The aggregation and surface activity ofN-butyl-2-pyrrolidone (BP) in water is investigated using surface tension, solubilization, fluorescence and NMR methods. The micelle point is high (-1.0 M) as expected for the butyl chain length, and the aggregation number is approximately 5, with pre-association below the c.m.c. Despite the low aggregation number, the fluorescence data indicate that the interior of the aggregate is similar to pure liquid BP. The adsorption ofBP at the solution/air

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... e goes to a maxmum near the c.m.c., corresponding to an area of 4S ~ 1 N/molecule. Introduction Because of the industrial interest in these compounds, the adsorption behavior, surface activity and micellization properties ofN-aikyl-2-pyrrolidones have attracted recent attention [l]. Phase separation, surface activity and micellization ofN-hexyl-2-PYTTOlidone in aqueous solution have recently been reported[2]. Unlike the longer chain pyrrolidones such as the N-octyl or Ndodecyl compounds, which have been reported to show no micelles in solution[3-S], N-hexyl-2pyrrolidone fonDS micelles in aqueous solution at all temperatures from the freezing point up to the 569 www.dekker.com Copyri&ht 01999 by Marcel Dekker. Inc. . 570 LOU ET AL. lower consolute temperature LCT (19.1 .C) and up to 25 OC in the water-rich phase above the LCT. Surface activity and adsorption of a series ofN-aikyl-2-pyrrolidones at the air/solution interface at low surface pressures have been reported and interpreted in terms of two-dimensional virial coefficients and lateral intennolecular forces in the mor.olayers[6]. The properties of the interfaces of mixtures of the partially miscible short chain alkyl pyrrolidones with paraffInS have been studied and the adsorption of the longer-chain pyrrolidones at these non-aqueous liquid/liquid interfaces have also been reported[7]. In this work, the surface activity and micellization ofN-butyl-2-pyrrolidone (BP) in aqueous solution are reported using surface tension, NMR and fluorescence spectroscopic methods. It is found that BP forms micelles at a relatively high concentration and pre-aggregates below the apparent critical micellar concentration (c.m.c.). A small aggregation number is estimated from the 13C NMR chemical shifts of the BP solutions and confinned by the surface tension measurements. Materials and Metho!!M aterials: The N-butyl-2-pyrrolidone was provided by International Specialty Products Inc. with a purity of 99.6% from peak areas using GLC. The residual impurities were short chain homo logs or compoWlds with a methyl group substituted in the pyrrolidone ring. These impurities are unlikely to interfere with the surface and micellar properties. Pyrene (Aldrich Chemicals) was recrystallized from ethanol-water mixtures. Water was triply distilled with the second stage from alkaline pennanganate. Methods: Surface tension Surface tensions were measured at room temperature (23.5:1: 0.1 °C) mostly with the drop volume method using the Harkins and Brown corrections [8] . Measurements were also made with a platinum Wilhelmy plate coupled to a Cabo microblance. It was found that at higher BP concentrations, the plate method was inapplicable due to the onset of a finite contact angle. At lower concentrations (0.1 molal and below) the two methods for the surf~ce tension agreed well. The