Colloids and Surfaces
`A: Physicochemical and Engineering Aspects 186 (2001) 129–136
`
`www.elsevier.nl/locate/colsurfa
`
`Effect of inorganic additives on solutions of nonionic
`surfactants — XVI. Limiting cloud points of highly
`polyoxyethylated surfactants
`
`Hans Schott *
`School of Pharmacy, Temple Uni6ersity, Philadelphia, PA 19140, USA
`
`Abstract
`
`As their degree of polyoxyethylation is increased, the cloud points (CPs) of three homologous series of nonionic
`surfactants (NSs) rise steeply at first but then level off, converging asymptotically to the temperature range of
`114–118°C, which corresponds to the CP of polyethylene oxide with a molecular weight of 104. Six of the 14
`surfactants investigated were octoxynols, five were nonoxynols and three were polyoxyethylene dodecyl ethers. Ten
`had an average number of oxyethylene units per molecule p\20 and a CP\100°C. Three of these CPs were
`measured directly on 2.00% surfactant solutions sealed into ampoules. To avoid additional measurements under
`pressure, the other surfactants had their CPs lowered below the normal boiling point of their 2.00% solutions by
`adding the salting-out, CP-lowering salt NaNO3 at various concentrations, measuring the depressed CPs and
`extrapolating them to zero salt concentration. The CPs decrease linearlv with increasing molality m of NaNO3 up to
`ca. 2.5 m and more steeply at higher concentration. For a given concentration of NaNO3, larger p values result in
`more extensive salting out: The more highly polyoxyethylated a surfactant is, the greater the reduction in CP it
`undergoes. As p increases, the curves of CP in water versus p for the octoxynols and nonoxynols rise steeply and
`nearly linearly, reaching 100 and 103°C at p=16, and then begin to level off. The sharpest inflections are at
`p=22–25. The nonoxynol curve becomes horizontal at p$40 and 115°C while the octoxynol curve rises gently from
`3°C below the nonoxynol curve to merge with it at p=90 and 114°C. The CPs of the three polyoxyethylene dodecyl
`ethers are slightly higher than those of the polyoxyethylated alkylphenols with comparable p values, peaking at
`118°C. The most highly polyoxyethylated surfactant of each series contains 95% polyoxyethylene. Their CP range of
`114–118°C equals that of polyethylene oxides with molecular weights between 8000 and 20 000. Published quantita-
`tive CP–p relations for polyoxyethylated NSs cover only CP values below 100°C and p values below 16. They fail to
`predict the levelling off of the CPs as the p values rise above 16. © 2001 Elsevier Science B.V. All rights reserved.
`
`Keywords: Cloud point limits; Highly polyoxyethylated nonionic surfactants; Nonionic surfactants; Nonoxynol cloud points;
`Octoxynol cloud points
`
`* Tel.: +1-2158870116; fax: +1-2157073678.
`
`0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
`PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 4 9 1 - 5
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`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`1. Introduction
`
`1.1. Cloud points
`
`Water-soluble polyoxyethylated nonionic sur-
`factants (NSs) are more soluble in cold than in
`hot water. When heated above a characteristic
`lower consolute temperature or cloud point (CP),
`their aqueous solutions separate into a surfactant-
`rich phase and a very dilute solution as the NSs
`change from being primarily water-soluble to be-
`coming primarily oil-soluble. Cooling below the
`CP reverses this process without hysteresis.
`Because of such phase separation, NSs cease to
`perform many of their intended functions above
`the CP: Suspensions stabilized with NSs flocculate
`or coagulate, foams collapse, and O/W emulsions
`break. Incidentally, many of the organic liquids
`being emulsified raise or lower the characteristic
`CP values substantially.
`CPs are commonplace. Most NSs have low
`critical micelle concentrations: At practical use
`levels, the bulk of the surfactants are aggregated
`into micelles. High micellar molecular weights
`cause the dissolved NSs to behave like polymers:
`Low entropies of mixing render micellar solutions
`far more prone to demixing or phase separation
`than solutions of nonaggregated molecules at
`comparable volume fractions.
`
`1.2. Predicting cloud point 6alues
`
`Because of the practical and theoretical impor-
`tance of the CP, procedures for predicting its
`values from the structure of NSs are of interest.
`CPs generally increase with increasing hydrophile-
`lipophile balance (HLB) of the NSs, correspond-
`ing either
`to an increasing number p of
`oxyethylene units per surfactant molecule or a
`shorter hydrocarbon moiety [1,2].
`Within several homologous series pertaining to
`different categories of NSs, the CP was reported
`to be proportional to log p [3,4] and inversely
`proportional to the number n of carbon atoms in
`the hydrocarbon moiety [4]. This
`empirical
`method for estimating CPs was extended and
`refined by including topological indices character-
`izing the hydrocarbon moiety of the surfactants.
`
`Thus, it became feasible to commingle different
`categories of NSs, resulting in a universal relation
`for estimating all CPs based on the p value and
`size and structure of the hydrocarbon moieties [5].
`A statistical approach consisted in using nonlin-
`ear regression analysis to correlate the CP with p
`at fixed n values within different categories of
`linear and branched polyoxyethylated aliphatic
`alcohols. The CP versus p plots were slightly
`concave towards the p axis [6].
`These studies [1,3–6] have a common short-
`coming: they are based on NSs with p015 and
`CPB100°C, presumably because CP data beyond
`these limits are scarce. However, NSs with p
`values up to 100 are commercially available and
`used in industrial
`formulations. Furthermore,
`temperatures exceeding 100°C are routinely used
`during the manufacture of aqueous disperse sys-
`tems stabilized with NSs. For instance, steam
`sterilization of pharmaceutical and cosmetic sus-
`pensions, emulsions and ointments is generally
`carried out by autoclaving atE121°C. Ultrahigh-
`temperature pasteurization employs temperatures
`in the 87–132°C range. In order to prevent desta-
`bilization of such disperse systems by high pro-
`cessing temperatures through surfactant phase
`separation, they should be formulated with NSs
`whose CPs exceed even these high temperatures.
`This points to the importance of knowing the
`CP values of NSs with p in the 16–100 range,
`which exceed the normal boiling points of the
`aqueous surfactant systems. Estimating these CP
`values by extrapolating the published relations
`[1,3–6] would be particularly useful because the
`pertinent CP measurements, which involve pres-
`surized systems, are more difficult to carry out.
`Two observations suggest that the published
`relations between CP and p [1,3–6] cannot be
`extrapolated indefinitely with increasing p values.
`(i) While the CPs within individual homologous
`series of both normally distributed and homoge-
`neous NSs always increase with p, the CP incre-
`ments
`for
`each additional oxyethylene unit
`decrease [1]. (ii) A plot of CP versus p for 7
`normally distributed polyoxyethylated octylphe-
`nols with p values ranging from 7 to 34 rises
`rapidly (although less than linearly) from p=7
`(CP=17°C) to p=19 (CP=101°C), but shows
`
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`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`131
`
`signs of levelling off at p=30 (CP=108°C) and
`p=34 (CP=110°C) [7]. This very informative
`1959 publication [7]
`is being completely over-
`looked in the current literature, even though it
`was the first and almost only one to publish CPs
`above 100°C.
`
`1.3. Research plan
`
`The purpose of the present work is to investi-
`gate the CPs of extensively polyoxyethylated NSs,
`which are merely listed as\100°C in the litera-
`ture. Two homologous series based on alkylphe-
`nols were chosen because they are widely used in
`formulations and include surfactants with the
`highest p values available.
`The CPs of highly polyoxyethylated NSs exceed
`the normal boiling points of their solutions. To
`avoid working with pressurized systems, the CPs
`of the solutions were depressed below 100°C by
`adding various amounts of a salting-out elec-
`trolyte and extrapolating them to zero electrolyte
`concentration. This approach is facilitated by the
`linear dependence of CP values on the concentra-
`tions of many 1:1 electrolytes up to ca. 3 m [8,9].
`
`2. Experimental
`
`2.1. Materials
`
`The two series of polyoxyethylated alkylphenols
`used are commercially available in a wide range of
`p values. These surfactants are normally dis-
`tributed. It was not feasible to use monodisperse
`(homogeneous) polyoxyethylated surfactants be-
`cause these are only available with pB9 and have
`CP values well below 100°C.
`Polyoxyethylated octylphenols are designated
`as octoxynols [10,11]. The numbers following the
`word, octoxynol, represent average p values. The
`branched octyl moiety is an isobutylene dimer.
`Octoxynols are supplied by Union Carbide Corp.
`under the tradename Triton X. The highest p
`value of the series is 53 (Surfactant No. 5 in Table
`1).
`Polyoxyethylated nonylphenols are designated
`as nonoxynols [10,11]. The branched nonyl moiety
`is a propylene trimer. Nonoxynols are supplied by
`Rhodia Inc. under the tradename Igepal CO. The
`highest p value of the series is 94 (Surfactant No.
`11 in Table 1).
`
`Table 1
`US Pharmacopeial names, trade names, average number of oxyethylene units per molecule (p) and hydrophile-lipophile balance
`(HLB) of the surfactants
`
`Trade namea
`
`Triton X-102
`Triton X-165
`Triton X-305
`Triton X-405
`Triton X-705
`–
`Igepal CO-730
`Igepal CO-880
`Igepal CO-970
`Igepal CO-987
`Igepal CO-997
`–
`Brij 35
`–
`
`p
`
`12
`16
`30
`35
`53
`90
`15
`29
`47
`70
`94
`11
`23
`80
`
`HLB
`
`14.4
`15.5
`17.3
`17.6
`18.4
`19.0
`15.0
`17.1
`18.1
`18.7
`19.0
`14.4
`16.9
`19.0
`
`Surfactant number
`
`US pharmacopeial name
`
`Octoxynol 12
`Octoxynol 16
`Octoxynol 30
`Octoxynol 40
`Octoxynol 70
`Octoxynol 90
`Nonoxynol 15
`Nonoxynol 30
`Nonoxynol 50
`Nonoxynol 70
`Nonoxynol 100
`Polyoxyl 11 dodecyl ether
`Polyoxyl 23 dodecyl ether
`Polyoxyl 80 dodecyl ether
`
`1
`2
`3
`4
`
`56
`
`78
`
`9
`10
`11
`12
`13
`14
`
`a Triton, Igepal and Brij are registered trademarks of Union Carbide Corp., Rhodia Inc. and ICI Americas Inc., respectively.
`
`Page 3 of 8
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`

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`132
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`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`Manufacturers obtain the p values from the
`weight increase during polyoxyethylation of the
`alkylphenols and/or by determining the hydroxyl
`number of the finished products. The surfactants
`contain small amounts of polyethylene glycols.
`These impurities generally have lower molecular
`weights than the surfactants and have two terminal
`hydroxyl groups compared to the surfactants’ one.
`Therefore, the reported p values represent lower
`limits. Polyethylene glycol impurities have negligi-
`ble effects on the CPs of highly polyoxyethylated
`surfactants [7].
`The commercial surfactants were supplied either
`neat or as 70% aqueous solutions. Their moisture
`contents were determined by drying at room tem-
`perature first over Drierite and then over P205. All
`neat products contained B0.1% water.
`Octoxynol 90 (Surfactant No. 6 in Table 1) was
`prepared by polyoxyethylation of molecularly dis-
`tilled octoxynol 3 (Triton X-35) to p=90. The
`catalyst was 0.7% NaOH; the temperature in the
`stirred autoclave was 160–180°C and the pressure
`2–3 atm. The weight
`increase was 11.3-fold
`(1130%). The hydroxyl number of Surfactant 6 was
`13.45 mg KOH/g surfactant.
`Surfactants No. 12 and 14 were made by poly-
`oxyethylating 1-dodecanol under similar condi-
`tions to p=11 and 80, respectively. They are
`designated polyoxyl 11 and 80 dodecyl ether [10] or
`laureth-11 and -80 [11].
`The other chemicals were ACS reagent grade.
`The water was double distilled. NaNO3 was se-
`lected as the salting-out electrolyte to lower the CPs
`- anion occupies an intermediate
`because the NO3
`position in the lyotropic series and neither raises
`nor lowers the CPs of NSs [12]. Thus, the CP
`reductions are due entirely to the salting-out activ-
`ity of the Na+ ions. Plots of CP versus mNaNO3
`are
`linear up to 2.5 m, which simplifies the extrapola-
`tion to m=0.
`
`2.2. Solution preparation
`
`Solutions containing surfactant and salt were
`prepared by adding water and analyzed, ca. 8 m
`NaNO3 solutions to 20.00% surfactant solutions,
`all weighed out to the nearest milligram. As a rule,
`the CPs of normally distributed NSs depend only
`
`slightly on surfactant concentration in the range of
`0.2–7.5% [1,7,13]. The final concentration of all
`surfactants was 2.00% by weight; their percentage
`is based on the weight of water present. All
`mixtures were stored in the dark at 6–9°C for E20
`h prior to measuring the CP to ensure full hydration
`of the micelles.
`
`2.3. Cloud point detemination
`
`CPs were measured visually in triplicate as the
`temperatures at which the solutions suddenly
`turned nearly opaque on heating and the tempera-
`tures at which they just as suddenly cleared on
`cooling, without hysteresis. Observing the precau-
`tions described [14], the six CP values for a given
`solution agreed within 0.2°C.
`Even though the CPs of Surfactants No. 6, 13
`and 14 exceeded the normal boiling points of their
`2.00% aqueous solutions, they were measured di-
`rectly. Salt-free solutions sealed inside glass am-
`poules were heated and cooled at the rate of 0.1°C
`min − 1 in the vicinity of the CP.
`The CPs of aqueous NS solutions are known to
`increase with pressure. However, the rate of in-
`crease, ca. 1×10 − 7°C Pa − 1 [15] is three orders of
`magnitude smaller than the rate of increase of
`saturated steam pressure with temperature. There-
`fore, the CP increase due to pressure developed in
`the sealed ampoules is negligible.
`
`3. Results and discussion
`
`3.1. Cloud points 6ersus molality of NaNO3
`
`The Na+ ions salt out the NSs, lowering their
`CPs. The CPs decrease monotonically with increas-
`ing molality m of NaNO3, linearly at first but more
`steeply at higher NaNO3 concentrations. For Sur-
`factant No. 1, which has the lowest degree of
`polyoxyethylation, the linear range extends only up
`to 1.0 m NaNO3. The plots of all other surfactants
`are linear up to ca. 2.5 m (see Figs. 1 and 2).
`The equation for the linear segments is:
`CP=CP0+bm
`(1)
`The intercept CP0 is the (extrapolated) CP at m=0.
`This is the desired quantity.
`
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`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`133
`
`variance (ANOVA) employing small-sample t-
`and F-tests, the residual or error variances must
`be homogeneous. It then becomes permissible to
`pool the estimated variances of the slopes and of
`the intercepts of the individual regression lines
`when using ANOVA [16,17]. Since the CP values
`of all octoxynols and nonoxynols except those of
`Surfactants No. 6 and 12–14 were measured by
`the same procedure and with the same precision,
`the expected homogeneity of the residual vari-
`ances was observed.
`
`3.3. (Extrapolated) intercepts: cloud points in
`water
`
`The increase in the intercepts (CP0 values) of
`the octoxynols with increasing p values shown in
`Tables 1 and 2 is significant at the 5% probability
`level. The intercepts of nonoxynols 50, 70 and 100
`(Surfactants No. 9–11) do not differ significantly
`at the 5% probability level. Likewise, the CP0
`value of Surfactant No. 6 is indistinguishable
`
`Fig. 2. Cloud point CP (°C) of nonoxynols as a function of the
`molality m of added NaNO3. The linear segments used for
`extrapolation are shown as solid lines. The dashed lines repre-
`sent linear extrapolation to m=0. Key: , nonoxynol 15
`(Surfactant No. 7); , nonoxynol 30 (No. 8); , nonoxynol
`50 (No. 9); , nonoxynol 70 (No. 10);
`, nonoxynol 100 (No.
`11).
`
`Fig. 1. Cloud point CP (°C) of octoxynols as a function of the
`molality m of added NaNO3. The linear segments used for
`extrapolation are shown as solid lines. Nonlinear segments are
`shown as dotted lines. The dashed lines represent linear ex-
`trapolation to m=0. Key: , octoxynol 12 (Surfactant No.
`1);
`, octoxynol 16 (No. 2); , octoxynol 30 (No. 3); ,
`octoxynol 40 (No. 4); , octoxynol 70 (No. 5).
`
`The empirical extrapolation procedure is vali-
`dated by the following observations: Since the
`CPs of the salt-free solutions of Surfactants No. 1,
`2 and 7 were below the respective normal boiling
`their CP0 values were also measured
`points,
`directly.
`The measured CP0 values agree with corre-
`sponding extrapolated CP0 values within an ex-
`perimental
`error of
`0.2°C (see Table
`2).
`Furthermore, all negative correlation coefficients
`for the linear segments of Fig. 2, described by Eq.
`(1) and based on 4–6 points each, have absolute
`valuesE0.999. This confirms the high precision of
`the CP extrapolations to m=0.
`
`3.2. Statistical comparison of the linear segments
`of the CP 6ersus m plots
`
`In order to compare the linear segments of Fig.
`1 and those of Fig. 2 by means of analysis of
`
`Page 5 of 8
`
`

`
`134
`
`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`Table 2
`Data for linear portions of the plots of cloud points versus
`molality of NaNO3 (m) in Fig. 1: Number of points (n),
`intercepts (CPO, extrapolated to m=0) and slopes (b)a
`
`Surfactant number
`
`1
`
`2
`
`3
`4
`5
`6
`7
`
`8
`9
`10
`11
`12
`13
`14
`
`n
`
`5
`–
`4
`–
`4
`4
`6
`–
`5
`–
`4
`4
`4
`4
`–
`–
`–
`
`CPO (°C)
`
`b (°C/m)
`
`89.26
`89.2b
`99.70
`99.5b
`110.09
`111.34
`112.16
`114b
`96.85
`96.8b
`113.45
`115.75
`114.65
`114.28
`98.8b
`113b
`118b
`
`−6.040
`–
`−6.800
`–
`−7.363
`−8.040
`−9.454
`–
`−6.866
`–
`−8.553
`−9.742
`−10.776
`−11.909
`–
`–
`–
`
`a b=d CP/d m from Eq. (1).
`b Cloud points were actually measured in salt-free solutions
`instead of being calculated by extrapolation to m=0.
`
`from those of Surfactants No. 9–11. The highest
`CP0 values of nonoxynols, characteristic of those
`with pE47, are in the range of 11591°C. This
`range also includes the limiting CP0 value for the
`octoxynol series, namely, 114°C of Surfactant No.
`6. The CP of Surfactant No. 14 is only slightly
`though significantly higher.
`As their p values increase, the NS micelles
`increasingly resemble coils of polyethylene glycol
`or polyethylene oxide (PEO) chains. Hence, their
`CPs approach the CPs of the latter.
`The CPs of PEOs decrease with increasing
`molecular weight. Based on interpolation on the
`nearly linear plots of the reciprocal of the CP in
`degrees Kelvin versus the reciprocal of the square
`root of the molecular weight [18], normally dis-
`tributed PEOs with CPs of 115°C and 118°C have
`molecular weights of 18 300 and 14 600, respec-
`tively [19]. For fractionated PEOs, CPs of 115°C
`and 118°C correspond to molecular weights of
`9600 and 8300, respectively [20]. The latter are
`perhaps more representative of the PEO shells of
`NS micelles.
`
`3.4. Slopes and susceptibility to salting out
`
`The slopes of the linear segments in Figs. 1 and
`2, b=dCP/dmNaNO3
`,are listed in Table 2. For
`both octoxynols and nonoxynols, the slopes be-
`come increasingly more negative as the p values
`increase. This trend is significant at the 5% proba-
`bility level. The water solubility of NSs is due to
`their polyoxyethylene moieties. Apparently, the
`bigger these moieties are, the stronger is the salt-
`ing-out or CP-lowering effect of a given increment
`in the concentration of the added Na+ ions.
`This novel effect of the degree of polyoxyethy-
`lation on the surfactants’ susceptibility to being
`salted out,
`is illustrated below: The difference
`DCP between the CP in water (CP0) and the CP in
`2.00 m NaNO3 is proportional to p. The least-
`squares equation of the linear relation is, for the
`five octoxynols of Table 2:
`DCP=11.3+0.139 p (n=5, r=0.987)
`
`(2)
`
`and for the five nonoxynols:
`DCP=13.0+0.120 p (n=5, r=0.980).
`
`(3)
`
`It is unlikely that these two empirical equations
`can be extrapolated to much higher p values.
`
`3.5. Relation between cloud points and degrees of
`polyoxyethylation
`
`According to Fig. 3, the increase in the CP0
`values of the octoxynols and nonoxynols with
`increasing p is nearly linear up to p$16. Both
`curves then begin to level off, with the sharpest
`curvature at p$22–25. They become nearly hori-
`zontal at p$40, where they are only 3°C apart,
`and eventually merge at p=90–94, in the vicinity
`of the CP of low molecular-weight PEO. The
`points for the polyoxyethylated dodecanols (not
`shown) lie at slightly higher temperatures.
`Even when the CP values are plotted against
`the surfactants’ LB instead of their p values to
`compensate for the difference in the size of their
`hydrocarbon moieties, the octoxynol curve lies
`slightly below the nonoxynol curve while the
`curve for the polyoxyethylated dodecanols lies
`slightly above it. This plot is not shown.
`
`Page 6 of 8
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`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`135
`
`4. Conclusions
`
`4.1. Validity of published cloud point relations
`
`The published relations [1,3–5] are based solely
`on surfactants with p015 and CPB100°C. Their
`points fall exclusively on the steeply ascending,
`nearly linear segments in Fig. 3. Extrapolations
`towards higher p values made on that basis alone
`could do nothing but to extend these steep seg-
`ments: Below p=15, the curves give no hint of
`levelling off. Therefore, such extrapolations are
`bound to produce unrealistically high CP values.
`For instance, Eq. (7) of [4] gives CP estimates of
`179°C, 298°C and 328°C for Surfactants No. 13,
`14 and 6 or 11, respectively, instead of the experi-
`mental values of 113°C, 118°C and 114°C (Table
`2). This shortcoming relegates the published rela-
`tions [1,3–5] to a CP rangeB100°C andp B16,
`for which CP values are easily measured and are
`already extensively tabulated [1,2]. They are use-
`less for estimating CP values\100°C, pertaining
`to NSs with p values between 16 and 100, which
`are more difficult to measure experimentally and
`for which there is, consequently, a dearth of pub-
`lished data.
`
`Fig. 3. Cloud point CPo in water (°C) of octoxynols and
`nonoxynols as a function of the number p of oxyethylene units
`per molecule. Key: , nonoxynols, present work; ,
`nonoxynols, Ref. [1]; , octoxynols, present work; , oc-
`toxynols, Ref. [7]; , octoxynols, Ref. [1].
`
`4.2. Upper cloud point limits for highly
`polyoxyethylated surfactants
`
`The three most highly polyoxyethylated surfac-
`tants, Nos. 6, 11 and 14, have an HLB value of
`19.0,
`indicating that their molecules consist of
`95% polyoxyethylene and 5% alkylphenol or alco-
`hol. Their hydrocarbon moieties are large enough
`to confer surface activity in aqueous solution but
`too small, on a volume-percent basis, to substan-
`tially affect solubilities or CPs: The limiting CPs
`of highly polyoxyethylated NSs are in the 114–
`118°C range, which equals the CPs of PEOs of
`relatively low molecular weight.
`
`4.3. Practical considerations
`
`NSs with CPsE121°C, which remain func-
`tional at steam sterilization temperatures, cannot
`be prepared by more extensive polyoxyethylation
`of alkylphenols or long-chain alcohols: Their CPs
`are capped by the CP of PEO.
`An alternate route for raising the CPs of NSs is
`to shorten their hydrocarbon chains, reducing m
`[1,2,4]. However, this route is severely limited
`because the m and p values of NSs are balanced
`to achieve optimum surface activity. Reducing m
`while maintaining (or even increasing) the value
`of p would soon reduce the surfactants’ surface
`activity excessively, so that they could no longer
`function as suspending agents, emulsifiers, solubi-
`lizers etc.
`One can, however, boost the CP of a NS sub-
`stantially by adding very small amounts of ionic
`surfactants. They form mixed micelles with the
`NS which, by virtue of their slightly ionic charac-
`ter and the resultant intermicellar electrostatic
`repulsion, have considerably higher CPs [21]. For
`instance, either a mere 5×10 − 4 m (0.014%)
`sodium dodecyl sulfate or 5×10− 3 m (0.097%)
`decylammonium chloride boosted the CP of a
`2.00% solution of octoxynol 9 by 26°C. Higher
`concentrations of ionic surfactants produced com-
`mensurately larger CP increases [22].
`In the case of pharmaceutical disperse systems,
`it is germane that the majority of prescription
`drugs are cationic [10]. Most of them are either
`water soluble and form mixed micelles with NSs
`
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`H. Schott /Colloids and Surfaces A: Physicochem. Eng. Aspects 186 (2001) 129–136
`
`or are solubilized by NS micelles. Both processes
`confer a slight positive charge to the nonionic
`micelles, boosting their CPs very effectively.
`
`References
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`.
`
`Page 8 of 8