50
3 Confinement Effects on the Reactivity in Ultrathin …
carbonyl band at 1737 cm
−1 . Since the contribution of the bands of the monomeric
and dimeric acid can be expected to be very small compared to the changes in
absorbance of the succinimidyl carbonyl band and to be approximately equal to the
decrease in absorbance of the ester carbonyl bands, we have estimated the changes
in film composition by analyzing the integrated absorbance of the entire carbonyl
region. This procedure takes the decrease of both the ester and the succinimidyl
carbonyl bands, as well as the increase of the carboxylic acid band, into account.
The initial course of the reaction, as followed by both FTIR and CA measurements,
is summarized in Fig. 3.4a. It is evident that the reaction, as sampled by CA, seems
to proceed more rapidly than sampled by IR spectroscopy. This difference can be
attributed to the different information depth of the methods. While the water droplets
in the CA measurements probe the outermost 5–10 Å [16], IR spectroscopy probes
all chromophores in the irradiated area throughout the entire depth of the film. Since
the reaction must start at the polymer–solution interface and IR spectroscopy is not
very surface-sensitive, IR provides information about the reaction at the surface only
at the beginning of the reaction.
For short reaction times (<120 s) we observe an exponential decrease of the
NHS ester coverage (Fig. 3.4b). This behavior is consistent with a pseudo-first-order
reaction kinetics at the early stage of the reaction and the presence of a homogeneous reaction (vide supra). From the slopes of the linearized plot in Fig. 3.4b we
can estimate the apparent pseudo-first-order rate constants k
of the hydrolysis at
21 °C, which can then be converted to concentration-independent second-order rate
constants. The rate of the reaction is described by:
−
d [NHS]
dt
= k
[OH
−
][NHS] = k
[NHS].
(3.1)
where [NHS] and [OH
− ] denote the concentration of NHS ester and hydroxide ions,
respectively, k
and k
are the second-order and pseudo-first-order rate constants,
respectively ([OH
− ] ≈ constant; k
= k
[OH
− ]).
For comparison, we show previously reported data for the hydrolysis of 11,11
-
dithiobis(N-hydroxysuccinimidylundecanoate) (NHS-C10) films in 1.00 × 10
−2 M
aqueous solution of NaOH on gold at 21 °C in Scheme 3.2. Similar to PNHSMA
films, the progress of the reaction was conveniently and rapidly followed by CA
measurements at 20 °C.
The corresponding rate constants for both SAMs and PNHSMA films are summarized in Table 3.1. The apparent rate constant for the SAMs (5 × 10
−2 L/mol s)
is five times smaller than that for the surface of the polymer films (26 × 10
−2
L/mol s). However, the rate constants for both systems are much smaller compared
to similar reactions in solution. These results indicate that the reaction is affected by
confinement effects both in the SAMs and the ultrathin polymer films.
3 Confinement Effects on the Reactivity in Ultrathin …
carbonyl band at 1737 cm
−1 . Since the contribution of the bands of the monomeric
and dimeric acid can be expected to be very small compared to the changes in
absorbance of the succinimidyl carbonyl band and to be approximately equal to the
decrease in absorbance of the ester carbonyl bands, we have estimated the changes
in film composition by analyzing the integrated absorbance of the entire carbonyl
region. This procedure takes the decrease of both the ester and the succinimidyl
carbonyl bands, as well as the increase of the carboxylic acid band, into account.
The initial course of the reaction, as followed by both FTIR and CA measurements,
is summarized in Fig. 3.4a. It is evident that the reaction, as sampled by CA, seems
to proceed more rapidly than sampled by IR spectroscopy. This difference can be
attributed to the different information depth of the methods. While the water droplets
in the CA measurements probe the outermost 5–10 Å [16], IR spectroscopy probes
all chromophores in the irradiated area throughout the entire depth of the film. Since
the reaction must start at the polymer–solution interface and IR spectroscopy is not
very surface-sensitive, IR provides information about the reaction at the surface only
at the beginning of the reaction.
For short reaction times (<120 s) we observe an exponential decrease of the
NHS ester coverage (Fig. 3.4b). This behavior is consistent with a pseudo-first-order
reaction kinetics at the early stage of the reaction and the presence of a homogeneous reaction (vide supra). From the slopes of the linearized plot in Fig. 3.4b we
can estimate the apparent pseudo-first-order rate constants k
of the hydrolysis at
21 °C, which can then be converted to concentration-independent second-order rate
constants. The rate of the reaction is described by:
−
d [NHS]
dt
= k
[OH
−
][NHS] = k
[NHS].
(3.1)
where [NHS] and [OH
− ] denote the concentration of NHS ester and hydroxide ions,
respectively, k
and k
are the second-order and pseudo-first-order rate constants,
respectively ([OH
− ] ≈ constant; k
= k
[OH
− ]).
For comparison, we show previously reported data for the hydrolysis of 11,11
-
dithiobis(N-hydroxysuccinimidylundecanoate) (NHS-C10) films in 1.00 × 10
−2 M
aqueous solution of NaOH on gold at 21 °C in Scheme 3.2. Similar to PNHSMA
films, the progress of the reaction was conveniently and rapidly followed by CA
measurements at 20 °C.
The corresponding rate constants for both SAMs and PNHSMA films are summarized in Table 3.1. The apparent rate constant for the SAMs (5 × 10
−2 L/mol s)
is five times smaller than that for the surface of the polymer films (26 × 10
−2
L/mol s). However, the rate constants for both systems are much smaller compared
to similar reactions in solution. These results indicate that the reaction is affected by
confinement effects both in the SAMs and the ultrathin polymer films.