O miniemulsions

Journal of Colloid and Interface Science 393 (2013) 203–209

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Cleavable surfactants to tune the stability of W/O miniemulsions C. Belenki a, M. Winkelmann b, M. Nieger d, W. Gerlinger c, B. Sachweh c, H.P. Schuchmann b, T. Muller a, S. Bräse a,⇑ a

Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, D-76131 Karlsruhe, Germany Institute of Process Engineering in Life Sciences, Section I: Food Process Engineering, Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany c BASF SE, GCP/TP – L540, 67056 Ludwigshafen, Germany d Laboratory of Inorganic Chemistry, University of Helsinki, 00014 Helsinki, Finland b

a r t i c l e

i n f o

Article history: Received 31 August 2012 Accepted 31 October 2012 Available online 8 November 2012 Keywords: Water-in-oil miniemulsion Cleavable emulsifier Molecular characteristics High pressure homogenization Droplet size Target phase separation

a b s t r a c t Recently, there has been growing interest towards the formation and use of miniemulsions as nanoreactors for polymerization and precipitation reactions. Regarding precipitation reactions in miniemulsions, emulsifiers are required that on the one hand stabilize droplets in a size range <1 lm and on the other hand allow break-up of the miniemulsion into its two initial phases after particle synthesis for purification reasons. In this work we report the synthesis and emulsifying abilities of low-mass cleavable emulsifiers based on monoesters of oxalic and malonic acids for the stabilization of water-in-oil miniemulsions. A systematic screening of compounds with respect to different polar groups as well as length, molecular branching and type of alkyl chains and their suitability as emulsifiers was performed. Our results show that the size of droplets stabilized by these emulsifiers strongly depends on the nature of the polar group and the length of the lipophilic chain. The targeted phase separation of the emulsions was triggered by the addition of a base cleaving the emulsifiers. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Recently, there has been a growing interest towards the formation and use of miniemulsions. A miniemulsion is an osmotically stable emulsion with droplet sizes ranging between 50 nm and 500 nm [1,2]. Miniemulsions are used in chemical processes for polymerization [3,4], in the pharmaceutical field as drug delivery [5,6] and in cosmetics as personal-care formulations [7]. For the formation of a miniemulsion from two liquid phases, energy input is required [4,8]. High-energy emulsification methods include high-shear stirring, high-pressure homogenizers and ultrasound generators [8]. In the low-energy emulsification methods, chemical instead of mechanical energy is used [9,10]. In the present study high pressure homogenization was used as a continuously working, well scalable process, allowing intensive energy input. In high pressure emulsification processes, a coarse pre-emulsion (premix) is exposed to a pressure p of up to several hundred to thousand bar (conventional machines work at up to 2000 bar) and decreased by passing this premix through a homogenization valve or orifice of varying geometry [11]. Droplet break-up mechanisms in homogenizing valves have already been intensely investigated in former studies [2,11–13] and are a topic of on-going research [14,15].

⇑ Corresponding author. Fax: +49 721 608 48581. E-mail address: [email protected] (S. Bräse). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.10.072

Once droplets are broken up, the interfaces have to be stabilized by emulsifiers. These are interfacial active molecules. They adsorb at hydrophilic/lipophilic interfaces and stabilize them dynamically and statically. Requirements to the molecular structure and resulting properties of emulsifiers are (1) their interfacial activity in general (often measured via their ability to decrease the interfacial tension between the phases) [11] and (2) fast kinetics of adsorbing at new interfaces, resulting in a fast stabilization of the newly formed droplets [16]. Fast stabilization kinetics of emulsifiers are strongly governed by their molecular size [17,18]. The latter is especially important for emulsification processes that produce very small droplets in short times, e.g. high-pressure homogenization [11,12]. There are two different kinds of emulsions: oil-in-water and water-in-oil emulsions. Whether an oil-in-water (O/W: direct) or a water-in-oil (W/O: inverse) emulsion is formed depends, amongst others, on the hydrophilic–lipophilic balance (HLB) of the emulsifier. Usually, the phase in which the emulsifier is soluble presents the continuous phase of the emulsion (‘Bancroft rule’) [19]. Oil-in-water miniemulsions are well-known and frequently applied in the food and cosmetic industry [20,21]. Recently, the application of an O/W emulsion as reaction medium for polymerization reactions has been introduced [1,22]. Thus, a whole series of emulsifiers are already established and commercially available for O/W miniemulsions. In contrast, inverse, i.e. miniemulsions of water-in-oil (W/O) type, were described for the first time no longer

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than 10 years ago [11,13,17,23]. Therefore emulsifiers for inverse miniemulsions are still relatively rare and not systematically investigated. It is common knowledge that the emulsifier is an essential part of an emulsion system. The emulsifier destines the property and the nature of the emulsion. Tan and Nakajima investigated the effect of different polyglycerol esters of fatty acids on stability and physicochemical properties of nanodispersions. They reported that particle size and stability of b-carotene nanodispersions were affected significantly by the type of fatty acid and also by the polar group of the emulsifier [24]. Identifying a suitable emulsifier with specific properties for particular applications is still a big challenge. The physicochemical properties of emulsifiers are significantly affected by the molecular size of the emulsifier. The micellization ability [25,26], micellar size [27] and surface activity for lowering the surface or interface tension [28] increase systematically with increasing hydrophobic chain length of surfactants. Traditionally, emulsifiers are stable compounds. However, the demand for cleavable emulsifiers has increased in recent years (for reviews about cleavable emulsifiers see [29–31]). Such interest is mainly due to the growing environmental concerns, i.e. the biodegradation rate of emulsifiers is a major issue [30]. Technical applications of emulsions also demand cleavable emulsifiers: especially applications where emulsions of decent stability are required in an early step, whereas at a later stage, this stability causes problems. This is the case in nanoparticle precipitation within miniemulsion droplets. In this process, droplets of a size below 1 lm serve as a reactor for the formation of nanoparticles. However, once the particles are formed, they have to be purified by breaking the miniemulsions into separate phases. The use of cleavable emulsifiers can solve this problem. After the emulsifiers have fulfilled their function, they are cleaved in order to lose their interfacial activity. In 2004 Eastoe et al. reported that ultraviolet radiation can trigger the destruction of microemulsions containing photodestructible surfactants [32]. A few years later Hayes et al. investigated acid-cleavable emulsifiers in microemulsion [33]. In the present study we focus on the synthesis and investigation of alkali-cleavable emulsifiers. The aim of this work was to develop cleavable emulsifiers for water-in-oil miniemulsion, so far unknown in literature. The next purpose of the study was to investigate the influence of the chemical structure of the emulsifier on the droplet size of the miniemulsion. The main challenge of this work is finding a molecular structure that allows: (1) Stabilization of water droplets in lipophilic phases (W/O type emulsions). (2) Stabilization of droplet sizes in a size range of 50 nm to 500 nm (W/O-miniemulsions), being required for nanoparticle precipitation. (3) Production of these W/O-miniemulsions via high pressure homogenization, as this is a scalable process ensuring droplet sizes <1 lm in high throughput applications. Thus, the synthesized emulsifiers are required to be low-mass emulsifiers to allow for fast stabilization kinetics. (4) Controlled break-up of its molecular structure in order to destabilize the emulsion in a controlled way. Here, we report the synthesis and emulsifying abilities of structurally simple low-mass emulsifiers for the production and stabilization of water-in-oil-miniemulsions via high-pressure homogenization. Several examples of the synthesized emulsifiers containing a hydrophobic tail and a polar head group are presented. A systematic screening of the molecules regarding the function of different polar groups as well as length, molecular

branching and type of alkyl chains and their ability to act as emulsifiers was performed. The obtained results served as basis for chemical modification and structural optimization of the molecules. In addition to the structure–activity relationship study of synthesized emulsifiers, we were also able to develop emulsifiers, which combine customized properties in order to selectively stabilize and destabilize inverse miniemulsions.

2. Experimental In the following section the chemical synthesis and physical properties of synthesized water-in-oil emulsifiers are described. Furthermore, the experimental procedure to produce and characterize water-in-oil emulsions is given. For all experimental procedures, for the detailed preparation of emulsifiers and their spectral data, please see the supporting information. 2.1. Chemical synthesis Five different classes of compounds were synthesized and studied regarding their applicability to stabilize a water-in-oil miniemulsion. In addition the carbon chain length of these compounds was varied and their influence on the resulting droplet size was characterized. Although some of the synthesized compounds are already known 3{8} [34,35], 3{9} [36], 3{10} [37], 3{12} [38], 3{13} [39], 3{14} [40], 3{16} [40], 3{18} [37], 5{6} [41], 12{10} [42], 12{11} [43], 12{12} [44], 12{14} [45], 12{16} [45], 13{14} [35,46], 17{16} [35,47], to the best of our knowledge they have not been used as emulsifiers so far. 2.1.1. Synthesis of oxalic acid monoesters One of the investigated classes of compounds are oxalic acid monoesters 3{n}.1 The structure of the synthesized cleavable emulsifiers contains an oxalic acid moiety as polar head and an alkyl chain as hydrophobic tail. In addition, the monoester is a group with limited stability. The cleavage of the ester bond can be alkali catalyzed and leads to loss of emulsifying abilities of the molecule, followed by destabilization of the emulsion. Although some members of these monoesters have been known for decades and their behavior in films and in hydrolysis [48] has been studied, they have not been used so far as emulsifiers to our knowledge. Oxalic acid monoesters 3{n} were easily prepared by the esterification of commercial available alcohols with oxalyl chloride2 (2) in absence of a base (Scheme 1) [49]. The known synthesis from Bartel et al. [49] was optimized and improved which allowed us to obtained monoesters with linear alkyl chains in multigram scale with yields up to 94% (Table 1). In addition to oxalic acid monoesters with a linear alkyl chain 3{n}, cycloalkyl oxalic acid monoesters 5{n} were synthesized in good yields (up to 75%) using the same reaction conditions (Scheme 2) [49]. Base-catalyzed saponification causes the cleavage of the monoesters into the corresponding alcohols and oxalic acid. By this the monoester loses its amphiphilic properties and the emulsion breaks up into two phases (Scheme 3). The physical properties of oxalic acid monoesters are highly dependent on the length of their alkyl chains. Regarding the aggregate state, while oxalic acid monoesters with alkyl chains C-8 to C10 (3{8}–3{10}) are oils, the esters with longer alkyl chains (3{11}– 3{18}) are white solids and their melting points increase with the 1 The first number herein presents the class of compound and the second number the alkyl chain length. 2 Safety notice: As with all acyl chlorides, oxalyl chloride reacts with water liberating HCl gas. Overall, its effects are comparable to those of phosgene.

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1) Et 2O, reflux, 15 h 2) H2O

O n-5 1{n}

OH

+

Cl

Cl O

O n-5 O

OH O

3{n}

2 Scheme 1. Synthesis of oxalic acid monoesters 3{n} [49].

Table 1 Synthesized linear oxalic acid monoesters. Substrate

Product

OH

3

OH

8

O

O OH

O

3

[34]

91

[36]

87

[37]

O

3{13}

OH

4

OH

O

8

O

3{8}

OH

9

1{14}

1{9}

OH

O

4

10

1{10}

OH O

OH

O

10

O

3{10}

11

1{11}

OH

O O

3{11}

O

[39]

85

[40]

O

OH

11

1{16}

O

1{12}

11

O OH

O O

13

OH

[40]

89

[37]

O

1{18}

length of the alkyl chains (from 30 to 50 °C). The synthesized cycloalkyl oxalic acid monoesters 5{6}–5{8} are slightly colored oils. The oxalic acid monoesters 3{n} are stable at 7 °C under argon for at least 14 days. At room temperature under air, the disproportionation of the octyl, nonyl, decyl and undecyl oxalates (3{8}–3{11}) and the cycloalkyl oxalates (5{6}–5{8}) to the corresponding oxalic acid diesters and oxalic acid was observed (Scheme 4), already after a few hours. However, the stability of the oxalic acid monoesters increases with the raise of the alkyl chain length. For instance, no decomposition of the octadecyl oxalate (3{18}) occurred, even after several weeks at room temperature.3 All synthesized oxalic acid monoesters 3{n} are soluble in Disproportionation was determined by 1H NMR.

94

OH

O

3{16}

3{12}

89

OH

OH

7

92

1{16} O

6

[38]

OH

3{15}

OH

6

88

1{15}

O 5

O

3{14}

OH

5

OH

O

9

O

3{9}

71

O

O

3

Reference

90

1{13}

1{8}

7

Yield (%)

n-decane, which was chosen as oil phase for our target water-inoil emulsion and not soluble in water. 2.1.2. Synthesis of 1,12-dodecylen bisoxalate 1,12-Dodecylen bisoxalate (10{12}) with two polar oxalic acid groups was prepared according to Scheme 5. First, in order to obtain a less polar compound and increase its solubility in diethylether, the 1,12-dodecandiol (8{12}) was transformed to its disilylether (9{12}). By the addition of oxalyl chloride (2), the labile protecting group was cleaved in situ as desired and 1,12-dodecylen bisoxalate (10{12}) was obtained in 60% yield (Scheme 5). 1,12-Dodecylen bisoxalate (10{12}) is a white solid with a melting point of 78 °C, which is considerably higher than the melting points of oxalic acid monoesters 3{n}. Furthermore, no

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1) Et 2O, reflux, 15 h 2) Et 2O

O + n-5OH

Cl

Cl

O n-5

O 2

4{6}: n=6 4{7}: n=7 4{8}: n=8

OH

O O

5{6}: n=6 (71%) 5{7}: n=7 (75%) 5{8}: n=8 (61%)

Scheme 2. Synthesis of cycloalkyl oxalic acid monoesters 5{n}[41,49].

O n-5 O

OH

O

OH n-5

O

3 {n}

OH

+

OH

HO O 6

1{n}

Scheme 3. Base-catalyzed hydrolysis of oxalic acid monoester 3{n} into alcohol 1{n} and oxalic acid (6).

decomposition of the bisoxalate 10{12} could be observed at room temperature. Due to its higher polarity, the bisoxalate 10{12} is not soluble in n-decane. 2.1.3. Synthesis of malonic acid monoesters The structure of the synthesized malonic acid monoesters 12{n} differs from oxalic acid monoesters 3{n} in only one carbon atom in the polar head. The conversion of commercially available alcohols with Meldrum’s acid (11) without solvents at 120 °C afforded malonic acid monoesters 12{n} in good to excellent yields (Scheme 6) [50]. Although structures of the synthesized malonic acid monoesters are already known [42–45], they have not been used as emulsifiers, to the best of our knowledge. Furthermore, a crystal structure for undecyl malonate (12{11}) was obtained for the first time (Figs. 1 and 2). The aggregate state changed from oil for decyl malonate (12{10}) to a solid state for malonic acid monoesters 12{n} with longer alkyl chain lengths. Malonic acid monoesters 12{n} are more stable than oxalic acid monoesters 3{n}. Thus, no decomposition at room temperature was observed. All synthesized malonic acid monoesters 12{n} are soluble in n-decane and insoluble in water. 2.1.4. Synthesis of N-hydroxycarbamic acid esters A further class of compounds, the N-hydroxycarbamic acid esters 13{n}, was synthesized and their emulsifying abilities were evaluated. The influence of a nitrogen atom in the polar head group on the emulsifying abilities was investigated. N-hydroxycarbamic

acid esters 13{n} were synthesized from corresponding fatty alcohols, hydroxylamine hydrochloride and CDI (N,N0 -carbonyldiimidazole) [51]. Using reaction conditions described by Lebel et al. only 35% conversion could be achieved. By using 3.0 equivalents instead of 1.1 equivalents CDI the yields of octyl N-hydroxycarbamate (13{8}) and tetradecyl N-hydroxycarbamate (13{14}) could be improved to 51% and 58% respectively [51]. Furthermore, the reaction time after addition of hydroxylamine hydrochloride was reduced from 6 to 4 h, which also resulted in better yields (Scheme 7). Hexyl N-hydroxycarbamate- is reported to be mutagen to Escherichia coli. [52]. Thus, special attention should be taken when handling these series of compounds although all of them have longer alkyl chains. All synthesized N-hydroxycarbamic acid esters 13{n} are soluble in n-decane. Regarding the aggregate state, a similar trend to the one observed for oxalic and malonic acid monoesters (3{n},12{n}) was noticed: melting points increase with the increase of alkyl chain lengths from 48 °C for decyl N-hydroxycarbamate (13{10}) to 64 °C for tetradecyl N-hydroxycarbamate (13{14}). N-hydroxycarbamic acid esters 13{n} are stable compounds, no decomposition at room temperature was observed. 2.1.5. Synthesis of N-alkyl oxamic acids In addition N-alkyl oxamic acids 17{n} were synthesized. They were prepared in a two-step synthesis. In the first stage the aliphatic amines 14{n} reacted with ethyl oxalyl chloride (15) under basic conditions with the formation of N-alkyl oxamic acid ethyl esters 16{n} in yields up to 95% (Scheme 8) [53]. The next step was the saponification of ethyl N-alkyl oxamic acid esters 16{n} with LiOH. This reaction afforded the desired 17{n} N-alkyl oxamic acids in excellent yields (90%) (Scheme 9). All of the synthesized N-alkyl oxamic acid ethyl esters 16{n} are solids with melting points between 41 °C and 60 °C. After saponification, the melting points of the corresponding N-alkyl oxamic acids 17{n} increase up to 100 °C. Both, N-alkyl oxamic acid ethyl esters 16{n} and N-alkyl oxamic acids 17{n} are stable at room temperature and N-alkyl oxamic acids 17{n} are soluble in n-decane. 2.2. Emulsification procedure The continuous phase of the emulsion consisted of n-decane (VWR, Germany), whereas a water based 0.1 M zinc-(II)-chloride solution (Merck, Germany) served as dispersed phase. All chemicals were used as purchased without further purification. The emulsion consisted of 58 w/w% n-decane and 40 w/w% 0.1 m zinc-(II)-chloride solution. As shown for a conventional emulsifier

O n-5 O

O

O OH

n-5 O

O

3{n}

O n-5

O 7{n}

OH

HO

+

O 6

Scheme 4. Disproportionation of oxalic acid monoesters 3{n} to oxalic acid diesters 7{n} and oxalic acid (6).

HO

OH

8

TMSCl, NEt3, Et2O, r.t., 2 h

Me Me Si O Me

8 {12} Me Me Si O Me

8

9{12}

8

9 {12}

1) Et2O, r.t., 2 d O Me 2) H2O Cl O Si Me + Cl Me O 2

Me O Si Me Me O HO

O O

O

8

OH

O O

10{12} (60% over 2 steps)

Scheme 5. Synthesis of 1,12-dodecylen bisoxalate (10{12}).

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O n-5

OH

O

+ O

1{10}: n = 10 1{11}: n = 11 1{12}: n = 12 1{14}: n = 14 1{16}: n = 16

120 °C, 3 h

O

n-5

O

O

O

OH

12{10}: n = 10 (96%) 12{11}: n = 11 (80%) 12{12}: n = 12 (81%) 12{14}: n = 14 (69%) 12{16}: n = 16 (91%)

11

Scheme 6. Synthesis of malonic acid monoesters 12{n} [42–45,50].

Fig. 1. Molecular structure of one independent molecule of 12{11} (displacement parameters are drawn at 50% probability level).

The ability of the synthesized emulsifiers to stabilize a waterin-oil-emulsion was investigated according to the following procedure: First the emulsifier was completely dissolved in n-decane. The n-decane/emulsifier solution (120 g) was mixed with 80 g 0.1 m zinc-(II)-chloride solution and stirred with a magnetic stirrer for two minutes at 400 rpm. This pre-emulsion was then high pressure homogenized with a MicrofluidizerÒ M – 110Y (Microfluidics, USA) using a Z–Y chamber at pressures between 200 bar and 1000 bar (single passage). First, it was studied whether a W/O or O/W emulsion was produced. For this, the generated emulsion was dispersed in n-decane. In case of an instant easy dispersing of the emulsion in n-decane it was approved that the emulsion is a W/O-type emulsion. The droplet size distribution of the miniemulsion was determined by laser diffraction (Mastersizer X, Malvern Instruments GmbH, Germany). From this, the Sauter mean diameter d3,2 was extracted as characteristic droplet diameter. This characteristic value was chosen as it is a good measure for the total volume related surface area Sv created by emulsification: Sv = 6/d3,2 The cleavability of the emulsifiers was investigated by adding an aqueous 1 M NaOH solution to the emulsion. In case of phase separation of the emulsion into its two initial phases, n-decane and water, upon addition of NaOH solution, the cleavability of the emulsifier is given.

3. Results and discussion

Fig. 2. Crystal packing of 12{11} showing the intermolecular hydrogen bond pattern.

by Winkelmann and Schuchmann [54], 2 w/w% of emulsifier proved to stabilize a miniemulsion. For this reason, we applied 2 w/w% of the synthesized emulsifiers in our experiments.

n-5 1{8}: n = 8 1{10}: n = 10 1{12}: n = 12 1{14}: n = 14

OH

The emulsifying abilities of the synthesized compounds were tested by the above-described procedure. All of the synthesized compounds besides 1,12-dodecylen bisoxalate (10) are soluble in n-decane and stable homogeneous emulsions could be obtained. 1,12-Dodecylen bisoxalate (10) with two oxalic acid groups is too polar to be dissolved in n-decane, thus, it cannot act as an inverse emulsifier. The synthesized cycloalkyl oxalic acid monoesters 5{n} are soluble in n-decane, but they were unable to stabilize a water-in-oil emulsion. Presumably, the cycloalkyl groups are too small to effectively stabilize the emulsion. All of the other synthesized compounds were able to stabilize a water-in-oil-emulsion. However, not all of them afforded emulsions with the target droplet size of 100–1000 nm. Strong discrepancies on the droplet size were observed within emulsifiers with different polar groups. Furthermore, a dependence of the droplet size of the emulsion on the organic chain length was observed (Fig. 3).

O

1) CDI, THF, r.t., 2 h 2) NH2OH·HCl, pyridine, r.t., 6 h n-5

O

N H

13{8}: n = 8 (58%) 13{10}: n = 10 (94%) 13{12}: n = 12 (73%) 13{14}: n = 14 (51%)

Scheme 7. Synthesis of N-hydroxycarbamic acid esters 13{n}[46,51].

OH

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O n-5

NH2

+

Cl

O

n-5

O 14{12}: n = 12 14{14}: n = 14 14{16}: n = 16

O

NEt3, CH2Cl2, 0 °C − r.t., 3 h

N H

O

O 16{12}: n = 12 (84%) 16{14}: n = 14 (95%) 16{16}: n = 16 (83%)

15

Scheme 8. Synthesis of N-alkyl oxamic acid ethyl esters 16{n} [53].

O n-5 N H

O O

16{12}: n = 12 16{14}: n = 14 16{16}: n = 16

LiOH, H2O/THF (1:1), r.t., 15 h

O n-5 N H

OH O

17{12}: n = 12 (91%) 17{14}: n = 14 (91%) 17{16}: n = 16 (95%)

Scheme 9. Saponification of N-alkyl oxamic acid ethyl esters 16{n} to N-alkyl oxamic acids 17{n}[35,47].

In Fig. 3, the Sauter mean diameter d3,2 is depicted as a function of the number of carbons in the emulsifier’s alkyl chain. Among the oxalic acid monoesters 3{n}, the droplet sizes obtained for emulsifiers with chain lengths between 9 and 15 carbon atoms were within a similar range (between 435 and 495 nm). Compounds with either shorter or longer chain lengths generated larger droplets. With increasing alkyl chains, the size of emulsion droplets rose to 1000 nm in the case of hexadecyl oxalate (3{16}) and to 1050 nm for octadecyl oxalate (3{18}). With a decreasing alkyl chain, an increase of droplet sizes was observed as well. In stabilizing the emulsion with octyl oxalate (3{8}), the Sauter mean diameter of the droplets was found to be 670 nm. Among the malonic acid monoesters 12{n}, the global minimum was found at 334 nm for tetradecyl malonate (12{14}). This compound afforded the smallest droplets of all synthesized emulsifiers. Furthermore, we could observe that with increasing alkyl chains, the size of emulsion droplets rose strongly, up to 1500 nm in case of hexadecyl malonate (12{16}). With a decreasing alkyl chain length, a continuous increase of droplet sizes up to 1130 nm for decyl malonate (12{10}) was observed additionally. One exception of this steady increase was found in dodecyl malonate (12{12}), its Sauter mean diameter of 450 nm constitutes a local minimum. The sizes of emulsion droplets with N-alkyl oxamic acids 17{n} have a global minimum at 1000 nm for N-tetradecyl oxamate (17{14}). Since the droplet sizes of N-dodecyl and N-hexadecyl oxamates (17{12}, 17{16}) with 1500 nm and 15000 nm confirm

the global minimum and all droplet sizes reached with this series of compounds are much bigger than those generated with the other emulsifiers, no further examples of this substance class were prepared and investigated. In contrast to malonic acid monoesters 12{n} and N-alkyl oxamic acids 17{n}, N-hydroxycarbamic acid esters 13{n} lead to a droplet size minimum of 442 nm for an alkyl chain length of 10 carbon atoms, instead of 14 carbon atoms like in the former series. The size of the emulsion droplets rose rapidly to 1500 nm for the shorter octyl N-hydroxycarbamate (13{8}), whereas the increase of droplet size seems slower for longer alkyl chain lengths. For all synthesized and tested emulsifiers, the smallest droplets were found for lipophilic chain lengths of 9–14 carbon atoms. Concerning the studied polar head groups, this seems to be the optimal lengths to penetrate into the lipophilic phase stabilizing the emulsion. In three of the four tested compounds, a global minimum of the Sauter mean diameter of the emulsion droplets was found for a certain chain length. This optimum was at 14 carbon atoms for malonic acid monoesters 12{n} and N-alkyl oxamic acids 17{n}. This might be explained considering that smaller molecules have faster stabilization kinetics resulting in a rapid coverage of the water/n-decane interface; whereas steric stabilization is favored for longer lipophilic chains. Thus, it can be concluded that for the investigated compounds, a chain length of 14 carbon atoms provides on the one hand adequate stabilization kinetics and on the other hand sufficient steric stabilization. In the case of N-hydroxycarbamic acid esters 13{n}, the smallest droplets were obtained with a lipophilic chain of 10 carbon atoms. Accordingly, the steric stabilization is effective for smaller chain lengths. Different adsorption behavior could be an explanation. For oxalic acid monoesters 3{n}, the polar group plays, apparently, a more prominent role than the length of the lipophilic chains, because among this class of emulsifiers, droplet sizes for chain lengths between 9 and 15 carbon atoms were found to be relatively similar (430–500 nm). Outside this chain length range, however, the droplet size increased more strongly. Further research is needed in order to identify responsible factors for the observed tendencies. The cleavability of the synthesized compounds was investigated by addition of aqueous 0.1 M NaOH solution to the emulsion, which resulted in its break-down. In case of oxalic and malonic acid monoesters, complete phase separation could be observed. The emulsions, which were stabilized with N-hydroxycarbamates 13{n} and N-oxamates 17{n}, remained stable even after addition of NaOH, as expected. 4. Conclusions

Fig. 3. Sauter mean diameter d3,2 of the droplets generated by the different synthesized emulsifiers as a function of the number of carbons in their alkyl chain.

Novel low-mass emulsifiers for the production of water-in-oil miniemulsions [3,23,24] were synthesized and evaluated. The influence of different lipophilic tails, chain lengths and hydrophilic groups on the emulsifying properties – droplet size after production at standardized process conditions and stability of the emulsion – were investigated (Fig. 3). It was shown that droplet sizes of stabilized emulsions strongly depend on the nature of the polar groups of the emulsifiers. These results are in accordance with those of Tan and Nakajima [24]. Within each class of compounds, huge discrepancy of droplet sizes were observed depending on the length of the lipophilic chains. These observations correlate well with

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previous publications, which reported strong dependence of micellization ability [25,26], micellar size [27] and surface activity [28] on hydrophobic chain length of emulsifier. The optimal combination of lipophilic and hydrophilic parts for inverse emulsifiers for miniemulsions was found to be malonic acid moiety as polar head group and a chain length of 14 carbons as hydrophilic part. Furthermore, oxalic acid monoesters containing 9–15 carbon atoms, could stabilize emulsions with droplet sizes of 430–500 nm. The stabilization optimum is reached for chain lengths that are on the one hand short enough to provide stabilization for high pressure homogenization (as is was already observed by) [17] and on the other hand long enough for a good steric stabilization. In addition, the structure of oxalic and malonic acid has an alkali-labile cleavage site. The miniemulsions could easily be broken-up into two distinct phases by addition of 0.1 M NaOH. In conclusion, oxalic and malonic acid fatty esters combine customized properties and allow for selective stabilization and destabilization of inverse miniemulsions. Such novel emulsifiers can be applied for stabilizing miniemulsions in industrial processes like miniemulsion polymerization [3]. Moreover cleavable emulsifiers can prevent complications encountered in traditional emulsifier formulation such as foaming or formation of unwanted, stable emulsions [30,32,33]. Notes Crystal structure determination: The single-crystal X-ray diffraction study was carried out on a Bruker-Nonius Kappa-CCD diffractometer at 123(2) K using Mo Ka radiation (k = 0.71073 Å). Direct Methods (SHELXS-97 [55]) were used for structure solution and refinement was carried out using SHELXL-97a (full-matrix leastsquares on F2). Non hydrogen atoms were refined anisotropically, hydrogen atoms were refined using a riding model. 12{11}: Colorless crystals, C14H26O4, M = 258.35, crystal size 0.21  0.09  0.03 mm, triclinic, space group P  1 (No. 2), a = 4.861(1) Å, b = 7.712(2) Å, c = 39.788(8) Å, a = 95.41(2)°, b = 91.39(2)°, c = 100.26(2)°, V = 1459.9(6) Å3, Z = 4, q(calc) = 1.175 Mg m3, F(0 0 0) = 568, l = 0.084 mm1,13424 reflections (2hmax = 50°), 4780 unique [Rint = 0.103], 327 parameters, R1 (I > 2r(I)) = 0.115, wR2 (all data) = 0.316, S = 1.05, largest diff. peak and hole 0.443 and 0.339 e Å3. Crystallographic data (excluding structure factors) for the structure reported in this work have been deposited with the Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC 851792 12{11}. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge DB2 1EZ, UK (Fax: +44 1223 336 033; e-mail: [email protected]). Acknowledgments This project is part of the JointLab IP3, a joint initiative of KIT and BASF. Financial support by the Federal Ministry of Education and Research (Project 13N10296) is gratefully acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.10.072. References [1] M.S. El-Aasser, E.D. Sudol, J. Coat. Technol. Res. 1 (2004) 21.

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