Conformation Of Adsorbates

CONFIGURATIONAL MODEL OF POLYMER ADSORPTION

AND

ADSORPTION OF THIOL DERIVATIVES ON GOLD

Maxwell’s demon

Configurations of tetramer and dimer adsorbed at interface. The solvent is a monomer. All segments are assigned positions in a close-packed hexagonal lattice.

1 2 3 4 5

54 36 48 36 24

6 7 8 9 10 11

24 36 24 36 27 36

12 13 14 15 16 17

48 36 36 24 36 27

1 2 3

5 3 3

config

ω

config

ω

config

ω

J. Bladon, et al, J. Colloid & InterfaceSci., 52, 14 (1975)

Statistical Thermodynamics Used to Calculate Equilibrium Adsorption

State equations for free energy,

Chemical potential of polymer in bulk solution and at or near surface,

XA = - 4 kcal/molXB = XC = 0EAA = EBB = ECC = EAB = EBC = EAC = 0

CASE 1

A B B B C

J. Bladon, et al, J. Colloid & InterfaceSci., 52, 14 (1975)

Initial Slope =

is the ratio of all possible configurations with the end group at the interface to all the configurations in the bulk.

Adsorption curve

dimer

tetramer

Adsorption curve for each configuration of the tetramer

-XA / kNT

CASE 1, con’t…

XA = - 4 kcal/mol

All other interactions are zero

ADSORBATE MOLECULES IN THE INITIAL SLOPE WILL CONSIST OF ALL NINE CONFIGURATIONS WITH THE END GROUP IN CONTACT WITH THE SURFACE

# SEGMENTS IN POLYMER

is the ratio of all possible configurations at the interface to the configurations in the bulk and R is the number of segments in the polymer

J. Bladon, et al, J. Colloid & InterfaceSci., 52, 14 (1975)

A VERY FLEXIBLE MOLECULE AT AN INTERFACE ONLY HAS ABOUT HALF THE CONFIGURATIONS THAT IT HAS IN THE BULK SOLUTION.

• THIS IS STRICTLY TRUE FOR INFINITELY LONG LINEAR MOLECULES

• APPROXIMATELY TRUE FOR SHORT CHAIN ALMOST-LINEAR MOLECULES

• SHOULD BE CONSIDERED IN DESIGNING ADSORBATES

J. Bladon et al, J. Colloid & InterfaceSci., 52, 14 (1975)

CASE 1, con’t…

XA = - 4 kcal/mol

All other interactions are zero

THE “WEAK” PLATEAU WILL BE A MIXTURE OF THE FOLLOWING CONFIGURATIONS:

XA = - 4 kcal/molEBC = +1 kcal/mol (POOR SOLVENT)

CASE 2

A B B B C

THE INITIAL SLOPE:

THE PLATEAU:

HALF WAY UP THE SLOPE:

CONCLUSION

IN ORDER TO ACHIEVE A WELL ORDERED SAM:

1. THE END GROUP MUST BE STRONGLY ADSORBED

2. THE MEDIUM MUST BE A RELATIVELY POOR SOLVENT FOR THE ADSORBATE.

3. MULTILAYER ADSORPTION MAY RESULT AT HIGHER CONCENTRATIONS AFTER MONOLAYER IS ACHIEVED.

Alkyl Sulfide Adsorption onto Gold Substrate

• Lattice Model is replaced with schematic construct of weakly adsorbing segmented alkyl tail and strongly adsorbing sulfide end segment.

• Source: S.M.Wetterer, Thesis, Princeton Univ., (1998)

physisorbed thiol groupchemisorbed sulfur

Temperature Programmed Desorption

(TPD)

S.M.Wetterer, Thesis, Princeton Univ., (1998)

10 min

15 min

60 min

0 min

physisorbed thiol group

chemisorbed sulfur

Time held at 208oK

TYPICAL TIME FOR MONOLAYER COVERAGE IS ~1 MIN.

DURING THIS TIME, 45-65% OF THE ADSORBATE IS SKATING AROUND THE SURFACE IN THE PHYSISORBED STATE

SULFUR-GOLD SUBSTRATE INTERACTION ENERGIES (XA)

S.M.Wetterer, Thesis, Princeton Univ., (1998)

n-ALKYLTHIOLS DIETHYLSULFIDE

DIBUTYLSULFIDE

THIOPHENEXAPHYS = 126 kcal/mol

XACHEM = 33 kcal/mol

XAPHYS = 24XACHEM = NONE steric hindrance

DIALKYLDISULFIDE

XACHEM = 124

XAPHYS = NONE

DIMERIZE ON CONTACT TO PHYSISORBED n-ALKYLSULFIDES

Steric hindrance decreases chemisorption strength

2-PROPANETHIOL

T-BUTANETHIOL

NEOPENTANETHIOL

XACHEM = 107XAPHYS = 33

XACHEM = 107XAPHYS = 33

XACHEM = 126XAPHYS = 33

CHELATING DITHIOL SHOULD ADSORB BETTER FOR TWO REASONS: GOOD AROMATIC INTERACTIONS AND ENTROPY LOSS IS LESS.

N.Garg, T.Lee, Langmuir, 14, 3815 (1998)

ASYMMETRIC FUNCTIONALIZATION WITHOUT FEAR OF “ISLANDS”

SPIROALKANEDITHIOL SHOULD ADSORB BETTER THAN TWO n-ALKYLTHIOLS BECAUSE ENTROPY LOSS IS LESS

Y. Shon, S. Lee, S. Perry, T. Lee, J. Amer. Chem. Soc.,122, 1278, (2000)

ASYMMETRIC FUNCTIONALIZATION WITHOUT FEAR OF “ISLANDS”

THE SECRET TO SELF ASSEMBLED

MONOLAYERS

Maxwell’s demon

I know you guys are only physisorbed,

but you’ll like it better standing up!