POLYMER-SURFACTANT INTERACTION 27 0.1 0.01 o.ool o oo o o [3o o oo o ..-o [] oO o-' o o oO' ,C,,,• 0.1 0.001 0.01 10 CMC of SDS Sodium dodecyl sulfate, % Figure 2. Pseudo-phase diagram of the system Polyquaternium 10/sodium dodecyl sulfate/water. Symbols indicate: O clear solution ß precipitate ¸ slight precipitate '.g• hazy solution [] gel ...... theoretical charge neutralization -- maximum precipitation observed. mer and surfactant •0.200 and 0.100%, respectively. At polymer concentrations 0.200%, the concentration of SDS necessary to precipitate the Polyquaternium 10- SDS complex is independent of polymer concentration. This result agrees with results observed previously for Polyquaternium 10 and SDS (8,10). The charge density for MQNNED is 467 g/mol. therefore, theoretical charge neutral- ization should occur at a weight ratio of 1.6:1. The results for this system (Figure 3) indicate that this relationship holds true for concentrations of polymer and surfactant •0.020 and 0.015%, respectively. At polymer concentrations •0.010%, the concen- tration of SDS necessary to precipitate the MQNNED-SDS complex is independent of polymer concentration. The charge density for DQNNED is 569 g/mol. Since the graft on DQNNED contains two cationic charges, two theoretical charge neutralization weight ratios can be calcu- lated. The first weight ratio, 2.0:1, is related to the addition of one SDS molecule per two cationic charges. The second weight ratio, 1:1, is related to the addition of two SDS molecules per two cationic charges. The results (Figure 4) indicate that maximum precipitation of the DQNNED-SDS complex occurs between these two theoretical charge neutralization lines. Therefore, for concentrations of polymer and surfactant •0.010%, the DQNNED-SDS complex precipitates maximally when approximately 1.5 SDS molecules neutralize the charge of the two cationic sites. At low polymer concentrations (0.010%), the concentration of SDS necessary to precipitate the DQNNED-SDS complex is independent of polymer concentration.

28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.1.1 z z o 0.1 0.01 o o' o o o- ß --o o o o-o-ooo--o o o o o o o , o--,--C,,, • ........ 0.001 0.001 0.01 10 CMC of SDS 0.1 i Sodium dodecyl sulfate, % Figure 3. Pseudo-phase diagram of the system MQNNED/sodium dodecyl sulfate/water. Symbols indicate: O clear solution ß precipitate © slight precipitate [] hazy solution [] gel ...... theoretical charge neutralization -- maximum precipitation observed. The observed maximum precipitation patterns for the three systems is shown (Figure 5). The results indicate that the polymers precipitate at their respective theoretical charge neutralization ratio when the concentrations of polymer are •0.010, 0.020, and 0.200% for DQNNED, MQNNED, and Polyquaternium 10, respectively. However, at polymer concentrations •0.010%, the limiting SDS concentration necessary to precipitate in- creases as the structure of the polymer is varied: DQNNED-SDS MQNNED-SDS Polyquaternium 10-SDS. Therefore, when precipitation is independent of polymer con- centration, a polyelectrolyte containing two cationic charges per residue will precipitate at lower percent SDS than its monoquaternary analog, and a monoquat closely associated with the backbone will precipitate before a monoquat attached to a flexible spacer arm. PRECIPITATION STUDIES WITH SDS AND OCTOXYNOL Factors affecting polymer-surfactant association are the micelie surface charge density ((y), polymer linear charge density (•), and the Debye-HQckel ion atmosphere thickness (K -1) (25). The corresponding experimental variables are the mole fraction of charged monomers in the micelie (Ysr)s), structural polymer charge density (l/b) where b is the distance between charges on the polymer backbone, and the ionic strength of the solution (I) (25). The flexibility of the polyelectrolyte is believed to be another factor (4). For a given polyelectrolyte at constant I and b, a critical mole fraction (Yc) correspond- ing to a critical micelie surface charge density may be determined (25). Yc corresponds to the reversible formation of soluble polyelectrolyte-mixed surfactant micelie com- plexes as determined by initial increase in solution turbidity. Beyond this phase tran-