The rheological behavior of a 1% w/w commercial hydrophobically modified (hydroxypropyl)guar (HMHPG) in water solution is examined in the presence of a low molecular weight nonionic surfactant. The linear and nonlinear response to steady and oscillatory shear flow, at different surfactant concentrations around the cmc, is mainly compared with the response of a 1% w/w aqueous solution of HMHPG alone, following a rheological approach previously used by the authors. Below the cmc of the free surfactant in water, the rheological results show a reinforcement of the reversible associating network mainly due to an increase of the number of intermolecular hydrophobic associations, with hardly no modification of the average ‘‘strength’’ of the hydrophobic junctions. Above the cmc, the rheological data show that there is a progressive weakening leading to the disruption of the intermolecular hydrophobic junction network. Those results are interpreted in terms of interactions of surfactant molecules with the regions of intermolecular hydrophobic association.

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