POLYMER/SURFACTANT INTERACTION 25 of polymers tends to be irreversible. It should be noted that both the above tendencies, i.e., to self aggregate or adsorb, are increased if the polymer contains hydrophobic groups. EVIDENCE OF POLYMER/SURFACTANT INTERACTION It is now of interest to discuss what occurs when these two entirely different types of material co-exist in the same aqueous solution. There is much evidence that they tend to associate. Many techniques have been employed to study the association of polymers and surfactants (3). Here we choose a half dozen or so for description and illustration of the phenomena that can occur. SURFACE TENSION A very simple and, as will be seen, informative technique for the study of polymer/sur- factant pairs involves ordinary measurements of surface tension. In these mixtures, it is often the case that the polymer is feebly surface-active hence, the measured surface tension will be a sensor of the free or uncomplexed surfactant in solution. Departure of the mixed systems from the surface tension behavior of polymer-free surfactant solutions is an indication of binding of the surfactant by the polymer or of complex formation. An ideal system is illustrated in Figure 2. Here, as the surfactant concentration is increased, the with- and without-polymer curves will coincide until a critical concentration (T•) for binding is reached. If the binding energy is high enough (and constant), the surfactant concentration, and hence the surface tension, will remain constant until the polymer binding sites are saturated. The resultant plateau will end at the saturation concentration T2'. Beyond this point the concentration of free surfactant will rise and the surface tension will drop along curve T 2' T 2 until regular micelies form, i.e., rejoin the curve of the polymer-free system. The concept of transition concentrations, T• and T2, was introduced by Jones (6) in his studies of polyethylene oxide (PEO)/sodium dodecylsulfate (SDS) pairs. His findings were confirmed and extended by Schwuger (7), who established that a minimum PEO molecular weight of 600 was necessary for the association reaction to occur. At MW 1550 the interaction was more pronounced, and beyond 4000 it was strong and essen- tially independent of molecular weight. See below. Another system that has been widely studied by the surface tension method [e.g., by Lange (8), Schwuger and Lange (9), and Arai and co-workers (10, 11)] is polyvinyl pyrrolidone (PVP)/SDS. Features, essentially similar to those noted with PEO, were obtained (Figure 3) and are summarized as follows: 1. Transition concentrations T• and T 2 straddled the CMC of SDS. 2. T 2 increased directly with the amount of polymer, while T• varied very little. 3. The presence of salt markedly reduced T•, i.e., association occurred at a much lower surfactant concentration in its presence. 4. Constancy of surface tension, implying constant SDS activity, was more evident at higher polymer concentration. The picture that emerges is that an association reaction (see below) of surfactant occurs in the presence of polymer. Since the association reaction starts at a lower concentration

26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Z z I- LOG CONCENTRATION Figure 2. Diagrammatic surface tension/concentration plot of a surfactant in the presence of a complexing polymer. than the CMC, it can be deduced that it is more energetically favorable than micelie formation. Only when the polymer is saturated will regular micelies form (on further addition of surfactant). A special case of polymer/surfactant interaction occurs if the polymer is charged and its charge is opposite in sign to that of the surfactant. Such a case occurs for the system polyquaternium 10 (PQ-10), a cationic cellulosic, and SDS (12). This polymer by itself is very weakly surface-active at the air/water interface. Figure 4 shows strong synergistic surface activity of the surfactant/polymer combination over a wide range of added SDS. These results, together with those obtained from model experiments with insoluble monolayers of sodium docosyl sulfate (13) spread on an aqueous solution of PQ-10, show clearly that the polymer/surfactant complex that forms is highly surface-active. It is of interest that the "parent" molecule of PQ-10, viz. uncharged hydroxyethyl cellu- lose (HEC), shows no association tendency with SDS by the surface tension method. SURFACTANT BINDING STUDIES Direct measurements of surfactant binding by polymers afford a very convincing dem- onstration of the formation of polymer/surfactant complexes. Dialysis equilibrium, tra-