Colloids and Surfaces A: Physicochem. Eng. Aspects 294 (2007) 221–227 Sorption characteristics of CTMA–bentonite complexes as controlled by surfactant packing density Runliang Zhu, Lizhong Zhu ∗ , Liheng Xu Department of Environmental Science, Zhejiang University, Hangzhou 310028, China Received 27 June 2006; received in revised form 8 August 2006; accepted 8 August 2006 Available online 14 August 2006 Abstract This work was to examine the relationship between the configuration and sorption characteristics of surfactant–clay complexes. Various amounts of cetyltrimethylammonium bromide (CTMAB) were intercalated into the bentonite matrixes with different layer charges. Packing densities of the adsorbed surfactants and sorption characteristics of the obtained CTMA–bentonite complexes towards phenol and naphthalene were examined. Experimental results indicated that packing density of the adsorbed surfactant was proportional to the surfactant loading amount and layer charge of the bentonite, and sorption capacities of these complexes had a close relationship with the surfactant packing density. That was, with the increase of surfactant packing density, the organic-carbon normalized sorption coefficient (K oc ) first rose till the maximum, and then began to decrease as the packing density further increased. This could be interpreted that increase of surfactant packing density would render the surfactant phases more hydrophobic environment, and the hydrophobic affinity of the surfactant phases towards the solutes thus increased accordingly. However, in the high surfactant packing density region, the densely packed surfactants reduced the available free space for the solutes, resulting in decrease of sorption capacity for these complexes. Hence, with the increase of surfactant packing density, the adsorbed surfactants would form a series of partition phases showing different affinity to the solutes. © 2006 Elsevier B.V. All rights reserved. Keywords: Sorption; Organic clay; Surfactant; Hydrophobic organic compounds; Partition phase; Layer charge 1. Introduction Numerous studies have investigated the sorption characteris- tics of surfactant–clay complexes towards hydrophobic organic compounds (HOCs) because of their wide variety of applications [1–7]. The hydrophobic medium created by the long alkyl chain of the adsorbed surfactant shows high affinity to HOCs, and thus the complexes are considered as potential sorbents in wastew- ater treatment [1–4], appropriate landfill liner [5] and effective barrier to prevent down-gradient pollution of groundwater and aquifer from organic pollutants [6,7]. Partition of HOCs to the organic phase created by the adsorbed surfactants was considered to be the predominant mechanism controlling sorption of HOCs on surfactant–clay complexes [4,8], and the sorption coefficient, K d (the ratio of solutes concentration on the sorbents to that in solution), was ∗ Corresponding author. Tel.: +86 571 88273733; fax: +86 571 88273450. E-mail address: zlz@zju.edu.cn (L. Zhu). suggested to be proportional to the amount of the adsorbed surfactant [9,10]. However, an increasing number of inves- tigations have observed that sorption characteristics of these complexes are strongly dependent on their configurations, and the organic-carbon content (f oc ) normalized sorption coefficient (K oc = K d /f oc ) can be quite different accordingly [11–14]. Smith et al. [11] observed that the surfactant–clay complexes prepared from the surfactant with relatively longer alkyl chain had larger K oc for HOCs. Esumi et al. [12] found that silica modified from the surfactant with two alkyl chains had larger K oc than that with one or three alkyl chains in sorption of 2-naphthol. Even for the surfactant–clay complexes synthesized from the same surfac- tant, K oc can be greatly influenced by the surfactant loading amount [3,13,14]. Although the idea that K oc can vary with the configurations of the surfactant–clay complexes has gradually been accepted, the relationship between the configuration and sorption characteris- tics of these complexes is still ambiguous. Sorption properties of the surfactant–clay complexes should be ascribed to their struc- tural difference. In fact, microstructures of the surfactant–clay 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.08.016