Dissipativeparticledynamicsstudyofmulticompartmentmicellarsolutionsinaslit1.doc

精品論文推薦dissipative particle dynamics study of multicompartment micellar solutions in a slit1cui yuanyuan, liu dahuan*, zhong chonglidepartment of chemical engineering, the key lab of bioprocess of beijing, beijing university ofchemical technology, beijing, p. r. china (100029)e-mail：abstractmulticompartment micelles are a new family of micelles that may find wide applications; however, due to their complexity, the knowledge on them is quite limited to date. in this work, dissipative particle dynamics simulations were performed to investigate the morphology and structure of multicompartment micellar solutions confined in two hydrophilic walls, as a function of slit width. the results show that pore width can influence largely the morphology and structure of multicompartment micellar solutions confined in hydrophilic walls, the commonly observed “confinement-induced alignment” also occurs in multicompartment micellar solutions, and confinement can induce new morphologies, which can also speed up and enhance the formation of large multicompartment micelles. in addition, this work shows that a strong external field can change the alignment of multicompartment micelles, leading to new well-defined ordered structures. the information obtained may be useful for experimental investigations, as well as for understanding multicompartment micelles at molecular level.keywords: dissipative particle dynamics, multicompartment micelles, slit.1. i n tr o d u c tio namphiphilic block copolymers can self-assemble into various structures such as micelles and vesicles in a selective solvent.1-3 recently, a novel class of micelles called multicompartment micelles explode an intriguing field of nanotechnology that shows great potential applications in drug delivery,4-6 as well as in many other fields such as multi-functional nano-reactors.in the past decade, several groups have performed investigations on preparing and characterizing multicompartment micelles in aqueous medium.4,7-10 recently, lodge and co-workers not only prepared stable multicompartment micelles through the self-assembly of miktoarm star copolymers, but also succeeded in evidencing visually the formation of segregated compartments in a micellar core by means of cryo-transmission electron microscopy (cryo-tem).11 however, the rigorous assessment of the morphology of inner core and the evolution of multicompartment micelles by experiments are very difficult due to the structural complexity of the micelles, to which molecular simulation is a powerful tool. unfortunately, molecular simulations on multicompartment micelles are very scarce to date,12-14 limiting to some extent the precise understanding of their morphologies as well as the corresponding evolution mechanisms at a microscopic level.a systematic research toward a understanding of the formation and morphological transitions in multicompartment micelles at molecular level is being performed in our laboratory by using the dissipative particle dynamics (dpd) simulation technique.12-14 in our previous works,12,13 dpd simulations were performed to investigate the effect of block compositions and shear on the morphologies of multicompartment micelles formed from abc miktoarm star copolymers in water. in this work, it was extended to study the effect of confinement on multicompartment micellar solutions. it is well known that confinement plays an important role in the mesoscale morphology formation due to the surface interactions and the geometrical constraints, and new morphologies and ordered1 support by specialized research fund for the doctoral program of higher education of china (20040010002)-12-structures can be formed in confinement. although there have been extensive investigations on the effect of confinement on nonionic and ionic micellar solutions, both experimentally15-21 and theoretically,22-28 investigations on the effect of confinement on multicompartment micelles have not been performed. therefore, it is valuable to perform a dpd simulation study to reveal the confinement effects as well as to make a comparison with other micellar solutions. this preview can provide useful information for both experiments and toward the complete understanding of the characteristics of multicompartment micelles.2. me th od and sim u la t i on det a i l s2.1 diss ip ativ e p a rtic le d y n a m i c s m e th o dthe dpd method, introduced by hoogerbrugge and koelman,30,31 is a particle-based mesoscopic simulation technique particularly suitable for complex fluids. it can be used to study systems over larger length and time scales than classical molecular dynamics and monte carlo simulations that has been successfully used to study microstructures and properties of polymers in melt state and in solvent.12-14,32-39 in this method, a number of soft particles are considered interacting with each other and each particle represents a small volume of fluid containing many atoms. details of the dpd method are given elsewhere,32,40 and only a brief introduction is given here.in the dpd method the force acting on a particle contains three parts, each of which is pairwiseadditive:40ij ij ijfi = ( f c + f d + f r )(1)j i where the sum runs over all other particles within a certain cutoff radius rc. the conservative force is asoft repulsion acting along the line of centers and is given bya (1 rf c =ij ijrc )rij(rij rc , is the frictioncoefficient, is the noise amplitude, and ijstatistics.the two weight functions can be taken simply as2is a randomly fluctuating variable with gaussian2 (1 r r ) (r r ) d (r ) = r (r ) = b 2 = 2 k t ij c ij c0 (rij rc )(5) (6)to determine the conservative force fc, the repulsion parameter aij has to be known. in this work, the relationship between aij and the flory-huggins -parameter proposed by groot and warren isadopted:40aii + 3.27ij = 3aij = a+ 1.45 = 5(7) ii ijwhere is the density, aii is the repulsion parameter between particles of the same type, and its value isderived from the compressibility of pure component byaii = 75kbt 2.2 m o del s a nd pa ra m e t e r s(8)in our previous work,12 the self-assembly of abc star copolymers (fig. 1) in water into multicompartment micelles was studied, and various morphologies were identified as a function of block composition. in this work, we focused on the study of the effect of confinement on wormlike multicompartment micelles. therefore, the a4b10c2 (a: weak hydrophobic block, b: hydrophilic block, and c: strong hydrophobic block) copolymer was selected since our previous work showed this copolymer could self-assemble into well-defined wormlike multicompartment micelles in the bulk.12 in the simulations, all the dpd parameters that describing the interactions between blocks and between block and water remained unchanged as before.12fig. 1: schematic structure for the abc star triblock copolymersin this work, the confinement was considered by applying parallel planar walls, and each wall interacted with each bead in the system with a potential of the same form as the bead-bead conservative force. in the present work, two kinds of walls were studied: the hydrophilic walls and the walls with ultra-strong interactions that selectively absorb the hydrophilic block and water. the latter is a simulation to a thin film under a strong external field. for the hydrophilic walls, the interactionsbetween the walls and the blocks and water were set to aawacw50, abwasw27 (w: wall, s:water); while for the ultra-strong walls, we set the parameters as: aawacw200, abwasw150.2. 3 si mu l a t i o n de ta i l sthe dpd simulations were performed in boxes of size lxlylz rc3 , where lz , equaling to the pore width of the planar walls (film width), is a variable that is set to lz=nd, where d is the average diameterof the wormlike multicompartment micelles formed in the bulk without confinement (d=4.6 dpd unit),and n was varied from 1.0 to 10.0 the lx =ly can be calculated for a given n with the dpd beads in the box being kept around 81000 (bead density was taken as 3.0). details of the sizes of the simulation boxes and the corresponding numbers of beads are shown in table 1. for convenience, the cutoff radius rc, the particle mass m and kbt were all taken as unity, and periodic boundary conditions were applied along the x- and y- directions. the volume fraction of the copolymer was set to be 0.1 as before toensure that enough micelles can be formed in these systems.12 the time step t was taken as 0.05, andadjacent particles in the polymer chain interacted via a linear spring with the harmonic spring constantof 4.0. about 2105 to 4105 dpd steps were carried out for each dpd simulation to guarantee theequilibration, depending on the system concerned as done in our previous work. 12-14,38,39table 1: sizes of simulation boxes and the corresponding numbers of beadssize of simulation boxeslxlylznumber of beads30.030.030.08100074.074.05.0 (1.0 d) 8200062.062.07.0 (1.5 d) 8100052.052.010.0 (2.0 d) 8100047.047.012.0 (2.5 d) 8000044.044.014.0 (3.0 d) 8100040.040.017.0 (3.5 d) 8200038.038.019.0 (4.0 d) 8200034.034.023.0 (5.0 d) 8000031.031.028.0 (6.0 d) 8100029.029.033.0 (7.0 d) 8300027.027.037.0 (8.0 d) 8100025.025.042.0 (9.0 d) 7900024.024.046.0 (10.0 d) 790003. r e su l t s a n d d i s c u s si o nin this work, we considered the star abc copolymers12 (fig. 1) and focused on the study of the morphology and the formation process of multicompartment micelles confined in slit pores, as a function of pore width, for which two kinds of walls were considered: hydrophilic walls and ultra-strong repulsive walls, the latter can mimic thin films under a strong external field.3.1 mu lticom p a rt m e nt m i c e lla r s o lutio n s c o n f ine d i n h y dr o philic w a lls 3. 1 . 1 e f f ect o f p o re w i d t hwe firstly discuss the effects of the pore width of hydrophilic walls on the morphology of multicompartment micelles formed in water. the models and the dpd parameters for the copolymers and the walls used are given in section “method and simulation details”, and the pore width, lz, was varied from 1.0 d to 10.0 d continuously. some typical results are shown in fig. 2, where the multicompartment micelles formed in water without confinement are also given for comparison (fig. 2a). for each picture, two views were given for clarity (top: perpendicular to the walls; bottom: along the walls).(a) unconfined (b) lz=1.0 d (c) lz=1.5 d (d) lz=2.0 d(e) lz=2.5 d (f) lz=3.0 d (g) lz=3.5 d (h) lz=4.0 d(i) lz=5.0 d (j) lz=6.0 d (k) lz=7.0 d (l) lz=8 d(m) lz=9 d (n) lz=10 dfig. 2: morphologies of multicompartment micelles formed from star triblock copolymers a4b10c2 in water confined in the hydrophilic walls with various lz (b blocks and water were omitted for clarity; a, red; c, green; two views are given: top: perpendicular to the walls; bottom: along the walls)from fig. 2, it is clear that the pore width of the walls influences the morphology as well as the structure of the multicompartment micellar solutions largely, and the effects can be divided into three stages approximately: lz=1.0 d-1.5 d, 2.0 d-3.5 d, and 4.0 d-10.0 d. for the pore width of 1.0 d-1.5 d, only discrete “hamburger” micelles can be formed, mainly due to the frustration effects of the walls; when lz=2.0 d, long wormlike multicompartment micelles parallel to the walls were formed, and the micellar solution is a mixture of wormlike and “hamburger” micelles. this structure remains up to lz=3.5 d. interestingly, the micelles presented were self-organized into one layer parallel to the walls, and the most well-aligned layer occurred at lz=2.5 d (fig. 2 e); the third stage starts from lz=4.0 d, the well-defined one-layer structure was destroyed, and the micelles aligned randomly between the walls due to the decrease in the strength of confinement; however, at lz=8 d (fig. 2 l), it seems a orderedstructure of two layers nearly parallel to the walls was formed. generally, with increasing in pore width, the morphology and structure of the multicompartment micellar solutions approached to that formed in the system without confinement (fig. 2 a). a comparison with fig. 2 a shows that several confinement-induced new morphologies were formed, such as “y-shaped” (lz=4.0 d, 6.0 d and 9.0 d) and “sphere-on-sphere” (lz=3.0-4.0 d, 7.0d) micelles, etc.the simulations show that confinement-induced alignment also occurs in multicompartment micellar solutions, which have been extensively observed in common nonionic and ionic micellar solutions.15,16,23,25,26 this, together with the previous works on micelles, comes to the conclusion that“confinement-induced alignment” is a general phenomenon that occurs in fluids confined in slit pores.3. 1 . 2 f o rm a t i o n m e cha n i s m o f m u l t i c o m pa rtm e n t m i c e l l e s i n co n f i n em en tour previous work12 shows that dpd simulation is a powerful tool to elucidate the underlying formation mechanisms of multicompartment micelles at a molecular level by direct visualization of the snapshots of the corresponding evolution processes. in this work, the evolution processes for three representative pore widths, lz=1.0 d, 2.0 d, and 4.0 d, corresponding to the three stages were studied, and the snapshots are shown in fig. 3-5, respectively.fig. 3: evolution of a short wormlike multicompartment micelle simulated by dpd with lz=1.0 d (bblocks and water were omitted for clarity; a, red; c, green)the evolutions of a short wormlike micelle at lz =1.0 d, long wormlike micelles at lz=2.0 d, and a “y-shaped” micelle at lz =4.0 d shown in fig. 3-5 indicate that all the large micelles were formed following the fusion process.for lz=1.0 d, the star triblock copolymers assembled into discrete “hamburger” micelles firstly (fig. 3 b, viewing along the walls), then some of them approached each other until contact (fig. 3 c), and combined into a short wormlike multicompartment micelle (fig. 3 d), which, however, could not grow longer because of the strong confinement.as lz increases to 2.0 d, similar phenomenon also occurred; short wormlike micelles were formed by the fusion of discrete “hamburger” micelles (fig. 4 b, c, viewing perpendicular to the walls), then, they combined with discrete “hamburger” micelles or short wormlike micelles to grow into long wormlike multicompartment micelles.fig. 4: evolution of a long wormlike multicompartment micelle simulated by dpd with lz =2.0 d (b blocks and water were omitted for clarity; a, red; c, green)for lz=4.0 d, long wormlike micelles were formed firstly as shown in fig. 4, then, they growed into “y-shaped” multicompartment micelle by fusing with discrete micelles as shown in fig. 5 (viewing perpendicular to the walls).fig. 5: evolution of a “y-shaped” multicompartment micelle simulated by dpd with lz=4.0 h (bblocks and water were omitted for clarity; a, red; c, green)3. 1 . 3 c o m p a r i s o n o f th e fo r m a t i o n o f m i ce l l e s i n co n f i n ed a n d un co nfi n e d c o n d it io n sour previous work12 shows that wormlike multicompartment micelles in unconfined condition are formed following the fusion process, it seems that confinement does not change this evolution mechanism. to investigate the effects of confinement on the dynamics of micelle formation, we studied the snapshots of the evolution of multicompartment micelles formed both in confined and unconfined conditions using the same size of simulation box, as shown in fig. 6. the snapshots show that confinement can enhance the formation of multicompartment micelles, which is consistent with the observations in polymer aqueous films 23 as well as polymer melt films.29 in addition, the micelles are longer in confined evolution, mainly due to the geometric obstacle to the growth of wormlike micelles to make them grow preferentially along the unconfined direction.fig. 6: snapshots of wormlike multicompartment micelles formed in the system of 303030.(a) unconfined, (b) confined in parallel walls (b blocks and water were omitted for clarity; a, red; c,green)3.2 mu lticom p a rt m e nt m i c e lla r s o lutio n s c o n f ine d i n u l t r a-s t r o n g re puls i ve w a lls it is interesting to see the morphology and stru