Surface Structure of Solutions of Poly(vinyl alcohol) in Water

We use surface-specific heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG) and surface tension measurements to investigate the molecular structure of the surface of aqueous solutions of poly(vinyl alcohol) (PVA) polymers with average molecular weights of 10000 and 125000 g/mol. We find that the interfacial water molecules have a preferred orientation with their hydrogen-bonded O-H groups pointing away from the bulk, for both PVA₁₀₀₀₀ and PVA₁₂₅₀₀₀. This observation is explained from the ongoing hydrolysis of the acetyl impurities on the PVA polymer chains. This hydrolysis yields negatively charged acetate ions that have a relatively high surface propensity. For both PVA₁₀₀₀₀ and PVA₁₂₅₀₀₀ the strong positive signal vanishes when the pH is decreased, due to the neutralization of the acetate ions. For solutions with a high concentration of PVA₁₀₀₀₀ the interfacial water signal becomes very small, indicating that the surface gets completely covered with a disordered PVA polymer film. In contrast, for high concentrations of PVA₁₂₅₀₀₀, the strong positive water signal persists at high pH, which shows that the water surface does not get completely covered. The HD-VSFG data combined with surface tension data indicate that concentrated PVA₁₂₅₀₀₀ solutions form a structured surface layer with pores containing a high density of interfacial water.