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Journal of Colloid and Interface Science220,338–346 (1999)Article ID jcis.1999.6514, available online at http://www.idealibrary.com on

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A Study on Organized Assemblies in the Aqueous Systemsof Alkylammonium Chlorides

Xu He, Bu-Yao Zhu,1 Jianbin Huang, and Guo-Xi Zhao

Institute of Physical Chemistry, Peking University, Beijing 100871, People’s Republic of China

Received April 28, 1999; accepted August 31, 1999

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Various types of organized assemblies, including micelle, planarulti-bilayer, bent multi-bilayer, fingerprint-like multi-bilayer,

nd multilamellar vesicles, were found in the aqueous systemrepared by only one simple surfactant—alkylammonium chlo-ide with a hydrocarbon chain length of 10 or 12. Electron micro-copic images of the assemblies are provided. The properties of theystem were characterized by DSC, fluorescence, and absorptionrobing techniques. It was found that the structure and shape ofolecular assemblies in this system can be adjusted by very simple

hysico-chemical measures, such as pH adjustment and alcohol oralt addition. The mechanisms of formation and transformation ofarious molecular assemblies were discussed based on the mixingffect of the cationic amphiphile and the nonionic amphiphilehich is only a hydrolysis product of the former. © 1999 Academic Press

Key Words: alkylammonium salt; organized assembly; micelle;ulti-bilayer; multi-lamellar vesicles.

Organized assemblies from surfactants, such as micesicles and liquid crystals, have been widely used as membranes of biomembranes, as capsules for agents innd drug delivery (1), and as microreactors and templateynthesizing fine particles with particular sizes and shape). It is well known that the structure of organized assemb

s directly related to the function or application of the systherefore, it is interesting and important in science and tology to construct a self-assembly system possessingired structure and morphology. Thus, the self-assemblihe aqueous systems of primary alkylamines were investign this work.

Primary alkylamine and alkylammonium chlorides are con compounds and have been used as collectors in flo

4). Their properties have been studied rather fully (4–7ecent years, they were used as one of the additives ireparation of some functional materials, such as aluminophorate and tin sulfide, with a mesostructure of lamellae0), coaxial cylindrical bilayers (10), or honeycomb-like p

erns (11). It is well known that lamellae, vesicles, and oesostructures were usually formed in the system of amhiles with complicated structure, e.g., double-chained am

1 To whom correspondence should be addressed.

338021-9797/99 $30.00opyright © 1999 by Academic Pressll rights of reproduction in any form reserved.

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hiles (1, 12–14). The formation of vesicles from alkylamas even considered to be theoretically unexpected (9,hus, the role of alkylamine in the formation of functioaterials with the special mesostructures mentioned abo

till vague. Besides, alkylamine has a much simpler struhan the amphiphiles used to form mesostructures, andheaper and easier to obtain. It has been thought that provhe various structures of organized assemblies in such aactant system would be an attractive project. However, shere has not been systematical report on the formationroperties of organized assemblies from alkylamine analt.In the present work, microstructures formed in alkylam

ium systems under various physico-chemical conditionsnvestigated. The influences of alkyl alcohol, salt, acid, alnd the length of hydrophobic chain of alkylammonium cide on the organized assemblies were studied. The mechf structural variation of assemblies was proposed, andeasures for controlling the formation of various assemere explored.

EXPERIMENTAL

aterials

Primary alkylammonium chlorides (CmNH3Cl, m 5 8, 10,2) were synthesized from alkylamine by reaction with hyhloric acid. Alkylamines were fractionated by vacuum disation before use. The products of alkylammonium chlorere recrystallized three times from a mixed solvent ofnol–ethyl ether and three times from ethanol. Their purere proved by the absence of a minimum in the sur

ension vs concentration curve.The hydrochloric acid, sodium hydroxide, and sodium c

ide were products of Beijing Chemical Reagents Fachina, A.R. degree, and used as received.Water was redistilled from potassium permanganate-d

zed water solution.

ethods

Solution preparation. The systems investigated werequeous solutions of CmNH3Cl (m 5 8, 10 and 12) at variou

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339ORGANIZED ASSEMBLIES OF ALKYLAMMONIUM CHLORIDES

H. The pH of the liquid phase was adjusted by additioodium hydroxide or chloric acid solution and was monitoith a pH meter (type PHS-2, Shanghai 2nd Analytical Insents Factory, China). In consideration of the solubilitylkylammonium chlorides, particularly that of C12H25NH3Cl,

he experimental temperature was kept at about 30°C.

Observation by transmission electron microscopy (TEamples were prepared by the negative staining method% uranyl acetate solution (13) or by the freeze-fracethod as described in Ref. (14), using a Hitachi HUS-5evice. They were then observed on an electron microsJEM- 100CXII).

Differential scanning calorimetry (DSC).DSC was perormed on Universal V1.10B TA instruments at a heatingf 2°C/min.

Micropolarity measurement.Two probes were used in thork. (a) Pyrene was used as a fluorescence probe to me

he micropolarity of the organized assemblies. The fluoence spectra of pyrene was recorded on a Hitachi F-luorescence Spectrophotometer with scan speed 2400 nnd slit(ex/em) 5.0 nm/2.5 nm. The concentration of pyas 33 1027 mol/L. The ratio of intensities of the first to th

hird vibronic peaks of the fluorescence spectrum of pyI 1/I 3) was determined. This value provides an estimate oolarity sensed by pyrene in its solubilization site (15, 16)ethyl Orange (MO) was used as an absorption probe.

ently, the maximum absorption wavelength of MO in soluas found to be a function of the micropolarity of the en

onment that MO encountered (17). The absorption specas recorded on a Shimadzu UV-250 UV-Visible Recordpectrophotometer; each reference sample was the sameeasured, except without MO. The concentration of MO.5 3 1025 mol/L.

RESULTS AND DISCUSSION

ormation of the Organized Assemblies

Assembly formations in aqueous systems of alkylamium chlorides were investigated. The concentrations of8-,10-, and C12-ammonium chlorides were fixed at 0.60, 0.nd 0.05 mol/L, respectively. Each was a few times higher

ts own cmc. The cmcs of C8-, C10-, and C12-ammoniumhlorides in neutral aqueous solution at 303 K are 0.18,nd 0.015 mol/L, respectively, determined according to

nflection point in surface tension curves. These cmc valuen good coincidence with those in the literature. Aggregatearious shapes and structures, including micelle, planar milayer, bent multi-bilayer, fingerprint-like multi-bilayer, aultilamellar vesicles, were found in the aqueous systelkyl-ammonium at various physico-chemical conditions.esults are shown in Figs. 1 and 2, where A, B, C, and D delanar multi-bilayer, bent multi-bilayer, fingerprint-like muilayer and multilamellar vesicles, respectively. It is kno

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hat these structures of assembly have been found separahe systems of amphiphiles with complicated molecular sures, e.g., double long-chained amphiphiles and those wigid group in the hydrophobic chain (1, 12–14). Particularls very rare that so many types of assemblies appear in a srepared by a very simple amphiphile and can be controlleery simple physico-chemical measures. It should be mioned that the formation of vesicles from alkylammonium sas even considered to be unexpected in theory (8, 10).The influence of pH was as follows. The solutions

mNH3Cl (m 5 8, 10, and 12) at neutral pH (;5.2) werelear, but they were translucent at higher pH, the valuhich varied with the hydrocarbon chain length of the amhile. TEM images of alkyl-ammonium chloride solutionsarious pH are shown in Fig. 1. The images provided herf the samples prepared by the negative staining mehich is similar to the counterpart obtained by the free

racture method, as shown in Figs. 2a and 2b, but in metail.In the pH range 0.1; 9.0, various microstructures we

bserved, and they are listed in Table 1. There are multi-biMBL) structures with different shapes (A and B in Figs.b, 1d, 1e), multilamellar vesicles (MLV) with 50; 100 nmiameter (D in Figs. 1c, 1d, 1f), and some fingerprint-ggregates (C in Fig. 1f). It is seen that the bilayer is the bnit for various assemblies observed. The thickness of ite estimated from the images. Thicknesses of about 4 nm

12NH3Cl system and slightly less in the C10NH3Cl systemere found. These results are in good agreement with

ength of the bimolecular layer of the amphiphiles. Tonowledge, this is the first time the fine structures of so minds of organized assemblies in a simple conventional suant system have been revealed, although these structureeen found in various systems of complex amphiphiles (12eparately. For example, MBL and MLV were formed frany single-tailed amphiphiles with rigid segments and ters disk-like structures and multiwalled vesicles, respectiy Kunitakeet al. (12).From Table 1, it is obvious that MBL and MLV structur

ppear only when the number of carbons in the hydrophhain of alkylammonium is 10 or larger. This is in accordaith the results found previously in other surfactant and lystems (18, 19).The microstructure varied with pH. At very low pH (,1),BL and a sparse amount of MLV (Fig. 1a, 1d) were

erved. As pH increased to pH 2; 5 in the C12NH3Cl systemnd to pH 2; 7.5 in the C10NH3Cl system, no structureilayers or vesicles was observed. However, as pH incre

urther, MBL occurred again. Then, MLV appeared in lamounts and the system looked translucent. It is seenable 1 that the pH values at which MBL, MLV, and transence appear decrease with the increase of chain length

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340 HE ET AL.

pproximately pH 8.0, pH 8.5, and pH 8.0 for the C10NH3Clystem and pH 5.2, pH 7.1, and pH 7.3 for the C12NN3Clystem.

FIG. 1. Negatively stained TEM images of alkylammonium chloride sysd) C10NH3Cl, [HCl] 5 1.25 mol/L; (e) C10NH3Cl, pH 8.2; (f) C10NH3Cl, pH 8ulti-bilayer, and multi-lamellar vesicles, respectively. The concentration is

The phenomena mentioned above show that alkylamium salt is an interesting system in which a variety of sssemblies, including micelles, bilayers, and vesicles,

s at various pH. (a) C12NH3Cl, pH 0.5; (b) C12NH3Cl, pH 5.2; (c) C12NH3Cl, pH 7.1;, B, C, and D represent planar multi-bilayer, bent multi-bilayer, fingerprin50 mol/L in the C12NH3Cl system and 0.10 mol/L in the C10NH3Cl system.

tem.6. A

0.0

dition. (a)C( oholw

341ORGANIZED ASSEMBLIES OF ALKYLAMMONIUM CHLORIDES

FIG. 2. Negatively stained and freeze-fractured TEM images of alkylammonium chloride systems at pH 3.0 with fatty alcohol and NaCl ad

12NH3Cl–C12OH (5%); (b) the same as (a), freeze-fractured TEM; (c) C12NH3Cl–C12OH (22%); (d) the same as (c), freeze-fractured TEM; (e) C10NH3Cl–C10OH8%); (f) C10NH3Cl–C10OH (33%); (g) C10NH3Cl, [NaCl] 5 0.1 mol/L; (h) C10NH3Cl, [NaCl] 5 0.2 mol/L. The total concentrations of surfactant and alcere 0.050 mol/L (m 5 12), 0.10 mol/L (m 5 10).

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342 HE ET AL.

ormed. Particularly, the microstructure of the system cadjusted through the pH—a very simple measure. This scity is important for both theory and application and is wonvestigating further.

It is well known that simple single-chained ionic surfactare micelles forming amphiphiles, and they can not filayer or vesicle structures, due to the relatively largeccupied by the charged hydrophilic headgroup (20). Ischvili (21) adopted molecular packing parameterP to de-cribe this characteristic of the surfactant in the system.P wasefined as

P 5 Vc/l ca0, [1]

here Vc and l c are the volume and length of the stretcydrocarbon chain anda0 is the appropriate area for the hrophilic group. As a rule, micelles would be preferred w, 1/3, while bilayers and vesicles would be preferred

.5 , P , 1. It is seen, among the quantities determiningalue of P, i.e., Vc, l c, and a0, only a0 has the characterarying with the physico-chemical environment of the systbviously, for a definite amphiphile, any factor which cakea0 decrease must be beneficial in the transformatio

he molecular assembly from micelles to vesicles. For exameither an anionic nor a cationic surfactant with a singlean form vesicles in aqueous solution, but their mixture18).

Alkylammonium chloride is a kind of ionic surfactant wsingle tail; hence itsa0 is quite large and should foricelles in a solution of concentration higher than its cmc

act, this is true only in the solution of lower pH, becauselkylammonium ion undergoes hydrolysis in water to folkylamine:

CmNH31 5 CmNH2 1 H1. [2]

he pK a is ;10.63 (5); thus

@CmNH2#/@CmNH31# 5 Ka/@H1#. [3]

TABLE 1MBL and MLV in the Dispersion of CmNH3Cl

(m 5 10, 12) at Various pH

pH: ,1 2 ; 4 5.2 7.1 7.5 8.0 8.5 9.

10 MBLMLV

— — — — MBL,t MBLMLV,t

—,t

12 MBLMLV

— MBL MBLMLV

MBLMLV,t

MBLMLV,t

MBLMLV,t

—,t

Note. (MBL) multibilayer; (MLV) multilamellar vesicles; (t) transluceolution; (—), no bilayer structure. Concentrations: 0.050 mol/L (m 5 12),.10 mol/L (m 5 10), 0.60 mol/L (m 5 8).

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fle,iln

ne

ere [CmNH2], [CmNH31], and [H1] are the concentration

mNH2, CmNH31, and H1, respectively. Obviously, the low

he pH, the more alkylammonium ions appear. At pH 1; 9,he ratios of alkylamine to alkylammonium ions in solut[CmNH2]/[CmNH3

1]) were calculated to be 2.33 10210

2.3 3 1022. That is to say, in this range of pH, [CmNH2] isery low and cationic CmNH3

1 dominated in the system. In observations, at an acidic solution of pH 2; 5 for12H25NH3Cl and pH 2; 7.5 for C10H21NH3Cl, the organizessemblies were mainly in the form of micelles which coot be observed by TEM, just as in the case of the micolution of alkyltrimethylammonium salt (12). Micelle formion in the solution of C12H25NH3Cl at pH 2 ; 6 was alsoeported in the literature (5, 22).

It is worth noting that although the equilibrium concenion of alkylamine is very low, and even can be neglectedffect on aggregation is stronger than expected basedoncentration in the bulk phase. This is due to the mtronger surface activity of neutral alkylamine comparedhat of the corresponding cation, as well as to the possibiliydrogen bond formation between the headgroups of CmNH3

1

nd CmNH2 in the oriented molecular layer of the organizssemblies. These effects decrease the energy levelrganized molecular layer and make it stable. Actually,ystem is a mixed surfactants system and not a simplelthough only one surfactant was used to prepare the syence mixed assemblies of alkylamine and alkylammonhloride resulted. With the increase of pH, the amounonionic CmNH2 increased; meanwhile cationic CmNH3

1 de-reased. The nonionic amphiphiles participate in the orieolecular layer, reducing the electrostatic repulsion am

onic surfactants and enhancing the density of amphiphilhe oriented molecular layer. Accordingly, the averagea0 ofhe mixed system goes down, which is beneficial to lamelesicle formation. Therefore, it is understandable that thylammonium chlorides form bilayers and vesicles in theem at higher pH. This is similar to the situation in the aqueystem of carboxylate (19), except that the carboxylatenionic amphiphile and lower pH would be beneficial foresicle formation.

TABLE 2MBL and MLV in the CmNH3Cl System (m 5 10, 12) at pH 3.0

with Various Amounts of Alcohol and Salt

CmOH (%) NaCl (mol/L)

0 5 8 10 20 70 0.1 0.2

5 10 — — MBL MBL,t MBLMLV,t

MBLMLV,t

MBL MBLMLV

5 12 — MBL MBLMLV,t

MBLMLV,t

MBLMLV,t

MBLMLV,t

MBL MBLMLV

Note.See note to Table 1.

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343ORGANIZED ASSEMBLIES OF ALKYLAMMONIUM CHLORIDES

According to the above analysis, the fact that vesiclesilayers were not observed in the system at pH 3.0 woulttributed to the lack of nonionic amphiphile, the alkylamine

his is true, addition of nonionic amphiphile should indilayer or vesicle formation. It was considered that since alcohol has a molecular structure similar to that of alkylamsimilar effect on the formation of MBL or MLV would b

rought about by alkyl alcohol addition. In addition, alcoas the advantage that its concentration in the aqueous sf alkylammonium salt can be adjusted independently, reg

ess of the pH of the system. Therefore, alkyl alcohol CmOHas added into the aqueous systems of CmNH3Cl at pH 3.0 and

he systems were observed by TEM. The experimental rere shown in Fig. 2, and the microstructure change withddition of alcohol is shown in Table 2. It is seen that the raf alkyl alcohol to total amphiphiles at which MBL and ML

ormed are about 5% (MBL) and 8% (MLV) in the C12NN3Clystem and 8% (MBL) and 20% (MLV) in the C10NH3Clystem. The fundamental structural units of MBL and MLVhese systems are also bilayers with a thickness of abouto bilayer structure was found in the C8NN3Cl–C8OH systemhich is similar to the case of pH adjustment and shoulttributed to too weak a hydrophobic interaction of the amhiles. These results support our inference very well.It is strange that the MBL and MLV structures also appe

n both C12NH3Cl and C10NH3Cl systems at pH, 1. Accord-ng to the hydrolysis equilibrium, the amount of noniolkylamine in these cases should be much less than thatame systems at pH 2; 4. Obviously, there should be nnough alkylamine for bilayer and vesicle formation. Howe

t is well known that ionic strength has a significant effecthe self-assembly of ionic surfactants (23) through counteinding. The addition of an inorganic salt of a common co

erion always strengthens the counterion binding to the aslies and decreases the repellence between the amphiph

he oriented molecular layer, which results in a smallera0, andence a stronger tendency of the assembly to transformicelles to bilayers or vesicles. This would be right for the cccurring in the CmNH3Cl system at pH, 1, since when thH is less than 1, a large amount of HCl exists in the sys

FIG. 3. Schematic illustration of the organized assembly variation w

mNH2 or CmOH.

de

f

l,

ltemd-

aking the ionic strength quite high. The headgroup arelkylammonium chlorides was obviously lowered by the co

erion binding at high ionic strength. This is beneficialilayer and vesicle formation, similar to the effect of salt

onic surfactant (23). To support this explanation, the effeodium chloride was investigated. Addition of sodium chlonto both C10NH3Cl and C12NH3Cl systems at pH 3.0, whereesicles or bilayers could be found, did induce MBL and Mormation. The MBL appeared at an NaCl concentration ool/L and MLV at 0.2 mol/L. TEM images of these solutiore shown in Figs. 2g and 2h. The microstructures of tystems are listed in Table 2.

he increase of nonionic CmNH2 or CmOH: (open circles) CmNH3Cl, (filled circles)

ith t

m.

ei-

d

the

r,

n-m-s in

me

,FIG. 4. The fluorescence spectra of pyrene in the C10NH3Cl system a

arious pH: (1) pH 3.0, (2) pH 8.9, (3) pH 7.9, (4) pH 8.2.

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344 HE ET AL.

It is worth noting that in addition to the methods ofdjustment and alkyl alcohol addition, sodium chloride a

ion is another simple method to get the desired organssemblies in an aqueous system of CmNH3Cl.Oliver et al. (8) and Sayariet al. (9, 10) showed simila

esults. They prepared lamellar aluminium phosphorate ulkylamine as an additive. A lamellar structure (similar toBL structure here) and a coaxial cylindrical bilayer (sim

o MLV in Fig. 1d (D) and Fig. 2h) were obtained fromixture of alkylamine and phosphoric acid. It should be

iced that under their experimental conditions, alkylaminelmost totally transformed into alkylammonium salt.Detailed examination of the microstructure shown in Fig

nd 2 and Tables 1 and 2 provided more information. Withncrease of pH and alcohol addition, the structure and shaelf-assemblies in the system at pH 3.0 change, from micelanar multi-bilayer (MBL(I)) to bent multi-bilayer (MBL(II)

o multilamellar vesicles (MLV), as shown in Fig. 3.The influences of various factors on the transformatioicelles to bilayers and vesicles can be explained by

urvature variation of the oriented molecular layer. Miceith high curvature were the main form of assemblies at.0, because in this case CmNH3

1 was the dominant amphiphilpecies, which exhibited a large value ofa0 due to the electriepellence between the headgroups. With the increase ohe amount of nonionic species CmNH2 increases while that o

mNH31 decreases, resulting in weaker electric repellence

ence smaller average molecular areaa0. Consequently, thicelles grew and changed to lamellar structure. Bilayeellae further aggregated to form MBL. They stacked par

o each other in the middle part and were curved and closhe edge or end of MBL. Obviously, the curvature ofriented molecular layer would be different at the two part

he same aggregate. To satisfy the curvature requireme

FIG. 5. I 1/I 3 of fluorescence spectra as a function of pH in the CmNH3Clystem (m 5 10, 12).

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ach part, more ionic amphiphiles would exist in the curart than in the planar part. As for MBL (I) and MBL (II) bore aggregates of bilayers, except MBL (I) is smaller and migid while MBL (II) is larger and more flexible. The radicifference between MBL (I) and MBL (II) can be thought ofdifference in average curvature. In other words, they

ifferent ratios of the planar part to the curved part. At loH, lower concentration of alkyl alcohol, or lower iontrength, the ratio of alkylammonium cations to nonoiomphiphiles is larger; hence the averagea0 and the averagurvature of the oriented molecular layer are larger too.

FIG. 6. The absorption spectra of MO in the C10NH3Cl system: (1) pH 5.22) pH 7.9, (c) pH 8.9.

FIG. 7. The lmax in MO absorption spectra as a function of pH in

mNH3Cl system (m 5 10, 12).

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345ORGANIZED ASSEMBLIES OF ALKYLAMMONIUM CHLORIDES

esult, the molecular assemblies have a larger area ofurvature and MBL (I) will be preferred. At higher pH, highoncentration of alkyl alcohol, or higher ionic strength,mount of nonoionic amphiphiles increases, resulting

arger planar part and a smaller curved part in the assemhus, MBL (II) will be preferred. As MBL (II) becomes largnd larger, it will be flexible enough for the ends of the saBL (II) to combine with each other to take advantage of

educing area of higher curvature. Then, closed bilayerLV forms. In other words, a system containing a lesmount of the cation is suitable for MLV formation. Thisight in the case of the alkylammonium chloride systemigher pH or alcohol concentration.It can be seen from Figs. 1 and 2 and Tables 1 and 2 th

H value and alcohol (CmOH) quantity required for the formion of MBL and MLV, and for the appearance of translucen the CmNH3Cl system, increase with the decrease ofydrocarbon chain length of alkylamine chlorides and aols. This is similar to findings revealed by Hargreavesthers for a system of fatty acids (19). In their case, ve

ormation appeared at higher pH with the increase ofydrocarbon chain length of fatty acids, which was attribu

o a higher packing density of amphiphiles also. Theylained that the higher packing density of amphiphilic ireated a higher surface charge density, and thus a greaten the apparent pK a occured for the assembled fatty acidsnother word, at a certain pH, the longer the chain length oydrocarbon is, the higher the packing density is, and the l

he amount of nonionic fatty acids. Things would be similarhe alkylammonium salt system, except the amphiphilicre oppositely charged. Therefore, the apparent pK a shouldary with the hydrocarbon chain length of alkylamine inode contrary to that of fatty acids. As a result, MBL, MLnd translucence would occur at lower pH with the increas

he hydrocarbon chain length of alkylamine.

he Properties of the Organized Assemblies in the CmNH3ClSystem

Micropolarity of the organized assemblies.Pyrene is aydrophobic compound, usually used as a fluorescence

o measure the polarity of its environment. When pyrenolubilized in assemblies of amphiphiles, the micropolarit

TABLE 3The Micropolarity in the C12NH3Cl System at pH 3.0

as a Function of Alkyl Alcohol and NaCl

C12OH (%) [NaCl] (mol/L)

0 5 10 15 50 0.1 0.2

1/I 3 1.072 1.011 0.993 0.956 0.956 1.020 1.0

max (nm) 424 422 414 414 420 418

gh

aes.

e

orr

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ee-dleed-

shift

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of

beisf

ssemblies can be probed according to theI 1/I 3 value of theuorescence spectra of pyrene. It has been shown that dificropolarities probed in an aqueous solution of surfacould indicate a difference in microstructure. For exampl

s well known that theI 1/I 3 values are always different at toncentrations lower and higher than the cmc, since the sure of the solution changes from molecular to micellar.

The fluorescence spectra of pyrene andI 1/I 3 as a function oH in the CmNH3Cl system are shown in Figs. 4 and 5.It can be seen that theI 1/I 3 value increased first and th

ecreased with the increase of pH. The highestI 1/I 3 valueorresponds to pH range 3; 6.7 in the C12NH3Cl system anH range 3; 7.4 in the C10NH3Cl system. These results aregreement with those for the transformation of microstrucbserved by TEM.As described above, in the pH range 3; 6.7 in the

12NH3Cl system and the pH range 3; 7.4 in the C10NH3Clystem, the organized assemblies are mainly in the foricelles, while beyond this pH range MBL and MLV stru

ures formed in large numbers. It is seen that the resultshe fluorescence spectra of pyrene provide the same infoion.

Recently, the maximum absorption wavelength of MOolution was found to be a function of the micropolarity ofnvironment too (17). The absorption spectra of MO andaximum wavelength (lmax) in the MO absorption spectra a

unction of pH in the CmNH3Cl system are shown in Figs.nd 7. It was found that thelmax of the MO absorption spectruid not change in aqueous solution within the pH range 5; 12,ut it shifted to 500 nm at pH below 4 because of the formaf an acidic type of MO molecule in acidic solution. Thusur experiments MO absorption spectra were measured omNH3Cl systems with pH higher than 5.It can be seen from the results thatlmax decreased with p

ncrease, indicating that the micropolarity of the organssemblies decreased. The pH range at which micropo

FIG. 8. DSC chart of the C12NH3Cl (- - -) and C10NH3Cl (—) systemsith various pH: (1) pH 0.7, (2) pH 6.7, (3) pH 7.3, (4) pH 7.4, (5) pH

6) pH 8.2.

ds y.T ec( La

hod dei tut Ba

andNd or-m Ba ab3

icm the tedt (1)S tv le

tC tuw ucsfs can temM5s ectT ntM tod actm ag

narm r,a stemp iumc hes stemsc ures,s wass clesc li iesi iallya

turalS

82.

m,

.

.

.,

11

1 rabe,

11 W.,

111112 Aca-

2 .

22 rsity

2

.0

m

m

0

346 HE ET AL.

ecreased rapidly corresponded to pH 7; 8 for the C12NH3Clystem and pH 8; 8.5 for the C10NH3Cl system, respectivelhis is consistent with the results in the fluorescence spsee Fig. 5), and in such a pH range, a large quantity of Mnd MLV existed.In a word, both fluorescence and absorption probe met

emonstrated that the micropolarity decreased abruptly innite pH ranges, which correspond to those for the strucransformation of organized assemblies from micelles to Mnd MLV.Furthermore, it was inferred that addition of alcoholaCl in the micellar solution of CmNH3Cl would result in theecrease ofI 1/I 3 andlmax, since they also induce the transfation of organized assemblies from micelle structure to Mnd MLV structure. The experiment results are shown in T, and they do support this inference.

Phase transition. The restricted motion of amphiphilolecules in MBL and MLV can be measured by DSC, andndothermic peaks in the DSC diagram have been attribu

he gel-to-liquid crystal phase transition of the bilayersome typical DSC thermograms of the CmNH3Cl system aarious pH are shown in Fig. 8 and are summarized in TabDistinct peaks were observable in the range pH 6; 7.4 for

he C12NH3Cl system and in the range pH 8.6; 8.9 for the10NH3Cl system. More than one phase transition temperaas detected, which is typical of a mixed system (24). In systems, both MBL and MLV exist abundantly. At pH. 8.6or the C12NH3Cl system and pH. 9.0 for the C10NH3Clystem, no phase transition temperature was detected, beo bilayer existed in such systems. Although in some sysLB was found by electron microscope, e.g., at pH, 1, pH.2 for the C12NH3Cl system and pH 8.2 for the C10NH3Clystem, the phase transition temperature was not dethere are three possible reasons for this. First, the amouLB structures in those systems was not large enoughetected by the DSC equipment used. Second, the surfolecular packing is loose in these situations. Third, theregate disintegrated during heating.

TABLE 4Phase Transition Temperature in the CmNH3Cl

(m 5 10, 12) System at Various pH

pH: 0.1; 5 6.7 6.9 7.3 7.4 8.2 8.7 8.9 9

5 12 — 49.082.6

76.0 41.252.074.4

50.862.187.0

— — — —

5 10 — — — — — — 63; 79 46.5 —

Note.(—) No peak in DSC chart. Concentrations: 0.050 mol/L (m 5 12),.10 mol/L (m 5 10).

traB

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eto.

4.

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uses

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beantg-

CONCLUSIONS

Various types of self-assemblies, including micelle, plaulti-bilayer, bent multi-bilayer, fingerprint-like multi-bilayend multilamellar vesicles were found in the aqueous syrepared by only one simple surfactant—alkylammonhloride with hydrocarbon chain length of 10 or 12. Ttructure and shape of molecular assemblies in these syan be adjusted by very simple physico-chemical measuch as pH adjustment and alcohol or salt addition. Ithown that multi-bilayer structures and multilamellar vesian be formed in a wide range of pH (0; 9) and alkyl alcoho

nclusion (about 0; 70% in amphiphiles). These propertmply the great importance of this simple and commercvailable surfactant in applications.

ACKNOWLEDGMENT

The authors gratefully acknowledge the support of the National Nacience Foundation of China (NSFC, No. 29733110).

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