2.2 Surface Reactions of Organic and Polymeric Films
19
2.2.1.4 Confined Reactions of Monolayers
Thanks to their well-defined structure, low defect density, and chemical stability,
SAMs offer unique opportunities to probe the mechanistic details of reactions at
interfaces. However, there are various factors that result from the structure and
highly ordered arrangement of SAMs and related systems that have an impact on the
reactivity observed.
Steric effects: The decreased reactivity of SAMs as has been studied in most
mechanistic studies is ascribed to partially blocked access of an external reagent to
the monolayer-embedded reaction center. Houseman and Mrksich found for enzymebased reactions that the extent of reaction drops dramatically if the surface concentration of functional groups is higher than a certain threshold (70%). Low yields at
high coverage were ascribed to steric crowding at the surface, which inhibits the
enzymatic reaction. The reactivity in monolayers deposited on gold colloids was
also studied as a function of size of the incoming nucleophile and steric crowding
around reaction center. Steric effects were reported to be important in these reactions,
and that the reaction rate was substantially diminished with bulky nucleophiles or in
short-chain monolayers on gold colloids, where the density of the reaction centers is
the highest.
Stirling and co-workers studied the reactivity of surface-attached esters toward
base-catalyzed hydrolysis. They showed that the carbonyl groups buried well below
the surface [HS(CH 2 ) 10 OCO(CH 2 ) 8 CH 3 ] of well-packed monolayers of aliphatic
esters are very resistant to hydrolysis, while the closer the esters are with their
carbonyl function to the monolayer surface [HS(CH 2 ) 10 OCOCH 3 ], the more rapid
the hydrolysis progresses. In such systems, the reaction is thought to start at defect
sites and domain boundaries because access of an external reagent is blocked. The
reaction can then grow and coalesce as the reaction proceeds. For these reactions,
the initial induction of hydrolysis will be a slow reaction at scarce defect sites and
be followed by a more rapid process with the leaving of the blocking groups from
the monolayer by increased conversion [32].
Dordi et al. have investigated the reactivity of NHS esters in SAMs of the corresponding disulfides on gold systematically as a function of chain length [33]. A
markedly different reactivity of the NHS monolayers with different chain lengths
was found, which was attributed to differences in orientation, packing, and conformational order of the molecules confined in the monolayers. By IR spectroscopy
and contact angles measurements, it was found that SAMs with short chain length
exhibited increased disorder and a more facile reorientation of functional groups at
the surface of the SAMs. Increasing disorder, reduced steric hindrance, and reduced
crowding at the SAM surface were argued to release the constraint of the local environment and to facilitate the attack of the hydroxide ions and thus increase the rate
of the reaction.
Ryswyk and co-workers reported on the influence of surface density of the isonicotinate ester groups on the reactivity on the mixed monolayers of isonicotinate
19
2.2.1.4 Confined Reactions of Monolayers
Thanks to their well-defined structure, low defect density, and chemical stability,
SAMs offer unique opportunities to probe the mechanistic details of reactions at
interfaces. However, there are various factors that result from the structure and
highly ordered arrangement of SAMs and related systems that have an impact on the
reactivity observed.
Steric effects: The decreased reactivity of SAMs as has been studied in most
mechanistic studies is ascribed to partially blocked access of an external reagent to
the monolayer-embedded reaction center. Houseman and Mrksich found for enzymebased reactions that the extent of reaction drops dramatically if the surface concentration of functional groups is higher than a certain threshold (70%). Low yields at
high coverage were ascribed to steric crowding at the surface, which inhibits the
enzymatic reaction. The reactivity in monolayers deposited on gold colloids was
also studied as a function of size of the incoming nucleophile and steric crowding
around reaction center. Steric effects were reported to be important in these reactions,
and that the reaction rate was substantially diminished with bulky nucleophiles or in
short-chain monolayers on gold colloids, where the density of the reaction centers is
the highest.
Stirling and co-workers studied the reactivity of surface-attached esters toward
base-catalyzed hydrolysis. They showed that the carbonyl groups buried well below
the surface [HS(CH 2 ) 10 OCO(CH 2 ) 8 CH 3 ] of well-packed monolayers of aliphatic
esters are very resistant to hydrolysis, while the closer the esters are with their
carbonyl function to the monolayer surface [HS(CH 2 ) 10 OCOCH 3 ], the more rapid
the hydrolysis progresses. In such systems, the reaction is thought to start at defect
sites and domain boundaries because access of an external reagent is blocked. The
reaction can then grow and coalesce as the reaction proceeds. For these reactions,
the initial induction of hydrolysis will be a slow reaction at scarce defect sites and
be followed by a more rapid process with the leaving of the blocking groups from
the monolayer by increased conversion [32].
Dordi et al. have investigated the reactivity of NHS esters in SAMs of the corresponding disulfides on gold systematically as a function of chain length [33]. A
markedly different reactivity of the NHS monolayers with different chain lengths
was found, which was attributed to differences in orientation, packing, and conformational order of the molecules confined in the monolayers. By IR spectroscopy
and contact angles measurements, it was found that SAMs with short chain length
exhibited increased disorder and a more facile reorientation of functional groups at
the surface of the SAMs. Increasing disorder, reduced steric hindrance, and reduced
crowding at the SAM surface were argued to release the constraint of the local environment and to facilitate the attack of the hydroxide ions and thus increase the rate
of the reaction.
Ryswyk and co-workers reported on the influence of surface density of the isonicotinate ester groups on the reactivity on the mixed monolayers of isonicotinate