Thermodynamics and kinetics of vesicleºmixed micelle transitions of sodium tridecyl-6-benzene sulfonate/sodium dodecyl sulfate surfactant systems Ute Brinkmann,a Eberhard Neumanna and Brian H. Robinsonb,* a Faculty of Chemistry, University of Bielefeld, P.O. Box 100131, D-33501 Bielefeld, Germany b School of Chemical Sciences, University of East Anglia, Norwich, UK NR4 7T J When aqueous salt solutions ([NaCl]P0.02 mol dm~3) of the double-chain surfactant sodium tridecyl-6-benzene sulfonate (STBS) are titrated with solutions of sodium dodecyl sulfate (SDS) at low concentrations (BmM) of the two surfactants, mixed micelles of STBS/SDS are formed at the expense of STBS bilayer vesicles.The extent and the kinetics of the interconversion are readily followed by the change in light scattering (turbidity) of the system at j\300 nm. The data re—ecting the surfactant self-assembly/disassembly processes can be analysed in terms of phase-transition-like conversions between vesicles and mixed micelles.The conversions are characterised by the critical free concentration [Dc] of the single-chain surfactant required to eÜect STBS vesicle dissolution. For example, for addition of SDS at [NaCl]\0.05 M, the characteristic lyotropic transition constant is [Dc]\0.48^0.03 mM, and for ethanol [Dc]\0.60^0.05 M at 293 K (20 °C). The thermodynamic analysis also yields the critical ratio Rc of bound single-chain surfactant for vesicle dissolution A similar value can be obtained for the mixed (RV c ).(RM c ) micelle. For instance, when SDS is used, the values are and This means that within the transition region RVc \0.10 RMc \0.71. one bound SDS molecule per 10 STBS molecules eÜects vesicle dissolution and the mixed micelles which are formed following dissolution contain ca. 70 SDS molecules per 100 STBS molecules. The kinetics of surfactant-induced vesicle dissolution are associated with a highly cooperative surfactant/vesicle interaction which provides a lag phase followed by an exponential decay phase associated with vesicle disintegration/mixed micelle formation and characterised by a Hill coefficient of ca. 8. The nH overall rate constants for micelle formation from vesicles are dramatically dependent on the SDS concentration, decreasing from s~1 at mM to s~1 at mM. kexp\0.090 [SDST]\0.65 kexp\0.023 [SDST]\0.55 The surfactant sodium tridecyl-6-benzene sulfonate (STBS) forms micelles in aqueous solution in the absence of added salt (e.g.NaCl)1 with a critical micelle concentration of 1.5]10~3 M at 25 °C. However, in the presence of salt the concentration of surfactant at which aggregation takes place is dramatically reduced and vesicles rather than micelles form spontaneously. These vesicles have been characterised by a range of techniques including dynamic light scattering, video-enhanced microscopy, neutron scattering and cryoelectron microscopy.2 Typically the vesicle diameter is small at ca. 0.5 lm and vesicles are polydisperse. Somewhat larger vesicles and more complex structures form at higher concentrations, but at low surfactant concentration (ca. 1]10~3 mM) such as is used in this work, the vesicles appear to be essentially single walled (unilamellar). The benzene chromophore of STBS absorbs light at wavelengths j\280 nm, with an absorption coefficient which changes when micelles are formed. This forms the basis for measurement of the c.m.c.However, the transition from monomers/micelles to vesicles on addition of salt is readily monitored by the increase in turbidity at j\300 nm, which is associated with light scattering from vesicles. There is a ì critical œ salt concentration needed to eÜect the micelle%vesicle transition, which is [NaCl]B20 mM for STBS.1 However, more recent studies have indicated the possibility of ì hysteresis œ eÜects in these systems; i.e. the salt concentration at which the transition occurs is dependent on whether the transition is followed in the direction micelles]vesicles (driven by an increase in salt concentration) or vesicles]micelles (driven by a decrease in salt concentration).In this paper we propose a mechanism by which singlechain surfactants like SDS fragment vesicles at concentrations below the c.m.c. of SDS at the added salt concentration used. The onset of the vesicle%micelle transition, as the concentration of SDS is changed, is rather abrupt and indicates cooperative, phase-transition-like behaviour.Structural transitions between diÜerent forms of surfactant assembly are of importance in connection with drug targeting, 3,4 which may be carried out using liposomes (vesicles) of phospholipid (PL) mixed with single-chain surfactants.5h7 Such mixed PL/surfactant vesicles are also of interest for membrane electroporation,8,9 a technique which can be used to release material from –lled lipid vesicles or to –ll vesicles with a substance which has been added to a vesicle suspension.In this paper, we present a novel approach to characterise the thermodynamics and kinetics of the vesicle to mixedmicelle conversion in terms of a theory of cooperative phase transitions. Materials and methods Sodium tridecyl-6-benzene sulfonate (STBS) was specially puri–ed by Unilever, Port Sunlight Laboratory (a gift from Dr Peter Garrett) and was used as received. Sodium dodecyl sulfate (SDS) was obtained from Sigma ([99% pure).Sodium 1-octyl sulfonate (99% pure) and sodium 1-dodecyl sulfonate (99% pure) were obtained from Lancaster Chemicals, UK. Deionised water (Fisons) was used throughout. Preparation of STBS vesicles The following stock solutions were used: I STBS in water (10 mM), II stock solution of NaCl (1 M). The vesicles were prepared by mixing respective aliquots of each of the two stock solutions and water at 20 °C in two diÜerent ways. J. Chem. Soc., Faraday T rans., 1998, 94(9), 1281»1285 1281Procedure A.Mixing the stock solution of STBS (I) and water to appropriate concentrations of STBS and then addition of the stock solution of NaCl (II) to the –nal concentration. Procedure B. Mixing the stock solution of NaCl (II) and water to appropriate concentrations of NaCl followed by addition of the stock solution of STBS (I) to the –nal concentration. The solubilization experiments, by adding SDS, are carried out 3 h after mixing using procedures A and B to form vesicles.Typically, the formation of vesicles takes ca. 30 min, but there is a slow ageing process resulting in a small (ca. 5%) increase in turbidity over the following 150 min. After some hours, the appearance of fragments of lamellar phase, which results in an unstable turbidity signal, can sometimes be seen. However, on shaking, the lamellar —oc can be redispersed to reform simple vesicle structures. For both procedures, A and B, the same results are obtained for the transition curves with SDS.The turbidity q, in 180°-scattering mode, was determined by measurements of the absorbance or optical density (OD) at j\300 nm via q\OD/d, where d\1 cm is the optical path length. A Hewlett-Packard 8452A diode-array spectrophotometer with appropriate thermostating was employed. Absolute values of q were not determined using this instrument, which was used for kinetic measurements together with a Hi-Tech small-volume stopped-—ow system equipped with UV»VIS detection, the latter instrument being used to study the faster processes.Results A typical titration curve for the dissolution of STBS vesicles (initially at an STBS concentration of 0.5 mM) by addition of an equal volume of an aqueous solution of a single-chain surfactant is shown in Fig. 1. A dramatic decrease in turbidity occurs when the total concentration of SDS after mixing exceeds mM, i.e. at a total molar ratio [SDS]\[DT, V c ]\0.47 of SDS/STBS of 1.9.The turbidity values after each titration step were essentially unchanged over a period of up to one week after mixing. When the initial concentration of STBS used is higher, e.g. 1.5 mM, a somewhat higher concentration of SDS (ca. 0.54 mM) is needed to disrupt the vesicles. However, the amount of Fig. 1 Turbidity (q) at j\300 nm of an aqueous sodium tridecyl-6- benzenesulfonate (STBS) vesicle suspension as a function of the total concentration of sodium dodecyl sulfate (SDS) which shows the transition from vesicles to mixed STBS/SDS micelles ; [NaCl]\50 mM, T \293 K (20 °C), mM, 300 s after [LT]\[STBST]\0.25 (Ö): mixing, 6 days after mixing.(I) indicates the vesicle domain, (II) (K) : the coexistence of vesicles and micelles and (III) the domain, where only mixed micelles exist. The lines connecting the points are used (Ö) for the thermodynamic analysis ; (^0.03) mM, [DT, M c ]\0.57 (^0.03) mM. [DT, V c ]\0.47 SDS does not scale in proportion to the amount of STBS present ; the concentration ratio being reduced to 0.72 for a –nal STBS concentration of 0.75 mM.Various single-chain surfactants can be used to disrupt STBS vesicles. Table 1 shows that diÜerent single-chain surfactants and ethanol have diÜerent values of and where rep- [DT ,V c ] [DT, M c ], [DT, M c ] resents the concentration where only mixed micelles exist. It would appear that most other surfactants, which can be used as ìstructure-breakersœ of the vesicles, are less eÜective than SDS.The trend is essentially as expected from the hydrophobicity of the surfactant. For example, for sodium dodecyl sulfonate mM. Sodium n-octyl sulfonate [DT, V c ]\0.70^0.03 was ineÜective in disrupting vesicles up to an added concentration of 2.5 mM. Addition of ethanol is also eÜective in vesicle dissolution. The STBS vesicles are stable at \3.5% v/v ethanol»water, but breakdown is observed when the volume fraction of ethanol in the solvent exceeds 0.035, corresponding to a molar concentration of 0.6 M (Table 1).The kinetics of conversion of vesicles into micelles on addition of SDS was measured according to the mixing scheme given by eqn. (1). The decay in the turbidity in Fig. 2 indicates that dissolution of vesicles is taking place. It is also seen that the rate of the vesicle dissolution process is strongly dependent on the Table 1 Critical concentrations and for the solu- [DT, V c ] [DT, M c ] bilization of a 0.25 mM STBS-vesicle suspension in 50 mM NaCl by diÜerent single-chain surfactants and ethanol at 293 K (20 °C).At mM, the critical concentrations are mM [STBST]\0.75 [DT, V c ]\0.54 and mM for addition of SDS. [DT, M c ]\0.80 single-chain surfactant [DT, V c ]/mM [DT, M c ]/mM sodium dodecyl sulfonate 0.70^0.03 1.00^0.03 sodium octyl sulfonate [2.50 ethanol (cosolvent) 600^0.05 1120^0.05 Fig. 2 Turbidity (q) as a function of time after mixing an STBS vesicle solution with SDS in 50 mM added NaCl.mM [SDST]\0.55 0.60 mM 0.65 mM T \293 K (20 °C). The initial (=), (K), (Ö), q0 value and the values refer to the kinetic analysis ; represents the q= q= turbidity of the mixed micelles and that of the vesicles within the q0 transition range. 1282 J. Chem. Soc., Faraday T rans., 1998, V ol. 94Fig. 3 Turbidity (q) as a function of time, after mixing SDS/salt solutions of diÜerent concentrations, 0.45 mM 0.50 mM and 0.55 (Ö), (K) mM concentration), with an aqueous STBS solu- (=) ([SDST]\ritical tion (without salt), indicating the initial formation of vesicles and the subsequent transition from vesicles back to small aggregates (mixed micelles).mM, [NaCl]\50 mM, T \293 K (20 °C). [STBST]\0.25 total concentration of SDS in the range of [SDST]\ M. An increase in enhances the rate of 0.55»0.65 [SDST] vesicle dissolution considerably. Approaching the transition region the kinetics are associated with a sigmoidal onset phase, which is also shown in Fig. 2. The interactions of SDS monomers with STBS monomers, measured according to the following mixing scheme given by eqn. (2) are re—ected in two kinetic phases (Fig. 3). Fairly rapid vesicle formation is followed by a slower dissolution of the vesicles and formation of mixed micelles. An increase in accelerates both vesicle formation as well as the fol- [SDST] lowing mixed micelle formation process. These data do not relate particularly well to the ìequilibriumœ diagram as shown in Fig. 1 because of possible hysteresis eÜects, which will be considered in more detail in a separate paper. Discussion and theory The data points as presented in Fig. 1 may be connected by straight lines. The experimental results are consistent with phase-transition-like conversions between STBS-vesicles and STBS/SDS mixed micelles. In an ìall-or-noneœ transition model, the double-chain surfactant STBS is either part of a vesicle membrane phase or part of a mixed micelle.Mass conservation for the added single-chain surfactant D dictates that the total concentration is given by [DT] [DT]\[D]][Db] (3) where [D] is the concentration of free SDS in the solution and [Db]\[Db, V]][Db, M] (4) speci–es the concentrations which are bound to the vesicle and in mixed micelles respectively. [Db, V] [Db, M] The surfactant interactions are analysed in terms of a binding ratio R, analogous to that used in ref. 10»12.Here, however, we have to refer to such that : LV R\ [Db] [LV] (5) where and is the concentration of [LV]\([LT][c.v.c.), vesicle-bound STBS, the total concentration of STBS [LT] and c.v.c.\0.12 mM, where the c.v.c. is the critical vesiculation concentration of STBS at an added salt concentration of 50 mM NaCl 1 at 20 °C. Note that the c.v.c. is of the same order of magnitude as [LV]. The onset of vesicle dissolution at as the single- [DT, V c ], chain surfactant is added, is characterized by the critical ratio Rvc\[Db, V c ]/[LV] (6) and complete vesicle dissolution occurs at character- [DT, M c ], ized by the critical binding ratio : RMc \[Db, M c ]/[LV] (7) For dilution experiments, is the total concentration for [DT, M c ] the onset of mixed micelle dissolution and vesicle formation.In the transition range between and the R [DT, M c ] [DT, V c ] value is given by12 Rtrs\bMRMc ](1[bM)RV c (8) where is the fraction of mixed micelles. bM It is found that and depend linearly to a –rst [DT, M c ] [DT, V c ] approximation, on as shown in Fig. 4. Applying mass [LV], conservation to both experimental quantities we obtain [DT, M c ]\[Dc]]RM c [LV] (9) and [DT, V c ]\[Dc]]RV c [LV] (10) Data evaluation according to eqn. (9) and (10) yields the critical ratios (for dissolution) of vesicles and of mixed RVc \0.10 micelles respectively and the characteristic free sur- RMc \0.71, factant concentration as the lyotropic constant [Dc]\0.48^0.03 mM SDS at 50 mM NaCl and T \293 K. Usually, the concentration of surfactant [Db, V c ]\RV c [LV] bound to the vesicle before vesicle dissolution starts is very Fig. 4 The critical dissolution (total) concentrations both [DT c ], (\), and of STBS/SDS mixed micelles and vesicles, [DT, M c ] [DT, V c ] (Ö), respectively, as a function of the vesicular STBS concentration where c.v.c.\0.12 mM.1 A data evaluation [LV]\[STBST][c.v.c., according to eqn. (9) and (10) yields the critical ratios for dissolution of vesicles of and of mixed micelles respectively.RVc \0.10 RMc \0.71, The characteristic free surfactant concentration the lyotropic constant, [Dc]\0.48^0.03 mM SDS at 50 mM NaCl and T \293 K (20 °C). J. Chem. Soc., Faraday T rans., 1998, V ol. 94 1283small. For the case we may approximate [DT, V c ]A[Db, V c ], eqn. (10) by: [Dc]\[DT, V c ][[Db, V c ]B[DT, V c ] (11) to roughly characterize the phase behaviour of a mixed surfactant system.Data representation in terms of eqn. (9) and (10) yields the critical free surfactant concentration [Dc] and the slopes are the respective Rc values from which we obtain : [Db, M c ]\[LV]RM c and [Db, V c ]\[LV]RV c (12) where, for instance, at mM and c.v.c.\0.12 [STBST]\0.25 mM, mM. [LV]\[STBST][c.v.c.\0.13 For SDS we have [Dc]\0.48^0.03 mM (Fig. 4). Compared with mM, mM [see [DT, V c ]\0.47^0.03 [Db, V c ]B0.013 eqn. (12)] which is indeed small relative to [DT, V c ]. The data analysis provides us with the average numbers of single-chain surfactant D molecules adsorbed onto or inserted into the vesicle membranes and in mixed L/D micelles, respectively [Fig. 5(a)]. Within the transition region the molar ratios and are the ratios of the D and L molecules per RV c RM c average vesicle and mixed micelle, respectively. It is apparent [Fig. 5(b)], that, within the transition range, the free concentration [Dc] of the single-chain surfactant is a constant.[Dc] is a characteristic quantity, the lyotropic transition constant, for every single-chain surfactant D/STBS system. For instance, at mM mixed with SDS, the [STBST]\0.25 characteristic free concentration of SDS is [Dc]\0.48^0.03 Fig. 5 Thermodynamic analysis of the turbidity for the titration q300 of STBS with single-chain surfactant D in terms of (a) the binding ratio as a function of the total surfactant concentra- R\[Db]/[LV] tion and of (b) the concentration of bound surfactant as a [DT] [Db ] function of free surfactant concentration [D] (see text and Fig. 1). Note that this data representation clearly visualizes the –rst-order phase transition characterized by the lyotropic transition constant [Dc]. mM. Within the transition range any further addition of D does not change the free concentration [Dc], but rather it leads to more binding of D to increase the proportion of mixed micelles at the expense of the vesicle bilayer structures. Note that [Dc] characterizes the lyotropic phase transition equilibrium vesicle%mixed micelle in an analogous way to the transition temperature of a –rst-order thermal phase transition.Kinetic analysis For the kinetic analysis the data in Fig. 2 are replotted according to a –rst-order decay: lnAq=[q q=[q0B\kexp t (13) where denotes the rate constant for the system (Fig. 6). kexp Both a lag phase and an exponential phase are clearly visible. Obviously, the formation of mixed micelles (M) is preceded by an interaction phase of SDS surfactant monomers (D) with vesicles (V) leading to adsorption/absorption according to the scheme: nD]VH K VDn»»’ kM M (14) The rate equation for mixed micelle formation is d[M] dt \kM[VDn] (15) where is the rate constant for the last step of mixed micelle kM formation from a mixed surfactant vesicle.If the concentration of the intermediate is determined by a rapid pre-equilibrium involving cooperative single-chain surfactant interactions with the vesicle, we may apply the approximation of the Hill model of cooperativity and obtain : [VDn]\[VDn]max [D]n [D]n]Kn (16) where Kn is the equilibrium constant for the reactions which are associated with the lag phase and which do not have a signi–cant light scattering change associated with them. On insertion of eqn.(16) into eqn. (15) the experimental rate constant kexp\kM[VDn] [D]n [D]n]Kn (17) is obtained. For the three SDS concentrations used, the kexp values are s~1 at mM, kexp\0.09 [SDST]\0.65 kexp\0.045 s~1 at mM and s~1 at [SDST]\0.60 kexp\0.023 [SDST]\ mM. 0.55 Fig. 6 The relative turbidity (logarithmic scale) as a function of time, replotted from Fig. 2 for the kinetic analysis of the dissolution process of STBS vesicles at diÜerent SDS concentrations, 0.55 mM 0.60 (=), mM and 0.65 mM according to eqn. (13). Linear –tting yields (K) (Ö) the lag time and the rate constants s~1 tlag kexp\0.023 (=), kexp\ s~1 and s~1 0.045 (K) kexp\0.090 (Ö). 1284 J. Chem. Soc., Faraday T rans., 1998, V ol. 94Fig. 7 Formal kinetic data analysis according to eqn. (18), providing the Hill coefficient nH\8 Eqn. (17) shows that, for low [D] values, i.e., if [D]@K, increases parabolically with [D]. The Hill coefficient kexp nH can be derived for these conditions from a single plot since : log(kexp/s~1)\nH log([D]/c0)]logC(kM/s~1)([VDn]max/c0) (K/c0)n D (18) where c0\1 mol l~1. Note that in Fig. 7, Data [D]B[DT]. analysis with eqn.(18) yields This means that at least nHB8. eight bound single-chain surfactant molecules have cooperated to disrupt an STBS vesicle. So far, the analysis refers to the exponential part of mixed micelle formation according to [M] [M]max \1[exp[[kexp(t[tlag)] (19) In this paper, the lag phase is not analysed in more detail. However, it can be noted that the lag time also depends on [SDS]: s at 0.65 mM SDS, s at 0.60 mM SDS tlag\6 tlag\27 and s at 0.55 mM SDS. tlag\115 The data from Fig. 3 suggest that in a mixture of SDS with STBS, STBS vesicle formation is more rapid than mixed micelle formation.However, the thermodynamically more stable structure is the mixed micelle, so that the STBS vesicles are transient metastable structures under the conditions applied. In summary, the onset of vesicle dissolution is thermodynamically speci–ed by (such that one in ten of the RVc \0.10 molecules in the unstable vesicle are single-chain) and kinetically by Both values suggest vesicle dissolution nH\8.by single-chain surfactants like SDS initially involves adsorption and insertion of SDS into the STBS vesicles up to a critical value The cooperative SDS aggregate is at least 8 RV c . single-chain surfactant molecules. At [D]O[Dc], local micellar domains may be formed within the vesicle membrane, which then at [D]\[Dc] become mixed micelles. Similar structures of lipids and detergents have been postulated for the disruption of natural membranes. 13 thank Dr Peter Garrett (Unilever, Port Sunlight We Laboratory) for supplying us with a puri–ed sample of STBS, Anke Gieselmann for excellent technical help and EPSRC for provision of laboratory equipment and other resources. Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged (DFG-grant Ne 227/9-2 to E. N.). Ute Brinkmann thanks the University of Bielefeld for support during a stay in Norwich when the experimental work was carried out (grant 21940/39 to E.N.).Abbreviations c.m.c. : critical micelle concentration c.v.c. : critical vesiculation concentration [DT] : total concentration of single-chain surfactant [Db], [Db, V], [Db, M] : concentration of bound single-chain surfactant, bound to vesicle and bound to micelles [Dc] : critical concentration of free singlechain surfactant (lyotropic transition constant) [DT, V c ] : critical total concentration of singlechain surfactant for the onset of vesicle dissolution [DT, M c ] : critical total concentration of singlechain surfactant for the onset of mixed micelle formation [LT] : total concentration of double-chain surfactant (STBS) [LV] : concentration of double-chain surfactant, bound in vesicles nH : Hill coefficient PL: phospholipid RV c : critical binding ratio of single-chain surfactant to STBS in vesicles RM c : critical binding ratio of single-chain surfactant to STBS in micelles SDS: sodium dodecyl sulfate (single-chain) STBS: sodium tridecyl-6-benzene sulfonate (double-chain) q: turbidity References 1 K.D. 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Paper 7/08798E; Received 8th December, 1997 J. Chem. Soc., Faraday T rans., 1998, V ol. 94 1285