CONFIGURATIONAL MODEL OF POLYMER ADSORPTION AND ADSORPTION OF THIOL DERIVATIVES ON GOLD Maxwell’s demon
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!