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The characterization of oil in water emulsions by means of a dielectric technique
The Characterization of Oil in Water Emulsions by Means of a Dielectric Technique R. M. HILL, *'1 E. S. BECKFORD,* R. C. ROWE,]" C. B. JONES,]" AND L. A. DISSADO* *The Dielectrics Group, Department of Physics, King's' College London, The Strand, London WC2R 2LS, and 4fICI Pharmaceuticals, Alderley Park, Macclesfield, Cheshire SKIO 2NA, United Kingdom Received October 23, 1989; accepted February 2, 1990 Dielectric spectroscopical investigation of an oil in water emulsion in the frequency range from mHz to kHz has shown that the response below about I Hz is dominated by the physical barriers formed by the lecithin emulsifier whereas the aqueous bulk properties of the emulsion control the response above 1 Hz. Unlike the more complex gel systems already examined using this technique the ac response of the emulsion can be characterized by only five physical parameters, i.e., a high-frequency ac conductance in parallel with a capacitance, the magnitude and fractional power law exponent of the barrier capacitance, and the barrier leakage conductance. This has allowed semi-quantitative assessment of the roles of the individual constituents of the emulsion and the effect of introducing a substituted phenol to the system. The technique has been found particularly useful in observing the stability of the system to both temperature cycling through the freezing point and exposure to air. © 1990AcademicPress,Inc.
conditions or exposure to air. This makes the technique particularly useful for the observaThe use of dielectric spectroscopy in the low tion or monitoring of processes that can lead frequency range below 10 4 Hz for the examto deterioration such as globule size coarsening ination of the physical structure of gel and a n d / o r phase separation. In order to underemulsion systems in a noninvasive manner has stand and characterize the dielectric response already been established (1, 2). Here we dein terms of the known physical structure of scribe the application of the technique to an the emulsion it is necessary to have available oil in water emulsion which has been designed information from a range of structures/comas a carrier for a lipophillic substituted phenol. positions. This is most conveniently done, as In particular we report on the stability of the here, by examining the effects of concentration emulsion system under freeze/thaw cycling changes of the constituents of the emulsion and exposure to air. and relating the observed dielectric response Unlike most other techniques used for the changes to the individual constituents. In adexamination of emulsion systems, for examdition we have examined the effect of using ple, photon correlation spectroscopy and ~" constituents from different suppliers and show potential measurement which both require dithat there are measurable differences. lution, the dielectric technique is applied to Unlike our earlier work on gels prepared the material to be investigated while it is in its from cetostearyl alcohol (1, 2) preliminary normal, as-prepared state. Changes in the diwork on the lecithin-based emulsions indielectric response can be observed directly as cated that, in the frequency range used, the the material is subjected to either freeze/thaw dielectric response was relatively simple and could be characterized in electrical terms by To whom all correspondence should be addressed, the series connection of a pair of parallel conINTRODUCTION
521 0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
Copyright © 1990 by Academic Press, Inc, All rights of reproduction in any form reserved.
522
HILL ET AL.
nected conductance and capacitance elements. This type of construct is c o m m o n l y termed a Maxwell-Wagner response (3). The only complication observed here was that the larger magnitude capacitive element was power-law dispersive in frequency; i.e., Cs = Cl(iw) n-I,
[1]
where C1 is the magnitude of the capacitance at 1 radian s -1 frequency and ~o is the frequency in the same units. The fractional power-law response, characterized by the exponent n with 0 < n < 1, has been generally accepted as a relaxation characteristic (4, 5) and has been shown to arise from cooperative interactions within the medium (6). The observation of such responses was general in our investigations of the gel systems and the more limited contribution observed here is most likely to be due to the absence of a gel network in this system. The simplest, low-concentration, particle-in-a-medium models reviewed by van Beck (7) and Hanai (8), can be described reasonably well in terms of only conductances and capacitances in parallel/series combination with a low complexity in their connection. The characteristic of Eq. [ 1] however requires a high connection complexity if it were to be obtained in this manner (9) and it is conceptionally useful to consider that the basis of such a response lies in a cooperative process within the medium.
EXPERIMENTAL
PROCEDURE
All emulsions were manufactured on a 2L scale by mixing the oil phase (soybean oil, lecithin, and the substituted phenol) and the aqueous phase (glycerol, NaOH, and distilled water) with a high shear mixer (Hydroshear HSI) and then homogenizing for 10 rain using a Manton Gaulin homogenizer (Model 15M). All processing was performed under nitrogen at approximately 70°C. The emulsions were filled into 15-ml ampoules under nitrogen and sterilized using an autoclave. All emulsions, as prepared, had a mean globule size between 0.21 and 0.24 ~tm as measured by photon correlation spectroscopy. After freezing, the sizes increased to greater than 3 #m, above the range of the instrument. The full range of sample compositions is listed in Table I. The dielectric sample cell is shown in section in Fig. 1. Because of the alkalinity the emulsion samples were held in a polypropylene thimble into which the metal electrodes were inserted. The thimble and electrode assembly were set in a copper vessel which was flushed with nitrogen and sealed. The complete cell was mounted on a h o t / c o l d stage of a vacuum cryostat and the temperature of the emulsion was measured by means of an internal thermocouple. The temperature was maintained to + I ° C during measurement. Initially problems of stability were encoun-
TABLEI Sample Compositionin Percentage w/v Composition
Sample no.
Soybean oil (Source A) Soybean oil (Source B) Lecithin (Source A) Lecithin (Source B) Glycerol Substituted phenol Oleic a c i d Distilled water to measured pH
1
10.0 . 1.2 . 2.25 -. 100 8.2
2
10.0 .
. 1.2 .
. 2.25 1.0
.
. 100 8.0
3
4
5
6
7
10.0 . 1.2 . 2.25 2.0 . 100 8.1
10.0
-10.0 1.2
10.0 -1.2
2.25 1.0
10.0 --1.2 2.25 1.0
100 7.5
100 8.9
2.4 . 2.25 2.0 . 100 7.1
.
2.25 1.0 0.085 100 7.3
Note. NaOH has been added in sufficient quantity to give the measured pH values after autoclaving. Oleic acid is used as an emulsifier and is often added in order to increase the stability of the formulation. Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
523
EMULSION CHARACTERIZATION
ELectrode leads J ~ ..._---Thermocouple E
~N~/Copper CeLt -'VII I/Potypropytene ' Liner
7~--~
plates (Pf)
FIG. 1. Cross section of the dielectric cell. The emulsion sample is held in the propylene thimble within the copper cell which is in turn mounted on the hot/cold stage of a vacuum cryostat. The cell is flushed with nitrogen and sealed at atmospheric pressure by the insulating block that carries the electrode leads and the measuring thermocouple. The cell plates are approximately 0.01 by 0.01 m and are immersed in the emulsion.
tered but it was found that these arose from exposure to air or from chemical action between the emulsion and the cell plates. It was found that neither copper nor stainless-steel plates were satisfactory but that platinum plates, with scrupulous cleaning, removed this problem so that both stability and reproducibility of the responses from sample to sample and over extended measurement periods were excellent. The platinum electrodes and their mounting legs were thin and slightly flexible. This was considered necessary in order to give a degree of flexibility as the sample was cycled through the freezing/melting point. However, it did mean that the electrode spacing was variable from run to run and during cycling. For this reason it has not been possible to determine the cell constant (electrode area/ spacing) and hence to determine from the measured capacitance, C(w) the relative permittivity, ~(w), of the samples nor has it been possible to make absolute comparisons of capacitances from run to run. For these reasons the analysis of the data is based on the f o r m s of the observed dispersions rather than their magnitudes.
The frequency response analyzer and the associated equipment has been described in detail elsewhere (1, 10) and the interested reader is referred to these papers. THE ANALYSIS OF THE DIELECTRIC RESPONSE
The dielectric technique measures the response of the sample to an impressed ac voltage of radian frequency ~o. For a perfect insulator the response is capacitive in nature and reflects the change in magnitude or orientation of fixed charge dipoles under the action of the field. As the response is not instantaneous there are both in-phase and out-of-phase components to the capacitance, i.e., C(w) = C'(w) - iC"(w),
[2]
where i = ( - 1 ) 1/2. In particular when the sample exhibits a conductance G this contributes a component of magnitude i G / w to the imaginary part of the complex capacitance. As the term C"(~0) + G / w results in power loss it is commonly referred to as the loss component of the capacitance. In the aqueousbased emulsions considered here the frequency response will be dominated by the effect of the continuous aqueous phase which has the anomalously high relative dielectric permittivity of S0 so that for the geometry of the cell used, with plates of approximately 10 -4 m 2 area spaced about 10-3 m apart, the magnitude of the capacitance is given by C'(o~) = eeoA/d
[3]
as about 7 × 10 -11 F, where ~0(=8.854 × 10 -12 F m-~ ) is the absolute permittivity of vacuum and A / d is the cell constant. In parallel with this we have the loss component which will be dominated by the conductance of the NaOH solution. Taking the conductivity of the solution as 2.5 10 -2 ohms -I m -I gives the loss component at 104 Hz as in the region of 4 × 10-8 F, almost three orders of magnitude greater than the capacitance and hence the high-frequency response will be expected to be dominated by the conductance of the aqueous phase. Journal of ColloM and Interface Science,
Vol. 138,No. 2, September1990
524
HILL ET AL.
In our earlier work on gels prepared from cetostearyl alcohol (1, 2) we found evidence for the coating of the electrodes by a layer which exhibited all the properties of a single layer of the complex physical structure. In the work reported here, because of the strong negative charge of the oil droplets and the complex nature of the surfactant based coating around the droplets, we expect a similar deposition onto the electrodes but now with a more or less closely packed array of complete globules in which the oil droplets are separated from themselves, and from the electrode by a surfactant-water layer. If the packing were perfect, such a boundary layer would completely block the movement of charges from the bulk of the emulsion to the electrodes. We do observe, and will report, such blocking which indicates the formation of at least a monolayer of globules. The effective permittivity of such a layer will be dominated by the water bound into the double surface layers. Estimating the thickness of these to be 3 n m and taking the permittivity to be that of water gives a barrier capacitance of about 1.3 10 -5 F. This is approximately two to five times less than actually observed and hence the effective barrier layer is thinner than we have estimated and likely to be dominated by the presence of imperfect packing of the globules. Thus although the observed response is simpler than that found for the gel systems previously studied the descriptions of changes observed in the response will necessarily be less direct. Experimentally we shall show that the barrier layer capacitance is not independent of frequency but takes the dispersive characteristic of Eq. [1] with n = 0.8, indicative of a small degree of disorder in the surfactant-water surface layers when the globules themselves are imperfectly packed. Nevertheless the capacitance is higher than that of the bulk of the emulsion and hence there will be a large dispersion, of about five orders of magnitude, in the capacitance and the frequency at which the dispersion would be expected to reach half its m a x i m u m value will be given by the inverse of the resistancecapacitance time constant as about 10 2 Hz, Journal of Colloid and Inte~face Science, Vol. 138, No. 2, September 1990
which is within our measuring range. The seties/parallel connection of the circuit elements is indicated in Fig. 2 ( b ) as an inset to the frequency response characteristic of the complex capacitance derived from the circuit. In the log/log plot used here the individual components are easy to pick out. The bulk capacitance Cb is constant and the bulk and barrier conductances, Gb and Gs, give responses in the plot that are inversely proportional to the frequency. The barrier capacitance is characterized by the real and imaginary components being of the same gradient with frequency, n 1, and with the separation given by tan(nTr/ 2) for ( i w ) n-I = w(n-l)exp(i(n - 1)7r/2) = w(n-l)[sin(nTr/2)
- i cos(nrc/2)].
[41 In parallel connection the individual components sum together but in series connection it is the reciprocals that sum and hence the smaller component dominates. It is for this reason that the bulk properties develop out of the barrier response as the frequency is increased. A full description of the capacitive response of the model is given in the Appendix
) \
o/
g
_3 °
C1(ioj f.~(.hl Cb t
L'-'I
Ch ~
t
Log[frequency
t-
x...~._, I
i
x I
]
FIG. 2. The calculated frequency response (a) for the circuit shown at (b). Gband Cb referto the low-magnitude conductance and capacitance of the bulk of the sample; the power-lawdispersivecapacitanceCl(io~)'-1 blocksthe bulk conductanceat frequenciesbelowo~x and is in parallel connection with the barrier leakage conductance Gs. A full analysisof this responseis givenin the Appendixwhere the low-and high-frequencylimitingbehaviorsare derived.
525
EMULSION C H A R A C T E R I Z A T I O N
and has been used to give the curve fits to the experimental data in the following figures. E X P E R I M E N T A L RESULTS
Characterization of the Emulsions Following the schematic representation of the expected response of Fig. 2 we present the experimental data in the form of log/log plots of complex capacitance as a function of frequency. Where possible we shall compare data sets for which known differences in the emulsion make up or treatment occur. This eliminates, to a large degree, our problem of plate stability. The parameters used in the curve fitting are given in Table II. For example, in Fig. 3 we present two data
sets, measured at four points per decade in frequency, for sample 2 which contains 1% w/ v of the substituted phenol. The only difference between the data sets occurs above the crossover frequency Wx and can be fully characterized by a decrease in the bulk conductance from that indicated by the crosses by a factor of 1.70. Experimentally the only difference was that the plate separation from one run to the other was increased by about two. Notice that the conductivity of the bulk, o-b, has not changed--it is the conductance given by Gb = abA/dwhich is inversely proportional to the plate separation. The crossover frequency is also reduced, through Gb, as is the high-frequency bulk capacitance Cb and it is these two factors together that give the decrease
TABLE II Dielectric Response Parameters
Sample
1 2 2 (a) 3 4 5 6 7
(mhos)
4.42 2.8 3.38 4.42 1.33 5.98 9.88 2.32
10-6 10 -6 10 -6 10 -6 10 -6 10 -6 10 -6 10 -6
(mhos)
3.1 5.8 3.4 4.42 7.98 8.97 1.46 4.65
10 -3 10 -3 10 -3 10 -3 10 -3 10 -3 10 -3 10 -3
n
0.87 0.87 0.87 0.76 0.80 0.87 0.87 0.87
(F)
4.16 3.63 3.93 2.92 2.06 3.98 3.67 4.22
10 -5 10 -5 10 5 10 5 10 -5 10-5 10 -5 10 -5
3.07 2.36 1.37 2.73 3.21 2.77 2.30
10 5 10 -s 10 -5 10 -5 10 -5 10-5 10 5
Fig. no.
4 3, 4, 5, 7 3 4 5 7a 7a 7b
Measured after three freeze/thaw cycles 1 2 3 4 5 6 7
2.0 1.0 2.05 2.67 2.76 5.28 1.6
10 -6 10 6 10 -6 10 -6 10 -5 10 -6 10 -6
2.0 2.0 2.05 1.07 1.84 1.42 3.20
10 -3 10 3 10-3 10 -1 10 -2 10-3 10 -3
0.80 0.87 0.80 0.80 0.87 0.87 0.76
m
8b
Material exposed to air before measurement 4 5 6
<3 2.3 1.6
10 -6 10 -6 10 -6
6.6 4.5 1.6
10 -3 10 -3 10 -3
0.69 0.80 0.76
4.3 1.91 2.49
10 -5 10 -5 10-5
7
1.7
10 -6
7.4
10 -3
[ 0.80 L0.4
2.3 5.3
10-5 ~ ] 10_5 j
9a 9b
9c
There is evidence in this response for more than one barrier relaxation process although all the materials exposed to air showed responses that were indicative of distributed properties in the barrier components. Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
526
HILL ET AL. -3 A LL
-5 tJ t-
3 o
f "2
f
K
i
0
Log [frequency ]
i
I
2
I 4
(Hz)
FIG. 3. The dielectric response measured from an emulsion containing 1.2%substituted phenol (sample 2). Two data sets are given, that indicated by the symbol O was obtained with the cell plates spaced further apart than the set indicated by the crosses. Note that the cell spacing has no effect on the response for frequencies below the crossover frequency, wx.
in C'(~0) that is observed for w > Wx. The data presented in Table II were obtained by curve fitting to both data sets using the full response expression given in the Appendix although, for clarity, only one of the theoretical response functions is given in the figure. The diagram clearly shows the dominance of the bulk properties for w > Wxas well as the insensitivity to the plate separation at the lower frequencies. In the low-frequency region the fractional power law is well developed for the real component of the capacitance but masked, at the lowest frequencies, by the bartier conductance contribution to the loss component. From the gradient of the log/log plot the exponent n has been determined as 0.87, indicative of a high degree of order within the surfactant-water surface layers of the globules packed against the electrode ( 1, 2, 9). The effect of including a substituted phenol in the emulsion response is indicated in Fig. 4. The three data sets here correspond to zero, 1, and 2% w / v of the c o m p o u n d with the highest capacitance in the barrier corresponding to zero concentration. The magnitudes of the barrier capacitances, at 1 r a d - s -~ are in Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
the ratio 1:0.95:0.70 so that purely in terms of effective thickness of the barrier adding 1% of the compound has a very small effect (N5%) but at 2% concentration there has been a 30% increase in the barrier thickness. It is reasonable to assume, from this plot, that the substituted phenol resides, at least in part, in the barrier as there has been no significant change in the bulk conductance and we can deduce from the value of n that at the 1% level the c o m p o u n d has no significant effect on the order within the barrier. Distortion is however present at the higher concentration of the c o m p o u n d and we would expect that at some even higher concentration the stability of the system would become affected. We have, however, implicitly assumed that the substituted phenol does not displace water from the barrier. If this were to be the case we would expect a significant decrease in the permittivity of the barrier and hence a decrease in the bartier capacitance but this would be accompanied by a decrease in the stability of the barrier which has not been observed. It has been implied in the above that the barrier layers can be associated with the lecithin emulsifier. That this is correct is shown -3 I.I..
-5 8
--1. 9
I "2
I
i
I
0
Log[frequency]
2
4
(Hz)
FIG. 4. Comparison of the responses measured from emulsions with zero(©), 1.0 (+), and 2.0%(X) substituted phenol (samples 1, 2, and 3, respectively). The curves of C'(w) and C"(w) have been fitted to the 1.2%data points and are taken as the standard response in a number of the followingfigures.
527
EMULSION CHARACTERIZATION
by the data in Fig. 5 where a comparison is given between the response measured from a formulation containing 1.2% lecithin (sample 2) and that prepared with twice this concentration (sample 4). There is little change in the bulk conductance but the nature of the response in the barrier region has been altered. From the tabulated values we see that the bartier conductance has decreased by a factor of 2.1 and the barrier capacitance by 1.76. As for the addition of the substituted phenol there is a decrease in the order parameter n to 0.8 and, finally, in the frequency region around 10 -2 Hz there is a secondary dispersion in the barrier response. We deduce, then, that doubling the concentration of the lecithin has not affected the bulk properties, it has, however, affected all the three parameters of the barrier in such a way that the apparent thickness of the barrier has increased, its physical order has reduced, and a secondary relaxation process has been introduced. Summarizing the experimental evidence presented in Figs. 3, 4, and 5, we can define the role of each of the individual components of the oil in water emulsion. The soybean oil forms droplets which are coated with lecithin emulsifier. Although the range of compositions
x 106
9 8 o +E
7 6
vI L.~ Q/ u t-
5
"t~O i._J
3 (
2
0 0
I
I
I
I
L
1
2
3
4
5 x 10 5
Capacitance
C 1 (F)
FIG. 6. A plot of the barrier conductance Gs as a function of the barrier capacitance magnitude CI for samples with 1.2% w / v lecithin grouped in substituted phenol concentration; zero phenol ( ~ ) , 1% phenol ( × ) , and 2% phenol (©). The samples indicated with the squares were prepared with source B lecithin. The data are taken from Table II and contain both the as-prepared and the freeze/thaw cycled samples. Data points lying on a line that passes through the origin would indicate that the conductance is intrinsic to the barrier layer. Note that the dashed line has half the gradient of the full line.
-3 o
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° ~
~
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~
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~-7 o ._A
.i
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I "2
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Log[frequency ]
I
I~ =
*
2
4.
(Hz)
FIG. 5. A comparison between the responses observed between the sample containing 1.2% lecithin (sample 3) and that prepared with twice the amount of lecithin (sample 4). It can be seen that the capacitance of the blocking barrier layer has decreased by a factor of about two. Note that the change in C'(~o) at high frequencies is due to the change in the low-frequency, barrier capacitance.
has been limited the 1.2% w / v concentration gives a physically well-ordered barrier around the oil particles. The low-frequency, barrier conductance can be associated with the lecithin as shown in Fig. 6. In this figure we plot the barrier conductance, Gs, as a function of the barrier capacitance (at 1 rad- s l ) for the range of samples listed in Table II from sample groups 1 to 5 for both the as-prepared samples and those subjected to freeze-thaw cycling. A straight line in this plot, passing through the origin, would indicate that the basis of the barrier conductance lay ~n the barrier material itself and not in the substituted phenol. Such a relationship is clearly seen for the 2% substituted phenol samples and a dotted trace of half the gradient of the trace that passes Journal of Colloid and Interface Science, Vol. 138,No. 2, September1990
528
HILL ET AL.
through the 2% data points is a reasonable fit to the 1% substituted phenol data. Two anomalous data sets are apparent, that at zero substituted phenol content (A) and the samples prepared with source B lecithin. The basis of this analysis is that only for a homogeneous barrier will the relationships [5]
Gs = a A / d = ~C1/~o oc Ca
hold where o- is the bulk conductivity of the material for which the relative permittivity is E. We deduce, then, that doubling the substituted phenol content increases the conductivity of the barrier with a slight decrease in the barrier order but for phenol concentrations up to the 1% level there is no evidence of such an increase in conductance and hence the barrier itself has a conductivity level equivalent to that introduced with the addition of 2% concentration of the substituted phenol. As the final sample characterization plot we give, in Fig. 7, that measured for a sample prepared from an alternative source of soybean oil (sample 5 ) and a sample prepared with lecithin from the second source (sample 6, already referred to), both of which contain the standard 1% of substituted phenol, and finally a sample to which 0.085% w / v of oleic acid has been added (sample 7 ). Comparison with
a
-3
the equivalent "standard" plots shows that the addition of the acid has had little effect on the bulk conductance, while the barrier conductance has decreased and the capacitance increased slightly. The use of a second source of soybean oil has made trivial differences to the response but the lecithin obtained from the alternative source, as noted in Fig. 6, gives a high barrier conductance notwithstanding that the capacitance magnitude and spectral response parameter n are very similar to those observed in the original material. We note, too, that the relationship (5) is not particularly well behaved in this material, which might indicate that a high content of mobile charge species is present. Temperature Cycling
An essential feature of this work was the investigation of the effects of temperature cycling the emulsions through their freezing point. Using the standard sample holder (Fig. 1 ) it was possible to cool the emulsions below the liquid/solid transition temperature and heat back to room temperature (16°C), without temperature overshoot, within an hour. The rate of heating and cooling was controlled by the thermal lag of the copper pressure vessel. As we wished to make a number of mea-
b
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-5
i
8 t-
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£
m
o
o
-9
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I -2
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I
L0g[frequency]
I 2
I
I
~.
(Hz)
I -2
I
I
I
0
L0g[frequency]
I
I
I
2
(Hz)
FIG. 7. Comparisons of the dielectric response from sample 2, given by the continuous curves and (a) samples prepared with soybean oil from a second source (O), sample 5, and with lecithin from the alternative source ( X ), sample 6. (b) Sample 7, the sample to which oleic acid had been added to improve the stability. Journal of Colloid and Interface Science, Vol. 138,No. 2, September1990
529
EMULSION CHARACTERIZATION
surements at a single frequency during the heat/cool cycle we chose the highest available f r e q u e n c y , 10 4 H z , to monitor the response. A typical time/capacitance plot is presented in Fig. 8. In the plot both the real and loss components of the capacitance are given. The real component has a maximum value, on freezing, equivalent to the high-frequency bulk capacitance Cb as the relaxation rate decreases significantly. The loss component is not limited in this way and large changes in the loss were observed. As shown in the figure each sample was cycled three times and then the full frequency response was measured at room
temperature. In the particular case shown after this measurement the sample was cycled a further three times and then left at room temperature for a period of 6 h to allow recovery. Two features in the plot are of interest. Firstly the response exhibits a change even after a single cycle and although subsequent cycles give further changes the first cycle invariably gave rise to the largest change. Second, although there is some recovery it is small in comparison to the degradation that occurs. The frequency response plots measured before temperature cycling and after three cycles are shown in Fig. 8b. It is clear that the forms
-7
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/+
(Hz)
FIG. 8. The effects of temperature cycling through the freezing point. (a) The real and imaginary components of the complex capacitance measured at 10 kHz. (b) The frequency response at zero time, the continuous curves, and after three cycles (©) for sample 2. The response after cycling of a sample prepared with the lecithin from the second source (sample 6) is given by the ×'s, although the original data for this sample were the same as that of sample 2. Journal of Colloid and Interface Science, Vol.
138, N o . 2, S e p t e m b e r 1990
530
HILL ET AL.
of the responses are equivalent--it is the magnitudes of the response elements that have changed. Table II lists these magnitudes for a range of samples that have been temperature cycled and a comparison can be made with the equivalent magnitudes measured on the uncycled material in the same table. For the particular sample shown in Fig. 8b the bulk conductance has decreased by 64%, the barrier capacitance by 35%, the barrier conductance by 66%, and the dispersion parameter n has not changed. As indicated in Fig. 6 the difference in the changes in the barrier properties indicates a change in the conductivity of the barrier. It is with this series of samples that we observed an increase in plate separation with temperature cycling and the effect of this can be seen in the decrease in the bulk conductance. The change in conductance, however, is greater than the increase in plate separation and it is likely that a decrease in the bulk conductivity has taken place. Although a number of the samples indicate changes in the magnitude of the dispersion parameter n there is no evidence of a catastrophic breakdown of the structure, for example, complete phase separation, on repeated freezing. For the sample shown in the figure the data begin to look like that obtained from the 2.4% lecithin sample. This would suggest that the observed changes are due to a decrease in the number of emulsion particles with a general coarsening of the emulsion structure consistent with the increase in globule size. It is clear that this coarsening was initiated in the first freeze/thaw cycle. Exposure to Air
During the preliminary work in this investigation it was observed that the low-frequency dielectric response was difficult to characterize and to reproduce. The use of nitrogen flushing of the sample vessel, of platinum electrode plates, and of rigorous cleaning of the cell and plates between runs removed these problems. In Fig. 9a we show comparative data for a sample measured under nitrogen and a second Journal of Colloid and Interface Science, Vot. 138, No. 2, September 1990
"+I
•
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.+.
' ," •
eL- 5
(el
- * e . ,'~ 0
•
8 .._J
i 8
-8
I -2
I
I
I
0
Log[frequency]
I 2
I
(Hz}
FIG. 9. The standard response is given in (a) by the continuous curves. The three data sets in the figure were all measured on samples that had been exposed to air. (a) sample 4, (b) sample 5, (c) sample 7. For all the samples that were exposed to air there is a collapse in the barrier characteristic. A perfectly diffusive charge transport would exhibit a characteristic in which the real and imaginary components have identical magnitudes and lie along a line of gradient -0.5 in these log / log plots.
sample from the same batch which was measured without prior nitrogen flushing of the cell. The difference is so extreme that it was not found possible to characterize the data from the latter using either our standard model of Fig. 2b or of any reasonable modification of it. This particular sample contained 2.4% lecithin but the responses (b) and (c), for samples 5 and 7, respectively, are indicative of those observed over the whole range of emulsions. All these responses show a weakening of the clear Maxwell-Wagner barrier characteristics which can be described as the development of a distribution of relaxation times or the onset of diffusion-like behavior. Under three-dimensional classic diffusion conditions the square root time dependence transforms as an (ion)-(t/2) frequency dependence with coincidence of the real and imaginary components of capacitance along a line of gradient - 0 . 5 in the log/log representation used here. Some evidence of diffusion-like behavior can be seen
EMULSION
CHARACTERIZATION
in Fig. 9 and this would imply a random motion of charges. In either case we have evidence in the figure for an almost complete breakdown of the lecithin-based barrier structure. It is clear that on the time scale of the barrier response, i.e., a second or so, we could use the fixed frequency technique to follow the dynamics of the breakup of a lecithin barrier in the presence of air. DISCUSSION
The principal feature of the dielectric properties of the oil in water emulsion which has been investigated is that it can be described, almost, by the simplest possible electrical barrier layer model of two parallel conductance and capacitance elements connected in series. As shown in Fig. 2 the single change that has had to be made was to let the barrier capacitance have a weak fractional power-law dependence on frequency. This has allowed us to relate the individual constituents of the formulation to the electrical elements. It should be realized, however, that this description is not in itself a model of the structure. In order to determine that, we have to consider how the electrical properties arise. The analysis of the response in terms of electrical series elements has been possible because the characteristic or crossover frequency, Wx, lies near the middle of our frequency range. If we had been limited to frequencies in excess of 100 Hz or so then all we would have observed would have been the bulk capacitance in parallel with the bulk conductance with perhaps, at the lowest frequencies, the onset of a massive increase in capacitance. In the past such an effect would have been loosely termed an electrode polarization, which it is, but which we have shown contains useful information about the system and which should not be ignored. It is only because with this system we have a relatively thick barrier and a high conductance in the bulk that both the low-frequency barrier response and the high-frequency bulk response appear in our frequency window.
531
By examining a number of formulations we have shown that the barrier layer, together with its fractional power law dispersion, is based in the surfactant coating of the oil droplets. This is in agreement with the known properties of the structurally complex barriers formed around oil globules by lecithin. Although the barrier that has been examined here has been formed around an oil globule and the droplet has been deposited onto a platinum surface as part of an irregular array, we have found it possible to characterize, in at least a semiquantitative manner, the effects of varying the composition of the emulsion, the source of the lecithin, and the introduction of a substituted phenol. If the surface layer formed were to be a perfect barrier then it would have had a frequency-independent permittivity and, hence, capacitance. The fractional power-law behavior actually observed is an indication that instead the charged moieties within the barrier can respond to an ac field, but only weakly. As the power-law parameter n becomes smaller the dispersion increases and hence we see that n defines the degree of order within the barrier itself (6). It is in this sense that we have made use of the dispersion parameters to describe the barrier properties. Because of the dispersion in capacitance there is a concomitant dispersion in the loss but this is hidden at the lowest frequencies investigated by the onset of a leakage conductance through the barrier. Again we have observed that this is an intrinsic property of the lecithin used rather than due entirely to the introduction of the substituted phenol to the dispersed phase. These results illustrate the flexibility of the dielectric technique in that it is not limited to the investigation of systems that give simple, well-ordered, barrier layers. It might have been expected that there could have been two components in the high-frequency response. One arising, as here, from the large water content within which the oil droplets are suspended, and the other from the conductance within the droplets. Here we have seen no evidence for the latter probably because the conductance of the aqueous phase Journal of Colloid and Interface Science, VoL 138, No. 2, September 1990
532
HILL ET AL.
is greatly in excess of that of the oils used. For a water in oil emulsion the situation would be reversed and the dominant high-frequency response would be expected to arise from the water within the droplets. As our high-frequency range is limited we have not been able to reveal the bulk capacitance which would undoubtedly have been given by the cell constant and the permittivity of the macroscopic emulsion which would have been almost that of the water. The technique of dielectric response was particularly suited to the examination of the stability of the emulsion in that instabilities could be followed without having to change the material in the measuring cell. Indeed the dielectric signature from the material exposed to air is so different from that which has been observed from the ordered lecithin barrier structure that the method forms a sensitive analytical test. The effects of exposure to air were found to be rapid and well within the spectral measuring period of up to 2 h. As the main effect is observed at the lower frequencies only a limited number of measurements can be made within the time period of the change in the response and hence only samples exposed for relatively long periods have been reported here. A similar problem arose with the changes in response induced by freeze/thaw cycling but there was sufficient change in the bulk conductance and in the capacitance dispersion at high frequencies to allow characterization at 10 kHz and hence the individual cycling events could be followed in time. The system is sensitive to freezing and the largest effect was observed on the first cycle although deterioration was cumulative with the number of cycles. In both cases the deterioration has been observed to be based in the barrier. CONCLUSIONS
The dielectric properties in the frequency range from 10 .2 to 10 4 Hz of a lecithin-based oil in water emulsion have been reported and analyzed in terms of a simple series/parallel electrical circuit. The effects of introducing a JournalofColloidandInterfaceScience,Vol. 138,No. 2, September1990
substituted phenol into the emulsion, of using materials from different sources, of exposure to air, and of freeze/thaw cycling have been investigated and it has been shown that the dielectric technique is particularly useful as a means of examining the fundamental nature of the macroscopic effects that have been observed. APPENDIX
For the two parallel capacitive/conductive elements of Fig. 2a the impedances are ( 1 ) Z s = [ i ~ ( C l ( i w ) "-1 - i G s / w ) ] -1
[A.1]
and Zb = [ k 0 ( C b - i G b / w ) ] - l .
[A.2]
As these are electrically in series the total impedance is given by Z=
Zs+ Zb
from which we obtain the capacitance C = ( i w Z ) -1 as C
l(sin ,cos ) - iGs/w)
(Cb --
(
Cb + Clio ~-1 sin ~
iGb/w)
i cos - i(Gs + Gb)/W _
[A.3] The crossover frequency, Fig. 2b, is given by
Gu
n . ~Ox = CI" sin(n~r/2)
[A.4]
For w ~> Wx C..-.~ C l ( i w ) n - l ( Cb - i G b / W ) / ( C l ( io)) n - l ) G
-
iGb/O., [A.51
and for w ~ Wx,
EMULSION CHARACTERIZATION
C--~ ( Cl( iw) n-1 - iGs/w)( Cb - iGb/O~)/ (Cb --
--~ C l w "-1
sin ~
i cos --
-
iGb/w)
iGs/o~. [1.61
REFERENCES 1. Dissado, L. A., Rowe, R. C., Haidar, A., and Hill, R. M., J. Colloid Interface Sci. 117, 310 (1987). 2. Rowe, R. C., Dissado, L. A., Zaidi, S. H., and Hill, R. M., J. Colloid lnterface Sci. 122, 354 (1988). 3. Maxwell, J. C., "Treatise on Electricity and Magne-
533
tism," 3rd ed. Dover Press, New York, 1954; Wagner, R. J.,Ann. Phys. 40, 817. 4. Jonscher, A. K., "Dielectric Relaxation in Solids." Chelsea Dielectrics, London, 1983. 5. Nicklasson, G. A., J. Appl. Phys. 62, R1, (1987). 6. Dissado, L. A., and Hill, R. M., Proc. R. Soc. London A 390, 131 (1983). 7. van Beek, L. K. H., in "Progress in Dielectrics" (J. B. Birks, Ed.), Vol. 7, p. 69. Heywood Press, London, 1967. 8. Hanai, T., in "Emulsion Science" (P. Sherman, Ed.), Chap. 5, p. 353. Academic Press, New York/London, 1958. 9. Dissado, L. A., and Hill, R. M., Phys. Rev. B 37, 3434 (1988). 10. Pugh, J., "Proceedings, IEE DMMA Conf." IEE Conf. Publ. No. 234, p. 247, 1984. 11. Hill, R. M., and Pickup, C., J. Math. Sci. 20, 4431 (1985).
Journal of Coltoid and Interface Science, Vol. 138, No. 2, September 1990
conditions or exposure to air. This makes the technique particularly useful for the observaThe use of dielectric spectroscopy in the low tion or monitoring of processes that can lead frequency range below 10 4 Hz for the examto deterioration such as globule size coarsening ination of the physical structure of gel and a n d / o r phase separation. In order to underemulsion systems in a noninvasive manner has stand and characterize the dielectric response already been established (1, 2). Here we dein terms of the known physical structure of scribe the application of the technique to an the emulsion it is necessary to have available oil in water emulsion which has been designed information from a range of structures/comas a carrier for a lipophillic substituted phenol. positions. This is most conveniently done, as In particular we report on the stability of the here, by examining the effects of concentration emulsion system under freeze/thaw cycling changes of the constituents of the emulsion and exposure to air. and relating the observed dielectric response Unlike most other techniques used for the changes to the individual constituents. In adexamination of emulsion systems, for examdition we have examined the effect of using ple, photon correlation spectroscopy and ~" constituents from different suppliers and show potential measurement which both require dithat there are measurable differences. lution, the dielectric technique is applied to Unlike our earlier work on gels prepared the material to be investigated while it is in its from cetostearyl alcohol (1, 2) preliminary normal, as-prepared state. Changes in the diwork on the lecithin-based emulsions indielectric response can be observed directly as cated that, in the frequency range used, the the material is subjected to either freeze/thaw dielectric response was relatively simple and could be characterized in electrical terms by To whom all correspondence should be addressed, the series connection of a pair of parallel conINTRODUCTION
521 0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
Copyright © 1990 by Academic Press, Inc, All rights of reproduction in any form reserved.
522
HILL ET AL.
nected conductance and capacitance elements. This type of construct is c o m m o n l y termed a Maxwell-Wagner response (3). The only complication observed here was that the larger magnitude capacitive element was power-law dispersive in frequency; i.e., Cs = Cl(iw) n-I,
[1]
where C1 is the magnitude of the capacitance at 1 radian s -1 frequency and ~o is the frequency in the same units. The fractional power-law response, characterized by the exponent n with 0 < n < 1, has been generally accepted as a relaxation characteristic (4, 5) and has been shown to arise from cooperative interactions within the medium (6). The observation of such responses was general in our investigations of the gel systems and the more limited contribution observed here is most likely to be due to the absence of a gel network in this system. The simplest, low-concentration, particle-in-a-medium models reviewed by van Beck (7) and Hanai (8), can be described reasonably well in terms of only conductances and capacitances in parallel/series combination with a low complexity in their connection. The characteristic of Eq. [ 1] however requires a high connection complexity if it were to be obtained in this manner (9) and it is conceptionally useful to consider that the basis of such a response lies in a cooperative process within the medium.
EXPERIMENTAL
PROCEDURE
All emulsions were manufactured on a 2L scale by mixing the oil phase (soybean oil, lecithin, and the substituted phenol) and the aqueous phase (glycerol, NaOH, and distilled water) with a high shear mixer (Hydroshear HSI) and then homogenizing for 10 rain using a Manton Gaulin homogenizer (Model 15M). All processing was performed under nitrogen at approximately 70°C. The emulsions were filled into 15-ml ampoules under nitrogen and sterilized using an autoclave. All emulsions, as prepared, had a mean globule size between 0.21 and 0.24 ~tm as measured by photon correlation spectroscopy. After freezing, the sizes increased to greater than 3 #m, above the range of the instrument. The full range of sample compositions is listed in Table I. The dielectric sample cell is shown in section in Fig. 1. Because of the alkalinity the emulsion samples were held in a polypropylene thimble into which the metal electrodes were inserted. The thimble and electrode assembly were set in a copper vessel which was flushed with nitrogen and sealed. The complete cell was mounted on a h o t / c o l d stage of a vacuum cryostat and the temperature of the emulsion was measured by means of an internal thermocouple. The temperature was maintained to + I ° C during measurement. Initially problems of stability were encoun-
TABLEI Sample Compositionin Percentage w/v Composition
Sample no.
Soybean oil (Source A) Soybean oil (Source B) Lecithin (Source A) Lecithin (Source B) Glycerol Substituted phenol Oleic a c i d Distilled water to measured pH
1
10.0 . 1.2 . 2.25 -. 100 8.2
2
10.0 .
. 1.2 .
. 2.25 1.0
.
. 100 8.0
3
4
5
6
7
10.0 . 1.2 . 2.25 2.0 . 100 8.1
10.0
-10.0 1.2
10.0 -1.2
2.25 1.0
10.0 --1.2 2.25 1.0
100 7.5
100 8.9
2.4 . 2.25 2.0 . 100 7.1
.
2.25 1.0 0.085 100 7.3
Note. NaOH has been added in sufficient quantity to give the measured pH values after autoclaving. Oleic acid is used as an emulsifier and is often added in order to increase the stability of the formulation. Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
523
EMULSION CHARACTERIZATION
ELectrode leads J ~ ..._---Thermocouple E
~N~/Copper CeLt -'VII I/Potypropytene ' Liner
7~--~
plates (Pf)
FIG. 1. Cross section of the dielectric cell. The emulsion sample is held in the propylene thimble within the copper cell which is in turn mounted on the hot/cold stage of a vacuum cryostat. The cell is flushed with nitrogen and sealed at atmospheric pressure by the insulating block that carries the electrode leads and the measuring thermocouple. The cell plates are approximately 0.01 by 0.01 m and are immersed in the emulsion.
tered but it was found that these arose from exposure to air or from chemical action between the emulsion and the cell plates. It was found that neither copper nor stainless-steel plates were satisfactory but that platinum plates, with scrupulous cleaning, removed this problem so that both stability and reproducibility of the responses from sample to sample and over extended measurement periods were excellent. The platinum electrodes and their mounting legs were thin and slightly flexible. This was considered necessary in order to give a degree of flexibility as the sample was cycled through the freezing/melting point. However, it did mean that the electrode spacing was variable from run to run and during cycling. For this reason it has not been possible to determine the cell constant (electrode area/ spacing) and hence to determine from the measured capacitance, C(w) the relative permittivity, ~(w), of the samples nor has it been possible to make absolute comparisons of capacitances from run to run. For these reasons the analysis of the data is based on the f o r m s of the observed dispersions rather than their magnitudes.
The frequency response analyzer and the associated equipment has been described in detail elsewhere (1, 10) and the interested reader is referred to these papers. THE ANALYSIS OF THE DIELECTRIC RESPONSE
The dielectric technique measures the response of the sample to an impressed ac voltage of radian frequency ~o. For a perfect insulator the response is capacitive in nature and reflects the change in magnitude or orientation of fixed charge dipoles under the action of the field. As the response is not instantaneous there are both in-phase and out-of-phase components to the capacitance, i.e., C(w) = C'(w) - iC"(w),
[2]
where i = ( - 1 ) 1/2. In particular when the sample exhibits a conductance G this contributes a component of magnitude i G / w to the imaginary part of the complex capacitance. As the term C"(~0) + G / w results in power loss it is commonly referred to as the loss component of the capacitance. In the aqueousbased emulsions considered here the frequency response will be dominated by the effect of the continuous aqueous phase which has the anomalously high relative dielectric permittivity of S0 so that for the geometry of the cell used, with plates of approximately 10 -4 m 2 area spaced about 10-3 m apart, the magnitude of the capacitance is given by C'(o~) = eeoA/d
[3]
as about 7 × 10 -11 F, where ~0(=8.854 × 10 -12 F m-~ ) is the absolute permittivity of vacuum and A / d is the cell constant. In parallel with this we have the loss component which will be dominated by the conductance of the NaOH solution. Taking the conductivity of the solution as 2.5 10 -2 ohms -I m -I gives the loss component at 104 Hz as in the region of 4 × 10-8 F, almost three orders of magnitude greater than the capacitance and hence the high-frequency response will be expected to be dominated by the conductance of the aqueous phase. Journal of ColloM and Interface Science,
Vol. 138,No. 2, September1990
524
HILL ET AL.
In our earlier work on gels prepared from cetostearyl alcohol (1, 2) we found evidence for the coating of the electrodes by a layer which exhibited all the properties of a single layer of the complex physical structure. In the work reported here, because of the strong negative charge of the oil droplets and the complex nature of the surfactant based coating around the droplets, we expect a similar deposition onto the electrodes but now with a more or less closely packed array of complete globules in which the oil droplets are separated from themselves, and from the electrode by a surfactant-water layer. If the packing were perfect, such a boundary layer would completely block the movement of charges from the bulk of the emulsion to the electrodes. We do observe, and will report, such blocking which indicates the formation of at least a monolayer of globules. The effective permittivity of such a layer will be dominated by the water bound into the double surface layers. Estimating the thickness of these to be 3 n m and taking the permittivity to be that of water gives a barrier capacitance of about 1.3 10 -5 F. This is approximately two to five times less than actually observed and hence the effective barrier layer is thinner than we have estimated and likely to be dominated by the presence of imperfect packing of the globules. Thus although the observed response is simpler than that found for the gel systems previously studied the descriptions of changes observed in the response will necessarily be less direct. Experimentally we shall show that the barrier layer capacitance is not independent of frequency but takes the dispersive characteristic of Eq. [1] with n = 0.8, indicative of a small degree of disorder in the surfactant-water surface layers when the globules themselves are imperfectly packed. Nevertheless the capacitance is higher than that of the bulk of the emulsion and hence there will be a large dispersion, of about five orders of magnitude, in the capacitance and the frequency at which the dispersion would be expected to reach half its m a x i m u m value will be given by the inverse of the resistancecapacitance time constant as about 10 2 Hz, Journal of Colloid and Inte~face Science, Vol. 138, No. 2, September 1990
which is within our measuring range. The seties/parallel connection of the circuit elements is indicated in Fig. 2 ( b ) as an inset to the frequency response characteristic of the complex capacitance derived from the circuit. In the log/log plot used here the individual components are easy to pick out. The bulk capacitance Cb is constant and the bulk and barrier conductances, Gb and Gs, give responses in the plot that are inversely proportional to the frequency. The barrier capacitance is characterized by the real and imaginary components being of the same gradient with frequency, n 1, and with the separation given by tan(nTr/ 2) for ( i w ) n-I = w(n-l)exp(i(n - 1)7r/2) = w(n-l)[sin(nTr/2)
- i cos(nrc/2)].
[41 In parallel connection the individual components sum together but in series connection it is the reciprocals that sum and hence the smaller component dominates. It is for this reason that the bulk properties develop out of the barrier response as the frequency is increased. A full description of the capacitive response of the model is given in the Appendix
) \
o/
g
_3 °
C1(ioj f.~(.hl Cb t
L'-'I
Ch ~
t
Log[frequency
t-
x...~._, I
i
x I
]
FIG. 2. The calculated frequency response (a) for the circuit shown at (b). Gband Cb referto the low-magnitude conductance and capacitance of the bulk of the sample; the power-lawdispersivecapacitanceCl(io~)'-1 blocksthe bulk conductanceat frequenciesbelowo~x and is in parallel connection with the barrier leakage conductance Gs. A full analysisof this responseis givenin the Appendixwhere the low-and high-frequencylimitingbehaviorsare derived.
525
EMULSION C H A R A C T E R I Z A T I O N
and has been used to give the curve fits to the experimental data in the following figures. E X P E R I M E N T A L RESULTS
Characterization of the Emulsions Following the schematic representation of the expected response of Fig. 2 we present the experimental data in the form of log/log plots of complex capacitance as a function of frequency. Where possible we shall compare data sets for which known differences in the emulsion make up or treatment occur. This eliminates, to a large degree, our problem of plate stability. The parameters used in the curve fitting are given in Table II. For example, in Fig. 3 we present two data
sets, measured at four points per decade in frequency, for sample 2 which contains 1% w/ v of the substituted phenol. The only difference between the data sets occurs above the crossover frequency Wx and can be fully characterized by a decrease in the bulk conductance from that indicated by the crosses by a factor of 1.70. Experimentally the only difference was that the plate separation from one run to the other was increased by about two. Notice that the conductivity of the bulk, o-b, has not changed--it is the conductance given by Gb = abA/dwhich is inversely proportional to the plate separation. The crossover frequency is also reduced, through Gb, as is the high-frequency bulk capacitance Cb and it is these two factors together that give the decrease
TABLE II Dielectric Response Parameters
Sample
1 2 2 (a) 3 4 5 6 7
(mhos)
4.42 2.8 3.38 4.42 1.33 5.98 9.88 2.32
10-6 10 -6 10 -6 10 -6 10 -6 10 -6 10 -6 10 -6
(mhos)
3.1 5.8 3.4 4.42 7.98 8.97 1.46 4.65
10 -3 10 -3 10 -3 10 -3 10 -3 10 -3 10 -3 10 -3
n
0.87 0.87 0.87 0.76 0.80 0.87 0.87 0.87
(F)
4.16 3.63 3.93 2.92 2.06 3.98 3.67 4.22
10 -5 10 -5 10 5 10 5 10 -5 10-5 10 -5 10 -5
3.07 2.36 1.37 2.73 3.21 2.77 2.30
10 5 10 -s 10 -5 10 -5 10 -5 10-5 10 5
Fig. no.
4 3, 4, 5, 7 3 4 5 7a 7a 7b
Measured after three freeze/thaw cycles 1 2 3 4 5 6 7
2.0 1.0 2.05 2.67 2.76 5.28 1.6
10 -6 10 6 10 -6 10 -6 10 -5 10 -6 10 -6
2.0 2.0 2.05 1.07 1.84 1.42 3.20
10 -3 10 3 10-3 10 -1 10 -2 10-3 10 -3
0.80 0.87 0.80 0.80 0.87 0.87 0.76
m
8b
Material exposed to air before measurement 4 5 6
<3 2.3 1.6
10 -6 10 -6 10 -6
6.6 4.5 1.6
10 -3 10 -3 10 -3
0.69 0.80 0.76
4.3 1.91 2.49
10 -5 10 -5 10-5
7
1.7
10 -6
7.4
10 -3
[ 0.80 L0.4
2.3 5.3
10-5 ~ ] 10_5 j
9a 9b
9c
There is evidence in this response for more than one barrier relaxation process although all the materials exposed to air showed responses that were indicative of distributed properties in the barrier components. Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
526
HILL ET AL. -3 A LL
-5 tJ t-
3 o
f "2
f
K
i
0
Log [frequency ]
i
I
2
I 4
(Hz)
FIG. 3. The dielectric response measured from an emulsion containing 1.2%substituted phenol (sample 2). Two data sets are given, that indicated by the symbol O was obtained with the cell plates spaced further apart than the set indicated by the crosses. Note that the cell spacing has no effect on the response for frequencies below the crossover frequency, wx.
in C'(~0) that is observed for w > Wx. The data presented in Table II were obtained by curve fitting to both data sets using the full response expression given in the Appendix although, for clarity, only one of the theoretical response functions is given in the figure. The diagram clearly shows the dominance of the bulk properties for w > Wxas well as the insensitivity to the plate separation at the lower frequencies. In the low-frequency region the fractional power law is well developed for the real component of the capacitance but masked, at the lowest frequencies, by the bartier conductance contribution to the loss component. From the gradient of the log/log plot the exponent n has been determined as 0.87, indicative of a high degree of order within the surfactant-water surface layers of the globules packed against the electrode ( 1, 2, 9). The effect of including a substituted phenol in the emulsion response is indicated in Fig. 4. The three data sets here correspond to zero, 1, and 2% w / v of the c o m p o u n d with the highest capacitance in the barrier corresponding to zero concentration. The magnitudes of the barrier capacitances, at 1 r a d - s -~ are in Journal of Colloid and Interface Science, Vol. 138, No. 2, September 1990
the ratio 1:0.95:0.70 so that purely in terms of effective thickness of the barrier adding 1% of the compound has a very small effect (N5%) but at 2% concentration there has been a 30% increase in the barrier thickness. It is reasonable to assume, from this plot, that the substituted phenol resides, at least in part, in the barrier as there has been no significant change in the bulk conductance and we can deduce from the value of n that at the 1% level the c o m p o u n d has no significant effect on the order within the barrier. Distortion is however present at the higher concentration of the c o m p o u n d and we would expect that at some even higher concentration the stability of the system would become affected. We have, however, implicitly assumed that the substituted phenol does not displace water from the barrier. If this were to be the case we would expect a significant decrease in the permittivity of the barrier and hence a decrease in the bartier capacitance but this would be accompanied by a decrease in the stability of the barrier which has not been observed. It has been implied in the above that the barrier layers can be associated with the lecithin emulsifier. That this is correct is shown -3 I.I..
-5 8
--1. 9
I "2
I
i
I
0
Log[frequency]
2
4
(Hz)
FIG. 4. Comparison of the responses measured from emulsions with zero(©), 1.0 (+), and 2.0%(X) substituted phenol (samples 1, 2, and 3, respectively). The curves of C'(w) and C"(w) have been fitted to the 1.2%data points and are taken as the standard response in a number of the followingfigures.
527
EMULSION CHARACTERIZATION
by the data in Fig. 5 where a comparison is given between the response measured from a formulation containing 1.2% lecithin (sample 2) and that prepared with twice this concentration (sample 4). There is little change in the bulk conductance but the nature of the response in the barrier region has been altered. From the tabulated values we see that the bartier conductance has decreased by a factor of 2.1 and the barrier capacitance by 1.76. As for the addition of the substituted phenol there is a decrease in the order parameter n to 0.8 and, finally, in the frequency region around 10 -2 Hz there is a secondary dispersion in the barrier response. We deduce, then, that doubling the concentration of the lecithin has not affected the bulk properties, it has, however, affected all the three parameters of the barrier in such a way that the apparent thickness of the barrier has increased, its physical order has reduced, and a secondary relaxation process has been introduced. Summarizing the experimental evidence presented in Figs. 3, 4, and 5, we can define the role of each of the individual components of the oil in water emulsion. The soybean oil forms droplets which are coated with lecithin emulsifier. Although the range of compositions
x 106
9 8 o +E
7 6
vI L.~ Q/ u t-
5
"t~O i._J
3 (
2
0 0
I
I
I
I
L
1
2
3
4
5 x 10 5
Capacitance
C 1 (F)
FIG. 6. A plot of the barrier conductance Gs as a function of the barrier capacitance magnitude CI for samples with 1.2% w / v lecithin grouped in substituted phenol concentration; zero phenol ( ~ ) , 1% phenol ( × ) , and 2% phenol (©). The samples indicated with the squares were prepared with source B lecithin. The data are taken from Table II and contain both the as-prepared and the freeze/thaw cycled samples. Data points lying on a line that passes through the origin would indicate that the conductance is intrinsic to the barrier layer. Note that the dashed line has half the gradient of the full line.
-3 o
LL.
° ~
~
_
~
-5 r'-
u
~-7 o ._A
.i
t
I "2
I
I 0
Log[frequency ]
I
I~ =
*
2
4.
(Hz)
FIG. 5. A comparison between the responses observed between the sample containing 1.2% lecithin (sample 3) and that prepared with twice the amount of lecithin (sample 4). It can be seen that the capacitance of the blocking barrier layer has decreased by a factor of about two. Note that the change in C'(~o) at high frequencies is due to the change in the low-frequency, barrier capacitance.
has been limited the 1.2% w / v concentration gives a physically well-ordered barrier around the oil particles. The low-frequency, barrier conductance can be associated with the lecithin as shown in Fig. 6. In this figure we plot the barrier conductance, Gs, as a function of the barrier capacitance (at 1 rad- s l ) for the range of samples listed in Table II from sample groups 1 to 5 for both the as-prepared samples and those subjected to freeze-thaw cycling. A straight line in this plot, passing through the origin, would indicate that the basis of the barrier conductance lay ~n the barrier material itself and not in the substituted phenol. Such a relationship is clearly seen for the 2% substituted phenol samples and a dotted trace of half the gradient of the trace that passes Journal of Colloid and Interface Science, Vol. 138,No. 2, September1990
528
HILL ET AL.
through the 2% data points is a reasonable fit to the 1% substituted phenol data. Two anomalous data sets are apparent, that at zero substituted phenol content (A) and the samples prepared with source B lecithin. The basis of this analysis is that only for a homogeneous barrier will the relationships [5]
Gs = a A / d = ~C1/~o oc Ca
hold where o- is the bulk conductivity of the material for which the relative permittivity is E. We deduce, then, that doubling the substituted phenol content increases the conductivity of the barrier with a slight decrease in the barrier order but for phenol concentrations up to the 1% level there is no evidence of such an increase in conductance and hence the barrier itself has a conductivity level equivalent to that introduced with the addition of 2% concentration of the substituted phenol. As the final sample characterization plot we give, in Fig. 7, that measured for a sample prepared from an alternative source of soybean oil (sample 5 ) and a sample prepared with lecithin from the second source (sample 6, already referred to), both of which contain the standard 1% of substituted phenol, and finally a sample to which 0.085% w / v of oleic acid has been added (sample 7 ). Comparison with
a
-3
the equivalent "standard" plots shows that the addition of the acid has had little effect on the bulk conductance, while the barrier conductance has decreased and the capacitance increased slightly. The use of a second source of soybean oil has made trivial differences to the response but the lecithin obtained from the alternative source, as noted in Fig. 6, gives a high barrier conductance notwithstanding that the capacitance magnitude and spectral response parameter n are very similar to those observed in the original material. We note, too, that the relationship (5) is not particularly well behaved in this material, which might indicate that a high content of mobile charge species is present. Temperature Cycling
An essential feature of this work was the investigation of the effects of temperature cycling the emulsions through their freezing point. Using the standard sample holder (Fig. 1 ) it was possible to cool the emulsions below the liquid/solid transition temperature and heat back to room temperature (16°C), without temperature overshoot, within an hour. The rate of heating and cooling was controlled by the thermal lag of the copper pressure vessel. As we wished to make a number of mea-
b
-3
ii -5
,x
."x
-5
i
8 t-
i.J - 7 ng {:3.
£
m
o
o
-9
-9 x
I -2
I
I 0
I
L0g[frequency]
I 2
I
I
~.
(Hz)
I -2
I
I
I
0
L0g[frequency]
I
I
I
2
(Hz)
FIG. 7. Comparisons of the dielectric response from sample 2, given by the continuous curves and (a) samples prepared with soybean oil from a second source (O), sample 5, and with lecithin from the alternative source ( X ), sample 6. (b) Sample 7, the sample to which oleic acid had been added to improve the stability. Journal of Colloid and Interface Science, Vol. 138,No. 2, September1990
529
EMULSION CHARACTERIZATION
surements at a single frequency during the heat/cool cycle we chose the highest available f r e q u e n c y , 10 4 H z , to monitor the response. A typical time/capacitance plot is presented in Fig. 8. In the plot both the real and loss components of the capacitance are given. The real component has a maximum value, on freezing, equivalent to the high-frequency bulk capacitance Cb as the relaxation rate decreases significantly. The loss component is not limited in this way and large changes in the loss were observed. As shown in the figure each sample was cycled three times and then the full frequency response was measured at room
temperature. In the particular case shown after this measurement the sample was cycled a further three times and then left at room temperature for a period of 6 h to allow recovery. Two features in the plot are of interest. Firstly the response exhibits a change even after a single cycle and although subsequent cycles give further changes the first cycle invariably gave rise to the largest change. Second, although there is some recovery it is small in comparison to the degradation that occurs. The frequency response plots measured before temperature cycling and after three cycles are shown in Fig. 8b. It is clear that the forms
-7
• *
.-
*
"*
*
["(co)
e~ -9 ~*'~o U
•*
,0
*o
o¢~o * ~ - - - - * " ~
¢
__
o, -11
r~',co,
o
c9
o3
N°
~z
LN
'g
100
200 0
Time
~lsoo
100
(mins.l
5-3 ii
-5 t-._ -7 n t17 t.J O "J
"9
I
-2
I
I
I
0
I
I
2
Log[frequency]
I
/+
(Hz)
FIG. 8. The effects of temperature cycling through the freezing point. (a) The real and imaginary components of the complex capacitance measured at 10 kHz. (b) The frequency response at zero time, the continuous curves, and after three cycles (©) for sample 2. The response after cycling of a sample prepared with the lecithin from the second source (sample 6) is given by the ×'s, although the original data for this sample were the same as that of sample 2. Journal of Colloid and Interface Science, Vol.
138, N o . 2, S e p t e m b e r 1990
530
HILL ET AL.
of the responses are equivalent--it is the magnitudes of the response elements that have changed. Table II lists these magnitudes for a range of samples that have been temperature cycled and a comparison can be made with the equivalent magnitudes measured on the uncycled material in the same table. For the particular sample shown in Fig. 8b the bulk conductance has decreased by 64%, the barrier capacitance by 35%, the barrier conductance by 66%, and the dispersion parameter n has not changed. As indicated in Fig. 6 the difference in the changes in the barrier properties indicates a change in the conductivity of the barrier. It is with this series of samples that we observed an increase in plate separation with temperature cycling and the effect of this can be seen in the decrease in the bulk conductance. The change in conductance, however, is greater than the increase in plate separation and it is likely that a decrease in the bulk conductivity has taken place. Although a number of the samples indicate changes in the magnitude of the dispersion parameter n there is no evidence of a catastrophic breakdown of the structure, for example, complete phase separation, on repeated freezing. For the sample shown in the figure the data begin to look like that obtained from the 2.4% lecithin sample. This would suggest that the observed changes are due to a decrease in the number of emulsion particles with a general coarsening of the emulsion structure consistent with the increase in globule size. It is clear that this coarsening was initiated in the first freeze/thaw cycle. Exposure to Air
During the preliminary work in this investigation it was observed that the low-frequency dielectric response was difficult to characterize and to reproduce. The use of nitrogen flushing of the sample vessel, of platinum electrode plates, and of rigorous cleaning of the cell and plates between runs removed these problems. In Fig. 9a we show comparative data for a sample measured under nitrogen and a second Journal of Colloid and Interface Science, Vot. 138, No. 2, September 1990
"+I
•
-61 o ~-4
.+.
' ," •
eL- 5
(el
- * e . ,'~ 0
•
8 .._J
i 8
-8
I -2
I
I
I
0
Log[frequency]
I 2
I
(Hz}
FIG. 9. The standard response is given in (a) by the continuous curves. The three data sets in the figure were all measured on samples that had been exposed to air. (a) sample 4, (b) sample 5, (c) sample 7. For all the samples that were exposed to air there is a collapse in the barrier characteristic. A perfectly diffusive charge transport would exhibit a characteristic in which the real and imaginary components have identical magnitudes and lie along a line of gradient -0.5 in these log / log plots.
sample from the same batch which was measured without prior nitrogen flushing of the cell. The difference is so extreme that it was not found possible to characterize the data from the latter using either our standard model of Fig. 2b or of any reasonable modification of it. This particular sample contained 2.4% lecithin but the responses (b) and (c), for samples 5 and 7, respectively, are indicative of those observed over the whole range of emulsions. All these responses show a weakening of the clear Maxwell-Wagner barrier characteristics which can be described as the development of a distribution of relaxation times or the onset of diffusion-like behavior. Under three-dimensional classic diffusion conditions the square root time dependence transforms as an (ion)-(t/2) frequency dependence with coincidence of the real and imaginary components of capacitance along a line of gradient - 0 . 5 in the log/log representation used here. Some evidence of diffusion-like behavior can be seen
EMULSION
CHARACTERIZATION
in Fig. 9 and this would imply a random motion of charges. In either case we have evidence in the figure for an almost complete breakdown of the lecithin-based barrier structure. It is clear that on the time scale of the barrier response, i.e., a second or so, we could use the fixed frequency technique to follow the dynamics of the breakup of a lecithin barrier in the presence of air. DISCUSSION
The principal feature of the dielectric properties of the oil in water emulsion which has been investigated is that it can be described, almost, by the simplest possible electrical barrier layer model of two parallel conductance and capacitance elements connected in series. As shown in Fig. 2 the single change that has had to be made was to let the barrier capacitance have a weak fractional power-law dependence on frequency. This has allowed us to relate the individual constituents of the formulation to the electrical elements. It should be realized, however, that this description is not in itself a model of the structure. In order to determine that, we have to consider how the electrical properties arise. The analysis of the response in terms of electrical series elements has been possible because the characteristic or crossover frequency, Wx, lies near the middle of our frequency range. If we had been limited to frequencies in excess of 100 Hz or so then all we would have observed would have been the bulk capacitance in parallel with the bulk conductance with perhaps, at the lowest frequencies, the onset of a massive increase in capacitance. In the past such an effect would have been loosely termed an electrode polarization, which it is, but which we have shown contains useful information about the system and which should not be ignored. It is only because with this system we have a relatively thick barrier and a high conductance in the bulk that both the low-frequency barrier response and the high-frequency bulk response appear in our frequency window.
531
By examining a number of formulations we have shown that the barrier layer, together with its fractional power law dispersion, is based in the surfactant coating of the oil droplets. This is in agreement with the known properties of the structurally complex barriers formed around oil globules by lecithin. Although the barrier that has been examined here has been formed around an oil globule and the droplet has been deposited onto a platinum surface as part of an irregular array, we have found it possible to characterize, in at least a semiquantitative manner, the effects of varying the composition of the emulsion, the source of the lecithin, and the introduction of a substituted phenol. If the surface layer formed were to be a perfect barrier then it would have had a frequency-independent permittivity and, hence, capacitance. The fractional power-law behavior actually observed is an indication that instead the charged moieties within the barrier can respond to an ac field, but only weakly. As the power-law parameter n becomes smaller the dispersion increases and hence we see that n defines the degree of order within the barrier itself (6). It is in this sense that we have made use of the dispersion parameters to describe the barrier properties. Because of the dispersion in capacitance there is a concomitant dispersion in the loss but this is hidden at the lowest frequencies investigated by the onset of a leakage conductance through the barrier. Again we have observed that this is an intrinsic property of the lecithin used rather than due entirely to the introduction of the substituted phenol to the dispersed phase. These results illustrate the flexibility of the dielectric technique in that it is not limited to the investigation of systems that give simple, well-ordered, barrier layers. It might have been expected that there could have been two components in the high-frequency response. One arising, as here, from the large water content within which the oil droplets are suspended, and the other from the conductance within the droplets. Here we have seen no evidence for the latter probably because the conductance of the aqueous phase Journal of Colloid and Interface Science, VoL 138, No. 2, September 1990
532
HILL ET AL.
is greatly in excess of that of the oils used. For a water in oil emulsion the situation would be reversed and the dominant high-frequency response would be expected to arise from the water within the droplets. As our high-frequency range is limited we have not been able to reveal the bulk capacitance which would undoubtedly have been given by the cell constant and the permittivity of the macroscopic emulsion which would have been almost that of the water. The technique of dielectric response was particularly suited to the examination of the stability of the emulsion in that instabilities could be followed without having to change the material in the measuring cell. Indeed the dielectric signature from the material exposed to air is so different from that which has been observed from the ordered lecithin barrier structure that the method forms a sensitive analytical test. The effects of exposure to air were found to be rapid and well within the spectral measuring period of up to 2 h. As the main effect is observed at the lower frequencies only a limited number of measurements can be made within the time period of the change in the response and hence only samples exposed for relatively long periods have been reported here. A similar problem arose with the changes in response induced by freeze/thaw cycling but there was sufficient change in the bulk conductance and in the capacitance dispersion at high frequencies to allow characterization at 10 kHz and hence the individual cycling events could be followed in time. The system is sensitive to freezing and the largest effect was observed on the first cycle although deterioration was cumulative with the number of cycles. In both cases the deterioration has been observed to be based in the barrier. CONCLUSIONS
The dielectric properties in the frequency range from 10 .2 to 10 4 Hz of a lecithin-based oil in water emulsion have been reported and analyzed in terms of a simple series/parallel electrical circuit. The effects of introducing a JournalofColloidandInterfaceScience,Vol. 138,No. 2, September1990
substituted phenol into the emulsion, of using materials from different sources, of exposure to air, and of freeze/thaw cycling have been investigated and it has been shown that the dielectric technique is particularly useful as a means of examining the fundamental nature of the macroscopic effects that have been observed. APPENDIX
For the two parallel capacitive/conductive elements of Fig. 2a the impedances are ( 1 ) Z s = [ i ~ ( C l ( i w ) "-1 - i G s / w ) ] -1
[A.1]
and Zb = [ k 0 ( C b - i G b / w ) ] - l .
[A.2]
As these are electrically in series the total impedance is given by Z=
Zs+ Zb
from which we obtain the capacitance C = ( i w Z ) -1 as C
l(sin ,cos ) - iGs/w)
(Cb --
(
Cb + Clio ~-1 sin ~
iGb/w)
i cos - i(Gs + Gb)/W _
[A.3] The crossover frequency, Fig. 2b, is given by
Gu
n . ~Ox = CI" sin(n~r/2)
[A.4]
For w ~> Wx C..-.~ C l ( i w ) n - l ( Cb - i G b / W ) / ( C l ( io)) n - l ) G
-
iGb/O., [A.51
and for w ~ Wx,
EMULSION CHARACTERIZATION
C--~ ( Cl( iw) n-1 - iGs/w)( Cb - iGb/O~)/ (Cb --
--~ C l w "-1
sin ~
i cos --
-
iGb/w)
iGs/o~. [1.61
REFERENCES 1. Dissado, L. A., Rowe, R. C., Haidar, A., and Hill, R. M., J. Colloid Interface Sci. 117, 310 (1987). 2. Rowe, R. C., Dissado, L. A., Zaidi, S. H., and Hill, R. M., J. Colloid lnterface Sci. 122, 354 (1988). 3. Maxwell, J. C., "Treatise on Electricity and Magne-
533
tism," 3rd ed. Dover Press, New York, 1954; Wagner, R. J.,Ann. Phys. 40, 817. 4. Jonscher, A. K., "Dielectric Relaxation in Solids." Chelsea Dielectrics, London, 1983. 5. Nicklasson, G. A., J. Appl. Phys. 62, R1, (1987). 6. Dissado, L. A., and Hill, R. M., Proc. R. Soc. London A 390, 131 (1983). 7. van Beek, L. K. H., in "Progress in Dielectrics" (J. B. Birks, Ed.), Vol. 7, p. 69. Heywood Press, London, 1967. 8. Hanai, T., in "Emulsion Science" (P. Sherman, Ed.), Chap. 5, p. 353. Academic Press, New York/London, 1958. 9. Dissado, L. A., and Hill, R. M., Phys. Rev. B 37, 3434 (1988). 10. Pugh, J., "Proceedings, IEE DMMA Conf." IEE Conf. Publ. No. 234, p. 247, 1984. 11. Hill, R. M., and Pickup, C., J. Math. Sci. 20, 4431 (1985).
Journal of Coltoid and Interface Science, Vol. 138, No. 2, September 1990