Colloids and Surfaces A: Physicochemical and Engineering Aspects 88 (1994) 27-32 COLLOIDS AND SURFACES zyxwvuts A An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil-in-water emulsions 1. Poly(acrylic acids) and hydroxyethyl celluloses Robert Y. Lochhead, Christopher J. Rulison * zyxwvutsrqponmlkjihgfedcbaZ Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406, USA Received 29 July 1993; accepted 5 January 1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUT Abstract Pseudophase diagrams for the system cyclohexane-water-polymer were constructed and the compositional regions of emulsion stability and coalescence were identified. The polymeric stabilizers investigated were hydrophobically modified poly(acrylic acid) (HMPAA), hydrophobically modified hydroxyethyl cellulose (HMHEC) and analogous poly(acrylic acid) (PAA) and hydroxyethyl cellulose (HEC) as controls. The critical overlap concentration c* of these polymers and their interfacial activities at the cyclohexane/water interface were determined. In the case of the acrylic acid polymers, c* varied with pH but a critical condition for emulsion stability appeared to be the c* value of the unneutralized polymer (c&). It was found that when HMPAA was incorporated as the emulsifier the emulsions coalesced when the pH was greater than 6.5 and simultaneously the HMPAA concentration was less than c&.+ The emulsions were stable above C& at all pH values and below pH 6.5 when the polymer concentration was less than c&. On the basis of these results, it is postulated that stabilization in the lower concentration range occurs by an electrosteric stabilization mechanism and in the upper concentration range by trapping the discrete oil droplets in a continuum of hydrophilic polymer segments. PAA stabilizes only below pH 6.5 and below c&. Both HMHEC and HEC are ineffective below c* and only HMHEC is effective as an emulsifier above c*. Keywords: Associative thickening; Hydrophobically modified hydrophilic polymers; Hydroxyethylcellulose; Poly(acrylic acid); Steric stabilization 1. Introduction The use of hydrophobically modified hydrophilic polymers as primary emulsifiers for oil-in-water systems has been described previously [l-3]. It is well known that polymers adsorbed on a disperse phase can prevent flocculation by the mechanism of steric stabilization. The steric stabilization mech- *Corresponding author. 0927-7757/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0927-7757(94)02818-D anism depends on the polymer being strongly anchored to the surface of the dispersed droplets. When the continuous phase is aqueous and the disperse phase is hydrophobic, anchoring of hydro- phobically modified hydrophilic polymers can be achieved through hydrophobic interaction, with the hydrophilic portions of the polymer extending into the aqueous phase. When the droplets diffuse into close proximity, osmotic and entropic effects between the polymer fringes cause repulsion between adjacent droplets [4]. Existing theory