J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2071-2076 207 1 Structure and Transport in Concentrated Micellar Solutions with a Lower Consolute Boundary J. M. Keller and H-D. Ludemann Universitat Regensburg, lnstitut fur Siophysik und Physikalische Biochemie ,8400 Regensburg, Germany Gregory G. Warr* Department of Physical and Theoretical Chemistry, The University of Sydney, NSW 2006,Australia Self-diffusion coefficients and viscosities of aqueous solutions of dodecyltributylammonium bromide (C,,NBu,Br) have been measured as a function of concentration and temperature by pulsed-field-gradient 'H NMR and couette viscometry. These solutions form no liquid-crystalline phases, remaining isotropic at all com-positions, but demix above a lower critical temperature of 58°C between 8 and 80 wt.% solution.Diffusion results are first interpreted using the conventional approach in which the partition of both surfactant and water between bulk and micelle bound states is examined. An alternative approach to self-diffusion in concentrated surfactant solutions incorporating the viscosity results has proven more fruitful, revealing a change in solution structure near the high-concentration side of the two-phase body. Based on this a mechanism for the phase separation involving hydration of the surfactant is proposed and compared with previous results on this and related systems. The usual sequence of self-assembly phases encountered with increasing concentration in cationic surfactant systems such as dodecyltrimethylammonium bromide is micellar (spheres and/or rods), hexagonal, (cubic) lamellar.This is understood to be due to changes in the surfactant packing parameter, u/uol,, where u is surfactant tail volume, a, is the area per molecule at the aggregate surface and-I, is the fully extended chain length of the surfactant tail. As concentration is increased, so is the ionic strength, leading to more effective screening of the electrostatic interactions between head groups which largely determine uo. However, when the tri- methylammonium head group is replaced with a bulky, hydrophobic tributylammonium group, this sequence is inter- rupted.' Steric interactions between tributyl head groups within the same micelle constrain the surfactants to form spherical micelles at all concentrations.The result is a solu- tion that contains no liquid-crystalline mesophases, but only a single, isotropic solution from the c.m.c. up to at least 90 wt. % surfact ant. Small-angle neutron scattering (SANS) has confirmed that only small, spherical micelles are present in dodecyltributyl- ammonium bromide (C,,NBu,Br) solution between 1 and 80 wt.% over a wide temperature range.2 At moderate concen- trations and elevated temperatures (>58 "C in H,O or 48 "C in D20) the solution displays an unanticipated demixing into two conjugate solutions ca. 10 and 80 wt.% in surfactant. SANS spectra display the characteristics of long-range attrac- tions between micelles near the two-phase region. Recent modelling of the scattering spectra3v4 using a dispersion force interaction has shown an increase in the magnitude of the attractions with increasing temperature.This is hardly sur- prising, as lower consolute behaviour can scarcely arise from anything but an increasing attraction of some kind. The diffi- culty lies in the interpretation of this as an effective potential between micelles, without really understanding the mecha- nism. The same question has dogged non-ionic systems, which undergo a similar demixing on warming. Although they also have the added complication of asphericity, the molecular origin of the effective intermicellar attraction is still a subject of C,,NBu,Br solutions are thus of interest both for their existence as concentrated dispersions of spherical micelles and for the interactions which lead to lower consolute behav- iour.Both SANS and preliminary diffusion coefficient measure- ments by NMR have shown that the monomer concentration in C, ,NBu,Br systems increases with increasing total sur-factant concentration above the c.m.c.,p9 The resultant screening of electrostatic interactions between micelles by the equilibrium monomer solution nullifies electrostatic stabiliza- tion of the dispersion at concentrations above CU. 30 wt.%. Above this concentration the Debye length is only a few A. The result is that a short-range attraction is sufficient to cause the phase separation. (Note that steric interactions within micelles provided by the butyl arms about the quat- ernary nitrogens still maintains the sphericity of the micelles.) The monomer behaviour explains why such an attractive interaction can play such an important role in the phase behaviour of C,,NBu,Br solutions, but not its origin.In this study we have used pulsed-field-gradient NMR to measure the self-diffusion coefficients of both surfactant and water in C,,NBu,Br solutions over a wide range of tem- peratures and compositions in order to examine structural changes in solution which may be masked in SANS or QELS studies" by long-range concentration correlations near the phase boundary. Solution viscosities have also been deter- mined in order to provide further information on solution structure, and a basis for temperature scaling of the diffusion.Experimental Dodecyltributylammonium bromide (C ,NBu,Br) was pre-pared by reaction of bromododecane (Aldrich) with tri-n- butylamine (Aldrich) in acetonitrile. The crude product, which is extremely hygroscopic, was recovered by rotary evaporation followed by freeze drying. This was then purified by recrystallization from acetone-ether, and stored in a desic- cator out of the light. Viscosities were measured in a Deer constant stress rheo- meter using Couette geometry. The shear stresses were in the range 0.01-2.0 Pa, corresponding to shear rates of between 0.5 and 200 s-'. The maximum shear rate provides a guide to the shortest timescale probed, which in this study was 0.01 s. Samples in the rheometer were jacketed and thermostatted to within 0.2"C.All solutions studied were found to be Newto- nian over the entire range investigated. Self-diffusion coefficients of the solutions were studied in glass capillaries with inner diameter 1.5 mm and outer diam- eter of 7 mm. Details of the apparatus" and the filling procedure' have been published previously. The self-diffusion coefficients were obtained in a Bruker MSL-300 spectrometer operating at a proton frequency of 300.1 MHz in a home-built probe head in a Hahn spinwxho pulse sequence by the pulsed-field-gradient method introduced by Stejskal and Tanner.13 In the presence of the field pulses, the decay of the echo amplitude A is given by ~(2z)= A(o)~xP(- WG)~XPC-((Y~~)~D(A (1)-8/31] where z is the time between the 90" and 180" pulses, D the self-diffusion coefficient, 6 the duration of the gradient pulse, A the time between the two gradient pulses and g the gra- dient strength, given by g = kl.Here I is the current intensity and k the coil constant, which is obtained from the known diffusion coefficient of water at ambient pressure and 298 K14 and from benzene data, known from tracer measurements.' In all experiments z was between 100 and 300 ms, this corre- sponding to the time over which diffusion is probed. D was determined by recording 10-15 spin+choes with increasing g values while holding all other parameters con- stant. Subsequent Fourier-transformation of the echoes facili- tates the analysis of the data and permits detection of impurities.The measured diffusion coefficients are regarded as reliable to f5%. Results and Discussion Fig. 1 shows the self-diffusion coefficients of water and C12NBu3+ ion as a function of composition at various tem- peratures. Results at all temperatures and compositions studied are listed in Table 1. These results agree with prelimi- lo-* (a) .-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 nary ~tudies,~ with both solute and solvent diffusion coeffi- cients decreasing as the surfactant content increases. The water self-diffusion coefficient decreases gradually across the composition range examined, whereas that of the surfactant decreases rapidly up to ca. 10 wt.%, then levels off and decreases only slowly with further increase in surfactant con- centration. All C12NBu3Br solutions demix on warming to above 60°C between 10 and 80 wt.% surfactant.Diffusion coefficients of both surfactant and water increase smoothly with increasing temperature as the phase boundary is approached (Table 1). It is usual to treat self-diffusion in surfactant solutions by considering the distribution of material between micellar and monomer states, together with diffusion in each of these states. In the analysis which follows we first take this approach, but finding the information obtained to be limited we also examine the effect of intermolecular interactions on the concentration and temperature dependence of the self- diffusion coefficients using a more general approach which incorporates solution viscosities.Based on these results, we propose a mechanism for the observed phase separation which is consistent also with previous data. Self-diffusionin Micellar Solutions :Two-state Model The measured self-diffusion coefficient of a surfactant in micellar solution is an average of its diffusion coefficients in the micellar and monomer states : D(Cl,NBu,) = f'mic 'mic(C12NB~J + (1 -f'mic)Dmonorner(C12NB~3) (2) where Pmicis the (mole) fraction of solute present in micelles and 1-Prnic is the fraction present as monomer.16 The diffusion of water in aqueous solutions has also been analysed using this two-state approach.' 7*18 Some fraction of the water is regarded as being bound to or associated with the micelle, f'hund, and is constrained to diffuse with the micelle.The rest, l-f'bund, is all viewed as free or bulk water. Thus we also have D(H20) = Pbound Dhund(H20) + (l -'bo~nd)~free(~2~)(3) Dfree(H20) and 'rnonorner(C12NBu3) in eqn. (2) and (3)describe the diffusion of small molecules in an environment containing various obstructions, which in this case are mono- meric surfactant molecules and micelles. Obstructions are a v) simple way of introducing interactions into diffusion behav- .^g3 10-12Iiour within a liquid which has been widely applied to micellar wt.% surfactant solutions. Jonsson et al. have made an extensive analysis of the self- diffusion of small molecules in colloidal systems' incorpor-ating the effect of large, essentially stationary obstructions like micelles.Large, spherical obstructions were found to reduce the diffusion coefficient of small molecules as Di = D,J(l + 4/2), where 4 is the volume fraction of obstructing particles. This agrees well with Monte Carlo simulations of diffusion in hard-sphere systems, but experimental results for model colloidal dispersions have shown that Difalls away more steeply than predicted above 4 = 0.2.'77'9 This expres- sion is independent of the size of the obstructions, although other forms apply for non-spherical species. 100 In these surfactant solutions, a two-state model including wt.% surtactant obstructions may provide a measure of changes in water Fig. 1 Self-diffusion coefficients of (a) C,,NBu,+ and (b)water as a structure with temperature, which has been postulated as the source of lower consolute behaviour in surfactants systems.function of concentration determined at various temperatures : .,20°C; 0,30°C; +, 40°C; 0,50°C; A, 60°C; A, 70°C; 0,80°C; Because the diffusion of CI2NBu3 is much slower than that 0,90"C; x ,100"C.Lines are included as guides to the eye. of water at all concentrations measured we neglect the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Diffusion coeficients of dodecyltributylammonium bromide-water mixtures‘ ~ ~~ surfactant : concentration (wt.%) T/”C 0.5 1.9 5 20.1 30.4 46.3 64.4 80.5 20 2.33( -10) 9.94( -11) 7.95( -11) 1.98(-11) 21.5 1.29(-11) 9.9q -12)25 2.w-10) 1.1q- 10) 30 2.84(- 10) 1.28( -10) 1.q- 10) 4.17( -11) 2.83( -11) 1.88(-11) 1.q- 11) 7.77( -12) 34 2.49( -11)40 3.43( -10) 1.63(-10) 1.26(-10) 5.46( -11) 3.91(-11) 3.35( -11) 2.59( -11) 1.25(-11) 50 4.07(-10) 2.w-10) 1.58(-10) 6.9q -11) 5.q-11) 4.q -11) 3.77( -11) 1.94(-1I) 60 4.94(- 10) 2.58( -10) 1.9q- 10) 8.91(-11) 6.98(-11) 5.94( -11) 5.06( -11) 2.92( -11) 61 7.21(-11) 62 7.45(-11) 6.06(-11)63 7.52(-11) 5.45(-11) 64 9.77( -11) 7.61( -11) 5.51( -11) 65 1.00(-10) 7.72(-11) 66 1.03(-10) 8.35(-11) 5.9q -11) 70 6.09- 10) 3.2q-10) 2.361-10) 4.38(-11) 80 7.95( -10) 4.2q-10) 2.82( -10) 5.89( -11) 90 1.08( -09) 5.77(-10) 3.55( -10) 8.26(-11) 100.5 1.58( -09) 8.41(-10) 4.48( -10) 1.14(-10) water: concentration (wt.%) T/“C 0.5 1.9 5 20.1 30.4 46.3 64.4 80.5 10 1.5q-09) 20 2.02( -09) 1.9q-09) 1.53( -09) 1.1q -09) 1.08(-10) 21.5 7.6q -10) 3.92(-10) 25 2.3q -09) 30 2.59( -09) 2.42( -09) 1.87( -09) 1.47(-09) 9.63(-10) 5.01( -10) 1.73(-10) 34 1.13(-09) 40 3.22(-09) 2.35( -09) 2.35( -09) 1.86(-09) 1.29(-09) 6.73( -10) 2.38( -10) 45 3.56( -09) 50 3.91( -09) 3.69( -09) 2.92( -09) 2.32( -09) 1.w-09) 8.52( -10) 3.24( -10) 53.5 1.73( -09) 55 2.52( -09) 1.78(-09) 9.55(-10) 56 1.83( -09) 9.9q -10) 57 2.62( -09) 1.87(-09) 1.02(-09) 58 2.73( -09) 1.87( -09) 1.04(-09) 59 1.94(-09) 1.w-09) 60 4.68(-09) 4.47( -09) 3.53( -09) 2.8q -09) 1.97(-09) 1.09(-09) 4.9q -10) 61 2.86( -09) 1.12(-09) 62 2.89( -09) 2.02( -09) 63 2.94( -09) 1.13( -09) 64 3.74( -09) 2.99( -09) 9.41( -10) 65 3.87( -09) 3.18( -09) 1.04(-09) 66 3.q -09) 67 3.47( -09) 70 5.45(-09) 5.5q -09) 6.3q -10) 80 6.29( -09) 6.13( -09) 8.15( -10) 90 7.42( -09) 1.06(-09) 100.5 8.10(-09) 8.41( -09) 1.49( -09) Numbers in parentheses are exponents.motion of these obstructions as both monomer and micelle In our previous investigation of this system, a hydrophobic and use the following form of eqn. (3) molecule was solubilised into the micelles, providing a tracer for their diff~sion.~ Whilst helpful at low concentrations, the probe was found to diffuse faster than the surfactant at high concentrations. This was attributed to solute transport by a jump mechanism between colliding micelles, due to the where D0(H20) is the diffusion coefficient of bulk water” strong attractions between them. It is therefore not possible and Dbound(H20) is the experimental diffusion coefficient of to measure a reliable value for Dmic in these solutions.In the surfactant at the composition and temperature of interest. order to obtain an estimate for Pmicwe have assumed that $ is the volume fraction of surfactant (in either monomer or micelles are stationary, or at least move so slowly as to act micelles) and is approximated by the weight fraction. Values only as obstructions to the diffusion of the monomer, i.e. of Pbound calculated from eqn. (3’) are shown in Fig. 2. Over is almost con- D(C12NBU3) = (l -Pmic)D0(C12NBu3)/(1 + $/2) (2’)the entire temperature range studied Pbound stant, decreasing only slightly with increasing temperature.where Do(C,,NBu3) is the diffusion coefficient of monomeric There is no major change in the amount of bound water as surfactant at infinite dilution, based on measurements made the consolute boundary is approached. below the c.m.c. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.00, i AA AAAAI I -0.80 z)c 0 0 0 3 'D 0.60 C 30 * * * * * I)* 0.40- 0.-Y i,2 w- I0.20 C c z T O .a. .a .. ,a, TrC Fig. 2 Fraction of bound water in C,,NBu,+ solution, as a func- tion of temperature for various concentrations of surfactant (wt.%): a,5.0;0,20.1; +,30.4; 0'46.3; A, 64.4;A, 80.5 This assumption is probably reasonable in concentrated solution, but less so in more dilute systemsg Surfactant diffu- sion in concentrated solutions is addressed in more detail below.Like Phund,we find that Pmicis temperature indepen- dent within experimental uncertainty. Pmicand Phundas a function of solution composition at 30 "C are plotted together in Fig. 3. These results are representative of those obtained at all temperatures examined (cf. Fig. 2). Note the qualitatively different behaviour of the two components : the surfactant is partitioned mainly into micelles even at very low concentra- tions, whereas with water the binding of water to the micelles is more gradual. Nevertheless, even as low as 20 wt.% sur- factant over 40% of the available water is apparently bound to surfactant. At 80 wt.% solution there is essentially no unbound water according to this interpretation.An 80 wt.% surfactant solution corresponds to a surfactant : water mole ratio of 1 :6, which is less than enough water to hydrate a tributylammonium group fully.2 At such high surfactant concentrations there is no 'bulk' water, so conclusions based on binding and obstruction of water molecules are rendered meaningless. Similarly, the divi- sion of diffusion into free and bound components becomes less appropriate as surfactant concentration increases. When there is no bulk water in the solution, through what do we imagine surfactant monomers and water molecules now diffuse? Whatever the medium is, it is not the one we chose as our reference point for Do(C12NBu,), so little can be con- cluded about the structure of concentrated solutions from this approach.We therefore take an alternative approach to 1.oo A t IA A A A0.75I A A U A -23 0.50 C .-A4-2 0.25-A I0.00 0 25 50 75 100 surfactant (wt.%) Fig. 3 Fraction of bound water (A) and fraction of micellised sur- factant (A) in C,,NBu3+ solution, as a function of concentration of surfactant (wt.%) at 30 "C diffusion process, examining the interactions affecting D(C,,NBu,) and D(H20). Diffusion in Concentrated Interacting Systems In hydrodynamics it is usual to deal with gradient diffusion processes, which for a solution of components i,j, k, . . . has a unique mutual diffusion ~oefficient~~-~~ (4) where q is the viscosity, Rh the hydrodynamic radius, 4 the volume fraction, and pi the chemical potential of species i, providing the chemical potential gradient or 'thermodynamic force' for diffusion.K(i$, t) is a mobility coefficient which describes the hydrodynamic interactions in the system.2s Self- diffusion then is the motion of a tracer molecule (i$tracerz 0) identical with other molecules. It thus generates neither chemical potential gradient nor backflow or pressure gra- dient by its motion, and hence self-diffusion coefficients are largely independent of thermodynamic interactions in the system, except through K(i$,t). In general the self-diffusion coefficient of a component of a solution at a particular concentration i$ may therefore be written as Di = DoiKi(i$, t), where Doi = k, T/67rqRh is an infinite-dilution or Stokes' law diffusion coeficient.Each tracer particle i has its own unique mobility function Ki. The general problem of the calculation of Di or Ki remains unsolved, although for model hard-sphere dispersions good agreement has been found between theory and experiment for short time diffusion coefficient^.^^^^^ However, theory and experiment for long-time diffusion coefficients differ substan- tially even for these model system^.^' As it is the long-time diffusion result measured in this study, we must therefore confine ourselves to a less quantitative interpretation. The ratio of water to surfactant self-diffusion coefficients is W2O) -RlI(C12NBU3) K(H2O) D(C12NBu3) -Rh(H20) K(C12NBU3) (5) At infinite dilution this measures the radius ratio of the entities present.From previous work2 the radius of C,,NBu,Br micelles is known to be between 18 and 20 1$ throughout the single-phase region, giving a micelle :water radius ratio of 15-18, depending on the extent of hydration of the micelles. For monomeric C12NBu3 this ratio should be around 5-8; however, this would only be evident for experi- ments below the c.m.c. of the surfactant. Fig. 4 shows D(H20)/D(C12NBu,) over the full range of solution composi- tions and temperatures examined. As the concentration tends 20 40 60 80 '1 10 surfactant (wt.%) Fig. 4 Ratio of self-diffusion coefficients, water : C12NBu,+, as a function of concentration determined at various temperatures : .,20°C; 0,30°C; +, 40°C; 0,50°C; A, 60°C; A, 70°C; 0,80°C; 0,90"C; x ,100"C.Lines are included as guides to the eye. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to zero, D(H,O)/D(C,,NBu,) decreases steeply towards ca. 15, then dipping suddenly to between 5 and 10 at 0.5 wt.%, consistent with expectations. The results at different tem- peratures all converge at low concentration, which is also expected since molecular sizes are independent of tem-perature. As surfactant concentration is increased two effects are observed. First, the D(H20)/D(C12NBu3) ratio increases and goes through a maximum near the critical composition of 46 wt.%.2 Secondly, the magnitude of this maximum decreases by almost 50% as temperature increases from 20 to 60°C.At even higher concentrations D(H20)/D(C, ,NBu,) again decreases at all temperatures and ventures into the upper range of the anticipated molecular size ratio. Compare Fig. 4 with the expected behaviour for a colloidal disper~ion.'~*'~?~~As the volume fraction of dispersed phase increases, diffusion coefficients of both the particles and solvent decrease until the particles become immobile at a volume fraction of 0.55. At this point the solvent diffusion coefficient is still at least half of its infinite-dilution value and the ratio, if plotted as in Fig. 4, would increase with volume fraction and diverge at 4 = 0.55. The present system differs from a colloidal dispersion in two ways: at low concentra- tions there is a changing partition of surfactant between monomer and micelle, causing the initial rapid rise in D(H20)/D(C1,NBu3).At higher concentrations the dynamic nature of the micelles permits enhanced diffusion of sur-factant by an exchange mechanism not available to the water.' This leads to the observed decrease in D(H20)/D(Cl 2NBU3). It is known from previous studies that there is an effective attractive potential between micelles which increases as the lower consolute boundary is approached.2 Without yet addressing the mechanism of attraction, it can be concluded that the frequency of sticky collisions between micelles must increase with temperature, and we would expect monomer transfer between micelles to increase, increasing D(C12NBu,) relative to that of water. This reduces D(H2O)/D(Cl2NBu,) with increasing temperature, as is observed (Fig.4). A more direct measure of interactions in solution may be obtained by combining diffusion coefficients of each com- ponent i of the solution with temperature and viscosity by comparing values of Diq/T. This quantity should reflect only changes in the (mean) size of the units, and their interaction potential through Ki.The results of Fig. 4 and a previous SANS investigation2 both suggest that micelle sizes are nearly constant up to at least 65 wt.% and are independent of temperature. Fig. 5 shows the reduced viscosity, qlq,,,,, ,of C,,NBu,Br solutions as a func- tion of concentration and temperature. Viscosity increases with concentration, as would be expected for any dispersion.For moderately concentrated solutions, the reduced viscosity decreases as temperature is increased, again suggesting the influence of micellar dynamics. Dq/T for both surfactant and water at various tem-peratures as a function of composition is shown in Fig. 6. Data for pure water are interpolated from ref. 20. Curves at all temperatures have a characteristic shape for each of the two species. For the surfactant the data at all temperatures lie on a master curve within experimental error, whereas for water trends with temperature remain evident, especially at high surfactant concentrations. For the surfactant [Fig. qa)] in the low-concentration region, Dq/T decreases with increas- ing concentration as the surfactant redistributes itself weight fraction surfactant Fig.5 Viscosity of C,,NBu,Br solutions as a function of concen- tration and temperature: W, 20°C; 0,30"C; +,40°C; 0,50"C; A, 60"C;A, 70 "C;a,80 "C. Lines are included as guides to the eye. between monomer and micelles. Above around 5 wt.% sur- factant, Dq/T increases monotonically with concentration at all temperatures studied, and any temperature trends are obscured by experimental scatter. This indicates a common structural progression with concentration for the surfactant aggregates, and that water plays the major role in determin- ing the temperature behaviour. In contrast with the surfactant behaviour, Dq/T for water [Fig. qb)] increases with surfactant concentration to a maximum at around 65 wt.%, before decreasing again.This decrease signals a change in R,(H,O), or more probably K(H,O), and hence in the solution structure below the two- phase body which has not previously been recognised. The temperature dependence of Dq/T of water, which can be seen from the spread of the data at a particular surfactant concentration in Fig. qb),is also revealing of solution struc- ture. Dq/T of water is independent of temperature over the range investigated up to 30 wt.% surfactant. This is typical of micellar solutions : the micelle/monomer partition of sur-factant is unchanged with temperature, hence the average Y 7 160 :120 Y I-7 80 02 40--.r-s 01 surfactant (wt.%) NI I I 60 E Y0 : 40 Iz I 20 40 60 80 100 surfactant (wt.%) Fig.6 Self-diffusion coefficients scaled by temperature and viscosity as Dq/T for (a) C,,NBu,+ and (b)water as a function of concentra- tion determined at various temperatures: W, 20°C; 0, 30°C; +, 40°C; 0,5OoC;A, 60°C; A, 70°C; a,80°C hydrodynamic size of the solute molecules remains the same, and intermolecular interactions are dominated by long-range electrostatic effects. The temperature dependence of Dg/T increases upon nearing the two-phase body, and is greatest at 65 wt.% where it decreases by a factor of two between 20 and 60°C. This also reflects the progressive failure of the conven- tional micellar solution description employed in the two-state model, eqn.(2’). In contrast with the qualitative differences in Dg/T, note that Pbunddisplays no temperature dependence (Fig. 2). There is also some evidence for a substantial change in solution structure in this concentration range from earlier SANS experiments on this system. In 80 wt.% solutions of C12NBu3Br in D20, the scattered intensity is far lower than would be expected compared with more dilute solutions of the same substance.2 In fact it is 20 times less than in an 8 wt.% solution, which is impossible to reconcile with a simple dispersion of micelles in water. An increasing monomer con- centration with increasing total surfactant adequately suc-cessfully rationalises the low scattering intensity, but at 80 wt.% surfactant this means a dispersion of micelles in a ‘solvent’ which is itself ca.50% by volume surfactant., At 65 wt.% total surfactant, the ‘solvent’ is only ca. 25% surfactant. The behaviour of Dq/T demonstrates that intermolecular interactions between surfactant molecules are only very little affected by temperature, whereas K(H20) is strongly tem- perature dependent. This provides direct evidence for the central role of water structure effects in the phase equilibria of this system, as we have previously suggested.’ In 80 wt.% solution there is insufficient water to solvate fully all of the surfactant present; the overall water :surfactant mole ratio is 6 : 1 and in the ‘solvent’ it is 24 : 1. Neither of these is suffi- cient to solvate the hydrophobic chains of the dodecyltri- butylammonium ion fully, so we envisage the H-bond network of water to be significantly disrupted.At 65 wt.%, the mole ratios are 13 : 1 overall, and 72 : 1 in the ‘solvent’. 72 is just the number of water molecules required to hydrate each of the monomers in the solvent fully.,* While the level of agreement is fortuitous, it does correlate with the high level of bound water found at this composition using the two-state approach. The water network at these intermediate composi- tions must be highly strained and ordered, as almost all water molecules are fully occupied solvating surfactant alkyl groups. We propose that phase separation occurs in order to relieve the strain in this fragile network, and that increasing temperature disrupts the order so that the network can no longer satisfy the solvation requirements of all the solute present.Evidence from previous work demonstrates that ‘v29’ solutions too dilute to phase separate are electrostatically sta- bilked, and contain a large amount of ‘bulk’ water. This work has shown that C,,NBu,Br solutions undergo a struc-tural change between 65 and 80 wt.%, and that this change affects water molecules more significantly. At 80 wt.% an extended H-bond network cannot be supported throughout the liquid phase. Mixtures below the two-phase body are thus trapped: they remain stable as long as the hydration struc- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ture of the surfactant molecules can be maintained. 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