Structural Properties of Pure Simple Alcohols from Ethanol, Propanol, Butanol, Pentanol, to Hexanol: Comparing Monte Carlo Simulations with Experimental SAXS Data

The primary liquid alcohols from ethanol to 1-hexanol were studied utilizing the configurational-bias Monte
Carlo (MC) simulations of the modeled alcohols (transferable potential for phase equilibria−united atom
model) and the small-angle X-ray scattering (SAXS) method. A novel approach for calculating the scattering
intensities from the theoretically obtained MC data by utilizing the Debye equation and their further validation
with experimental results was introduced. This procedure is important, since the common problem of how to
initially separate the intra- and intermolecular contributions to the scattering when comparing the calculated
and experimental data was successfully avoided. Nevertheless, the intra- and intermolecular contributions to
the scattering were able to be investigated directly from the MC results. The most pretentious task of the
procedure was the suppression of the MC box background scattering, which was solved by utilizing the
averaging of the scattering intensities over the different box sizes. This method of the scattering intensity
calculations enabled us to make a theoretical analog to the well-known small-angle neutron scattering contrast
matching experiment that, in our case, nicely revealed the origin of the two alcohol scattering peaks in the
SAXS regime of the scattering curves (0.3 Å-1 < q < 3 Å-1). For the example of butanol, the outer alcohol
scattering peaks at ∼1.40 Å-1 were unambiguously ascribed to the correlations between the alcohol hydrocarbon
tails described by the gCHxCHx(r) pair correlation function. Similarly, the inner alcohol scattering peaks that
shift from ∼0.8 to ∼0.4 Å-1 with an increasing alkyl chain length of the alcohol molecule are mainly the
consequence of the O−O correlations. These findings were tested on pentanol/water mixtures and further
applied to the results of the structural investigations on the binary and ternary microemulsion systems of the
nonionic surfactant Brij 35 (Tomšič, et al. J. Phys. Chem. B 2004, 108, 7021; Tomšič, et al. J. Colloid Interface
Sci. 2006, 294, 194), which were in fact the actual motivation for this present study.