Microgel systems containing phospholipid: Role of component interactions on rheology and tribology

Microgel systems containing phospholipids are often used in personal care products due to their gelling and emulsifying properties. Herein, we investigate how the type of microgel (hydrophobically modified versus hydrophilic) and its interaction with phospholipid manifest itself on the tribology and rheological behavior of the system. We examine two different polymers one forming a microgel with hydrophobic moieties on the surface (Pemulen) and the other being a hydrophilic microgel (Carbopol). While both polymers exhibit gel-like features rheologically, their mode of interaction with phospholipid is different. Phospholipid addition to the Pemulen leads to an increase in elastic modulus because of the interaction of the hydrophobic moieties with the phospholipid tails. Such active participation is verified using heat of interaction measured through isothermal calorimetry. Tribological behavior, measured with a soft model polydimethylsiloxane (PDMS) contact, reveals lower friction coefficients in the boundary regime for the hydrophobic Pemulen with and without phospholipid. Adsorption of phospholipids and Pemulen on the PDMS substrate are atrributed to the decreased friction coefficient, which we verify through optical microscopy and with Quartz Crytal Microbalance measurements.