Hydrotropic Properties of Alkyl and Aryl Glycerol Monoethers

Article pubs.acs.org/JPCB

Hydrotropic Properties of Alkyl and Aryl Glycerol Monoethers Laurianne Moity,† Yan Shi,‡ Valérie Molinier,*,† Wissam Dayoub,‡ Marc Lemaire,‡ and Jean-Marie Aubry† †

Laboratoire de Chimie Moléculaire et Formulation, E.A. 4478, Université Lille Nord de France, USTL, ENSCL, Cité Scientifique, 59652 Villeneuve d’Ascq Cedex, France ‡ Laboratoire de Catalyse et Synthèse Organique, UMR CNRS 5246, Institut de Chimie et Biochimie Moléculaire et Supramoléculaire, Université Lyon 1, Bâtiment Curien-CPE, 43 bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France ABSTRACT: Three pentyl and three aryl 1-O-monoglyceryl ethers prepared via a green catalytic route were investigated as hydrotropic solvents. Their amphiphilicities were quantified by comparing their so-called “fish” diagrams constructed with oils of increasing hydrophobicity (EACN). For the same number of carbon atoms, the presence of a methyl substituent in β position makes the hydrotrope slightly more hydrophobic, as evidenced by the more hydrophobic optimal oil. Branched isomers are also less efficient since they require higher concentrations to get microemulsions. The presence of an aromatic moiety within the hydrophobic chain increases the solubility of the hydrotrope in water in comparison to the alkyl derivative that has the same number of carbon atoms. It also modifies significantly the associative behavior in oil/water systems: Benzylglycerol monoether is able to form Winsor III systems, just as the pentyl derivatives, but with much more polar oils, whereas phenylglycerol is not. In oil/water systems, all glycerol-derived amphiphiles exhibit a twice-lower temperaturesensitivity compared to their ethyleneglycol counterparts. The pentyl and benzyl 1-O-monoglyceryl ethers can be classified as amphiphilic solvents, or “solvo-surfactants”, as regards to their surface-active properties and good solubilizing abilities.

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INTRODUCTION In the past few years, so-called “green” or “sustainable” solvents have aroused a growing interest in both industrial and academic communities.1 The former need to find alternatives to in-place petrochemical solvents in applications such as coatings, hardsurface cleaning, agrochemicals, organic synthesis; for ecological, ethical, or marketing reasons; and, over all, because of the ever more restrictive legislations. In the academic community, the search for new alternative solvents with improved ecological footprints and better performances is a good opportunity to revisit the physicochemical and predictive tools for solvents characterization2−4 and to explore the properties of unconventional solvents such as liquid polymers, supercritical fluids, or ionic liquids. 5 Developing new biosourced or biobased solvents, which are obtained from renewable building blocks, is also the subject of many research efforts.6,7 An interesting class of solvents is the family of amphiphilic solvents that are sometimes named “solvo-surfactants”.8 The main amphiphilic solvents are currently the short-chain ethyleneglycol and propyleneglycol ethers. They are used as coalescing agents in paints and as solvents or cosolvents in inks and hard-surface cleaning formulations. Their unique solvent properties result from their amphiphilic structure that makes them miscible with water and many organic solvents and confer them hydrotropic properties. However, as the shortest © 2013 American Chemical Society

members of the ethyleneglycol ethers family are reprotoxic and also because the ethylene and propylene glycol moieties are of petrochemical origin, finding greener alternatives obtained from biomass is of great interest. Monoethers of glycerol9,10 and isosorbide,11−14 two major building blocks derived from the vegetable oils and starch feedstocks, have been developed at the laboratory scale and possess effective solvo-surfactive properties. However, the current synthetic pathway for the preparation of monoethers of such polyols (R1−OH) is the Williamson alkylation that has limited yields and a bad environmental footprint since it starts from halogenoalkanes (R2−X), requires the use of strong mineral bases (MOH) and generates stœchiometric amounts of salts (MX; eq 1). R1OH + MOH + R2X → R1OR2 + H 2O + MX

(1)

To become competitive alternatives to the glycol ethers, such solvo-surfactants should be prepared using an efficient, upscalable, and eco-friendly synthetic pathway. An attractive option is the catalytic reductive alkylation of polyols starting from aldehydes,15,16 ketones,17,18 or carboxylic acids,19 which works with good selectivities and high yields (Scheme 1). It Received: April 4, 2013 Revised: June 17, 2013 Published: July 10, 2013 9262

dx.doi.org/10.1021/jp403347u | J. Phys. Chem. B 2013, 117, 9262−9272

The Journal of Physical Chemistry B

Article

About 20 mg of sample were deposited on a platinum sample holder. A temperature ramp was applied from room temperature to 300 °C at a rate of 10 °C min−1. DSC. Differential scanning experiments were performed with a DSC Q1000 from TA Instruments. The sample (ca. 3 mg) was placed in a hermetic aluminum pan that was flushed with high purity nitrogen gas (50 mL/min) during the experiment. The sample was first cooled to −90 °C, left to equilibrate at this temperature for 3 min, and then underwent a temperature increase at 5 °C/min up to 20 °C. Phase Diagrams in Water. Tubes containing different ratios of the compound from 0 to 100% (w/w) in water were prepared, shook manually, and then placed in a thermostatted bath at (25.0 ± 0.1) °C. The temperature was then increased gradually with 0.5 °C increments and the temperature at which a change was observed was recorded to plot the boundaries of the phase diagram. The change in aspect of each sample was evaluated visually on heating and cooling, and the transition temperature was taken as the average of the two values recorded. No liquid crystal phases were encountered on the whole concentration range as ascertained by visualization of the samples between crossed polarizers. Surface Tension Measurements. The surface tensions were measured at (25.0 ± 0.1) °C with a K100MK2 Krüss tensiometer using a platinum rod as the probe. A total of 2.5 mL of a concentrated solution of hydrotrope was prepared at a concentration of approximately 40% w/w of hydrotrope (20% w/w for PhGly). When the surface tension was stable (standard deviation of the 5 final steps of 0.1 mN m−1), a manual dilution keeping the volume constant was performed. Measurements of Self-Diffusion Coefficients. The selfdiffusion coefficients were measured by the PGSE-NMR technique on a Bruker Avance 300 spectrometer equipped with a field gradient probe unit, following the method first introduced by Stejskal and Tanner24 and using a BPP-STELED sequence25 combining constant time, stimulated echo, bipolar pulse, and longitudinal eddy current delay method. At the end of the sequence the echo attenuation follows eq 2:

Scheme 1. Direct Catalytic Etherification of Glycerol Starting from an Aldehyde

goes through the reduction of acetals to ethers, which can be performed in high yields and allows the preparation of a range of products via an atom-economical pathway (Scheme 1).20 Here we report on the aqueous phase behavior of alkyl and aryl 1-O-monoglyceryl ethers that have been prepared using this green synthetic pathway. For the alkyl derivatives, the work has been focused on linear and branched pentyl ethers because previous studies have shown that five carbons is the limit to ensure water solubility for 1-O-monoglyceryl ethers.9 The surface-active properties of aqueous solutions of the compounds were evaluated by tensiometry, and their abilities to solubilize organic compounds in water were compared. To evaluate the hydrophilic/lipophilic balance of these amphiphilic compounds, formulation scans were performed to find out their optimal oil in a homogeneous family. The sensitivity to temperature has also been investigated.

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MATERIALS AND METHODS Chemicals. All chemicals were used as received. The 1-Omonoglyceryl ethers were synthesized as previously described15−20 and were obtained with a purity >98%, as determined by GC and NMR analyses. Solubilization experiments were performed with 2-[4-(2-chloro-4-nitrophenylazo)N-ethylphenylamino]ethanol (Disperse Red 13) (Acros, 97%). For spectroscopic quantification, absolute ethanol (Merck) was used for dilutions. For the oil scan (Fish diagrams), the 1monochlorinated and α,ω-dichlorinated alkanes were obtained from Aldrich or Alfa Aesar (purity >98%). For all experiments, ultrapure water (resistivity 18.2 MW cm) was used. log P Calculations. The 1-octanol/water partition coefficients were calculated using COSMO-RS. The molecular geometries of each compound have been sketched as threedimensional structure with Arguslab (http://www.arguslab. com/, Release 2004). Conformational analysis has been carried out for each solvent using the COSMOconf script.21 It involves semi-empirical AM1 calculations via MOPAC 7 locally modified by COSMOlogic, followed by a more accurate density functional theory (DFT) treatment of the most important AM1 conformers. The turbomole program was used to perform the DFT/COSMO geometry optimizations according to the standard quantum chemical method for COSMO-RS, i.e., the DFT functional B88-PW8622 with a triple-ζ valence polarized basis set (TZVP). Subsequent to this conformational search and optimization, partition coefficients (log P) were calculated using COSMOtherm (C21_0111 version).23 The relative contributions of each conformer have been determined by an iterative procedure using the Boltzmann-weight of the free energies of the conformers. Volatility. Volatility was evaluated by thermogravimetric analysis by recording the weight loss when imposing a temperature increase to the pure samples from room temperature to 300 °C. To compare quantitatively the various hydrotropes, the times necessary to reach 50% and 90% weight loss were recorded for each hydrotrope. Thermogravimetric analyses were performed on a TGA Q50 from TA Instruments. The atmosphere of the measuring chamber was pure nitrogen to avoid possible oxidation of the samples at high temperatures.

A = A 0 exp[−γH 2δ 2G2(Δ − δ /3)D]

(2)

where A is the echo amplitude in the presence of the gradient pulse, A0 is the echo amplitude in the absence of the gradient pulse, γH is the proton gyromagnetic ratio, δ is the gradient pulse length, G is the strength of the applied field gradient, Δ is the interval between two field gradient pulses, and D the selfdiffusion coefficient. By varying the field gradient amplitude G, a series of experiments is collected and the self-diffusion coefficient D can be extracted with a simple three-parameter fit for well-separated resonances. In our experiments, the gradient strengths were varied from 2 to 52 G cm−1, δ was fixed at 1 ms, and Δ was adjusted for each sample to obtain a full decrease of the echo signal (from Δ = 100 to 600 ms for the more concentrated samples). All NMR signals of the compound can be used to compute the selfdiffusion coefficient and give consistent values ±0.1.10−10 m2/s. To compare all samples, the values obtained from the resonance of the main methylene peaks (1.15−2.00 ppm) was chosen to fit eq 2. The temperature control of the probe was fixed at 300 K, with a precision within ±0.5 K. The field gradient was calibrated with the diffusion of H2O in a H2O/ D2O 90/10 v/v mixture. 9263



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Scheme 2. Alkyl and Aryl α-Glyceryl Ethers under Study

Solubilization of a Hydrophobic Dye. Aqueous solutions of the substance to be tested were prepared at different concentrations, and Disperse Red 13 was added until reaching saturation, i.e., until excess powder remained undissolved. The solutions were kept under stirring at room temperature during 24 h. After this period, the solutions were filtered by means of a syringe equipped with a 0.45 μm pore-size filter. The amount of hydrophobic dye dissolved was determined by UV−visible absorption of the solutions at 503 nm (Varian Cary 50 spectrometer). Prior to the measurement, a calibration curve was established at this wavelength. The solutions were diluted in absolute ethanol before measurement. Fish Plots and Determination of the Optimal Oils. The so-called “Fish” plots are 2D-graphs with the hydrotrope concentration (%) as the ordinate axis and the oil chain length as the abscissa axis on which the different types of Winsor systems formed are indicated. The frontier between the different Winsor systems has a γ (or “Fish”) shape and gives its name to this kind of representation. Ternary hydrotrope/ oil/water systems were prepared in test tubes with the studied hydrotrope and chosen oil. Equal weights of oil and water were first introduced (typically 200 mg each), and increasing amounts of hydrotrope were added. After each addition, the test tubes were gently shaken and placed in a thermostatted bath at (25.0 ± 0.1)°C until attainment of equilibrium. The types of Winsor systems (Winsor I, II, III, and IV) were determined by visual observation. The critical point at the frontier of all domains is called the X-point of the diagram and the abscissa of this point gives the carbon number of the socalled “optimal oil”. To draw finely the gamma plots, mixtures of oils have been used to attain intermediate polarities. The equivalent number of carbons of the alkyl chain of the oil mixture has then been calculated on a mass average.

in order to establish structure-property relationships: n-pentyl, 1-methylbutyl, and 2-methylbutyl α-glyceryl ethers have been chosen to evaluate the effect of chain branching on the properties of these short-chain amphiphiles. Phenyl, benzyl, and 3-phenylpropyl α-glyceryl ethers have been studied to evaluate the effect of the aromatic ring and of its distance to the polar glyceryl head on the hydrotropic properties. Thermophysical Properties: Melting and Volatility. One of the most important characteristics of solvo-surfactants is to exhibit surface-active properties while possessing some characteristic features of solvents, such as fluidity and volatility. Several methods allow evaluating and comparing the volatility of a family of compounds, among which thermogravimetric analyses that provide a quick and reliable estimation of volatility. All glyceryl ethers are liquid at room temperature except 1-Ophenylglycerol (PhGly) that is a solid with low melting point (54 °C). For some samples, DSC measurements were performed in order to find out whether these compounds could easily crystallize at low temperatures. With the conditions detailed in the experimental section, a glass transition temperature was encountered for PhC1Gly at −62 °C, whereas in the same conditions, nC5Gly and 2iC5Gly did not show any phase transitions in the temperature-range studied. For comparison purposes, the same temperature scan was performed on diethyleneglycol hexyl ether (C6E2), for which melting occurred at −36.9 °C (litt.: − 40 °C).26 These results indicate that the compounds do not freeze or crystallize easily down to −90 °C. Figure 1 shows the evaporation profiles in TGA for the five pure hydrotropes subjected to a temperature increase from room temperature to 300 °C under N2 atmosphere. The temperatures at which 50% and 90% of the compounds have been evaporated are reported in Table 1. They are well-above the temperatures corresponding to dimethylisosorbide (DMI), a biobased solvent that is known to be at the VOC limit.27 Among the pentyl derivatives, those having a branched chain (1iC5Gly and 2iC5Gly) are more volatile, which can be related to the decreased intermolecular forces in the liquid state resulting from disfavored chain−chain interactions. On the contrary, compounds having an aromatic ring (PhGly, PhC1Gly, and PhC3Gly) are much less volatile, which is in turn due to strong interactions in the liquid state via π−π interactions. It is interesting to notice that, even if PhGly is a solid at room temperature (mp = 54−56 °C), it is much more volatile that PhC1Gly, since 90% weight is lost at a temperature 22 °C lower. Phase Diagrams in Water. All alkyl derivatives (nC5Gly, 1iC5Gly, and 2iC5Gly) and benzyl α-glyceryl ether (PhC1Gly) are miscible with water in the whole concentration range.

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RESULTS Scheme 2 recaps the 1-O-monoglyceryl ethers studied in this work. n-Pentyl α-glyceryl ether (nC5Gly), 1-methylbutyl αglyceryl ether (1iC5Gly), 2-methylbutyl α-glyceryl ether (2iC5Gly), and 3-phenylpropyl α-glyceryl ether (PhC3Gly) were synthesized by catalytic reductive alkylation of glycerol starting from the corresponding aldehyde, as described in references.15,16 Benzyl α-glyceryl ether (PhC1Gly) was obtained by reduction of benzyl glyceryl acetal using 1,1,3,3-tetramethyldisiloxane (TMDS) with Pd/C in the presence of a Brønsted acid (camphorsulfonic acid).20 Phenyl α-glyceryl ether (PhGly) was obtained by dehydrogenative alkylation of cyclohexanone with glycerol, using Pd/C as catalyst, in solventfree conditions.18 These catalytic routes allow efficient and up-scalable access to a variety of amphiphilic structures.15−20 In the present work, only the compounds presented in Scheme 2 have been studied 9264



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Figure 1. Thermogravimetric analyses of the five compounds under study with a temperature increase from room temperature to 300 °C at 10 °C/min under a N2 atmosphere. From left to right (more volatile to less volatile): 1iC5Gly, 2iC5Gly, nC5Gly, PhGly, PhC1Gly, and PhC3Gly. A reference compound that is known to be at the VOC limit (dimethylisosorbide) is also presented (dotted curve).

Table 1. Volatilities of the Compounds Studied: Temperatures at Which 50% (T50%) and 90% (T90%) of the Samples Have Been Vaporized in the Thermogravimetric Analyses T50%/°C T90%/°C

1iC5Gly

2iC5Gly

nC5Gly

PhGly

PhC1Gly

PhC3Gly

155 169

162 177

167 182

200 214

220 236

235 254

Figure 2. Binary phase diagrams of the PhGly-water system (top) and PhC3Gly-water system (bottom). Symbols L and L′ refer to monophasic liquid regions.

Phenyl α-glyceryl ether (PhGly) is water-soluble up to 20% w/w. For higher concentrations, a biphasic system is obtained, with excess crystals in equilibrium with the aqueous phase. This system becomes homogeneous at higher temperatures, as indicated in the binary phase diagram presented in Figure 2 (top). 3-Phenylpropyl α-glyceryl ether (PhC3Gly) is a viscous liquid showing a miscibility gap as indicated in Figure 2 (bottom). The solubility limit in the dilute region was determined visually and was found at a concentration close to 0.4% w/w. For further investigation of the hydrotropic properties, only the water-soluble compounds were considered, and thus PhC3Gly was not studied further. Surface Tension of Aqueous Solutions. The surface tension of aqueous solutions containing increasing concentrations of each compound was measured by the plate method using a platinum rod as probe. The data are plotted in Figure 3. For all compounds, reduction of the surface tension of water is observed and the saturated value is reached at quite high concentrations, after a smoother decrease than in the case of real surfactants, which is typical of this kind of short-chain amphiphiles. For PhC1Gly, a dip is observed before the saturated value, which interferes with the accurate determination of the aggregation concentration. The minimum aggregation concentrations (MAC) determined from the curves are given in Table 2. The pentyl derivatives appear to be more efficient in reducing the water surface tension with a saturated value of 28 mN/m approximately. This value is in accordance with what is obtained with short chain ethylene glycol derivatives and with our previous data on monoalkylglycerol.9 Chain branching does not seem to affect much the surface characteristics of the products. The aryl derivatives are less efficient, with a saturated

Figure 3. Surface tension vs concentration of the glycerol-derived hydrotropes. MAC (minimum aggregation concentrations) determinations are indicated by small arrows. nC5Gly (■), 1iC5Gly (□), 2iC5Gly (gray square) , PhGly (◊), PhC1Gly (⧫).

surface tension of 40 mN/m, somewhat lower than the values obtained with typical aromatic anionic hydrotropes, like sodium xylene sulfonate (ca. 42 mN/m) and sodium cumene sulfonate (ca. 52 mN/m).28 Solubilization Experiments. The solubilization of a typical hydrophobic dye (Disperse Red 13) in water was investigated to highlight the hydrotropic properties of the compounds (figure 4). For all compounds, the typical curve for hydrotropic solubilization is observed: the solubilization of the hydrophobic dye in water increases strongly after a critical concentration of hydrotrope is reached. This concentration is called the minimum hydrotropic concentration (MHC) and is usually related to the MAC obtained by tensiometry.29 A value of MHC can be worked out for all compounds by zooming in the low concentration region and looking for the intersection of the tangent of the curve at low concentrations with the abscissa 9265



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Table 2. Water Solubility Limit, Minimum Aggregation Concentrations (MAC) Determined by Tensiometry, Minimum Hydrotropic Concentrations (MHC) Determined by Solubilization Experiments, Amount of Dispersed Red 13 Solubilized in a 1 M Solution of Hydrotrope ([DR 13]1M), and Concentration of Hydrotrope Required to Solubilize 1 mmol/L of Disperse Red 13 ([Hydrotrope]1 mM) water solubility limit/% w/w MAC/mol/L/% w/w MHC/mol/L/% w/w [DR 13]1M/mmol/L [Hydrotrope]1 mM/mol/L

nC5Gly

1iC5Gly

2iC5Gly

PhGly

PhC1Gly

PhC3Gly

complete 0.25 4.1 0.33 5.3 1.1 0.86

complete 0.25 4.1 0.69 10.2 0.5 1.20

complete 0.37 6.0 0.34 5.5 0.6 1.30

20.0 0.32 5.4 0.48 8.1 1.9 0.76

complete 0.30 5.5 0.49 8.9 1.4 0.80

0.4

existing between the improved solubilization of the hydrotrope solutions and the supposed associative phenomena. Self-Diffusion of nC5Gly in D2O. The self-diffusion coefficient of nC5Gly in D2O was measured by PFGSE NMR as a function of concentration (Figure 5). These data give an

Figure 4. Solubilization of Disperse Red 13 by aqueous solutions containing increasing concentrations of the glycerol-derived hydrotropes nC5Gly (■), 1iC5Gly (□), 2iC5Gly (gray square), PhGly (◊), PhC1Gly (⧫).

axis. The values obtained by this method are reported in Table 2. To compare the solubilizing capacities of the aqueous solutions of hydrotropes, the data were modeled with polynomial curves and the amount of Disperse Red 13 solubilized in 1 M solutions was computed. Conversely, the amount of hydrotrope required to solubilize 1 mmol/L of Disperse Red 13 was also extracted from the data. The results are reported in Table 2. The most efficient hydrotrope for solubilization of the model dye is PhGly followed by PhC1Gly, which is consistent with the expected favorable interactions between the hydrotropes and the solute through π−π interactions (cf. chemical structure of Disperse Red 13 presented in Scheme 3). The pentyl derivatives are less efficient, all the more that the chain is branched.

Figure 5. Variation of the self-diffusion coefficient D of nC5Gly as a function of concentration in D2O. The surface tension data presented in Figure 3 are reproduced for comparison.

insight on what occurs within the solution when the concentration of hydrotrope increases. In Figure 5 is also reproduced the evolution of the surface tension of aqueous solutions of nC5Gly to facilitate the comparison between bulk and surface properties. The self-diffusion coefficient D of nC5Gly has a constant value of 6.7.10−10 m2/s for concentrations lower than ca. 0.25 mol/L and it then gradually decreases down to 1.0.10−10 m2/s at 3.4 mol/L (50 wt %). This behavior is comparable to what occurs in the case of surfactants when they start forming micelles, but at much lower concentrations.30 The concentration at which D starts to decrease corresponds to the point where the surface tension reaches a plateau. It is also the point where slightly turbid samples are observed, whereas clear solutions are obtained for concentrations lower and higher than 0.25 mol/L. All these data support the hypothesis that these short-chain amphiphiles do undergo a self-association in water at the MAC. Amphiphilic Behavior in Polar Oil/Water Systems. Fish Diagrams of 1iC5Gly, 2iC5Gly, nC5Gly, and PhC1Gly. The amphiphilic properties of the compounds have been evaluated in water/oil systems, and particularly, the ability to form a third-phase microemulsion in given conditions has been investigated. Indeed, it has been shown for other short-chain amphiphiles that the formation of a third-phase microemulsion can be a way to distinguish the behavior of solvo-surfactants from the one of common solvents.31

Scheme 3. Chemical Structure of Disperse Red 13, 2-(4-(2Chloro-4-nitrophenylazo)-N-ethylphenylamino)ethanol

The minimum aggregation concentrations (MAC) worked out from the surface tension curves and the minimum hydrotropic concentrations (MHC) obtained from the solubilization curves do not match perfectly, particularly for 1iC5Gly (see Table 2), which questions once again the link 9266



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Figure 6. Fish diagrams of 1iC5Gly (□), 2iC5Gly (gray square), nC5Gly (■), and PhC1Gly (⧫) at 25 °C. 1/1 w/w oil/water systems, the oils chosen are 1-monochlorinated alkanes Cl−CnH2n+1 and α,ω-dichlorinated alkanes ClCnH2nCl of increasing carbon numbers. For nC5Gly, the characteristic points are indicated.

zone are characteristic data of the systems. All these figures are given in table 3 for the three pentyl derivatives.

In ternary water/oil/amphiphile systems at equilibrium, the type of system formed depends on several formulation variables, among others the chemical structures of oil and amphiphile, temperature and salinity.32 Near the so-called “optimal formulation”, when the amphiphile has a balanced affinity for oil and water and decreases the oil/water interfacial tension sufficiently, a bicontinuous microemulsion in equilibrium with excess water and oil phases is formed. It has been demonstrated that solvo-surfactants show a similar phase behavior as “true” surfactants, except that a much higher concentration of amphiphile is required to possibly obtain these three-phase systems. Moreover, the minimal interfacial tension between the excess oil and water phases attained at the optimal formulation is much higher (∼0.1 mN m−1) for solvosurfactants than in the case of real surfactants (∼10−3−10−4 mN m−1).33 The behaviors of the five glycerol ethers have been investigated in 1/1 w/w oil/water systems at 25.0 °C. The nature of the oil has been chosen as the formulation variable to tune the relative affinity of the amphiphile for oil and water. To scan the oil polarity, 1-mono and α,ω-dichlorinated alkanes of increasing carbon numbers have been selected and the appearance of the one, two or three phase systems has been observed. Chlorinated oils were chosen because the compounds studied are short-chain amphiphiles with a fairly polar glyceryl head and cannot microemulsify alkanes. The pentyl derivatives (1iC5Gly, 2iC5Gly, and nC5Gly) exhibit a typical phase behavior, forming, depending on the oil, either so-called Winsor I (microemulsion with excess oil), Winsor II (microemulsion in equilibrium with excess water) or Winsor III (microemulsion with excess oil and water) systems, and then a monophasic Winsor IV system at high hydrotrope concentrations. With short-alkyl chain oils, Winsor II systems are formed, whereas Winsor III systems are obtained close to the optimal oil, then Winsor I systems when the oil is too long. This succession of phase behaviors can be represented in a socalled “Fish diagram”, as presented in Figure 6. The coordinates of the so-called “Fish tail”, at which the minimum amount of hydrotrope is needed to attain a monophasic Winsor IV system, give the number of carbons of the optimal oil of the hydrotrope (nC*) and its efficiency (C*). Also, the minimum amount of hydrotrope to form Winsor III systems (C0) and the number of carbons of the shortest (nClo) and longest (nCup) oils delimiting the 3-phase

Table 3. Characteristic Data of the 1iC5Gly, 2iC5Gly, and nC5Gly/Water/1-Chloroalkane CnCl Systems and of the PhC1Gly/Water/α−ω-Dichloroalkane ClCnCl Systems at 25.0 °C (Water/Oil Ratio 1/1 w/w) C0/% w/w C*/% w/w ΔC = C* − C0 n C* nloC nup C lo ΔnC = nup C − nC

1iC5Gly

2iC5Gly

nC5Gly

PhC1Gly

9.0 38.0 29.0 5.55 5.20 5.80 0.60

3.5 38.5 35.0 6.55 5.95 7.60 1.65

2.5 34.0 31.5 5.50 4.95 6.05 1.10

6.5 33.5 27.0 5.25 4.90 5.75 0.85

The aryl derivatives PhGly and PhC1Gly show different behaviors in oil/water systems. Even with the shorter 1-chloroalkane (1-chloropropane), PhC1Gly forms Winsor I systems, which means that 1chloropropane is already too lipophilic to balance PhC1Gly. When screening the more polar α,ω-dichlorinated alkanes family, it was found that PhC1Gly forms balanced Winsor III systems with a limited range of oils mixtures between α−ωdichloropropane and α−ω-dichlorohexane (figure 6). The characteristic data for the phase behavior of PhC1Gly are also presented in table 3. Finally, PhGly does not show an associative behavior in oil/ water systems, neither with the 1-chloroalkanes, nor with the α,ω-dichlorinated alkanes. Whatever the oil, this amphiphile partitions between the oil and water phases, without promoting a cosolubilization. At high concentration, PhGly precipitates without leading to a homogeneous Winsor IV system. Temperature-Sensitivity of the Phase Behavior of nC5Gly. For nC5Gly, the position of the Fish tail (nC*, C*) was also determined at 17.0, 36.5, 45.0, 58, and 76 °C in order to evaluate the sensitivity to temperature. The values are indicated in Table 4.

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DISCUSSION Alkyl Glycerol Ethers: Effect of Chain Branching and Hydrophilicity of the Gly Polar Head. nC5Gly, 1iC5Gly, and 2iC5Gly exhibit MAC values of the same order of magnitude, as 9267



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Table 4. Position of the Fish Tail (nC*, C*) for nC5Gly/ Water/1-Chloroalkane CnCl Systems (Water/Oil Ratio 1/1 w/w) at Different Temperatures T*/°C n C* C*/% w/w

17.0 5.0 34.0

25.0 5.50 34.0

36.5 6.0 35.3

45.0 6.35 35.0

58.0 7.0 41.3

in household products in the 1960s and 1970s, were replaced by linear counterparts. Here, the surface tension and solubilization data show that methyl branching does not much affect the association of the amphiphiles in water, however it makes the 1iC5Gly and 2iC5Gly hydrotopes less efficient for solubilizing compared to nC5Gly. Chain branching impacts the volatilities of the pure compounds and makes 1iC5Gly and 2iC5Gly more volatile than nC5Gly. This trend is emphasized when the methyl substituent is directly linked to the carbon involved in the ether linkage (1iC5Gly). The increased volatility of the branched isomers can be related to the disfavored chain−chain interactions in the liquid state, which decreases the global attractive forces and hence the vaporization enthalpies. It is also likely that it results from the modification of the strengths of the intramolecular hydrogen bonds by the presence and the position of the methyl branching. Figure 7 shows the minimized structures of 1iC5Gly, 2iC5Gly, and nC5Gly computed by COSMO-RS showing the intramolecular hydrogen bonds. The favored conformer of 1iC5Gly exhibits a staggered conformation that strongly differs from eclipsed conformations of 2iC5Gly and nC5Gly. This staggered conformation is induced by the steric hindrance caused by the methyl group in the vicinity of the polar head. The so-called σ-potentials computed by COSMO-RS are presented in Figure 8 and indicate that the affinity of 1iC5Gly for electron poor regions, i.e. its electron donor ability is higher that the two other isomers. On the contrary, the affinity of nC5Gly for electron-rich regions, i.e. its hydrogen bond donor ability, is the highest, which indicates that this latter isomer is the one that can establish the more intermolecular hydrogen bonds, which decreases the volatility. Volatility is of great importance for some applications of hydrotropes in which the compound should be removed at the end of the process (paint coalescence, hard-surface cleaning, etc.). All compounds remain non-VOC, which is of primer interest for large-scale applications. n-Octanol/water partition coefficients P were also computed by COSMO-RS at 25.0 °C for the five water-soluble compounds. These data give an indication of the hydrophilicity of a compound since the lower log P, the more it partitions in

76.0 8.0 42.3

determined by tensiometry. It is slightly higher when chain branching is in the β position (2iC5Gly); however, no distinction could be made between nC5Gly and 1iC5Gly. The MAC value reported here for nC5Gly is slightly higher than the one reported previously (0.25 vs 0.15 mol/L9), which highlights the difficulty to determine accurately this aggregation concentration for short-chain amphiphiles. The one found for 2iC5Gly is close to what was reported for 3-methylbutylglycerol (3iC5Gly).9 Consequently, the MAC values are not reliable to compare the hydrophilic/lipophilic balance of the compounds. The concentrations at which the solubilization of hydrophobic Disperse red 13 starts to be effective (minimum hydrotropic concentrations, MHC) are of the same order of magnitude, somewhat higher in the case of 1iC5Gly this time. As regards to the amount of dye solubilized, the linear isomer nC5Gly is clearly the most efficient one, as indicated by the more important slope of the solubilization curves, and quantitatively observed in the data presented in Table 2: nC5Gly is twice as efficient as the branched isomers at the same concentration. The decreased solubilization efficiencies of 1iC5Gly and 2iC5Gly can be related to the decreased lipophilicities of the chains. To our knowledge, the effect of chain branching has not been much discussed in the case of short-chain amphiphiles. In the case of surfactants, chain branching has been reported to increase the CMC value and to lower the surface tension in the case of Guerbet surfactants (branched ethoxy sulfate and sulfate surfactants).34 Branching is also responsible for the more dense hydrophobic layer in the case of branched nonylphenol ethoxylates.35 It should also be recalled that in the case of surfactants, extensive branching of the hydrocarbon tail often leads to a reduced biodegradability.36 For that reason, branched alkyl benzene sulfonates, which were efficient surfactants used

Figure 7. Minimized structures and σ-surface of nC5Gly, 1iC5Gly, and 2iC5Gly proposed by COSMO-RS. 9268



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ethoxylated alcohols causes T* to fall (indicating a hydrophobic shift) and the surfactant to become less efficient for mixing oil and water (C* increase). In the same work, the authors also indicated that methyl branching has less impact than high branching of the chain. Our results on the pentyl glycerol ethers indicate that methyl branching has more impact when the substituent is in position 2 than when it is in position 1. This could be due here again to conformational effects that make the glycerol polar head more or less accessible for hydrogen bonding. The fact that 1iC5Gly and nC5Gly have nearly the same optimal oils tends to indicate that the decrease in the lipophilicity of the chain induced by branching is compensated by a decrease in the hydrophilicity of the polar head. For 2iC5Gly, conformational constraints would favor rotamers in which the glycerol polar head is even less accessible and consequently, the loss in lipophilicity of the chain is overtaken by the loss in hydrophilicity of the polar head. The amphiphile is overall more hydrophobic (longer optimal oil). The concept of optimal oil has already been used to characterize other short-chain nonionic amphiphiles presented in scheme 4.12 CnIso-endo and CnIso-exo are the two positional

Figure 8. σ-potentials of 1iC5Gly (···), 2iC5Gly (---), and nC5Gly (plain line) computed by COSMO-RS.

the water phase. log P are collected in table 5 for nC5Gly, 1iC5Gly, 2iC5Gly, PhGly, and PhC1Gly. Table 5. log P (n-Octanol/Water Partition Coefficients) Calculated for the Five Water-Soluble Glycerol Hydrotropes by COSMO-RS log P

nC5Gly

1iC5Gly

2iC5Gly

PhGly

PhC1Gly

2.26

1.80

2.11

1.43

1.71

Scheme 4. Nonionic Hydrotropes with Isosorbide (CnIsoendo and CnIso-exo) or Ethyleneglycol (C4E1 and C4E2) Polar Heads

There is a significant difference in the logP values for the three pentyl derivatives. 1iC5Gly has the lowest log P, which would indicate that it is the more hydrophilic one, a result in accordance with the higher MHC observed in solubilization experiments. This increased hydrophilicity could be explained by preferred conformations that make the glycerol moiety more accessible. It should be reminded however that these values have been computed from the conformations calculated by COSMO-RS for the various hydrotropes considered in their pure state, and they do not take into account the modification of intramolecular hydrogen bonding by the presence of water molecules. In oil/water systems, the determination of the optimal oil of an amphiphile, hydrotrope or surfactant, gives consistent information of its hydrophilic/lipophilic balance since it considers the molecule in its global environment. The optimal oil of an amphiphile is the oil with which a balanced Winsor III system (middle-phase microemulsion in equilibrium with equal amounts of excess oil and water) is obtained at 25 °C. For this system, the amphiphile has thus a balanced affinity for both phases and is able to cosolubilize as much oil as water. It corresponds to the situation at which its efficiency is optimum. The notion of “optimal oil” is equivalent to the concept of “optimal temperature” when a system composed of an ethoxylated amphiphile/oil/water is scanned at various temperatures. With the same number of carbon atoms, 2iC5Gly has the longest optimal oil (C6.55Cl), whereas 1iC5Gly and nC5Gly have very similar optimal oils (C5.55Cl and C5.50Cl, respectively), which tends to indicate in that case that 2iC5Gly is slightly more hydrophobic than the two other isomers. The fact that 2iC5Gly is more hydrophobic than the linear isomer nC5Gly is in accordance with what has been described previously for ethoxylated alcohols surfactants.37 Wormuth et al. have demonstrated that an increase of the degree of branching of the hydrocarbon chain of monodisperse

isomers obtained by the monoetherification of isosorbide, a diol obtained by the double dehydration of sorbitol. C4E1 and C4E2 are two common solvo-surfactants from petrochemical origin. The points corresponding to the “Fish tail” (nC*, C*) are presented in Figure 9 for the glycerol-derived hydrotropes under study and the isosorbide and ethyleneglycol derivatives presented in Scheme 4. Figure 9 indicates the hydrophilic/lipophilic balance of the various hydrotropes (nC*) and their efficiencies (C*). For nearly the same optimal oil (ca C5Cl), nC5Gly is much more

Figure 9. Optimal oils (nC*) and efficiencies (C*) of various nonionic hydrotropes in a 1-monochlorinated alkane series (oil/water 1/1 w/ w). 9269



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efficient than the butyl ethers of isosorbide (C4Iso-endo) and diethyleneglycol (C4E2). The linear pentyl chain is more efficient than its branched isomers, as already observed in the Disperse Red 13 solubilization experiments. The higher values of C* for the other hydrotropes should also be related to the higher values of C0, i.e., the minimum amount of amphiphile required to form a middle-phase microemulsion. These data are not reported on Figure 9 but scores respectively 9.0% and 2.5% for 1iC5Gly and nC5Gly (Table 3) and 12.0% and 22.0% for C4E238 and C4Iso-endo,39 which indicates an increased amount of amphiphiles “lost” as unimers solubilized in the oil. It has already been shown on homogeneous series of ethoxylated alcohols (CiEj) that the efficiencies of amphiphiles depend quasi linearly on its alkyl chain length. Figure 10 plots the C*

Figure 11. Efficiencies (C*) of various ethoxylated alcohols CiEj as a function of the length of the oil in n-alkane/water 1/1 w/w systems.

Figure 12. Temperature-variation of the optimal oil in a 1chloroalkane series (1-CnCl) for nC5Gly, C5Iso-endo (from ref 12) and C4E1 (from ref 12).

Figure 10. Efficiencies (C*) of a homogeneous series of ethoxylated alcohols CiEj in a n-decane/water 1/1 w/w system. The optimal temperatures of the systems T* are in the same range, T* = (45 ± 10) °C.

comparison.12 A linear evolution is obtained in all cases, and the sensitivity to temperature can be given by the slope of the straight line. All slopes are positive, which indicates that the compounds become more hydrophobic on heating. They are nearly the same for C5Iso-endo and nC5Gly (0.05 nC/K), whereas it is twice as important for C4E1 (0.11 nC/K). This means that the sensitivity to temperature of glycerol and isosorbide polar heads are close, and twice less important than an ethyleneglycol polar head. Finally, attention should also be paid to the shape of the Fish diagrams presented in Figure 6. With nearly the same optimal oil as nC5Gly, 1iC5Gly shows a Winsor III behavior in a less extended oil polarity-range (Δ(nC) = 0.60 for 1iC5Gly and Δ(nC) = 1.10 for nC5Gly, see Table 3). This could be related to the decreased amphiphilic strength of 1iC5Gly compared to nC5Gly: we have already shown on the example of short-chain ethylene glycol ethers and esters that, with the same optimal oil, the weakening of the strength of the amphiphile induces a shrinking of the 3-phase body until it disappears.31 Aryl Glycerol Ethers: Lipophilic Contribution of the Aromatic Ring. Aryl glycerol ethers are much less volatile than the pentyl glycerol ethers, as indicated by the TGA data presented in Figure 1 and in Table 1. This is the result of stronger interactions in the liquid state via π−π interactions. This decreased volatility should also be related to the higher surface tension of the pure liquids (ca. 40 mN/m). Indeed, when comparing solvents within the same solvent class, it has been found that faster evaporating solvents usually have lower surface tension values than their slower evaporating counterparts.36

values of CiEj of increasing chain length in a n-decane/water system.33,40−43 The number of ethoxylates of the amphiphiles were selected so that the optimal temperatures T* of the systems are in the same range, T* = (45 ± 10) °C, in order to be in similar conditions at the optimal formulation and exclude any concomitant temperature effects. These data indicate that the nature of the polar head does not have a strong impact on the efficiency of the amphiphile C*, but the length and degree of branching of its lipophilic chain does, which confirms what has been observed in the present work. The other hydrotropes, for which the optimal oil is longer, have higher C* but this should not be understood as a reduced efficiency of the amphiphile. It should rather be related to the greater difficulty to solubilize longer oils, this phenomenon being well documented for the CiEj series,33,40−43as illustrated in Figure 11. For a given amphiphile, not only the optimal temperature T* varies linearly with the length of the n-alkane chosen as the oil phase but C* also increases quasi-linearly. When comparing the lengths of the optimal oils of the nC5hydrotropes, the following hydrophilicity ranking of the polar heads can be proposed: Gly > Iso-endo > Iso-exo. As Iso-endo has already been reported to be close in hydrophilicity to a diethyleneglycol unit E212 (cf. also Figure 9 C4Iso-endo and C4E2), these results tend to indicate that a glycerol polar head should be closer to a triethyleneglycol E3. The experiments conducted for nC5Gly at 17.0, 25.0, 36.5, 45.0, 58.0, and 76.0 °C indicate a limited sensitivity to temperature. The evolution of the chain length of the optimal oil against temperature is plotted in Figure 12 and the data previously published for C5Iso-endo and C4E1 are recalled for 9270



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For the same number of carbon atoms in the alkyl chain, branching reduces the efficiency of the hydrotrope and modifies its hydrophilic/lipophilic balance: 1-methylbutyl glycerol (1iC5Gly) has the same optimal oil as n-pentylglycerol (nC5Gly) but is less efficient, whereas 2-methylbutyl glycerol (2iC5Gly) is more hydrophobic than its isomers since it forms microemulsions with slightly less polar oils. These modifications can be due to conformational effects linked to chain branching that make the glycerol polar head more or less accessible. The presence of an aromatic substituent increases the solubility in water and modifies the associative behavior in oil/ water systems. Benzylglycerol monoether (PhC1Gly) is able to form Winsor III systems, just as the alkyl derivatives but with much more polar oils, whereas phenylglycerol (PhGly) does not exhibit the usual Winsor-type behaviors encountered with short and long chain amphiphiles. The glycerol-based hydrotropes become slightly more hydrophobic with increasing temperature. This effect is of the same order of magnitude as in the case of hydrotropes derived form isosorbide, another biosourced polyol. It is however twice less pronounced than in the case of ethyleneglycol derivatives. As regards to their properties as pure compounds (fluidity and volatility) and in water and oil/water systems (solubilization and microemulsion formation), pentyl and benzyl glycerol ethers can be classified among amphiphilic solvents, also named “solvo-surfactants”.

The presence of an aromatic ring overall increases the hydrotrope water-solubility: for alkyl glycerol monoethers, an alkyl chain longer than five carbons makes the compound non water-soluble (nC6Gly has a solubility limit of 0.2 w/w %).9 Here, the propylphenylglycerol (PhC3Gly) has nearly the same water solubility (0.4 w/w %) with a total number of 9 carbons in the hydrophobic chain. The log P values presented in Table 5 indicate that, indeed, the aryl glycerol ethers PhGly and PhC1Gly are more hydrophilic than the pentyl ethers. In the surface tension data, PhGly and PhC1Gly exhibit a smoother decrease in the premicellar region compared to the pentyl derivatives, indicating a lower concentration of amphiphiles at the interface, even if no accurate calculation of the surface coverage can be performed. Their better ability to solubilize the model organic compound chosen is probably due to the improved interactions by π interactions with the aromatic dye. The mechanism of hydrotropic solubilization is still unclear, with the main hypotheses being (1) the formation of a complex between the solute and the hydrotrope, (2) a change in the solvent structure induced by the hydrotrope, and (3) more generally, the self-association of the hydrotrope molecules in micelle-like loose structures (dimers or trimers) that solubilize the hydrophobic compounds.44 The good efficiency of PhGly and PhC1Gly for the solubilization of model azo dye can be explained satisfactorily with mechanisms 1 and 3. The behaviors in oil/water systems indicate that PhGly is not amphiphilic enough to lead to self-associative phenomena and cosolubilization of oil and water, even with the most polar α,ωdichlorinated alkanes. On the contrary, PhC1Gly is able to cosolubilize water and polar oils, and its optimal oil is a mixture of α,ω-dichlorinated alkanes having a mean alkyl chain length of 5.24 carbons. Surface-active species containing an aromatic ring are widespread in industrial applications, mainly because of their ease of preparation from inexpensive petrochemical intermediates. Alkyl benzene sulfonates are still the most important anionic surfactants in volume today. The short-chain sulfonates (as for example sodium cumene sulfonate) are the wider class of hydrotropes on the market. Alkyl phenol ethoxylates are inexpensive and efficient surfactants that have been widely used in a variety of application fields for more than 50 years. However, they are biodegraded into nonylphenol that is considered as an endocrine disruptor able to mimic estrogen, and that is why they are replaced by alcohol ethoxylates in numerous consumer products.45 Few systematic studies of the effect of the aromatic ring on the amphiphilic behavior of nonionics have been published, mainly because alkyl phenol ethoxylates are commercial products that are mixtures of compounds. From an applicative point of view, alkyl phenol ethoxylates have lower freezing points compared to linear alkyl ethoxylates and less tendency to form gels in aqueous solutions.45 The synthetic method used in this work could allow access a range of well-defined aromatic amphiphiles with various nonionic polar heads that would probably exhibit unusual amphiphilic behaviors.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +33 (0)320336366. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS ANR (France) is acknowledged for a grant to L.M. in the frame of the InBioSynSolv project (ANR-CP2D-2009-08). We thank Mariá Soledad Ortiz for the establishment of the phase diagram of PhGly during her stay in Lille in the frame of the Postgraduate Cooperation Program (PCP) No. 2010000305 “Green Emulsions”. FONACIT (Venezuela) and the Ministère des Affaires Etrangères et Européennes (France) are thanked for providing her grant.We thank Dr. Frédéric Cazaux for ATG analyses and Dr. Jean-François Willart for DSC measurements and for fruitful discussions on glass transitions. Pr. Nathalie Azaroual, Dr. Eric Buisine, and Dr. Xavier Trivelli are acknowledged for their precious help in the self-diffusion measurements.

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REFERENCES

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CONCLUSION Alkyl and aryl monoethers of glycerol prepared by an environmentally friendly process have been evaluated as “greener” hydrotropic solvents compared to glycol ethers. 9271



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Hydrotropic Properties of Alkyl and Aryl Glycerol Monoethers

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