Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Comparative Study of Conventional/Ethoxylated/Extended n‑Alkylsulfate Surfactants Ji Chen, Xue-yi Hu, Yun Fang,* Huan-huan Liu, and Yong-mei Xia Key Laboratory of Synthetic and Biological Colloids (Ministry of Education); School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, P. R. China
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S Supporting Information *
ABSTRACT: A series of novel anionic e-surfactants n-CcPpS was molecular designed and synthesized from long-chain fatty alcohols by polypropoxylation and sulfation followed by neutralization. Excellent all-round performance of extended surfactants (esurfactants) interests us how a simple polypropylene oxide (PPO) spacer has great effects on properties. By a comparative study of conventional/ethoxylated/extended n-alkylsulfate surfactants, we were surprised to find that e-surfactants are in an obvious rugby shape at the air/water surface according to molecular surface area (am), and it comes down to the intramolecular PPO spacer coiling and surface-induced collapse. On the basis of the interfacial properties of the e-surfactants, it is found that the PPO spacer can provide both hydrophilic and lipophilic contributions to an esurfactant molecule. The synergism between PPO spacers and alkyl chains indicates that a certain PPO spacer can adjust the contributions in view of different alkyl chain lengths. Therefore, it is both the rugby-shaped molecular geometry of e-surfactants and the dynamic amphipathicity of a PPO spacer that makes esurfactants behave with excellent interfacial and solution properties for household cleaning. Therefore, this work gives us a hint that the molecular geometry of surfactants plays a vital role in interfacial and solution properties similar to molecular amphipathicity.
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water interface2,3,6 and thus producing ultralow interfacial tension.3,7−10 Additionally, e-surfactants can exhibit excellent solubilization and adsolubilization of both nonpolar and polar small molecules11−15 because the PPO spacer acted as a lipophilic alkyl chain to solubilize nonpolar molecules and also contributed weak polarity to solubilization of polar molecules. Meanwhile, use of e-surfactants also can facilitate the formation of microemulsion6,16−19 for high polarity oils, such as triglyceride and vegetable oils.2,20−23 In view of the abovementioned excellent properties, e-surfactants have been investigated in various fields such as extraction, 24−27 separation,28,29 and textile cleaning.30−33 As surfactants can significantly improve the interfacial properties because of molecular amphipathicity, many novel surfactants have been designed to meet needs of different fields, such as gemini surfactants,34−36 bola surfactants,37−39 double-tailed surfactants,40,41 dendritic surfactants,42,43 and so on. Behaviors of these surfactants, both adsorption at interfaces and self-assembly in solution, are different from those of csurfactants, producing superior properties because of their distinctive molecular frameworks. Previous papers have
INTRODUCTION Sodium laureth sulfate (SLE3S) is a typical anionic−nonionic hybrid surfactant, which is obtained by ethoxylation modification from a more commonly used conventional surfactant (c-surfactant) sodium dodecyl sulfate (SDS). SLE3S has being broadly applied in household cleaning and daily home care products because of its excellent performance worldwide, such as good low-temperature solubility, foamability, hard water resistance, and detergency, except for the adverse-effect caused by a carcinogenic byproduct dioxane though it was reported being minimized in or removed from SLE3S manufacture;1 therefore, people have paid close attention to substitutes for the controversial SLE3S for a long time. Extended surfactants (e-surfactants) contain an intermediate polarity part, a polypropylene oxide (PPO)-related spacer, located between hydrophile and lipophile groups of csurfactants.2 The general molecular structure of anionic esurfactants is similar to that of conventional anionic−nonionic hybrid surfactant, which makes them behave with properties of anionic and nonionic surfactants, such as Krafft point3 and cloud temperature.2,4,5 The introduction of a PPO spacer in the central part of an e-surfactant molecule causes a smoother transition between the lipophilic tail and hydrophilic anionic head, extending its lipophilic tail to an oil phase and maintaining good interaction with the water phase at the oil/ © XXXX American Chemical Society
Received: December 1, 2018 Revised: January 27, 2019
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DOI: 10.1021/acs.langmuir.8b04022 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Scheme 1. Synthetic Route and Schematic Illustration of the Molecular Architecture of Linear Alkyl PPOp Ether Sulfates (nCcPpS)
gas alternately three times. After the autoclave was heated to approximately 140 °C, 174 g of PO was added dropwise into it under the pressure no higher than 0.4 MPa, and then, the addition reacted till the pressure dropped to a constant value. After cooling, the mixture was neutralized with acetic acid to neutral pH, extracting with saturated brine after being dissolved by ethyl acetate. After ethyl acetate was removed by evaporation, the wanted intermediate ndodecyl PPO3 ether (n-C12P3) was obtained. The as-prepared n-C12P3 (0.05 mol) was diluted with some 1,2dichloroethane and cooled by an ice water bath, and then chlorosulfonic acid (0.075 mol) was added dropwise while the reaction temperature was kept about 0 °C. After the addition, the mixture was neutralized with sodium hydroxide in ethanol to pH 7−8, and then, 1,2-dichloroethane and ethanol were removed by evaporation. The mixture was dissolved into ethanol and filtered, and the crude n-C12P3S free from alcohol was obtained after evaporating and vacuum drying. The purified n-C12P3S sample was obtained by isolation through column chromatography for an identification purpose. The sulfation reaction was confirmed by FT IR (FTLA2000-104, ABB Bomen Corporation, Canada), and the result is seen in Figure S1 in the Supporting Information. The molecular structure of n-C12P3S was confirmed by a 1H NMR (AVANCE III HD 400 MHz nuclear magnetic resonance spectrometer, Bruker, Switzerland), and the result is seen in Figure S2. The process for synthesis and purification of the other 14 target esurfactants was similar to that of n-C12P3S. The 1H NMR data of all members of n-CcPpS could be seen in Figures S3−S16, and the average experimental values of p of the n-CcPpS homologues were calculated based on 1H NMR data as exampled in Table S1 and listed in Table S2. The general molecular formula of n-CcPpS is illustrated in Scheme 1 and the abbreviations of all n-CcPpS members are listed in Table 1.
reported that surfactant molecular geometry is very important for both adsorption at interfaces44 and self-assembly in solution,45 and molecular geometry of c-surfactants can be described by a packing (P) parameter46−48 that is obviously not applicable to the above surfactants with specific molecular frameworks. Simple geometrical considerations indicate that csurfactants are cone or truncated-cone shaped when P is less than 1. Although the dominant shape of e-surfactants seems to be a cone shape simply judged by P parameter, the geometry of their lipophilic tail on the characteristic curvature was studied,49 and that gave a hint that a certain special molecular geometry should occur and be responsible for their unique properties. Therefore, we assume that the PPO spacer of esurfactants might produce an unexpected “extended” behavior at the interface and in solution endowing e-surfactants with a recessive shape. Meanwhile, anionic e-surfactants containing PEO block or branched alkyl chain have been investigated for cleaning,30−33 but these used e-surfactants always involve concerns of safety and environmental friendliness. Therefore, this paper would create a green building block combination by long-chain fatty alcohols together with suitable PPO spacers to highlight the effect of PPO spacers on both molecular geometry and amphipathicity in comparison to conventional and ethoxylated n-alkylsulfate counterparts and further to investigate correlation between molecular structure and interfacial properties, which aims to develop new green and efficient e-surfactants used in household cleaning products.
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EXPERIMENTAL SECTION
Table 1. Abbreviation of all Members of n-CcPpS and the Corresponding Average Experimental Values of p
Materials. General. All reagents and solvents were obtained from commercial sources. n-Dodecanol, n-tetradecanol, n-hexadecanol, noctadecanol, propylene oxide (PO), chlorosulfonic acid, liquid paraffin (LP), and other reagents and solvents are all analytical grades and were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Soybean oil (SO) was purchased from Yihai Kerry Ltd. (Shanghai, China). SDS (min. 99 wt %) was purchased from Acros Organics Company (Geel, Belgium). Mixed C12 and C14 fatty alcohols (mdodecanol/mtetradecanol = 7:3) and sodium laureth sulfate (SLE3S) with average three EO units are of industrial grade and gifts from Sasol Chemical Ltd. (Nanjing, China). All above materials and other general reagents were used as received. To eliminate the interference of other ions, ultrapure water prepared by a water purification system (SIMSV0001, Millipore, France) was used to prepare all of the solutions in this study. Synthesis and Identification of n-Alkylsulfate e-Surfactants (n-CcPpS). The synthetic route of n-CcPpS is shown in Scheme 1. A typical sulfation reaction was described taking n-C12P3S as an example. About 186.0 g of n-dodecanol and a few of dry potassium hydroxide were added into a 0.5 L autoclave (WHF-0.5L, Weihai Automatic Control Reaction Kettle Co. Ltd., China). The mixture was heated to 90 °C and stirred until all of the potassium hydroxide dissolved, and the autoclave was vacuumed and purged with nitrogen
target products n-C12P3S n-C12P6S n-C12P9S n-C14P3S n-C14P6S n-C14P9S
target products
p 3 6 9 3 6 9
(3.0) (5.9) (8.8) (3.1) (6.4) (9.0)
a
n-C16P3S n-C16P6S n-C16P9S n-C18P3S n-C18P6S n-C18P9S
p 3 6 9 3 6 9
(3.1) (5.9) (8.7) (3.2) (5.6) (8.6)
target products
p
n-C1214P3S n-C1214P6S n-C1214P9S
3 (3.0) 6 (6.2) 9 (9.0)
a
The data in brackets are the average experimental values calculated from 1H NMR data in Table S2.
Methods. Tensiometry. Surface tension (γ) of aqueous surfactant solution was measured at 25 ± 0.1 °C by the drop volume method50 and recorded as the average value of three measurements. The surface tension is related to the volume (V) of drop formed at the end of the capillary tip of glass syringe by the following equation i ρVg zy zz γ = F jjj k r {
B
(1) DOI: 10.1021/acs.langmuir.8b04022 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. γ-log c curves of aqueous surfactant solutions at 25 °C. where ρ is the density of aqueous surfactant solution; g is the acceleration of gravity; r is the outer radius of the capillary tip; and F is the correction factor depending on the value of r/V. The saturated surface excess of surfactants Γm was derived from the following Gibbs adsorption isotherm equation51 Γm = −
1 jij dγ yzz jj zz 2.303nRT jk dlog c z{
T
formed dispersed in water of 330 ppm hardness expressed as calcium carbonate. LSDP (%) =
1 × 1020 NA Γm
(4)
where V is the volume of lime soap dispersing agent solution. Foamability. Initial foam height and residual foam height were measured by the Ross-Miles foam test.54 A 50 mL of aqueous surfactant solution (0.25 wt %) was put into the foam cylinder. Another 200 mL of the same solution was poured into the foam cylinder from the top of it. Foams were produced by collisions of solution. All tests were conducted at 40 ± 0.1 °C. The foam height at the initial time (H0) and at 5 min (H5) was determined. H0 was used to assess the foaming ability (FA, cm), and the ratio of H5 and H0 was used to assess the foam stability (FS). Five runs were made for each solution, and the FA and FS are averages of values from these runs. Emulsifying Power. The emulsifying power (EP, min) test was evaluated at 20 ± 0.1 °C. Aqueous surfactant solution (40 mL, 0.1 wt %) was poured into a 100 mL cylinder with a stopper containing 40 mL of oil. The stoppered cylinder was inverted up and down 10 times successively. The time for separation of 10 mL of water was recorded as EP in seconds and the experiment was repeated five times for each tested surfactant. Wettability. Wettability (W) is presented as the textile wetting time, which was measured using the dip-wetting method55 at 30 ± 0.1 °C. A piece of standard canvas (0.39 ± 0.01 g, 30 cm in dimension) was suspended in an aqueous sample solution with a standard hook, and the sinking time of the canvas sheet touching the bottom of container was recorded as the wetting time, all data presented were the average values from five parallel tests. Detergency. Detergency (D) testing was conducted following the procedures described by Tongcumpou et al.56 A series of beakers containing surfactant solutions fixed at 0.2 wt % were prepared. The different kinds of soiled fabrics were cut into a 6 × 6 cm swatches in the warp and weft direction. Experiments were conducted using a Terg-O-Tometer (RHLQ-III, Shanghai Yinze Equipment Co., Ltd., China). Three kinds of typical soiled fabrics were washed for 20 min at 30 ± 1 °C in 1 L of surfactant washing solution, respectively. The swatches were then rinsed twice with 1 L of deionized water. All washing and rinsing cycles were conducted at 120 rpm agitation speed. The treated swatches were hung to dry overnight prior to evaluating the values of D, which based on reflectance of the prewash and the postwash soiled fabrics at 457 nm using the Brightness tester (DRK103A, Drick Instruments Co., Ltd., China).
(2)
where Γm (mol/m2) is the surface excess concentration at the critical micelle concentration (cmc); γ (mN/m) is the surface tension; R (J/ (mol K)) is the gas constant; T (K) is the absolute temperature; c (mol/L) is the surfactant concentration; (dγ/dlog c) is the slope below the cmc in the surface tension plots; and the value of n is taken as 2 for the e-surfactants studied.
am =
V × 0.25% × 100 5 × 0.50%
(3)
where am (Å2/molecule) is the occupied surface area per molecule at the cmc and NA is the Avogadro number. Critical Micelle Temperature. Critical micelle temperature (cmt, °C) was determined by slowly heating 1.0 wt % of the surfactant solution until a clear solution was obtained52 or by cooling the 1.0 wt % surfactant solution to a cloudy state first and then heating until a clear solution was obtained. If the solution remained clear even below −4 °C, it would be marked as cmt < 0 °C. Electrolyte Tolerance. Electrolyte tolerance (ET) was determined by putting the electrolyte into the aqueous surfactant solution until a cloudy solution was first obtained.3 ET of the surfactants was obtained by observing the precipitation boundary of each surfactant solution upon the addition of NaCl or CaCl2. A series of beakers containing the surfactant solutions with fixed concentration at 0.3 wt % were prepared. Different dosages of NaCl or CaCl2 were added into the surfactant solutions separately, and the mixtures were stirred at 45 ± 1 °C for at least 24 h, and then, the turbidity of the solutions was measured by a UV−vis spectrometer (Libra S80 UV−vis Spectroscopy, Biochrom, USA). As the dosage of NaCl or CaCl2 gradually increased, the turbidity of the mixed solutions slowly increased first and then abruptly increased and reached a maximum value where apparent precipitation was observed. Beyond the maximum, turbidity sharply decreased because the precipitates deposited to the bottom or stuck to the wall of containers. Thus, ET was expressed as the salt tolerance (ST, g/L) for NaCl and the calcium tolerance (CT, g/L) for CaCl2 and precipitation boundaries started to appear correspondingly. Lime Soap Dispersing Power. Lime soap dispersing power (LSDP) was run according to the method of Borghetty and Bergman53 at ambient temperature (15−25 °C). The lime soap dispersing agent solution (0.25 wt %) was added to 5 mL of sodium oleate solution (0.5 wt %) to keep the calcium and magnesium soaps
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RESULTS AND DISCUSSION Estimate of Molecular Geometry of e-Surfactants at the Air/Water Surface in Comparison to c-Surfactants by Tensiometry. Plots of surface tension versus logarithmic C
DOI: 10.1021/acs.langmuir.8b04022 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Table 2. Surface Active Parameters of Surfactants at 25 °C surfactant the n-C12PpS series
the n-C14PpS series
the n-C16PpS series
the n-C18PpS series
reference samples
n-C12P3S n-C12P6S n-C12P9S n-C14P3S n-C14P6S n-C14P9S n-C16P3S n-C16P6S n-C16P9S n-C18P3S n-C18P6S n-C18P9S SDS SLE3S
Γm (10−6 mol/m2)
am (Å2/molecule)
S (am/aPP)
γcmc (mN/m)
cmc (10−3 mol/L)
pC20
cmc/C20
cmt (°C)
1.78 1.42 0.95 1.56 1.21 0.85 1.55 1.19 0.81 1.62 1.20 0.85 2.40 2.11
93.3 117 175 106 138 196 107 139 204 103 139 196 69.2 79.1
4.66 5.85 8.77 5.32 6.88 9.79 5.34 6.95 10.2 5.14 6.94 9.79 3.46 3.95
32.5 32.3 32.0 31.2 30.2 29.3 30.4 29.3 29.1 28.9 27.8 28.4 35.0 40.8
0.63 0.30 0.18 0.30 0.15 0.081 0.15 0.078 0.043 0.062 0.026 0.013 8.0 0.69
4.17 4.74 5.60 4.58 5.15 5.96 4.92 5.46 6.27 5.23 6.09 7.00 2.51 3.64
9.26 16.3 72.0 11.5 21.1 73.6 12.5 22.3 81.1 10.5 32.1 130 4.21 3.00
