This behaviour could be explained considering that the hydrophilic centers are distributed relatively homogeneously on the surface of the lignin (Fig. ). These centres are occupied in the initial phase with increasing relative humidity. With increasing relative humidity, water-water interactions increase, water molecules then also occupy the interstices. At extremely high humidity, different water layers are likely to form and interact with each other. During desorption, these layers are gradually removed, leaving a true monolayer that forms the knee-like formation in the desorption curves.