Surface tensions of aqueous solutions of lithium dodecyl sulfate, sodium oleate, and dodecylbenzene sulfonic acid in contact with methane under hydrate-forming conditions

Abstract

This paper presents experimental surface tension data of separate aqueous solutions of lithium dodecyl sulfate, sodium oleate, and dodecylbenzene sulfonic acid, each in contact with methane at a pressure of 3.90 MPa or 4.00 MPa and a temperature of 275 K (i.e., a condition in which a clathrate hydrate of methane is thermodynamically stable). These data were obtained by use of the pendant drop method in the metastable absence of any hydrate in the experimental system (i.e., every measurement was accomplished during the induction time for the hydrate formation). The surfactant concentration ranged up to 5.0 g kg⁻¹ for lithium dodecyl sulfate, 0.6 g kg⁻¹ for sodium oleate, and 1.0 g kg⁻¹ for dodecylbenzene sulfonic acid. Based on the obtained surface-tension versus surfactant-concentration data, we estimated the critical micelle concentrations for lithium dodecyl sulfate, sodium oleate and dodecylbenzene sulfonic acid to be about 3.2 g kg⁻¹, 0.12 g kg⁻¹ and 0.39 g kg⁻¹, respectively.

Introduction

Clathrate hydrates (abbreviated hydrates, hereafter) are crystalline solid compounds formed from water and various guest substances including light hydrocarbons, carbon dioxide and some fluorocarbons. Various industrial applications of such hydrates have been proposed and are currently under study; for example, the storage and transport of natural gas or hydrogen, the separation of carbon dioxide from flue gas at coal-fired power plants, the recovery of clean water from the waste water generated at paper-making mills, and the cool storage for residential air conditioning are typical applications. One of the technical requirements common to these applications is to provide a sufficiently high rate of hydrate formation. A promising means for drastically increasing, even with no aid of mechanical agitation, the rate of hydrate formation is empirically known; that is to add a small amount of an appropriate surfactant (for example, sodium dodecyl sulfate or its homologue) to the liquid water which is to be brought into contact with a guest substance (mostly in the gaseous state) in a hydrate-forming reactor [1], [2], [3], [4], [5], [6], [7], [8]. There has been a controversy about the role of the added surfactant on the hydrate formation. A hypothesis was once proposed that claimed the surfactant micelles encapsulating the guest-gas molecules to be the primary promoter for hydrate formation [3]. This hypothesis was soon spread in the hydrate research community and has been occasionally cited in the literature. However, this hypothesis has been disputed and denied by several research groups on the basis of different arguments [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The most straightforward argument against the hypothesis was probably that micelles could not be formed by many surfactants including sodium dodecyl sulfate (SDS), an anionic surfactant most extensively used in hydrate studies, at temperatures used in ordinary hydrate-forming operations (typically less than 283 K) [4], [9], [13], [14]. That is, the lowest micelle-forming temperature, known as the Krafft point, for each of such surfactants added to water is generally higher than the temperatures used in the hydrate-forming operations. Brief but reasonably detailed surveys of the controversy about the “surfactant micelle hypothesis” and the working mechanism of enhanced hydrate formation by the surfactant addition were given by Okutani et al. [8] and Ribeiro and Lage [15].

Although the “surfactant micelle hypothesis” as a plausible model for interpreting the ever observed promotion of hydrate formation by surfactant addition [1], [2], [3], [4], [5], [6], [7] has already been, in our opinion, scientifically invalidated, an interesting question is still left to be clarified; i.e., if surfactant micelles were actually formed in an aqueous phase under hydrate-forming conditions, how they could affect (or not affect) the hydrate formation. Experiments to reveal this point must be done at a temperature T higher than the Krafft point for a given surfactant and at a pressure p higher than the three-phase (hydrate + water-rich liquid + gas) equilibrium pressure corresponding to the experimental-system temperature T. If we use SDS in a methane-hydrate-forming system, the required conditions are: T > 289 K [4], [13] and p > 14.2 MPa [16]. The experimental observation of methane-hydrate formation at such a high pressure condition is technically difficult and of little concern from an engineering viewpoint. Fortunately, there are some, though not very many, surfactants that have sufficiently low Krafft points, thus allowing the micelle formation at hydrate-forming conditions under moderate pressures. Thus, it is meaningful to determine the surface tensions of aqueous solutions of such a low-Krafft-point surfactant over a wide concentration range including the critical micelle concentration (CMC) under a p–T condition approximating those to be set in ordinary hydrate-forming operations for laboratory research or for possible industrial use. This idea has been implemented in this study, using three anionic surfactants – lithium dodecyl sulfate (LDS), sodium oleate (SO), and dodecylbenzene sulfonic acid (DBSA). The formation of micelles of SO and DBSA under a methane-hydrate-forming condition (p = 4 MPa, T = 275 K) was first confirmed by Di Profio et al. [10] using the electrical-conductometric technique. As for LDS, no such information has yet been reported. In this paper, we present surface tension data that we have obtained by applying the pendant drop method to LDS, SO or DBSA solutions each equilibrated with methane gas in a closed system controlled at p = 3.90 MPa or 4.00 MPa and T = 275.0 K.