Monohydroxy alcohols (MAs) with methyl and hydroxyl side groups attached to the same carbon atom in the alkyl backbone can display very weak structural and supramolecular dielectric relaxation processes when probed in the regime of small electrical fields. This can render their separation and assignment difficult in the pure liquids. When mixing with bromoalkanes, a faint Debye-like process can be resolved dielectrically for 4-methyl-4-heptanol. To achieve a separation of structural and supramolecular processes in pure 4-methyl-4-heptanol and 3-methyl-3-heptanol, mechanical experiments are carried out in the linear-response regime as well as using medium-angle oscillatory shear amplitudes. It is demonstrated that first-order and third-order nonlinear mechanical effects allow for a clear identification of supramolecular viscoelastic modes even for alcohols in which they leave only a weak signature in the linear-response shear modulus. Additionally, the nonlinear rheological behavior of 2-ethyl-1-hexanol is studied, revealing that its linearly detected terminal mode does not coincide with that revealed beyond the linear-response regime. This finding contrasts with those for the other MAs studied in this work.
REFERENCES
According to Ref. 22, at temperatures near 180 K, 2-methyl-3-heptanol (2M3H) displays an even lower static dielectric constant than 3M3H. This contrasts with the dielectric results of Ref. 22. Near 180 K, the 2M3H data (from stated 90% pure samples) are reported to more closely resemble those of 4M3H. See footnote 14 in Ref. 15 concerning the difficulties to separate 2M3H from 4M3H chemically.
Rather small gK factors are also reported for 2-methyl-2-hexanol (see Ref. 17).
Figure 53(a) of Ref. 9 shows room temperature OH stretching overtones for 3M3H, 4M3H, 5M3H, and 6M3H.
See, e.g., the supplementary information given in Ref. 52.
In preliminary experiments, plates with rough and with smooth surfaces were used. We found that using the latter, the LAOS regime is typically entered more abruptly than using the former plates. Since otherwise no significant differences were found, the smooth-surface plates were preferred for the measurements since they define a uniform gap distance.
Let us emphasize again that 2E1Br mixes well with the studied octanol isomers. As shown in Ref. 70, even 2-hexyl-1-decanol mixes with 2E1Br if cooled with typical rates of 15 K/min. However, extended annealing at appropriate temperatures or use of longer MA chains leads to phase separation.
Master curves for 4M4H, 3M3H, and 3M4H were already reported in Ref. 70. In Figs. 3 and S2 of that work, one recognizes that 3M4H displays exceptional behavior. However, due to a typographical error, the text of that article says that 3M3H shows exceptional behavior where it should say that 3M4H shows exceptional behavior. Since the exceptional rheological behavior of 3M4H still awaits clarification, mechanical data from this substance are not included in Fig. 2 of the present article.
Our data for 3M3H fully agree with those published in Ref. 22.
Note that for 2r = 4 mm diameter plates and a gap width of g = 1 mm, this corresponds to angular excursion amplitudes of α = (g/r)γ0180°/π = γ028.65°.
From Eq. (7), |G*13| ∝ γ0–2 is expected. The slope of the red line in Fig. 4(a) slightly deviates from this expectation because |G*1| deviates positively and negatively from |G*11|. On a logarithmic representation, the negative differences are disregarded, thus resulting in a slightly distorted slope for a given dataset.
For example, Eqs. (17) and (18) in Ref. 80 provide a simple derivation of the factor of 3/2 for G13.