Aggregation phenomena in water-alcohol solutions. Thermodynamic and dynamic studies

Abstract

In order to elucidate the aggregation phenomena in alcoholic aqueous solutions, we present some experimental results from small-angle neutron scattering (SANS), elastic and quasielastic light scattering and ultrasonic spectroscopy. We find that, by increasing the alkyl chain length of the alcohol, the molecular aggregates change from small fluctuating wateralcohol groups to micelle-like structures. The role of temperature and concentration is also discussed.

Molecular aggregations in water–2-butoxyethanol mixtures by ultrasonic and Brillouin light-scattering measurements

“…A similar conclusion was reached by Ito et al [10,11] by interpreting Rayleigh scattering spectra and by Kato [29] by evaluating the Kirkwood-Buff parameters of the mixtures. More direct evidence for BE aggregations is given by recent SANS [13] and light-scattering experiments [12]. According to the above indications, we consider the narrow concentration range around X, * as reminiscent of the critical micelle concentration (CMC) of a binary surfactant system.…”

“…Apparently this conclusion disagrees with SANS experiments [13], which show the presence of large BE aggregates whose size monotonically decreases as T decreases. However, recent QELS experiments [12] show noticeable increases of both scattered intensity and mean hydrodynamic radii (up to -2000 A) for 0.03~X (0.07 as the temperature decreases below -25'C. This apparent contradiction with SANS results was ascribed to the different scale lengths probed by neutronand light-scattering experiments and interpreted as due to an interaction process among micellelike alcohol aggregates that gives rise to the formation of large clusters of correlated micelles.…”

“…A recent and accurate determination of the low-temperature portion of the phase diagram [12] enables us to locate the lower critical solution temperature at T, =49.2'C and X, =0.052 (see Fig. 8).…”

“…In light of recent structural and dynamical indications from SANS [13] and light scattering [12] experiments, this paper extends our ultrasonic investigations to a wider composition range and, in addition, we have performed Brillouin light-scattering (BLS) measurements. From these experiments we hope to gain a better understanding of the ultrasonic dynamics and the related aggregation processes over large temperature and concentration ranges.…”

“…These experiments support the presence of alcohol aggregates similar to micelles. The ability of BE amphiphilic molecules (C&H9OC2H&OH) to form topologically ordered surfactantlike aggregates is also suggested by their chemical structure and phase diagram [12], which are similar to those of long-chain nonionic surfactants C"Hz"+,(OCH2CH2) OH (polyoxyethylene monoalkyl ethers, usually denoted as C"E ). BE can be considered as a short (C4E, ) homologous member of the C"E series, so that one expects that static and dynamic properties of its aqueous solutions exhibit behaviors similar to those in the corresponding C"E solutions.…”