ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 6, с. 924-929
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КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 543.4
EFFECT OF DIPALMITOYLPHOSPHATIDYLCHOLINE ON A MICROEMULSION
© 2015 г. Soheil Sharifi* and Aboozar Nasrollahi**
* Department of Physics, Faculty of Sciences, Ferdowsi University ofMashhad, Mashhad 91775-1436, Iran ** Department of Physics, University of Sistan and Baluchestan, 98135-674 Zahedan, Iran E-mail: ssharifi@ferdowsi.um.ac.ir, soheil.sharifi@gmail.com Received December 18, 2014
In this work, the dynamic behavior of droplets under addition of Dipalmitoylphosphatidylcholine (lipid) and CTAB (salt) is studied by photon correlation spectroscopy. The collective diffusion coefficient (Dc) of Brownian motion of droplets/salt and droplets/lipid was explored by photon correlation spectroscopy (PCS). The PCS experiment demonstrated that Dc of AOT/H2O/n-decane microemulsion increased with increase of lipid and CTAB. For understanding the dynamic of droplets, we investigated viscosity and droplet interaction in Lipid/AOT/H2O/n-decane microemulsion. This system illustrated a well-known maximum in relative viscosity as a function of water/AOT molar ratio near X = 7. The maximum of viscosity can be varied by adding lipid at different concentration. Small angle X-ray scattering measurements revealed that the morphology of droplets changed from cylindrical to spherical with increase of lipid amount in the droplets. The same effect was observed in the mixture of water droplets with CTAB.
DOI: 10.7868/S0030403415060203
INTRODUCTION
Light scattered by an obstacle is related to its physical properties and, hence, in principle it is possible to obtain information about the scatter form analysis of the scattered light [1—3]. In the PCS, the normalized intensity time autocorrelation function g2(q, t) was measured [4, 5]
g 2 (q, t)
= (I (q,0) I (q, I)) T (q,0))2 ,
(1)
where I(q, t) is the scattered intensity at a given q and time t. The g2(q, t) function is related to the normalized electrical field correlation function g1(q, t) by the Siegert relation assuming that the system is an ergodic media [6, 7]
g 2 (q, t ) = 1 + Bg (q, t )|2 (2)
where B is the coherence factor of the equipment. In the case of Gaussian scatters the intensity correlation function g2(t) measured in a homodyne experiment is related to the field correlation function g1(t) by the Siegert relation [6, 7]
gi (t ) = exp (-t/т)
ß
(3)
where t is the delay time of light scattering. The stretched exponential function describes the decay processes that have a distribution of relaxation times (t). The parameter p (0 < p < 1) measures the width of the
distribution function. The mean value of the relaxation time is given by [6, 7]
<т> - J
exp
dt = тГ' 1
ß Iß
(4)
where r is the gamma function. The collective diffusion coefficient is calculated from Dc = 1/(q2(T)). The dynamic properties of microemulsion and telechelic polymer mixtures are not completely understood despite their impact on the rheological properties or the kinetics of solubilisate exchange [8, 9]. So far, most of the work regarding the dynamics was concerned with PCS experiments, which for the case of highly viscous networks showed two or three relaxation modes [10—12].
A ternary microemulsion (ME) system (water in-oil) is formed easily with the anionic surfactant Aerosol OT (sodium te-(2-ethylhexyl) sulphosuccinate, AOT) together with water and oil. The composition of each system is determined by the molar ratio X of water to surfactant molecules, X = [H2O]/[AOT] and the droplet mass fraction mfdop = (mass of droplet)/(total mass) which varies by the respective mass of the components: water (mH O), decane (mdec) and AOT(mAOT). The dynamic attributes of microemulsions can be explored by photon correlation spectroscopy (PCS). Furthermore, the morphology and structure of ME can be probed by small angle X-ray scattering (SAXS) [13-15].
0
Previous studies on AOT/H2O/n-decane ME for low molar ratio reported the spherical-cylindrical transition (effect of TBAC) [16]; however, for molar ratio of X = 40 and 10 were observed for different mass fractions remained the shape of droplets are spherical. Chen and coworkers found that the presence of attractive interaction resulted in an escalation in viscosity of solution. Bergenholtz et al. showed that the viscosity of AOT w/o MEs displayed a maximum as a function of water/AOT molar ratio at constant volume fraction could be related to an obvious maximum in the attraction between droplets [17]. It was investigated the origin of the maximum in viscosity by a direct correlation between the viscosity maximum and a maximum in inter-droplet attraction through dilute viscometry, static light scattering and dynamic light scattering [17].
The present work aims to investigate the correlation between the morphology of droplets and its collective diffusion of light scattering. We examine the behavior of the collective diffusion coefficient at the molar ratio X = 7 and low mass fraction (0.01 < mf < < 0.08) by addition of lipid and salt, by means of DLS and SAXS techniques. Moreover, viscosity measurements are performed to find out behavior of light scattering of ME at X = 7.
EXPERIMENTS Material and Preparations
Dipalmitoylphosphatidylcholine (DPPC), were ordered from Avanti Polar Lipids (Alabaster, AL). So-dium-2-diethylhexyl sulphosuccinate, or AOT 99%, n-decane (99%), and lipid were purchased from Sig-ma-Aldrich Company, Mainz, Germany. AOT 99% (an Alfa product), was dried in vacuum. Deionized and distilled water were employed to prepare the samples for the light scattering and SAXS measurements. The water-in-oil MEs were prepared by mixing of surfactants AOT, H2O and oil (n-decane) and waiting for several minutes until the samples were single phase and optically clear. The composition of the AOT/H2O/n-decane ME is given by the two parameters X and mfdrop. The mixing of lipid with MEs is described with the molar ratio of lipid to AOT, Y = [lip-id]/[AOT]. The mixing of CTAB with MEs is described with the molar ratio of CTAB to AOT, Y= = [CTAB]/[AOT]. The mass fraction of droplets
(mf,drop = (mH2O + mAOT)/ (mTotal) X which mH2O is
mass of water and mAOT is mass of surfactant and mTotal is the total mass.
Dynamic Light Scattering
The Malvern photon correlation spectroscopy instrument at Ferdowsi University of Mashhad was used in this experiment. The light source is a He—Ne laser, operating at a wavelength of 632.8 nm, with vertically
polarized light. The scattering angle 6 was constant at 90°, and the q is the scattering wave vectors (q) [6, 7]
* = X sin if',
(5)
where n is the refractive index of the solvent, X is the wavelength of the laser or X-ray, and 6 is the scattering angle. An alternative way of analyzing multimodal relaxation processes is by fitting g1 (t) to a multiexponen-tial function. For the cases discussed here a very suitable functional form was found to be stretched exponential decay describing the relaxation, which is given by Eq. 2.
Small-Angle X-Ray Scattering
Small-angle X-ray scattering (SAXS) measurements were performed using the pinhole SAXS instrument at Nanolab company (KNL Iran). The experiments were done at a fixed wavelength of A = 1.54 Â. The scattering intensity as function of the scattering vector I(q) of spherical particles can be described with a form factor component F(q), which is proportional to the scattering of a single particle, and a structure factor S(q) [18]:
I(q) = cF (q)S(q),
(6)
c being a prefactor, which contains the number density of scattering particles and S(q), describes the interaction effect. For the general case of n shells around a spherical droplet core the form factor reads [19, 20]
i=0
t
F(q) = 4nV Ap,. ( sin(qR) - qRi cos(qR)
I q
(7)
where Ri is the radius of the ith shell or, respectively, the core R0 and Ap,- is the electron density contrast between the shells i and i + 1 with pn +1 and p0 being the electron density of the solvent and the core, respectively. So, for a simple core-shell micelle n = 1. The structure factor is the Fourier transform of the pair correlation function g(r) [20]:
S (q) = 1 + 4nn f (g (r) -1) r2 dr.
J qr
(8)
The pair correlation function gives the probability to find another particle at a distance r from the center of a given particle, relative to the probability to find a particle at this distance in an ideal gas. The pair correlation function is related to the total correlation function h(r) = g(r) — 1 and it can be calculated with the Ornstein—Zernike equation [20].
82 0.8 0.01Y-. \\
0.4 0
82 0.8 0.4 0.009^"'^ T •
0
10-6 10-5 10-4 10-3 10-2 Time, ms
Fig. 1. Autocorrelation function of solutions, (a) AOT/H2O/«-decane/lipid ME with [lipid]/[AOT] = 0.0 and 0.01 molar with X = 7 and mfdrop = 0.07 and (b) AOT/H2O/^-decane/CTAB ME with [CTAB]/ [AOT] = 0.0 and 0.009 molar with X = 7 and mfdrop = 0.07.
The simple expression for the scattered intensity can be obtained by applying appropriate averages in Eq. 9 [20]
I (q ) = c {\F (q )\) 2 5 (q ) + (\F (q )\2) -- (\F(q)\) 2} + Ib9
(9)
where (• ••) denotes an average over a distribution of radii, i.e., we only consider size polydispersity, as size and shape polydispersity cannot be distinguished in the SAXS experiment. To calculate the averages we choose a gamma distribution for the distribution of radii.
Models that incorporate orientation correlations are available for slender rigid rods [21—24]
Viscosity
Viscosity measurements were carried out using three dilution-type Ubbelohde capillary viscometers (Cannon Instrument Company), covering roughly the viscosity range 1.6—35 mPa s. The temperature of the viscosity measurements was controlled to within ±0.03°C by keeping the viscometers submerged in a constant-temperature water bath [16].
RESULTS AND DISCUSSION
The ME were prepared by mixing AOT with ^-de-cane and H2O at the constant molar ratio of water to AOT (X = 7). A
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