquitous! e.g., oxides of Co, Ti, Cr, Fe, etc. H2O cleavage with H+ transfer to surface basic O2-site & OH-coordination with M2+ H H O-H+H •O • O2-M2+ O2-M2+ O2-M2+ O2-M2+ M2+ acts as Lewis acid (e-pair acceptor) for oxygen lone pairs5 3.051J/20.340J 2-ii) CO2,adsorbed + O2-lattice→ CO3 (carbonate formation) Experimentally seen, e.g., on TiO2 (110) iii) w/ Hydrocarbons: Alcohols (similar to HOH): ROHadsorbed + O2-lattice → RO-+ OH--Carboxylic Acids: RCOOHadsorbed + O2 lattice → RCOO-+ OH-c) Redox (Oxidation/Reduction) Reactions on Oxides Example Alcohol dehydrogenation to aldehyde: O RCH2OH + 2O2-→ RCH + 2OH-+ 2e-alcohol is oxidized electrons reduce Mz+ at surface reference: V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides, Cambridge Univ. Press: 19946 3.051J/20.340J 2. Aqueous Corrosion of Metals In water or in vivo, even a “passive” oxide layer (terminated by bound water) becomes susceptible to corrosion. Why? • Mz+ diffusion will always occur • oxide may dissolve • damage to oxide layer 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=consumption of e-) in different regions -anodic rxn: M → Mz+ + ze cathodic rxns: O2 (dissolved) + 4H+ + 4e-→ 2H2O in acidic 2H+ + 2e-→ H2 (g) O2 (dissolved) + 2H2O + 4e-→ 4OH-in neutral or basic 2H2O + 2e-→ H2 (g) + 2OH-Locally, a biological environment can be neutral, acidic or basic.7 3.051J/20.340J Metals in aqueous solution Metal ions leave surface; surface H2 H+ Mz+ 2e-H+ Metal M becomes negatively charged. Mz+ are attracted back toward surface, establishing a dynamic equilibrium. The resulting charged double layer exhibits a characteristic E. Values of Ε measured relative to a reference electrode (e.g., std. H2 electrode = SHE), give metric of reactivity in aqueous soln. on M electrode: -M → Mz+ + ze aM=1 on reference electrode: H2 → 2H+ + 2e-PH2 1atm Inert Pt reference electrode aM z+=1 aH +=1 V Semi-permeable membrane8 3.051J/20.340J emf series (Table 5, p. 432 of text) Standard electrode potentials E0 measured at unit activity (~1 N) anodic of Mz+ ions in soln. with SHE ref. Metal Potential (V) Au 1.43 Pt 1.2 Ag 0.79 H 0.0 Sn -0.14 Mo -0.20 Co -0.28 Fe -0.44 Cr -0.73 Al -1.33 Ti -1.63 Li -3.05 If the activity of Mz+ is less than unity, the electrical potential across the metal/solution interface is modified according to the Nernst equation: EE0 RT ln aoxidized species = + zF areduced species Equating activity to concentration: z+ ] M aoxidized species = [M areduced species = [] = 1 z+ ]( EV ) = E0 + 0.059log[ M z At this potential, the system is in dynamic equilibrium, i.e., equal metal dissolution and deposition rates (the exchange current density).9 3.051J/20.340J 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 -EA > -EB (i.e., A more neg./anodic), B becomes an e-“sink” ⇒ accelerating corrosion of A B + + + A e -The ↓ in e- from A surface allows ↑ release of Az+ 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 • Metal grains of different orientation Fretting Corrosion: