Langmuir 1992,8, 1464-1469
1464
Surface Charge of Alkali Halide Particles As Determined by Laser-Doppler Electrophoresis J. D. Miller' and M. R. Yalamanchili Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112
J. J. Kellar Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701 Received August 26,1991. In Final Form: November 15, 1991
Nonequilibrium electrophoretic-mobilitymeasurements by laser-Doppler electrophoresis are reported for alkali halide particles. These measurements have been made for the first time and allow for the sign of the surface charge to be predicted for alkali halides in their saturated brines. In general, the results can be explained from the simplified lattice ion hydration theory; however, potassium chloride is, at least, one noteworthyexception. On the basis of this analysis,particle dispersion/aggregationbehavior in saturated brines can be explained. Further, the nature of collector (surfactant) adsorption and the flotation behavior of the alkali halide salts from their saturated brines can now be described more accurately.
Introduction The surface charge of particulates in suspension is of particular significance in phenomena such as adsorption, crystallization, and coagulation/dispersion. Generally the surface charge of such particulates is described by measurement of their electrophoretic mobility and the calculation of the corresponding zeta potential, the potential in the region of shear some distance away from the particle surface. The measured zeta potential then provides information regarding the sign of the prevailing charge at the particles' surface.' Electrophoretic-mobility measurements are limited to M)due to relatively low ionic strength solutions (I< electrode polarization, conductivity limitations, and the generation of thermally induced convective flow in the sample capillary.2 For these reasons and because of complete double-layer collapse in high ionic strength solutions, experimental electrophoretic-mobility measurements for soluble-salt particles such as alkali halide particles have not been reported in the literature. In this regard, laser-Doppler electrophoresis offers two particular advantages over traditional electrophoretic measureFirst, small values of the electrophoretic mobility (even smaller than 0.1pm/(s V cm)) can be measured with internal referencing to an applied modulation frequency. Second, with the membrane capillary cell, higher ionic strength solutions can be accommodated without inducing unwanted electrode polarization and other effects associated with the higher conductivity. In view of the foregoing, laser-Doppler electrophoresis was used to determine the electrophoretic mobilities of soluble-salt particles (NaC1,KC1, and other alkali halides) and thus predict the sign of their surface charge in a saturated brine. Such information is of technological importance in the areas of corrosion, tertiary oil recovery, crystallization, coagulation, and flotation. Of course zetapotential measurements in a saturated brine are impossible because the double layer is collapsed and the zeta potential does not exist, even though a surface charge may still be (1)Hunter, R.J. Zeta Potential in Colloid Science; Academic Press: London, 1981. (2) McNeil-Watson, F. K. Particle Size Analysis; Lloyd, P. J., Ed.; John Wiley: New York, 1988.
0743-7463/92/2408-1464$03.00/0
present. In order to describe the electrokinetic behavior of soluble-salt particles, nonequilibrium electrophoreticmobility measurements of the alkali halide salt particles were made after injection into freshwater solutions. Such measurements are dynamic in the sense that the salt particles are dissolving in the aqueous phase during the course of the electrophoretic-mobility measurement by the laser-Doppler electrophoresis equipment. Importantly, however, these measurements should give a relative indication of the sign of the surface charge of the salt particles in their saturated brine. The utility of these experimental measurements was demonstrated in the case of LiF and NaF, because with these salts both equilibrum and nonequilibrium are possible and can be compared. The electrophoretic-mobility results are discussed in terms of the simplified lattice ion hydration theory. In addition, the results from selected particle interaction and flotation experiments are reported in order to demonstrate the relevance of these electrophoretic measurements.
Experimental Section The experimental work involved the determination of electrophoretic mobilities for various alkali halides, including fluorides,chlorides,bromides, and iodides. The importance of these measurements is demonstrated by particle interaction experimenta in which the extent of heterocoagulation was examined for selected systems by optical microscopy. In addition, the flotation response of KC1 and NaCl was determined using both dodecylamine hydrochloride and sodium laurate as collectors (surfactants). Materials. The alkali halides (ultrapuregrade, 99.9%)used in this work were purchased from Alfa Research Chemicals. The surfactants, dodecylamine hydrochloride and sodium laurate, were obtained from Kodak and were used as received (reagent grade). Milli-Q (18 Ma) water was used in all experiments. Glassware used in the experiments was soaked in chromic acid, rinsed with large amounts of pure water, soaked in 7 M KOH, rinsed with water, and dried just prior to use. Laser-Doppler Electrophoresis. Electrophoretric-mobility measurements were done by the laser-Doppler electrophoresis technique.3 The Zetasizer 3 (Malvern Instruments,La., London, UK) with the AZ4 standard cell was used for measuring elec(3) Maluern Instruments Zetasizer User's Manual; IM060, Issue 2,
1990.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 5, 1992 1465
Surface Charge of Alkali Halide Particles
r
Beam
FROTHPWE
Beam
+
ll
,n
\ Laser Intensity
C r o r r Beam SeiAction Shutter
Monitor
Figure 1. Schematic arrangement of the Zetasizer 3.
HoBlC PARTICLES
RED FROM FROTH
Figure 3. Schematic of the laboratory vacuum flotation cell.
\Secondary
I
E l e c t roder
Figure 2. Schematic diagram of the capillary electrophoresis cell for the Zetaaizer 3. trophoretic mobilities. The schematic arrangement of the Zetasizer 3 is shown in Figure 1. Two coherent laser beams of red light produced by splitting the output of a low-power He-Ne laser are focused and made to intersect withii the quartz capillary cell holding the particle suspension at a point of zero convective flow. As a result, a pattern of interference fringes is formed, and the particles move across the fringes under the influence of the applied electric field scatter light. The intensity of the scattered light varies with a frequency that is related to the velocity of the particles. A fast photomultiplier together with a digital correlator is used to analyze the signals, and the distribution of particle velocities (electrophoretic mobilities) and/or distribution of zeta potentials is thus determined. The sign of the zeta potential is determined by referencing the observed Doppler frequency of the light scattered by the particles moving through the fringes to the modulation frequency applied to one of the laser beams. Other details are given elsewhere.3 The nonequilibrium measurements for alkali halide particles take only a few seconds. Since the measurement is done on a large number of dissolving particles, a distribution of zeta potentials will be obtained, and a distinct electrokinetic characterization of the alkali halide particles is expected. Figure 2 presents a schematic diagram of the AZA standard cell used in the present study. This cell consists of a 4 mm diameter quartz capillary. The platinum electrodes are in compartments at each end of the cell. A semipermeable membrane separates the electrodes from the suspension sample to prevent the contamination of the electrodes by the sample. The electrode chambers are filled with an electrolyte which is at least as conducting as the sample itself. Polarization of the electrodes is prevented by the application of a periodically reversed field. A technique called 'duty cycling", which allows the voltage to be switched off for a short time between each cycle, is used to avoid heating of the suspension sample, especially for high-conductivity solutions. The problem of electroosmosis is overcome by measuring the velocity of particles at a point in the cell where the electroosmotic velocity is zero. Procedure for Electrophoresis. The solublesalts were sized and the 65 X 100 mesh fraction was used in this study for non-
equilibrium electrophoretic-mobility measurements. The electrode chambers were filled with an appropriate electrolyte of required ionic strength. The electrophoretic mobilities of the alkali halides were measured in the following manner: (1)The salt particles were added to 150 mL of high-purity water in a beaker. (2) The amount of the salt added (from 0.5 to 2.5 g, depending on density and solubility) was such that the Zetasizer 3 system would signal when an ideal level of particles in the solution was present after injection. (3) The suspension was stirred gently for a moment (-1 s) and then quickly injected into the cell and the electrophoretic mobility measured. Each measurement required approximately 15s. It should be mentioned that the starting size of the salt particles is a critical factor. If the salt particles are too small, they dissolve before the measurement can be completed. On the other hand, if the salt particles are too large, sedimentation occurs in the capillary. Great care was taken to avoid injection of air bubbles into the cell. The experiment was repeated at least 3 times for each salt under the same conditions to determine the reproducibility of the measurements. In the case of equilibrium measurements for LiF and NaF, the salts (-400mesh) were allowed to equilibrate at natural pH for at least 60 min prior to electrophoresis measurements. Particle Interactions. The significance of the sign of the surface charge was examined with respect to particle dispersion/ aggregation behavior. Experiments involved preparation of selected alkali halide particles in their saturated brines for examination by optical microscopy using a Zeiss Axioplan optical microscope. In some cases the particle size was controlled and narrow size fractions were prepared by dry screening of the salt prior to study of the extent of particle interaction. In this way different alkali halide particles could be distinguished in a binary suspension. In all cases, comparisonswere made at equal particle concentrations. Flotation Experiments. Flotation has been used by industry for more than 40 years for the separation and recovery of solublesalt minerals. The separation is based on the selectiveadsorption of collectors (surfactants) to create a hydrophobic surface state for a particular mineral phase. The hydrophobic salt particles thus established attach to air bubbles dispersed through the suspension and rise to the surface where they are separated as a froth phase. The most common example is the separation of KCl from NaCl. Some researchers4have suggested that collector adsorption is determined by the difference in surface charge (4) Roman, R.;Fuerstenau, M.C.; Seidel, D.Trans. AZME 1968,241, 56.
Miller et al.
1466 Langnuir, Vol. 8,No. 5, 1992
Table I. Results from Laser-Doppler Electrophorerir Experiments-Nonequilibrium Conditions
Sodium Chloride uEs 4.171 a0.148
1
.1
2
3
( ptn /secI V l c m )
Figure 4. Nonequilibrium electrophoretic mobility distributions
for KC1 and NaC1. between the mineral components, and for this reason some results from recent flotation experiments are r e p ~ r t e d . ~ The vacuum flotation technique was used to study the flotation response of KCl and NaCl particles using dodecylamine hydrochloride and sodium laurate as collectors (surfactants). A schematic diagram of the laboratory vacuum-flotation cell6used in this study is shown in Figure 3. In the experiment, a sample (0.5-3.0 g) of the particular alkali halide salt (200 X 270 mesh) was conditioned for about 1 h in a saturated brine containing the collector (surfactant) at a particular concentration. The cellwas then connected to avacuum. The flotation time was set at 3 min, and the vacuum was maintained at 1 X lO4Torr. Subsequently, the hydrophobic particles which floated were recovered, dried, and weighed. Similarly, the hydrophilic particles which did not float were recovered, dried, and weighed, in order to determine the flotation recovery of the alkali halide under consideration.
Experimental Results and Discussion Laeer-Doppler Electrophoresis. Following the procedure discussed in the previous section, the nonequilibrium electrophoretic mobilities of selected alkali halides were measured during dissolution by laser-Doppler electrophoresis. As stated, the measurements were made during a time interval of 10-15 s. The results were found to be quite reproducible with respect to the sign of the electrophoretic mobility. Unfortunately, the change of electrophoretic mobility with time during dissolution is a more difficult measurement and has not yet been accomplished. The laser-Doppler electrophoresis measurements give a distribution of electrophoretic mobilities. For example, typical electrophoreticmobility distributions for NaCl and KC1 are shown in Figure 4. Notice that, although small in magnitude, NaCl exhibits an electrophoretic mobility of +0.171 f 0.148bm/(sV cm), which is distinctly positive, while the electrophoretic mobility for KC1, -0.47 f 0.263 pm/(s V cm), is distinctly negative. Table I presents the experimentalresults for 21 different alkali halides. In all cases, excellent reproducibility was achieved with respect to the sign of the electrophoretic mobility, when ultrapure salts were used (299.9%). As many as nine replicate measurements were made, and it should be emphasized that for a given salt the measured electrophoreticmobility was consistentlyof the same sign. As indicated in Table I, the range of the electrophoretic ~
~
electrophoretic mobility, Pm/ (e V cm) range mean
5 4 5 9 3
+0.175 to +0.271 +0.112 to +0.171 +OM1 to +0.083 +0.04 to +0.135 +0.486 to +OB13
+0.221 +0.162
4 5 6 3 4
-0.497 to -0.733 +0.124 to +0.216 -0.33 to -0.467 +0.164to +0.27 +0.325 to +0.495
-0.641 +0.163 -0.421 +0.213 +0.397
7 3 3 5 3
-0.224 to -0.375 -0.252 to -0.342 -0.388 to -0.496 -0.078 to -0.106 +0+064to +0.081
4,320 -0,311 -0.459 +0.072
4 8 4 4 3 4
-0.741 to -0.803 -0.662 to -0.791 +0.057 to +0.109 -0.098 to -0.348 +0.025 to + O M 1 +0.026 to +0.05
-0.761 -0.741 +0.079 -0.253 C0.041 +0.039
+0.084 +0.065 +0.582
-0.086
Table 11. Comparison of Nonequilibrium Electrophoretic Mobilities with Equilibrium Electrophoretic Mobilities for Two Alkali Halide Salts
mean electrophoretic mobility," pm(s V cm) salt solubility nonequilibrium equilibrium LiF 0.27 g/100 cm3 +0.221 (5) +0.155 (8) NaF 4.22 g/100 cm3 +0.162 (4) +0.099 (6) a Number of observations reported in parentheses.
mobilities was in most cases well within a factor of 2 between minimum and maximum values. For example, statistical analysis of the NaI data shows that the mean electrophoretic mobility is -0.74 f 0.043 pm/(s V cm) at the 95% confidence level. Of course the ultimate demonstration of the validity of these nonequilibrium meaeurements is the measurement of corresponding equilibrium electrophoreticmobilities which was possible for LiF and NaF. The results are presented in Table I1and show the nonequilibrium electrophoretic mobilitiea to be equivalent in sign to the equilibrium electrophoreticmobilities. Although these results will be discussed in detail in the next section, Lattice Ion Hydration Theory, it is particularly significantto note with respect to flotation response that the measured electrophoretic mobility for NaCl is positive, while the measured electrophoretic mobility for KC1 is negative. Further, it is interesting that the measured electrophoretic mobility for NaI is negative, while that for the thermodynamically stable NaI-2H20 is positive. It would appear that the hydration of lattice ions does have a significant influence on the surface charge of alkali halides. Particle Interactions. An important consequence of the surface charge developed by alkali halide particles is the stability of such particulate suspensionsand the extent of particle interaction.' It has been shown that the DLVO
~
(5) Yalamanchili, M. R., Kellar, J. J., and Miller, J. D. Collector Adsorption Phenomena and Flotation Behavior of Soluble Salts. To be
submitted to Int. J. Mineral Process. (6) Dedek,F.In Tram.Instit. Geol.Ceotechnics,Rague 1961,4,116129.
alkali halide fluorides LiF NaF KF RbF CsF chlorides LiCl NaCl KCl RbCl CSCl bromides LiBr NaBr RbBr KBr CsBr iodides LiI NaI KI RbI CSI NaL2H20
no. of observations
(7) Yalamanchili, M. R.; Miller, J. D. Interaction of Alkali Halide Particles in Their Saturated Brines; to be preaentd at the Engineering Foundation Conference on Dispersion and Aggregation: Fundamentale and Applications, Palm Coast, Fl, March 1992.
Surface Charge of Alkali Halide Particles
theory is only of limited utility in describing suspension stability.8 This is especially true at high ionic strengths such as is the case for soluble salt particulate suspensions including the alkali halide systems. The extent of particle interaction has been studied by optical microscopy, which clearly shows that, when oppositely charged alkali halide particles are examined as a binary mixture of particles, significant aggregation occurs relative to the behavior of the individual salt particles at the same particle concentration, in which case they tend to remain dispersed. Some evidence for this particle interaction in the KCl/NaCl system has been given by Roman et al.4 and has been confirmed from our own laboratory observations. As another example, consider the KC1 (-0.421 pm/sec/v/cm)/ CsCl (+0.397 pm/sec/v/cm) system, where the apparent difference in surface charge is well established. As shown from the photographs presented in Figure 5, the mixed particles are distinctly aggregated and rather stable, as compared to a suspension consisting of only one salt which is clearly di~persed.~ These observationsprovide evidence that the DLVO theory is inadequate to describe particle interaction in these systems and that structural hydration forces must be considered in order to explain the dispersion state observed at these high ionic strengths. Further, in the case of heterocoagulation, it is important to note that the sign of the particle surface charge appears to have a very significant effect at these high ionic strengths. Flotation of Soluble Salts. In view of the foregoing, these nonequilibrium electrophoretic-mobilitymeasurements should provide a good estimate of the sign of the surface charge for alkali halides in their saturated brine and allow us to examine in more detail the previous speculationsregarding the role of surface charge in collector adsorption by alkali halides. Such analysis is underway, and it has already been found that the sign of the surface charge does determine collector (surfactant) adsorption and the hydrophobic/hydrophilic balance at the surface of alkali halide salts.5 Briefly, it is known that most amine and carboxylate surfactants used as collectors in alkali halide flotation systems form insoluble collector salt colloids (RNH3X(,) or RCOOM(,), where x is a halide anion and M is an alkali cation) in saturated brines. For example, the solubility of n-ddecylamine hydrochloride Ri2NH3C1 in a KC1saturated solution is -7 X low5M.9 Recent research5has shown that the adsorption of these collector colloids and subsequent flotation of alkali halide salts are determined by Coulombic forces associated with the surface charge of the collector colloid and the surface charge of the alkali halide salt. When these charges are opposite in sign, heterocoagulation occurs between the collector colloid and the alkali halide. The surface thus becomes hydrophobic, and effectiveflotation is realized. For example, it is known that RI~NH~CI(,) is positively charged at natural pH,l+12 and because it is Opposite in charge to KC' (see I), the positively charged R12NH3Cl(s)collector colloid adsorbs at the surfaceOf the charged KC1particle* This heterocoagulation event accounts for the flotation of KC1once the R12NH3Cl collector colloid precipitates from (8) Claesson, P. M. h o g . Colloid Polym. Sei. 1987, 74, 48-54. (9) Seidel, D.C.; Roman,R.J.; Fuerstenau,M. C. Trans. AIME 1968,
Langmuir, Vol. 8, No. 5, 1992 1467
1f
Figure 5. Particle inkaction photographs: (a) CScl(- 50 Pm) particles in saturated brine; (b) mixed KC1 (-100 pm)/CsCl (-75 Pm) particles in saturated brine.
solution as is evident from the results presented in Figure 6. Interestingly, the R12NH3+ cation does not induce flotation prior to precipitation of the collector colloid. It is important to note, also from Figure 6, that NaCl is not floated by the positively charged R12NH3Clb)collector colloid, because NaCl carries the same charge as the amine colloid (see Table I), and adsorption/ heterocoagulation does not occur. Similar arguments hold for the negatively charged R11COONa(,)collector colloid. In this case positively charged NaCl (see Table I) adsorbs the collector colloid and is floated, whereas negatively charged KClis not floated with the carboxylate collector as shown in Figure 7Other explanations, in addition to heterocoagulation with collector co~~oids, are, of course,possible, and further research on this topic is in progress. In any event, the significance of these Iaser-Doppler electrophoresis measuremenb is evident. The flotation behavior of soluble saltsis quite nicely explained from the sign of the surface charge as predicted from measured mobilities. A more detailed discussion on the flotation of soluble-salt minerals is presented in another c~ntribution.~
241,64.
(10) Laskowaki, J. S.;Vurdela, R. M.; Liu, Q. In XVI International Mineral Processing Congress; Forssberg, E., Ed., Elsevier Scientific Publishers: Amsterdam, 1988; p 706. (11) Castro,,S. H.:.Vurdela, R.M.; Laskowski. J. S. Colloid Surf. 1986.
21,87.
(12) Laskowski, J. S.In Challenges in Mineral Processing; Sastry,K. V. S., Fuerstenau, M. C., Eds.; Society of Mining Engineers: Littleton, CO, 1989; pp 15-34.
Lattice Ion Hydration Theory General Remarks. It is evident from our experimental results that surface charge analysis of soluble-sat systems is now possible using this nonequilibrium laser-Doppler electrophoresis methodology. Further, it has been shown
Miller et al.
1468 Langmuir, Vol. 8,No. 5, 1992 100-
-
I I
-
A
effects which were shown to be particularly significant for more complicatednonreactive ionic solids such as calcium fluoride.14-le Thus, a more complete theoretical treatment considers the lattice energy of surface ions which in turn is dependent on the crystal structure and preferential plane of cleavage. For uni-univalent ionic solids such as the alkali halides now being examined, the energetics can be represented, for the solid MX, as follows:
I
I
!
' lo 1
DODECYL AMINE CONCEMRATlON (M)
Figure 6. Flotation recovery of KCl and NaCl as a function of dodecylamine hydrochloride concentration. 0"
I
I
I I
-E
6
-1 ,/
KCI
!
SOLUBILITY LIMIT OF
R,,COONa
I I
I
NaC1
1
D
I
I
(13) De Bruyn, P. L.; Agar, G .E. In Froth Flotation-50th Annioersary Volume; Fuerstenau, D. W:,Ed.; American Institute of Mining, Metallurgical and Petroleum Engineers: New York, 1962; p 91.
If the hydration energy of the surface lattice cation is more negative than the hydration energy of the surface lattice anion, as described by reactions 3, then the surface of MX will carry a negative charge. The converse is also true. The complete analysis requires knowledge of the hydration free energies for gaseous ions (reactions 1)and the lattice energy for surface ions (reactions 2). The former is available from the literature as determined from the Born and Mayer theory,17 while the latter is determined from electrostatic principles, taking into consideration the geometric arrangement of cations and anions in the lattice as described by the surface Madelung c o n ~ t a n t . l ~ - ~ ~ Alkali Halides. For the alkali halides, the analysis appears to be rather simple. If the cation and anion are interchangeable, then the lattice energies of surface cation and anion are equivalent, and the surface charge should be established simply by comparing the gaseous ion hydration free energies of the respective lattice ions as done by Roman et aL4 Confirmation of this theoretical expectation was limited to indirect evidence based on the flotation response of these salts in their respective brines and microscopic examination of particle interaction phenomena which showed that heterocoagulation occurred in certain circumstances. Of course, such indirect analysis of the alkali halide system was dependent upon certain assumptions regarding collector (surfactant) adsorption. The results from this current investigation provide more direct evidence of the sign of the surface charge developed by alkali halides in their brines, and the results of nonequilibrium laser-Doppler electrophoresis measurements are summarized in Table 111,where they are compared to predictions using the simplified lattice ion hydration theory. The gaseous hydration free energy values given in Table I11were determined based on the Born model.17 The Born model, which has been examined in further detail, still allows for the calculation of free energies that are accurate to within a few percent.18 As can be seen when the difference in gaseous ion hydration free energies is large, the simplified theory accurately predicts the sign of the surface charge. For example, in all cases where the difference in gaseous ion hydration free energies is greater (14) Miller, J. D.; Calara, J. V. In Flotation, A.M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; American Institute of Mining, Metallurgical and Petroleum Engineers: New York, 1976; Vol. 1, p 66. (15) Calara, J. V.; Miller, J. D. J. Chem. Phy8. 1976, 65,843 (16) Calara, J. V.; Miller, J. D. Colloid and Interface Science, Proceedings of International Conference-50th Colloid and Surface Science Symposium, June 21-25, 1976, Puerto Rico; Kerker, M., Ed.; Academic Press: New York, 1976; Vol. 111, p 157. (17) Hunt, J. P. Metal Ions in Aqueous Solution; W. A. Benjamin, Inc.: New York, 1965. (18) Jayaram, B.; Fine, R.; Sharp, K.; Honig, B. J.Phys. Chem. 1989, 93, 4320.
Langmuir, Vol. 8, No. 5,1992 1469
Surface Charge of Alkali Halide Particles Table 111. Sign of the Surface Charge for Selected Alkali Halides negative gaseous hydration free energies,a kcal/mol cation anion fluorides LiF NaF KF RbF CsF chlorides LiCl NaCl KCl RbCl CSCl bromides LiBr NaBr KBr RbBr CsBr iodides LiI NaI KI RbI CSI NaI-2H20
sign of surface charge predictedb experimentalC
112.0 88.4 71.1 65.9 58.2
109.6 109.6 109.6 109.6 109.6
?
112.0
82.5
88.4 71.1
82.5 82.5
-
65.9 58.2
82.5 82.5
112.0 88.4
75.7 75.7
71.1 65.9
75.7 75.7
58.2
75.7
112.0 88.4
66.5 66.5
71.1
66.5
65.9 58.2 NA
66.5 66.5 NA
+ + + +
+ + + + + -
+ + +
+ + +
-
-
-
-
+ + +
?
+
NA
-
+
-
-
+
-
+
+
a Reference 17. b Simplified lattice ion hydration theory. Electrophoretic- mobility measurements by laser-Doppler electrophoresis.
than about 10 kcal/mol, the sign of the surface charge as measured is in agreement with that predicted from the simplified lattice ion hydration theory. When the difference in gaseous ion hydration free energies is less than about 10 kcal/mol, the experimental results are not always supported by the simplified theory but can be opposite to that predicted as shown by the rows in italic in Table 111. As is evident, five salts are in variance with theory. If we assume that the gaseous ion hydration free energies are accurate, then it seems that the anomalous results will have to be explained in terms of differences in lattice energy contributions. That is, the assumption of lattice energy equivalency for cation and anion is not valid. Sucha differencein lattice energy might arise from lattice defects and relaxation effects, and further analysis willbe necessary to explain these results. On the other hand, if gaseous ion hydration free energy values are used based on enthalpies reported by Kanevskii,'9 then the agreement between experiment and theory is even better, withKC1, KBr, and RbBr being the only exceptions to the simplified lattice ion hydration theory. (19) Kanevskii, E. A. Zh.Fiz. Khim. 1949,23, 723.
In any event, it seems that the experimental results are reliable and, in fact, are substantiated'by the results from flotation experiments. As a final point for discussion, the results for NaI and NaI.2H20 must be mentioned. Recall that the anhydrous NaI is negatively charged, whereas the NaI.2H20 is positively charged. Here is a clear indication of the significance of hydration in the charging process. The behavior of NaI is well described by the simplified lattice ion hydration theory and is expected to be negatively charged. See Table 111. On the other hand NaI.2H20 is triclinic,20 the anion and cation are not interchangeable, and the simplified lattice ion hydration theory is not applicable. Of course in such cases the respective lattice energies for the anion and cation (reaction 2) must be considered in order to make the proper calculation and predict the sign of the surface charge. Qualitatively it is expected that when water is introduced into the crystal lattice, the hydration of the Na+ in the lattice is substantial, and the lattice energy associated with the cation release for reaction 2 is reduced considerably; the iodide anion then has a relatively greater tendency to hydrate and leave the lattice surface site, thus resulting in a positive surface charge.
Summary and Conclusions For the first time, nonequilibrium electrophoreticmobility measurements for alkali halide salts have been made by laser-Doppler electrophoresis. The measurements, which were made during the initial stages of salt dissolution, were found to be reproducible, and the expected sign of the surface charge in a saturated solution can be described from the simplified lattice ion hydration theory in most cases. This analysis is supported by particle interaction experiments in which the strong aggregation of oppositely charged salt particles was quite evident from optical microscopy. Also, the flotation behavior of the alkali halide salts can be described from these electrophoretic measurements, if the flotation is assumed to involve collector (surfactant) adsorption at the salt surface by Coulombic attraction of the oppositely charged collector colloid. The adsorbed collector colloid thus renders the salt particle hydrophobic and accounts for the observed flotation response. Finally, the importance of surface charge for such short-range interactions at high ionic strength is of particular significance. Acknowledgment. This work was supported by the
U.S.Department of Energy, Grant DE-FG02-84ER13181, Basic Science Division.
(20) Wooster, W. A. Nature 1932,3288 (130), 698.
