Multiple molecular driving forces mediate protein stability, association, and recognition in concentrated solutions. Here we investigate the interactions that modulate the nonideal solution behavior of two immunoglobulins (IoG1s) in highly concentrated solutions using two-dimensional vibrational correlation spectroscopy (2D-COS) and principal components analysis (PCA). A specific sequence of changes is observed in the concentration-dependent vibrational spectra of the highly viscous IgG solution that deviates from ideality, whereas that sequence is reversed for all other conditions examined. The asynchronous spectra reveal variation in beta-sheet and turn regions occur before intensity variations in disordered and alpha-helical regions as the concentration is increased for the highly viscous regime. This is in contrast to the sequence observed for all other conditions studied and to the idea that beta-sheet regions are resistant to concentration-dependent affects. Finally, we show that increased hydrogen bonding and electrostatics primarily modulate the intermolecular association and nonideal behavior. Specifically, 2D-COS and PCA analysis of the amide II region suggests that Glu and Asp residues trigger the change resulting in increased viscosity and association of one IgG.