ACTA FACULTATIS PHARMACEUTICAE UNIVERSITATIS COMENIANAE Tomus LIV 2007
HYDROPHOBICITY AND CRITICAL MICELLE CONCENTRATION OF SOME QUATERNARY AMMONIUM SALTS WITH ONE OR TWO HYDROPHOBIC TAILS 1
Kopecký, F. – 1Fazekaš T. – 2Kopecká B. – 1Kaclík, P.
1
Department of Physical Chemistry of Drugs, 2Department of Pharmaceutical Analysis and Nuclear Pharmacy, Faculty of Pharmacy, Comenius University, Bratislava
Partition coefficients (P, octan-1-ol/water) of some micellizing quaternary ammonium salts were calculated (KowWin) as a measure of hydrophobicity and critical micelle concentrations (cmc) of these salts were compiled or determined in this work to scrutinize the relationship between log cmc and log P. The selected quaternary ammonium salts (cationic surfactants S+X-) were alkyltrimethyl ammonium chlorides and bromides CmH2m+1N+(CH3)3X- (m = 10 – 18) with one non-branched hydrophobic alkyl tail, alkyldodecyldimethyl ammonium bromides CmH2m+1N+(C12H25)(CH3)2X- (m = 2 – 8) with two non-branched alkyl tails linked to a positively charged nitrogen, and others were benzalkonium chloride and bromide, benzethonium chloride, and domiphen bromide. The structure of the last three surfactant cations is rather irregular, each of them is characterized by two hydrophobic tails of a mutually different nature, comprising also benzene rings and eventually etheric oxygen atoms. UV spectrophotometry was therefore used to determine cmc and concentration of the free non-micellized cations in the micellar solutions of benzethonium chloride and domiphen bromide. A simple empirical equation log cmc = a – blog P was suitable to correlate log cmc with log P, however the coefficients a, b for one-tailed and bi-tailed quaternary salts were mutually different and, furthermore, chlorides exhibited slightly higher cmc than bromides. The bi-tailed cationic surfactants, including benzalkonium chloride and bromide, benzethonium chloride and domiphen bromide, exhibited substantially higher cmc than one-tailed surfactants with similar log P value. Key words: cationic surfactants – micellization – cmc – hydrophobicity – log P – benzalkonium bromide – benzethonium chloride – domiphen bromide
INTRODUCTION In the series of surfactants with similar structure the micellization tendency increases and the critical micelle concentration (cmc) in aqueous solution generally decreases with
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increasing hydrophobic character of the amphiphilic surfactant. Thus in the homologous series of ionic surfactants with the surface-active ions consisting of one non-branched alkyl tail with one terminal ionic group, the cmc value is known to decrease in a regular pattern with the increasing number of carbon atoms in the alkyl chain, at least in the range of C10 – C18. The empirically found relationship between cmc and the number of carbon atoms (m) of the non-branched alkyl tail may be expressed in the form of the logarithmic Eq. 1: log cmc = A – Bm
(1)
Equation 1 or its variations [1-6] have often been applied to homologous series of micellizing quaternary ammonium salts like alkyltrimethylammonium chlorides or bromides representing cationic surfactants with one hydrophobic alkyl tail (CH3(CH2)m-1–). However, if we examine structurally more diverse cationic surfactants, eventually characterized by two hydrophobic tails, then the Equation 1 may become inadequate. Therefore we put under scrutiny the relationship between log cmc of some quaternary ammonium salts and their calculated log P, since the octan-1-ol/water partition coefficient P (or log P) is regarded as a useful measure of hydrophobicity [7]. In this paper we focused on the quaternary ammonium salts SX, composed of surfactant cation S+ with one positively charged quaternary nitrogen atom and a counterion X-, namely chloride or bromide anion. The quaternary ammonium salts selected for examination are summarized in Table 1, they are altogether a representative sample of cationic surfactants as well as antimicrobial and analytical agents. Cations of some interesting quaternary ammonium salts SX are characterized by two hydrophobic tails linked to one positively charged nitrogen. Therefore together with alkyltrimethylammonium chlorides and bromides we examined several alkyldodecyldimethyl ammonium bromides with two non-branched alkyl tails as well as salts of three other surfactant cations, namely benzalkonium chloride and bromide, benzethonium chloride, and domiphen bromide. The last three surfactant cations are characterized by two structurally different and rather irregular hydrophobic tails linked to one quaternary nitrogen (Fig. 1). The corresponding salts are known antimicrobials and benzethonium chloride has also been recently identified as an anticancer agent [8]. Besides of the micellization in aqueous solutions [1-3,9-14], the bi-tailed cationic surfactants are distinguished by their ability to form the reverse cationic micelles in non-aqueous media [15] as well as various colloidal microemulsions [16] while in analogous systems with one-tailed cationic or anionic surfactants the addition of non-ionic co-surfactant is often a necessary precondition for the formation of sufficiently stable microemulsions [17]. Hydrophobicity is also supposed to affect other important characteristics of cationic surfactants with both one and two tails, such as formation of mixed micelles, ion pairs and various associates as well as the sorption phenomena [9,18-21]. In this work we at first compiled cmc values of the selected quaternary ammonium salts listed in Table 1 and experimentally measured or re-measured cmc of benzethonium chloride and domiphen bromide. Due to the favorable aromatic structure of both the benzethonium and domiphen cations (Fig. 1) we could apply UV spectrophotometry to determine the respective cmc’s. The used spectral method was on one hand rather
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laborious but, on the other hand, besides of cmc, it also enabled us to estimate other interesting surfactant features, in particular the concentration of the free (non-micellized) surfactant cations in aqueous micellar solutions of benzethonium chloride and domiphen bromide, respectively. In the second part of this work, the octan-1-ol/water partition coefficients (P) of the studied quaternary ammonium salts were calculated and the values of log cmc and log P were mutually confronted. Table 1. Critical micelle concentration (cmc, mol/l) in the form of log cmc and calculated octan-1-ol /water partition coefficient (KowWin, log P) of the selected quaternary ammonium salts – cationic surfactants of the type S+X- (25 °C) One alkyl tail – alkyltrimethyl ammonium chlorides and bromides, CmH2m+1N+(CH3)3XAnion XAlkyl carbons, m Total carbons, m+3 log cmc Ref. to cmc log P Cl10 13 -1.167 [1] 0.238 Cl12 15 -1.678 [1] 1.220 Cl12 15 -1.642 [5] Cl12 15 -1.672 [9] Cl13 16 -1.951 [5] 1.712 Cl14 17 -2.347 [1] 2.203 Cl14 17 -2.276 [5] Cl 16 19 -2.854 [1] 3.185 Cl16 19 -2.824 [5] Cl 18 21 -3.461 [5] 4.167 Br10 13 -1.180 [1] 0.238 Br12 15 -1.796 [1] 1.220 Br14 17 -2.469 [1] 2.203 Br16 19 -3.036 [1] 3.185 Br18 21 -3.509 [1] 4.167 Two alkyl tails – alkyldodecyldimethyl ammonium bromides, CmH2m+1N+(C12H25)(CH3)2XAnion XAlkyl carbons, m Total carbons, m+14 log cmc Ref. to cmc log P Br2 16 -1.854 [1] 1.712 Br 2 16 -1.845 [9] Br4 18 -2.125 [1] 2.694 Br6 20 -2.509 [1] 3.676 Br8 22 -2.959 [1] 4.658 Two different (irregular) hydrophobic tails Name Abbreviation log cmc Ref. to cmc log P Benzalkonium chloride BzlkCl -2.056 [1] 2.93 Benzalkonium bromide BzlkBr -2.310 [2,10] 2.93 Benzethonium chloride BzthCl -2.553 this work 4.00 -2.745 [14] Domiphen bromide DmphBr 4.20 -2.770 this work
MATERIALS AND METHODS Materials and instruments Solutions of benzethonium chloride Fluka (BzthCl) and domiphen bromide Aldrich (DmphBr) in deionized water were spectrophotometrically measured in the UV spectral
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region with the UV-VIS Specord M 40 (Zeiss Jena) instrument. Due to the broad concentration range of the measured solutions, several quartz cells (0.1 – 5 cm) were used in the measurements (25 ºC); more details were described recently [13].
UV spectrophotometric determination of cmc and the free surfactant cation concentration in micellar solutions of BzthCl and DmphBr Benzethonium chloride and domiphen bromide exhibited UV absorption spectrum in aqueous solutions, corresponding to their aromatic structure (Fig. 1). The UV spectral parameters of BzthCl were described previously [13] and it appeared that both BzthCl and DmphBr showed a similar weak absorption band in the region of λ 250 – 300 nm. This weak and broad composite band proved to be convenient for the determination of cmc as well as the concentration of the free benzethonium or domiphen cations in the respective BzthCl and DmphBr micellar solutions since the micelle formation was indicated by the slight hyperchromic and bathochromic shifts at certain wavelengths. The rather low intensity of the selected spectral band was indeed advantageous because measurements could be stretched up to sufficiently high concentrations (10-2 mol/l). C12H25
C12H25 N+(CH3)2 X
N+ (CH3) 2 Br -
CH2
O
BzlkX
CH2
CH2
DmphBr CH3 CH3
H3C
C
C
O
CH2
CH2
O
CH2
CH2 N+ (CH3)2 Cl -
CH3 CH3
CH2
BzthCl Fig. 1. The examined quaternary ammonium salts, cationic surfactants SX with two different (irregular) hydrophobic tails; BzlkX – benzalkonium chloride (BzlkCl) and bromide (BzlkBr), DmphBr – domiphen bromide, BzthCl – benzethonium chloride
Measured absorbances A were transformed into molar absorption coefficients, ε = A/(l . cSX), where l is the optical length (0.1 – 5 cm) and cSX is the total concentration of the surfactant BzthCl or DmphBr. Because the observed spectral shifts of the used weak band were rather small, the investigated solutions had to be measured at two distinct wavelengths and the differences ∆ε between two molar absorption coefficients,
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ε1 and ε2, measured in the same solution at two selected wavelengths, were used for further evaluations: ∆ε = ε2 – ε1
(2)
The molar absorption coefficients ε1 and ε2 were measured at the respective wavelengths 265 nm and 278 nm in the solutions of BzthCl while at 264 nm and 270 nm in the solutions of DmphBr. Wavelengths selected for the ε1 measurements corresponded to a shallow minimum (or a shoulder) on the spectral absorption curves and for both the investigated surfactants the value of ε1 was nearly concentration independent, it was approximately 1060 l mol-1 cm-1 for BzthCl and 1050 l mol-1 cm-1 for DmphBr in the whole measured concentration range (10-5 – 10-2 mol/l). On the other hand, wavelengths selected for ε2 measurements corresponded to a local absorption maximum that remained continually unchanged only in dilute solutions below cmc (at cSX < cmc), there ε2 was 1620 l mol-1 cm-1 for BzthCl and 1250 l mol-1 cm-1 for DmphBr. As soon as the surfactant concentration reached the corresponding cmc, ε2 started to rise with the increasing concentration. From the dependence of ∆ε on the total surfactant concentration it was therefore possible to evaluate both cmc (Table 1) and the approximate concentration of the free (non-micellized) surfactant cations in the respective micellar solutions (cSX > cmc) of BzthCl and DmphBr (Fig. 6). More experimental details and calculations have been described recently [13].
Calculations of the logarithm of octan-1-ol/water partition coefficient (log P) The octan-1-ol/water partition coefficients of the selected quaternary ammonium salts were calculated using the KowWin program that estimates log P of a compound by summing atom/fragment contributions and corrections factors for various substructural effects occurring in the chemical structure [7]. In the used form, the program practically did not distinguish between chloride and bromide anions of the examined quaternary salts. The resulting log P values are summarized in Table 1.
RESULTS AND DISCUSSION Correlations of log cmc with structural parameters and with log P The relationship between log cmc and the number of carbon atoms (m) of the alkyl tail of the selected alkyltrimethylammonium chlorides and bromides (SX, Table 1) is seen in Fig. 2 in the form of two decreasing lines, the upper line is representing chlorides and the lower line bromides. The slightly lower cmc of bromides in comparison with chlorides is regarded as a consequence of stronger binding of bromide counterions to the cationic surfactant micelles that reinforces micellization in aqueous solutions [1-4]. Values of the coefficients A, B of Eq. 1, resulting from the least-squares method, can be obtained from the data seen Fig. 2 using a small rearrangement, for chlorides B = 0.291
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and for bromides B = 0.295. The two lines are thus nearly parallel and one more carbon, i.e. one more CH2 group in the alkyl tail always decreases cmc of both bromides and chlorides approximately by ½ (log ½ = -0.3). This is a well-known rule, although the coefficients A, B depend somewhat on the selection of the studied surfactants and the sources of cmc values [1-6]. -1 One-tailed SX
y = -0.291x + 1.798
-2
log cmc
R2 = 0.997
-3 y = -0.295x + 1.730 R2 = 0.996 -4 10
14
Alkyl carbons, m
18
Fig. 2. The plot of log cmc of the selected alkyltrimethyl ammonium salts SX (chlorides upper line, bromides lower line) vs. number of carbon atoms in the alkyl tail of the surfactant cation -1 One- and bi-tailed SX
y = -0.182x + 1.083 R2 = 0.990
log cmc
-2
-3
-4 12
16
20
24 Total carbons
Fig. 3. The plot of log cmc of alkyltrimethyl ammonium chlorides and bromides (one alkyl tail, solid lines) together with alkyldodecyldimethyl ammonium bromides (two alkyl tails, dashed line) vs. total number of carbon atoms of the surfactant cation
In the framework of the structural parameters a comparison between one-tailed alkyltrimethyl ammonium salts and bi-tailed alkyldodecyldimethyl ammonium bromides is also possible if log cmc is plotted against total number of carbon atoms, as it is seen in Fig. 3. Such comparison is well justified since in the compared quaternary ammonium
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salts all the carbon atoms are built in the CH2 and CH3 groups of the corresponding surface-active cations characterized by one positively charged nitrogen. By this way Fig. 3 illustrates the fact, that the bi-tailed ionic surfactants have generally higher cmc in aqueous solution than analogous surfactants with one hydrophobic tail, since one longer non-branched and flexible hydrophobic tail is apparently more suitable for micelle formation than two shorter tails [1,3,4]. However, if cationic surfactants with structurally more complicated (irregular) hydrophobic tails are to be included into consideration, such simple structural approach becomes inadequate. A convenient option is then the correlation between log cmc and logarithm of the partition coefficient (log P) as a measure of hydrophobicity in the form of the following empirical equation: log cmc = a – blog P
(3)
The plots of log cmc vs. log P for one-tailed alkyltrimethyl ammonium chlorides and bromides in Fig. 4 illustrate the cmc decrease with increasing hydrophobicity and the coefficients a, b both for chlorides (upper line) and bromides (lower line) can also be obtained with a bit care from Fig. 4. With regard to the connection between log P and structural parameters [7] it is indeed obvious that the plots shown in Fig. 4 are analogous to those in Fig. 2 and 3, in accordance with the analogy between Eq. 3 and Eq. 1. -1 One-tailed SX
-2 log cmc
y = -0.592x - 0.970 R2 = 0.997
-3
y = -0.600x - 1.076 R2 = 0.996
-4 0
2
4 Hydrophobicity, log P
Fig. 4. The plot of log cmc of alkyltrimethyl ammonium salts SX (chlorides upper line, bromides lower line) vs. log P as a measure of hydrophobicity
The dependence of log cmc on log P for all the examined bi-tailed cationic surfactants SX is seen in Fig. 5. In comparison with Fig. 4 it is apparent that the bi-tailed surfactants are more hydrophobic than one-tailed surfactants with comparable cmc. The decreasing dashed line in Fig. 5 and the coefficients printed below it were calculated using exclusively the data for alkyldodecyldimethyl ammonium bromides, in the diagram they are represented by the empty squares. The surfactants with two structurally irregular hydrophobic tails were not included in the evaluation of the dashed line but the are
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represented by circles in the Fig 5, the empty circles are chlorides BzlkCl and BzthCl, the full black circles are bromides BzlkBr and DmphBr. Rather unexpectedly, the structurally irregular bromides BzlkBr and DmphBr (Fig. 1) are quite in line with the simple bi-alkyldimethyl ammonium bromides when log cmc is plotted against log P. Furthermore, chlorides BzlkCl and BzthCl are located slightly above the line for bromides, as it is also the case with alkyltrimethyl ammonium chlorides and bromides (Fig. 4). Apparently, the correlation of log cmc with log P in the form of the simple Eq. 3 may be able to encompass the bi-tailed cationic surfactants of various structures. Another noticeable feature is the relatively small difference between the coefficients a (-0.970, -1.076 and -1.190) obtained from Figs. 4 and 5 that suggests not too big effect of the different ionic moieties on the process of micellization. -1.5
log cmc
Bi-tailed SX, bi-alkyl and irregular -2
-2.5 y = -0.370x - 1.190 R2 = 0.990
-3
-3.5 1
3
5 Hydrophobicity, log P
Fig. 5. The plot of log cmc vs. log P for alkyldodecyldimethyl ammonium bromides (two alkyl tails, dashed line, squares) together with the points representing cationic surfactants with two irregular tails – BzlkBr and DmphBr (full black circles) as well as BzlkCl and BzthCl (empty circles)
Hydrophobicity, cmc and the concentration limits of the free surfactant cations in aqueous solutions The available concentration of the free, non-micellized (non-associated) surfactant ions in aqueous solution is limited not so much by the solubility of the corresponding surfactant salt but primarily by the value of cmc [1-3,21]. The maximum possible concentration of the free surfactant cation ([S+]) is more or less equal to the corresponding cmc because beyond cmc, in the micellar solution (at cSX > cmc), the surfactant cation concentration starts to decrease with increasing total surfactant concentration as it is seen in Fig. 6. The curves 1 – 3 in Fig. 6 correspond to the cationic surfactants (SX) characterized by two hydrophobic tails, namely BzlkBr (1), BzthCl (2), and DmphBr (3) while the lowest curve 4 corresponds to hexadecyltrimethyl ammonium bromide characterized by one alkyl tail. The curves for BzthCl (2) and DmphBr (3) were
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determined in this work by the UV spetrophotometry while the curves 1 and 4 were evaluated from potentiometric data, the curve 1 (BzlkBr) from our older data [2,11] and the data for the curve 4 were taken from another source [21]. 6
1
Free [S+], mmol/l
4
2
2
3 4 0 0
4
8
Total SX , c SX , mmol/l
Fig. 6. The decreasing concentrations of the free surfactant cations ([S+]) in micellar solutions (cSX > cmc) plotted against total surfactant concentrations (cSX); the full black marks correspond to the respective cmc's of the surfactants 1) BzlkBr, 2) BzthCl, 3) DmphBr (1 – 3 with two tails) and 4) hexadecyltrimethyl ammonium bromide (with one tail)
It is interesting to compare hydrophobicity in the form of log P values summarized in Table 1 with the curves in Fig. 6. For example, hydrophobicity of DmphBr with two tails (curve 3, log P = 4.17) and that of hexadecyltrimethyl ammonium bromide with one tail (curve 4, log P = 4.20) are very similar but cmc and therefore also the maximum available concentration of the free surfactant cations is much higher for DmphBr with two hydrophobic tails. Thus in general, cationic surfactants with two hydrophobic tails are characterized by higher cmc as well as higher available concentration of free, non-micellized surfactant cations in aqueous solution than analogous one-tailed surfactants of the same hydrophobicity. The relatively high available concentration of the hydrophobic cations may be the very advantageous factor in ion-pair formation [18], sorption [19-21] and together with the favorable steric conditions it may facilitate the formation of reverse micelles [15] and microemulsions [16] of the bi-tailed surfactants as well. Acknowledgement: This study was supported by the VEGA agency of the Slovak Ministry of Education (grant no. 1/4297/07 and 1/4299/07).
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Registered: June 11, 2007 Accepted: June 20, 2007
Doc. RNDr. František Kopecký, CSc. Faculty of Pharmacy Comenius University Odbojárov 10 832 32 Bratislava Slovak Republic
[email protected]
HYDROFOBICITA A KRITICKÁ MICELOVÁ KONCENTRÁCIA VYBRANÝCH KVARTÉRNYCH AMÓNIOVÝCH SOLÍ S JEDNOU ALEBO DVOMA DLHŠÍMI HYDROFOBNÝMI SKUPINAMI 1
Kopecký, F. – 1Fazekaš, T. – 2Kopecká, B. – 1Kaclík, P.
1
Katedra fyzikálnej chémie liečiv, 2Katedra farmaceutickej analýzy a nukleárnej farmácie, Farmaceutická fakulta, Univerzita Komenského, Bratislava
Rozdeľovacie koeficienty (P, oktán-1-ol/voda) vybraných micelizujúcich kvartérnych amóniových solí boli vypočítané (KowWin) ako miera hydrofobicity a kritické micelové koncentrácie (cmc) týchto solí boli zhromaždené z rozličných zdrojov alebo odmerané v predloženej práci na preskúmanie vzťahu medzi hodnotami log cmc a log P a jeho dôsledkov. Vybrané kvartérne amóniové soli (katiónové tenzidy S+X-) boli alkyltrimetyl-amóniumchloridy a -bromidy CmH2m+1N+(CH3)3 X- (m = 10 – 18) s jedným dlhším nevetveným alkylovým reťazcom, alkyldodecyldimetylamóniumbromidy CmH2m+1N+(C12H25)(CH3)2 X- (m = 2 – 8) s dvoma nevetvenými dlhšími alkylmi pripojenými ku kladne nabitému atómu dusíka a ďalšie boli benzalkóniumchlorid a -bromid, benzetóniumchlorid a domifeniumbromid. Komplikovanejšia štruktúra posledných troch tenzidových katiónov sa vyznačuje prítomnosťou vždy dvoch hydrofóbnych reťazcov nerovnakého charakteru, súčasťou ktorých sú aj benzénové jadrá, prípadne éterové kyslíkové atómy. Bola preto použitá UV-spektrofotometria na stanovenie cmc a koncentrácie nemicelizovaných (voľných) katiónov v micelárnych roztokoch benzetóniumchloridu a domifeniumbromidu. Na koreláciu medzi log cmc a log P sa ukázala byť postačujúca jednoduchá empirická rovnica log cmc = a + blog P, ale regresné koeficienty a, b boli odlišné pre kvartérne soli s jedným hydrofóbnym reťazcom a s dvoma hydrofóbnymi reťazcami; okrem toho chloridy mali o niečo vyššie cmc ako bromidy. Pri podobných hodnotách log P mali soli tenzidových katiónov s dvoma hydrofóbnymi reťazcami, včítane benzalkóniumchloridu a -bromidu, benzethóniumchloridu a domifeniumbromidu, podstatne vyššie cmc ako soli katiónov s jedným hydrofobným reťazcom.
Acta Facult. Pharm. Univ. Comenianae 54, 2007, p. 84-94.
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