Syntheses and properties of novel non-ionic fluorinated multichains “star-like” surfactants

Journal of Fluorine Chemistry 119 (2003) 191±205

Syntheses and properties of novel non-ionic ¯uorinated multichains ``star-like'' surfactants M.-J. SteÂbeÂa,*, V. Istratovb, A. Langenfelda, V.A. Vasnevb, V.G. Babaka,b a

Equipe Physico-Chimie des ColloõÈdes UMR 7565, Universite H. Poincare Nancy 1/CNRS, Faculte des Sciences, BP 239, 54506 Vandoeuvre-leÁs-Nancy Cedex, France b A. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Street, Moscow 119991, Russia

Received 1 July 2002; received in revised form 21 October 2002; accepted 22 October 2002

Abstract A series of star-like nonionic surfactants (with two hydrophobic and two hydrophilic chains) with different lengths of hydrophilic and hydrophobic arms were synthesised on the basis of pyromellitic acid dianhydride. The hydrophilic arms were formed by polyoxyethylene and hydrophobic ones either by per¯uoro- or by alkyl chains. The adsorption monolayers (Gibbs monolayers) were studied by surface pressure (p) measurements as a function of time for different surfactant concentrations. For the spread monolayers (Langmuir monolayers), the measurements of the surface pressure (p) versus the molecular area (A) as well as the relaxation measurements of the area (A) as a function of time at constant surface pressure were performed. The comparison between the characteristic parameters of two types of monolayers was made in order to understand the effect of the preparation conditions on the structure of these monolayers. It was found that decreasing the ¯uoroalkyl chain length induced a systematical decrease in the stability of Langmuir monolayers, which is manifested as the Marangoni±Gibbs viscoelasticity of the monolayers. For the surfactants, which have a large number of oxyethylene groups, adsorption at the air/water interface from the bulk solution required extremely long times to reach equilibrium due to the diffusion from the solution and to the conformational rearrangements at the interface. The observation of a hysteresis in the compression/decompression curves for these compounds is explained by the presence of the residual organic solvent molecules absorbed by oxyethylenic chains. A novel model describing the kinetics of desorption or rearrangement of molecules during the lateral compression was suggested, allowing the estimation of both characteristic time of this process and areas per molecule at the equilibrium from the relaxation curves A(t). # 2002 Elsevier Science B.V. All rights reserved. Keywords: Star-like surfactant; Esteri®cation; Fluorinated surfactants; Spread and adsorption monolayers; Surface tension

1. Introduction Fluorinated amphiphiles have attracted particular interest because of their remarkable characteristics. The per¯uoroalkylated chains are more hydrophobic than hydrocarbon chains, and consequently the ¯uorinated surfactants display stronger surface activity and much smaller critical micelle concentration (CMC) values than their hydrogenated analogues [1,2]. The ¯uorinated amphiphiles present not only a hydrophobic but also a lyophobic character being able to lower the surface tension of organic solvents [3]. Per¯uorinated amphiphiles have signi®cant potentialities in the biomedical ®eld [4±6]. These amphiphiles are used for * Corresponding author. Tel.: ‡33-383-684343; fax: ‡33-383-684322. E-mail address: [email protected] (M.-J. SteÂbeÂ).

the preparation and the stabilisation of highly concentration emulsions [7,8], as blood substitutes and oxygen carriers [9], as biocides for water treatment [10], as membrane lipids in biophysical investigations on transmembrane permeability [11,12], and as protein-solubilising surfactants [13], among others. In order to increase the hydrophilicity of ¯uorocarbon amphiphiles, which is generally poorer than that of hydrocarbon surfactants, per¯uorinated and oxyethylene fragments have been combined in the amphiphilic molecule [14±19]. Star-branched poly(ethylene oxide) moieties are expected to provide not only an increased hydrophilicity of molecules, but also additional potentialities for their biomedical applications. The ¯uorinated surfactants most extensively studied are linear, whereas star-like ¯uorinated surfactants are less known. However, an interesting

0022-1139/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 2 7 8 - 6

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Table 1 List and conventional designation of new synthesised compounds Chemical formulae

Notation

Structural representation

E(EO)1F1

M(EO)7F1

E(EO)1F6H

M(EO)7H13

M(EO)7F6H

( ) Perfluoro-methylen or -methylenic carbon; ( ) methylen or methylenic carbon; ( ) oxyethylen group, and ( ) ethylen group. E and M mean ethyl and methyl and correspond to the terminal group of hydrophilic chains. (EO)n means ethylene oxide and n corresponds to the number of these groups H and F mean that the hydrophobic chains are hydrogenated or fluorinated respectively, and the following number corresponds to the number of carbon atoms of these chains. When the formula of the partially fluorinated compounds contains a terminal H, this means that the terminal methyl groups of the hydrophobic chains bear a hydrogen atom.

physicochemical study of ¯uorinated surfactants derived from mannitol with two hydrophobic chains and having a complex chemical structure has been reported [20]. At the same time the properties of star-shaped compounds can differ from their linear analogues and confer on compounds new valuable properties. The aim of the present work is to synthesise and examine surface properties of novel star-like per¯uorinated surfactants bearing nonionic oxyethylene hydrophilic chains. Basically, the syntheses of such hybrid structures can be divided into two steps. First, as the star-shaped architecture of surfactants is created on the basis of pyromellitic dianhydride (which can be considered as a four-functional monomer with two pairs of different functional groups), we focused on preparing several hybrid building blocks bearing ethoxyethylene and polyoxyethylene chains. Then, the appropriate hydrophobic substrates are reacted with the functionalised hydrophilic building blocks to achieve the star-like amphiphiles. Using a series of star-like amphiphilic compounds provides also a valuable opportunity to compare systematically the spread (Langmuir) and adsorbed (Gibbs) monolayers in order to evaluate differences or similarities of their properties. The formation of adsorbed monolayers was followed by

the recording of variations of surface pressure with time. The properties of monolayers spread at the air/water interface were monitored by recording the isotherms of surface pressure versus area per molecule. The results obtained have allowed us to evaluate the stability of such monolayers and to determine the packing and order of molecules in the monolayer. 2. Results and discussion The interfacial properties of the following series of ¯uorinated and hydrogenated compounds have been studied. Chemical formulae and notations are presented in Table 1. 2.1. Properties of Langmuir monolayers Among the ®ve compounds studied, only three, namely, E(EO)1F6H, M(EO)7H13 and M(EO)7F6H, formed stable monolayers when spread on an aqueous subphase (Fig. 1). The monolayer stability was con®rmed by p(t) measurements (Fig. 2, curves 3±5), which showed low variations of the area per molecule a at constant surface pressure p ˆ 15 mN/m, as well as very small hysteresis upon

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193

Table 2 Characteristic parameters (molecular area acoll, surface pressure pcoll and the elasticity module Ecoll at the collapse) for the Langmuir isotherms Formulaea

acoll (nm2)

pcoll (mN/m)

Ecoll (mN/m)

E(EO)1F6H M(EO)7H13 M(EO)7F6H

0.40 0.75 0.40

26 41 42

49 63 48

a

See Table 1.

the compound M(EO)7F1 from the surface during multiple compression/decompression cycles. The characteristic parameters of the isotherms of the stable compounds are given in Table 2. Fig. 1. Surface pressure vs. area isotherms for stable monolayers spread on water subphase for compounds: E(EO)1F6H (curve 1); M(EO)7H13 (curve 2); M(EO)7F6H (curve 3).

compression/decompression (Fig. 3a±c). On the other hand, the compounds E(EO)1F1 and M(EO)7F1 gave unstable monolayers and manifested a remarkable hysteresis during upon compression/decompression cycles (Fig. 3d and e). Submitted to a lateral compression, the monolayers of the compound E(EO)1F1, containing two short oxyethylene chains, were continuously destructed while the area per molecule a(t) decreased sharply to afford very low values (Fig. 2, curve 1). The compound M(EO)7F1 with two long oxyethylene chains also showed a remarkable decrease of the area per molecule under constant compression (Fig. 2, curve 2), but a tended to some ®nite value for longer times. Note that the hysteresis curves of the compound E(EO)1F1 shifted to smaller values of areas with an increasing number of compression/decompression cycles (Fig. 3d), whereas the second compound, M(EO)7F1, showed hysteresis curves almost coinciding at their top (Fig. 3e). This means that there is no irreversible loss of the molecules of

Fig. 2. Area/time dependencies for monolayers spread on water subphase at p ˆ 15 mN/m. E(EO)1F1 (curve 1); M(EO)7F1 (curve 2); E(EO)1F6H (curve 3); M(EO)7H13 (curve 4); M(EO)7F6H (curve 5).

2.1.1. Stable monolayers The values of the molecular area (acoll) corresponding to the collapse of the monolayers are the same for ¯uorinated compounds, E(EO)1F6H and M(EO)7F6H, and signi®cantly smaller compared to the hydrogenated one M(EO)7H13. This feature of (acoll) corresponds to the values found for the twochain compounds (the partially ¯uorinated amphiphilic derivatives of mannitol, studied in [20]) for which (acoll) values were smaller than those expected on the basis of the crosssection of their ¯uorinated carbon chains). These ¯uorinated or hydrogenated two-chain compounds have the structural formulae CH2OH±CHOH±[CHOC3H7CnF2n‡1]2±CHOH± CH2OH or CH2OH±CHOH±[CHOC3H7CnH2n‡1]2±CHOH± CH2OH with n ˆ 4, 6 or 8, and are designated as F4F4, F6F6, F8F8 or H4H4, H6H6, H8H8, for the ¯uorinated and hydrogenated compounds, respectively. According to [1] the minimal area per single-chain nonionic ¯uorinated surfactant molecule is about 0.30 nm2. For the single-chain compound C8F17C2H4SC2H4(EO)2 designed as (EO)2F8 we have found also an area acoll of 0.30 nm2 which con®rmed this observation. The same value has been given by [21] for the molecular area in the crystalline phase for a single-chain ¯uorocarbon. This may signify that when approaching the state of collapse, the ¯uorinated compounds studied form highly packed layer of molecules in which the ¯uorinated chains are oriented perpendicularly to the surface and extended in two opposite directions, one pointing into the water phase and the other into the air. If both ¯uorinated chains were oriented in the same direction, e.g. towards air, we should have obtained an area per molecule equal to 0.60 nm2 rather than the experimental value of 0.40 nm2. It is interesting to note that for the two-chain ¯uorinated compounds studied in [20] mentioned it has been found that the area (acoll) had a tendency to decrease from 0.51 to 0.48 nm2 on going from sample F6F6 to F8F8. These values are higher than that of 0.40 nm2 obtained for the stable ¯uorinated compounds E(EO)1F6H and M(EO)7F6H, but obviously smaller than the estimated value of 0.60 nm2 corresponding to the sum of the cross-sections of two ¯uorinated chains. It seems that there is a general tendency of the

194 M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

Fig. 3. Compression/decompression cycles for the compounds: E(EO)1F6H (a); M(EO)7H13 (b); M(EO)7F6H (c); E(EO)1F1 (d); M(EO)7F1 (e).

M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

¯uorinated compounds to form condensed structures in the monolayers, as the gain in free energy related to the maximal association of the ¯uorinated chains is higher than the free energy loss due to the partial immersion of these chains in the aqueous phase. When the length ¯uorinated chains decreases, the energy of hydrophobic interactions between these chains decreases, and we may expect a looser packing of the molecules in the monolayers (Fig. 4a and b). Indeed, for the compound F4F4 the area acoll was found to be equal to 0.60 nm2, which may signify that in this case the attraction between the ¯uorinated chains is not suf®cient to produce condensation and consequently both ¯uorinated chains are oriented at the interface in the same direction (to the air).

Fig. 4. Schemes illustrating the difference between the structures of the Langmuir monolayers of fluorinated E(EO)1F6H (a) and M(EO)7F6H (b) compounds, and the hydrogenated compound M(EO)7H13 (c).

195

On the other hand, the acoll value of 0.75 nm2 for the hydrogenated compound M(EO)7H13 is remarkably higher than the sum of calculated cross-sections of two carbon chains, which is expected to be equal to 0.40 nm2. Indeed, the minimal area for single-chain hydrogenated compounds in the monolayers is about 0.20 nm2 [1], which corresponds also to the molecular area in the crystalline phase for a simple hydrocarbon [21]. It is interesting to note that the same tendency increasing the area acoll has been found for the two-chain hydrogenated compounds studied in [20]. For example, the compound H6H6 had an area of 0.59 nm2. The result obtained can be rationally explained if we imagine that both alkyl chains of the hydrogenated compound M(EO)7H13 are parallel and oriented in the same direction (to the air) at the interface. Nevertheless, this is insuf®cient to explain the very remarkable increase in acoll for the compound M(EO)7H13. The long OE chains is probably inserted into the interface and thus contribute to the area acoll (Fig. 4c). The results obtained may be generalised as follows. Fluorinated compounds with suf®ciently long ¯uorinated chains tend to associate at the interface via hydrophobic interactions between the ¯uorinated chains. At the state of the maximal compression near the collapse, these compounds tend to extend their ¯uorinated chains in two opposite directions with respect to the central benzene ring, to acquire a maximal gain in the free energy of hydrophobic association. For these ¯uorinated compounds only a 30% increase of the area acoll was found with respect to the theoretical cross-sections of single ¯uorinated chains (0.30 nm2), which means that the majority of molecules extends fully their ¯uorinated chains and forms a very compact layer via the hydrophobic association of both chains. This requires the immersion of one of the ¯uorinated chains into the water subphase and probably induces these asymmetric molecules to orient their benzene ring perpendicularly to the interface to ®ll a smaller area. Unlike the ¯uorinated compounds, the hydrogenated ones show a much lower tendency to associate in the state of maximal compression. The alkyl chains ®ll a much larger area at the interface being pushed from the water into the air phase. Moreover, the relatively loose packing of hydrophobic hydrogenated chains allows the asymmetric molecules to orient their benzene ring parallel to the interface, which allows the hydrophilic moieties to be inserted into the interface. The assumption that ¯uorinated chains interact much more strongly at the interface than hydrogenated ones is in agreement with the results of simulations of molecular dynamics, showing that the collective tilt of the former is nearly zero, whereas it is substantial for the latter [22]. The surface pressure values (pcoll) corresponding to the collapse of the stable monolayers of the studied compounds and characterising their mechanical strength under compression are presented in Table 2. It is noteworthy that the strength of these four-chains surfactants (two hydrophobic and two hydrophilic chains) is systematically slightly smaller

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compared to the two-chains (one hydrophobic and one hydrophilic) ¯uorinated or hydrogenated compounds studied in [20], the pcoll values of which achieve 50±60 mN/m. It is also smaller than the strength of the one-chain ¯uorinated compound (EO)2F8 whose pcoll value affords 50 mN/m. This can be explained by a higher steric hindrance upon packing of the molecules with two polyoxyethylene chains. For the considered compounds, a decrease of the length of the hydrophilic chains leads to a drastic decrease of the monolayer strength upon compression. For example, replacing two hydrophilic M(EO)7 groups of the compound M(EO)7F6H by two short hydrophilic ethylene oxide chains in the compound E(EO)1F6H one decreases the pcoll value from 42 to 26 mN/m. It should be pointed out that the strength of the hydrogenated M(EO)7H13 and ¯uorinated M(EO)7F6H compounds, which have two hydrophilic chains of the same length, is almost the same (pcoll  40 mN/m). As far as the elasticity modules Ecoll of the four-chain ¯uorinated compounds studied here, as concerned their values of 50 mN/m are systematically much smaller than those of the two chain ¯uorinated compounds (the partially ¯uorinated amphiphilic derivatives of mannitol, F6F6 and F8F8 studied in [20]) having Ecoll values of 120 mN/m, or for the singlechain ¯uorinated compound (EO)2F8 for which Ecoll ˆ 65 mN/m. This may be explained by the relatively loose packing of the compounds considered due to the presence of two polyoxyethylene chains and a central benzene ring with four spacers. It is noteworthy that the four-chain hydrogenated compound M(EO)7H13 manifests a slightly increased elasticity modulus (Ecoll  63 mN/m) compared to that for the ¯uorinated homologues. This tendency to increase the elasticity module from ¯uorinated to hydrogenated compounds has been mentioned also for the derivatives of mannitol [20]. 2.1.2. Problem of leakage of molecules from the interface Fig. 3 shows that the hysteresis curves for both stable and unstable monolayers shift to smaller areas with an increasing number of cycles. As it has been mentioned, this is the

Fig. 5. The initial (a) and improved (b) p

manifestation of the irreversible loss of molecules from the interface with time due to dissolution in the aqueous phase or adsorption at the walls of the trough. While the number N(t) of surfactant molecules, which are retained at the surface of the area A(t) gradually decreases because of this desorption, the real area a0 …t† ˆ A…t†=N…t† per molecule will be always higher than the area a…t† ˆ

A…t† No

(1)

calculated making the assumption of a constant number No of molecules a…t† ˆ n…t†a0 …t†

(2)

where n…t† ˆ

N…t† No

(3)

is a function of time and describes the irreversible leakage rate of molecules from the surface. Parameter n(t) also depends on many factors including the solubility of the compounds in water, the lateral compression p, etc. The simplest way to explain the variation of the number of molecules N(t) is to assume that the rate of desorption of molecules is constant, independent of the surface pressure. This assumption corresponds to the differential equation dN ˆ

b N dt

(4)

where b is the rate constant for the desorption. The integration of this equation with the initial condition N ˆ No at t ˆ 0 gives N…t† ˆ No exp… bt†

(5)

So, the function n(t) is equal to exp( bt) and the real area per molecule a0 (t) increases with time as a0 …t† ˆ a…t† exp…bt†

(6)

The use of the isotherms p a0 instead of the isotherms p a allows us describe the irreversible leakage of the

a isotherms for the compound E(EO)1F1 (the best fit corresponds to b ˆ 8  10

5

s 1).

M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

Fig. 6. The initial (a) and improved (b) p

a isotherms for the compound M(EO)7H13 (the best fit corresponds to b ˆ 2  10

surfactant molecules from the interface. The parameter b is a ®tting parameter, which may be found from the best coincidence of the compression/decompression branches of the isotherms p a0 corresponding to different hysteresis cycles. Figs. 5 and 6 illustrate how the hysteresis curves may be improved by using the area a0 corresponding to the real number of molecules N(t) under the assumption that the law (4) is ful®lled. The compound E(EO)1F1 was noti®ed as possessing the less stable spread monolayers. Now, we can quantify this instability by estimating the rate constant (the parameter b) of the irreversible leakage of the molecules from the interface. The best ®t of the hysteresis curve has been obtained for b ˆ 8  10 5 s 1 that corresponds to the characteristic time of the irreversible desorption of molecules tir ˆ 1=b  104 s (Fig. 2). An analogous procedure may be applied to the ``stable'' monolayers (e.g. the best ®t for the compound M(EO)7H13 has been obtained with the parameter b ˆ 2  10 5 s 1, Fig. 5). Surprisingly the apparently unstable monolayer, which was formed by the compound M(EO)7F1 proves perfectly stable as it shows a very small irreversible loss of molecules during compression/decompression cycles (see Fig. 3e). The very small shift at the top of the hysteresis curves could be eliminated by the very small ®tting parameter b ˆ 10 5 s 1, which corresponds to the characteristic time of the irreversible desorption of the order of tir  105 s. The observed hysteresis between the compression and decompression branches comes from the viscoelastic effect, which is due to the adsorption/desorption of the hydrophobic groups of the molecules. We conclude that in spite of the apparent instability of the monolayers formed by the compound M(EO)7F1, its molecules remain at the interface during compression/decompression cycles. An interesting feature of the initial compression curve for the compound M(EO)7F1, which is above the following curves of the hysteresis cycles (Fig. 3e) should be pointed out. After the ®rst compression and the following

197

5

s 1).

decompression the surface pressure in the maximally extended monolayer decreases considerably. This effect, which is characteristic of all compounds with long OE chains, can only be explained by the substantial decrease of the radius of the molecules in the monolayer while there is practically no loss of molecules from the interface. After the ®rst compression the molecules undergo a collapse, and they keep this collapsed conformation during the following compression/decompression cycles. The following arguments may be proposed to rationally explain this effect. After spreading the organic solution of these compounds onto the water surface and after evaporation of the organic solvent, the OE chains could continue to absorb residual molecules of the organic solvent and thereby, manifesting hydrophobicity, keep the extended conformation at the air/water interface. During the ®rst compression, some of these OE chains are pushed into the water phase and become hydrophilic because of their hydratation by water molecules. During the following hysteresis cycles, the OE chains continue to be immersed in the water phase, a fact revealed by the apparent interfacial collapse of the molecules. This feature of nonionic surfactants having long OE chains has been mentioned by different authors [23] and is due to the more general difference between the spread and adsorption monolayers of these surfactants. The adsorption (Gibbs) monolayers are formed by molecules having hydrated (``hydrophilic'') OE chains via diffusion from the water phase whereas the spreading (Langmuir) monolayers are formed by the non-hydrated OE chains from the air phase impregnated by the residual organic solvent molecules (``hydrophobic''). The procedure described for the correction of the hysteresis curves allows us to quantify the effect of the irreversible desorption of the molecules from the interface during the compression of the monolayers. For example, the compounds studied have rather high characteristic times tir ˆ 1=b for the irreversible desorption, which is superior to 104 s (see Table 3).

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198

Table 3 Parameters of the theoretical relaxation curves for the compounds studied Compound

Aˆa (nm2)

B ˆ a‡ (nm2)

E(EO)1F1 M(EO)7F1 E(EO)1F6H M(EO)7H13 M(EO)7F6H

0.064 0.215 0.517 1.276 0.832

0.368 0.203 0.041 0.026 0.049

a

trev (103 s)

tir (104 s)

k ˆ 1/t (10 4 s 1)

A ‡ B ˆ a‡ (nm2)

Ep (m/mN)

Ecoll (m/mN)

3.0 1.3 1.4 0.9 3.5

1.3 3.3 5.0 2.3 >10

4 8 7 11 3

0.43 0.42 0.56 1.30 0.88

± ± 26 26 22

± ± 49 63 48

A, B and t are the parameters of the Eq. (7); a‡ and a are area occupied by one molecule in the monolayer at the beginning of the relaxation process and at the infinite time, respectively; Ep and Ecoll are the elasticity modules for the given surface pressure p and at the collapse; trev and tir are the characteristic times of the reversible and irreversible leakage of molecules from the monolayer.

2.1.3. Kinetics of the decrease of the monolayer area under constant surface pressure As it has been mentioned, the monolayers of all the compounds considered manifest elastic (in the meaning of Gibbs) properties. The ¯agrant viscoelasticity (in the meaning of Marangoni±Gibbs) is proper to the so-called ``unstable'' monolayers formed by the compounds E(EO)1F1 and M(EO)7F1, which show a remarkable relaxation leading to a decrease of the area per molecule a(t) in the monolayer submitted to the lateral compression p (Fig. 2). But even the so-called ``stable'' monolayers formed by the compounds E(EO)1F6H, M(EO)7F6H and M(EO)7H13, manifest a measurable decrease in the area a during a time of the order of 102± 103 s. This effect is mainly due to the reversible reconformation of the molecules at the interface and to their reversible desorption into the water or air subphases. As it will be shown later on, the characteristic time of this relaxation process is one decimal order of magnitude lesser than the time tir  104 ±105 s of the irreversible leakage of the molecules from the interface. This means that practically there is no leakage of the molecules from the interface to the bulk of the subphase solution, and we may consider that the number of molecules in the compressed monolayer remains constant during the relaxation experiments. The relaxation curves a(t) provide interesting information about the interaction between the molecules in the monolayer and between the molecules and the interface. Fig. 2 represents the kinetic observed during the compression of spread monolayers by a constant pressure p of 15 mN/m. The area per molecule a(t) calculated following Eq. (1) as a formal ratio of the macroscopic area A(t) to the number No of deposed molecules, was plotted as a function of time. All the relaxation curves a(t) could be satisfactorily approximated by an exponential decay function of the type  t a…t† ˆ A ‡ B exp (7) t

model uses several simplifying but realistic assumptions concerning the mechanism of the area per molecule relaxation of the monolayers. For example, it is assumed that the lateral pressure p produces the elastic compression of the liquid-like monolayers in the meaning of the Gibbs elasticity, and consequently the increase of the free energy (or the chemical potential) of the molecules at the interface. The possibility for the molecules to decrease their free energy by removing reversibly or irreversibly their hydrophobic groups from the interface by diffusion is usually described in term of the Marangoni viscoelasticity of the monolayers. The characteristic relaxation time is equal to t ˆ Zs =Es where Zs and Es are the effective viscosity and elasticity, respectively, in the range of the Maxwell rheological models of the monolayers. To describe the kinetics of the relaxation of the area per molecule a(t) in the liquid-like monolayer submitted to some constant lateral compression p, let us designate by a‡ the area, which is occupied by one molecule at the beginning of the relaxation process, and by a the in®nite time. During the relaxation process the molecules vary continuously their area in the range a‡ > a…t† > a . But in order to simplify the problem, we assume that only two different states are accessible to the molecules at the monolayer: one corresponding to the initial area a‡, and the other one to the ®nal area a . It should be pointed out that it is not dif®cult to generalise the problem by admitting some special distribution law p(a) for the molecules occupying the area a. During the compression by the constant lateral pressure p the molecules undergo a transfer from the initial to the ®nal state. Therefore, the number of molecules N‡ in the initial state decreases from No to zero, whereas the number of molecules at the ®nal state N increases from zero to No. According to this scheme, the area A(t) of the monolayer at any arbitrary time t can be expressed as

where A and B are constant parameters, and t the characteristic time of the relaxation presented in the Table 3. We suggest a model to describe the relaxation curves a(t) of the unstable monolayers on the basis of the kinetic and molecular parameters of the amphiphilic molecules. The

with N‡ ‡ N ˆ No ˆ constant. The kinetic equation for the transfer process N‡ ! N has the form

A…t† ˆ a‡ N‡ …t† ‡ a N …t†

dN‡ ˆ

kN‡ dt

(8)

(9)

M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

where k is the rate constant of the relaxation process. The solution of this equation satisfying the initial condition N‡ …t† ˆ No is given by N‡ …t† ˆ No exp… kt†

(10)

Consequently, the number of the molecules in the ®nal state is equal to N …t† ˆ No

N‡ …t† ˆ No ‰1

exp… kt†Š

(11)

Substituting (10) and (11) into (8) gives A…t† ˆ No fa‡ exp… kt† ‡ a ‰1

exp… kt†Šg

(12)

De®ning the effective area per molecule by Eq. (6) we obtain a…t† ˆ a ‡ …a‡

a †exp… kt†

(13)

Finally, comparing this expression to the ®tting function (7) we ®nd the following conformity between the parameters Aˆa ;

B ˆ …a‡

a †;

t ˆ k 1:

(14)

The corresponding kinetic parameters for all the compounds considered are represented in Table 3. It should be borne in mind that the values obtained for the areas a‡ and a correspond to the particular and not the critical surface pressure p ˆ 15 mN/m, which was chosen arbitrarily. At these area values the monolayers are still well compressible, i.e. the corresponding elasticity moduli Ep are not at their maximum (the maximal elasticity moduli Ecoll are also indicated in the Table 3). The most unstable monolayer corresponds to the compound E(EO)1F1, which has the minimal ®nal area a ˆ 0:064 nm2. This small value of the area per molecule cannot be reasonably attributed to the size of any group of atoms of the compound and this result may be interpreted as the fact that the molecules leave the surface during the compression apparently reversibly (by forming a second layer) and irreversibly (by dissolving in water). On account of the characteristic time of the reversible desorption trev  3000 s, which is several times lower than that for the irreversible leakage of the molecules tir ˆ 1:3  104 s, we conclude that during the compression step the main part of the molecules desorb reversibly from the interface by forming the second layer either above or below the initial monolayer. Only a very small number of molecules leave the interface de®nitely by dissolving in the water subphase. The monolayers of compound M(EO)7F1 also relax remarkably with a characteristic time trev  103 s tending to the minimal area a ˆ 0:215 nm2 at p ˆ 15 mN/m. In this case we ®nd also that the characteristic time of the irreversible desorption tir ˆ 3:3  104 s is more than 10 times higher than trev. Consequently, we conclude a practically reversible character of the desorption of the molecules. The calculated area (0.215 nm2) which corresponds to the cross-section of one OE chain [23,24], may signify that under the lateral compression mentioned the molecules of compound M(EO)7F1 desorb into the aqueous subphase by hanging at the interface with one OE chain or more realistic,

199

adopt a more stretched conformation and a tighter packing of their OE chains. Comparing the data for compounds M(EO)7F1, E(EO)1 F6H and M(EO)7F6H (see Table 3), we may conclude that the equilibrium area a increases with growing length of the ¯uorinated chains length of the compounds. The OE chains contribute also to the area per molecule at this surface pressure p ˆ 15 mN/m: the molecules of compound M(EO)7F6H occupy an area a of 0.832 nm2 whereas those of compound E(EO)1F6H have only an a of 0.517 nm2. It is interesting to note that at the collapse the OE chains are desorbed from the interface, while at this state of maximum packing these molecules will have an identical area acoll of 0.400 nm2. Unlike the ¯uorinated compound M(EO)7F6H, the hydrogenated compound M(EO)7H13 having the same length of OE chains is characterised by a much higher equilibrium area a of 1.30 nm2. Here, we ®nd again the ranking in the areas at the collapsed state for these two compounds: for the ¯uorinated compound M(EO)7F6H we have found a value acoll of 0.400 nm2, whereas for the hydrogenated compound we had an acoll of 0.75 nm2. For the collapsed state, this difference between the areas acoll has been explained above by the fact that the ¯uorinated molecules could form hydrophobic aggregates at the interface whereas the hydrogenated ones could not. It seems that still far before the collapsed state the ¯uorinated molecules begin to form associates at the interface whereas the hydrogenated ones remain in the gaseous state. 2.2. Properties of adsorbed monolayers 2.2.1. Kinetics of the formation of the adsorption layers In order to compare the properties of the adsorbed layers of the studied compounds, the surface pressure/time isotherms g(t) at the air/water interface were recorded. Figs. 7±9 show examples of these isotherms for different bulk concentrations Cs of the compounds M(EO)8F1, M(EO)8H13 and

Fig. 7. Dynamic surface tension curves of the compound M(EO)8F1 for different concentrations Cs (mol/l): 5:0  10 6 (curve 1); 1:0  10 5 (curve 2); 2:2  10 5 (curve 3); 1:0  10 4 (curve 4); 5:0  10 4 (curve 5).

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Fig. 8. Dynamic surface tension curves of the compound M(EO)8H13 for different concentrations Cs (mol/l): 5:0  10 6 (curve 1); 1:0  10 5 (curve 2); 5:0  10 5 (curve 3); 1:0  10 4 (curve 4).

M(EO)8F6H, which contain long OE chains. It has been found that the less hydrophilic compounds, E(EO)1F6H and E(EO)1F1 practically do not change their surface tension with time, whereas the compounds with long OE chains present a remarkably long relaxation behaviour of the surface tension. All the kinetic curves g(t) show a decrease in surface tension as a function of time and may be ®tted by the exponential functions h  t i g…Cs ; t† ˆ g…Cs ; 0† ‡ k 1 exp (15) t where g(Cs, 0) is the surface tension when the experiment begins, and where k and t are ®tting parameters. It can be

expected that the formation of the adsorption layers of the amphiphiles is a long process consisting of the diffusion of the molecules from the bulk of the solution to the surface, and of their further reconformation at the interface accompanied by steric interactions and by the structure formation inside the adsorption layer. The characteristic times t, which correspond to the formation of the adsorption layers by the mechanism of the diffusion of the molecules from the solution to the interface undergo a systematic decrease with the bulk concentration Cs of compounds. For example, these characteristic times are equal to t…s† ˆ 2:5  104 , 0:9  104 and 0:3  104 for the bulk concentrations of the compound M(EO)8F6H equal to Cs …mol=l† ˆ 10 6 , 5  10 6 and 10 5, respectively. For the compound M(EO)8F1 t…s† ˆ 2:7  103 , 250 and 60 for concentrations equal to Cs …mol=l† ˆ 5  10 6 , 10 5 and 2:2  10 5 , respectively. This probably means that the rate of the formation of the adsorption layers of these compounds is controlled by the diffusion of molecules from the bulk of the solution to the interface. The adsorption amount of molecules g(t), which is formed during a time t by the diffusion mechanism is related to the bulk concentration of molecules Cs and to the diffusion coef®cient D by the well-known Ward and Tordai equation [25±27] r D t (16) G…t†  2Cs p The diffusion coef®cient D in its turn may be estimated according to the Einstein relationship Dˆ

kB T 6pZRg

(17)

where Z stands for the viscosity of the aqueous solution, and Rg is the hydrodynamic radius of the molecule. The formula (16) is obtained under the assumption that all the molecules, which reach the surface by diffusion, remain there and do not desorb from this surface. This assumption is true in the case of the low degree of ®lling of the surface by the molecules (gaseous state). Admitting that this hypothesis is ful®lled up to the formation of an adsorption monolayer in its liquid-extended state, we may estimate from the Ward±Tordai Eq. (16) the characteristic time t of the diffusional stage of the adsorption. As the area per molecule corresponding to the transition between gaseous and liquid-extended state of the adsorption layer may be estimated as a  1 nm2, the corresponding adsorption amount may be expressed as Gˆ

1  10 6 …mol=m2 † aNA

(18)

where NA is the Avogadro number. The diffusion coef®cient may be approximated at D  10 10 (m2/s), Z  10 3 (Pa s) and Rg  1 (nm), the expression (19) is then obtained Fig. 9. Dynamic surface tension curves of the compound M(EO)8F6H for different concentrations Cs (mol/l): 1:0  10 6 (curve 1); 5:0  10 5 (curve 2); 1:0  10 5 (curve 3).

t  10

2

1 Cs2

(19)

M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

and relates the characteristic time t of the diffusional stage of the adsorption layer formation to the concentrations Cs of the compound. It must be here pointed out that, according to (16) and (17), the characteristic diffusion time scales as t / Rg

(20)

with the variation of the hydrodynamic radius of the molecule. For example, according to the Eq. (19), the characteristic time t  103 s can be obtained in the case of compound M(EO)8F1 for a concentration Cs equal to 5  10 6 (mol/l), which reasonably coincides with the experimental time t  2:7  103 s. The relationship t  1=Cs2 is also ful®lled for higher concentration of this compound: the predicted characteristic times t ˆ 100 and 20 s for the bulk concentrations Cs ˆ 10 5 (mol/l) and Cs ˆ 2:2  10 5 (mol/l) satisfactorily correspond to the experimental times t  250 and 60 s obtained for the same concentrations of this compound. For the more hydrophobic ¯uorinated M(EO)8F6H compound, the estimation time t  104 s seems to correlate with the experimental time t  2:5  104 s for a very small concentration Cs ˆ 10 6 (mol/l). But for the higher concentrations (Cs ˆ 5  10 6 (mol/l) and Cs ˆ 10 5 (mol/l)), there is a discrepancy between estimated and measured times: the expected values t  103 and 102 s are systematically much lower than the measured times t  0:9  104 and 0.3104 s, corresponding to these concentrations. This discrepancy between estimated and measured characteristic times may be related to the effect of the association of the molecules in the solution with increasing the concentrations. This association may lead to an increase in the hydrodynamic radius Rg and, consequently, to an increase in the characteristic diffusion time t according to the relationship (20). The eventual hydrophilisation of the associates (due to the screening of their hydrophobic moieties in the interior of the hydrophilic shell formed by the polyoxyethylene groups)

201

must also be taken into account. This may contribute to the decreasing of the ef®ciency of the adsorption of these associates at the interface, which also leads to an increase of time t. 2.2.2. Determination of molecular parameters of the adsorbed monolayers The surface tension (g)-bulk concentration (Cs) dependencies for the ®ve studied compounds are presented in Fig. 10. This results from the collection of the quasi equilibrium surface tension g data (corresponding to the adsorption layer formation times t > 104 s) for various bulk concentrations Cs of surfactants. The maximum surface excess concentrations (gmax) and the minimum area/molecule (amin) at the air/water interface were calculated using the Gibbs adsorption equation for non-ionic surfactants 1 @g (21) G…Cs † ˆ 2:3 RT @ log Cs T and the state equation for the adsorption monolayers of von Szyszkowski under the form g…Cs † ˆ go

A ln…1 ‡ BCs †

(22)

where A and B are the ®tting parameters satisfying to the Langmuir adsorption equation BCs 1 ‡ BCs

(23)

1 A ˆ amin NA RT

(24)

G ˆ Gmax with Gmax

where NA is Avogadro's number. The adsorption free energy of DGad of the compounds has been evaluated according to the following relationship [1]: DGad ˆ RT ln xCMC

pCMC ACMC

(25)

Fig. 10. Surface tension g vs. log Cs curves for aqueous solutions of (a) E(EO)1F1 (curve 1), M(EO)7F1 (curve 2) and (b) E(EO)1F6H (curve 3), M(EO)7H13 (curve 4), M(EO)7F6H (curve 5).

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202

Table 4 Maximum adsorption amount Gmax, critical agregation concentration CMC, surface tension gCMC and adsorption free energy DGad for the studied compounds Compounda

Gmax (106 mol/m2)

CMC (10 4 mol/dm3)

gCMC (mN/m)

E(EO)1F6H M(EO)7F6H M(EO)7H13 E(EO)1F1 M(EO)7F1

4.3 3.8 5.9 5.7 4.2

0.2 0.4 2.0 ± ±

33 28 44 ± ±

a

DGad (kJ/mol) 46.9 43.9 39.5 43.9 53.7

See Table 1.

with the molar fraction of the surfactant xCMC ˆ CMC=w, the molar area ACMC ˆ aCMC NA and the surface pressure pCMC ˆ go gCMC taken at the CMC, and w ˆ 55:5 mol/l, which equals the number of moles of H2O per litre of pure H2O at 25 8C. It must be noticed that the term CMC is used in the more extended meaning, i.e. of the concentration of monomers, which are in equilibrium with the aggregates. The formula (25) coincides with that used in [21,28] DGad ˆ RT ln xp

pNA amin

(26)

where xp ˆ Cs;p =w is the molar fraction of the compound in the aqueous solution at p (in the work [21] p was conventionally taken equal to 20 mN/m. Thus, xp ˆ Cs;20 =w, where Cs,20 is the surfactant concentration corresponding to p ˆ 20 mN/m). The calculated values of the maximum adsorption amount Gmax, the critical aggregation concentration CMC, the surface tension gCMC and the adsorption free energy DGad for all the compounds are given in the Table 4. The g(Cs) isotherms for the compounds E(EO)1F6H, M(EO)7H13 and M(EO)7F6H, which formed stable spread (Langmuir) monolayers, present a decrease with increasing the surfactant concentration Cs, and a break at the CMC. On the other hand, the surface tension isotherms for the samples E(EO)1F1 and M(EO)7F1 present a gradual decrease with increasing the bulk concentration Cs of these compounds and do not show any signi®cant break. Among the compounds E(EO)1F6H, M(EO)7H13 and M(EO)7F6H, which form aggregates, the ¯uorinated ones manifest the lowest CMC values with regards to the hydrogenated compound M(EO)7H13. It is interesting to note that the ¯uorinated compound M(EO)7F6H, which possesses the same length of OE chains as the hydrogenated compound M(EO)7H13, has a ®ve times lower CMC. This means that the hydrophobic interactions between the ¯uorinated chains composed of six per¯uoromethylene groups are much higher than for the longer alkyl chains with 13 methylene groups. The adsorption free energy DGad, which characterises the hydrophobic interactions of surfactants at the interface is equal to 43.9 kJ/mol for the ¯uorinated compound, whereas only equal to 39.5 kJ/mol in the case of the hydrogenated compound.

It is not surprising that the CMC value of the more hydrophobic compound E(EO)1F6H is low relatively to the compound M(EO)7F6H, which has the same ¯uorinated chains but does not possess long OE chains. The increment of the adsorption free energy DGad equal to approximately ‡0.5 kJ/mol per one OE group is in agreement with the empirical rule observed for the ¯uorinated nonionic surfactants [2]. Surprisingly the compound M(EO)7F1, which has two short ¯uorinated groups, is more surface active than the hydrogenated compound M(EO)7H13, whereas the latter possess much longer alkyl chains. The former compound is characterised by higher adsorption free energy DGad than the latter (see Table 4). It appears that the interactions between the short ¯uorinated chains at the interface of the less ``hydrophobic'' compound M(EO)7F1 give more considerable contribution to the adsorption free energy (DGad ˆ 53:7 kJ/mol) than the more bulky hydrophobic alkyl chains of more ``hydrophobic'' compound M(EO)7H13 (DGad ˆ 39:5 kJ/mol). In addition, the former ¯uorinated compound is characterised by a higher decrease of the surface tension (gmin ˆ 32 mN/m) than the hydrogenated compound (gmin ˆ 44 mN/m). It must be stressed that the kinetic effects are more pronounced for the hydrogenated compound, which has more bulky hydrophobic groups. It is possible that in aqueous solution, even at very small concentrations, the sample M(EO)7H13 forms associates, which undergo conformational rearrangements at the interface after adsorption. The comparison between the minimum molecular areas amin of the adsorption monolayers and the areas acoll of the spread monolayers at the collapse is given in Table 5. This Table also contains the data concerning the maximum surface pressure pCMC of the adsorption monolayers in comparison with the surface pressure values pcoll for the spread monolayers. As expected, the ¯uorinated compound E(EO)1F6H, which did not possess long OE chains, had almost coinciding areas amin and acoll per one molecule for the Gibbs and Langmuir monolayers. The same remark suits with the molecules of the ¯uorinated M(EO)7F6H and the hydrogenated M(EO)7H13 compounds with long OE chains occupied lesser area amin in the adsorption monolayers than in the spread monolayers acoll. The latter feature is common for all surfactants possessing long OE chains, as discussed in Table 5 Minimum molecular area in the adsorbed monolayer amin, molecular area at the collapse acoll, surface pressure at CMC pCMC and surface pressure at the collapse pcoll for the compounds studied Compound

Gibbs amin (nm2)

Langmuir acoll (nm2)

Gibbs pCMC (mN/m)

Langmuir pcoll (mN/m)

E(EO)1F6H M(EO)7F6H M(EO)7H13 E(EO)1F1 M(EO)7F1

0.43 0.33 0.49 0.72 0.90

0.40 0.40 0.75 ± ±

38 44 28 29 40

29 42 41 ± ±

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Section 2.1.2 in connection with the properties of the spread monolayers [23]. 3. Conclusions In the present paper, we describe the new ``star-like'' type of four chains compounds having two hydrophobic (¯uorinated or hydrogenated) and two OE chains. The comparison of properties of two types of monolayers (spread and adsorbed at the air/water interface) have been carried out. The results obtained allowed us to evaluate the stability of such monolayers and to determine the packing and order of molecules in the monolayer as well as the adsorption free energy at the air/water interface. Particularly it has been shown that decreasing the ¯uoroalkyl chain length induced a systematical decrease in the stability of Langmuir monolayers, which is manifested as the Marangoni±Gibbs viscoelasticity of the monolayers. For the surfactants, which have a large number of oxyethylene groups, the adsorption at the air/water interface from the bulk solution required extremely long times to reach equilibrium due to the diffusion from the solution and to the conformational rearrangements at the interface. For the low bulk concentration of surfactants, the effective diffusion coef®cients of molecules could be estimated via applying the Ward±Tordai equation to the p(t)-curves. The novel model describing the kinetics of desorption or rearrangement of molecules during the lateral compression was suggested, allowing the estimation of both the characteristic time of this process and the areas per molecule at the equilibrium from the relaxation curves A(t). 4. Experimental 4.1. Materials The monomer pyromelitic dianhydride, 2,2,2-tri¯uoroethanol (Merck) and 1H,1H,7H-dodeca¯uoro-1-heptanol (P&M Ltd., Russia) were used as obtained; 2-ethoxy-ethanol (Aldrich) was distilled at 408.1 K, 1-tetradecanol and monomethyl ether of the poly(ethylene glycol) (FW ˆ 350) (Aldrich) were dried by azeotropic distillation with benzene from solutions of the corresponding substances.

203

4.3. Formation of adsorbed (Gibbs) monolayers Surface pressure measurements of the solutions were carried out by the Wilhelmy plate method using a tensiometer K 100 (KruÈss, Germany). The temperature was kept constant at 23 8C. The kinetics of adsorption were measured for arbitrarily chosen concentrations both below and above the CMC for all surfactants, using freshly prepared solutions. The surface tension measurements were taken for a suf®ciently long time to approach the equilibrium value of the surface tension g. The time necessary was longer than 104 s for low concentration Cs of the amphiphiles in the water. The surface tension isotherms g(log Cs) were plotted using these equilibrium values of g. 4.4. Preparation of spread (Langmuir) monolayers Water for the subphase in the Langmuir trough and for surfactant solutions was distilled and passed through a MilliQ water puri®cation system (surface tension of 72.5 mN/m at 20 8C). Chloroform (Aldrich, A.C.S. spectrophotometric grade) and ethanol (Merck, 95%, distilled) were used to prepare the solutions. All other chemicals were of analytical grade. All solutions for the monolayer studies were prepared daily by dissolving the compounds in pure chloroform. Experiments were carried out in a KSV LB 5000 Te¯on trough equipped (KSV, Finland) with two hydrophilic Delrin barriers (symmetric compression) and a Wilhelmy plate as a surface-pressure sensor. The temperature was kept constant at 23 8C. KSV-5000 software was used to control the experiments. The compounds were spread from ca. 1 mg/ ml chloroform solutions onto an aqueous subphase. The equilibration time for surface monolayers before compression was 10 min. Monolayers were compressed at a constant rate of 0.048 nm2/molecule min, unless stated otherwise. Compression rates from 0.02 to 0.1 nm2/molecule min lead to similar results for all compounds forming stable Langmuir monolayers. 4.5. Synthesis of tetraesters of pyromellitic acid

4.2. Measurements and instruments

The synthesis was carried according to the following Scheme 1. At the ®rst step were obtained: di(monoethyl(ethylene glycol)) ester of pyromelitic acid (I), and di(monomethyl(polyethylene glycol)) ester of pyromelitic acid (II). Di(monoethyl(ethylene glycol)) ester of pyromelitic acid (I):

The IR spectra were obtained on the KBr plates with the ``Specord M80'' spectrophotometer (Karl Zeiss, Germany). NMR-1 H spectra of 4±10% solutions in acetone±d6 were recorded on a ``Bruker WP-200SY'' (200 MHz) spectrometer at room temperature using TMS as internal standard. Elemental analyses were performed in the laboratory for micro analyse of INEOS RAS.

Pyromelitic dianhydride (0.2 mol; (43.624 g)) was mixed with 2-ethoxy-ethanol (0.4 mol (36.05 g)) during 45 min. in an argon atmosphere at 423 K. The completion of reaction

M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

204

Scheme 1.

was followed by the disappearance of the absorbance peaks of carbonyl groups of pyromelitic dianhydride (v ˆ 1856 and 1770 cm 1) in the IR-spectrum of the reaction mixture. After completion of the reaction, the reaction mixture was cooled and diester recrystallised from chloroform. The diester was obtained with a yield of 95%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, para-isomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H) 4.50 (s, ±COOCH2± CH2±, 4H), 3.70 (m, ±CH2±O±, 28H), 3.54 (s, ±CH2±CH3, 4H), 1.20 (s, ±CH3, 6H). Req.: C 66.40%, H 8.30%, O 25.30%; Found: C 66.10%, H 8.6%, O 25.5%. Di (monomethyl(polyethylene glycol)) ester of pyromelitic acid (II):

The methodic of synthesis and puri®cation of diester was almost the same as for the synthesis of the compound (I). Compound (II): pyromelitic dianhydride (0.05 mole (10.91 g)), monomethyl ether of the poly(ethylene glycol)350 (0.10 mol (35.00 g)), temperature 423 K, 55 min. The diester was obtained with the yield of 95%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, para-isomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H), 4.50 (s, ±COOCH2± CH2±, 4H), 3.70 (m, ±CH2±O±, 28H), 3.40 (s, ±O±CH3,

6H). Req. for C40.9H67.8O14.4: C 67.14%, H 8.58%, O 29.28%; Found: C 66.15%, H 9.20%, O 24.65%. The reaction products were obtained as mixtures of the equivalent quantities of the ortho- and para- isomers, where the effect of the ortho-/para- substitution of the aromatic ring on the solubility of diesters was observed only for di(monoethyl(ethylene glycol)) ester of pyromelitic acid. The di(monomethyl(polyethylene glycol)) ester of pyromelitic acid showed no difference in solubility for different isomers. Because of this we did not separate isomers of diesters, and used for the following syntheses the mixture of equivalent quantities of isomers. For the reactions of diesters (I) and (II) with the 1H,1H,7H-dodeca¯uoro-1-heptanol, 2,2,2-tri¯uoroethanol and 1-tetradecanol were obtained correspondingly tetraesters designed as E(EO)1F6H, E(EO)1F1, M(EO)7F1, M(EO)7H13, M(EO)7F6H (Table 1). The methodic and conditions of all reactions were the same. E(EO)1F6H: The synthesis of this compound was carried according the following technique: to 0.005 mol of the diester of pyromelitic acid was added 20 ml of thionyl chloride (SOCl2) and the mixture was heated at 323 K for 1 h. The completing of reaction was controlled by disappearance of the absorbance peaks of hydroxyl groups of pyromelitic acid (v ˆ 945 cm 1) in the IR-spectra of reaction mixture. After completing of reaction the most of thionyl chloride was distilled under vacuum. For removal of the traces of thionyl chloride to the reaction mixture was added 50 ml of toluene, which with the traces of thionyl chloride was distilled later under vacuum. Then to the diester dichloride was added 8 ml of solvent (dichloroethane), and under stirring quickly was added the mixture of 0.01 mol (3.30 g) of 1H,1H,7H-dodeca¯uoro-1-heptanol alcohol with 0.015 mol (2.1 ml) of triethylamine in 17 ml of dichloroethane. The reaction was carried out for 1.5 h at 313 K in the argon atmosphere. After synthesis, the reaction mixture was cooled, ®ltered from the partially precipitated triethylamine hydrochloride and evaporated at the room temperature for 10 h. Obtained ester was dissolved in 100 ml of diethyl ether and washed with 200 ml of water. The ester solution in diethyl ether was dried overnight with Na2SO4, followed by distillation of diethyl ether. To remove the traces of the diethyl ether obtained star-shape ester was kept for 24 h in vacuum at 323 K. The ester was obtained with the yield of 94%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, para-isomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H), 6.05 (t, ±CF2±H, 4H), 4.85 (t, ±COO±CH2±CF2±, 4H), 4.45 (s, ±COOCH2± CH2±, 4H), 3.75 (s, ±COOCH2±CH2±, 4H), 3.54 (s, ±CH2± CH3, 4H), 1.20 (s, ±CH3, 6H). Req. for C32O10H26F24: C 37.44%, O 15.59%, H 2.55%, F 44.42%; Found: C 35.32%, O 15.29%, H 2.54%, F 46.85%. E(EO)1F1: This ester was obtained with the yield of 96%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, para-isomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H), 4.75 (t, ±CH2±CF3, 4H), 4.50 (s, ±COOCH2±CH2±, 4H), 3.70

M.-J. SteÂbe et al. / Journal of Fluorine Chemistry 119 (2003) 191±205

(s, ±CH2±O±, 28H), 3.54 (s, ±CH2±CH3, 4H), 1.20 (s, ±CH3, 6H), n20 d ˆ 1:461. M(EO)7F1: This ester was obtained with the yield of 94%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, para-isomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H), 4.75 (t, ±CH2±CF3, 4H), 4.50 (s, ±COOCH2±CH2±, 4H), 3.70 (m, ±CH2±O±, 28H), 3.40 (s, ±O±CH3, 6H). Req. for C44.8O22.4H69.6F6: C 49.80%, O 33.16%, H 6.50%, F 10.54%; Found: C 49.86%, O 34.08%, H 6.50%, F 9.56%; n20 d ˆ 1:469. M(EO)7F6H: This ester was obtained with the yield of 92%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, paraisomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H), 6.05 (t, ±CF2±H, 4H), 4.75 (t, ±CH2±CF3, 4H), 4.50 (s, ±COOCH2±CH2±, 4H), 3.70 (m, ±CH2±O±, 28H), 3.40 (s, ± O±CH3, 6H) 1.469, n20 d ˆ 1:426. M(EO)7H13: The ester was obtained with the yield of 93%, NMRÐ1 H (d, ppm): 8.14 (s, aromatic ring, para-isomer, 2H), 8.27 and 8.01 (s, aromatic ring, meta-isomer, 2H), 4.30 (s, ± COOCH2±(CH2)12±, 4H), 4.50 (s, ±COOCH2±CH2±O±, 4H), 3.70 (m, ±CH2±O±, 28H), 3.40 (s, ±O±CH3, 6H), 1.70 (s, ± COOCH2±CH2± CH2±, 4H), 1.25 (m, ±(CH2)11±CH3, 4H), 0.90 (s, ±CH2±CH3, 6H). Req. for C64.8O22.4H133.6: C61.71%, O 28.41%, H 9.88%; Found: C 62.85%, O 27.65%, H 9.50%, n20 d ˆ 1:478. The list and the designation of new synthesised compounds are presented in the Table 1. Acknowledgements The authors thank L. RodehuÈser for revising the English. References [1] J.C. Ravey, A. Gherbi, M.-J. SteÂbeÂ, Coll. Polym. Sci. 76 (1988) 234. [2] J.C. Ravey, M.-J. SteÂbeÂ, Coll. Surf. A: Physicochem. Eng. Aspects 84 (1994) 11.

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