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POE chain length selectivity in the clouding of a triton surfactant

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Abstract

The POE chain length distribution between the coacervate and aqueous phases of a clouded Triton X-114 solution was studied. The average chain length of the surfactant in the aqueous layer was determined by HPLC and compared to the distribution in the original (unclouded) solution. Both the clouding temperature and the equilibration time were found to govern the chain length selectivity. A slight excess of longer POE chains were encountered in the aqueous layer, thus enriching the coacervate layer with shorter, less soluble chains. Chain length selection appeared to occur when the surfactant particles were dispersed in the clouded suspension, rather than after the establishment of the aqueous and coacervate layers.

Introduction

The most widely used nonionic surfactants are those having polyoxyethylene (POE) polymers as their hydrophilic moieties. The hydration of these POE chains gives rise to the aqueous solubility of the molecules, and their temperature-induced dehydration is chiefly responsible for the inverse relationship between aqueous solubility and temperature observed among these amphiphilic compounds [1], [2], [3]. Surfactants with long POE chains have the greatest solubility in water because of extensive hydrogen bonding with the solvent. As the dielectric constant of water decreases with temperature, hydration of the POE chains diminishes when the solution is heated. At a temperature known as the cloud point, dehydration is sufficient to cause the formation of macroscopic surfactant aggregates, which give the solution a cloudy appearance. If the elevated temperature is maintained and adequate time is allowed, the aggregates will sink to the bottom of the vessel and form a coacervate (concentrated surfactant) layer. This attribute of nonionic surfactants has been exploited to remove organic contaminants from aqueous solution, and to preconcentrate analytes [4], [5], [6], [7], [8], [9].

Industrial production of POE surfactants results in a Poisson distribution of hydrophilic chain lengths, attached to a generally monodisperse hydrophobic portion [10], [11], [12], [13], [14]. The POE chain length quoted for a surfactant is therefore invariably a weighted average of all lengths present in the product. This average POE chain length (POE#), combined with the nature of the hydrophobic portion, gives each surfactant its unique character. The hydrophobic moiety governs its critical micelle concentration (cmc), and the cloud point is especially sensitive to the POE# [1]. For instance, the Triton surfactants TX-100 and TX-114 have identical hydrocarbon chains, while their POE#'s differ by 2 units: their respective cmc values are 0.25 and 0.27 mM, while their cloud points 65 and 23 °C [9].

Detailed studies of surfactant partitioning between water and oil phases in binary systems have revealed that compounds with short POE chains have a relative preference for the medium of lower polarity [15], [16]. Likewise, size selectivity was found in the adsorption of nonionic surfactants onto silica surfaces [17], [18], [19]. Thus it was observed that molecules with longer POE chains partitioned into the aqueous layers of oil–water systems and preferentially adsorbed onto polar surfaces. These findings provided the incentive for the present investigation, in which the role of the POE chain lengths in the clouding process in aqueous surfactants, and hence the distribution of POE#'s between the aqueous and coacervate layers of clouded systems, is considered.

Reverse-phase high-pressure liquid chromatography (HPLC) has been shown to be suitable for the separation of nonionic surfactants based on their POE chain lengths [13]. Dilute surfactant solutions close to the cmc can be successfully separated without any preconcentration steps. The nonionic surfactant TX-114 was chosen for this work because of its low cloud point (23 °C) and the presence of an aromatic ring in its structure. This latter allows for sensitive detection by absorption at 254 nm. This absorption has been shown to be independent of POE chain length [20] and can thus be used for quantitative monitoring of the separated components.

Section snippets

Reagents

The Triton surfactants TX-114, TX-100, and TX-15 were obtained from Sigma Chemicals (St. Louis, MO) and used as received. Stock surfactant solutions (20.4 mM) were prepared with doubly deionized water treated to a resistivity of at least 14 MΩ cm.

Clouding

The surfactant stock solution was diluted to give 10 ml of 10.2 mM TX-114 for all measurements. Approximately 1 ml of each solution was set aside to be used as a control for the determination of POE#'s before clouding. The remaining solution was placed

Results and discussion

Clouding of TX-114 solutions was allowed to proceed at 30 and 50 °C. At 30 °C, which is 7 °C above the cloud point, separation of the solution into two layers appeared to be complete after ca. 3 h, but a relatively high surfactant concentration remained in the aqueous layer (Fig. 2). Even after an equilibration period of 169 h (∼1 week) at this temperature, this concentration did not fall below 3 mM (cf. 10 mM surfactant concentration in the control solution). In contrast, a similar equilibration

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