The control of the colloidal stability of latex particles is essential to obtain homogenous films. The manufacture of latex gloves is required to control the flocculation of latex particles, induced by the addition of calcium salts. This flocculation can, however, be retarded by the addition of very small amounts of surfactants. The present publication describes the effect of Ca(NO3)2 on the stability and rheological properties of natural rubber latex in the presence of small concentrations of a hydrophobically modified polyfructose (Inutec NRA). This surfactant strongly adsorbs on the surface of the latex particles, forming a repulsion layer. However, addition of Ca(NO3)2 causes a shrinkage of this layer. The colloidal stability, as a function of surfactant and Ca(NO3)2 concentrations was studied by determining surfactant adsorption, zeta potential and rheological properties. The addition of Ca(NO3)2 increased surfactant adsorption, due to the poorer solvency of the medium for the polyfructose chains, which became less hydrated in the presence of Ca2+. The addition of calcium also reduced the absolute value of zeta potential, due to compression of the repulsion layer. However, as the surfactant was adsorbed onto the surface of the particles, the latex particles remained stable even after charge reversal at high Ca2+ concentration, due to the higher steric stabilisation conferred by the surfactant. Flow behaviour studies showed a reduction in the maximum packing fraction (ϕmax ) of latex with surfactant concentration, due to formation of the repulsion layer that increased the distance between particles. However, the presence of Ca(NO3)2 increased ϕmax , indicating a reduction in the interparticle distance caused by compression of the repulsion layer between particles. Moreover, the addition of Ca(NO3)2 also decreased the cohesive energy density of the latex, resulting in a more viscous behaviour. These results demonstrate the ability of calcium ions to dehydrate surfactant molecules, which is crucial for explaining the macroscopic properties of latex suspensions.

The authors would like to thank the Director General of the Malaysian Rubber Board for the permission to publish this work. Support from the Malaysian Rubber Board, Beneo (Belgium) and CreaChem (Belgium) for this collaborative project is highly appreciated. The authors also greatly acknowledge financial support from the Malaysian Rubber Board for funding the PhD studies of Dr Manroshan Singh, as well as the Spanish Ministry of Science and Innovation (CTQ2014-52687-C3-1-P and CTQ2017-84998-P grants). Support from Generalitat de Catalunya (2014SGR1655 and 2017SGR1778 grants) is recognised. The Grupo de Nanotecnología Farmacéutica, of the University of Barcelona (UB), which forms an Associated Unit to CSIC, is also commended. Authors are equally grateful to the European Regional Development Fund (Fondo Europeo de Desarrollo Regional, FEDER), and AEI (Agencia Estatal de Investigación).

Peer reviewed