J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2709-2715 Gel to Liquid Crystal Transitions for Vesicles in Aqueous Solutions prepared using Mixtures of Sodium Dialkylphosphates Michael J. Blandamer,* Barbara Briggs and Paul M. Cullis Department of Chemistry, The University, Leicester, UK LE1 7RH Jan B. F. N. Engberts, Anno Wagenaar and Elly Smits Department of Organic & Molecular Inorganic Chemistry, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands Dick Hoekstra Department of Physiological Chemistry, University of Groningen, Bloemsingel 10,9712 KZ Groningen, The Netherlands Anna Kacperska Department of Physical Chemistry, University of Lodz, Pomorska 18,9146 Lodz, Poland The scans recorded by differential scanning microcalorimetry (DSC) for aqueous solutions containing vesicles prepared from mixtures of two sodium dialkylphosphates are complicated where the first surfactant anion (R'O)(R2O)PO2-Na+ has alkyl groups R" and R2 with different chain lengths, and where the second surfactant anion (R30)2P02- has two alkyl groups R" with the same chain length.For mixed solutions where R' = C,,H,, , R2 = C14H29 or C,8H37 and R3 = C12H2,, C14H29, C1&!33 or CiBH37, the DSC traces can be understood in geometric terms. Where the chains can be assembled into bilayers with modest mismatch, the DSC traces show well resolved features. With increase in mismatch, the complexities of the DSC traces are consistent with the presence of bilayers in which the domains differ in composition. Poor chain packing and consequent weak intravesicular van der Waals forces between the alkyl chains favour low temperatures for gel to liquid crystal transitions.The characteristic gel to liquid crystal transitions for vesicles in aqueous solutions' prepared using sodium dialkyl-phosphates, (RO),PO, -Naf, depend2 significantly on the length of the chains in the alkyl group R. In the traces recorded by DSC,3,4 the gel to liquid crystal transition produces an extremum at a melting temperature, T,,analysis of the trace yielding the enthalpy changes for the transitions expressed in terms of surfactant monomer and patchnumber^.^ The latter describe the numbers of surfactant monomers which melt cooperatively. For vesicles formed by the surfactant2 (C,4H290)2P02-Na+, T', = 52.2 "C, the patch number n = 140 and the enthalpy of melting (fusion) Af,, H" = 5.6 kcal (mol monomer)- '.An important contribu- tion to this latter enthalpy is assigned to chain-chain van der Waals interactions within the bilayers. Interest in bilayers formed by these novel surfactants stems from their close relationship with biologically important bilayer systems. However, naturally occurring bilayer systems do not com-prise single substances. Consequently, we examined' the thermal stabilities of vesicles produced in aqueous solutions prepared using mixtures of soldium dialkylphosphates, (R'O)P02-Naf and (R30),P0,-Naf, in which the alkyl chains in each component surfactant are identical. When R1 and R3 differ by C2H4, the temperature T, of an equimolar mixture of the two surfactants is intermediate between the recorded T,s of the solutions prepared from one of the two surfactants.However, when R' and R3 differ by more than C2H4, the DSC traces are complicated. We suggested6 that such complexity is a consequence of a mismatch in chain lengths as the two surfactants associate to form bilayers within the vesicle structures. This conclusion is supported by the results reported here. Solutions containing vesicles pre- pared by mixing two sodium dialkylphosphates, (R'O)(R20)P02-Na+ and (R30),P02-Na+, where R' and R2 differ in alkyl chain length. The DSC traces for these systems are complicated but an underlying pattern emerges based on the lengths of alkyl chains R1,R2 and R3.If a bilayer can be modelled using the building blocks, R1,R2and R3 such that the mismatch is small, the pattern recorded by the calorimeter is simpler than when the mismatch is signifi- cant.If in the matching exercise holes are produced, the tran- sitions identified by the DSC are poorly defined and the enthalpy of transition is small suggesting that the van der Waals interactions between the alkyl chains are weak. Experimental Differential Scanning Microcalorimetry The DSC scans were recorded using a differential scanning microcalorimeter (MicroCal Ltd., USA) as previously de~cribed.~.~At regular intervals during this work (e.g. every 2 days) scans were recorded in which both sample and refer- ence cells were filled with water.The water-water baseline was a gentle concave-downwards curve. It was important to check this baseline at regular intervals because some sur- factant systems produced a deposit on the inner surface of the sample cell. The scans for these systems are not reported, but the presence of such a deposit was signalled by a sharp spike on the otherwise smooth water-water baseline. The deposits were removed by washing the cell repeatedly with hot (e.9. 90°C) concentrated HCl(aq). A clean cell produced a smooth water-water baseline and it was important to re-establish this baseline before the study of surfactant systems was continued. In the experiments reported here the sample cell contained either a single sodium dialkylphosphate (aq; 8.4 x mol dm-3) or an equimolar (4.2 x mol dm-3 for each surfactant) mixture of two sodium dialkylphosphates(aq).The usual scan rate was 60°C h-' and the scans were nor- mally recorded between 5 and 90°C. The exceptions to the latter generalisation involved a more complicated mixed solution prepared using (R'O)(R20)P02-Na+ and (R30)(R40)P02-Naf where R' = C10H2', R2 = C14H29, R3 = C10H2, and R4 = C18H37. In the last case, the DSC scan was recorded over the range 2-90 "C. Surfactants The dialkylphosphates were prepared as described pre- vio~sly.~In all cases, the surfactants were in the form of white solids. Preparation of Vesicles We have emphasised the importance of developing a protocol for the preparation of vesicle solutions if reproducible and repeatable DSC traces are required.Similar considerations apply to the preparation of mixed solutions used in this study. For ease of comparison, we report DSC traces for aqueous solutions containing a common total concentration of surfactant, 8.42 x mol dm-3. For the mixed solu- tions the required mass of each surfactant was weighed out to produce an aqueous suspension, volume, 2.2 cm3, containing equal concentrations (4.2 x mol dm-3) of each alkyl- phosphate. The aqueous suspension was heated to tcm-perature T* (see below) and held at that temperature for 1 h. In detail, temperatures T* for (R'O)(R20)P0, -Na+ and (R30)(R40)P02-Na+ were as follows: (a) T* 2 60°C for (i) R1= C10H2,, R2 = C14H29, R3 = R4 = C12H25; (ii) R' = Cl0HZ1, R2 = C14H29, R3 = R4 = C14H29; (iii) R' = C10H2,, R2 = C16H37, R3 = R4 = C12H25; (iv) R' = Cl0HZ1, R2 = C14H29, R3 = C10H2, and R4= C18H3,; (b) T* 2 70°C for (i) R' = CIOHZ1,R2 = C14H29, R3 = R4 = C16H33; (ii) R' = C10H21, R2 = C18H37, R3 = R4 = C16H33; (c) T* 2 75 "C for (i) R' = C10H2,, R2 = C14H29, R3 = R4 = C18H37; (ii) R' = C10H2', R2 = R3 = R4= C18H37.Protocol for DSC The surfactant solutions were placed in the sample cell in the manner de~cribed.~-~ The solution was cooled in the sample cell of the calorimeter to 5 "C. When thermal equilibrium had been attained, the scan was initiated to a maximum tem- perature of 90°C.After the first scan had been recorded, the solution was allowed to cool slowly in the calorimeter to 5°C. After a predetermined time (see captions to figures), a new scan was recorded from 5 to 90°C. For a few systems described below, the low-temperature limit was set at 2°C because there was clear evidence for an extremum in the recorded trace well below 20°C. Analysis of DSC Scans The scan data were recorded on 3.5 in? discs and later analysed using the Origin (MicroCal) software. The recorded quantity4 was the relative isobaric heat capacity, SC,. In the first stage of the analysis, the water-water baseline (see above) was subtracted from the DSC trace. For most systems reported here, the outcome from this step was a plot showing one or more extrema.A chemical baseline4 produced by a t 1 in = 2.54 an. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 gradual dependence of heat capacity on temperature wassub-tracted. Where an extremum was well defined, the depend- ence of heat capacity on temperature was fitted to an equation describing one or more independent processes of the general form, X(aq) Y(aq), where X(aq) and Y(aq) describe two states in equilibrium. Each such process can be described using eqn. (1) for the molar isobaric heat capacity, C,, m. Here AvH H" is the van't Hoff enthalpy for the transition, cal- culated from (C,, m),= at Tmwhere K is unity. The area under the curve characterised by Tmyields the calorimetric enthalpy of fusion, AfusHzal.The two enthalpies AVHH"and AfusHzal are brought into coincidence using the patch number n.In one limit, n is unity showing that in each vesicle and for each surfactant unit the alkyl chains gain liquid-like freedom independent of the changes taking place in neighbouring chains. With increase in n, the extent of cooperative melting increases within a patch (domain) of the bilayer forming a vesicle. A trace showing several extrema with differences in patch numbers is indicative of domains having very different proportions of the two surfactants. Results In line with our previous practice, we report (i) a comparison of DSC traces obtained for aqueous solutions containing various phosphate surfactants and (ii) repeat DSC traces for the surfactant solutions recorded over an extended period of time.The point of recording the latter set was to test the extent to which the transitions producing the extrema are reversible in the thermodynamic sense. Where the same trace was obtained in repeat scans recorded over a period of several hours it can be reasonably assumed that the gel to liquid transitions are reversible for that range and type of vesicles in solution. Moreover, the validity of using an analysis based on eqn. (1) is confirmed in that this equation assumes complete revqrsibility . Data conforming to these criteria are shown in Fig. 1 for three systems contain- ing (i) the single sodium dialkylphosphate (C10H2 10)(C18H370)P02 -Na+, (ii) the single surfactant (C14H2,0)P02-Na+ and (iii) an equimolar mixture of the two phosphate surfactants.The contrast between the scans for the single surfactants and the mixture was striking [Fig. l(a)J. Nevertheless, the repeat scans [Fig. l(b)] showed that the complicated scans for the mixture were reversible over a period of ca. 14 h. Scans for the surfactant with(CloH210)(C,8H370)P0,-Na+ T, = 20.9"C were analysed [cf:eqn. (l)] in terms of an overall enthalpy change of 5.4 kcal (mol monomer)- ',a patch number of 216 & 8 and two independent transitions. The scan for the surfactant (Cl4H2,0)PO2-Na+ with T, = 52.2"C had a similar enthalpy change of 5.6 kcal (mol monomer)- ', n = 140 and a single transition. In the scan for the mixture (C OH, O)(C ,H 3,O)PO, -Na + + (C 4H2g0)2P02 -Na + [Fig.l(a) and l(b)J, there are two important features at 11.5 and 28.6"C, but they are much less intense than the extrema observed for the component solutions. The feature centred on 28.6"C can be accounted for only by using three or more components having the form described by eqn. (1). The calcu- lated patch number is 222 with an integrated enthalpy of melting of 4.6 kcal (mol monomer)- '. Interestingly, the patch number is almost equal to that for vesicles formed from the pure surfactant, (C, ,H2 O)(Cl ,H3,0)P02 -Na +, although the enthalpy of fusion is lower. The scans for a similar set of solutions in which the surfactant (Cl,H,90),P02-Na+ was replaced by the surfactant (Cl,H,,0)(C,,H370)P02-Na+ (Fig. 2) showed a simpler scan for the mixed system except J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 271 1 0.06 A B 0 0.04 r cI IY Y--iu -0.010 $--. -.. 0.02 k -0.020Ot 20 40 60 0 20 40 60 TPC T/OC Fig. 1 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared (aq; 8.4 x rnol dm-3) where T, =from sodium dialkylphosphates. A, Scans recorded for: (a) surfactant (CloH210)(C18H3,0)P02-Na+ 20.9 k0.3 "C; (b) surfactant (Cl,H2,0),P02-Na+ (aq; 8.4 x mol dm-3) where T, = 52.2 0.1 "C and (c) equimolar mixtures (4.2 x rnol dm-3) of (Cl,H2,0),P02-Na+(aq) and (CloH210)(C18H,,0)2P02-Na+(aq).B, Scans (a) to (e) recorded consecutively for the system described in Fig.lA(c); for clarity the scans have been displaced on the heat capacity axis. that the extremum for the mixture was close to that for the single surfactant (C,,H,, O)(C,,H,,O)PO, -Na+, although again with much less intensity. The patch number, 395 f22, is much larger than that for either pure surfactant coupled with a significantly lower enthalpy of fusion, 1.8 kcal (mol monomer)-'. The scan patterns were reversible for five traces obtained over a period of 8 h; cf: Fig. l(b). The extremum could be fitted using three independent transitions having the form given by eqn. (1);Fig. 2(b).The enthalpy change defined by the extremum was much lower at 1.8 kcal (mol monomer)-' than for the two separate solutions; e.g. 3.9 kcal +(mol monomer) -for (C ,H2O),PO2-Na .By contrast, when the chain length of the symmetric anion was increased forming (C,,H,,O),PO,-Na+, the scan pattern of the mixed solutions showed a complex trace with poorly defined extrema, a trend which continued when a further change was made to solutions containing (C,,H,,O),PO,-Na~(aq); Fig. 3. Fig. 3 compares the DSC traces for the individual solutions and an equimolar mixture. The complex scan for the mixture was repeated in five scans recorded over a period of 19 h; Fig. 3(b). It is noteworthy that one of the extrema for the mixture (Cl,H,,O)(CI,H,,O)PO,-Na+-(Cl,H,,O),PO,-Na+was at the same temperature as T, for the solutions containing (C ,H, ,O)(C sH3,0)P0,-Na+, 20.6 "C. Other notable extrema occur near 24,66.5 and 73.6"C.TrC Fig. 2 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared from sodium dialkylphosphates. A, Scans recorded for: (a) surfactant (CloH,10)(C18H370)P02-Na+(aq; 8.4 x mol dm-3) where T, = 20.9 k0.3"C; (b) surfactant (Cl,H,,0)2P02-Na+ (aq; 8.4 x lop3rnol drnp3) where T, = 34.8"C and (c) equimolar mixtures (4.2 x mol where T, =dm-') of (C,,H,,O),PO,-Na+(aq) and (CloH210)(C18H370)P02-Na+(aq)21.7 "C. B, Dependences on temperature of the molar heat capacity of an aqueous solution described in Fig. 2A(c);dotted lines show calculated contributions (Origin software, MicroCal Ltd.) from three independent equilibria of the general form, X(aq) sY(aq).2712 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I B 7 0.04 Y-m \ clp O0 0.02 I I 1 1 I 20 40 60 80 20 40 60 80 T/T T/"C Fig. 3 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared (aq; 8.4 x lo-, mol d~n-~) from sodium dialkylphosphates. A, Scans recorded for: (a)surfactant (C,oH,10)(Cl,H,70)P0,-Naf where T, = 20.9 f0.3"C (cf: Fig. 1A); (b) surfactant (C,,H,,O),PO,-Na+ (aq; 8.4 x mol drn-,) where T', = 77.1 k0.1"C and (c) equimolar mix- tures (4.2 x mol dm-3) of (C,,H3,0),P02-Naf(aq) and (C,oH,,0)(C,,H370)P0,-Naf(aq).B, Scans (a)-@)recorded consecutively for the system described in Fig. 3A(c); for clarity the scans have been displaced on the heat capacity axis.A similar series of experiments used (C,,H,, O)(C14H,,0)P0, -Na + as a common alkyl-phosphate. In fact, the DSC trace for solutions containing only this surfactant [Fig. 4(a)] showed a much less intense but broad extremum with T, = 14.4 f0.1"C. The calori- metric enthalpy change calculated over the broad transition was small, 1.8 kcal (mol monomer)-'. This contrast with the DSC scan for (C,2H,,0)2P02-Na+(aq) was significant [Fig. qa)] where T', = 34.8 f0.2"C with a calorimetric enthalpy change of 3.9 kcal (mol monomer)-'. Moreover, the tran- sition could be accounted for using eqn (1) in conjunction with n = 168. A single broad extremum was recorded for the equimolar mixture (C ,O)(C 14H290)P02 -Na+-(C,,H,,O),PO,-Na+ at T', = 24.9 f0.1 "C, roughly midway between the T,s for the single surfactants.The calo- rimetric enthalpy for the overall transition is 2.0 kcal (mol monomer)-', n for both component equilibria being 606 & 11. For solutions containing the single surfactant with 3OC A"-"-(b) 200 I Y 7 I-E-100 cp (a) (c) 0 I I 20 40 T/"C longer dialkyl chains, (RO),PO,-Na+ where R = C,,H,, , C,,H,, and C18H37, the T, increases from 52.2 through 66.3 to 77.1 "C. The DSC traces recorded for the equimolar mix- tures containing the surfactant where R = C14H29 and (C ,H ,O)(C 4H,,O)PO, -Na showed a broad extremum + at 37.1"C between T,s or the two component surfactants, a pattern reproduced over five successive scans; Fig.5. The broad extremum was fitted to eqn. (1) using three independ- ent processes but with a common patch number, 148 f6, which is much lower than the number for the pure(Cl,H,10)(C14H,,0)P02~Na+system but is close to that for the (C14H,90)P0, -Na+ surfactant. The calorimetric enthalpy change is 2.6 kcal (mol monomer)-'. With increase in chain length of the surfactant with identical alkyl chains (Fig. 6) the extrema in scans for the mixture covered a broader temperature range and became less intense. At the next level of complexity, the scans were recorded for mixtures of surfactants where the alkyl chains in each sur- B 1 I I I 15 20 25 30 T/"C Fig. 4 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared (aq; 8.4 x mol d~n-~) from sodium dialkylphosphates.A, Scans recorded for: (a) surfactant (C,,H2,0)(C,4H,90)P0,-Naf where T, = 14.4 & 0.1 "C; (b) surfactant (C,,H,,O),PO,-Naf (aq; 8.4 x lo-, mol drn-,) where T, = 34.8 f0.2"C and (c) equimolar mixtures (4.2 x lo-' mol dm-,) of (C,oH,,0)(C,4H,90),P02~Naf(aq)and (C,,H,,O),PO,-Na+ where T, = 25.1 "C. B, Dependences on tem-perature of the molar heat capacity of an aqueous solution described in Fig. 4A(c); dotted lines show calculated contributions (Origin software, MicroCal Ltd.) from equilibria of the general form, X(aq) eY(aq). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.06 1 7 -c 0.04 I Y-(0 -Y Q"uo 0.02 --I I I I01 I I I I I J I 0 20 40 60 0 20 60 100 T/OC T/OC Fig.5 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared from sodium dialkylphosphates. A, Scans recorded for: (a)surfactant (C,oH,10)(C,,H,,0)P02-Na+(aq; 8.4 x lo-, mol drn-,) where T, = 14.4f 0.1"C; (b) surfactant (Cl,H,,O),PO,-Na+ (aq; 8.4 x lop3 mol dm-7 where T, = 52.2 k0.1"C and (c) equimolar mixtures (4.2x mol dm-') of (C,,H,,0),P02-Na+(aq) and (C,,H2,0)(C,,H,,0)P0,-Na+(aq).B, Scans (a)-(e) recorded consecutively for the system described in Figure 5A(c); for clarity the scans have been dispIaced on the heat capacity axis. 0.06 0.04 c I Y-7 0.04 m Y-Y (0 u"Y 0.02 0.02 I0 01 1 I 10 40 80 20 40 60 T/OC T/" C Fig.6 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared from sodium dialkylphosphates. A, Scans recorded for: (a) surfactant (C,oH210)(C1,H,,0)P0,-Na+ (aq; 8.4 x lod3mol dm-') where T, = 14.4 0.1"C; (6) surfactant (C,,H,,O),PO,-Na+ (aq; 8.4 x lo-, mol dm-,) where T, = 66.3 k0.1"C and (c) equimolar mixtures B,(4.2x rnol drn-,) of (C,,H,,O),PO,-Na+(aq) and (C,oH,,0)(C,4H,,0)P02-Na+(aq).Scans recorded for: (a) surfactant (C,oH210)(C,,H,,0)P0,-Na+(aq; 8.4 x mol dm-,) where T, = 14.4k0.1"C; (b) surfactant (C,,H,,O),PO,-Na+ (aq; 8.4 x lo-' rnol dm-3) where T, = 77.1"C and (c) equimolar mixtures (4.2x lop3 mol dm-3) of (C,,H,,O),PO,-Na+(aq) and (CloH2,O)(C,,H2,O)PO,-Na+(aq)where T, = 67.3 "C.0.012 B 0.04 I 7 0.008 Y Y-m -Y 0.02 '0.004 0 0 I I I I 1 I 20 40 60 0 10 20 30 40 50 TPCT/OC Fig. 7 Dependences on temperature of the differential heat capacities (reference = water) for aqueous solutions containing vesicles prepared from sodium dialkylphosphates. A, Scans recorded for: (a)surfactant (C,,H2,0)(C,,H2,0)P0,-Na+(aq; 8.4 x rnol dm-') where T, = 14.4 0.1"C; (b) surfactant (CloH,,O)(C,,H,,O)PO,-Na+ (aq; 8.4 x lo-, mol dm-,) where T, = 20.9 k0.3"C and (c) equimolar mixtures (4.2 x lop3rnol dm-3) of these two surfactants where T, = 17.9 & 0.1"C. B, Scans (a)-(e) recorded consecutively for the system described in Fig. 7A(c); for clarity the scans have been displaced on the heat capacity axis. factant differed in length.For the systems described in Fig. 7, T, for the equimolar mixture was approximately midway between the T,s for the individual solutions, the scan being retraced over five consecutive scans. Nevertheless, the scan pattern for each system is complicated but could be accounted for using three independent values of the patch numbers. Discussion The background to the story described here centred on the complex reorganisation which takes place when the tem-perature of lipid bilayers is raised. It is well established that bilayers formed from phospholipids are responsible for bio-chemically important supramolecular structures.The inten-tion was, therefore, to probe the extent to which reorganisation within these layers, particularly the gel to liquid phase transition, depends on the presence of mixtures of surfactants. However, the thermal stabilities of lipid bilayers are complex. An analogy was to be drawn with the structural transitions undergone by vesicles containing mix-tures of surfactants where the chain lengths differ both between and within the surfactants. The differential scanning microcalorimeter has the necessary sensitivity to probe gel to liquid transitions in quite dilute solutions where we can be reasonably confident that intravesicular processes are responsible for the scan patterns and that the contributions from intervesicular interactions are small.The pattern formed by the differential scans for the mix-tures are clearly complicated; Fig. 1-7. However, an import-ant observation concerns the extent to which scan patterns for the mixtures were repeatable. Therefore, we conclude that the melting processes responsible for the extrema are limited to localised domains (patches) in the vesicles. An alternative explanation links the extrema to a massive reorganisation and disruption on increasing the temperature, leading to new structures which, subsequent to cooling back to, say, 5"C, would be characterised by new scan patterns on reheating. Therefore, the data show that an explanation based on the latter model is probably incorrect. Nevertheless, an under-J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 directly to the increasing difference in the total number of carbon atoms in the dialkyl chains; e.g. 24 andfor (C10H210)(C14H250)P02-Na+(aq)36 for (c 1gH3 7O)2'O2 -Na The latter observation prompts us to propose a general explanation based on a model for the bilayers which explores how these dialkyl chains can pack together. We illustrate the point by reference to two examples. In bilayers formed by the symmetric dialkyl surfactants, (C,,H,,O),PO,-Na+(aq) or (C18H370)2P02-Na+(aq),the alkyl chains can be arranged in just one configuration. Hence a single sharp extremum (Fig. 8) is accounted for in terms of bilayers having C24 and C,, carbon number widths. However, for the single dialkyl surfactant (C ,H, , OXC, 4H290)P02-Na (aq) models of the+ bilayers can be drawn having widths with carbon numbers, C,, having short alkyl chains opposite long chains and C,, where long chains are opposite long chains; Fig.9. In Fig. 9 we illustrate two of the four possible arrangements in which the total carbon widths are C24 and c28. The mismatch between the chain lengths is identified by a shaded area in the c28 arrangement. Therefore, van der Waals cohesions in the bilayers are smaller, the cohesion being weaker in the c28 system. In these terms the four components of the scan pattern are separated by small differences in T,; Fig. 4(a). The mismatch in the models for the mixed solutions is more significant. For the mixed system, (CloH,,O) (C14H2,0)P0,-Na+(aq) + (C18H,,0),P0,-Na+(aq)a large number of structures can be constructed with several different widths defined by the carbon number.These models show that if a surfactant can be accommodated within the bilayer of a host surfactant with minimum disruption, the scan shows well defined extrema. Where the chain lengths are incompatible, the melting occurs over a broad temperature I I ; 12 12 ; I I lying trend can be discerned in which extrema for mixttres;-c24 )I I I Ispan a difference in alkyl chain lengths. For example, the 1 I extremum recorded for the mixture comprising I I 18 [('I 2 HZ 5 O)2 -Na and (c1OH, 1o)(c14H290) IPO,-Na+(aq)] is distinct and midway between extrema Irecorded for the individual solutions.We note here that both I 18 I I broader temperature range with increase in the components contain 24 carbon atoms in the dialkyl chains. But with increase in the number of carbon atoms in the ;-c36 W I 1 I symmetric dialkyl surfactant, the scan pattern broadens, Fig. 8 Diagrammatic representation of the arrangements in bilayerscovering a larger temperature range. This trend is linked formed by symmetric dialkyl phosphates I I I ! Id 14 !. .-I I I 14 I I I I I I I I I I I I I * I-i :-I c28Cza 4 I I I I I I I I I I I I Fig. 9 Diagrammatic representation of the arrangements in bilayers formed by (C,,H, ,O)(C,,H,,O)PO, -Na+(aq) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 range and the patches probably differ considerably in com-position. We thank the SERC for their support through the Molecular Recognition Initiative and both the British Council and the CEC for an award under the ‘Go-West’ Scheme to A.K. References 1 T. Kunitake, Angew. Chem., Znt. Ed. Engl., 1992, 13,709. 2 M. J. Blandamer, B. Briggs, P. M. Cullis, J. B. F. N. Engberts and D. Hoekstra, J. Chem. SOC.,Faraday Trans., 1994,90,2703. 3 M. J. Blandamer, B. Briggs, P. M. Cullis, J. A. Green, M. Waters, G. Soldi, J. B. F. N. Engberts and D. Hoekstra, J. Chem. SOC., Faraday Trans., 1992,88,3431, 4 M. J. Blandamer, B. Briggs, J. Burgess, P. M. Cullis and G. Eaton, J. Chem. Soc., Faraday Trans., 1992,88,2874. 5 C. Gutierrez-Merino, A. Molina, B. Escudero, A. Diez and J. Laynez, Biochemistry, 1989,244, 3398. 6 M. J. Blandamer, B. Briggs, M. D. Butt, P. M. Cullis, J. B. F. N. Engberts and D. Hoekstra, submitted. 7 A. Wagenaar, L. A. M. Rupert, J. B. F. N. Engberts and D. Hoekstra, J. Org. Chem., 1983,54,2638. Paper 4/02345E; Received 20th April, 1994, 1994