Chapter 2: Techniques
II. 3. 4. Contribution of 1-pentanol molecules to the dielectric relaxation
Surfactant enhanced ionization may increase the electrostatic repulsion between micelles and raise their total number229. Other authors213, 227 suggested that the penetration of alcohol molecules into the outer hydrophilic region should lower the electrostatic interaction between the surfactant headgroups, and makes the surfactant more energetically favourable for being a part of the micelles, and consequently a lowering the mixing free energy and cmc occurs. The presence of 1-pentanol molecules in the palisade layer promotes an increase of the effective packing parameter in the micelle (table (III. 12) and a swelling of the particle (the mean curvature tend to be more positive). As a consequence, shape transition that may occur should be of different types, for example to rod-like222, 230-233
or perhaps to oblate ellipsoidal (disks) micelles225. These possible structural change would render inappropriate Grosse’s theory and the model of Pauly and Schwan in this surfactant and cosurfactant concentration range, although a certain agreement between experiment and theory could be found. Note also that micellar interaction would play an important role in the deviation between our experimental results and the theory, since this effect is ignored by both models of Grosse and Pauly and Schwan.
Considering that all 1-pentanol molecules are located on the palisade layer and that the interaction between micelles remain weak, we made an evaluation of the effective packing parameter peff (equations (I. 1) to (I. 4)) with aSDS = 60 Å2 and a1-pentanol = 16 Å2. We observed that the effective packing parameter increased with increasing W1-pentanol (from 0 % wt to 7 % wt, peff passes from 0.34 to 1.16) supposing a transition spherical to elongated and bicontinuous structures (table (III. 16)). This observation is consistent with the apparition of bicontinuous structures (expected with the conductivity results) at higher WSDS and W1-pentanol
(~21 % wt). Another cause of experimental deviations to the model of Grosse may arise from the dielectric relaxation of alcohol molecules in both disperse and continuous phases. This point is treated in the next paragraph.
II. 3. 4. Contribution of 1-pentanol molecules to the dielectric relaxation
It has been shown that 1-pentanol present several relaxation processes in our frequency range. Garg and Smyth234 first analyzed their data in terms of three different relaxation times (three Debye model). This 3D model has been also succesfully applied to DRS spectra of shorter n-alkanols235, 236. In this spectral function the main dispersion located at several hundred of picoseconds is due to the cooperative process of hydrogen bonds (O-H· · · O) in the long-chain alcohol multimer. This cooperation induces a large dipole moment change so that the reorientation of monomers and dimers becomes less important in comparison; these high-frequency dispersion steps 2 and 3 were interpreted in terms of monomer relaxation and monomeric –CH2OH or –OH group rotation respectively234-236. With addition of nonpolar solvent some hydrogen bond in the alcohol multimer are broken and the chain length of multimers becomes shorter resulting in a decrease in the main relaxation time237-240. The same modifications in the main relaxation process are observed when substituting non-polar solvent by water241, 242.
Since they could cause interpretations errors of our dielectric spectra, dielectric dispersions arising from 1-pentanol at various states (solubilized in the palisade layer, located in the micelle hydrophobic core, and solubilized in the water continuous medium) were evaluated.
DRS measurements of pure pentanol, and of an aqueous solution of pentanol at 2 % wt 1-pentanol have been carried out. The complex permittivity spectra of both samples have been measured in the frequency range 0.008 ≤ ν / GHz ≤ 89 at 25 °C (figure (III. 26) and table (III. 18)). The DRS spectrum of 1-pentanol was analyzed in terms of 3 Debye relaxation processes since other spectral fuctions gave higher values of s2. The first dispersion step centred at τ1 = 673 ps (cooperative process of hydrogen bonds of alcohol multimers) was found in agreement with literature results241; relaxation processes 2 and 3, both with low amplitude (S2 + S3 < 1) are centred at 26 ps and 3.7 ps, respectively. In pure 1-pentanol, dispersions steps 2 and 3 are related to the reorientation of monomers and –OH groups, respectively234-240. 2 Debye fit model has been found (other spectral fuctions gave also higher values of s2) for 2 % wt pentanol in water, the main dispersion step 2 (centred at ∼ 8 ps) is attributed to the cooperative hydrogen-bond network of water molecules, while relaxation process 1 (centred at ∼ 80 ps) should be due to water/alcohol interactions242 with different hydration structures243.
Table (III. 18) DRS parameters of pure 1-pentanol and 2 % wt of 1-pentanol in pure water at 25°C; relaxation parameters εi and τi of 3D and 2D models with corresponding variance, s2.
Pure 1-pentanol
ε1 τ1 (ps) ε2 τ2 (ps) ε3 τ3 (ps) ε s2 15.15 673 3.22 25.87 2.75 3.69 2.33 0.0031
water/1-pentanol mixture
ε1 τ1 (ps) ε2 τ2 (ps) ε s2 77.28 82.28 75.23 8.78 5.37 0.1496
0,01 0,1 1 10 100 wt 1-pentanol in pure water (b) at 25°C. 1-pentanol spectrum has been fitted with a 3 Debye model whereas 2 % wt 1-pentanol in pure water spectrum has been fitted with a 2 Debye model (see table (III. 18)).
For 1-pentanol molecules solubilized within the micellar hydrophobic core, a dielectric relaxation, S1m−pentanol, may be evaluated with a maximal theoretical amplitude of
m whereφ1m−pentanol is the volume fraction of 1-pentanol within the micelle; v1-pentanol is here chosen as 105.5 cm3.mol-1 corresponding to the alcohol partial volume measured in micellar solutions244. We found that at maximal surfactant and cosurfactant concentration (W1-pentanol = 7 % wt) that φ1m−pentanol ~ 0.05 so that a total dispersion of “bulk” alcohol S1-pentanol = (S1 + S2 + S3) scaled by φ1m−pentanol gives S1m−pentanol ~ 0.7. A contribution of the alcohol solubilized in the
micelle should theoretically happen, but remember that a certain amount of this 1-pentanol should be located at the micelle surface and the alcohol molecules present within the core may interact with surfactant hydrophobic tails probably like in n-alkane/alcohol mixtures237-239. Therefore a dielectric relaxation arising from 1-pentanol within the core is considered as negligible.
The alcohol molecules located at the micellar surface should also show a dielectric relaxation related to the rotation of their hydroxyl goups. This relaxation process in the case of pure 1-pentanol solutions is centred at around τ3 ∼ 3 ps with a dispersion amplitude of about 0.4. We suppose the alcohol molecule which hydrophobic tail “fixed” in the core and its C—OH group, orientated toward the external field. This assumption yields a theoretical amplitude,S1m−pentanol, that can be calculated (with help of equation (III. 30)) as
( ) ( )
was evaluated following Exner245 considering the semi-axes of the polarizability. —OH group was assumed as a sphere of radius 0.9 Å and of dipole moment µOH corrected by the direction cosinus determined by the angle (θ-90) (where θ is the CCOH bond angle, equal to 107.4247°) of the dipole moment vector µOH and the given coordinates as( )
µa µ θ
m
= OH cos −90 (III. 68)
Sam was found varying from 0.0025 (2 % wt 1-pentanol) to 0.1 (74 % wt 1-pentanol).
Therefore the dielectric increment related to a rotation of an alcohol hydroxyl group at the interface could be ignored. On the other hand the contribution of water/1-pentanol interaction in water can have a significant influence especially for high-frequency permittivity data. For 2
% wt 1-pentanol, the relaxation process centred at τ1 = 80 ps indicates hydrogen bonding247 resulting from water/1-pentanol interaction. This contribution has an amplitude of S1 ∼ 2 at alcohol concentration close to its solubility limit (at 2.2 % wt from Ginnings and Baum101).
This makes that in path B, between 2 and 7 %wt 1-pentanol, the relaxation process centred at
∼ 50 ps may include both water-SDS headgroups and water-1-pentanol interactions.