Article pubs.acs.org/Langmuir
Adsorption and Aggregation Properties of Homogeneous Polyoxypropylene−Polyoxyethylene Alkyl Ether Type Nonionic Surfactants Shiho Yada,† Toshiyuki Suzuki,‡ Satoru Hashimoto,‡ and Tomokazu Yoshimura*,† †
Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan Nikkol Group Cosmos Technical Center Co., Ltd., 3-24-3 Hasune, Itabashi-ku, Tokyo 174-0046, Japan
‡
S Supporting Information *
ABSTRACT: Homogeneous polyoxypropylene (PO)−polyoxyethylene (EO) alkyl ether type nonionic surfactants comprising alkyl, EO, and PO chains with identical chain length distributions (CnEOxPOy; alkyl chain length n = 10, 12, 14, or 16; EO chain length x = 4, 6, or 8; and PO chain length y = 1, 2, or 3) were synthesized from homogeneous polyoxyethylene alkyl ether bromide and monosodium polyoxypropylene by Williamson ether synthesis. The adsorption and aggregation properties of these surfactants were characterized (cloud point, surface tension, dynamic light scattering, smallangle X-ray scattering, polarization microscopy, and cryogenic transmission electron microscopy) and compared to those of conventional homogeneous EO alkyl ether type nonionic surfactants (CnEOx). The introduction of a PO chain to the EO terminal group of the CnEOx species lowered the cloud points, especially for x = 6. Contrary to our expectations, the CnEOxPOy surfactants adsorbed efficiently at the air/water interface, despite their complex structure (hydrophobic alkyl chain/hydrophilic EO chain/hydrophobic PO chain). They also displayed excellent micelle-forming ability in solution. Furthermore, the CnEOx species formed small micelles in solution at low concentrations and the structures transformed to hexagonal liquid crystals as the surfactant concentration increased. Conversely, CnEOxPOy maintained a micellar structure even at high concentrations. Notably, the introduction of a PO chain into the CnEOx surfactant controlled the formation of aggregates with a higher-order structure (hexagonal liquid crystals).
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INTRODUCTION Polyoxyethylene (EO) alkyl ether type nonionic surfactants (CnEOx; n and x represent alkyl and EO chain length, respectively), comprising an EO chain as the hydrophilic group, are generally safe, nontoxic, and nonirritant. Because their hydrophilic/lipophilic balance (HLB,1 degree of surfactant affinity for water/oil) can be easily changed by altering both the alkyl and EO chain length, these surfactants have been employed in a wide range of fields (e.g., detergents and cosmetics). Thus, much research has been dedicated toward the design, development, and analyses of novel surfactant structures with improved properties and greater functionality. These include EO type nonionic surfactants with amino acids,2,3 sorbitan fatty acid esters,4 or fatty acid methyl esters;5 polyoxypropylene (PO) adducts where a PO chain is introduced to the end of the EO chain of the surfactant;6 and triblock copolymers (Pluronic-type surfactant) comprising hydrophilic EO chains and a hydrophobic PO chain.7 CnEOx surfactants are generally synthesized by the addition polymerization of higher alcohols and ethylene oxide. Further, alkaline catalysts yield CnEOx with broad chain length distributions. Moreover, Hama et al. reported that the chain © XXXX American Chemical Society
length distribution was narrowed when a metal or acid catalyst was applied. 6 Subsequently, Tabata et al. synthesized homogeneous CnEOx by Williamson ether synthesis using a higher alcohol and poly(ethylene glycol) with a single chain length instead of ethylene oxide.8 CnEOx surfactants with a broad EO chain length distribution are used in many industrial fields. However, the EO chain length distribution as well as the lengths of the alkyl and EO chains must also be taken into account when investigating the surface-active and aggregation properties of these surfactants in aqueous solution. Consequently, interpretation of these surfactant properties has proven to be difficult. In contrast, because CnEOx surfactants with a single chain length have a homogeneous chain length distribution, their properties can be determined precisely. Thus, in recent years, the development of homogeneous surfactants has been a prime objective. PO−EO alkyl ether type nonionic surfactants (CnEOxPOy, y represents PO chain length), synthesized from propylene oxide Received: January 11, 2017 Revised: March 27, 2017 Published: March 28, 2017 A
DOI: 10.1021/acs.langmuir.7b00104 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
and the solution was filtered to remove any undissolved material. The removal of the solvent afforded the crude product that was then purified by silica gel column chromatography (hexane:acetone = 8:2 and then 6:4) yielding homogeneous CnEOxPOy as a liquid (Supporting Information: yields, 1H NMR data, elemental analysis, and ESI-MS data). Measurements. CnEOxPOy and CnEOx surfactant solutions were prepared using Milli-Q Plus water (resistivity = 18.2 MΩ cm), and the measurements, excluding cloud point determinations, were performed at 15.0 ± 0.5 °C for C14EO6 and C14EO6PO3; 20.0 ± 0.5 °C for C10EO6, C10EO6PO3, C12EO6, and C12EO6PO3; and 25.0 ± 0.5 °C for C12EO8, C12EO8PO1, C12EO8PO2, and C12EO8PO3. General Methods. Viscosity measurements to determine the cloud point of the CnEOxPOy and CnEOx surfactants in aqueous solution (1.0 wt %) were performed on a Brookfield DV-2T (Middleboro, MA). The surface tensions of the surfactants in aqueous solution were measured with a Krüss K122 tensiometer (Hamburg, Germany) using the Wilhelmy plate technique. Dynamic light scattering (DLS) measurements were performed on an ALV-5000E spectrophotometer (Hessen, Deutschland). ALV-5000/E for installed Windows software was used to calculate the particle size distribution (CONTIN analysis).11,12 Small-angle X-ray scattering (SAXS) measurements were conducted on an SAXS instrument installed at the BL40B2 beamline in SPring-8 (Hyogo, Japan). The cryogenic transmission electron microscopy (cryo-TEM) measurements were performed on a JEOL JEM-2100F(G5) (Tokyo, Japan). The polarization of the liquid crystal structure was observed using an OLYMPUS BX53 (Tokyo, Japan) microscope under a crossed Nicol prism. The details of these measurements are shown in the Supporting Information.
and CnEOx species with a distribution of chain lengths, have also been investigated. Narrowing the chain length distributions of both the EO and PO chains afforded excellent foaming and antifoaming properties.6 CnEOxPOy surfactants with single EO and PO chain lengths have not been investigated to date. The analyses of these surfactants would allow the precise determination of their physicochemical properties. Herein, we report the synthesis of novel C n EO xPO y surfactants with a single chain length (no chain length distribution; n = 10, 12, 14, and 16; x = 4, 6, and 8; y = 1, 2, and 3; Figure 1). Their surface-active properties and the
Figure 1. Structures of CnEOxPOy and CnEOx.
structure of the assemblies formed in aqueous solution were investigated and compared to those of homogeneous CnEOx surfactants. The effects of the alkyl, EO, and PO chain lengths on the physicochemical properties were also studied.
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EXPERIMENTAL SECTION
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Materials. Homogeneous nonionic surfactants with a single EO chain length distribution (polyoxyethylene alkyl ether; C10EO6, C12EO4, C12EO6, C12EO8, C14EO6, and C16EO6) were supplied by Nikko Chemicals Co., Ltd. (Tokyo, Japan), and used as received. Propylene glycol, dipropylene glycol, tripropylene glycol, methanesulfonic anhydride, N-ethyldiisopropylamine, acetone, diethyl ether, dimethyl sulfoxide (DMSO), hexane, anhydrous tetrahydrofuran (THF), anhydrous magnesium sulfate, and sulfuric acid (0.5 mol dm−3) were obtained from Wako Pure Chemical Ind., Ltd. (Osaka, Japan). Sodium was purchased from Kanto Chemicals Co., Inc. (Tokyo, Japan), and lithium bromide was obtained from SigmaAldrich Co., LLC. (St. Louis, MO). All chemicals were reagent grade commercial materials used without further purification. Synthesis of Homogeneous Polyoxyethylene Alkyl Ether Bromide.9 Methanesulfonic anhydride (1.5 equiv) and N-ethyldiisopropylamine (2.0 equiv) were added to a stirred homogeneous nonionic surfactant CnEOx (1.0 equiv) dissolved in anhydrous THF. The mixture was stirred at 0 °C for 1 h and successively at room temperature for 2 h. The solution was filtered, and the filtrate was concentrated in a rotary vacuum evaporator. Next, lithium bromide (8.0 equiv) was added to the resultant residue dissolved in acetone, and the mixture was refluxed for 12 h. The solution was filtered to remove the unreacted lithium bromide, and the filtrate was concentrated under reduced pressure. The afforded residue was poured into pure water, and the dissolved solution was extracted twice with diethyl ether. Sulfuric acid (0.5 mol dm−3) was then added to the diethyl ether solution. Subsequently, anhydrous magnesium sulfate was added to remove the water phase from the solution, and the excess anhydrous magnesium sulfate was removed by filtration. Finally, diethyl ether was removed by rotary evaporation to afford the polyoxyethylene alkyl ether bromide as a liquid. Synthesis of Homogeneous Polyoxypropylene−Polyoxyethylene Alkyl Ether.10 Sodium (2.2 equiv) followed by polyoxyethylene alkyl ether bromide (1.0 equiv) was added to stirred monopropylene glycol, dipropylene glycol, or tripropylene glycol (5.1 equiv) at 120 °C. DMSO was then added to the mixtures and stirred at 120 °C for 20 h. After the solution was concentrated under reduced pressure to remove the unreacted polypropylene glycol and DMSO, diethyl ether was poured into the afforded residue, and the solution was filtered to remove the undissolved salt; the solvent was removed by rotary vacuum evaporation. Next, hexane was added to the residue,
RESULTS AND DISCUSSION Cloud Point. The cloud points of the CnEOxPOy and CnEOx surfactants in aqueous solution at 1.0 wt % were determined from the viscosity−temperature relationship. As the temperature increased, the viscosity initially remained constant and the aqueous solution clear. However, at a specific temperature, the viscosity suddenly decreased and the solution became cloudy. This temperature corresponds to the cloud point. The cloud points of C14EO6PO3 and C16EO6PO3 were
