Similar to Ethanol and water, Acetic Acid is a protic (polar) solvent. This means that it dissolves both polar compounds, like salts and sugars, but also non-polar compounds, such as oils.
Peptide solubility characteristics vary strongly from one peptide to another. Residues such as Ala, Cys, Ile, Leu, Met, Phe, and Val will increase the chance of the peptide having solubility problems. Some peptides have low solubility in water and must be dissolved in other solvents such as Acetic Acid for positively charged peptides or 10 % Ammonium Bicarbonate solution for negatively charged peptides.
Related specifically to the peptides that we sell, IGF1 Long R3 does not dissolve well in water. Depending upon the volume of water vs the quantity of peptide, and the time available, IGF1 Long R3 will eventually dissolve in water, but it is much more reliable using 0.6% Acetic Acid to dissolve it.