Oct., 1961
MICELLARCHARACTER OF NON-IONIC DETERGENT SOLUTIONS
1787
TiLTRACESTRIFUGAL DETERMINATION OF THE MICELLAR (.IHA4.RACTEROF NON-IONIC DETERGENT SOLUTIONS. TI' BY C. W. DWIGGINS,JR.,AND R. J. BOLEN Petroleum Research Center, Bureau of Mines, Bartlesz,ille, Oklahoma Received March 7, 1961
h r i analytical ultracentrifuge was used to determine the temperature dependence of the anhydrous, weight-average, micellar molecular weight of a polyoxyethylated nonylphenol in aqueous solutions. Micellar molecular weights were determined several times under different conditions a t each temperature. Partial specific volumes were determined a t each temperature. The micellar molecular weight of this detergent increased a t a rate of 2217 ( f 8 6 ) molecular weight units per degree in the temperature range of 10.5 to 35.0'.
a good quantitative theory is not available. The statistical mechanical treatment of micelle formationlo is perhaps the most complete theory that has been proposed. However, it would be very difficult to calculate micellar molecular weights or their temperature dependence using this theory. Many of the parameters required for solving the equations would be difficult to determine with the necessary accuracv. In addition, because of the complexity of micelle formation, 3everal assumptions are necessary, and the theory is semiquantitative at present. The statistical mechanical theory does include a temperature term, suggesting that a temperature dependence of micellar molecular weight is likely. The molecules of the detergent studted contain varying numbers of oxyethylene groups, and the number of molecules of varying size should hc represented by Poisson's distribution formula. Fortunately, the transient-state ultracentrifuge methods used allow determination of weight-average, anhydrous, micellar molecular weights.*-P Theoretical -1chain-length distribution is not necessarily a disThe trans,ient-statemethod of determining molec- advantage since nearly all commercially available ular weights in the analyt'ical ultracentrifuge may non-ionic detergents are of this type. Studies of be used to avoid independent determinations of dctergcrits having a narrow or no chain-length disdiffusion cc~efficient~s,~~~ and synthetic boundary trihution would he of interest also, hut such polymethods n u y be used to determine relative optical ouvcthylcne detergents are difficult to ot~taiir concentrations without independent determinaExperimental t,ions of specific refractive increment^.^ The 4 Spinco Model E analytical ultracentrifuge, equipped treatment of the data obt>ainedfrom the Schlieren w t h Schlieren optics including a phase-plate diaphragm optics analytical ultracent'rifuge for non-ionic was used. A rotor temperature indicator control (RTIC) detergents has hecn given in detail6 and will not was used, and temperature control to 005" was obtained. Double-sector, 12-mm., cells were used for all of the work. be repeat'ed here. The experimental conditions, equipment and techniques The micellar molecular weights of non-ionic have been described.6 detergents in 2Lqueous solutions are expected to The detergent studied is a polyoxyethylated nonylphenol change with temperature. Phase separation at and is more than 99% pure surfactant.6 This detergent is manufactured under the trade name of Igepal CO-710 t'he cloud-point' temperatures of the detergents by the General Aniline and Film Company. suggest's, but does not prove, that a significant Solutions of the detergent had to be kept for several days temperature effect is present. for the numerous studies a t various temperatures. It had Although qualitative and semiquantit'ative treat- been noted previously that molds grew in several stock desolutions after several days of storage a t room temments of micelle formation have been proposed, tergent perature.6 During early experiments, good reproducibility
Introduction llicelle formation is one of the factors that affect the cleaning efficiency of detergents, and studies of this phenomenon should lead to a more complete understanding of detergency action. Among other applications non-ionic det'ergents have been used by the petroleum industry as water addit,ives? to increase the production of pet'roleum. I3ot,h light s ~ a t t e r i n g ~ -and ~ ultracent
