- Email: [email protected]
Surfactant deposition on surfaces: An ESR technique
Surfactant Deposition on Surfaces: An ESR Technique BRET B E R N E R The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, Ohio 45247 Received May 28, 1981; accepted August 10, 1981 An ESR technique is developed which distinguishes surfactant deposition onto from penetration into a material; this technique also allows classification of surfactant films as packed or expanded. In this technique, the spectral line width AH of a surfactant, spin-labeled at different positions along the chain, is measured as functions of the concentration, Cp, of added paramaguetic ions and Xs, the mole ratio of labeled:unlabeled surfactant. When the absorbent material is impermeable to the ions, dAHIdCp is related to the exposure of that part of the surfactant to the bulk solution, dAHIdXs is related to the packing of the surfactant, and the spectral lineshape contains information about the "rigidity" and ordering of the labeled surfactant. From analysis of experimental results for stearic acid in ethanol, latex suspension, and egg lecithin liposomes, the distinct effects of a variety of environments on a spin label have been documented. INTRODUCTION
Understanding surfactant deposition onto and penetration into a material can help develop processes which control coating, binding, and partitioning of surfactants. It is useful to distinguish surfactant penetration into a substance from deposition onto that substance; furthermore, we would like to classify surfactant on the surface as a packed or expanded film or as isolated adsorbed molecules. To distinguish among these possibilities, we developed an electron spin resonance (ESR) technique which utilizes spin-labeled surfactant, unlabeled suffactant, and paramagnetic ions. In this technique, the ESR line width is measured as a function of the concentration of paramagnetic ions and the ratio of labeled:total (labeled + unlabeled) surfactant. The ESR spectral line shape, from which (spin label)-(spin label) and (ion)(spin label) interactions can be deduced, contains information about surfactant deposition. The use of this ESR method to understand surfactant deposition assumes that: (1) the spin label accurately probes the same
arrangement as the unlabeled surfactant, (2) addition of paramagnetic ions does not appreciably modify surfactant sorption, (3) the substrate material with which the surfactant interacts is impermeable to paramagnetic ions, (4) a single type of surfactant sorption predominates, and (5) exchange of surfactant among distinct environments is slow on the ESR time scale. These assumptions are generally not very restrictive. The ESR spectrum of the spin-labeled surfactant is broadened if the spin label is exposed directly to the paramagnetic ion and unaffected if it is not. Consequently, if the above assumptions are satisfied, we can determine which parts of the surfactant molecule are "hidden" from the paramagnetic ions by varying the site of the spin label on the surfactant molecule and by the proper choice of paramagnetic ion. By studying (spin label)-(spin label) interactions, we can learn about the packing of the surfactant. The combination of these two types of information can help us understand surfactant sorption phenomena. Paramagnetic ions are used as shift rea-
422 0021-9797/82/040422-10502.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
SURFACTANT DEPOSITION ON SURFACES
423
some model systems and a brief summary of experimental methods.
z a)
b)
1 e)
d)
ei
FIG. 1. The types of deposits. (a) Surfactantin bulk, homogeneoussolution;(b) surfactantin the interior of the absorbent; (c) surfactant in a packed monolayer on surface;(d) surfactantin a slightlyexpandedmonolayer; (e) surfactant adsorbed as isolated molecules or as a greatly, expanded monolayer. gents in NMR (I), to distinguish inside from outside of membranes by NMR (2-4) and by ESR (5, 6), and to study active surface sites in heterogeneous catalysis by ESR (7, 8). We generalize the inside-outside method for membranes by including the dependence of ESR spectral line broadening on the position of the spin label in the surfactant molecule. The next section contains a list of rules for distinguishing among the types of surfactant sorption. These rules are based on the theory presented in the following two sections, in which the results of the theory for homogeneous isotropic solutions are summarized and the dependence of the spectral line broadening on the distance of closest approach between a spin label and a paramagnetic ion is discussed. The final two sections contain the results for ethanol, latex, and egg lecithin lipo-
A SCHEME FOR CLASSIFYING SURFACTANT DEPOSITS We measure the line width, AH, of an ESR derivative spectrum of a lipid spin label as a function of Cp, the concentration of paramagnetic ions, and as a function of X=, the mole fraction of spin-labeled (to unlabeled + labeled) surfactant. These measurements allow us to distinguish among interior surfactant, surfactant in the bulk, homogeneous solution and surfactant deposits which are packed monolayers, slightly expanded monolayers, or isolated adsorbed surfactant (see Fig. 1). In order to classify stearic acid sorption we use the labeled stearic acids, 2-(3-carboxypropyl)-4,4'-dimethyl-2-tridecyl-3-oxazolidinyloxyl (5-NS) and 2-(14-carboxytetradecyl)-2-ethyl-4,4'-dimethyl-3-oxazolidinyloxyl(16-NS); in addition, a Cls surfactant spin-labeled in the polar head region, analogous to the lecithin spin label used by Kornberg and McConnell (9), is required to fully utilize this technique. 12-NS is not useful because this spin label may not reflect accurately the arrangement of the unlabeled surfactant (10-12). The scheme for classifying surfactant sorption is: (I) Spin label situated farther than 5 It in the interior of the substrate from the boundary with the homogeneous solution. The spectrum of the spin label is unaffected by the paramagnetic ions regardless of the position of the label along the stearic acid molecule. Information about the interior may be obtainable from the spectral line shape. (2) Spin label in a packed monolayer adsorbed to the surface. Only polar head spin labels exhibit line broadened spectra in the presence of paramagnetic ions. AH varies with X=. The spectrum of 5-NS and 16-NS may be polycrystalline depending on the viscosity of the monolayer. The Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
424
BRET BERNER
surfactant chains are assumed to pack toward the substrate, but other orientations may be determined. (3) Spin label in a slightly expanded monolayer on the surface. AH for 5-NS and probably 16-NS broadens in the presence of paramagnetic ions. AH is a function of Xs and the spectrum is still polycrystalline. (4) A greatly expanded monolayer or isolated molecules adsorbed to the surface. AH for all label positions is broadened, Flo. 2. (Paramagneticion)-(spin label) interactions. but AH is independent of Xs and over some low concentration range is independent of The exchange and dipolar interactionsdepend on the Cs, the spin label molarity in the overall approach of the ion and the spin-labeled surfactant. The distance between the pair is 2re and 2re~ for the solution. The spectrum is polycrystalline. dipolar and exchangeinteractions, respectively. (5) Spin label in bulk, homogeneous solution. Paramagnetic ions broaden AH for all lipid spin label positions. AH varies the following section discuss the excluded with Cs; the spectrum resembles that of an volume complication introduced by an adisotropic liquid, depending on the viscosity. sorbent which is impermeable to the ions. Spectra of multilayer deposits should depend Let us examine the exchange interaction. To on Xs and Co as a combination of monolayer a first approximation, the exchange integral, and interior-surfactant would. Note that the J, is a constant, J0, whenever the ion and choice of paramagnetic ion depends on the the radical are within a collision diameter, viscosity of the solution. If the solution is 2rex and J is zero if the pair is further apart quite viscous (Tin < 3000°K/P), even those (13). ProvidedJ~-$ ~> 1, where zl is the collispin labels exposed to the ions are un- sion duration, the exchange contribution to affected by NiC12; CuC12 or some other para- AHex is (13-15): magnetic ion with a similarly long spin2 3 lattice relaxation time must be used to see an effect in these viscous solutions. THE THEORY OF HOMOGENEOUS, ISOTROPIC SOLUTIONS The line width, AH, of an ESR derivative spectrum of the nitroxide is measured as a function of the concentration Cp of paramagnetic ion. Cp is sufficiently dilute that only two particle (ion-radical) interactions need be considered. The additional line broadening, dAHMCo, due to the presence of these ions arises from Heisenberg exchange and dipolar interactions between the electron spins on the ion and the radical (see Fig. 2), We shall first consider the case of a homogeneous, isotropic liquid and then, in Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
where 3~eis the electron gyromagnetic ratio, Na is Avogadro's number, f accounts for the ionic interaction, and D is the diffusion constant defined as: D = 1/2(Dp + Da).
[2]
The subscripts P and R indicate the paramagnetic ion and the radical, respectively. Note that when the radical is bound or immobilized, that is, DR ~ Dp, D = 1/2Dp. We have yet to include the dipolar interaction. There are two sources of modulation of this interaction, the relative motion of the two interacting spins and the spin-lattice relaxation of the electron. Spin-lattice relaxation of the dipolar interaction must be treated
425
SURFACTANT DEPOSITION ON SURFACES
in both the fast and slow relaxation limits; the static and motionally narrowed limits of the modulation of the dipolar interaction by the relative translational motion of the spins need to be considered. In the static, slow relaxation limit for the case of spin 1/2 (1): dAHdip
dCp
3.8
- V~ btNA10-3
[3]
where /.t is the effective magnetic moment of the ion. Equation [3] is valid provided: Tie
~>1
/'2
Ze
--~
I
[4]
T2 where T~e is the electron spin-lattice relaxation time for the ion and T2 is the transverse relaxation time for the electron on the nitroxide; ze is the characteristic time for translational diffusion of the ion-radical pair and is given by 2r~ ~-c -
D
[5]
where rc is one half the distance of closest approach of the ion and the nitroxide. On the other hand, in the motional, fast relaxation limit, that is Tie
T2 T-L ~ 1
[61
r~
we find that (16): dAHdip
2~2TeNA × 10-3Tie
dC
45r 3 2V 3 [1 - l'2 - (1 + g)2e-2Vl
[71
where V = (zJTle) lj2. In the static, fast relaxation limit, that is, rJT2 >> 1 and TIe~T2 <~ 1 (17, 18):
dAHdip 327rTe/.t2 TxeNA I0 -3 dCp 9V~ r~
- -
[8]
If translational motion is the dominant source of relaxation, that is, V2 ~ 1 and Te/T 2 ~ 1,
dAHdlp 4Te/X2NA10-3 -dCp 75X/'3
[Drc] -1.
[9]
At low viscosities, experiments on Ni2÷(Tle 4 × 10-12 sec) (19) are best described by Eq. [7], while those on Cu2+(T~e = 3 × 10-9 see) (19) may be approximated by Eq. [9]. In conclusion, dAH/dCp is given by the sum of the exchange and the appropriate dipolar contributions. In low viscosity, homogeneous solutions, the exchange contribution dominates, and the broadening due to either Cu2(H20)2w+ o r Ni2(N20)zz+ should be identical; however, the dipolar contributions dominate in viscous solutions. The dipolar contributions to dAH/dCp for Ni2(H20)~ + and Cu~(HzO)22+ should be approximately 5 G/M and 50 G/M, respectively (see Eqs. [8] and [3]). THE DEPENDENCE OF z2d-/ON rc AND rex
(Spin label)-(ion) interactions for those spin labels which are free to collide with paramagnetic ions, i.e., re = 0, can be understood in terms of the theory presented in the preceding section. Spin labels in homogeneous solution and those in expanded monolayers both fall in this category. We now calculate the broadening of the spectrum of radicals in the "interior" of the absorbent, and we demonstrate that spectral broadening occurs only for radicals which can approach the ionic solution within a collision diameter. Since the absorbent is impermeable to the ions, this calculation is an excluded volume problem, and we must determine the dependence of dAH/dCp on rex and re. Spin exchange is an extremely short-range interaction; to a first approximation dAHexMC,-~ 0 whenever re > rex. The dipolar interaction, on the other hand, is a long-range interaction. According to Eqs. [8] and [9], dAHaip/dCp falls off as r~3 and r~-1 in the static, fast Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
426
BRET BERNER
relaxation and the motional, slow relaxation limits, respectively. The dependence o f dAHdip/dCp on rc must be calculated for the static, slow relaxation limit. Then the dependence of rc on the angle between the center of the absorbent, the ion on the surface, and the interior radical can be derived. We demonstrate that spectra for radicals on the absorbent surface are broadened whereas spectra for those in the interior are not. We calculate the dependence of dAHdip/ dCp on r~ in the static, slow relaxation limit. Equation [3] for dAHd~p/dCp in this limit is independent ofr~, because in the derivation o f this equation we assumed that rc = 0. The use of this assumption predicts a spectrum with a Lorentzian lineshape; including the r e dependence in the derivation causes the Lorentzian lineshape to have a finite cutoff (I). When rc is allowed to vary, we find that for the case of spin I/2 the spectral intensity, K, is proportional to:
label in the interior o f the absorbent, the center o f the absorbent, and the paramagnetic ion (Fig. 3). r~(O) varies with the geometry of the substrate particles. For convenience, we consider spherical absorbent particles of radius r which are impermeable to ions; we find that rc(0) = cos 0(rc0 - r) + {cos ~ O(r~o - r) z
-rZeo + 2rrco}112 [12]
where re0 denotes rc(O = 0). rc ~ 2re0 when 0 = +-1.1; consequently, dAHdip/dCp is a small fraction of the value calculated for re = 0 and an isotropic solution. For the low concentrations of paramagnetic ions and broad spectra involved in this study, dAH/dCp is effectively zero for r~ > 5 A. The spin label must be exposed directly to the homogeneous solution containing the ions in order to detect a broadening for the viscosities involved in this study; at these viscosities, we expect the dipolar contribution to be negligible even if rc ~ 0. Finally, another possible source of moK(~) I dt f d 0 { 2 r ~ ( 1 - cos b/rae) ,1 J tional relaxation o f the dipolar interaction + 2bSi(b/r~)} [101 is introduced by the 0 dependence of r~m the rotation of the absorbent particle. Abwhere t is the time, 0 is the angle between sorbent rotation becomes important whentwo spins and the magnetic field, Si repreever ~'~A, the characteristic time for absents the sine integral function, and sorbent rotation, is fast on an ESR time scale, i.e., b = 3/4ty~1~(3 cos ~ 0 - 1). [11] Equation [10] may be used to estimate the dependence of the Lorentzian cutoff frequency on r~. For rc ~ 5 A, Eq. [3] is an excellent approximation for dAHdip/dCp; for re ~ 10 A, by a frequency of 1 G from the center of the spectrum, the spectral intensity, K --~ 0. Since for the polycrystalline spectra in this study, A H -> 3 G, we shall let rc = 5/~ be an upper bound for the range o f the dipolar interaction in the static limit. We have yet to consider an additional complication caused by the excluded volu m e - - r ~ becomes angle dependent. To see this, let 0 be the angle formed by a spin Journal o f Colloid and Interface Science, Vol. 86, No. 2, April 1982
"ira
--
T~
>> 1
[13]
where 47VqKr 3
~'rA -- - -
3kBT
[14]
"0 is the solvent viscosity, T is the temperature, r is the absorbent radius, kB is Boltzmann's constant, and K is the ratio o f the torques to the forces on the solute (20). For small vesicles (r ~ 100 A and K = 0.5), ZrA/T2 ~ 5000 */; thus, even in water, rotation of the absorbing substance does not significantly modulate the dipolar interaction.
SURFACTANTDEPOSITION ON SURFACES
427
f
Ion
FIG. 3. The angle dependenceof re. MODEL SYSTEMS We have characterized stearic acid deposition in ethanol, Dow 288 latex, and egg lecithin. Let us first discuss the simplest model system, the homogeneous ethanol solution; this system is understandable in terms o f the theory presented above. As predicted by the theory, AH for both 5-NS and 16-NS in ethanol is a linear function of CNiC~, (Figs. 4 and 5). According to this theory, for low viscosity liquids, the spin exchange term, Eq. [1], is the dominant contribution to AH. In order to calculate dAH/dCNioz for 5-NS or 16-NS in ethanol, we need to estimate D and use Eq. [1]. To approximate D, we relate the solvent viscosity, rl, 0.012 P at 293°K (CRC Handbook), to D through the Stokes-Einstein relation,
respectively;, consequently, f or (rex/rT) varies considerably from unity and differs for the different spin labels. The difference between dAH/dCNJc~,for 5-NS and 16-NS may be due to variation in re~, i.e., rex (5-NS)/rex (16-NS) = 0.7. Since spectra for both spin label positions are broadened by NiC12, in ethanol solution the entire lipid chain can approach the paramagnetic ions to within a collision diameter. We now consider deposition of 5-NS or 16-NS in latex suspensions. To control for extraneous ionic effects on deposition, we
' ~ 3 t~
D-
kB_____fT 67r'r/rT
[15]
where rT is the solute radius for translational diffusion. We predict that dAH/dCNio~ =f(rJrT) 180 G/M. The experimental values of dAH/dCN~cl,for 5-NS and 16-NS are, however 37 _+ 4 G/M and 51 _+ 3 G/M,
L '/
0
, 10
' -
20
5o
30
go
--
CNiCI 2 (rnM)
FIG. 4. A H vs CNlo,
5-NS in ethanol.
Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
428
BRET BERNER
kept the total cation (paramagnetic plus diamagnetic) concentration in the latex or egg lecithin suspensions constant; in addition, the same counterion, CI-, was used. The AH vs CNich results for the latex system are presented in Figs. 6 and 7; dAH/dCNich is 138 +__ 11 G/M and 92 +__8 G/M for 5-NS and 16-NS in latex, respectively. Since CNICl, is actually a number density, we must correct the above values for the excluded volume (41%) of the latex; the corrected values of dAH/dCr~jcl,for 5-NS and 16-NS in latex are 83 G/M and 55 G/M, respectively. To interpret these results further, we again utilize Eq. [1] for the spin exchange contribution to AH; the dipolar contribution need not be considered since it i s " averaged out" by spin-lattice relaxation of the Ni 2+ ions. D in Eq. [1] must be modified to account for adsorption of the spin label onto the latex. If the spin label is adsorbed onto the latex and if the viscosity of ethanol and the latex suspension were comparable, D and consequently, dAH/dCN~cl~would be approximately one-half its value in ethanol, 180 G/M. Furthermore, the viscosity of latex is undoubtedly higher than that for ethanol, and therefore, the predicted value of dAH/dCmch for latex suspensions, is less than 9Of(rex~r) G/M. Instead the experimental values of dAH/dCn~ch for 5-NS
"I" 2
J
12
10
8
6
4
J
2
0
I 10
/ 20
I I 30 40 CNJCI2 (raM)
I 50
I 60
FIG. 6. AH vs Croci, 5-NS in latex.
and 16-NS in latex are respectively 2.2 and I times their respective values in ethanol. In addition, dAH/dCsicl, for 5-NS in ethanol is smaller than dAH/dCNic~ for 16-NS in ethanol whereas the reverse is true in latex. We do not understand the large value of dAH/dCr~iCl, for 5-NS in latex. Possibly (rexlrr),f, and the local ionic concentration at the latex surface may vary. In spite of these uncertainties, it is clear that the entire lipid chain can approach the paramagnetic ions to within some small distance, 2rex, because the spectra for both 5-NS and 16-NS are broadened by the NiC12. The polycrystalline shape of the ESR spectra of 5-NS and 16-NS in a latex suspension also provides information about stearic acid deposition onto latex. To exhibit a polycrystalline spectrum, the rotation of the spin label must be slow on an ESR time scale, that is, rn
--
>> 1
[16]
T2
I
1
I
I
I
I
10
20
3O
4O
50
60
C NiC t 2 (raM)
FIG. 5. AH vs
Croci,
16-NS in ethanol.
Journal of Colloid and Interface Science, V o l . 86, N o . 2, April 1982
where ~R is the rotational correlation time for the lipid spin label and is given by Eq. [14]. For the rotation to be sufficiently slow that the spectrum is polycrystalline, the lipid spin label must either be in an extremely viscous liquid in the latex solu-
429
S U R F A C T A N T DEPOSITION ON SURFACES
then the exchange contribution to dAH/ dCNicl2 would be negligible and the dipolar contribution to dAH/dCNic]2 would be
12
10
8
•J • 9,"" •
6
js 7 4
0
I
I
10
20
FIG. 7. AH vs
I
l
30 40 C NiCI2(mM)
CNiCl, 16-NS
I
50
6I
0
in latex.
tion or adsorbed onto the latex and immobilized. Let us first consider the viscosity of the latex suspension. For AH = 5 G, r -~ 0.5, r ~ 10/~, and T = 300°K, we find that 7 / = 10 P is required to satisfy condition [16]. While the macroscopic viscosity of the latex suspension is almost certainly large enough to account for a polycrystalline spectrum, rotational diffusion of the lipid spin labels is governed by some microscopic viscosity, which for latex suspensions may be very different from the macroscopic viscosity (21). For example, spectra of 2,2,6,6-tetramethyl piperidine N-oxyl (TEMPO) in various polymer solutions resemble that of an isotropic liquid (22, 23). The rotational microscopic viscosity of a small solute such as TEMPO may differ, however, from that for the lipid spin labels. The rotational microscopic viscosity for a lipid spin label in latex solution is probably less than or equal to that for the translational diffusion of the spin label (23). If the lipid spin label were in bulk latex solution, we could use the (paramagnetic ion)(spin label) data to estimate the translational microscopic viscosity, and consequently, to set an upper limit for ~'a. If the translational microscopic viscosity for the lipids in the latex were to satisfy condition [4],
"averaged out" by spin-lattice relaxation of the electron or the ion. Thus, if the translational microscopic viscosity were to satisfy condition [4], then dAH/dCNicl~ ~ O, but dAH/dCNic]~ >>O. Then the translational microscopic viscosity, and therefore, also the rotational microscopic viscosity for the lipid label in latex is much less than required by conditions [4] and [16]. Since the latex suspension is not viscous enough to account for the polycrystalline spectra, the lipid spin label is adsorbed onto the latex. We now consider the dependence of AH on Xs, the mole fraction of labeled: (unlabeled + labeled) lipid for 16-NS in latex. As shown in Fig. 8, AH is independent of Xs; therefore, the spin labels are effectively isolated. In summary, at the fractional coverage studied, spin-labeled stearic acid adsorbs onto the surface of the latex as isolated molecules or a greatly expanded monolayer. We now turn to the results for 5-NS in egg lecithin liposomes. According to extensive studies on the order parameter, hyperfine splittings, and rotational correlation times of spin-labeled stearic acids in egg lecithin (24-26), the spin label mimics the egg lecithin in a packed arrangement within the liposomes. If spin labels are situated in such an arrangement, paramagnetic ions should only broaden the spectrum of polar-head spin labels. As shown in Fig. 9 d2d-1/dC,,
I
4~ I I
L
i 01
i 02
i 03
~ 04
i 05
J 06
i 07
I
.08
16 - NS: Stear/c Acid Mole Ratio
FIG. 8. AH vs Xs 16-NS in latex. Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
430
BRET BERNER
0 for 5-NS in egg lecithin; both CuClz and NiCI~ are used as the paramagnetic ions. Therefore, this portion of the chain of the spin label is further than rex from the ionic solution. The criterion, developed above, that a spin label is interior to the absorbent from the ionic solution if re > 5/~, appears to be a good one. On the other hand, the spectrum of lecithin spin-labeled at the polar head group is abolished by sodium ascorbate (9). Presumably, a polar head spin label stearic acid analog would be broadened by paramagnetic ions. As shown in Fig. I0, AH for 5-NS increases monotonically with Xs:
dAH - 17 ± 2G. dXs The combination of the dAH/dXs and of dAH/dCp data substantiates the hypothesis that the 5-NS molecules are within the liposomes in a packed arrangement. In summary, 5-NS in ethanol, latex, and egg lecithin liposomes are examples of an isotropic solution, isolated molecular sorption, and incorporation into a packed layered structure, respectively. A polar head spin label is necessary to distinguish between interior absorption and a packed monolayer.
• o£0
•
4,
'~
2
5
|l
~
J
B
•
ISV •
~3 I
1
I
.05
.110 .lt5 X-NS: Lecithin Mole Ratio
I
.20
FIG. 10. AH vs Xs 5-NS in lecithinliposomes. EXPERIMENTALMETHODS
Latex Samples The 5-NS and 16-NS (SYVA) and stearic acid were combined with a weighed amount of latex (Dow 288) sonicated at a very low power for 10 rain in an Ultramet III sonic cleaner, and mixed on a rotary mixer overnight. A weighed portion of this mixture was added to a predetermined amount of NiC12.6H~O plus CaC12.2H20 (Fisher). The sum of the concentration of CaC12 and NiC12 in the sample was kept as close to 30 mM as possible. The stearic acid:latex suspension ratio (w/w) used was 1:8, for those experiments in which Xs was not varied, Xs ~ 0.02. These samples were then sonicated as previously described; five glass beads were added and they were then mixed for 2 days.
Egg Lecithin Samples
The egg lecithin was purified in a column by Dr. E. R. Cooper. 5-NS and 16-NS and the egg lecithin were mixed with chloroI I I I 0 form and then evaporated under nitrogen. 10 20 30 40 50 The last traces of chloroform were removed CNiCI2 (mM) CCuCl 2 (mM) by placing the samples under vacuum for 2 FIG. 9. AH vs Croci, or Ccucl, 5-NS in lecithin hr. A predetermined amount of NiCI2.6H20 liposomes. or CuCI~ •2H20 plus CaCI2 •2H20 was added Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
SURFACTANT DEPOS~ION ON SURFACES
to the samples. The (NiCI2 plus CaCID or (CuCI2 plus CaCID concentration was kept as close to 30 mM as possible. Five glass beads were put in each sample. The appropriate amount of Dulbecco's phosphate buffer (Grand Island) was added to make a 1% liposome suspension, and the samples were mixed on a rotary mixer overnight. Thus, the ions were on the inside as well as the outside of the liposomes. For those experiments in which Xs was not varied, the ratio (w:w) of spin label to lecithin was 1:20.
The ESR Spectra All samples were drawn into 50 /~1 capillaries and sealed. The spectra were taken on a Varian E-9 spectrometer. Two spectra were taken of the mid-field line of each latex sample and of the high-field line of each lecithin sample. All spectra were taken at room temperature.The microwave power was well below saturation; the modulation amplitude was less than 10% of the line width. ACKNOWLEDGMENTS The author wishes to thank Drs. Eugene R. Cooper, John Lang, and Steve A. Goldman for their helpful discussions. REFERENCES 1. Abragam, A., "The Principles of Nuclear Magnetism," p. 128, 292, 296. Oxford Press, London, 1961. 2. Dekruiff, B., Cullis, P. R., and Radda, G. K., Biochim. Biophys. Acta 406, 6 (1975). 3. Lawaczeck, R., Kainoshu, M., and Chan, S. I., Biochim. Biophys. Acta 443, 313 (1976). 4. Hutton, W. C., Yeagle, P. L., and Martin, R. B., Chem. Phys. Lipids 19, 225 (1977).
431
5. Keith, A. D., and Snipes, W., Science 183, 666 (1974). 6. Dix, J. A., Kivelson, D., and Diamond, J. M., J. Memb. Biol. 40, 315 (1978). 7. Lunsford, J. H., J. Chem. Phys. 46, 4347 (1967). 8. Lunsford, J. H., in "Chemistry and Physics of Solid Surfaces" (K. Vanselow and S. Y. Tong, Eds.). CRC Press, Cleveland, 1977. 9. Kornberg, R. D., and McConnell, H. M., Biochemistry 10, 1111 (1971). 10. Cadenhead, D. A., and Muller-Landau, F., Biochim. Biophys. Acta 307, 279 (1973). 11. Tinoco, J., Ghosh, D., and Keith, A. D., Biochim. Biophys. Acta 274, 279 (1972). 12. Muller-Landau, F., Ph.D. dissertation, SUNY Buffalo, 1978. 13. Kivelson, D., J. Chem. Phys. 33, 1094 (1960). 14. Eastman, M. P., Bruno, G. V., and Freed, J. H., J. Chem. Phys. 52, 2511 (1970). 15. Berner, B., and Kivelson, D., J. Phys. Chem. 83, 1406 (1979). 16. Pfeifer, H., Ann. Phys. Lpz. 7, 1 (1961). 17. Anisimov, O. A., Nikitaev, A. T., Zamaraev, K. K., and Molin, Y. N., Theoret. Exp. Chem. 7, 556 (1971). 18. Klauder, J. R., and Anderson, P. W., Phys. Rev. 125, 912 (1962). 19. Raitsimring, A. M., Salikhov, K. M., Zhidomirov, G. M., and Tsvetkov, Y. D., Dokl Akad Nauk 183, 389 (1968). 20. Kivelson, D., Kivelson, M., and Oppenheim, I., J. Phys. Soc. Japan Suppl. 26, 288 (1979). 21. Crank, J., and Park, G. S., "Diffusion in Polymers." Academic Press, London, 1968. 22. Vasserman, A. M., Kovarskii, A. L., Yasina, L. L., and Bucachenko, A. L., Theoret. Exp. Chem. 13, 19 (1977). 23. Keith, A. D., Snipes, W., Mehlhorn, R. J., and Gunter, T., Biophys. J. 19, 205 (1977). 24. Schreier-Mucillo, S., Marsh, D., Dugas, H., Scheider, H., and Smith, I. C. P., Chem. Phys. Lipids 10, 11 (1973). 25. Stockton, G. W., Polnaszek, C. F., Tulloch, A. P., Hasan, F., and Smith, I. C. P., Biochemistry 15, 954 (1976). 26. Gaffney, B. J., and Chen, S. C., Methods Membr. Biol. 8, 291 (1977).
Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
Understanding surfactant deposition onto and penetration into a material can help develop processes which control coating, binding, and partitioning of surfactants. It is useful to distinguish surfactant penetration into a substance from deposition onto that substance; furthermore, we would like to classify surfactant on the surface as a packed or expanded film or as isolated adsorbed molecules. To distinguish among these possibilities, we developed an electron spin resonance (ESR) technique which utilizes spin-labeled surfactant, unlabeled suffactant, and paramagnetic ions. In this technique, the ESR line width is measured as a function of the concentration of paramagnetic ions and the ratio of labeled:total (labeled + unlabeled) surfactant. The ESR spectral line shape, from which (spin label)-(spin label) and (ion)(spin label) interactions can be deduced, contains information about surfactant deposition. The use of this ESR method to understand surfactant deposition assumes that: (1) the spin label accurately probes the same
arrangement as the unlabeled surfactant, (2) addition of paramagnetic ions does not appreciably modify surfactant sorption, (3) the substrate material with which the surfactant interacts is impermeable to paramagnetic ions, (4) a single type of surfactant sorption predominates, and (5) exchange of surfactant among distinct environments is slow on the ESR time scale. These assumptions are generally not very restrictive. The ESR spectrum of the spin-labeled surfactant is broadened if the spin label is exposed directly to the paramagnetic ion and unaffected if it is not. Consequently, if the above assumptions are satisfied, we can determine which parts of the surfactant molecule are "hidden" from the paramagnetic ions by varying the site of the spin label on the surfactant molecule and by the proper choice of paramagnetic ion. By studying (spin label)-(spin label) interactions, we can learn about the packing of the surfactant. The combination of these two types of information can help us understand surfactant sorption phenomena. Paramagnetic ions are used as shift rea-
422 0021-9797/82/040422-10502.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
SURFACTANT DEPOSITION ON SURFACES
423
some model systems and a brief summary of experimental methods.
z a)
b)
1 e)
d)
ei
FIG. 1. The types of deposits. (a) Surfactantin bulk, homogeneoussolution;(b) surfactantin the interior of the absorbent; (c) surfactant in a packed monolayer on surface;(d) surfactantin a slightlyexpandedmonolayer; (e) surfactant adsorbed as isolated molecules or as a greatly, expanded monolayer. gents in NMR (I), to distinguish inside from outside of membranes by NMR (2-4) and by ESR (5, 6), and to study active surface sites in heterogeneous catalysis by ESR (7, 8). We generalize the inside-outside method for membranes by including the dependence of ESR spectral line broadening on the position of the spin label in the surfactant molecule. The next section contains a list of rules for distinguishing among the types of surfactant sorption. These rules are based on the theory presented in the following two sections, in which the results of the theory for homogeneous isotropic solutions are summarized and the dependence of the spectral line broadening on the distance of closest approach between a spin label and a paramagnetic ion is discussed. The final two sections contain the results for ethanol, latex, and egg lecithin lipo-
A SCHEME FOR CLASSIFYING SURFACTANT DEPOSITS We measure the line width, AH, of an ESR derivative spectrum of a lipid spin label as a function of Cp, the concentration of paramagnetic ions, and as a function of X=, the mole fraction of spin-labeled (to unlabeled + labeled) surfactant. These measurements allow us to distinguish among interior surfactant, surfactant in the bulk, homogeneous solution and surfactant deposits which are packed monolayers, slightly expanded monolayers, or isolated adsorbed surfactant (see Fig. 1). In order to classify stearic acid sorption we use the labeled stearic acids, 2-(3-carboxypropyl)-4,4'-dimethyl-2-tridecyl-3-oxazolidinyloxyl (5-NS) and 2-(14-carboxytetradecyl)-2-ethyl-4,4'-dimethyl-3-oxazolidinyloxyl(16-NS); in addition, a Cls surfactant spin-labeled in the polar head region, analogous to the lecithin spin label used by Kornberg and McConnell (9), is required to fully utilize this technique. 12-NS is not useful because this spin label may not reflect accurately the arrangement of the unlabeled surfactant (10-12). The scheme for classifying surfactant sorption is: (I) Spin label situated farther than 5 It in the interior of the substrate from the boundary with the homogeneous solution. The spectrum of the spin label is unaffected by the paramagnetic ions regardless of the position of the label along the stearic acid molecule. Information about the interior may be obtainable from the spectral line shape. (2) Spin label in a packed monolayer adsorbed to the surface. Only polar head spin labels exhibit line broadened spectra in the presence of paramagnetic ions. AH varies with X=. The spectrum of 5-NS and 16-NS may be polycrystalline depending on the viscosity of the monolayer. The Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
424
BRET BERNER
surfactant chains are assumed to pack toward the substrate, but other orientations may be determined. (3) Spin label in a slightly expanded monolayer on the surface. AH for 5-NS and probably 16-NS broadens in the presence of paramagnetic ions. AH is a function of Xs and the spectrum is still polycrystalline. (4) A greatly expanded monolayer or isolated molecules adsorbed to the surface. AH for all label positions is broadened, Flo. 2. (Paramagneticion)-(spin label) interactions. but AH is independent of Xs and over some low concentration range is independent of The exchange and dipolar interactionsdepend on the Cs, the spin label molarity in the overall approach of the ion and the spin-labeled surfactant. The distance between the pair is 2re and 2re~ for the solution. The spectrum is polycrystalline. dipolar and exchangeinteractions, respectively. (5) Spin label in bulk, homogeneous solution. Paramagnetic ions broaden AH for all lipid spin label positions. AH varies the following section discuss the excluded with Cs; the spectrum resembles that of an volume complication introduced by an adisotropic liquid, depending on the viscosity. sorbent which is impermeable to the ions. Spectra of multilayer deposits should depend Let us examine the exchange interaction. To on Xs and Co as a combination of monolayer a first approximation, the exchange integral, and interior-surfactant would. Note that the J, is a constant, J0, whenever the ion and choice of paramagnetic ion depends on the the radical are within a collision diameter, viscosity of the solution. If the solution is 2rex and J is zero if the pair is further apart quite viscous (Tin < 3000°K/P), even those (13). ProvidedJ~-$ ~> 1, where zl is the collispin labels exposed to the ions are un- sion duration, the exchange contribution to affected by NiC12; CuC12 or some other para- AHex is (13-15): magnetic ion with a similarly long spin2 3 lattice relaxation time must be used to see an effect in these viscous solutions. THE THEORY OF HOMOGENEOUS, ISOTROPIC SOLUTIONS The line width, AH, of an ESR derivative spectrum of the nitroxide is measured as a function of the concentration Cp of paramagnetic ion. Cp is sufficiently dilute that only two particle (ion-radical) interactions need be considered. The additional line broadening, dAHMCo, due to the presence of these ions arises from Heisenberg exchange and dipolar interactions between the electron spins on the ion and the radical (see Fig. 2), We shall first consider the case of a homogeneous, isotropic liquid and then, in Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
where 3~eis the electron gyromagnetic ratio, Na is Avogadro's number, f accounts for the ionic interaction, and D is the diffusion constant defined as: D = 1/2(Dp + Da).
[2]
The subscripts P and R indicate the paramagnetic ion and the radical, respectively. Note that when the radical is bound or immobilized, that is, DR ~ Dp, D = 1/2Dp. We have yet to include the dipolar interaction. There are two sources of modulation of this interaction, the relative motion of the two interacting spins and the spin-lattice relaxation of the electron. Spin-lattice relaxation of the dipolar interaction must be treated
425
SURFACTANT DEPOSITION ON SURFACES
in both the fast and slow relaxation limits; the static and motionally narrowed limits of the modulation of the dipolar interaction by the relative translational motion of the spins need to be considered. In the static, slow relaxation limit for the case of spin 1/2 (1): dAHdip
dCp
3.8
- V~ btNA10-3
[3]
where /.t is the effective magnetic moment of the ion. Equation [3] is valid provided: Tie
~>1
/'2
Ze
--~
I
[4]
T2 where T~e is the electron spin-lattice relaxation time for the ion and T2 is the transverse relaxation time for the electron on the nitroxide; ze is the characteristic time for translational diffusion of the ion-radical pair and is given by 2r~ ~-c -
D
[5]
where rc is one half the distance of closest approach of the ion and the nitroxide. On the other hand, in the motional, fast relaxation limit, that is Tie
T2 T-L ~ 1
[61
r~
we find that (16): dAHdip
2~2TeNA × 10-3Tie
dC
45r 3 2V 3 [1 - l'2 - (1 + g)2e-2Vl
[71
where V = (zJTle) lj2. In the static, fast relaxation limit, that is, rJT2 >> 1 and TIe~T2 <~ 1 (17, 18):
dAHdip 327rTe/.t2 TxeNA I0 -3 dCp 9V~ r~
- -
[8]
If translational motion is the dominant source of relaxation, that is, V2 ~ 1 and Te/T 2 ~ 1,
dAHdlp 4Te/X2NA10-3 -dCp 75X/'3
[Drc] -1.
[9]
At low viscosities, experiments on Ni2÷(Tle 4 × 10-12 sec) (19) are best described by Eq. [7], while those on Cu2+(T~e = 3 × 10-9 see) (19) may be approximated by Eq. [9]. In conclusion, dAH/dCp is given by the sum of the exchange and the appropriate dipolar contributions. In low viscosity, homogeneous solutions, the exchange contribution dominates, and the broadening due to either Cu2(H20)2w+ o r Ni2(N20)zz+ should be identical; however, the dipolar contributions dominate in viscous solutions. The dipolar contributions to dAH/dCp for Ni2(H20)~ + and Cu~(HzO)22+ should be approximately 5 G/M and 50 G/M, respectively (see Eqs. [8] and [3]). THE DEPENDENCE OF z2d-/ON rc AND rex
(Spin label)-(ion) interactions for those spin labels which are free to collide with paramagnetic ions, i.e., re = 0, can be understood in terms of the theory presented in the preceding section. Spin labels in homogeneous solution and those in expanded monolayers both fall in this category. We now calculate the broadening of the spectrum of radicals in the "interior" of the absorbent, and we demonstrate that spectral broadening occurs only for radicals which can approach the ionic solution within a collision diameter. Since the absorbent is impermeable to the ions, this calculation is an excluded volume problem, and we must determine the dependence of dAH/dCp on rex and re. Spin exchange is an extremely short-range interaction; to a first approximation dAHexMC,-~ 0 whenever re > rex. The dipolar interaction, on the other hand, is a long-range interaction. According to Eqs. [8] and [9], dAHaip/dCp falls off as r~3 and r~-1 in the static, fast Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
426
BRET BERNER
relaxation and the motional, slow relaxation limits, respectively. The dependence o f dAHdip/dCp on rc must be calculated for the static, slow relaxation limit. Then the dependence of rc on the angle between the center of the absorbent, the ion on the surface, and the interior radical can be derived. We demonstrate that spectra for radicals on the absorbent surface are broadened whereas spectra for those in the interior are not. We calculate the dependence of dAHdip/ dCp on r~ in the static, slow relaxation limit. Equation [3] for dAHd~p/dCp in this limit is independent ofr~, because in the derivation o f this equation we assumed that rc = 0. The use of this assumption predicts a spectrum with a Lorentzian lineshape; including the r e dependence in the derivation causes the Lorentzian lineshape to have a finite cutoff (I). When rc is allowed to vary, we find that for the case of spin I/2 the spectral intensity, K, is proportional to:
label in the interior o f the absorbent, the center o f the absorbent, and the paramagnetic ion (Fig. 3). r~(O) varies with the geometry of the substrate particles. For convenience, we consider spherical absorbent particles of radius r which are impermeable to ions; we find that rc(0) = cos 0(rc0 - r) + {cos ~ O(r~o - r) z
-rZeo + 2rrco}112 [12]
where re0 denotes rc(O = 0). rc ~ 2re0 when 0 = +-1.1; consequently, dAHdip/dCp is a small fraction of the value calculated for re = 0 and an isotropic solution. For the low concentrations of paramagnetic ions and broad spectra involved in this study, dAH/dCp is effectively zero for r~ > 5 A. The spin label must be exposed directly to the homogeneous solution containing the ions in order to detect a broadening for the viscosities involved in this study; at these viscosities, we expect the dipolar contribution to be negligible even if rc ~ 0. Finally, another possible source of moK(~) I dt f d 0 { 2 r ~ ( 1 - cos b/rae) ,1 J tional relaxation o f the dipolar interaction + 2bSi(b/r~)} [101 is introduced by the 0 dependence of r~m the rotation of the absorbent particle. Abwhere t is the time, 0 is the angle between sorbent rotation becomes important whentwo spins and the magnetic field, Si repreever ~'~A, the characteristic time for absents the sine integral function, and sorbent rotation, is fast on an ESR time scale, i.e., b = 3/4ty~1~(3 cos ~ 0 - 1). [11] Equation [10] may be used to estimate the dependence of the Lorentzian cutoff frequency on r~. For rc ~ 5 A, Eq. [3] is an excellent approximation for dAHdip/dCp; for re ~ 10 A, by a frequency of 1 G from the center of the spectrum, the spectral intensity, K --~ 0. Since for the polycrystalline spectra in this study, A H -> 3 G, we shall let rc = 5/~ be an upper bound for the range o f the dipolar interaction in the static limit. We have yet to consider an additional complication caused by the excluded volu m e - - r ~ becomes angle dependent. To see this, let 0 be the angle formed by a spin Journal o f Colloid and Interface Science, Vol. 86, No. 2, April 1982
"ira
--
T~
>> 1
[13]
where 47VqKr 3
~'rA -- - -
3kBT
[14]
"0 is the solvent viscosity, T is the temperature, r is the absorbent radius, kB is Boltzmann's constant, and K is the ratio o f the torques to the forces on the solute (20). For small vesicles (r ~ 100 A and K = 0.5), ZrA/T2 ~ 5000 */; thus, even in water, rotation of the absorbing substance does not significantly modulate the dipolar interaction.
SURFACTANTDEPOSITION ON SURFACES
427
f
Ion
FIG. 3. The angle dependenceof re. MODEL SYSTEMS We have characterized stearic acid deposition in ethanol, Dow 288 latex, and egg lecithin. Let us first discuss the simplest model system, the homogeneous ethanol solution; this system is understandable in terms o f the theory presented above. As predicted by the theory, AH for both 5-NS and 16-NS in ethanol is a linear function of CNiC~, (Figs. 4 and 5). According to this theory, for low viscosity liquids, the spin exchange term, Eq. [1], is the dominant contribution to AH. In order to calculate dAH/dCNioz for 5-NS or 16-NS in ethanol, we need to estimate D and use Eq. [1]. To approximate D, we relate the solvent viscosity, rl, 0.012 P at 293°K (CRC Handbook), to D through the Stokes-Einstein relation,
respectively;, consequently, f or (rex/rT) varies considerably from unity and differs for the different spin labels. The difference between dAH/dCNJc~,for 5-NS and 16-NS may be due to variation in re~, i.e., rex (5-NS)/rex (16-NS) = 0.7. Since spectra for both spin label positions are broadened by NiC12, in ethanol solution the entire lipid chain can approach the paramagnetic ions to within a collision diameter. We now consider deposition of 5-NS or 16-NS in latex suspensions. To control for extraneous ionic effects on deposition, we
' ~ 3 t~
D-
kB_____fT 67r'r/rT
[15]
where rT is the solute radius for translational diffusion. We predict that dAH/dCNio~ =f(rJrT) 180 G/M. The experimental values of dAH/dCN~cl,for 5-NS and 16-NS are, however 37 _+ 4 G/M and 51 _+ 3 G/M,
L '/
0
, 10
' -
20
5o
30
go
--
CNiCI 2 (rnM)
FIG. 4. A H vs CNlo,
5-NS in ethanol.
Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
428
BRET BERNER
kept the total cation (paramagnetic plus diamagnetic) concentration in the latex or egg lecithin suspensions constant; in addition, the same counterion, CI-, was used. The AH vs CNich results for the latex system are presented in Figs. 6 and 7; dAH/dCNich is 138 +__ 11 G/M and 92 +__8 G/M for 5-NS and 16-NS in latex, respectively. Since CNICl, is actually a number density, we must correct the above values for the excluded volume (41%) of the latex; the corrected values of dAH/dCr~jcl,for 5-NS and 16-NS in latex are 83 G/M and 55 G/M, respectively. To interpret these results further, we again utilize Eq. [1] for the spin exchange contribution to AH; the dipolar contribution need not be considered since it i s " averaged out" by spin-lattice relaxation of the Ni 2+ ions. D in Eq. [1] must be modified to account for adsorption of the spin label onto the latex. If the spin label is adsorbed onto the latex and if the viscosity of ethanol and the latex suspension were comparable, D and consequently, dAH/dCN~cl~would be approximately one-half its value in ethanol, 180 G/M. Furthermore, the viscosity of latex is undoubtedly higher than that for ethanol, and therefore, the predicted value of dAH/dCmch for latex suspensions, is less than 9Of(rex~r) G/M. Instead the experimental values of dAH/dCn~ch for 5-NS
"I" 2
J
12
10
8
6
4
J
2
0
I 10
/ 20
I I 30 40 CNJCI2 (raM)
I 50
I 60
FIG. 6. AH vs Croci, 5-NS in latex.
and 16-NS in latex are respectively 2.2 and I times their respective values in ethanol. In addition, dAH/dCsicl, for 5-NS in ethanol is smaller than dAH/dCNic~ for 16-NS in ethanol whereas the reverse is true in latex. We do not understand the large value of dAH/dCr~iCl, for 5-NS in latex. Possibly (rexlrr),f, and the local ionic concentration at the latex surface may vary. In spite of these uncertainties, it is clear that the entire lipid chain can approach the paramagnetic ions to within some small distance, 2rex, because the spectra for both 5-NS and 16-NS are broadened by the NiC12. The polycrystalline shape of the ESR spectra of 5-NS and 16-NS in a latex suspension also provides information about stearic acid deposition onto latex. To exhibit a polycrystalline spectrum, the rotation of the spin label must be slow on an ESR time scale, that is, rn
--
>> 1
[16]
T2
I
1
I
I
I
I
10
20
3O
4O
50
60
C NiC t 2 (raM)
FIG. 5. AH vs
Croci,
16-NS in ethanol.
Journal of Colloid and Interface Science, V o l . 86, N o . 2, April 1982
where ~R is the rotational correlation time for the lipid spin label and is given by Eq. [14]. For the rotation to be sufficiently slow that the spectrum is polycrystalline, the lipid spin label must either be in an extremely viscous liquid in the latex solu-
429
S U R F A C T A N T DEPOSITION ON SURFACES
then the exchange contribution to dAH/ dCNicl2 would be negligible and the dipolar contribution to dAH/dCNic]2 would be
12
10
8
•J • 9,"" •
6
js 7 4
0
I
I
10
20
FIG. 7. AH vs
I
l
30 40 C NiCI2(mM)
CNiCl, 16-NS
I
50
6I
0
in latex.
tion or adsorbed onto the latex and immobilized. Let us first consider the viscosity of the latex suspension. For AH = 5 G, r -~ 0.5, r ~ 10/~, and T = 300°K, we find that 7 / = 10 P is required to satisfy condition [16]. While the macroscopic viscosity of the latex suspension is almost certainly large enough to account for a polycrystalline spectrum, rotational diffusion of the lipid spin labels is governed by some microscopic viscosity, which for latex suspensions may be very different from the macroscopic viscosity (21). For example, spectra of 2,2,6,6-tetramethyl piperidine N-oxyl (TEMPO) in various polymer solutions resemble that of an isotropic liquid (22, 23). The rotational microscopic viscosity of a small solute such as TEMPO may differ, however, from that for the lipid spin labels. The rotational microscopic viscosity for a lipid spin label in latex solution is probably less than or equal to that for the translational diffusion of the spin label (23). If the lipid spin label were in bulk latex solution, we could use the (paramagnetic ion)(spin label) data to estimate the translational microscopic viscosity, and consequently, to set an upper limit for ~'a. If the translational microscopic viscosity for the lipids in the latex were to satisfy condition [4],
"averaged out" by spin-lattice relaxation of the electron or the ion. Thus, if the translational microscopic viscosity were to satisfy condition [4], then dAH/dCNicl~ ~ O, but dAH/dCNic]~ >>O. Then the translational microscopic viscosity, and therefore, also the rotational microscopic viscosity for the lipid label in latex is much less than required by conditions [4] and [16]. Since the latex suspension is not viscous enough to account for the polycrystalline spectra, the lipid spin label is adsorbed onto the latex. We now consider the dependence of AH on Xs, the mole fraction of labeled: (unlabeled + labeled) lipid for 16-NS in latex. As shown in Fig. 8, AH is independent of Xs; therefore, the spin labels are effectively isolated. In summary, at the fractional coverage studied, spin-labeled stearic acid adsorbs onto the surface of the latex as isolated molecules or a greatly expanded monolayer. We now turn to the results for 5-NS in egg lecithin liposomes. According to extensive studies on the order parameter, hyperfine splittings, and rotational correlation times of spin-labeled stearic acids in egg lecithin (24-26), the spin label mimics the egg lecithin in a packed arrangement within the liposomes. If spin labels are situated in such an arrangement, paramagnetic ions should only broaden the spectrum of polar-head spin labels. As shown in Fig. 9 d2d-1/dC,,
I
4~ I I
L
i 01
i 02
i 03
~ 04
i 05
J 06
i 07
I
.08
16 - NS: Stear/c Acid Mole Ratio
FIG. 8. AH vs Xs 16-NS in latex. Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
430
BRET BERNER
0 for 5-NS in egg lecithin; both CuClz and NiCI~ are used as the paramagnetic ions. Therefore, this portion of the chain of the spin label is further than rex from the ionic solution. The criterion, developed above, that a spin label is interior to the absorbent from the ionic solution if re > 5/~, appears to be a good one. On the other hand, the spectrum of lecithin spin-labeled at the polar head group is abolished by sodium ascorbate (9). Presumably, a polar head spin label stearic acid analog would be broadened by paramagnetic ions. As shown in Fig. I0, AH for 5-NS increases monotonically with Xs:
dAH - 17 ± 2G. dXs The combination of the dAH/dXs and of dAH/dCp data substantiates the hypothesis that the 5-NS molecules are within the liposomes in a packed arrangement. In summary, 5-NS in ethanol, latex, and egg lecithin liposomes are examples of an isotropic solution, isolated molecular sorption, and incorporation into a packed layered structure, respectively. A polar head spin label is necessary to distinguish between interior absorption and a packed monolayer.
• o£0
•
4,
'~
2
5
|l
~
J
B
•
ISV •
~3 I
1
I
.05
.110 .lt5 X-NS: Lecithin Mole Ratio
I
.20
FIG. 10. AH vs Xs 5-NS in lecithinliposomes. EXPERIMENTALMETHODS
Latex Samples The 5-NS and 16-NS (SYVA) and stearic acid were combined with a weighed amount of latex (Dow 288) sonicated at a very low power for 10 rain in an Ultramet III sonic cleaner, and mixed on a rotary mixer overnight. A weighed portion of this mixture was added to a predetermined amount of NiC12.6H~O plus CaC12.2H20 (Fisher). The sum of the concentration of CaC12 and NiC12 in the sample was kept as close to 30 mM as possible. The stearic acid:latex suspension ratio (w/w) used was 1:8, for those experiments in which Xs was not varied, Xs ~ 0.02. These samples were then sonicated as previously described; five glass beads were added and they were then mixed for 2 days.
Egg Lecithin Samples
The egg lecithin was purified in a column by Dr. E. R. Cooper. 5-NS and 16-NS and the egg lecithin were mixed with chloroI I I I 0 form and then evaporated under nitrogen. 10 20 30 40 50 The last traces of chloroform were removed CNiCI2 (mM) CCuCl 2 (mM) by placing the samples under vacuum for 2 FIG. 9. AH vs Croci, or Ccucl, 5-NS in lecithin hr. A predetermined amount of NiCI2.6H20 liposomes. or CuCI~ •2H20 plus CaCI2 •2H20 was added Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
SURFACTANT DEPOS~ION ON SURFACES
to the samples. The (NiCI2 plus CaCID or (CuCI2 plus CaCID concentration was kept as close to 30 mM as possible. Five glass beads were put in each sample. The appropriate amount of Dulbecco's phosphate buffer (Grand Island) was added to make a 1% liposome suspension, and the samples were mixed on a rotary mixer overnight. Thus, the ions were on the inside as well as the outside of the liposomes. For those experiments in which Xs was not varied, the ratio (w:w) of spin label to lecithin was 1:20.
The ESR Spectra All samples were drawn into 50 /~1 capillaries and sealed. The spectra were taken on a Varian E-9 spectrometer. Two spectra were taken of the mid-field line of each latex sample and of the high-field line of each lecithin sample. All spectra were taken at room temperature.The microwave power was well below saturation; the modulation amplitude was less than 10% of the line width. ACKNOWLEDGMENTS The author wishes to thank Drs. Eugene R. Cooper, John Lang, and Steve A. Goldman for their helpful discussions. REFERENCES 1. Abragam, A., "The Principles of Nuclear Magnetism," p. 128, 292, 296. Oxford Press, London, 1961. 2. Dekruiff, B., Cullis, P. R., and Radda, G. K., Biochim. Biophys. Acta 406, 6 (1975). 3. Lawaczeck, R., Kainoshu, M., and Chan, S. I., Biochim. Biophys. Acta 443, 313 (1976). 4. Hutton, W. C., Yeagle, P. L., and Martin, R. B., Chem. Phys. Lipids 19, 225 (1977).
431
5. Keith, A. D., and Snipes, W., Science 183, 666 (1974). 6. Dix, J. A., Kivelson, D., and Diamond, J. M., J. Memb. Biol. 40, 315 (1978). 7. Lunsford, J. H., J. Chem. Phys. 46, 4347 (1967). 8. Lunsford, J. H., in "Chemistry and Physics of Solid Surfaces" (K. Vanselow and S. Y. Tong, Eds.). CRC Press, Cleveland, 1977. 9. Kornberg, R. D., and McConnell, H. M., Biochemistry 10, 1111 (1971). 10. Cadenhead, D. A., and Muller-Landau, F., Biochim. Biophys. Acta 307, 279 (1973). 11. Tinoco, J., Ghosh, D., and Keith, A. D., Biochim. Biophys. Acta 274, 279 (1972). 12. Muller-Landau, F., Ph.D. dissertation, SUNY Buffalo, 1978. 13. Kivelson, D., J. Chem. Phys. 33, 1094 (1960). 14. Eastman, M. P., Bruno, G. V., and Freed, J. H., J. Chem. Phys. 52, 2511 (1970). 15. Berner, B., and Kivelson, D., J. Phys. Chem. 83, 1406 (1979). 16. Pfeifer, H., Ann. Phys. Lpz. 7, 1 (1961). 17. Anisimov, O. A., Nikitaev, A. T., Zamaraev, K. K., and Molin, Y. N., Theoret. Exp. Chem. 7, 556 (1971). 18. Klauder, J. R., and Anderson, P. W., Phys. Rev. 125, 912 (1962). 19. Raitsimring, A. M., Salikhov, K. M., Zhidomirov, G. M., and Tsvetkov, Y. D., Dokl Akad Nauk 183, 389 (1968). 20. Kivelson, D., Kivelson, M., and Oppenheim, I., J. Phys. Soc. Japan Suppl. 26, 288 (1979). 21. Crank, J., and Park, G. S., "Diffusion in Polymers." Academic Press, London, 1968. 22. Vasserman, A. M., Kovarskii, A. L., Yasina, L. L., and Bucachenko, A. L., Theoret. Exp. Chem. 13, 19 (1977). 23. Keith, A. D., Snipes, W., Mehlhorn, R. J., and Gunter, T., Biophys. J. 19, 205 (1977). 24. Schreier-Mucillo, S., Marsh, D., Dugas, H., Scheider, H., and Smith, I. C. P., Chem. Phys. Lipids 10, 11 (1973). 25. Stockton, G. W., Polnaszek, C. F., Tulloch, A. P., Hasan, F., and Smith, I. C. P., Biochemistry 15, 954 (1976). 26. Gaffney, B. J., and Chen, S. C., Methods Membr. Biol. 8, 291 (1977).
Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982