Herein we report the first in-depth structural characterisation of simple linear carboxylic acids with alkyl tail length ranging from one to six carbon atoms. By means of the SWAXS technique, a pronounced nanoscopic heterogeneity evolving along the aliphatic portion of the molecule is highlighted. Via classical molecular dynamics, the origin of such heterogeneity is unambiguously assigned to the existence of aliphatic domains resulting from the self-segregation of the polar and apolar portions of the molecules. Furthermore, the structural correlation of aliphatic-separated polar domains is responsible for observing the so-called “pre-peak” in the SAXS region.