Aggregative processes in aqueous solutions of isomeric 2-butoxyethanols at 298.15 K

Abstract

Ultrasound speed measurements across the entire composition range of aqueous mixtures of both isobutoxyethanol (iC₄E₁) and tert-butoxyethanol (tC₄E₁) have been made at 298.15 K with a sonic solution monitor that employs a “pulse–echo–overlap” technique. In addition, densities of aqueous mixtures of tC₄E₁ were determined using a vibrating tube densimeter. These new data were complemented with literature values for densities of aqueous mixtures of tC₄E₁ and for densities and sound speeds of aqueous mixtures of n-butoxyethanol (nC₄E₁). In all cases, density values were converted to molar volumes, V_m, and excess molar volumes, V_m^E. Estimates of the isentropic molar compression, K_S,m [= −(∂V_m/∂p)_S], and of its excess counterpart, K_S,m^E, were obtained from the combination of the ultrasound speeds and density values. Data reduction procedures were used to generate consistent sets of values for thermodynamic properties of isomeric amphiphiles with increasing degree of alkyl branching. The graphs for the composition dependence of excess partial molar volumes and isentropic compressions of water show enhanced visual impact. These graphs are used for presenting evidence for identifying the prevailing patterns of molecular aggregation. Segmented-composition models, including a version onto which a mass action component has been grafted, are employed, together with a simplified pseudo-phase model, to analyse the various excess molar quantities. The experimental evidence thus obtained is used to relate the effect of chain branching with the degree of self-aggregation of amphiphiles in aqueous solution. An unexpectedly low self-aggregation among iC₄E₁ molecules is found and discussed in terms of vicinity to the lower critical solution temperature.