The effect of different liquids on paper strength demonstrates the central role of hydrogen bonding.

Paper derives its mechanical integrity from a complex network of interactions between cellulose fibres, yet the relative importance of hydrogen bonding, hydrophobic interactions and electrostatic forces remains debated. Here, the wet tensile strength of paper sheets are investigated when exposed to water, a series of organic solvents with differing hydrogen-bonding characteristics, and aqueous salt and urea solutions designed to modify electrostatic and hydrophobic interactions. By systematically varying the liquid environment surrounding the fibre network, we directly probe the mechanisms governing inter-fibre adhesion. The results show that solvents capable of disrupting hydrogen bonding produce a pronounced loss of strength, whereas changes in ionic strength and electrostatic screening have only minor effects. These findings provide strong experimental evidence that hydrogen bonds constitute the dominant contribution to paper strength, in agreement with the classical model proposed by Campbell in 1933, but in contrast to more recent interpretations that downplay their significance. Hydrophobic interactions and van der Waals interactions appear to contribute secondarily to fibre bonding, although to a smaller extent than hydrogen bonding. The observed trends further reveal striking parallels between the loss of strength in different liquid environments and the hornification phenomenon occurring during drying of wet pulp fibres. Together, the results establish hydrogen bonding as the central mechanism underlying paper strength and suggest a broader conceptual link between fibre bonding and irreversible structural changes in cellulose networks.