Interactions between selected bile salts and Triton X-100 or sodium lauryl ether sulfate
© Ćirin et al 2011
Received: 5 November 2011
Accepted: 29 December 2011
Published: 29 December 2011
In order to develop colloidal drug carriers with desired properties, it is important to determine physico-chemical characteristics of these systems. Bile salt mixed micelles are extensively studied as novel drug delivery systems. The objective of the present investigation is to develop and characterize mixed micelles of nonionic (Triton X-100) or anionic (sodium lauryl ether sulfate) surfactant having oxyethylene groups in the polar head and following bile salts: cholate, deoxycholate and 7-oxodeoxycholate.
The micellization behaviour of binary anionic-nonionic and anionic-anionic surfactant mixtures was investigated by conductivity and surface tension measurements. The results of the study have been analyzed using Clint's, Rubingh's, and Motomura's theories for mixed binary systems. The negative values of the interaction parameter indicate synergism between micelle building units. It was noticed that Triton X-100 and sodium lauryl ether sulfate generate the weakest synergistic interactions with sodium deoxycholate, while 7-oxodeoxycholate creates the strongest attractive interaction with investigated co-surfactants.
It was concluded that increased synergistic interactions can be attributed to the larger number of hydrophilic groups at α side of the bile salts. Additionally, 7-oxo group of 7-oxodeoxycholate enhance attractive interactions with selected co-surfactants more than 7-hydroxyl group of sodium cholate.
Bile salts are steroid compounds with planar amphiphilic properties . These surfactant characteristics result from the combination of the polar hydroxyl groups on the concave α-face and the methyl groups on the convex β-face [1, 2]. Bile salts in aqueous solution form aggregates mainly due to hydrophobic attractive interactions of the apolar β-faces .
However, further stabilisation and aggregation occurs through intermolecular hydrogen bonds formation [4, 5]. It is known that bile salts form micelles at concentrations above the critical micelle concentration (cmc) [3, 6, 7]. The more hydrophobic bile salt is, the lower critical micelle concentration it has, due to more stable hydrophobic aggregations [6, 7].
Bile salts are known as permeability enhancers to increase drug penetration through various biological membranes by interacting with phospholipids in cell membranes [8, 9]. Micelles of bile salts have been investigated in different pharmaceutical formulations where they solubilise poorly soluble drugs [10–13]. The ability of bile salts to achieve this is mainly dependent on their hydrophobicity. Bile salts with more hydrophobic micellar core have greater ability to accept hydrophobic pharmaceutical ingredients . Solubilisation capacity of the micellar systems can be increased by chemical modification of the bile salt steroid skeleton. Another method to improve solubilisation properties of micellar systems is to produce mixed micelles by combining individual surfactants with different aggregation properties (cmc values) [14, 15]. Furthermore, intermolecular hydrophilic interactions between polar groups in certain bile salt mixed micelles are known to greatly increase stability of these binary systems .
In the pharmaceutical area, various nonionic co-surfactants have been used in bile salt-nonionic binary systems due to ability of these mixtures to improve solubilisation of poorly soluble drugs [17–23]. However, in studies of bile salt-anionic binary mixtures, sodium dodecyl sulfate has been predominantly used as an anionic co-surfactant [24–27].
Triton X-100 (TRX) is a nonionic surfactant with a hydrophilic polyethylene oxide group (on average it has 9.5 oxyethylene groups (OE)) and a hydrophobic group. TRX (p-(1,1,3,3-tetramethylbutyl)phenoxypolyoxyethylene glycol) differs from conventional nonionic surfactants, having hydrophilic part longer than the hydrophobic region of the molecule. Sodium lauryl ether sulfate (SLES) is an anionic surfactant with one to four hydrophilic OE groups adjacent to the anionic sulfate group. Its basic structure is C12H25(OCH2CH2)xOSO3Na where x represents the number of OE groups in the chain. For this work, SLES having two OE groups was used.
To the best of our knowledge, work on physico-chemical characterisation of binary mixtures consisting of bile salt and polyoxyethylene type of co-surfactant has been done only with individual bile salts [17, 28, 29]. However, there is no comparative data on the physico-chemical properties of mixed micelles of TRX or SLES with different bile salts. According to the authors' knowledge investigations of SLES-bile salt binary systems have not been reported previously.
In the present work, we have tested a set of three bile salts (sodium cholate (C), sodium deoxycholate (D) or sodium 7-oxodeoxycholate (OD)) to interact with TRX or SLES. The results of the study have been analysed using Clint's, Rubingh's, and Motomura's theories for mixed binary systems [30–32].
The first aim of this study was to investigate how different combinations of hydroxyl and oxo groups on the concave α- face of the bile salt skeleton can influence the interactions with anionic and nonionic co-surfactant having polar OE groups in the hydrophilic region. The second goal was to evaluate how structural differences in selected anionic and nonionic co-surfactant impact micelization behaviour of developed mixed micelles.
It is envisioned that insight on the formation of the micellar structures obtained in this study could be used to develop drug delivery formulations with optimized physico-chemical properties.
2. Results and Discussion
2.1. Determination of the cmcvalues of individual surfactants
Experimentally obtained critical micelle concentrations of the individual surfactants compared to the literature values
cmc ex (mM)
cmc lit (mM)
4.30 ± 0.07
4.25 ± 0.05
12.90 ± 0.16
12.78 ± 0.14
61.10 ± 0.9
59.50 ± 1.2
0.80 ± 0.01
0.82 ± 0.02
0.240 ± 0.004
2.2. Determination of the cmcvalues of the binary mixtures
Phillips' method was employed as proposed by Mosquera et al . The Runge-Kutta integration was applied and a least squares fitting method, according to Levenberg-Maquardt, was used. The usage of Phillips' method for determination of cmc values has been reported in previous studies, as well [39, 40]. Figure 1 shows the application of Phillips' method.
In order to evaluate reproducibility of used methods, all conductivity and surface tension measurements were repeated three times. The cmc determination error did not exceed 3%. Extended t-test (p < 0.05) shows that the mean cmc values determined using conductivity and surface tension measurements belong to the same population.
Comparision of the experimentally obtained critical micelle concentrations of investigated TRX-bile salt binary systems
cmc ex (mM)
4.30 ± 0.07
4.20 ± 0.05
1.545 ± 0.023
1.533 ± 0.021
0.832 ± 0.017
0.821 ± 0.019
0.617 ± 0.015
0.605 ± 0.013
0.498 ± 0.012
0.491 ± 0.011
0.416 ± 0.012
0.420 ± 0.010
12.90 ± 0.16
12.78 ± 0.14
1.565 ± 0.020
1.579 ± 0.018
0.848 ± 0.019
0.855 ± 0.018
0.633 ± 0.014
0.619 ± 0.012
0.510 ± 0.013
0.501 ± 0.010
0.423 ± 0.011
0.410 ± 0.09
61.10 ± 0.20
59.50 ± 0.25
2.021 ± 0.029
2.035 ± 0.025
1.040 ± 0.020
1.028 ± 0.022
0.723 ± 0.017
0.711 ± 0.016
0.560 ± 0.013
0.551 ± 0.011
0.464 ± 0.011
0.448 ± 0.010
Comparision of the experimentally obtained critical micelle concentrations of investigated SLES-bile salt binary systems
cmc ex (mM)
4.30 ± 0.07
4.20 ± 0.05
2.857 ± 0.041
2.845 ± 0.037
2.203 ± 0.035
2.193 ± 0.036
1.845 ± 0.032
1.841 ± 0.034
1.528 ± 0.031
1.517 ± 0.030
1.322 ± 0.029
1.305 ± 0.032
12.90 ± 0.16
12.78 ± 0.14
4.461 ± 0.058
4.477 ± 0.065
2.864 ± 0.045
2.875 ± 0.050
2.165 ± 0.039
2.181 ± 0.042
1.730 ± 0.036
1.725 ± 0.043
1.427 ± 0.035
1.438 ± 0.039
61.10 ± 0.20
59.50 ± 0.25
6.511 ± 0.088
6.395 ± 0.081
3.598 ± 0.054
3.620 ± 0.049
2.493 ± 0.050
2.475 ± 0.052
1.906 ± 0.041
1.885 ± 0.044
1.549 ± 0.034
1.529 ± 0.038
As seen in Tables 2 and 3 the cmc values obtained in conductivity and surface tension studies are in good agreement. Since there is no statistically significant difference between the cmc values determined by means of the two methods, the cmc values acquired by conductivity measurements were used in further calculations.
2.3. Determination of the physico-chemical parameters
In order to study the influence of the structure of bile salts on the formation of mixed micelles with TRX or SLES, the physico-chemical parameters of the micellar systems were calculated using experimentally obtained cmc values. The determined physico-chemical parameters are: the critical micelle concentration of ideal mixtures (cmc id ), the mole fraction of the more hydrophobic surfactant in the ideal mixed micelle (x id ), the mole fraction of the more hydrophobic surfactant in the real mixed micelle (x 1 ) and the β parameter.
Physico-chemical parameters of mixed micelles
The critical micelle concentration of ideal mixtures (cmc id ) according to Clint 
The mole fraction of the more hydrophobic surfactant in the ideal mixed micelle (x id ), according to Motomura:
The mole fraction of the more hydrophobic surfactant in the real mixed micelle according to Rubingh 
The β interaction parameter, according to Rubingh 
Comparison of experimental with ideal parameters and interaction parameter (β) of the TRX-bile salt mixtures
Comparison of experimental with ideal parameters and interaction parameter (β) of the SLES-bile salt mixtures
The results show deviation of the cmc ex values from those calculated according to Clint's theory indicating nonideal behaviour of examined mixtures and existence of mutual interactions of the components in the micelles. The difference between x id and x1 also indicates nonideal behaviour of the mixtures.
Average β interaction parameters (β av ) of analysed mixed binary systems
The average interaction parameter values show that TRX generates stronger sinergistic interactions with investigated bile salts than SLES. Many recent studies have confirmed that synergistic effect generally occurs in anionic-nonionic surfactant mixtures, improving stability of the system [41–48]. In previously investigated sodium deoxycholate-Triton X-100 binary system, synergistic interaction is reported, with β av = -0.9 . This value is in good agreement with the average value of the interaction parameter reported in our study (-0.88). It is known that in mixed miceles composed of two anionic amphihpiles, repulsive electrostatic interactions between anionic heads are important source of destabilisation [49, 50]. Hence, weak synergistic or antagonistic interactions between individual surfactants are frequently reported in anionic-anionic surfactant mixtures [26, 49, 50]. Therefore, weak synergism that has been noticed for anionic-anionic surfactant mixtures in this article is in agreement with previous studies.
Obviously, the larger number of hydrophilic groups on α side of the steroid ring generates stronger synergism, since C and OD have one polar group more than DC (7-hydroxyl and 7-oxo group, respectively).
Namely, it is possible that α-hydroxyl groups on the steroid skeleton create hydrogen bonds with proton acceptor OE groups of the polar head of TRX and SLES. It is known that DC lacks 7-hydroxyl group, thus most likely forms fewer hydrogen bonds.
We assume that this spatial relation of the oxygen atom enables polar head (OE groups) of the co-surfactants to be oriented towards the α-axial hydroxyl groups of the OD steroid skeleton, since hydrophilic region of the co-surfactants is probably located at the micellar surface.
This spatial orientation of TRX is probably achieved through the water molecules, as seen in Figure 3. We assume that similar positioning is attained with SLES molecules, through the dipole interactions between the OE group distal to sulfate ion and the 7-oxo group of OD. According to the results, TRX shows stronger synergism with investigated bile salts than SLES. Apart from difference in hydrophobic region, this variation can be explained due to the smaller number of OE groups in hydrophilic region of SLES, forming fewer attractive interactions with the polar part of selected bile salts. Additionally, negatively charged sulfate ion of SLES creates repulsive interactions with carboxylate anion of selected bile salts, decreasing stability of mixed micelles.
We have studied a set of three bile salts (sodium cholate, sodium deoxycholate or sodium 7-oxodeoxycholate) to interact with Triton X-100 or sodium lauryl ether sulfate, by conductometric and tensiometric method. The results of the study have been analysed using Clint's, Rubingh's, and Motomura's theories for mixed binary systems. The critical micelle concentration of the ideal mixed micelle, the mole fraction of the more hydrophobic surfactant in the ideal mixed micelle, the mole fraction of the more hydrophobic surfactant in the real mixed micelle, and the β interaction parameter of the mixed micelles were calculated using experimental data.
Based on the calculated values of the β parameter we have concluded that TRX and SLES generate the weakest interactions with D, while OD creates the strongest interaction with investigated co-surfactants. Most probably, hydrogen bonds between α-hydroxyl groups of steroid skeleton and proton acceptor OE groups of the polar head of selected co-surfactants are formed and contribute to the synergism. Since D lacks one polar group compared to C and OD, it showed the weakest interactions.
We have also noticed that OD generates stronger synergism than C in all investigated binary systems. This is probably due to attractive interactions between 7-oxo group of OD and the oxygen atom of the phenoxy ring of TRX through the water molecules. This effect favours attractive intermolecular hydrophilic interactions and formation of hydrogen bonds.
It is assumed that attractive interaction between 7-keto group of OD and of the second OE group distal to sulfate group in SLES promotes formation of hydrogen bonds and stabilisation of the binary system.
Sodium 7-oxodeoxycholate was synthetized by selective oxidation of the 7α-hydroxy group of cholic acid, following the Tullar procedure . All bile acids were transformed to the sodium salt by known procedure .Cholic and deoxycholic acids were purchased from Sigma-Aldrich (Germany). The degree of purity is >99%. Triton X-100 was obtained from J.T. Baker (Holland). The degree of purity is >99%. Sodium lauryl ether sulfate having two oxyethylen groups was provided by Stepan Company, USA. Surfactant solutions were prepared by dissolving accurately weighed quantities of surfactants in requisite volumes of deionised water. Deionised water (conductivity < 1 μS cm-1, at 25°C) was used for all purposes.
4.2.1. Critical micelle concentration determination
The cmc values of pure C, D, OD and SLES were acquired through conductivity and surface tension measurements. The cmc value of TRX was obtained through surface tension studies only, since this surfactant is nonionic amphiphile and does not have influence on electric conductance. The critical micelle concentrations of all investigated binary mixtures (D-TRX, C-TRX, OD-TRX, D-SLES, C-SLES, and OD-SLES) were determined by means of conductometry and tensiometry.
4.2.2. Conductivity measurements
Conductivity measurements were carried out on aqueous solutions of pure C, D, OD and SLES. Aqueous solutions of mixtures of C, D or OD and one of two co-surfactants (TRX or SLES) were also analysed measuring conductivities. The cmc values of the binary systems were obtained for the different molar ratios of the components. Conductivities were measured by gradual dilution of surfactant solutions with the deionised water. The data were acquired using Consort C 860 conductometer. The cell containing solutions was immersed in a water bath, controlling the temperature variation at ± 0.1°C. The temperature was kept constant at 25°C. Trials were repeated three times for reproducibility. The cmc determination error did not exceed 3%.
4.2.3. Surface tension measurements
Surface tension measurements were carried out on aqueous solution of TRX in order to determine its individual cmc. Surface tension measurements were carried out on a Sigma 703D tensiometer (Finland) using a du Nouy ring method. All measurements were repeated three times. In all measurements temperature was kept constant at 25 ± 0.1°C. The cmc determination error did not exceed 3%.
This work was financially supported by the Provincial Secretariat for Science and Technological Development, AP Vojvodina, Republic of Serbia, Grant No.114-451-2113/2011-01
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