The Effect of Biocompatible Esters and Alcohols as Cosurfactants on

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The Effect of Biocompatible Esters and Alcohols as Cosurfactants on Structure and Solubilization Behavior of the Zwitterionic Surfactant Tetradecyldimethylamine Oxide Michael Gradzielski, Klaus Horbaschek, and Bruno Demé Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05232 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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Industrial & Engineering Chemistry Research

The Effect of Biocompatible Esters and Alcohols as Cosurfactants on Structure and Solubilization Behavior of the Zwitterionic Surfactant Tetradecyldimethylamine Oxide Michael Gradzielski1,*, Klaus Horbaschek2, Bruno Deme3 1: Stranski-Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie, Technische Universität Berlin, D-10623 Berlin, Germany 2. Hochschule Coburg, Friedrich-Streib-Straße 2, D-96450 Coburg, Germany 3: Institute Laue-Langevin (ILL), F-38042, Grenoble, France email: [email protected]

Abstract In this work we compare the effect of different monoterpenoid alcohols that differ with respect to their number of double bonds and simple aromatic esters of variable molecular architecture as cosurfactants on the phase behavior of the zwitterionic surfactant tetradecyldimethylamine oxide (TDMAO) and its solubilization behavior with respect to decane as a model paraffin oil. The esters are shown to be potent cosurfactants but require higher concentrations to achieve similar effects with respect to structural changes and solubilization enhancement. Compared to the alcohols they solubilize somewhat smaller amounts of decane, do reduce the interfacial tension substantially less, and also do not form an isotropic phase of unilamellar vesicles (L4) but directly multilamellar vesicles (Lαl). A very interesting effect is the significance of the detailed molecular architecture of the esters, as ethyl benzoate and benzyl acetate, both having the same sum formula, differ significantly with respect to their cosurfactant properties. However, all systems allow to incorporate relatively large amounts of the oil. For the case of the esters this always leads to the formation of oil-in-water (O/W) microemulsion droplets while the alcohols can build in relatively large amounts of oil within their vesicular structures. These findings show that these biofriendly cosurfactants allow to formulate structurally rather versatile systems and efficiently enhance oil solubility for the given surfactant system.

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1. INTRODUCTION Cosurfactants are amphiphilic molecules that by themselves do not form micelles in aqueous solution. Typical representatives are medium chain alcohols, fatty acids, or amines. However, in the presence of surfactants they become readily incorporated into their micelles thereby largely affecting the self-assembly behavior. Accordingly cosurfactants are often employed in order to control self-assembled surfactant structures and to modify their properties1-5, one very important property here being the solubilization behavior6,7. Solubilization often leads to the formation of microemulsions, which are frequently observed for classical nonionic surfactants8 but for other surfactants often require the presence of a cosurfactant, that enhances the solubilization capacity of surfactant systems. For these reasons cosurfactants are important in formulation science and are of high relevance for applications of surfactants9-11. The typical effect of the incorporation of cosurfactants into micelles is to change the spontaneous curvature of the surfactant system12. This can be discussed in the context of the packing parameter concept13 according to which the packing parameter p is given as: p = vh/(ah∙lc), where vh is the volume of the hydrophobic chain, ah: the head group area of the surfactant, and lc the effective length of the hydrophobic chain. It should be noted here that in systems containing cosurfactant one has, of course, to employ here an effective packing parameter where the vh includes the hydrophobic volume of the cosurfactant and ah its area requirement at the interface (and as similarly employed in the description of other microemulsions contained mixtures of amphiphiles for stabilization14). These parameters then determine the spontaneous curvature of the surfactant film and thereby also their solubilisation capacity15. Addition of a cosurfactant will mainly add to the hydrophobic volume v h but typically has little effect on the head group area ah, as cosurfactants at the micellar interface mainly replace water there that is bound to the surfactant head group16. Accordingly, the addition of medium chain alcohols like hexanol was shown to induce a sphereto-rod transition in mixtures of cationic and nonionic surfactant, and for still higher cosurfactant concentration the formation of bilayer phases has been observed17,18. Quite a number of investigations have been done to elucidate the effect of straight chain alcohols and for instance for alcohols ranging from butanol to dodecanol their cosurfactant properties and ability to affect the formation and properties of microemulsions was studied, showing the increasing cosurfactant effectiveness with increasing chain length19. However, for practical applications such straight chain alcohols are often not so attractive, due to their molecules typically not so pleasant smell. In that respect, monoterpenoids are attractive alternatives as they derive from 2 ACS Paragon Plus Environment

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natural resources, have a positive smell and are well biocompatible. Accordingly, they are also employed in formulations from cosmetics and serve as fragrance materials20. However, despite that fact a much smaller number of systematic studies has been done on them with respect to their cosurfactant properties. For the surfactant octyl monoglucoside (C8G1) the addition of geraniol was shown to be quite efficient with respect to forming microemulsions with alkanes21. The detailed structural progression upon geraniol addition resembles that observed for conventional nonionic surfactants of the CiEj type upon increasing the temperature22. Of course, one may not restrict the use of cosurfactants just to alcohols but also other polar/amphiphilic compounds could be interesting here, where in general for applications biocompatible compounds are preferable. Therefore we were interested in comparing the effect on phase and solubilization behavior of different monoterpenoid cosurfactants with those of amphiphilic esters, i. e. ones that have rather short hydrocarbon moieties (as ones with longer chains basically just function as more polar oils23,24. Esters have the interesting property that they are intrinsically well biodegradable, where the hydrolysis products themselves will still have some amphipilic character. It might be noted that esters have not been explored so far extensively in that respect. One study looked at mixtures of differently polar esters with decane, using SDS as surfactant and 1-hexanol as cosurfactant, but this study did not report an enhancement of decane solubilization by the presence of ethylbenzoate or ethylbutyrate25. It also has been reported that the presence of esters in the Brij30/n-octane/water system had only a rather small effect on the phase inversion temperature, while that of alcohols was very marked26. Depending on the polarity of such an additive its localization within the amphiphilic monolayer will be at the interface or in the core of the formed aggregates27. Accordingly in this work we were now interested in addressing the question of the effect of different alcohols and amphiphilic esters on the phase behaviour of the zwitterionic surfactant tetradecyldimethylamine oxide (TDMAO) and the corresponding ability to solubilize decane as a reference oil, thereby forming oil-in-water (O/W) microemulsion droplets. TDMAO has been investigated intensively before with respect to its phase behavior in the presence of straight chain alkyl alcohols of different chain length28,29 and also with respect to the microemulsions that can be formed with it without30,31 and with alcohol16 as cosurfactant. It might be noted that alkyl amine oxides, including TDMAO, are frequently employed in cosmetic or detergent formulations, where they show a thickening effect and are employed as perfume solubilizers32. In our work we employed a constant concentration of 100 mM TDMAO and studied quaternary systems where we varied the amount of added cosurfactant and decane as solubilized oil. 3 ACS Paragon Plus Environment


2. EXPERIMENTAL SECTION 2.1 Materials Tetradecyldimethylamine oxide (TDMAO) was obtained as 25 wt% solution from Clariant (Gendorf, Germany) and was purified by freeze-drying and recrystallising twice from acetone (p.a.). Geraniol (trans-3,7-dimethyl-2,6-octadiene-1-ol; purum; 96%), citronellol ((R), trans3,7-dimethyl-6-octen-1-ol; pract.; 90-95%), 3,7-dimethyl-1-octanol (purum; 96%), ethyl benzoate (purum; 99%), and n-decane (purum, free of olefins; > 98% (GC)), as well as the esters ethyl benzoate (purum, > 99% (GC)), ethyl salicylate (purum, > 99% (GC)), and benzyl acetate (purum, > 99% (GC)) were obtained from Fluka Chemie AG (Buchs, Germany). D2O (99.9% atomic purity), employed for the SANS experiments, was from Euriso-top (Gif-surYvette, France).

a)

d)

b)

e)

c)

f)

Figure 1. Chemical structure of the different cosurfactants employed: a) 3,7-dimethyl-1octanol, b) β-citronellol, c) geraniol, d) ethyl benzoate, e) ethyl salicylate, f) benzyl acetate. 2.2 Methods Interfacial tension measurements have been done with a spinning-drop interfacial tensiometer of the type Site 2 (ITE-Engineering, Göttingen, Germany), which allows to measure interfacial tensions in the range of 10-4 to 5 mN/m. All samples were saturated with the oil (decane) prior to the experiments, which means that they were containing just a small bit of an excess decane phase. The static light scattering experiments were done with a Chromatix KMX-6 instrument, which uses a wavelength λ of 632.8 nm and measures at an angle of 6-7 and 173-174°. From the measured intensity (Rayleigh factor Rθ) the effective molecular weight Mw,eff of the contained aggregates was calculated via: 4 ACS Paragon Plus Environment

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M w ,eff 

N Av  4  R 



4   2  n 0  dn / dc g 2

2  cg

(1)

where NAv is the Avogadro constant, n0 the refractive index of the solvent, cg the concentration in weight/volume, and dn/dcg the refractive index increment. For Freeze-Fracture Transmission Electron Microscopy (FF-TEM) specimens were prepared by placing a small amount of the sample on a 0.1 mm thick copper disk covered with a second copper disk. The sample was rapidly frozen by propelling this sandwich into liquid propane, which itself was cooled with liquid nitrogen. Fracturing and replication were carried out in a freeze-fracture apparatus (Balzer BAF 400, Germany) at -140 °C and Pt/C was deposited at an angle of 45°. The replicas were examined with a CEM 902 electron microscope of Zeiss (Germany). Small Angle Neutron Scattering (SANS) experiments were done at the instrument D11 of the ILL (Grenoble, France). As configurations a wavelength of 0.48 nm together with sample-todetector distances of 1.1, 5.0 and a collimation distance of 8.0 m and one with a sample-todetector distance of 20.0 m and a collimation distance of 20.5 m were chosen. The data were azimuthally averaged, corrected for the sensitivity of the detector elements and scaled on absolute intensity by comparison with the scattering of a 1 mm H2O sample, after subtracting the scattering of the empty cuvette and taking into account the transmissions33. For all samples measured by SANS H2O was substituted by D2O in order to enhance contrast. 2.3 Classification of Phases In our phase studies normal micellar phases or oil-in-water (O/W) microemulsion droplets are denominated as L1-phase. Similarly isotropic and low viscous, but somewhat more translucent is the L4-phase which contains unilamellar vesicles. Birefringent and somewhat more turbid are the Lα-phase which one may subdivide into the Lαl-phase which consists of multilamellar vesicles and the Lαh-phase which is built from planar lamellae. 3. RESULTS AND DISCUSSION In all our experiments we worked with a fixed surfactant concentration of 100 mM TDMAO (~2.6 wt%). First we elucidated the effects of the addition of the different alcohols and esters on the self-assembled structures and then in a second and main step we focused on the effect of adding decane as oil, thereby moving from the ternary to a quaternary system, in order to see



how the alcohols modify the solubilization behavior and what oil containing structures are formed. 3.1 Ternary Systems of TDMAO/Alcohol(Ester)/Water In this part we describe the effect of adding the different alcohols geraniol, β-citronellol, and 3,7-dimethyl-1-octanol, which are very similar in chemical structure (Fig. 1a, identical branching and total length) and only differ with respect to the number of double bonds, being 2 for geraniol, 1 for citronellol and 0 for 3,7-dimethyl-1-octanol (tetrahydrogeraniol), thereby reducing the polarity of the alcohol cosurfactant, as double bounds introduce some more polar character to organic molecules. The observations then are directly compared to those for adding the esters ethyl benzoate, benzyl acetate, and ethyl salicylate, which are very similar in their structure, differing by the direction of their ester bond, respectively having an additional phenolic OH-group. 3.1.1 Phase Behavior and Rheology The phase behavior and macroscopic properties of the systems at a given constant concentration of 100 mM TDMAO was studied as a function of the concentration of the different cosurfactant additives and subsequently also for the addition of oil. The pseudo-ternary phase diagrams obtained at 25 °C are shown in Figure 2a for the three different alcohols employed. Here one observes always that a certain amount of the alcohol can become incorporated within the micellar L1-phase, where this amount is highest for geraniol, lower for citronellol, and lowest for 3,7-dimethyl-1-octanol. At higher alcohol content an L4phase is formed, where the location is similarly placed in relative concentration for the different alcohols, as for the extent of the L1-phase. However, the situation then becomes quite different for still higher alcohol concentration, where only citronellol shows the formation of a birefringent Lαl-phase (Figure 2a). This phase progression for citronellol is the same as for instance seen in straight chain alcohols, like hexanol16,28, where with increasing concentration one has a certain range of a micellar L1-phase, followed by a vesicle phase, but which at still higher concentration is transformed into a classical Lαh-phase. The situation is different for geraniol and 3,7-dimethyl-1-octanol, where for similar concentrations L1- and L4-phases are formed (Figure 2a), but the Lαl-phase is absent (was checked up to 180 mM alcohol). It might be noted that the Lαl-phase is turbid, isotropic and shows weak flow birefringence.


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The corresponding systems with the different esters instead of an alcohol show a similar phase behavior (Figure 2b) and retain an L1-phase up to a concentration of 35-55 mM (somewhat higher than for the investigated alcohols). Then in the range of 60-120/150 mM an Lαl-phase is formed for ethyl salicylate and ethyl benzoate, but no such phase is seen for the benzyl acetate (which however has a larger L1-phase extending up to 60 mM). In contrast to the alcohol system no phase of unilamellar vesicles (L4) is formed by any of the esters, which is a markedly different behavior compared to the alcohols. At lower ester concentration the Lαl-phase is viscoelastic (and shows a rheological behavior similar to Maxwellian; Figure S1a) but then for higher ester concentration becomes a vesicle gel with a constant storage modulus of ~ 8 Pa (for 123 mM ethyl benzoate), as confirmed by rheological experiments shown in Figure S1b.

a)



b) Figure 2: Phase diagrams for 100 mM TDMAO as a function of concentration of cosurfactant and decane at 25 °C for a) different alcohols, b) different esters. 3.1.2 Structural Characterization – Small Angle Neutron Scattering (SANS) A further structural characterization of the system with ethyl benzoate as a function of cosurfactant concentration was done by means of SANS experiments. Within the L1-phase one observes an increase of intensity at low q and a longer q-1 behavior (Fig. 3), which indicates the elongation of the already present rodlike micelles of the pure TDMAO system34. Fig. 3 also contains data from a sample in the 2-phase region (61.2 mM ethyl benzoate), which shows that the surfactant containing upper phase contains the maximally elongated cylindrical micelles, but not yet bilayers (the lower phase is richer on D2O and also seems to contain rodlike structures but to a much lesser extent, potentially phase separation simply was not efficient). The curves obtained for higher ethyl benzoate concentration in the range of 92-184 mM are markedly different to those at lower concentration but amongst each other very similar. They show a correlation peak around 0.01 Å-1, which is in good agreement with a stacked multilamellar structure with an average spacing of ~ 65-70 nm. Furthermore for q values larger than the peak a q-2 slope is observed, which indicates the presence of a locally lamellar structure. The results of the detailed analysis of the scattering curves are given in Table 1 and show that the thickness D (determined by fitting a model of a planar lamellae; for details see SI) increases with increasing amount of ethyl benzoate, i. e. the bilayers swell by the incorporation of the 8 ACS Paragon Plus Environment

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ethyl benzoate. Also the product of spacing d times the volume fraction amph increases, which effectively is another measure of the bilayer thickness but not taking into account undulations. The values (which basically are a projected thickness) are significantly larger than D, which indicates a substantial effect of undulations and the difference increases with increasing ethyl benzoate content. This means that the bilayers become softer and therefore undulate more with increasing ethyl benzoate incorporation, in a manner as similarly seen for benzyl alcohol35.

Figure 3. SANS curves for systems with 100 mM TDMAO and different amounts of added ethyl benzoate (in D2O measured at 25 °C; the sample with 61.2 mM was in the two-phase region and upper and lower phase were measured separately). The blue dashed line indicates a q-2 scaling, the red dotted line a q-1 scaling. Table 1. Lamellar spacing d and lamellar thickness D obtained from the SANS experiments for samples of the lamellar phase of the system 100 mM TDMAO / ethyl benzoate / D2O at 25 C (Φamph: volume fraction of dispersed amphiphilic material, TDMAO and ethyl benzoate). c(ethyl benzoate) [mM]

92.0

122.6

153.0

183.6

d [nm]

73.2

66.3

65.7

65.7

damph [nm]

3.08

3.08

3.34

3.63

D [nm]

2.58

2.68

2.77

2.79



3.2 Quaternary Systems of TDMAO/Alcohol(Ester)/Decane/Water The main focus of this study was on the effect of the different cosurfactants on the systems ability to solubilize oil, where we employed decane as a reference hydrocarbon. When adding decane the general phase sequence remains and a certain amount of decane can become solubilized until the solubilization limit is reached, which in general increases with increasing cosurfactant content. 3.2.1 Interfacial Tension A key parameter that determines the solubilization of oils in surfactant solutions is the interfacial tension γ observed between the surfactant solution and the oil to be solubilized 36. The lower the interfacial tension, the higher the solubilization capacity of the system studied and the larger the formed microemulsion droplets/domains37-39. For alcohols of different chain length the interfacial tension of TDMAO solutions has been shown to go through a marked minimum with increasing alcohol concentration, where with increasing alcohol chain length this minimum becomes deeper and appears at lower concentration, being able to yield a substantial lowering interfacial tension by a factor larger than 10040 (and also for water-in-oil microemulsions a higher fluidity of the interfacial film has been claimed for lower alcohol chain lengths41).

a)


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b) Figure 4. Interfacial tension  at 25 °C against n-decane for systems with 100 mM TDMAO and different amounts of a) geraniol, -citronellol, 3,7-dimethyl octanol and b) ethyl benzoate benzyl acetate, and ethyl salicylate. The interfacial tension γ was measured by the spinning-drop method against decane as oil for equilibrated samples and is given as a function of the concentration of contained cosurfactant in Fig. 4 (in all cases we measured until the phase boundary was reached). For all the different alcohols one observes a pronounced decrease of the interfacial tension upon increasing the concentration down to 0.15-0.4 mN/m (Figure 4a), where apparently the geraniol is most effective in reducing the interfacial tension and the citronellol least so. Such rather low values indicate a marked tendency for good solubilisation properties. In contrast, the amphiphilic esters also lead to a reduction of the interfacial tension but to a much lesser extent of a factor just about 4, and therefore at lowest show values of 0.8 mN/m. However, in general the esters require much higher concentrations then the alcohols to reach their lowest value of interfacial tension, which typically is achieved around the phase boundary of cosurfactant solubility in the L1-phase (cf. Fig. 2b). It is interesting to note that benzyl acetate, that differed substantially with respect to its phase behaviour (Figure 2b) and basically does not enhance oil solubilization, nonetheless leads to a substantial reduction of the interfacial tension and this even at lower concentrations than observed for the other two esters. It is also interesting to point out the relation that only the alcohols, which also show the lower interfacial tension, are able to form the L4 phase of unilamellar vesicles, while this is not observed for the esters.



3.2.2 Phase Behavior – Solubilization of Decane In the next step we want to look at the quaternatery phase diagrams as given in Fig. 2. In general one observes that upon addition of decane the generic phase behavior of the ternary systems is retained and into the existing phase always a certain amount of decane can become incorporated. As expected from the interfacial tension data, where the alcohols showed much lower values, substantially larger amounts of decane can become solubilized by them, where citronellol shows the highest value (of about 3.4 decane molecules per TDMAO molecules). Interestingly the highest values of solubilization are observed for alcohol concentrations where in the ternary systems an L4-phase is present (it might be noted that the concentration steps taken for determining the phase diagrams were typically about 10 mM, sometimes 5 mM, for the alcohol/ester concentration, while the solubilization capacity for decane was determined to an error of less than 5 mM by drop-wise addition of decane, until phase separation was observed). Quite interesting is the phase behavior of the different esters. Here the L1-phase, which for higher decane content is an oil-in-water microemulsion, has a markedly different extent with respect to the amount of ester for which it can be formed. In terms of extension with respect to ester concentration the L1-phase grows substantially upon addition of decane and one sees a transition from the Lαl-phase to the L1-phase with increasing decane content. This is different to the behavior of the alcohols where the oil becomes incorporated within the vesicle phase, which indicates a higher rigidity of the amphiphilic interface in this case. The maximum amount to be solubilized increases from benzyl acetate over ethyl salicylate to ethyl benzoate and this coincides with the maximum amount of ester in the L1-phase. Interesting to note here is that for ethyl benzoate and ethyl salicylate the maximum concentration of solubilized ester increases markedly upon the addition of decane. This synergistic solubilization behavior points clearly to a cosurfactant function of these esters. However, they might also be partly solubilized within the microemulsion core and apparently the benzyl acetate is basically just acting as a somewhat polar oil, being a bit better solubilized than decane. 3.2.3 Structural Characterization 3.2.3.1 Static Light Scattering The structure of the aggregates formed upon solubilization of oil into the different cosurfactant containing surfactant samples was studied by means of light scattering. This was done for samples saturated with decane, i. e., at the line of emulsification failure, where one sees the 12 ACS Paragon Plus Environment

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maximum size of droplets that can be formed at that cosurfactant concentration. Fig. 5 shows the Rayleigh factor for the different samples based on the alcohols (a) or esters (b) as cosurfactants. Except for benzyl acetate one observes immediately in increase in scattering intensity upon addition of the ester or alcohol. For the different alcohols this increase is generally similar for all of them, but one can notice an increase in size of the microemulsion droplets with increasing degree of polarity of the alcohol, i. e. with increasing number of double bonds. Quite differently, for benzyl acetate one even observes a slight decrease and only for the highest concentrations an increase of intensity by about a factor 2. In contrast, for ethyl benzoate and ethyl salicylate a maximum increase by about a factor 10 is seen, which indicates that the microemulsion droplets formed here are much larger than the ones present for the pure TDMAO, i. e. these both esters are working as cosurfactants while benzyl acetate is not.

a)

b)

Figure 5. Rayleigh factor from static light scattering for microemulsions saturated with ndecane as a function of the amount of contained alcohol (a) or amphiphilic ester (b). 13 ACS Paragon Plus Environment


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Table 2: For the systems with different alcohols and esters (and also pure 100 mM TDMAO): the maximum ester concentration ccs,max in the microemulsion phase, maximum molecular weight Mw,max, and corresponding droplet radius Rmax as derived from static light scattering. ccs,max/mM

Mw,max/104 g/mol

Rmax/nm

-

4.1

2.6

geraniol

32

24.3

4.8

citronellol

31

18.4

4.4

3,7-dimethyl-1-octanol

21

9.2

3.5

benzyl acetate

118

5.7

2.9

ethyl salicylate

150

29.4

4.8

ethyl benzoate

238

31.3

5.0

cosurfactant - (pure TDMAO)

From the light scattering data we determined the molecular weight (eq. 1), from which in turn (using the densities of 0.891 and 0.730 g/ml for TDMAO and decane, as well as the corresponding values for the alcohols and esters, respectively, and assuming additivity of volumes) an effective droplet radius was calculated. In these calculations we assumed all of the surfactant, cosurfactant, and decane to be contained in the aggregates, thereby neglecting the amount of molecularly dissolved material (but compared to the TDMAO concentration of 100 mM its cmc of 0.12 mM42 is basically negligible). These values are given for the maximum size (highest ester concentration) in Table 2. They show that for benzyl acetate the size even decreases compared to the pure TDMAO (the increase in intensity simply comes from the fact that substantially more organic material is dispersed). In contrast for ethyl salicylate and ethyl benzoate much bigger microemulsion droplets are observed, which have 4.8 and 5.0 nm radius, respectively. This demonstrates that ethyl salicylate and ethyl benzoate really function as a cosurfactant, while benzyl acetate does not. The small size of the droplets for the case of benzyl acetate can be explained such that it is a polar oil that enlarges the interfacial area of the microemulsion by being solubilized in the palisade layer but only enhances little the solubilization capacity of the system. Based on this geometric argument one would expect basically only a small increase in size, as seen experimentally 3.2.3.2 Small Angle Neutron Scattering (SANS) More detailed structural information regarding the ethyl benzoate/decane system was obtained by SANS experiments43, again done for samples of 100 mM TDMAO containing various 14 ACS Paragon Plus Environment

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amounts of cosurfactant and saturated with decane. The SANS curves are depicted in Figure 6 and nicely show the scattering pattern of globular aggregates that grow in size with increasing content of ethyl benzoate, as indicated by an increasing scattering intensity and the shift of the form factor minimum to lower q (it might be noted here that in the SANS experiments we employed D22-decane and D2O and in the resulting shell contrast the form factor minimum is much more pronounced than for a bulk sphere37.

Figure 6. SANS curves for systems of 100 mM TDMAO and different amounts of added ethyl benzoate and D22-decane (in D2O measured at 25 C; the absolute intensity applies to the sample with 182 mM ethyl benzoate (eb), subsequent samples were multiplied by a factor 3 for better legibility (fits according to eq. 6 are given as solid lines)). The curves were analyzed by a polydisperse spherical core-shell modell in an identical fashion as done previously16,37, for which the SANS form factor P(q, R1, R2) is given as:





P(q, R1 , R 2 )  16 2  SLD sh  SLD s   R 2  f 0 q  R 2   A  R1  f 0 q  R1  2

with f 0 x  

3

sin x  x  cos x x3

3

2

(2) (3)

A

SLD sh  SLD c SLD sh  SLD s

(4)

q

4  sin  / 2 

(5) 15 ACS Paragon Plus Environment


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where R2 and R1 are the outer and inner radius, respectively. SLDsh, SLDc and SLDs are the scattering length densities of shell, core, and solvent, respectively, and q the modulus of the scattering vector with λ being the wavelength and θ the scattering angle. The scattering intensity for a polydisperse system is then given as: 

I(q) N   P(q, r )  f (r )  S(q, )  dr 1

(6)

0

where S(q,σ) is the structure factor that accounts for the interparticle interactions (for which in our case we employed the hard sphere model, as described by Ashcroft&Lekner44) and f(r) the distribution function of the mean radius (r = (R1+R2)/2) for which we employed the log-normal function as defined by:

 t 1  f r    R  m

t 1

 t 1  rt   exp   r  ( t  1) R m  

(7)

In our model we allowed for a distribution of the ethyl benzoate between the (oil) core of the microemulsion droplets and the amphiphilic monolayer (the distribution being controlled by A, eq. 4). This will affect substantially the SLD of the core and is a likely scenario as ethyl benzoate is well (fully) soluble in decane. That leads to marked changes of the scattering pattern (moving from a pure shell contrast more to solid sphere contrast) and therefore can be deduced reliably from the scattering patterns. This analysis shows nicely the growth of the droplets upon increasing the ethyl benzoate concentration and the rather low polydispersity index p of 0.12 for the pure TDMAO that increases only slightly to 0.15 upon incorporation of the ethyl benzoate. The thickness of the amphiphilic film decreases somewhat from 1.1 to 0.9-0.95 nm, a reduction that may be understood such that the ethyl benzoate intercalates into the amphiphilic monolayer, thereby opening some space behind for hydrophobic material. This will be filled up by D22-decane thereby reducing the contrast and making the shell effectively look thinner (one could account for this effect by using a second layer in the shell for the SLD profile of the microemulsion droplet or in general introducing a more complex profile, but we refrained from doing so as attempts showed that this invites for the problem of overfitting, thereby reducing the reliability of the deduced structural parameters). It is also interesting to notice that with increasing content of ethyl benzoate its fraction in the core (xc(EtBz)) is increasing. This is to be expected as the amphiphilic interface will become saturated with it and then more and more of the additional ethyl benzoate will have to reside within the microemulson core. However,


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the clear preference of the ethyl benzoate to be solubilized in the amphiphilic palisade layer at lower content, is a clear confirmation of its cosurfactant character. Table 3: Mean shell radius Rm, shell thickness D, and polydispersity index p for samples in the system 100 mM TDMAO / ethyl benzoate (eb) / D22-decane / D2O. Given are the concentrations of ethyl benzoate and decane, the value for the sum 2κ +κ of the bending moduli, and the mole fraction xc(EtBz) of the ethyl benzoate that is localized within the core of the microemulsion droplets. c(eb)/mM

0

54

94

138

182

c(decane)/mM

30

35

86

92

71

Rm in nm

2.52

2.87

4.40

5.15

5.19

D in nm

1.10

0.918

0.952

0.949

0.905

p

0.1165

0.1252

0.1221

0.1351

0.1511

2κ +κ in kT

3.28

2.87

2.97

2.47

2.03

xc(eb)

-

0.13

0.41

0.59

0.58

As microemulsions are largely controlled by the bending moduli of their amphiphilic monolayers45,46 we then also used the polydispersity index p determined by SANS to calculate the sum 2κ +κ of mean bending modulus κ and saddle-splay modulusκ. This was done in a fashion described in ref. 37 and using the formula:

2   1 ln()  (1/   1)  ln(1  )   2 k T 2 4  p

(8)

where Φ is the volume fraction of the dispersed microemulsion. The obtained values are summarized in Table 3 and show a systematic decrease with increasing content of ethyl benzoate. Compared to the behavior of alcohols it can be stated that the ethyl benzoate has a behavior in between that of an aliphatic alcohol like 1-hexanol and that of benzyl alcohol40. This is interesting as that means that this ester is leading to a more rigid monolayer than benzyl alcohol and therefore also has a more marked cosurfactant property than that alcohol 3.2.3.3 Freeze-Fracture Transmission Electron Microscopy (FF-TEM) While SANS is perfectly suited to investigate small structures in the range of 0.5-500 nm complementary information regarding structural details and for larger structures can be obtained from FF-TEM. Accordingly some FF-TEM experiments for various amounts of added 17 ACS Paragon Plus Environment


cosurfactant and decane were done. Figure 7 shows FF-TEM micrographs for samples with different amounts of added alcohols and decane in the L4-phase which demonstrate the presence of unilamellar vesicular structures here. FF-TEM images of samples of geraniol and 3,7-dimethyl octanol in the L4-phase show that even for relatively high content of decane, close to reaching the phase boundary, still unilamellar vesicles are present with diameters of 150-200 nm (Figure 7 a-d). In images b) and d) one can also discern smaller spherical objects, which might be microemulsion droplets formed here in addition. In contrast the preparation of a sample with 75 mM citronellol with 101 mM decane, which is in the Lαl-phase, shows clearly much bigger multilamellar vesicles with about 2-3 µm diameter (Figure 7 e,f). The multilayers look like being easily deformable which indicates a rather low bending modulus of the bilayers. a)

b)

c)

d)


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e)

f)

Figure 7. FF-TEM images of a, b) 100 mM TDMAO/61 mM geraniol/142 mM decane/water. The scale bars indicate 1.0 µm (a) and 200 nm (b), respectively. c, d) 100 mM TDMAO/45 mM 3,7-dimethyl-1-octanol/89 mM decane/water. The scale bars indicate 0.5 µm (c) and 200 nm (d), respectively. e, f) 100 mM TDMAO/75 mM β-citronellol/101 mM decane/water. The scale bars indicate 0.5 µm (e) and 1.0 µm (f), respectively.

4. Conclusions In our investigations we studied the effect of different monoterpenoid alcohols and polar esters with an aromatic and a C2-unit (ethyl benzoate, ethyl salicylate, and benzyl acetate) with respect to their cosurfactant properties and thereby their ability to enhance the solubilization capacity of surfactant systems. As surfactant we choose the zwitterionic tetradecyldimethylamine oxide (TDMAO) and decane as reference oil. These investigations showed that the alcohols are much more effectively reducing the interfacial tension between surfactant solution and decane, and also smaller amounts of them are required. However, both types of cosurfactants lead to similar amounts of solubilized oil at maximum solubilization (only benzyl acetate is not enhancing oil solubility and behaving not as a cosurfactant but an oil). However, there is a big difference with respect to the phase behavior. While for the alcohols typically an isotropic L4-phase containing unilamellar vesicles is formed for high enough concentration, for the esters the formation of a birefringent Lαl-phase containing large multilamellar vesicles (MLVs) is observed (except for benzyl acetate where only a micellar L1-phase is observed). The marked difference in behavior for benzyl acetate and ethyl acetate is interesting as it shows that the detailed molecular architecture of ester is very important for its cosurfactant properties and no bigger droplets are formed for benzyl acetate (while it might be noted that for instance almost no difference in the microemulsion behavior of a nonionic surfactant has been observed for isomeric butanols47). 19 ACS Paragon Plus Environment


OH

OH

OH

OH

OH

OH

decane

OH

OH

OH

OH

OH

a)

decane

decane

decane OH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

b)

c)

d)

Figure 8. Scheme of localization and the effect on solubilization for the different alcohols at low (a) and high (b) concentration and of esters like ethyl benzoate or ethyl salicylate (c) or benzyl acetate (d). For low alcohol concentration simply larger microemulsion droplets with the alcohol in the surfactant monolayer are formed (Figure 8a). Interestingly the addition of decane to the L4phase of the alcohols leads just to a swelling of the bilayers (Figure 8 b) but largely retaining the vesicle structure. This indicates a higher stiffness of the amphiphilic film, as seen similarly for straight chain alcohols40, that stabilizes a swollen bilayer. In contrast, for the esters the L1phase becomes much larger and the Lαl-phase becomes transformed to an L1-phase (oil-in-water microemulsion) upon decane addition. As seen by SLS and SANS the size of the microemulsion droplets at the phase boundary is correspondingly increasing with increasing content of ester and decane at the phase boundary of emulsification failure. SANS in the shell contrast shows nicely how with increasing content of ethyl benzoate more and more of it becomes contained in the core of the microemulsion droplets thereby indicating that the amphiphilic interface of the droplets becomes saturated with the ester cosurfactant (Figure 8c). Apparently the esters ethyl benzoate and ethyl salicylate play a double role, acting as cosurfactant at low concentration and behaving more like an oil at higher concentration. In contrast, the benzyl acetate is only solubilized like a polar oil (Figure 8d). In summary, this shows that esters can be potent cosurfactants that allow to control the size of the formed microemulsion droplets. In contrast, the monoterpenoid alcohols allow for a surprisingly marked swelling of their vesicle phases by decane incorporation. Both types of employed cosurfactants have the advantage that they are easily biodegradable, thereby giving their formulations have a high application potential (e. g. directly as fragrances). The structural


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versatility of these systems renders them interesting for biofriendly formulations that require a rather high degree of oil solubilization. Supporting information Rheological measurements on the Lα,l-phase, SANS analysis of lamellar structures, and static light scattering results. Acknowlegdements The authors are grateful to Tanja Leicht for performing some of the experimental work and to ILL for allocating SANS beamtime for this investigation.

References (1)

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(8)

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(9)

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decane

SANS

decane


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The Effect of Biocompatible Esters and Alcohols as Cosurfactants on

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