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Low angle laser light scattering and photon correlation spectroscopy in surfactant vesicles
1 July 1979
CHE51ICAL PHYSICS LElTERS
Volume 64, number 1
LOW ANGLE LASER LIGHT SCATTERING AND PHOTON CORRELATION IN SURFACXANT
VESICLES
Ucio HERRMANN
* and Janos H. FENDLER
irztitut de chinie plxysi@ue. Ecot
SPECTROSCOPY
**
Po&vtechnique de Ltuuanne. Lausanne. Switze&nd
Rccehed 28 March 1979 diffusion coefticienu, and h) drodynamic 0 xteight nvemgxi molecular xseights.&. Usingzlow angk Iaser tight scatterin,. radii. RH, ha-e brxn drtermined for completely synthrtic surhcmnt vesicles, prepared by ultrasonic irradiation of dioctsdcr~ldimethyIammonium chloride. DODAC. and diheladecyIphosphate. DHP dispersion% Both the_ww and RH values acre found to dewxse exponentially as a function of sonication time. At the limit..?,v and RH for DODAC vesicles are 12.6 X 10’ daIcon and 396 A, and those for L)HP vesicIcs are 13 X 106 dzdton and 59.5 A. Calcuhtions indicate both vesicles to bs
prokl:ti-
I_ Introduction Sonic dispersal of compIeteIy synthetic dioctadecyldimethylammonium chloride, DODAC [l-61 ~ or dihexadecy Iphosphate, DHP [7], resuhs in the formation of smectic mesophases of surfactant bilayers with water interspaced between
them_ These surfactant
vesicles are
stable in the pH 2- 12 range for weeks, osmotically active, undergo thermotropic phase transitions and, most importantly, are able to entrap and retain molecules [4-7]_ They resemble, therefore, the more complex phospholipid Iiposomes [S,9] md thus represent the simplest functional model for the biological membrane_ For this reason surfactant vesicIes have been selected to be the subject of the present investigations. Electron micrographs indicated the presence of closed single compartment bilayer DODAC [3] and DHP [7] vesicles having average diameters of 300 A and 500 A, respectively_ In spite of their potential utilization in catalysis [IO] and energy conversion [I I- 131 no information is available on the size and shape of surfactant vesicIes in sohtion and on factors which influence them. Elastic and quasielastic Iighhtscattering spectroscopy have been advantageously utilized for the structural in* On kve from the Max-Pkmck-institut fiir Bioph>siklische Chemie. G~ttingen. Germany_ ** On frxwe from the Depzrtment of Chemistry, Texts A&M University. Collr%e Station. Texas 77843, USA. 270
vestigstions of aqueous micellar solutions [I4-161. The present work reports data, for the first time, on diffusion coefficients, weight averaged moIecuIar weights and hydrodynamic radii of completely synthetic DODAC and DHP surfactant vesicIes. These parameters have been determined by means of low angle laser Iight scattering measurements and photon correlation spectroscopy-
2_ Experimental Preparations, purifications and physical-chemical characterizations of DODAC and DHP have been described [4,5,7,1 I] _ Stock solutions of vesicles were prepared by the ultrasonic dispersal of 12.0 mg DODAC or 5.0 mg DHP in 2.0 ml triply distilied water at 55°C or 60°C, respectively- The microtip of 3 Branson B-12 sonifier was used. Measurements were carried out on vesicIe solutions having concentrations less than 4 X 10-S g/mI. Low angle laser light scatterings were determined on a Chromatix K&IX-6 instrument equipped with 3 model 64 digital correlator. Vesic!c solutions were introduced into the thermostatted cell compartment through a O-45 micron blillipore filter_
CHEYICAL
Volume 64, number 2
is performed
3_ Theory The physical characteristics of macromolecules in solution are related to the Rayleigh factor,R@), by the equation [ 171 I
(I)
where c is the concentration in g/ml and m,s is the weight average molecular weight of the scattering particles;P(fI) is the particle scattering function; the terms A ?, A 3, ___are virial coefficients and K is given by : K = (25s2,Z2
(drr/dc)2(1
/A4N)
f co&),
(2)
where PZis the refractive index and dlz/dc is the differential index of refraction; X is the wavelength in vacua, N is Avogadro’s number. The high sensitivity of the Chromatix allows the dete_mination of the Rayleigb factor in extremely dilute solutions (see section 2) which, in turn, justifies the simplification of eq. (1) to: KC/R@)
=
l/i&
f
u,C.
0)
Further, in dilute solutions the value for JZ can be taken for that in water (JZ = 1333) and &z/dc = 0.12. The time dependency of the fluctuation of the scattered light intensity is related to the diffusion coeffcient, D, of the scattering particle. Values for D can be evaluated by determining the autocorrelation function of the scattered light-intensity; since its decay, re, in acqueous solutions under homodyne conditions is given by [18] :
ic = 1/2DK’, where the scattering K = 47i sin($l)fhjz,
(4) vector, K, is calculated
I July 1979
PHYSICS LETTERS
from:
by the equation
In [G(r) - G(m)]
= 2(.lcu - x1 t + x2t2 - x3 t’),
]191For a monodisperse solution of noninteracting macromolecules the time dependent term is a single exponential with a timeconstant given by eq. (4) For a polydisperse solution the decay is non-exponential_ Further, deviation from a single exponential contains information about the size distnbution. A cubic polynomial fit to the logarithm of the time dependent part of the measured correlation function, In [G(t) - G(m)] ,
(6)
where x1 = K’D. 2-x2 = K4D% and 6x, = K6D3s; v and s represent the relative variance and skewness from monodispenity. Assuming the vesicles to be spherical, the measured diffusion coefficient, D, in a medium of viscosity q is related to the hydrodynamic radius, RH _ of the scattering particle by the Stokes-Einstein equation: D=kTj6jinlRH.
(7)
The possible shapes of vesicles can be calculated by assuming negligible solvation and that the observed value ofDO/D is caused by the asymmetry of the vesicles. The Perrin equation [2 I] for prolate ellipsoid (semiaxes a,b,b) is: D,/D
= RfRo
and for oblate ellipsoid (semi-axes DoID = R/R,, =
(&$
-
a,a,b)
1)‘E
(~/~)‘15tan-‘($/~’
--_ I)‘;”
(9)
where R, is the radius of a sphere of equal volume to the ellipsoid, i.e...$srR~ = 4iiab’ (prolate ellipsoid) or $rK; = $r~‘b (oblate ellipsoid)_ Vesicle shapes can be confirmed by independent measurements of the radius of gyration. R, _According to the equation: Eo
Kc/R(@) = (l/z,\)[l
f (16&h’@
(5)
where 8 is the scattering angle (4.5”). Under the present condition K was taken to be 1.039 X IO4 cm-l
[20] :
sin2(0;2)] (10)
plots Of KC/R(@) against sin”(e/2) at given concentrations of vesicle yield straight lines whose slope is related to R, by [19] : Rg = (3X’/16~~)(1~~)
(slope).
(11)
4_ Results and discussion Fig. l_ illustrates typical light scattering data according to eq. (3) Table 1 summarizes the results for DODAC 271
1000 34,l 21,G 33s
0 OS5 IO,0 IO,0
DOD/K
DOD/K!
DODAC 6)
12,o 24.0 4880 6060 6080
DllP
DIR
DllP
DtlP
DWi) m-“,.1
97
29q2
23,6
358
64,8
11s
S60
.
.
.
10’D
-_.
3,56
4s3
480
402
3,lO
2446
2,RS
5,92
5,77
7871
St25
2435
(cm* s”‘)
__”
I_“_
669
525
595
592
772
968
843
396
407
309
447
_.*I*..-..-
1000‘-I
Rt[ (A)
.
2.4
2.7
3,3
2.9
3.1
302
1.7
2.8
208
-
1.6
2,6
5.1
-
-..
R/R0c,
I
--__
1200
1488
1275
___I
II
12
225
3s 42
630
II
461
687
__-_
2740
2320
2930
2680
3552
4690
2647
1760
la10
951
1850
-*--
atit)
n (A) /J(A)
prokttc d)
92
s!i
45
56
66
76
225
41
42
89
53
/J(A)
_....-l”,...“..-”
obllltc (1)
Cnlculatcd shpo _-....__-
2090
2136
2640
2453
3200
3900
187
74
58
79
103
140
862
49 862
44 1680
126
G5
b (A)
2200
812
1860
a(A)
prcdt\tc Q)
9.
a) At 2S,0°C!, unless stntcd otkcrwlse, cstlmntctl errors nrc IO% b, Using n hlson 11.12 sonificr, DODAC dispcrsh wcrc sonicatcd at 55% nt II scttlng of 3; DW dispersions wcrc sonlcntcd nt GS’C! nt a scttlny of 7, c, Mcn~ur~d hydrodynnmicradiusclrvldcd by the rndi~~sof n splrcro ~nlc~~luted from the dotcrtt~lned ntolcculnr w~i6ltt [see eqs, (8) nnd (9)], d, Cnlculatcd from cqs, (8) nnd (9), nssuminl: thnt tbcrc Is no wntcr irirrdc the vesicles (La., thy nrc complctcly frlletl with surfuctnnts), c, Cnlculntcd by nsswmlng slnglc compnrtmcnt bllnyor vcsiclcs, hnving bllaycr thlckncss of 60 A, f) Too big for corrclntian, g) Dctcrmincd at 35% D vnluc chnngcs from 4S3 X 10” cm* it to 2.1 X lo”” cm2 <’ durlnb ln~ns~lr~r~tcnt, i, Vcsich 1~1sbean ngcd for 5 days,
-.“....1__--e-w..--
3,o 640
12,G
60,O
DODAC
DIIP
13,4
DODAC
DIil’
31s
12,o so,0
DODAC
n’i\y
DODAC
I CP
(~lnltotl)
Sauicntion tiwz (rytirl) b)
V&AC
Surfnctnnt
Volume 64, number 2
0
...*‘****‘.****
1
CHEMICAL PHYSICS LIZTIERS
1 Jul> 1979
OO 0
b
2 3 WbJHi
l_ Plots of data according to eq_ (3) for 12 minute sonicated DODAC and DHP vesicles_
2
\,
....‘.. 0
50
Fig.
time. In the absence of sonication extremely large non-uniform particle formation is seen In general, formation of DODAC vesicles requires less applied sonic power than that needed to form DHP vesicles. Increasing the sonication time, at a given power setting, results in an exponential decrease of the size of the vesicles, down to a point beyond which further sonication does not alter the size of the vesicles_ Weight averaged molecular
0
lm
. . .
1
60
.
.
I-l,xxx
2mn!.E [o-q
Fig. 2. Plots of data according to eq. (6) for 12 minute sonicated DODAC and DHP vesicles
and DHP vesicles as a function of the sonic&ion
weights and hydrodynamic radii of DODAC and DHP vesicles at their plateau values are 12.6 X lo6 and 23 X IO6 dalton; and 396 and 595 A, respectiveby. It is interesting to note that at 35”C, close to the phase transition temperature, vesicles are unstable. AIso on standing, both M,,, and RH increase (see table l), presumably due to vesicle-vesicie fusion. Fig. 2 illustrates typical plots of the data according to eq. (6). We calculate the polydispersitjr indices, uvalues, to be 0.26 and 0.22 for DODAC and DHP vesicles, respectively_ These data Indicate DODAC vesicIes to be more polydisperse than their anionic counterparts_ The calculated shape factors, R/R0 values in table 1, indicate pronounced deviations from sph=ricaI structures_ Neither DODAC nor DHP vesicles can be described as ablates since the calculated semi-axes are less than the thickness of a single bilayer, 60 A [5,6] _ The present data are most compatible with the formation of single compartment prolate DODAC and DHP vesicles at sufficiently high applied ultrasonic irradiations_ The determined values of RG [using eqs. (10) and (1 l)] are in accord with this postuIate_ In general, discrepancies between the calculated and determined ii?,-values ex-
elude spherical shapes. However there is good agreement between the calculated and determined values if prolate or rod-like shapes are assumed for these synthetic surfactant vesicles_ Thus, for example, using the relationship R& = L2/12 (where L is the length of a rod) [21], L = 4020 A is obtained for aged DHP vesicles_ This value differs from *hat calculated for a filled vesicle (2~ = L = 2 X 2740 = 5480 A, see table i), but it is in a very good agreement with that calculated by assuming single compartment bilayer vesicles (2~ = L = 2 X 2090 = 4180 A, see table 1).
Acknowledgement Financial support of the Swiss National Fund is gratefully acknowledged_ We thank Professor M. Gritzel for his interest and Dr. J. Kiwi for setting up the instrument and for helpful discussions_
References [ 1] K. Tarrrki. F. Kumamuru and T. Takayanagi, Chem. Letters (1977) 387. [2] T_ Kunitake and y_ Okahata. J. Am. Chem. Sot. 99 (1977) 3860. [3] K- Deguchi and J. Mine. J. CoIIoid Interface Sci. 65 (1978) 155. [4] C-D_ Tran, P-L. Klabn, A_ Romero and J.H. Fendier, J. Am. Chem. Sot. 100 (1978) 1672. [S] Y-Y. Lim and J.H. Fendler, J. Am. Chem. Sot. 101 (1979), to be published-
273
Volume 64. number 7,
CHEMICAL
PHYSICS LETTERS
[6] IL I&no. A_ Romero, B_ Djermouni. H_ Ache and J-H_ Fendler, J_ Am_ Chem. Sot. 101 (1979). to be pubIIshed[7] R-A. hlortam, F-H. Quina and H. Chaimovich. BiochimBiophys. Res_ Commun. 81 (1978) 1080. [S] A-D. Bangham, Pmgr. Biophys. hIo1.Biol. 18 (1968) 29. 191 D_ Papahadjopoulos and XX Kimelbcrg. Progr. Surface sci4 (1973) 141. [IO] T_ Sun&k+ and T_ Sakamoto. J_ Am_ Chem- Sot- 100 (1978) 4615. [ 1I) J-R. Escabi-Perez, A. Romero. S. Lukac and J.H. Fcndler. J. Am. Chcm. Sot. 101 (1979). to be pubIished. [ 121 A_ HengIein. Th_ Proske and W_ Schnccke. Ber. Bunserges. Physik. Chem. 82 (1978) 956. [ 131 hl_ Gr&el. J-H_ Fondler and coIIsbomtors. unpublished result2
274
1 July 197b
[ 14) N-A_ hfazer. G-B. Benedek and MC_ Carey. J. Phys. Chem. 80 (1976) 1075. (IS] hi. Corti and V_ Degiorgio, Opt. Commun. 14 (1975) 358. [ 161 hl. Corti and V. Degiorgio, Chem. Phys. Letters 53 (1978) 237. [ 171 D. hfcintyre and F. Gomick. edr. Light scattering from dilute poIymer solutions (Gordon and Breach. New York, 1964). [IS] hI_A_ CIark. J-H. Lunacek and G-B. Bcnedek, Am. J. Phys. 38 (1970) 575. [ 191 Chromatic KMX-6 Application notx LSI and LS8, Chromatiu. 560 O&mead Parkway. Sunnyvale. California 94086, USA. [20] D-E. Koppel. J. Chcm. Phys. 57 (1972) 4814. [?I] F. Pen-in, J. Phys. Radium 7 (1936) 1.
CHE51ICAL PHYSICS LElTERS
Volume 64, number 1
LOW ANGLE LASER LIGHT SCATTERING AND PHOTON CORRELATION IN SURFACXANT
VESICLES
Ucio HERRMANN
* and Janos H. FENDLER
irztitut de chinie plxysi@ue. Ecot
SPECTROSCOPY
**
Po&vtechnique de Ltuuanne. Lausanne. Switze&nd
Rccehed 28 March 1979 diffusion coefticienu, and h) drodynamic 0 xteight nvemgxi molecular xseights.&. Usingzlow angk Iaser tight scatterin,. radii. RH, ha-e brxn drtermined for completely synthrtic surhcmnt vesicles, prepared by ultrasonic irradiation of dioctsdcr~ldimethyIammonium chloride. DODAC. and diheladecyIphosphate. DHP dispersion% Both the_ww and RH values acre found to dewxse exponentially as a function of sonication time. At the limit..?,v and RH for DODAC vesicles are 12.6 X 10’ daIcon and 396 A, and those for L)HP vesicIcs are 13 X 106 dzdton and 59.5 A. Calcuhtions indicate both vesicles to bs
prokl:ti-
I_ Introduction Sonic dispersal of compIeteIy synthetic dioctadecyldimethylammonium chloride, DODAC [l-61 ~ or dihexadecy Iphosphate, DHP [7], resuhs in the formation of smectic mesophases of surfactant bilayers with water interspaced between
them_ These surfactant
vesicles are
stable in the pH 2- 12 range for weeks, osmotically active, undergo thermotropic phase transitions and, most importantly, are able to entrap and retain molecules [4-7]_ They resemble, therefore, the more complex phospholipid Iiposomes [S,9] md thus represent the simplest functional model for the biological membrane_ For this reason surfactant vesicIes have been selected to be the subject of the present investigations. Electron micrographs indicated the presence of closed single compartment bilayer DODAC [3] and DHP [7] vesicles having average diameters of 300 A and 500 A, respectively_ In spite of their potential utilization in catalysis [IO] and energy conversion [I I- 131 no information is available on the size and shape of surfactant vesicIes in sohtion and on factors which influence them. Elastic and quasielastic Iighhtscattering spectroscopy have been advantageously utilized for the structural in* On kve from the Max-Pkmck-institut fiir Bioph>siklische Chemie. G~ttingen. Germany_ ** On frxwe from the Depzrtment of Chemistry, Texts A&M University. Collr%e Station. Texas 77843, USA. 270
vestigstions of aqueous micellar solutions [I4-161. The present work reports data, for the first time, on diffusion coefficients, weight averaged moIecuIar weights and hydrodynamic radii of completely synthetic DODAC and DHP surfactant vesicIes. These parameters have been determined by means of low angle laser Iight scattering measurements and photon correlation spectroscopy-
2_ Experimental Preparations, purifications and physical-chemical characterizations of DODAC and DHP have been described [4,5,7,1 I] _ Stock solutions of vesicles were prepared by the ultrasonic dispersal of 12.0 mg DODAC or 5.0 mg DHP in 2.0 ml triply distilied water at 55°C or 60°C, respectively- The microtip of 3 Branson B-12 sonifier was used. Measurements were carried out on vesicIe solutions having concentrations less than 4 X 10-S g/mI. Low angle laser light scatterings were determined on a Chromatix K&IX-6 instrument equipped with 3 model 64 digital correlator. Vesic!c solutions were introduced into the thermostatted cell compartment through a O-45 micron blillipore filter_
CHEYICAL
Volume 64, number 2
is performed
3_ Theory The physical characteristics of macromolecules in solution are related to the Rayleigh factor,R@), by the equation [ 171 I
(I)
where c is the concentration in g/ml and m,s is the weight average molecular weight of the scattering particles;P(fI) is the particle scattering function; the terms A ?, A 3, ___are virial coefficients and K is given by : K = (25s2,Z2
(drr/dc)2(1
/A4N)
f co&),
(2)
where PZis the refractive index and dlz/dc is the differential index of refraction; X is the wavelength in vacua, N is Avogadro’s number. The high sensitivity of the Chromatix allows the dete_mination of the Rayleigb factor in extremely dilute solutions (see section 2) which, in turn, justifies the simplification of eq. (1) to: KC/R@)
=
l/i&
f
u,C.
0)
Further, in dilute solutions the value for JZ can be taken for that in water (JZ = 1333) and &z/dc = 0.12. The time dependency of the fluctuation of the scattered light intensity is related to the diffusion coeffcient, D, of the scattering particle. Values for D can be evaluated by determining the autocorrelation function of the scattered light-intensity; since its decay, re, in acqueous solutions under homodyne conditions is given by [18] :
ic = 1/2DK’, where the scattering K = 47i sin($l)fhjz,
(4) vector, K, is calculated
I July 1979
PHYSICS LETTERS
from:
by the equation
In [G(r) - G(m)]
= 2(.lcu - x1 t + x2t2 - x3 t’),
]191For a monodisperse solution of noninteracting macromolecules the time dependent term is a single exponential with a timeconstant given by eq. (4) For a polydisperse solution the decay is non-exponential_ Further, deviation from a single exponential contains information about the size distnbution. A cubic polynomial fit to the logarithm of the time dependent part of the measured correlation function, In [G(t) - G(m)] ,
(6)
where x1 = K’D. 2-x2 = K4D% and 6x, = K6D3s; v and s represent the relative variance and skewness from monodispenity. Assuming the vesicles to be spherical, the measured diffusion coefficient, D, in a medium of viscosity q is related to the hydrodynamic radius, RH _ of the scattering particle by the Stokes-Einstein equation: D=kTj6jinlRH.
(7)
The possible shapes of vesicles can be calculated by assuming negligible solvation and that the observed value ofDO/D is caused by the asymmetry of the vesicles. The Perrin equation [2 I] for prolate ellipsoid (semiaxes a,b,b) is: D,/D
= RfRo
and for oblate ellipsoid (semi-axes DoID = R/R,, =
(&$
-
a,a,b)
1)‘E
(~/~)‘15tan-‘($/~’
--_ I)‘;”
(9)
where R, is the radius of a sphere of equal volume to the ellipsoid, i.e...$srR~ = 4iiab’ (prolate ellipsoid) or $rK; = $r~‘b (oblate ellipsoid)_ Vesicle shapes can be confirmed by independent measurements of the radius of gyration. R, _According to the equation: Eo
Kc/R(@) = (l/z,\)[l
f (16&h’@
(5)
where 8 is the scattering angle (4.5”). Under the present condition K was taken to be 1.039 X IO4 cm-l
[20] :
sin2(0;2)] (10)
plots Of KC/R(@) against sin”(e/2) at given concentrations of vesicle yield straight lines whose slope is related to R, by [19] : Rg = (3X’/16~~)(1~~)
(slope).
(11)
4_ Results and discussion Fig. l_ illustrates typical light scattering data according to eq. (3) Table 1 summarizes the results for DODAC 271
1000 34,l 21,G 33s
0 OS5 IO,0 IO,0
DOD/K
DOD/K!
DODAC 6)
12,o 24.0 4880 6060 6080
DllP
DIR
DllP
DtlP
DWi) m-“,.1
97
29q2
23,6
358
64,8
11s
S60
.
.
.
10’D
-_.
3,56
4s3
480
402
3,lO
2446
2,RS
5,92
5,77
7871
St25
2435
(cm* s”‘)
__”
I_“_
669
525
595
592
772
968
843
396
407
309
447
_.*I*..-..-
1000‘-I
Rt[ (A)
.
2.4
2.7
3,3
2.9
3.1
302
1.7
2.8
208
-
1.6
2,6
5.1
-
-..
R/R0c,
I
--__
1200
1488
1275
___I
II
12
225
3s 42
630
II
461
687
__-_
2740
2320
2930
2680
3552
4690
2647
1760
la10
951
1850
-*--
atit)
n (A) /J(A)
prokttc d)
92
s!i
45
56
66
76
225
41
42
89
53
/J(A)
_....-l”,...“..-”
obllltc (1)
Cnlculatcd shpo _-....__-
2090
2136
2640
2453
3200
3900
187
74
58
79
103
140
862
49 862
44 1680
126
G5
b (A)
2200
812
1860
a(A)
prcdt\tc Q)
9.
a) At 2S,0°C!, unless stntcd otkcrwlse, cstlmntctl errors nrc IO% b, Using n hlson 11.12 sonificr, DODAC dispcrsh wcrc sonicatcd at 55% nt II scttlng of 3; DW dispersions wcrc sonlcntcd nt GS’C! nt a scttlny of 7, c, Mcn~ur~d hydrodynnmicradiusclrvldcd by the rndi~~sof n splrcro ~nlc~~luted from the dotcrtt~lned ntolcculnr w~i6ltt [see eqs, (8) nnd (9)], d, Cnlculatcd from cqs, (8) nnd (9), nssuminl: thnt tbcrc Is no wntcr irirrdc the vesicles (La., thy nrc complctcly frlletl with surfuctnnts), c, Cnlculntcd by nsswmlng slnglc compnrtmcnt bllnyor vcsiclcs, hnving bllaycr thlckncss of 60 A, f) Too big for corrclntian, g) Dctcrmincd at 35% D vnluc chnngcs from 4S3 X 10” cm* it to 2.1 X lo”” cm2 <’ durlnb ln~ns~lr~r~tcnt, i, Vcsich 1~1sbean ngcd for 5 days,
-.“....1__--e-w..--
3,o 640
12,G
60,O
DODAC
DIIP
13,4
DODAC
DIil’
31s
12,o so,0
DODAC
n’i\y
DODAC
I CP
(~lnltotl)
Sauicntion tiwz (rytirl) b)
V&AC
Surfnctnnt
Volume 64, number 2
0
...*‘****‘.****
1
CHEMICAL PHYSICS LIZTIERS
1 Jul> 1979
OO 0
b
2 3 WbJHi
l_ Plots of data according to eq_ (3) for 12 minute sonicated DODAC and DHP vesicles_
2
\,
....‘.. 0
50
Fig.
time. In the absence of sonication extremely large non-uniform particle formation is seen In general, formation of DODAC vesicles requires less applied sonic power than that needed to form DHP vesicles. Increasing the sonication time, at a given power setting, results in an exponential decrease of the size of the vesicles, down to a point beyond which further sonication does not alter the size of the vesicles_ Weight averaged molecular
0
lm
. . .
1
60
.
.
I-l,xxx
2mn!.E [o-q
Fig. 2. Plots of data according to eq. (6) for 12 minute sonicated DODAC and DHP vesicles
and DHP vesicles as a function of the sonic&ion
weights and hydrodynamic radii of DODAC and DHP vesicles at their plateau values are 12.6 X lo6 and 23 X IO6 dalton; and 396 and 595 A, respectiveby. It is interesting to note that at 35”C, close to the phase transition temperature, vesicles are unstable. AIso on standing, both M,,, and RH increase (see table l), presumably due to vesicle-vesicie fusion. Fig. 2 illustrates typical plots of the data according to eq. (6). We calculate the polydispersitjr indices, uvalues, to be 0.26 and 0.22 for DODAC and DHP vesicles, respectively_ These data Indicate DODAC vesicIes to be more polydisperse than their anionic counterparts_ The calculated shape factors, R/R0 values in table 1, indicate pronounced deviations from sph=ricaI structures_ Neither DODAC nor DHP vesicles can be described as ablates since the calculated semi-axes are less than the thickness of a single bilayer, 60 A [5,6] _ The present data are most compatible with the formation of single compartment prolate DODAC and DHP vesicles at sufficiently high applied ultrasonic irradiations_ The determined values of RG [using eqs. (10) and (1 l)] are in accord with this postuIate_ In general, discrepancies between the calculated and determined ii?,-values ex-
elude spherical shapes. However there is good agreement between the calculated and determined values if prolate or rod-like shapes are assumed for these synthetic surfactant vesicles_ Thus, for example, using the relationship R& = L2/12 (where L is the length of a rod) [21], L = 4020 A is obtained for aged DHP vesicles_ This value differs from *hat calculated for a filled vesicle (2~ = L = 2 X 2740 = 5480 A, see table i), but it is in a very good agreement with that calculated by assuming single compartment bilayer vesicles (2~ = L = 2 X 2090 = 4180 A, see table 1).
Acknowledgement Financial support of the Swiss National Fund is gratefully acknowledged_ We thank Professor M. Gritzel for his interest and Dr. J. Kiwi for setting up the instrument and for helpful discussions_
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