12354
J. Phys. Chem. B 2008, 112, 12354–12360
Solubilization in Alkanes by Alcohols as Reverse Hydrotropes or “Lipotropes” P. Bauduin,† F. Testard,*,† and Th. Zemb‡ CEA, IRAMIS, SCM, LIONS, F-91191 Gif-sur-YVette, France, and CEA/CNRS/UniVersite´s UMR 5257, ICSM, F-30207 Bagnols sur Ce`ze ReceiVed: May 27, 2008; ReVised Manuscript ReceiVed: July 25, 2008
Hydrotropes in aqueous systems do not aggregate in micelles, inhibit presence of mesophases and allow significant and progressive solubilization of “insoluble” molecules in water. It was shown that n-alcohols in alkanes develop the same properties, including the power-law for maximum solubilization of “hydrophilic” molecules. The aim of this paper is to highlight properties of reverse hydrotropes or “lipotropes” by taking n-alcohol/alkane mixtures as model systems. So as to establish a clear parallel between lipotropes and hydrotropes the same methodology used to characterize hydrotropes was applied to these systems. The solubilization of solutes insoluble in alkane, i.e. water and a hydrophilic dye in dodecane, enabled by the addition of n-alcohols (n ) 2, 3, 4 and 7) was studied. In parallel, the nonmicellar aggregation state of butan-1-ol and heptan-1-ol in dodecane was investigated by small-angle X-ray scattering. By applying the Porod’s treatment the specific area of the H-bond network formed by heptan-1-ol and the area occupied by hydroxyl group in this network were determined as a function of concentration. A correlation between the aggregation of alcohols in dodecane and the solubilization was made. The disrupting of concentrated mesophases by a lipotrope was illustrated by studying the effect of adding n-alcohols to water/oil/extractant ternary systems used in liquid/liquid extraction. Under some conditions the organic phase splits up into two phases: an extractant mesophase and nearly pure oil. The amount of n-alcohols required to make the extractant mesophase disappear was determined for water/alkane/malonamide extractant systems. The influence of the chain length of the n-alcohol on the efficiency as lipotrope was also experimentally studied. The trend obtained was similar to the one observed with the solubilization experiments. Introduction The term hydrotrope was first introduced nearly a century ago and referred to water-soluble organic compounds whose addition in water leads to an increased solubilization of “hydrophobic” compounds i.e. being insoluble in pure water.1 Since then, hydrotropes were extensively studied and they were found to have numerous industrial applications.2,3 As in the case of surfactants, the solubilization induced by hydrotropes is a consequence of the hydrotrope aggregation state in water. For surfactants, the formation of micellar aggregates leads to solubilization.4,5 The class of hydrotrope refers to short amphiphiles showing three common features: (i) They do not aggregate in ordered structures, such as spherical micelles, but rather form dimers, trimers,... to connected structures in a pairwise manner, via weak interactions such as hydrogen binding. Due to their shorter and/or branched alkyl chains hytrotropes show weaker hydrophobic effects compared to surfactants in water. (ii) Above a given concentration in water, hydrotropes enable the solubilization of large quantities of hydrophobic compounds in water: the solubilization vs hydrotrope concentration curves follow power-laws. (iii) Hydrotropes break ordered lyotropic phases (mesophases) formed by surfactants at high concentrations. * Corresponding author. Telephone: +331 6908 9642. Fax: +331 6908 6640. E-mail:
[email protected]. † CEA, IRAMIS, SCM. ‡ CEA/CNRS/Universite ´ s UMR 5257, ICSM.
In water when the surfactant concentration is increased above the critical micellar concentration (CMC) surfactant molecules self-aggregate into micelles. Because of their short hydrocarbon parts, leading to a weaker hydrophobic effect compared to true surfactants, hydrotropes show a weak tendency to self-aggregate in water and therefore do not form micelles. This is proved by routine scattering experiments. Hydrotropes are sometimes characterized by a high apparent “critical aggregation concentrations”, i.e., above 0.2 M, usually determined by surface tensionor conductivity measurements. At such high concentrations the concept of critical aggregation is controversial.6 The simplest way to illustrate this remark is to consider that hydrotrope molecules become “ordered” in the solution at high concentrations with sterical and entropic constraints being involved.7 Most reasonable is to depict the hydrotrope aggregation through a stepwise self-aggregation process for example in a pairwise manner. Due to this progressive formation of labile nonmicellar structures, no clear break-point can be detected in the solubilization curves at the apparent hydrotrope “critical aggregation concentration”. Moreover exponential increases in the solubility of hydrophobic compounds are observed as a function of hydrotrope concentration.3 Recently, it was demonstrated that the solubilization is related to the extent of the hydrophobic part brought about by hydrotropes molecules in water.7 Thus the aggregation of hydrotrope molecules was not taken into account to describe the solubilization process. Nevertheless the aggregation of hydrotropes in water is still actively under debate.8,9 Another generic and important property of hydrotropes lies on their ability to break ordered phases, such as hexagonal or
10.1021/jp804668n CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
Solubilization in Alkanes lamellar phases, formed by surfactants in water.10,11 This property is used for example to lower the viscosity of liquid detergents composed of lamellar phase forming surfactants such as polyethoxylated surfactants. In such formulations, hydrotropes lead to decrease substantially the elastic modulus of the surfactant film. Consequently the curvature is increased. This molecular packing is similar to a cosurfactant effect. Owing to this property it was possible to produce “on demand” thermodynamically stable vesicles in simple surfactant systems only by replacing counter-ions by hydrotropes in ionic micelles.3 Without hydrotropes, thermodynamically stable vesicles in binary surfactant systems described so far rely on the equilibrium between bound and free states of counter-ions.12 Compared to oil in water (o/w) dispersions, reverse systems such as reverse micelles characterized by polar regions dispersed in a continuous oil phase have attracted relatively less attention in the literature.13,14 Few phase diagrams can be found, and thermodynamics of aggregation is still under debate.13 Surfactants with bulky hydrophobic parts or suitable surfactant/ cosurfactant combinations tend to form large domains of water nanodroplets covered by a surfactant film, i.e., typical w/o microemulsions referred to sometimes as “reverse micelles” in apolar solvents. In the case of ionic surfactants forming reverse micelles, a minimum of water is often needed to enable the solubilization of the surfactant. Hence the solubilization of water as a third component leads to the formation of swollen reverse micelles i.e. microemulsions. On the contrary direct micelles are formed in water without the requirement of adding oil. In this work, the terms “reverse hydrotrope” or “lipotrope” will be used in opposition to the term hydrotrope. [Note that in biology the term “lipotrope” refers to a compound acting on fat metabolism by hastening the removal or by decreasing the deposit of fat in the liver, e.g. choline or betaine.] It defines amphiphilic compounds having hydrophilic/lipophilic ratio lower than the ones of reverse micelles forming surfactants. These are sometimes called cosurfactant or “lipophilic linker”.15-17 Their main feature is to comicellize with a surfactant and hence can be clearly distinguished from lipotropes.18 The term lipophilic linker (LL) was first introduced by Graciaa et al.17 to describe amphiphilic substances having a small hydrophilic group and a large hydrophobic part. LLs were extensively studied in surfactant–oil-water systems mainly because they increase substantially solubilization. According to Sabatini et al., the difference between a cosurfactant and a LL lies in the fact that the first one participates to the formation of the surfactant film whereas the second one does not.15 To our knowledge no clear experiment has evidenced such a difference.15 Nevertheless it has to be noticed that cosurfactant and lipotrope properties are not mutually exclusive and that the cosurfactant property is only relevant when a surfactant is present in solution. The aim of the present paper is to emphasize a parallel between hydrotropes and lipotropes by studying model systems composed of n-alcohols (n ) 2-4 and 7), propylene glycol propyl ether (C3PO1) as lipotropes, and an apolar solvent i.e. dodecane. In comparison to the studies made usually on LL and cosurfactants the systems chosen do not contain any surfactant. Hence, the formation of the so-called “mixed micelles” detected via X-ray or neutron scattering do not interfere with properties studied. Here the three common properties of hydrotropes mentioned above are transposed to lipotropes. The examined properties are as follows:
J. Phys. Chem. B, Vol. 112, No. 39, 2008 12355 (i) The ability of solubilizing hydrophilic compounds in apolar solvent. The solubilization curves of water and hydrophilic dye in dodecane by addition of lipotropes, here alcohols, are determined. (ii) The low aggregation ability in apolar solvent. The microstructure adopted by alcohol molecules in dodecane is investigated by small angle X-ray scattering. (iii) The ability of breaking ordered phases such as hexagonal or lamellar phases. To illustrate this property, we choose an example taken from industrial applications: liquid/liquid extractant systems used for the extraction/separation of minor actinides. Those systems are composed of an aqueous phase containing salts to be extracted and an organic phase being a solvent/extractant mixture. It was shown recently that the undesired formation of a third phase, being the splitting of the organic phase, may be caused by an increase in the organization state of extractant molecules in dodecane from spherical to cylindrical micellar aggregates.19 In this context, addition of octanol to extractant/dodecane mixtures proved to break the extractant supramolecular structure efficiently and hence acts to prevent the “third phase” formation.20 In that sense, n-octanol and more generally n-alcohols display the lipotropic property of breaking ordered reverse aggregates. To illustrate the lipotropic efficiency, the minimum quantity of n-alcohols required to make disappear the third phase of model extraction systems composed of N,N′-dimethyl-N,N′dibutylpentyl malonamide (DMDBPMA)/dodecane or heptane/ water is determined. By varying the alkyl chain length of the n-alcohol added (n ) 5, 7, 10 and 12) the lipotropic efficiency of n-alcohols is compared and discussed. Experimental Section Materials. Dodecane (99%), 4-(2-Hydroxy-1-naphthylazo)benzenesulfonic acid sodium salt (hydrophilic dye Orange II, noted as OII hereafter), ethanol (anhydrous, >99.8%), propan-1-ol (anhydrous, >99.5%), butan-1-ol (>99.5%), pentan1-ol (>99.5%), heptan-1-ol (>99.5%), decan-1-ol (>99.5%), dodecan-1-ol (>98.5%), propylene glycol propyl ether (C3PO1, >98.5%) were obtained from Sigma-Aldrich, deuterated octan1-ol (99%) from Eurisotop and N,N′-dimethyldibutylpentyl malonamide (DMDBPMA) from Panchim. Determination of the Maximum Solubilization Curve: Water Solubility in n-Alcohol/Dodecane Mixtures. Solutions of n-alcohols (n ) 4 and 7) in dodecane at different concentrations were saturated with water. The amount of water added was adjusted in order to prevent the formation of large quantity of an excess aqueous phase which would lead to partitioning of the alcohol between coexisting phases. The change in the density of the n-alcohol/dodecane mixtures following the addition of water is small hence the concentration of n-alcohol in dodecane is considered to be constant. The Karl Fischer method was used then to determine the concentration of water solubilized in the organic phases. Titrations were run on a Metrohm KF-684 coulometer that can detect between 10 µg and 10 mg of water. The reliability of the method was checked by repeated measurements of the amount of water contained in standard samples (Hydranal-water standard 0.1). The solubilization curves are expressed as S/Smax, S being the measured solubility and Smax the maximum amount of water solubilized in the solution at a given lipotrope concentration. Orange II Solubility in Alcohol/Dodecane Mixtures. Solutions of n-alcohol (n ) 2 and 3), propylene glycol propyl ether (C3PO1) in dodecane at different concentrations were saturated with the hydrophilic dye Orange II and left equilibrated 24 h at
12356 J. Phys. Chem. B, Vol. 112, No. 39, 2008 21 °C. The solutions were then filtered in order to separate the excess dye from the solutions. The optical density (OD) of the filtered solutions were measured using 1 cm path length quartz cells with a UV-visible spectrophotometer UVIKON-932 at a wavelength of 483nm corresponding to the wavelength λmax of the Orange II where the dye has his adsorption maximum. Before each measurement, a zero absorbance was done with the corresponding solution without dye. For the lipotropes studied λmax value was found to be insensitive to the chemical nature of these compounds ((5 nm). When the measured OD was above a critical value, suitable dilutions were done by using the same solution but without dye. Through the Beer-Lambert relation and the extinction coefficient of the Orange II ( ) 21744 L · mol-1 · cm-1) the measured OD were converted into Orange II concentration. The results will be presented by plotting S/Smax as a function of lipotrope concentration, S being the measured solubility and Smax the maximal amount of dye solubilized in the solution. Determination of the Minimum Quantity of n-alcohols Required To Make the Extractant Third Phase Disappear. Three phase extractant systems were formulated by contacting 1 mL of milli-Q water and 1 mL of a solution containing DMDBPMA (C ) 1 M) in dodecane19 or heptane. After mixing the two phases, a third phase forms immediately. Then aliquots of lipotropes, namely pentan-1-ol, heptan-1ol, decan-1ol, and dodecan-1-ol, were added until the third phase completely disappears and only an oil-rich and a water-rich phase coexist. Small Angle X-ray Scattering (SAXS). SAXS experiments were performed in flat cells of 0.1 and 0.2 mm thicknesses with Kapton windows on the Huxley-Holms high flux camera.21,22 The X-ray source is a copper rotating anode operating at 15 kW. The KR1 radiation (λ ) 1.54Å) is selected by a Xenocs monochromator mirror. Spectra were recorded with a twodimensional gas detector of 0.3 m in diameter giving an effective q-range of 0.02-0.35 Å-1 (q ) 4π/(λ sin θ) where θ is the scattering angle and λ the wavelength of the incident beam). Data correction, radial averaging, and absolute scaling were performed using routine procedures.23 To access a larger q range, additional spectra were obtained with a “Guinier-Mering” setup23,24 using a two-dimensional image plate detector.25 The X-ray source is a molybdenum rotating anode operating at 3 keV. The q-range covered by this instrument is 0.06 to 2 Å-1. Data correction, radial averaging, and absolute scaling were performed using routine procedures. Methods SAXS Data Treatment. Quantitative analysis of SAXS (and SANS) absolute scale spectra can be obtained by modeling the intensity scattered by the objects, i.e., by assuming a specific shape for the molecular aggregates26 as well as for interactions between them. Recently, the calculation of the SAXS scattering intensities of pure alcohols has been analyzed from theoretical considerations using Monte Carlo simulation.27 Hence the structure of pure alcohols has been resolved confirming and completing previous studies28-32 by showing that alcohols molecules are sequentially H-bonded to flexible linear and even cyclic aggregates of different sizes associated in a progressive pair-wise manner: dimer, trimer, tetramer,..., and n-alcohols mixed with dodecane are likely to form similar aggregates. The addition of dodecane to pure alcohol comes to “dilute” and change compositions in aggregates of different lengths. Alcohol/dodecane mixtures27 and extractant (malonamide)/ dodecane mixtures19 show some evident similarities: both extractant and alcohol show a microstructure in alkanes which
Bauduin et al. is the origin of the existence of a correlation peak in the q-range 0.15-0.4 Å-1 observed in their SAXS spectra at high concentrations (c > 40% w/w). This broad peak comes from an average distance between high electronic density regions in the solution. Recently, the structure of highly concentrated extractant solutions was resolved by determining both the specific surface of the aggregate/alkane interface in the sample and the volume fraction of the scattered objects in the solution from SAXS spectra.19 The specific surface is related to the surface occupied by extractant molecules at the interface between dodecane and the polar region of extractant aggregates while the volume fraction of the scattered objects takes into account the polar region of extractant taking part to the formation of aggregates in the solution. A similar approach is adopted here to investigate the structure of heptan-1-ol/dodecane mixtures. The specific surface Σ is obtained trough the Porod law (eq 1).26,33
lim (Iq4) ) 2π2∆F2Σ
qf∞
(1)
with Σ (cm-1) the total interface of the scattering objects per unit volume which is related to the average concentration of heptanol molecules participating to the aggregates and to the specific surface per alcohol molecule, σ, through: Σ ) [heptanol]agg × σ. The limit term can be determined from the SAXS spectra in the large q-range where the intensity follows a q-4 behavior. This regime is only obtained if a thin interface separates two media of different scattering length densities F (cm-2) which is proportional to electronic density (eq 2).26
ft FRX )
∑ Zi i
Vm
(2)
where ft is the Thomson constant (0.282 10-12cm), Zi the atomic number of the ith atom and Vm the molecular volume (cm3). ∆F corresponds to the difference in the scattering length densities between the polar part of higher electronic density Fpolar formed by the hydrogen bonded hydroxyl groups of the n-alcohol molecules and the apolar part of lower electronic density Fapolar composed of the alkyl chain of the n-alcohol and of the dodecane. Fapolar is estimated to be the scattering length density of a mixture of dodecane and heptane in a similar C12/C7 alkyl chain ratio as in the sample giving:
Fapolar ) FdodecaneΦdodecane + FhepteneΦc7
(3)
with Φdodecane and ΦC7 being respectively the volume fraction of dodecane and C7 alkyl chain of heptanol in the aliphatic part of the sample. To determine Fpolar, the volume of the hydroxyl group VOH in heptan-1-ol/dodecane mixtures at a given ratio must be known. Hence partial molar volumes of heptan-1-ol (Vheptanol) are determined as a function of concentration in the mixture with dodecane by density measurement (Anton Paar-DM5000) according to a standard procedure.35 On Figure 1, Vheptanol is plotted versus the mass fraction of heptan-1-ol. Assuming with good confidence that the volume of the C7 alkyl chain (VC7) is independent of the heptan-1-ol/dodecane ratio, partial molar volumes of the hydroxyl group (VOH) in the mixtures may be estimated by VOH ) Vheptanol - VC7, with VC7 ) 215.7 Å3 estimated with the Tanford expression for linear alkyl chains V (Å3) ) 27.4 + 26.9 n.35 Σ, Fpolar, and Fapolar are listed in Table 1 for the concentrations tested.
Solubilization in Alkanes
J. Phys. Chem. B, Vol. 112, No. 39, 2008 12357
Figure 1. VC7OH (Å3/molecule), the partial molar volume of heptan1-ol in the heptan-1-ol/dodecane mixture, is plotted versus XC7OH mass fraction of heptan-1-ol in dodecane.
The volume fraction of the heterogeneities in the solution Φscatt., i.e., H-bonded hydroxyl groups, contributing to the scattering intensity can be obtained using a general theorem of SAXS spectra (eq 4) ∞
∫ i(q)q2 dq ) 2π∆F2Φscatt(1 - Φscatt)
(4)
0
The so-called “invariant”, QExp, is an integral deduced from the complete scattering spectra measured on absolute scale (left term of eq 4). On the other hand the invariant may be calculated theoretically (right term in eq 4). Hence the volume fraction Φscatt of the scattering object is obtained from eq 4 using the scattering length density contrast (∆F). In our case, the Porod regime is observed only after subtraction of the background and QExp is calculated in a q-range from 0.06 to 0.7 Å-1. Φscatt and ΦOH the total hydroxyl volume fraction in the samples, calculated from the weighted amounts of dodecane/ heptan-1-ol and VOH values, are listed in Table 1. To evaluate σ from Σ, [Heptanol]agg is required and is calculated directly from Φscatt with eq 5
[Heptanol]agg )
Φscatt 1027 × VOH N
(5)
For butan-1-ol/dodecane mixtures the background subtraction could not be done properly in a reliable interval of q; thus this makes the Porod’s treatment unreliable. Results and Discussion Solubilization Property of Lipotropes. Water Solubilization. In Figure 2 the water solubility curves in 1-butanol/ dodecane and 1-heptanol/dodecane mixtures is plotted as S/Smax Vs volume fraction of the hydroxyl part in n-alcohol/dodecane mixtures. Monotonous increases in the water solubility are observed by increasing alcohol volume fraction. Compared to a classical micellization process no break in the curves subsequent to a sudden aggregation process occurs. The solubility curves for C4OH and C7OH can be described by power laws (see Figure 2) with exponents ranging from 1.5 to 1.6 and differ from typical curves determined when micelles are at the origin of solubilization. The exponent values are reminiscent of normal percolation laws. Recently Glatter et al.27 studied carefully the structure of pure alcohols using SAXS measurement and Monte Carlo simulation. This study and others28-32 propose that pure alcohols are mixtures of oligomers held through hydrogen bonds and of varying lengths. Very long
percolating structures of alcohol molecules through hydrogen bond formation were also shown in molecular dynamics simulations. Hence it is likely that the percolation laws shown in Figure 2 reflect the insertion of the added water molecules in the n-alcohol percolating hydrogen bond network formed while n-alcohol volume fraction is increased. From the difference between the curves in Figure 2 it seems that by increasing n from 4 to 7 the lipotropic efficiency increases; i.e., the water solubilization starts at lower hydroxyl volume fraction. The more hydrophobic environment brought by C7OH compared to C4OH may reinforce the hydrogen bond network making it more favorable to accommodate water molecules. This is in agreement with the argument of Iglesias et al.36 who have shown through excess permittivity measurements that the breaking of hydrogen bonds through dilution with alkane is more efficient for shorter alcohols. Dyes Solubilization. In order to show that the solubilization in oil by lipotropes is a general process, we compare the solubilization limit of the most frequent “polar” solute, i.e., H2O, with a typical ionic water-soluble organic dye (Orange II). This dye was chosen as a solute because it is both insoluble in dodecane and only partially soluble in n-alcohols. Shorter n-alcohols (n ) 2 and 3) and C3PO1 were used to ensure solubilization of the dye. For longer chain alcohols (n > 4) the OII solubility becomes insignificant. The solubilization curves of OII in dodecane, shown in Figure 3, display a similar shape as the one observed for the solubilization of water by longer n-alcohols (Figure 2). Power laws enable also the description of the solubilization curves but here the exponents are ranging from 2 to 2.2. This may reflect the larger connectivity of the hydrogen bond network of shorter alcohols. Specific lipotrope/solute interactions based on hydrophobic effect and/ or van der Waals interactions are likely to be involved in the change of the exponent. C2OH, C3OH, and C3PO1 show increasing lipotropic efficiencies in terms of solubilization of the OII dye in the following order: C2OH < C3OH < C3PO1. This follows the trend classifying short amphiphiles according to their hydrophobicity which was recently determined for CiPOj and n-alcohols from different types of methods.7,37,38 Moreover, as it was observed for the water solubilization (Figure 2) that an increase in the hydrophobicity of the lipotrope leads to increase the lipotropic efficiency as measured in the present study. It should be noticed that CiPOjs were found to be efficient also as “direct” hydrotropes.7 Thus n-alcohols as well as CiPOj can be used as both hydrotropes and as lipotropes. Nevertheless, if water is present as a second liquid phase C2OH, C3OH, and C3PO1 are preferentially partitioned in water because their partition coefficients are largely in favor of water due to their high hydrophilic/lipophilic balance. Hence in the presence of an excess water phase no lipotropic effect could be observed in the systems studied. Nonmicellar Aggregation of Lipotrope Molecules. SAXS measurement was used to investigate the supra-molecular structure of n-alcohols (n ) 4, 7) in dodecane; see Figure 4. Two peaks can be observed on the spectra. The broad peak observed in the spectra at around 1.4 Å-1 refers to an average correlation distance (4.5 Å) between CH2 groups in dodecane and alcohol. This peak is always observed when alkyl chains are present in the sample and does not give any direct information on the aggregation state of the alcohol molecules in dodecane. As already observed and described by Glatter et al.27 in the case of pure alcohols, the peak at lower q values is due to local
12358 J. Phys. Chem. B, Vol. 112, No. 39, 2008
Bauduin et al.
TABLE 1: Data Obtained for the Heptan-1-ol/Dodecane System Studied (See the Text for Details)a CHeptanol (mol/L) XHeptOH
total
interface
VOH (Å3)
Fpolar (cm-2) × 10-11
Fapolar(cm-2) × 10-10
Σ (Å2/Å3) × 103
σ (Å2/mol)
QExp. (cm-4) × 103
Φscatt.× 102
ΦOH × 102
D* (Å)
0.401 0.601 0.796 1
2.687 4.104 5.530 7.074
0.148 0.688 2.657 5.408
11.2 15.3 18.2 19.1
2.27 1.66 1.40 1.33
7.68 7.61 7.54 7.45
5.7 29.3 105.8 264.0
25.3 38.3 42.6 54.9
0.251 1.168 4.511 9.194
1.11 1.85 3.51 5.61
4.6 7.0 9.4 12.1
15.5 15.6 14.5 14.1
a VOH is deduced from density measurements, Fpolar and Fapolar are respectively the scattering length densities of polar (OH) and apolar regions (heptan-1-ol alkyl chain/dodecane), specific area Σ between regions of high electron-density and low electron-density is deduced from asymptotes of scattering at high-q, volume fraction Φscatt of high-electronic density of connected OH domains comes from value of invariant and peak position, directly read on the SAXS spectrum. In supplementary materials the peak position is compared to the prediction of packing and cubic random models.
Figure 2. Water solubility, expressed as S/Smax, as a function of volume fraction of the hydroxyl part in n-aclohol/dodecane mixtures: butan1-ol (b), S/Smax ) 20.19ΦOH1.49 (R2)0.9942); heptan-1-ol (9), S/Smax ) 12.00ΦOH1.61 (R2 ) 0.9982).
Figure 3. OII solubility, expressed as S/Smax, as a function of volume fraction of the hydroxyl part in alcohol/dodecane mixtures: ethanol (2), propan-1-ol (b), and C3PO1 (9).
electron density excess formed by the hydroxyl groups hold together by hydrogen bonds in aliphatic hydrocarbon medium of lower electronic density. Additional SANS experiment on deuterated octan-1-ol (see Supporting Information) confirmed the presence of this correlation peak at the same qpeak value as observed in SAXS by Glatter et al. and also by Abe´cassis et al.20 Compared to SAXS where electronic density heterogeneities scatter, SANS experiment is sensitive to H/D contrast. Despite the difference in absolute unit of scattered length density between SAXS and SANS, these experiments reflect the same contrast limit between the chains and the hydrogenated polar head of octanol. Thus the peak corresponds to an average distance between concentrated zones in hydroxyl groups among alkyl chains and gives information on oligomers that held through hydrogen bonds.
Figure 4. SAXS spectra of 1-heptanol and 1-butanol/dodecane mixtures at different ratios. The total alcohol mass fractions (and the volume fractions of the hydroxyl part) in the mixtures for 1-butanol are 0.4 (0.071), 0.6 (0.108), 0.8 (0.146), and 1 (0.186) and for 1-heptanol, they are 0.4 (0.046), 0.6 (0.070), 0.8 (0.094), and 1 (0.120). The spectra that show the lowest intensity on both graphs correspond to pure dodecane.
The maximum of the peak qpeak is related to the correlation length D* through the expression: D* ) 2π/qpeak, D* corresponding to the average distance between polar heterogeneities in the solution. These peaks are clearly detectable for C4OH and C7OH (see Figure 4) for volume fractions above around 0.07 and 0.04, remarkably well above the dimerization threshold.39 This is coherent with the absence of detection of dimers or short oligomers in the mixtures by SAXS. Hence, at higher concentrations only larger aggregates contribute to the scattering intensity in the vicinity of the correlation peak of the H-bonded aggregates. This is confirmed by Φscatt < ΦOH (see Table 1). Remark that the higher the concentration the higher the Φscatt/ ΦOH ratio, reflecting the progressive aggregation, i.e., the growth of the oligomers, when concentration is increased. The values of qpeak (and D*) shift slightly by increasing volume fractions: 0.59-0.6 Å-1 (10.7-10.5 Å) and 0.4-0.44
Solubilization in Alkanes Å-1 (15.5-14.1 Å) for C4OH and C7OH. The decrease in D* as a function of volume fraction may be related to the supramolecular structure formed by the hydrogen bonds. Different models were tested: 1-, 2-, and 3-D dilution laws, cubic random model (CRC model), and DOC models for disordered lamella and cylinder (for heptanol with Φscatt and Σ as input parameters). However, none of them enabled us to reproduce the slight but measurable D* shifts (see details in Supporting Information). A more general approach is based on the determination of both the specific surface and the volume fraction of scattered objects. The aim is to characterize “compacity” or “connectivity” in order to determine the variations in the structure of alcohol/ alkane mixtures through dilution (see the Methods section). The method used presents the major advantage of being independent of the structure of the aggregates and is particularly well adapted to the study of the present system whose supra-molecular structure is not well defined, i.e. composed of different shaped and sized aggregates. The area occupied per heptan-1-ol molecule at the interface σ of the aggregate are obtained from Porod limits (see Table 1). σ increases with concentration from 25.3 Å2 (Φscatt ) 0.011) to 54.9 Å2 (Φscatt ) 0.056) for pure heptan-1-ol. The formation of new H-bonds due to the elongation alcohol imers is consistent with an increase in the averaged σ value. At low volume fractions σ is comparable to the area occupied by fatty alcohol molecules in a compressed monolayer just before collapse (≈20 Å2). This value represents the lower limit of σ and corresponds roughly to the cross section of a single alkyl chain. Thus alcohols are self-organized in alkane but the aggregates formed are not organized into well defined micellar aggregates as in surfactant solutions. The solubilization of water or the ionic dye when alcohols are used as lipotrope is related to this supramolecular organization and also is the origin of the solubilization mechanism. This is consistent with the mechanism given by Neuman40 to explain the solubilization properties of bis(2-ethylhexyl) hydrogen phosphate (HDEHP), used as a chelating and extracting agent. Neumann proposed that the presence of opened connected “polar” networks in organic solutions is implied in the incorporation of metal ions in apolar medium as occurring in liquid/ liquid separation processes. In alcohol/alkane systems a cohesive network hold by hydrogen bonds permit to solubilize polar solutes nevertheless the solubilization ability is limited. This remark holds also for hydrotropes; hence, they also have a limited ability in solubilizing hydrophobic compounds in water. In that sense, a parallel can be done between hydrotropes and “lipotropes”. Disrupting Property of Ordered Condensed Phases by “Lipotropes”. In hydrometallurgy, organic molecules composed of a complexing polar part and of an apolar part are used to extract specifically salts from water toward the organic solution. Malonamides are used specifically for the treatment of radioactive waste issued from the reprocessing of the spent nuclear fuel. As for many liquid/liquid extraction systems a third phase, i.e., the splitting of the organic phase into two phases, a diluted one and a condensed one, may form under certain conditions. Third phases are concentrated extractant phases which are organized in connected lamellar or cylindrical microstructures.19 Hence, to demonstrate the disrupting property of lipotropes, we have determined the minimum amount of n-alcohol sufficient to make the third phase disappear (Cmin). n-alcohols were added to three phase systems composed of water/DMDBPMA (C ) 1 M in alkane)/dodecane or heptane (see Figure 5). For both
J. Phys. Chem. B, Vol. 112, No. 39, 2008 12359
Figure 5. Minimum concentration (Cmin) of n-alcohol which is sufficient to make the third phase of water/DMDBPMA (1 M in alkane)/ dodecane (top) or heptane (bottom) systems disappear.
systems, a general trend in the lipotropic efficiency is observed following the series: C5OH < C7OH < C10OH < C12OH, C12OH being the most efficient lipotrope in breaking ordered phase meaning it is efficient at lower concentrations. As it was observed for water and dye solubilization experiments, an increase in the hydrophoby of the lipotrope leads to increase lipotropic efficiency. Conclusion It was shown in the case of alcohol/alkane mixtures that the three common properties characterizing hydrotropes, namely weak ability to aggregate, solubilization property and ability to break ordered phases, could be transposed to nonaqueous apolar systems i.e. to reverse systems. As the term hydrotrope, the proposed term “lipotrope” does not represent a specific class of chemicals but rather regroups a class of molecules showing the three similar properties investigated here. It was shown evidently that the solubilization of polar compounds in dodecane by addition of alcohols is related to the increase in size of alcohol aggregates which is hold by hydrogen bonds. Finally, it was found that by changing the chain length of the n-alcohol the efficiency in solubilizing polar solutes in alkanes and the efficiency in breaking ordered phases follows a similar order. Acknowledgment. The authors thank: Dr. Laurence Berthon and Dr. Claude Berthon for fruitful discussions and for supplying the DMDBPMA diamide extractant, F. Marchal and Dr. P. Guenoun (DSM/IRAMIS/SCM) for the SANS spectrum of fully deuterated octanol, the Nuclear Fission Safety Program of the European Union for support under the EUROPART (F16WCT-2003-508854), and the CEA DSOE/CHSOL program for support. Supporting Information Available: Text discussing and figures showing the SANS spectrum of deuterated octan-1-ol as well as models used for the prediction of D* variation as a
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