Journal of Colloid and Interface Science 255, 403–409 (2002) doi:10.1006/jcis.2002.8616 Interactions between a Nonionic Gemini Surfactant and Cyclodextrins Investigated by Small-Angle Neutron Scattering E. Alami, ∗,1 S. Abrahms´ en-Alami, ∗,2 J. Eastoe, ∗ I. Grillo,† and R. K. Heenan‡ ∗ School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom; †ILL, BP 156-X, F-38042 Grenoble Cedex, France; and ‡ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX United Kingdom Received February 8, 2002; accepted July 16, 2002 The microstructure of complexes between hydroxypropyl– cyclodextrins (HPCDs) (α, β , and γ ) and a novel gemini surfactant has been investigated by small-angle neutron scattering (SANS). This nonionic hetero-gemini surfactant (denoted NIHG750) con- tains two hydrophobic groups and two hydrophilic groups. One is a methyl-capped polyoxyethylene chain with 16 oxyethylene units and the other is a secondary hydroxyl group. Various form factor models have been considered for fitting the SANS data. Spheri- cal aggregates (25 to 40 ˚ A) with a size slightly larger than that of NIHG750 micelles (about 23 ˚ A) appear in mixed systems. These could be micellar aggregates partly covered with a few cyclodextrin molecules. In addition, the results indicate rod formation (r ∼ 8 ˚ A, L ∼ 70 ˚ A) for the NIHG–HPCD complexes. This result is consistent with the threading of HPCDs onto NIHG750 to such an extent that the surfactant molecule takes an extended conformation at high levels of HPCD. Also, the results indicate that HPCDs may interact with the oxyethylene groups of the spherical micellar aggregates leading to an increase in micelle size and a gradual transformation to rod-shaped aggregates. The tendency to form rods increases in the order γ -CD <α-CD <β -CD. An increase in HPCD concentration results in an increased amount of rods in the system. All spheri- cal aggregates disappear at relative amounts of HPβ CD above 4 to 5 (molar ratio). However, for HPαCD and HPγ CD spherical ag- gregates coexist with rod shaped aggregates in the whole range of concentrations investigated here. C  2002 Elsevier Science (USA) Key Words: gemini surfactant; hydroxypropyl–cyclodextrin; in- clusion complexes, small-angle neutron-scattering; micellar model. INTRODUCTION Over the past two decades the field of supramolecular chem- istry has developed new “intelligent” or “smart” materials that exhibit large property changes in response to small physical or chemical stimuli. Inclusion complexes consisting of cyclic molecules and surfactants or polymers represent such a new type of molecular assembly often termed molecular necklaces (MN). Cyclodextrins (CDs) are rigid “molecular doughnuts” composed 1 To whom correspondence should be addressed. Current address: Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden. E-mail: ealami@surfchem.chalmers.se. 2 Current address: AstraZeneca R&D M ¨ olndal, SE-431 83 M ¨ olndal, Sweden. of six to eight (α-1,4)-linked α-D-glucopyranose units with inter- nal cavities ranging from 5 to 8 ˚ A(α-, β -, and γ -cyclodextrin). Owing to this structure CDs readily form inclusion complexes through noncovalent interactions with molecular hosts of ap- propriate size. They are used in a variety of applications as sol- ubilizing agents, for example, in pharmaceutical formulations where they may be used in combination with nonionic surfac- tants, which are employed due to their low toxicities (1, 2). One reason for mixing two solubilizing agents is to minimize the con- centration of each component so that toxic levels are not reached. Interactions between cyclodextrin and nonionic surfactants have been studied previously using methods such as fluorescence (3, 4), nuclear magnetic resonance (4, 5), and surface tension (5–8). Native cyclodextrins often form solid crystalline complexes with polymers; for example, polyethyleneoxide (PEO) is known to form crystalline complexes with α-CD, polypropyleneoxide (PPO) forms crystalline complexes with β -CD, and the larger cavity γ -CD forms complexes with both PPO and two chains of PEO. The studies of Topchieva and Karesin imply that an interac- tion may also take place between α- or γ -CD and the hydrophilic part of surfactants containing much shorter oxyethylene groups (6). These complexes are termed pseudo-rotaxanes or, if poly- mers are the guest molecules, pseudo-polyroxatanes. Structures of crystalline CD host–guest complexes have been studied by methods such as X-ray crystallography and solid-state 1 H- and 13 C-NMR (6, 9, 10). In the literature less interest has been placed on soluble cy- clodextrin complexes formed with surfactants and polymers, al- though some studies with surfactants and Pluronic copolymers have been reported using surface tension, light scattering, and small-angle neutron scattering (6, 11, 12). In this paper a new type of nonionic gemini surfactant based on renewable resources has been investigated (13). The aggre- gation and adsorption properties of this surfactant have been thoroughly studied by our group previously (13). In addition, the aggregation behavior of complexes formed by this surfactant and cyclodextrins has been studied previously by PGSE NMR self-diffusion and surface tension (5). Studies of the interaction of this surfactant with cyclodextrin are particularly interesting due to its relatively bulky hydrophobic part. 403 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.