Compendium

Surface and Interface Chemistry

24 min read

Surface and Interface Chemistry

A Tier 2 deep-dive into the chemistry of interfaces — adsorption thermodynamics and isotherms (Langmuir, Freundlich, BET, Temkin, DR), surface area and porosimetry methods (N₂ and Ar physisorption, Hg intrusion), surface energy and wetting (contact angle, hysteresis, Wilhelmy plate), self-assembled monolayers (alkanethiol-Au, silanes on SiO₂, phosphonates, click SAMs), Langmuir-Blodgett films, the surface-analysis toolkit (XPS, AES, UPS, LEED, RHEED, AFM/STM, ToF-SIMS, GIXRD, IR/PM-IRRAS/SFG, ellipsometry, QCM-D), heterogeneous-catalysis surface science (Sabatier principle, BEP scaling, Nørskov microkinetic modeling), corrosion + passivation (Pourbaix, EIS, polarization), electrochemical interfaces (double-layer Stern + GCS, EDL capacitance, pzc), and coatings/thin-film deposition (PVD, CVD, ALD). Surface chemistry is where every macroscopic property of a material gets defined — adhesion, wetting, catalysis, corrosion, sensing, friction — and is central to semiconductors, catalysis, biomedical devices, batteries, and coatings.

See also

Adsorption isotherms

An adsorption isotherm is the equilibrium relationship at constant T between the amount of gas (or solute) adsorbed on a surface and the bulk pressure (or concentration).

Langmuir isotherm

Irving Langmuir 1916-1918 (Nobel 1932; GE Schenectady). Assumes: monolayer coverage, equivalent sites, no lateral interactions, dynamic equilibrium.

θ = (K_eq p) / (1 + K_eq p), where θ = fractional coverage.

Or equivalently n = n_m K p / (1 + K p), where n_m is monolayer capacity.

Linearization: 1/n = 1/n_m + 1/(n_m K p) → plot 1/n vs 1/p; slope = 1/(n_m K), intercept = 1/n_m. Extract n_m and K from low-pressure data; K relates to ΔG_ads via -RT ln K.

Used for: chemisorption with localized sites; dilute-gas adsorption on uniform surfaces. Reality check: real surfaces are heterogeneous, so Langmuir works best at submonolayer with strong site preference.

Freundlich isotherm

Empirical (Freundlich 1909):

n = K_F p^(1/n), with 1/n typically 0.1-1.0.

Linearized log n = log K_F + (1/n) log p. Captures heterogeneous surfaces with continuous distribution of adsorption energies, but has no monolayer plateau (unphysical at high p).

BET isotherm

Brunauer-Emmett-Teller 1938 (J Am Chem Soc 60:309). Multilayer adsorption — extends Langmuir to allow stacking. Assumes monolayer at energy E1, all subsequent layers at liquefaction enthalpy E_L.

p/(n(p₀-p)) = 1/(n_m C) + (C-1)/(n_m C) · (p/p₀)

where p₀ is saturation pressure, C ≈ exp((E1-E_L)/RT). Plot p/(n(p₀-p)) vs p/p₀ — linear range typically 0.05 < p/p₀ < 0.30; slope + intercept → n_m and C.

The basis for BET surface area: A_BET = n_m · N_A · σ, where σ is molecular cross-section (16.2 Ų for N₂ at 77 K; 14.2 Ų for Ar at 87 K). Reported in m²/g.

Temkin isotherm

θ = (RT/E₀) ln(K p)

Assumes adsorption energy decreases linearly with coverage. Used for chemisorption with strong lateral interactions or surface heterogeneity. Applied historically to ammonia synthesis on Fe (Temkin-Pyzhev kinetics).

Dubinin-Radushkevich (DR)

For micropore filling:

ln V = ln V₀ - (RT/E)² (ln(p₀/p))²

Used for microporous adsorbents (activated carbon, zeolites). V₀ = micropore volume; E = characteristic energy (~10-30 kJ/mol).

IUPAC isotherm classification

Type I (Langmuir-like; microporous), Type II (sigmoidal; macroporous + multilayer), Type III (no monolayer plateau; weak adsorbate-adsorbent), Type IV (hysteresis loop; mesoporous), Type V (Type III + hysteresis; mesoporous + weak interaction), Type VI (stepwise; stepwise filling of energetically distinct sites — e.g., Kr on graphite).

Hysteresis types (H1-H4): H1 — narrow, parallel branches (cylindrical mesopores); H2 — broad (ink-bottle); H3 — narrow at low p/p₀ (slit-like); H4 — broad with steep desorption at p/p₀ ~0.4 (cavitation, tensile-strength).

Surface area and porosimetry

Gas physisorption

Standard method for porous-material characterization.

N₂ at 77 K (liquid-nitrogen bath): historical default. Limitations — slow diffusion in narrow micropores (<7 Å) because N₂ molecules barely fit. σ(N₂) = 16.2 Ų.

Ar at 87 K (liquid-Ar): preferred for microporous zeolites and MOFs. Smaller quadrupole moment than N₂ → less specific interaction with charged surfaces. σ(Ar) = 14.2 Ų. Faster equilibration in micropores.

CO₂ at 273 K: ultra-micropore probe (<7 Å); fast diffusion at low p/p₀.

Instruments: Micromeritics ASAP 2020, 3Flex, TriStar II; Quantachrome Autosorb iQ, Nova; Anton Paar Autosorb 1MP; BELSORP-max (Microtrac MRB).

NLDFT and QSDFT pore-size analysis

Non-local density functional theory and quenched solid DFT — replace classical Kelvin-equation pore-size analysis. Provide accurate distributions for cylindrical, slit, spherical pore models in slits 4-50 nm. Native in instrument software (MicroActive, ASiQwin, BELMaster).

Mercury intrusion porosimetry

Hg forced into pores under pressure; non-wetting (θ ~140° on most oxides). Washburn equation: r = -2γ cos θ / P. Pressure 0.01-413 MPa probes pores 3 nm to 300 µm. Micromeritics AutoPore IV/V; Quantachrome Poremaster. Used for macro+meso pore characterization of catalysts, soil, ceramics, paper.

Caveat: Hg is toxic (regulated under REACH; many labs phasing out). Replacement methods (water intrusion, He pycnometry, CT scan, NLDFT) increasingly used.

t-plot and αs methods

t-plot (de Boer-Lippens 1964) and αs (Sing) — analyze multilayer thickness vs reference isotherm → distinguish micropore + external surface contributions. Standard processing on BET-N₂ data.

Surface energy and wetting

Young equation

γ_SV = γ_SL + γ_LV cos θ

where γ_SV, γ_SL, γ_LV are solid-vapor, solid-liquid, liquid-vapor interfacial tensions, and θ is the contact angle of a liquid drop on the solid surface. Static contact angle (sessile drop).

Wetting:

  • θ < 90° — wetting (hydrophilic for water).
  • θ > 90° — non-wetting (hydrophobic).
  • θ ~150-170° — superhydrophobic (lotus effect; rose petal effect).
  • θ → 0° — complete wetting.

Surface energy of solid γ_S can be decomposed (Owens-Wendt-Rabel-Kaelble; OWRK) into dispersive (γ_S^d) and polar (γ_S^p) components via two-liquid contact-angle measurement (water + diiodomethane standard).

Contact-angle hysteresis

Advancing θ_A and receding θ_R differ on real surfaces — hysteresis (θ_A - θ_R) reflects surface roughness, chemical heterogeneity, and contamination. Wenzel (1936) and Cassie-Baxter (1944) models describe rough surfaces:

  • Wenzel (homogeneous): cos θ_W = r cos θ_Y, where r is roughness factor. Roughness amplifies whatever wetting tendency exists.
  • Cassie-Baxter (heterogeneous; air pockets): cos θ_CB = f cos θ_Y + (f-1), where f is fraction of solid in contact. Air pockets dramatically increase apparent θ → superhydrophobicity.

Measurement

  • Sessile drop: deposit drop, image with goniometer; static, advancing, receding via volume addition/withdrawal. Krüss DSA100, Dataphysics OCA, Biolin Theta.
  • Wilhelmy plate: dip plate into liquid; measure force on balance → surface tension from γ = F/(p cos θ). Used in Langmuir trough for surface-pressure measurements.
  • Captive bubble: bubble at submerged surface; advantage for low-θ surfaces.
  • Tilting plate, drop-shape analysis: alternatives.

Surface tension and surfactants

Liquid-liquid and liquid-vapor surface tension γ_LV measured via Wilhelmy plate, du Noüy ring, pendant drop, spinning drop, maximum bubble pressure. Surfactants (SDS, CTAB, Triton X-100, PEG-fatty-acid esters) lower γ_LV by adsorbing at surface; critical micelle concentration (CMC) marks bulk micelle formation.

Self-assembled monolayers (SAMs)

Ordered organic monolayers on a substrate, assembled spontaneously from solution or vapor through anchor-group chemisorption. Whitesides (Harvard) and Nuzzo (Illinois) seminal work — alkanethiols on Au 1980s.

Alkanethiols on gold

Au(111) + HS-R → Au-SR (gold-thiolate). Anchor: S-Au bond ~40 kcal/mol; tilt angle ~30° from surface normal; molecular area 21.6 Ų/molecule; standing-up phase from upright self-assembly. Bare surface (cleaned by piranha or O₂ plasma) → 1 mM thiol in EtOH overnight → rinse → ready.

Standard thiols:

  • 1-dodecanethiol (C12; CAS 112-55-0, Sigma 471364). Hydrophobic test SAM.
  • 1-octadecanethiol (C18; CAS 2885-00-9, Sigma O1858). Higher hydrophobicity.
  • 11-mercaptoundecanoic acid (CAS 71310-21-9, Sigma 450561). Terminal -COOH; pH-responsive.
  • 11-mercapto-1-undecanol (CAS 73768-94-2, Sigma 447528). Terminal -OH; hydrophilic, anti-fouling.
  • HS-(CH₂)₁₁-EG₆-OH (oligo-ethylene-glycol terminated; Sigma 712574; ProChimia FT 037). Resists non-specific protein adsorption — basis for SPR/QCM biosensor matrices.

Mixed SAMs (binary thiols) tune wettability and functional density.

Silanes on SiO₂ / glass

Hydrolysis-condensation chemistry. Si-OH surface + (RO)₃-Si-R’ → (Si-O)₃-Si-R’ + 3 R-OH. Standard agents:

  • APTES (3-aminopropyltriethoxysilane; CAS 919-30-2; Sigma 440140). Amine-terminated; couples carboxylates, aldehydes, NHS-esters, biotin-NHS; biotemplating, DNA microarrays, biosensors.
  • APTMS (trimethoxy analog; CAS 13822-56-5; Sigma 281778). Faster hydrolysis; smaller leaving group.
  • GOPTS / GPS (3-glycidoxypropyltrimethoxysilane; CAS 2530-83-8; Sigma 440167). Epoxide-terminated; couples amines, thiols nucleophilically.
  • MPTMS (3-mercaptopropyltrimethoxysilane; CAS 4420-74-0; Sigma 175617). Thiol-terminated; tethers Au nanoparticles, organic thiol-ene click.
  • OTS (octadecyltrichlorosilane; CAS 112-04-9; Sigma 104817). C18 hydrophobic; classical hydrophobic glass coating; cleanroom-grade quality essential.
  • TMS (trimethylchlorosilane; CAS 75-77-4; Sigma 386529) and HMDS (hexamethyldisilazane; CAS 999-97-3; Sigma 379212). Adhesion promoter in photoresist process; deposits methylated surface.
  • Perfluorinated silanes (FDTS — 1H,1H,2H,2H-perfluorodecyltrichlorosilane; CAS 78560-44-8). Lowest-energy surfaces; used in MEMS anti-stiction, microfluidics. Strem 14-0570 and Sigma 448931.

Procedure: glass / Si wafer cleaned (piranha or O₂ plasma) → 1% silane in toluene or dry vapor → 1-12 h → rinse + bake 100-120 °C for cure (silanol-silanol condensation densifies monolayer).

Phosphonate SAMs

R-PO(OH)₂ on TiO₂, Al₂O₃, ITO, mica, ZrO₂, hydroxyapatite. T-BAG (tethering by aggregation and growth; Hanson-Schwartz-Bernasek 2003) — load surface from solvent of phosphonic acid, then heat to drive Si-O-P or M-O-P condensation. More stable on oxides than silanes in many cases; ALD-compatible.

  • Octadecylphosphonic acid (ODPA; CAS 4724-47-4, Sigma 715166).
  • Hexadecylphosphonic acid (CAS 4753-99-7).
  • (11-azidoundecyl)phosphonic acid — for click chemistry coupling.

Click chemistry SAMs

CuAAC (Cu-catalyzed azide-alkyne; Sharpless-Meldal-Tornøe; Nobel 2022) post-functionalization. Azide-terminated SAM + R-alkyne + CuSO₄/sodium ascorbate → 1,2,3-triazole linkage to surface. SPAAC (strain-promoted; BCN, DBCO) avoids Cu when needed for biology. Used in protein arrays, glycan microarrays, DNA-coated surfaces.

SAM characterization

  • Contact angle — surface wettability indicator.
  • Ellipsometry — film thickness ±0.1 nm.
  • XPS — elemental composition + bonding chemistry.
  • RAIR / PM-IRRAS — molecular orientation, conformation.
  • AFM — topography, defects.
  • STM — atomic resolution on conductive SAMs (alkanethiol/Au).
  • QCM-D — frequency + dissipation during assembly.
  • CV / EIS — defect density via redox probe (Ru(NH₃)₆³⁺) electron transfer through SAM.

Langmuir-Blodgett films

Insoluble amphiphile spread on water → compress with movable barrier → measure π-A isotherm (surface pressure vs molecular area; Wilhelmy plate). Transfer to solid substrate by vertical dipping (Y, X, Z deposition types depending on geometry).

Langmuir trough

Pockels (1891) original; Langmuir trough as Nobel-prize-winning instrument. Modern: KSV NIMA (Biolin Scientific), KSV Mini-, Mini-PMI; Nima 312D; Krüss LB-100.

π-A isotherm

Compress monolayer of stearic acid, oleic acid, fatty alcohol, phospholipid (DPPC, DPPE). Phases:

  • Gaseous (G) — A > 200 Ų/molecule, π ~0.
  • Liquid-expanded (LE) — A 60-200 Ų, π 1-15 mN/m.
  • Liquid-condensed (LC) — A 25-60 Ų, π 15-40 mN/m.
  • Solid (S) — A < 25 Ų, π > 30 mN/m.
  • Collapse at high π (often ~40-70 mN/m) — film buckles/squeezes out 3D structures.

Y, X, Z deposition

  • Y-type — monolayer deposited on both up-stroke and down-stroke; tail-tail, head-head bilayer stacking. Most common.
  • X-type — only on down-stroke (substrate enters hydrophobic-down); tail-head ordering.
  • Z-type — only on up-stroke (substrate exits hydrophilic-up); head-tail ordering.

Applications

Historical: LB-multilayers as model membranes (DPPC, DPPS bilayers). Photosynthetic mimics (chlorophyll-LB). Phthalocyanine-LB for gas sensors. Diacetylene-LB photopolymerized to conjugated polymer (Tieke-Wegner). Modern use modest; superseded for many purposes by self-assembled monolayers and dip-coating polyelectrolyte multilayers (LbL — Decher 1991).

Surface-analysis toolkit

X-ray Photoelectron Spectroscopy (XPS / ESCA)

Siegbahn (Uppsala; Nobel 1981) ESCA — Electron Spectroscopy for Chemical Analysis. Mg Kα (1253.6 eV) or Al Kα (1486.6 eV) X-ray ionizes core electrons; measure kinetic energy → binding energy E_B = hν - E_K - φ.

Surface sensitivity: inelastic mean free path λ ≈ 1-3 nm → top 3-10 nm probed at normal emission. Angle-resolved XPS (ARXPS) → depth profile by tilting sample.

Information:

  • Elemental composition (~0.1 at% sensitivity except H and He which are invisible).
  • Chemical state from binding-energy shifts (e.g., C 1s: 285.0 aliphatic, 286.5 C-O, 288.0 C=O, 289.3 -COOR; N 1s: 399.5 amine, 400.0 amide, 402 -NH₃⁺ / -NO₂; O 1s: 530 oxide, 532 carbonyl/hydroxyl).
  • Quantitative composition via Scofield cross-sections + transmission function.
  • Spatial mapping (~10-100 µm spot).

Instruments: Thermo Fisher K-Alpha, Nexsa; PHI VersaProbe; Kratos Axis Supra, AXIS Ultra; SPECS Phoibos; JEOL JPS-9200. Monochromated Al Kα standard.

Auger Electron Spectroscopy (AES)

Electron-beam excitation → Auger emission (core-hole filling + ejection of valence electron). Spatial resolution sub-µm; sensitive to surface (top 1-3 nm). Used for micro-area composition mapping, light elements, conductive samples. JEOL JAMP-9500F, PHI 700Xi, Bruker Quantax Auger. Often combined with SEM (Auger-SEM systems).

Ultraviolet Photoelectron Spectroscopy (UPS)

He I (21.2 eV) or He II (40.8 eV) photons → ionize valence electrons. Probes valence band structure, HOMO position, work function. Used heavily for OLED materials, organic semiconductors, surface dipole studies. Same instruments as XPS with He lamp.

LEED — Low-Energy Electron Diffraction

20-300 eV electrons elastically backscattered from periodic surface → diffraction pattern on phosphor screen. Probes surface periodicity, reconstructions (Si(100) 2×1, Au(111) herringbone). I-V analysis recovers atomic positions. Workhorse of surface science since Davisson-Germer 1927.

RHEED — Reflection High-Energy Electron Diffraction

10-30 keV electrons at grazing incidence. Streaks vs spots distinguish flat vs rough. Real-time monitor during epitaxial growth (MBE, sputtering); RHEED oscillations track monolayer growth periods.

AFM and STM

Cross-link characterization-methods for atomic-force and scanning-tunneling microscopy detail. Both invented 1980s (Binnig-Quate-Gerber AFM 1986; Binnig-Rohrer STM 1981, Nobel 1986). AFM: contact, tapping/intermittent, non-contact modes; PeakForce-QNM (Bruker) for mechanical property mapping; KPFM Kelvin probe for work function; conductive AFM (c-AFM) for electrical mapping.

STM: atomic resolution on conductive surfaces; spectroscopic STM (STS, dI/dV) measures local DOS; molecular manipulation (Eigler 1990 IBM 35-atom logo).

Instruments: Bruker Dimension Icon, MultiMode, BioScope; Asylum Research Cypher, MFP-3D; Park Systems NX-Hivac; Nanosurf Flex; Omicron, Createc UHV-STM.

ToF-SIMS — Time-of-Flight Secondary Ion Mass Spectrometry

Primary ion beam (Bi₃⁺, Ar gas cluster, C₆₀⁺, O₂⁺) sputters surface; secondary ions analyzed by ToF mass spec. Surface sensitivity top 1-2 nm; sub-µm spatial resolution; mass resolution m/Δm > 10,000. Detects molecular fragments + elemental. Depth-profile mode (dual-beam) reveals composition vs depth.

IONTOF M6, ION-TOF V; PHI nanoTOF 3. Used heavily in semiconductor failure analysis, polymer surface analysis, biomaterial characterization, drug-formulation mapping.

GIXRD / GIWAXS — Grazing-Incidence X-ray Diffraction

X-ray incident at low angle (<1°) → reduce penetration depth; probe top tens of nm. GIWAXS for thin polymer films, organic semiconductors; GISAXS for nanoscale morphology (block-copolymer self-assembly). Standard at synchrotron beamlines (NSLS-II, ESRF, Diamond, APS); benchtop Anton Paar SAXSpoint, Xenocs Xeuss, Bruker D8 Discover.

Infrared methods at surfaces

  • ATR-FTIR (attenuated total reflectance) — Ge or diamond crystal; evanescent wave penetrates ~0.5-5 µm into sample. Routine surface IR. Bruker Tensor, Thermo Nicolet, PerkinElmer Spectrum.
  • RAIR / RAIRS (reflection-absorption IR) — grazing incidence; polarization-modulation (PM-IRRAS) discriminates surface molecules from bulk gas. Standard for SAM characterization, adsorbed species on metals. Bruker Vertex 80v, Thermo Nicolet iS50.
  • DRIFTS (diffuse-reflectance FTIR) — powder catalysts in situ; cell heated + with gas flow. Harrick, Pike Technologies cells.
  • In situ / operando IR catalyst cells — Praying mantis, Spectra-Tech.

Sum-frequency generation (SFG) and second-harmonic generation (SHG)

Non-linear optical methods inherently interface-selective (forbidden in centrosymmetric bulk, allowed at interface where symmetry breaks). SFG probes vibrational modes of molecules at interfaces with sub-monolayer sensitivity. Used in catalysis (CO on Pt under reaction), electrochemical interfaces (water structure at electrode), polymer surfaces, lipid membranes. Shen-Somorjai pioneered.

Ellipsometry

Measure change in polarization (ψ, Δ) of light reflected from thin film → film thickness + complex refractive index. Sub-Å thickness resolution on uniform films. Spectroscopic ellipsometry (SE, 200-1700 nm) extracts optical constants n(λ), k(λ).

J.A. Woollam M-2000, VASE; Horiba MM-16, UVISEL; Sentech SE; Semilab SE-2000. Used heavily in semiconductor metrology (oxide thickness), photovoltaics, OLED, biological monolayers.

QCM and QCM-D — Quartz Crystal Microbalance

AT-cut quartz oscillator at ~5-10 MHz; Sauerbrey equation Δf = -(2 f₀²/A √(ρ_q μ_q)) Δm relates frequency shift to mass added on electrode (~0.1 ng/cm² sensitivity in air). In liquid: also measure dissipation D (energy loss per oscillation) → distinguishes rigid (low D) vs soft/viscoelastic (high D) films. Biolin Scientific QSense Explorer, Pro, Omni standard; Stanford Research Systems QCM200 for simple gas-phase.

Applications: protein adsorption kinetics, SAM assembly real-time, biofilm growth, polymer hydration, drug-membrane interaction.

Heterogeneous catalysis surface science

Sabatier principle

Optimum catalysis at intermediate adsorbate binding strength — too weak: no activation; too strong: no product release. Volcano plots correlate catalyst activity with descriptors (M-H, M-O, M-CO binding energy). Sabatier 1911 (Nobel 1912).

Brønsted-Evans-Polanyi (BEP) relations

Activation energy linearly correlates with reaction enthalpy across a series of related catalysts:

E_a = α ΔH_rxn + β

α ≈ 0.5-0.9 typical. BEP relations enable scaling-up from DFT-computed thermochemistry to rate predictions without computing every TS individually. Nørskov-Bligaard-Jaramillo et al. 2005-2015 codified for ammonia synthesis, HER, OER, CO₂ reduction.

Scaling relations

DFT binding energies of related adsorbates (CHO, CH, CO, OH; or *OH,*OOH, *O on OER) correlate linearly. Scaling-relation breaking is the path to better catalysts than the volcano predicts. Nørskov-Studt-Bligaard Fundamental Concepts in Heterogeneous Catalysis (Wiley 2014) — foundational.

Microkinetic modeling

Elementary steps with rate constants from DFT + transition-state theory + frequency calculation. Solve coupled ODEs → site coverage and turnover at conditions. CatMAP (Medford-Nørskov-Bligaard, Stanford); MKMCXX (Filot-Hensen Eindhoven); Cantera (Goodwin Caltech; broader scope including homogeneous combustion).

DFT for catalyst design

Periodic slab models (3-5 atomic layers; fix bottom layers; PBC). Adsorbate binding energies + zero-point energy + thermal corrections → free energy diagrams. Functional choice: PBE common despite known errors; revPBE or BEEF-vdW for better adsorption energies; HSE06 for semiconductor surfaces. Cross-link computational-chemistry-deep.

Production catalysts designed via DFT screening: HEAs for ammonia synthesis (Ulrich Mortensen), HER alternatives (MoS₂ Hinnemann-Nørskov 2005), OER electrocatalysts (Halck-Nørskov, Mefford-Boettcher), CO₂RR (Kanan oxide-derived Cu, Sargent gas-diffusion-electrode systems).

Corrosion and passivation

Cross-link to corrosion section of electrochemistry and electrochemistry-energy-storage. Surface chemistry of corrosion: anodic dissolution + cathodic reduction + passive-film growth.

Pourbaix diagrams (E-pH)

Marcel Pourbaix 1945 (CEBELCOR Brussels). Map of dominant species in E-pH plane at fixed temperature and ion activity. Identify immune (no corrosion), passive (oxide-protected), and active (corroding) regions. Standard reference: Atlas of Electrochemical Equilibria in Aqueous Solutions (NACE/CEBELCOR 1974). Fe Pourbaix: passive Fe₂O₃/Fe₃O₄ at high pH; passive in alkaline; active in acid; immune at very low E.

Passivation

Spontaneous formation of dense oxide film that blocks further oxidation. Cr-passivation of stainless steel — Cr₂O₃/Cr(OH)₃ a few nm thick; self-healing in air. Al-passivation — Al₂O₃ amorphous oxide 2-4 nm spontaneous (thicker via anodization). Ti, Nb, Ta — passivation key to medical implant biocompatibility.

Breakdown by Cl⁻ (pitting): chloride penetrates passive film at defects → autocatalytic pit growth. Pitting resistance equivalent number PREN = %Cr + 3.3 × %Mo + 16 × %N — quantitative comparison across stainless grades.

Electrochemical impedance spectroscopy (EIS) for corrosion

Equivalent circuit: R_s (electrolyte) + parallel (R_ct, C_dl). Polarization resistance R_p inversely proportional to corrosion rate (Stern-Geary). EIS Bode/Nyquist plots distinguish charge-transfer vs diffusion limitations. Cross-link electrochemistry-energy-storage EIS section for instrument detail.

Polarization curves

Potentiodynamic sweep ±200 mV around E_corr → Tafel slopes b_a, b_c; extrapolate to E_corr → corrosion current density i_corr → corrosion rate via Faraday:

CR (mm/yr) = (i_corr × M × 3.27) / (n × ρ)

ASTM G5, G59, G61 standard procedures. Solartron, Gamry, BioLogic potentiostats standard.

Salt spray and accelerated tests

ASTM B117 (neutral salt spray), ASTM G85 (cyclic), automotive J2334 — accelerated outdoor exposure equivalents. Pass criteria for coatings, fasteners, painted parts.

Atmospheric corrosion

ISO 9223-9226 classification CX-C5 by atmospheric corrosivity (rural vs marine vs industrial). Cor-Ten weathering steel (USS 1933) develops protective rust patina in atmospheric exposure — used in bridges (Tacoma, Chicago) and architecture (Pompidou Centre).

Electrochemical interfaces

Electric double layer

Helmholtz (1879) — capacitor-like charged plane facing solution. Gouy-Chapman (1910-1913) — diffuse cloud, Boltzmann distribution; thickness Debye length 1/κ = √(ε_r ε_0 RT / (2 F² I)) where I is ionic strength.

Stern (1924) — combined model: inner (compact) Helmholtz layer + outer diffuse Gouy-Chapman layer.

Differential capacitance

C_d = ∂σ/∂E (charge density vs potential). Direct measurement via AC impedance at high frequency. Bell-shaped curve with minimum at the potential of zero charge (pzc) — analytically identifies pzc.

Specific adsorption

Anions (Cl⁻, Br⁻, I⁻, SO₄²⁻) often specifically adsorb to metal electrode beyond outer Helmholtz — alters double-layer structure. Cl⁻ on Au promotes underpotential deposition of metals (Cu, Ag).

Stark-Einstein / Gouy-Chapman-Stern (GCS) for biological interfaces

Lipid bilayer membrane potential, ion adsorption to colloids and cells follows similar treatment. Boltzmann distribution + Poisson equation → numerical PB (Poisson-Boltzmann) solvers (APBS, DelPhi) standard in biomolecular electrostatics.

EDL capacitance in supercapacitors

Cross-link electrochemistry-energy-storage supercapacitor section. Ionic-liquid double layers, microporous-carbon ionic confinement, “superionic state” (Kornyshev) effects deviate from classical GCS in narrow pores.

Coatings and thin films

Physical Vapor Deposition (PVD)

  • Sputtering — Ar⁺ plasma bombards target; ejected atoms condense on substrate. DC sputtering (conductive targets), RF sputtering (insulating targets — oxides, nitrides), magnetron sputtering (magnetic field traps electrons → higher rate), HiPIMS (high-power impulse, dense plasma, high adhesion). Vendors: AJA International, Kurt J. Lesker, Angstrom Engineering, Denton Vacuum.
  • Thermal evaporation — resistive heating of source (Joule). Used for Ag, Au, Cu, Al, organics (small-molecule OLED). Edwards Auto306, Kurt Lesker NANO, MBraun.
  • E-beam evaporation — focused electron beam melts source in crucible. Refractory metals (Ti, Mo, W, Ta), oxides (SiO₂, Al₂O₃, HfO₂).
  • PLD (pulsed laser deposition) — KrF or Nd:YAG laser ablates target; plume condenses on substrate. Complex oxides (cuprates, manganites, ferroelectrics) — Twente PLD, Neocera.
  • MBE (molecular beam epitaxy) — UHV; effusion cells; atomically precise epitaxy. III-V semiconductors (GaAs/AlGaAs), 2D materials, oxide MBE. Veeco GEN10, Riber 200.

Chemical Vapor Deposition (CVD)

Gas-phase reaction at heated surface deposits solid film. Variants: APCVD (atmospheric), LPCVD (low-pressure), PECVD (plasma-enhanced; lower T), MOCVD (metal-organic; semiconductor heterostructures), HWCVD (hot wire), ICP-CVD.

Standard films:

  • Polysilicon — SiH₄ at 580-650 °C (LPCVD).
  • Si₃N₄ — SiH₂Cl₂ + NH₃ (LPCVD) or SiH₄ + NH₃ (PECVD).
  • SiO₂ — TEOS + O₃ (sub-atmospheric); SiH₄ + N₂O (PECVD).
  • Graphene — CH₄ on Cu foil at 1000 °C (LPCVD); 6”-200” sheets routine.
  • Diamond — CH₄/H₂ microwave plasma; CVD-grown gemstones and industrial.
  • TiN, TaN, WN — diffusion barriers via MOCVD precursors (TDMAT, PEMAT).

Atomic Layer Deposition (ALD)

Tuomo Suntola (Finland) 1974 — invented as ALE atomic layer epitaxy at Lohja Corp; modernized as ALD. Self-limiting sequential surface reactions:

A-pulse (precursor A, e.g., trimethylaluminum (TMA) for Al) → react with surface OH → form Al-CH₃ + CH₄. Purge. B-pulse (H₂O) → react with surface Al-CH₃ → re-create Al-OH + CH₄. Purge.

Net: monolayer Al₂O₃ per cycle (~1.0-1.2 Å); pinhole-free, conformal in deep trenches and porous 3D structures, sub-Å thickness control.

Standard films:

  • Al₂O₃ — TMA + H₂O (or O₃). Encapsulation, MOSFET high-κ gate (now HfO₂).
  • HfO₂ — HfCl₄ or TEMAH + H₂O. Logic-transistor high-κ since Intel 45 nm node (2007).
  • ZnO, ZrO₂, TiO₂ — various precursors. PV, catalysis.
  • Pt, Ru, Ni metals — MeCpPtMe₃ + O₃/H₂.
  • TiN, TaN, WN — for barriers and electrodes.

Vendors: Beneq, Picosun (now Picosun-Applied Materials), Veeco Fiji and Savannah, Cambridge NanoTech (now Veeco), Ultratech. Throughput limited by sequential dosing — high-volume manufacturing uses spatial ALD (different precursors at different regions, substrate moves).

Sol-gel coatings

Metal alkoxide hydrolysis-condensation (M(OR)₄ + H₂O → M-OH → M-O-M) at low T → glass-like oxide films/networks. Tetraethyl orthosilicate (TEOS, Si(OEt)₄), Ti(OPr-i)₄, Zr(OPr-n)₄. Used for AR (anti-reflection) coatings (eyeglasses, solar glass), self-cleaning TiO₂ on glass (Pilkington Activ), hard-coat scratch-resistant.

Polyelectrolyte multilayers (LbL)

Decher 1991 Macromol Chem Macromol Symp — sequential dipping of substrate alternately into cationic (PDDA, PAH, polylysine) and anionic (PSS, PAA, alginate) polymer solutions builds bilayer-by-bilayer film. Cheap, scalable, additive (functional polymers, particles, biomolecules layered in). Used in drug-delivery capsules (Möhwald), corrosion protection, membranes, antibacterial coatings.

Spray and dip coatings

Industrial scale — spin-coating in microfabrication; spray-pyrolysis for transparent conductive oxides (ITO, FTO, AZO); roll-to-roll for flexible electronics, PV panels, food packaging.

Anodization

Electrochemical oxide growth on Al, Ti, Ta. Al anodization in H₂SO₄ or oxalic acid → ordered nanoporous Al₂O₃ (anodic alumina; AAO) with cylindrical pores 20-300 nm — template for nanowire arrays, nanofilters. Ti anodization → colorful oxide layers (used in titanium jewelry, medical implants).

Industrial applications

Semiconductor manufacturing

Surface and interface chemistry is the foundation of CMOS:

  • Gate oxide / high-κ: SiO₂ → HfO₂ at 45-nm node (Intel 2007); now HfSiON, HfZrO₂, Al/Hf doped at advanced nodes. ALD-grown.
  • Diffusion barriers: TaN, TiN, WN — ALD or PVD.
  • Interconnect metallization: Cu damascene with Ta/TaN barrier + Cu seed (PVD) + Cu electroplating fill.
  • Photoresist surface preparation: HMDS adhesion promoter (Sigma 379212; CAS 999-97-3) vapor-prime before resist coat.
  • Surface cleaning: RCA1 (NH₄OH/H₂O₂/H₂O 1:1:5) for organic; RCA2 (HCl/H₂O₂/H₂O 1:1:6) for metals; HF dip for native oxide.

Biomaterial surfaces

  • PEGylation of implants and drug carriers — surface-grafted PEG resists protein adsorption (“stealth” surfaces).
  • Bioactive coatings on Ti implants — hydroxyapatite (HA) plasma-spray or sol-gel; RGD peptide-grafted SAMs for osseointegration.
  • Antifouling marine coatings — Cu-based biocides (regulated), silicone-based fouling-release (SeaSpeed; Hempel; AkzoNobel Intersleek), zwitterionic poly(sulfobetaine).
  • Drug-eluting stents — polymer coating (PLGA, PVDF, parylene) with rapamycin / paclitaxel.

Catalyst surface chemistry

Cross-link catalyst-instrumentation-and-monomers. Industrial heterogeneous catalysts characterized by BET surface area (m²/g), pore-size distribution, dispersion (active-site density), and operando spectroscopy. Pt/Al₂O₃ reforming, V₂O₅/TiO₂ SCR, Fe-K-Al₂O₃ ammonia synthesis, Cu/ZnO/Al₂O₃ methanol synthesis, Co/SiO₂ Fischer-Tropsch.

Battery interface chemistry

Cross-link electrochemistry-energy-storage SEI/CEI section. Solid-electrolyte interphase on graphite anode, cathode-electrolyte interphase on NMC — both surface chemistry challenges at active/electrolyte interface. XPS depth profiling + ToF-SIMS + cryo-EM characterize SEI mosaic.

Sensors

Surface-functionalized electrodes underpin glucose meters, lateral-flow assays, biosensors. SAM-modified Au or screen-printed C electrodes + biorecognition layer + electrochemical readout. Cross-link electrochemistry-energy-storage sensor section.

Anti-corrosion and protective coatings

Industrial sector ~$30 B/yr globally. Epoxy primers (epichlorohydrin-bisphenol-A + amine cure) over phosphate conversion coatings + topcoat polyurethane/polyester. Cathodic e-coat for automotive bodies (PPG, BASF, Axalta, Henkel suppliers).

Workflows

Characterize a new catalyst material

  1. BET surface area + N₂ physisorption isotherm → m²/g + pore-size distribution.
  2. XRD (Rietveld) → bulk phase and crystallinity.
  3. XPS → surface composition + oxidation states.
  4. HAADF-STEM + EDX → atomic-resolution morphology + elemental map.
  5. TPR, TPD, TPO (temperature-programmed reduction / desorption / oxidation) → active-site reducibility + adsorbate desorption energies. Micromeritics AutoChem, Quantachrome ChemBET.
  6. DRIFTS or operando IR with probe molecule (CO, NO, pyridine) → surface acid/base site characterization.
  7. Catalytic test rig (fixed-bed micro-reactor) → activity + selectivity + stability under realistic conditions.
  8. DFT model → confirm/refute mechanism + suggest variations.

Build a SAM-based biosensor

  1. Substrate prep: Au-coated glass slide (template-stripped or PV-deposited); ozone/UV clean; immerse in piranha or 1:1 H₂SO₄/30% H₂O₂ (caution — strongly oxidizing).
  2. Self-assembly: 1 mM thiol mix (e.g., 1:9 11-MUA : 1-mercaptoundecanol) in EtOH, 18 h, RT.
  3. Activation: 11-MUA -COOH → NHS ester via EDC + sulfo-NHS (200 mM/50 mM in MES pH 6.0, 15 min).
  4. Coupling biorecognition element: antibody, aptamer, lectin, MIP, or DNA in PBS — 30-60 min.
  5. Blocking: ethanolamine pH 8.5 quench remaining NHS; BSA, casein for nonspecific blocking.
  6. Characterization: SPR (Biacore, Reichert), QCM-D, EIS, fluorescence to confirm functionalization.
  7. Test analyte response: dose-response curves, LOD, selectivity vs interferents.

Wettability tuning via plasma treatment

  1. Hydrophobic polymer (PDMS, PE, PP) baseline: θ ~100-110°.
  2. O₂ plasma (Harrick PDC-32G, Diener Pico, Plasma Etch PE-50; 30-100 W, 30-180 s): introduce -OH, -COOH → θ < 20° (hydrophilic).
  3. Hydrophobic recovery time-dependent (mins-hours in air) as low-MW oligomers migrate to surface; counteract with grafting or wet chemistry overlay.
  4. F-plasma (CF₄, SF₆): introduce fluorinated surface → θ > 120° superhydrophobic when combined with roughening.

Adjacent

Further reading

  • Somorjai, G.A., Li, Y. — Introduction to Surface Chemistry and Catalysis, 2nd ed. Wiley 2010 — canonical textbook from a founding figure.
  • Atkins, P.W., de Paula, J. — Physical Chemistry, 11th ed. Oxford 2018 — surface and interface chapters.
  • Adamson, A.W., Gast, A.P. — Physical Chemistry of Surfaces, 6th ed. Wiley 1997 — comprehensive classical reference.
  • Israelachvili, J.N. — Intermolecular and Surface Forces, 3rd ed. Academic 2011 — foundational forces and interfaces.
  • Anslyn, E.V., Dougherty, D.A. — Modern Physical Organic Chemistry. University Science Books 2006 — non-covalent interactions at interfaces.
  • Crabtree, R.H. — The Organometallic Chemistry of the Transition Metals, 7th ed. Wiley 2019 — homogeneous-heterogeneous bridge for organometallic surface species.
  • Smith, M.B. — March’s Advanced Organic Chemistry, 8th ed. Wiley 2020 — surface-modification organic chemistry context.
  • Frenking, G., Shaik, S. — The Chemical Bond Across the Periodic Table. Wiley-VCH 2014 — bonding interpretation at surfaces (NBO, ELF, NCI).
  • Nørskov, J.K., Studt, F., Abild-Pedersen, F., Bligaard, T. — Fundamental Concepts in Heterogeneous Catalysis. Wiley 2014 — scaling relations, BEP, microkinetic modeling.
  • Hartman, M., ed. — XPS Handbook (Thermo Scientific 2023 edition) — practical XPS line tables.
  • Briggs, D., Grant, J.T., eds. — Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy. IM Publications 2003.
  • Schreiber, F. — “Structure and growth of self-assembling monolayers” Prog Surf Sci 2000, 65:151 — review.
  • Love, J.C., Estroff, L.A., Kriebel, J.K., Nuzzo, R.G., Whitesides, G.M. — “Self-assembled monolayers of thiolates on metals as a form of nanotechnology” Chem Rev 2005, 105:1103.
  • Stryer, L., Berg, J.M., Tymoczko, J.L. — Biochemistry, 9th ed. W.H. Freeman 2019 — for biological interfaces (membrane chemistry).

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