3.2 Determination of Potential-Driven Structural Changes
in Films of Amphiphilic Molecules: In Situ PM IRRAS
Adsorption of ionic and neutral surfactants on an electrode surface leads to a
decrease in the differential capacitance of the electrode|electrolyte interface [3, 10,
21]. Similar electrochemical behavior is observed when a monolayer or multilayers
of insoluble in water amphiphilic molecules adsorb on the electrode surface
[13, 22–24]. Therefore, capacitance-potential curves are often used to characterize
the potential-dependent adsorption-desorption process of organic molecules on an
electrode surface. Some capacitance-potential plots display pseudocapacitance
peaks which are assigned to phase transitions or adsorption/desorption of these
films. To understand nature of potential-dependent changes in the structure and
orientation of molecules present in a film adsorbed on the electrode surface the use
of in situ PM IRRAS offers a very promising experimental approach. Studies of
n-octadecanol bilayers adsorbed on the Au(111) surface is described as an explanatory example [25]. In a X-type bilayer [26] the hydrocarbon chains in both leaflets
are directed toward the electrode and hydroxy groups toward the electrolyte phase.
The X-type bilayer of n-octadecanol is characterized by the capacitance of
0.84 μF cm
À2 [25]. The low capacitance value indicates a full coverage of the
electrode surface by a defect-free bilayer. During the negative going potential scan
the capacitance remains constant, until E ¼ À0.63 V versus SCE (SCE: saturated
calomel electrode) when an abrupt desorption of the bilayer from the Au(111)
surface takes place. The re-adsorbed n-octadecanol film has a distinct electrochemical characteristic. The capacitance potential curve has two minima (1.5 μF cm
À2 at
À0.10 < E < 0.20 V and 2.6 μF cm
À2 at À0.45 < E < À0.20 V) which are separated
by a pseudocapacitive peak at E ¼ À0.14 V versus SCE. Electrochemical studies
indicate that after desorption of the freshly prepared X-type bilayer the hydroxyl
groups reorient and turn toward the aqueous phase. This reorientation in the inner
leaflet leads to the formation of a so called Y-type bilayer with polar head groups
turned toward the electrode and electrolyte solution in the inner and outer leaflet,
respectively [25]. These results suggest large potential-dependent rearrangements in
the structure of the n-octadecanol bilayer assembly. To follow these changes in situ
PM IRRAS with electrochemical control was used. Figure 3.2a shows the PM IRRA
spectra in the CH stretching modes region in the first and following negative going
potential scans, respectively.
Large differences are observed between the PM IRRA spectra recorded in the two
potential scans. In the first negative going potential scan, when n-octadecanol is
assembled into the X-type bilayer, the methylene stretching modes are very weak
(Fig. 3.2a). Their intensities are comparable to the intensities of the methyl stretching
modes. The n-octadecanol molecule contains one methyl and 17 methylene groups.
Such strong attenuation of the methylene stretching modes indicates a large degree
of order and an almost vertical to the Au surface orientation of the hydrocarbon
chains in the X-type bilayer. In the desorbed film, at E ¼ À0.85 V, the intensities of
the methylene stretching modes start to increase. In the re-adsorbed n-octadecanol
3.2 Determination of Potential-Driven Structural Changes in Films of Amphiphilic. . .
49
in Films of Amphiphilic Molecules: In Situ PM IRRAS
Adsorption of ionic and neutral surfactants on an electrode surface leads to a
decrease in the differential capacitance of the electrode|electrolyte interface [3, 10,
21]. Similar electrochemical behavior is observed when a monolayer or multilayers
of insoluble in water amphiphilic molecules adsorb on the electrode surface
[13, 22–24]. Therefore, capacitance-potential curves are often used to characterize
the potential-dependent adsorption-desorption process of organic molecules on an
electrode surface. Some capacitance-potential plots display pseudocapacitance
peaks which are assigned to phase transitions or adsorption/desorption of these
films. To understand nature of potential-dependent changes in the structure and
orientation of molecules present in a film adsorbed on the electrode surface the use
of in situ PM IRRAS offers a very promising experimental approach. Studies of
n-octadecanol bilayers adsorbed on the Au(111) surface is described as an explanatory example [25]. In a X-type bilayer [26] the hydrocarbon chains in both leaflets
are directed toward the electrode and hydroxy groups toward the electrolyte phase.
The X-type bilayer of n-octadecanol is characterized by the capacitance of
0.84 μF cm
À2 [25]. The low capacitance value indicates a full coverage of the
electrode surface by a defect-free bilayer. During the negative going potential scan
the capacitance remains constant, until E ¼ À0.63 V versus SCE (SCE: saturated
calomel electrode) when an abrupt desorption of the bilayer from the Au(111)
surface takes place. The re-adsorbed n-octadecanol film has a distinct electrochemical characteristic. The capacitance potential curve has two minima (1.5 μF cm
À2 at
À0.10 < E < 0.20 V and 2.6 μF cm
À2 at À0.45 < E < À0.20 V) which are separated
by a pseudocapacitive peak at E ¼ À0.14 V versus SCE. Electrochemical studies
indicate that after desorption of the freshly prepared X-type bilayer the hydroxyl
groups reorient and turn toward the aqueous phase. This reorientation in the inner
leaflet leads to the formation of a so called Y-type bilayer with polar head groups
turned toward the electrode and electrolyte solution in the inner and outer leaflet,
respectively [25]. These results suggest large potential-dependent rearrangements in
the structure of the n-octadecanol bilayer assembly. To follow these changes in situ
PM IRRAS with electrochemical control was used. Figure 3.2a shows the PM IRRA
spectra in the CH stretching modes region in the first and following negative going
potential scans, respectively.
Large differences are observed between the PM IRRA spectra recorded in the two
potential scans. In the first negative going potential scan, when n-octadecanol is
assembled into the X-type bilayer, the methylene stretching modes are very weak
(Fig. 3.2a). Their intensities are comparable to the intensities of the methyl stretching
modes. The n-octadecanol molecule contains one methyl and 17 methylene groups.
Such strong attenuation of the methylene stretching modes indicates a large degree
of order and an almost vertical to the Au surface orientation of the hydrocarbon
chains in the X-type bilayer. In the desorbed film, at E ¼ À0.85 V, the intensities of
the methylene stretching modes start to increase. In the re-adsorbed n-octadecanol
3.2 Determination of Potential-Driven Structural Changes in Films of Amphiphilic. . .
49