Surface methods-especially surface tension measurementshave long been used in the study of interaction between water soluble polymers and surfactants. Many factors control "reactivity" in such systems. For example, hydrophobicity (and surface activity) of the polymer can be a dominant factor in the case of uncharged polymer/ionic surfactant pairs, while oppositeness of charge can be dominant in polyelectrolyte/ionic surfactant systems. In the latter case, "ideal" surface behavior in the Gibbsian adsorption sense is often observed. Other "surface" methods used, and briefly alluded to, include neutron reflection, surface rheology, radio tagging, ellipsometry, X-ray reflectivity, and the study of foams. Interaction at the solid/water interface, traditionally studied in mineral dressing research, is receiving increasing attention. References to the use of 20 different instrumental methods are given, together with a brief discussion of the use of three of them, namely electron spin resonance, photon correlation spectroscopy, and atomic force microscopy. C 2002 Elsevier Science (USA)
I. INTRODUCTION
Recent years have witnessed an explosive growth in studies of the interaction of water-soluble polymers with surfactants, both in solution and at interfaces. It is with the latter category that this review is concerned. To curtail length, the "traditional" and still very active field of protein/surfactant interaction (1) is not treated. The beginnings of research on the interfacial properties of mixed synthetic water-soluble polymer, surfactant systems go back to the 1960s: they include studies at both the air/water interface and the solid/water interface. The former category receives emphasis here and only highlights of the rapidly expanding latter category are presented.
II. AIR/WATER INTERFACE
A. Uncharged Polymer/Charged Surfactant
1. Surface Tension
Considerable impetus was provided to research in the field as a whole by the surface tension studies of Jones (2). He showed 1 To whom correspondence should be addressed.
that in a mixture of the surfactant sodium dodecyl sulfate (SDS) and the relatively weakly surface-active polymer polyethyleneoxide (PEO), incorporation of the PEO led to two new transitions in the surface tension/logarithm of concentration (γ/ln c) plot of the surfactant. Jones labeled the transitions "T 1 " and "T 2 ," one below and one above the conventional critical micelle concentration (cmc), respectively. His interpretation was that T 1 signaled the initiation of interaction between the surfactant and the polymer (at a fixed level), and T 2 , the saturation point beyond which further addition of surfactant led to increasing formation of regular SDS micelles. Many workers today refer to T 1 as the critical association concentration (cac). An idealized depiction of this behavior is given in Fig. 1. A refinement is the inclusion of a point T 1 2 , which refers to "saturation" of the polymer. Thereafter there is a buildup of monomer concentration, and hence surface tension reduction, in the T 1 2 -T 2 region. Jones' own results, and subsequent work, especially by Lange (3) on the polyvinylpyrrolidone (PVP)/SDS system, led to further modification of the "model" interaction scheme (see Fig. 2). This version allows for the observation that in interacting systems in the dilute region (concentration < T 1 ) the surface tension is always below that of the surfactant solution by itself, implying that an interacting polymer is surface active in its own right. Furthermore, there is no reason, a priori, that a surface tension plateau exists in the T 1 -T 1 2 region, and, in fact, a gradual drop to the value at concentration T 2 is most often observed. A recent finding of great interest is that a maximum in surface tension can exist in the T 1 -T 1 2 region, for example, in the PVP-SDS system if the polymer concentration is ≥1% (4). (For illustration, see the lower and upper dotted lines in Fig. 2, respectively.) These behaviors are undoubtedly concerned with the changing composition (ratio and amounts of polymer and surfactant) of the surface in the T 1 -T 1 2 region. It should be mentioned that very precise measurements in bulk phase using microcalorimetry are providing evidence that the interaction patterns in comparable systems in the T 1 -T 2 region are themselves more complicated than previously thought (5,6). More work in this area can be expected. Direct measurements on surface composition are referred to below.
A potentially important role of the surface activity of the polymer itself is implied by the above. Indeed, this may be inferred from the listing by Breuer and Robb (7) of the "reactivity" 228