The interactions of amphiphilic latexes with surfaces: the effect of surface modi®cations and ionic strength Stuart W. Prescott a , Christopher M. Fellows a , Robert F. Considine b , Calum J. Drummond b , Robert G. Gilbert a, * a Key Centre for Polymer Colloids, School of Chemistry, Sydney University, Sydney, NSW 2006, Australia b CSIRO Molecular Science, Private Bag 10, Clayton South, Vic. 3169, Australia Received 3 September 2001; received in revised form 17 December 2001; accepted 11 February 2002 Abstract The effect of surface modi®cations brought about by a polymeric stabilizer on the interactions between polymer colloid particles and various substrates in aqueous media are directly measured using atomic force microscopy. The interactions of polystyrene particles with grafted hydrophilic `hairs' of hydroxypropyl cellulose denoted HPC/PS), of molecular weight ,10 5 , with mica, silica and graphite substrates are measured. HPC/PS is found to be compatibilized so that it will interact with both hydrophobic and hydrophilic substrates. The observed jump-to contact between HPC/PS and silica is characteristic of polymer solutions and is the result of the grafted hairy layer. Further direct evidence of HPC±substrate interaction is seen in a secondary adhesion with mica. The adhesion of the particles was found to follow the order silica . graphite . mica. The magnitudes of these interactions are rationalized in terms of the interactions of each of the substrate, core polymer and surface modi®cation. It is concluded that the combined effects of surface roughness and hairy layer collapse due to compression give rise to the observed trend. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Adhesion; Surface interactions; Colloid probe atomic force microscopy 1. Introduction There is as yet insuf®cient fundamental scienti®c under- standing of how the forces between latex particles and surfaces vary with particle coating e.g. the presence of a polymeric stabilizer). Nevertheless, many commercial products such as water-based adhesives and paints rely on the interactions between coated colloidal particles and substrates for their properties. A better understanding of the effects of surface coatings on adhesive interactions would allow the continued development of existing products and also open the way to new products such as protective coatings suitable for use on seeds in broad-acre agriculture. This study extends recent work measuring the interactions that occur between individual latex particles and various substrates using atomic force microscopy AFM) [1]. The direct measurement of the interactions between latex particles has recently been undertaken using AFM to assess the behavior of polymer colloids in relation to classical colloid theory [1]. Other studies have looked at the inter- actions between surfaces coated with adsorbed polymer chains [2±4], or a functional monolayer [5]. In contrast to many previous studies, the work reported here uses particles with known surface composition. This study involves the measurement of adhesive inter- actions between a polymer colloid and silica, mica and graphite surfaces. The effect of ionic strength on the inter- actions was also determined. The observed adhesive interactions are rationalized in terms of the interactions observed in studies of other systems. 1.1. Adhesion theory The interactions between surfaces in aqueous media are typically characterized by four distinct regions [2,5,6]: a region of zero force, a jump-to contact, a region of constant compliance and an adhesive hysteresis. Of particular inter- est in this study are the jump-to contact and the adhesive hysteresis. The jump-to contact is a result of the attractive forces between the surfaces becoming strong enough to give a region of mechanical instability, suddenly drawing the surfaces together. The adhesive hysteresis is due to the short-range forces between the surfaces holding the sur- faces together as the separating force is increased. The peak Polymer 43 2002) 3191±3198 0032-3861/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0032-386102)00141-6 www.elsevier.com/locate/polymer * Corresponding author. Tel.: 161-2-9351-3366; fax: 161-2-9351-8651. E-mail address: gilbert@chem.usyd.edu.au R.G. Gilbert).