Electrostatic Induced Molecular Tilting in Self-Assembled Monolayers of nOctadecylamine on Mica Jaime Oviedo,1 Miguel A. San-Miguel,1 José A. Heredia-Guerrero,2 José J. Benítez.2* 1
Physical Chemistry Department, University of Seville. Seville (Spain).
2
Materials Science Institute of Seville (ICMS), Spanish Research Council (CSIC)-
University of Seville. Americo Vespuccio 49. Isla de la Cartuja, 41092. Seville (Spain). *Corresponding author:
[email protected], phone: 34 954489551, fax: 34 954460665.
SUPPORTING INFORMATION
Figure S1. AFM topographic image and line profile (white line in the image) showing the coexistence of the h ≈ 17Å (light grey) and h ≈ 13Å (grey) phases in noctadecilamine self-assembled mononalayers on mica (ODA/mica SAMs) after exposition to air for 6 days. When ODA/mica SAMs are left in contact with the
atmosphere for periods below 5-7 days, only the h ≈ 17Å phase is observable. Above this ripening time, ODA molecules suddenly and spontaneously tilt by the effect of moisture to the h ≈ 13Å state. The simultaneous detection of both heights is a quite unusual case and may be caused by a deficient access of water molecules to some areas of the –NH2-mica interface. The observation of such discrete height values indicates that they are well defined configurations with no stable intermediates.
Figure S2. ODA/mica SAMs N1s XPS spectra decomposition into –NH2 (399.4 eV), NH-COO- (400.2 eV) and –NH3+ (401.3 eV). The –NH-COO- component is the result of amine carbamation in contact with atmospheric CO2 dissolved in the water adlayer on
mica. –NH3+ species are also produced by direct protonation of the amino groups by the water adlayer. The band evolution with contact time with air (days) indicates that no additional carbamation is produced. On the other side, water induced protonation is continous.
Table S1. Calculated percentage of nitrogenated species in n-octadecylamine SAMs on mica as a function of time in contact with air after preparation (ripening time). Data are obtained from the N1s XPS signal. Ripening time (days)
-NH2
-NH-COO-
-NH3+
0.1
45.54
17.15
37.31
1
41.75
17.66
40.59
2
39.24
17.81
42.95
7
36.82
17.41
45.77
DIPOLE TO DIPOLE ENERGY CALCULATION IN A HEXAGONAL PACKING Here the procedure to evaluate the electrostatic energy of an infinite set of interacting parallel dipoles is described. Dipoles are represented by circles arranged in a hexagonal packing in a two dimensional surface (Figure S3). The interaction energy between parallel dipoles at a D distance is U=
p2 4πε r ε 0 D 2
Where p, ε0 and εr are the dipole magnitude and the free space and relative permittivity respectively.
The energy interaction between one specific dipole (labeled as 0 in Figure S3) and the rest can be computed as an infinite series.
U=
N1 N 2 N3 3 + 3 + 3 + L 4πε r ε 0 D1 D2 D3 p2
Where Dl is the distance between dipole 0 and dipole l and Nl is the number of dipoles at the same distance because of symmetry. Hereon, the constant within the brackets (in distance-3 units) is called S and the procedure to compute is described below. N N N S = 13 + 23 + 33 + L D1 D2 D3 First, we define a cartesian coordinate system, where dipole 0 is at the origin (0,0). It is convenient to refer distances to the circle radius R. The position of each dipole (i.e. the circle centre) can be referred as (i,j). For instance, dipole 6 is located at (4,2). Scales are not the same in both axis; i axis is expressed in R units whereas j axis is expressed in 31/2 R units. The hexagonal packing presents two different symmetry (C6) axes that are shown in Figure S3 as bold lines. The two axes are defined by the first neighbor and the next neighbor positions respectively. Therefore, because of symmetry, the sum can be restricted to a 30° plane section. Dipole coordinates can be grouped in two cases: If i is an odd number, j is also odd and runs from 1 to j = i whereas if i is an even number, j is also even and runs from 0 to j = i. Thus, dipole positions in increasing i are written as: (1,1), (2,0), (2,2), (3,1), (3,3),
(4,0), (4,2), (4,4), (5,1), (5,3), (5,5), (6,0), (6,2), (6,4), (6,6), (7,1), (7,3), (7,5), (7,7), etc… The number of dipoles at a certain distance is 6 for elements on the symmetry axes (i.e. dipoles 5 or 10). However, for off-axes elements this number is doubled (i.e. dipoles 4 or 9). Thus, Nij is 6 if j = 0 or i = j and Nij is 12 otherwise. In order to calculate the distance between dipoles (in R units) the different scale in i and j has to be taken into account. For instance, distance between dipoles 0 (0,0) and 7 (4,4) is 8 (Figure S3) and can be calculated as Dij =
( 3i )
2
+ j 2 = 3 ×16 + 16 = 64 = 8 R
In general distances from 0 to a dipole at (i,j) are (in R units)
Dij = 3 × i 2 + j 2 Each particular contribution to S from dipole (i,j) is calculated as
Sij =
Nij Dij 3
Finally, a computer code adds all the contributions in a double loop that runs from i = 1 to Nmax and j taking all the possible values compatible with i. If Nmax is large enough (about 250), U is converged to: U=
1 p2 C 4πε 0 ε r R 3
( C = 1.3764641)
Figure S3. An infinite set of dipoles, represented as circles of R radii, arranged in an
hexagonal packing. Due to symmetry, only a fraction of circles (i.e. about one sixth) are shown. The centre is taken at i = j =0 and the dipole labeled as 0. Neighbor dipoles are labeled as 1, 2, 3, .. in an increasing distance order.
