State equation parameters of three homologous series: tetraalkyltin compounds, tetraalkoxysilanes, trialkylamines

Abstract

Expansion coefficients and thermal pressure coefficients γ have been measured at 25 °C for members of the following series of compounds: (1) SnR4 (R = CH3, C2H5, C3H7, C4H9, C12H25). (2) Si(OR)4 (R = being the same except for C6H13 and C8H17, instead of C12H25). (3)NR3, R from C2H5 to C12H25 except C9H19 and C11H23; two NR3′ where R′ is a branched (br) alkyl radical isopentyl and isooctyl and N(CH3)2(C12H25). (4) n-C7H16, 3-3-diethylpentane (3,3-DEP), dimethylsulfoxide (DMSO), 2,2,4,4,6,8,8-heptamethylnonane (br-C16). The variation of γ between 25 and 80 °C is obtained for the following compounds: br-Q6, N(isopent)3 and N(Dec)3. The comparison of the cohesive energies for the different members of a series or from one series to another is made with P* rather than with γ. [Formula: see text]. For the three series the limiting values, when n increases, of the pressure reduction parameter P* and of the expansion coefficients are very similar. The P* do not have regular values for the NR3, SnR4, and Si(OR)4 series for low values of the molecular weights. One explanation of these irregularities is the different shape of the molecules. The surface to volume ratios of the different molecules are calculated with a spherical model or a cylindrical model. Values of cohesive energies per unit surface P*/s are compared. P*/s for the linear and branched alkanes are on the same curve while the P* were not. However, no similar smoothing is accomplished for the lower members of the other series. The high values of P*/s for the sterically hindered SnEt4, SnPr4, and 3,3-DEP seem to indicate that measurements of y and P* can be a method to evaluate steric hindrance in the different members of a series. Report is made of calorimetric results on the same sterically hindered compounds in solution. Higher P*'s may also come from increased polarity in the lower members of a series.