Effects of structure variation on solution properties of hydrotropes: Phenyl versus cyclohexyl chain tips

Article abstract of DOI:10.1021/la301222m

The physicochemical behavior of the phenyl-n-alkanoate (PhenCx) and cyclohexyl-n-alkanoate (CyclohexCx) series has been investigated, supporting previous work on the understanding of hydrotropes (Hopkins Hatzopoulos, M.; Eastoe, J.; Dowding, P.J.; Rogers, S. E.; Heenan, R.; Dyer, R. Langmuir2011, 27, 12346-12353). Electrical conductivity, surface tension, ¹H NMR, and small-angle neutron scattering (SANS) were used to study adsorption and aggregation in terms of critical aggregation concentration (cac). The PhenCx series exhibited very similar d log(cac)/dn to n-alkylbenzoates (CnBenz), exhibiting two branches of behavior, with a common inflection point at four linear carbons, whereas the CyclohexCx series showed no break point. Electrical conductivity and ¹H NMR concentration scans indicate a difference in physicochemical behavior between higher and lower homologues in both the PhenCx and CyclohexCx series. Surface tension measurements with compounds belonging to either group gave typical Gibbs adsorption profiles, having d log(cac)/dn curves consistent with limiting headgroup areas in the region of (35-55 A²) indicating monolayer formation. SANS profiles showed no evidence for aggregates below the electrical conductivity determined cac values, inferring an "on-off" mode of aggregation. Analyses of SANS profiles was consistent with charged ellipsoidal aggregates, persisting from lower through to higher homologues in both the PhenCx and CyclohexCx series.

Full text of DOI:10.1021/la301222m

Article
pubs.acs.org/Langmuir
Effects of Structure Variation on Solution Properties of Hydrotropes:
Phenyl versus Cyclohexyl Chain Tips
Marios Hopkins Hatzopoulos,† Julian Eastoe,*,† Peter J. Dowding,‡ Isabelle Grillo,§ Bruno Deme,§
́
Sarah E. Rogers,∥ Richard Heenan,∥ and Robert Dyer⊥
†School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
‡Infineum UK Ltd., Milton Hill Business & Technology Centre, Abingdon, Oxfordshire OX13 6BB, United Kingdom
§ILL, BP 156-X, F-38042 Grenoble Cedex, France
∥ISIS-STFC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, United Kingdom
⊥Kruss U.K., School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
̈
S
* Supporting Information
ABSTRACT: The physicochemical behavior of the phenyl-n-
alkanoate (PhenCx) and cyclohexyl-n-alkanoate (CyclohexCx) series
has been investigated, supporting previous work on the understanding
of hydrotropes (Hopkins Hatzopoulos, M.; Eastoe, J.; Dowding, P.J.;
Rogers, S. E.; Heenan, R.; Dyer, R. Langmuir 2011, 27, 12346−
1
12353). Electrical conductivity, surface tension, H NMR, and small-
angle neutron scattering (SANS) were used to study adsorption and
aggregation in terms of critical aggregation concentration (cac). The
PhenCx series exhibited very similar d log(cac)/dn to n-alkylbenzoates (CnBenz), exhibiting two branches of behavior, with a
common inflection point at four linear carbons, whereas the CyclohexCx series showed no break point. Electrical conductivity
1
and H NMR concentration scans indicate a difference in physicochemical behavior between higher and lower homologues in
both the PhenCx and CyclohexCx series. Surface tension measurements with compounds belonging to either group gave typical
Gibbs adsorption profiles, having d log(cac)/dn curves consistent with limiting headgroup areas in the region of (35−55 Å2)
indicating monolayer formation. SANS profiles showed no evidence for aggregates below the electrical conductivity determined
cac values, inferring an “on−off” mode of aggregation. Analyses of SANS profiles was consistent with charged ellipsoidal
aggregates, persisting from lower through to higher homologues in both the PhenCx and CyclohexCx series.
“on−off” mechanism which is characteristic of surfactants. This
is considered as a distinguishing feature of hydrotropes
compared with classical surfactants. Balasubramanian and
Srinivas7 proposed a distinction between hydrotropes and
surfactants after examining a homologous series of n-
alkylbenzenesulfonates to find where hydrotropes “become”
surfactants: a clear point of departure could not be identified.
Recently, Hatzopoulos et al.8 studied aqueous solutions of the
sodium n-alkylbenzoate (CnBenz) series employing a range of
techniques. Classical methods such as electrical conductivity
and 1H NMR concentration scans showed that lower
homologues exhibit gradual changes in behavior, generally
observed previously for hydrotropes. However, when the alkyl
tail consisted of four or more methylene (CH2) groups,
behavior characteristic of surfactants was observed. Surface
tension measurements on the other hand produced typical
Gibbs adsorption profiles throughout the homologous series,
implying no distinction between the behavior for higher
homologues and the lower “hydrotropic” members of the
INTRODUCTION
■
The physicochemical behavior of short chain organic salts has
been the issue of much debate but of little scientific research in
comparison to common surfactants. An array of these organic
salts, commonly known as hydrotropes, has been observed to
facilitate increased solubilization of insoluble organic com-
pounds above a salt specific concentration. The broad class of
hydrotropes encompasses a wide variety of types (anionic,
cationic, neutral) and molecular structures, which has
hampered any general overview of behavior. The point of
contention has always been the mechanism of hydrotrope
action and how they impart increased solubility of hydrophobic
compounds. The formation of complexes between solute and
hydrotrope1 and action as structure makers or breakers2 have
been speculated before. On the other hand, association or
aggregation of hydrotropes has been supported by a number of
studies,3−5 being conclusively demonstrated by small-angle
neutron scattering (SANS) experiments on common ionic
hydrotropes.6 Having shown that hydrotropes aggregate, the
mechanism of association is under debate. A gradual change in
physicochemical properties is normally observed (e.g., electrical
conductivity and dye solubilization),3−5 leading to the view that
hydrotropes associate in a stepwise manner, rather than the
Received: March 22, 2012
Revised: May 22, 2012
Published: May 23, 2012
© 2012 American Chemical Society
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Article
series. SANS data also concurred with this, all recorded profiles
being consistent with charged ellipsoidal aggregates, suggesting
that “hydrotropes” do not differ in their overall physicochemical
properties to common surfactants. Scattering profiles also
indicated that no aggregates formed below the cac, again
characteristic of an “on−off” type of transition from a chaotic
solution to an aggregate dispersion. A further point was that for
such compounds not all carbon atoms in the molecule are
thermodynamically equivalent, though still hydrophobic
enough to drive aggregation.
This work extends that of ref 8, shedding more light on these
systems. It aims to probe the additive nature of hydrophobicity
by investigating an analogous series of alkylbenzoates, being
phenyl-n-alkanoates (PhenCx) or cyclohexyl-n-alkanoates
(CyclohexCx). The generic structures are shown in the inset
in Figure 1, where x is the number of linear carbons including
(phenyl vs cyclohexyl) differ by a mere five H atoms, as seen
below, there are some quite surprising effects on solution
properties.
EXPERIMENTAL SECTION
■
Materials. All chemicals were purchased from Sigma Aldrich at the
highest purity available. The sodium phenyl-n-alkanoates and cyclo-
hexyl-n-alkanoates were synthesized from the parent acids. Sodium
hydroxide was purchased from Fischer Scientific (97%). D2O for NMR
and SANS experiments was purchased from Aldrich (99.9%). EDTA
tetrasodium salt, used in surface tension experiments, was also
purchased from Aldrich (99.5%). Activated carbon was purchased
from Lancaster and used as received.
Methods. Compounds were prepared according to ref 8. Sodium
salts were generated by refluxing equimolar amounts of acid and
NaOH for 8 h, in approximately 1.5 times the minimum amount of
ethanol required to dissolve the acid. The hot solutions were passed
over carbon black and the collected filtrate was allowed to boil while
distilling excess ethanol. The solutions were then allowed to cool; the
products were collected by filtration and washed with cold ethanol.
The products were then dissolved in deionized water, any excess acid
was extracted by shaking with distilled diethyl ether, and then the
products were collected by evaporation of the aqueous phase. The
powders were placed in a vacuum oven at 40 °C for 24 h over
refreshed phosphorus pentoxide.
Electrical conductivities were determined using a Jenway model
4510 Conductivity/TDS meter with temperature controlled at 25
0.1 °C (thermostatic water bath).
NMR proton shifts were measured using a Varian 400 MHz
instrument at 25 °C, relative to HDO at 4.75 ppm.
Surface tension measurements were measured using the Wilhelmy
plate method with a Kruss K100 instrument at 25 °C. Protocols for
̈
purification of the amphiphilic salts are given in the Supporting
Information. Solutions were prepared with the necessary minimum
amount of the chelating agent EDTA to sequester divalent ions
introduced as impurities by NaOH in the preparation of the sodium
salts (Supporting Information). All glassware was prewashed with 50%
nitric acid and thoroughly rinsed with deionized water. The platinum
plate was held over a blue flame until red hot after every measurement
and then cooled to room temperature before every use.
Small-angle neutron scattering (SANS) was carried out on the D16
instrument at the Institute Laue Langevin (ILL). The wavelength at
take off was 4.752 Å. A sample to detector distance of 900 mm was
employed using three detector angles at 12°, 28°, and 44°, giving rise
to a Q range of 0.01−1.2 Å−1. The sample changer was set at a 21°
angle. Slits 1 were fully open at horizontally 15 mm and vertically 136
mm, whereas slits 2 (closest to the sample) were set at 5 mm
horizontally and 8.8 mm vertically.
Additional measurements were made on the SANS2d instrument at
ISIS at the Rutherford Laboratory. A simultaneous Q-range of 0.0045−
0.75 Å−1 on the SANS2d instrument was achieved utilizing an incident
wavelength range of 1.75−16.5 Å and employing an instrument set up
of L1 = L2 = 4 m, with the 1 M2 detector offset vertically 150 mm and
sideways 180 mm. The PhenCx series was measured on D16, as were
CyclohexC1 and CyclohexC5. The compounds CyclohexC2−4 were
measured on SANS2d.
Figure 1. Electrical conductivity profiles of examples of lower and
higher homologues of (a) sodium phenyl-n-alkanoates and (b) sodium
cyclohexyl-n-alkanoates.
The momentum transfer Q is defined as
4π sin(θ/2)
Q =
(1)
the headgroup carbon. This structural switch reveals the change
from cyclic aromatic to cyclic aliphatic chain tips, to explore the
distinction between these different but related groups.
Common commercial surfactants, like those from the Triton
series, can be found in aromatic and reduced cyclohexyl forms;
therefore it is of interest to isolate the effects of this general
structural switch on physicochemical behavior. The phenyl ring
is planar, and at 25 °C the cyclohexyl ring predominantly
adopts a chair conformation.9 Even though these two groups
λ
where θ is the scattering angle and λ is the incident neutron
wavelength.
D2O was used to prepare the solutions, providing the necessary
contrast. Samples were contained in 2 mm path length Hellma fused
silica cells. The SANS data were normalized for transmission and
subtraction of the empty cell and solvent background.
The FISH program was employed for data modeling.10 The
scattering profile curve is broadly described by
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I(Q) ∝ P(Q)S(Q)
where P(Q) is the form factor, which contains information about the
size and shape of the aggregates, and S(Q) is the structure factor
reporting on interactions. Here the ellipsoidal P(Q) model was used in
final fits, in combination with a Hayter-Penfold charge repulsion
S(Q).11 The ellipsoid aspect ratio X is characterized by two principal
radii R1 and R3
Article
(2)
once more, at four linear carbons. Whereas the aromatic
compounds appear to share similarities with each other with
respect to the behavioral gradients and breaks, the cyclohexyl-
tipped compounds appear different. The cacs of the
homologues, with the exception of the parent hydrotrope
(CyclohexC1), fall on a straight line.
The regular dlog(cac)/dn ≈ 0.285 is found for most of the
homologous series, with the first member being a slight outlier,
indicative of the similarity in hydrophobic contribution to the
linear carbons by those of the cyclohexyl ring. The equations
describing the regions of behavior are:
X = R3/R1
(3)
Other model parameters include the volume fraction ϕ, the Debye
length κ−1, and the effective radius of the charged micelle RS(Q). An
approximate value of κ−1 value can be calculated by
CnBenz log(cac) = −0.2866n + 0.4179
(6)
1/2
2
⎛
⎜
⎝
⎞
⎟
⎠
2F ρI
0
κ =
PhenCx log(cac) = −0.2714n + 0.5626
ε ε RT
(7)
(4)
r
F is the Faraday constant, ρ is the solvent density, I is the ionic
strength, ε0 is the permittivity of free space, εr is the solvent dielectric
constant, R is the gas constant, and T is the temperature.
The first approaches to fitting used hydrophobic tail lengths
calculated using the Tanford equation12 (eq 5)
CyclohexCx log(cac) = −0.2867n + 0.0752
(8)
These compare well to the Klevens equation (eq 9) for regular
sodium alkanoates,14 which is gratifying, considering the
difficulty in attaining precise cac values by conductivity for
such compounds, where ionic strength is very high and the cac
values are large.
lmax = 1.5 + 1.265n
(5)
where lmax is the fully extended chain and n is the carbon number of
the linear chain. It was assumed that a benzene ring ∼3.5 CH2 in
length.13 The micellar radii R1 and R3 were minimized in the final
analyses.
linear alkanoates log(cac) = −0.2832n + 1.5145
(9)
Skauge and Splitzer15 investigated CyclohexC1-CyclohexC4 by
surface tension, reporting cac values in good agreement to
those listed here for the higher homologues. Cyclohexanecar-
boxylate (CyclohexC1) data did not yield a plateau and hence a
cac for this compound was not reported. Interestingly the cac of
sodium cyclohexaneacetate (CyclohexC2) was reported having
a cac of 1 M. This is much higher than the value presented in
this study and closer to the cac value of the parent hydrotrope
reported here. Skauge and Splitzer15 constructed a Klevens-like
equation (eq 10) by adding all of the carbon atoms (ring
carbons inclusive) to n, which gave a very similar gradient to
the sodium alkanoate Klevens equation (eq 9)
RESULTS AND DISCUSSION
■
Electrical Conductivity. Electrical conductivity was
employed as a first estimation of the cacs, and the values are
shown in Table 1. When the cac values of compounds
Table 1. Critical Aggregation Concentration (cac) Values as
Determined by Electrical Conductivity
cac/M
carbon number
PhenCx
CyclohexCx
6
5
4
3
2
1
0.17
0.31
0.37
0.39
CyclohexCx* log(cac) = −0.29n + 2.03
(10)
0.08
0.18
0.28
0.60
0.97
As stated previously8 the difference in intercept values of
alkanoates, the compounds under study, arises from not
including the hydrophobic effect of the ring moieties into the
value of n in the form of neff. MacInnis et al.16 also investigated
these cyclohexane-tipped alkanoates by 13C NMR and 23Na-
NMR reporting cmc values in good agreement with those
found in15 and here. They also report a higher cmc value for
CyclohexC2 at 0.89 M by sodium chemical shift change and
1.02 M by 13C chemical shift change. A more detailed
discussion on the discrepancies can found in the Supporting
Information.
0.48*
*
Data from ref 8.
containing the same number of carbons are compared, it is
readily appreciated that the higher homologues (x = 4 and 5) of
the saturated compounds have significantly lower cac values to
the corresponding aromatic. The opposite trend however is
noted when comparing the lower homologues (x = 1−3). In
both classes of compounds, the conductivity profiles showed a
slightly curved behavior for the lower homologues, whereas the
higher ones exhibited a clear break, which is readily identifiable
as a critical point and is associated with reaching the cmc of
common surfactants. Figure 1 presents two example cases of
each category, illustrating examples of readily identifiable
changes in behavior as a function of concentration in the
higher homologue, but instead a gradual change in slope in the
lower homologues.
Examining the hydrophobic contribution of the cyclic
components of these compounds (Figure 2b), the benzene
ring in both classes reaches limiting values (nring) of 3−4 when
n has reached 4. It is appreciated that the saturated component,
cyclohexane, imparts a higher hydrophobic contribution to the
structures than the aromatic ring in either case. The stronger
hydrophobic contribution of the cyclohexyl group appears
constant at nring ≈ 5 and hence higher than the aromatic
analogues. The comparison of cyclohexyl to the phenyl ring
hydrophobic contribution has been demonstrated recently in
branched dodecylammonium bromide analogues containing
these ring moieties17 with the cyclohexyl-tipped compounds
possessing a lower cac to their phenyl-tipped analogues. As the
compared compounds are identical in all structural aspects
A Klevens type analysis shows two behavioral modes for the
phenyl-tipped alkanoates. Figure 1a shows that the phenyl-
tipped series mirrors the behavior of the alkylbenzoates,
examined in ref 8. The break in behavior seems to occur
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point when considering that the cac approximately doubles,
when the amphiphile “acquires” a double bond.18 The cac
values of sodium cyclohexanecarboxylate and sodium benzoate
do not compare directly, implying that aromaticity does play a
role in promoting aggregation. The possibility of π−π
interactions has indeed been suggested in the past;19 however,
the exact nature of this effect cannot be identified from the
results of this study.
The electrical conductivity profiles of all compounds are
similar with those of alkylbenzoates,8 in that lower homologues
appear to lack clear breaks, whereas the higher homologues
show a clear break in gradient. A Klevens type analysis of the
PhenCx series supports the finding of a change in behavior at
four linear carbons in line with the CnBenz series, which could
be seen as the transition point from hydrotropes to surfactants.
The CyclohexCx series however reveals that the common
Klevens behavior, encountered in linear alkanoates, is seen for
all cyclohexyl-n-alkanoates, whose lower homologues possess
cac values higher than those of the aromatic series; reinforcing
the idea of a hydrophobic, thermodynamic balance at play,
between saturated and unsaturated tail carbons.
Surface Tension Measurements. All compounds ex-
hibited surface tension behavior similar to that of common
surfactants. Figure 3 illustrates the behavior of surface tension
Figure 2. (a) Comparison of behavior of log(cac/M) with
hydrophobic chain length (n) for phenyl-n-alkanoates, cyclohexyl-n-
alkanoates, and n-alkylbenzoates. (b) Comparison of hydrophobic
contribution of ring moieties (nring) with hydrophobic chain length (n)
phenyl-n-alkanoates, cyclohexyl-n-alkanoates, and n-alkylbenzoates.
Data for n-alkylbenzoates from ref 8.
apart from the ring moieties, then it is reasonable to infer a
higher hydrophobic contribution by the cyclohexyl than the
phenyl ring.
Figure 3. Surface tension (γ) vs ln(α) of PhenCx homologues.
Quadratic equations were fitted to the pre-cac data, to determine
adsorption parameters. Surface tension profiles for other homologues
not available due to difficulty in purification.
An interesting point is that the aromatic ring in the parent
hydrotrope (sodium benzoate) has a higher hydrophobic
contribution than its saturated counterpart (sodium cyclo-
hexanecarboxylate). However when the second behavioral
region is reached, it appears that the saturated ring has a
higher contribution than the aromatic ring, in effect reversing
the trend of the first behavioral branch with respect to the ring
moiety. Though both governed by the hydrophobic effect, they
are thermodynamically different. The break in the aromatic
compounds can be seen as occurring due to a switchover in
thermodynamic behavior. In the first behavioral branch it is the
aromatic carbons that dominate the aggregation process,
switching over to the saturated linear tails in the second
branch. The saturated compounds however show no change.
This can be seen to be a result of the small difference
between the cyclic saturated carbons and those belonging to the
saturated linear tail, which may explain why the parent
compounds show such difference in hydrophobic contributions
across the three series. While sodium benzoate has a neff of
approximately 6.5 carbon atoms, sodium cyclohexanecarbox-
ylate has a neff of ∼5 carbon atoms. This becomes an interesting
(γ) with activity (a) of PhenCx compounds with the γ vs ln(α)
plots exhibiting a smooth “pre-cac” tension decrease followed
by a sharp break at the cac and a “post-cac” plateau. Data for
PhenC3 were of poor quality, with low reproducibility, but
broadly exhibiting similar features to the rest of the
homologous series, as was for the carbon number equivalent
C2Benz.8 Despite these difficulties, an approximate analysis still
yields parameters in accord with the rest of the homologous
series (Table 2). CyclohexCx compounds however do not
Table 2. Parameters for PhenCx Series, Derived from
Surface Tension Measurements Using the Gibbs Equation in
Terms of Activity
activity
(α)
γcac 0.1/(mN Γcac 0.1/(10‑6 mol
Acac
PhenCx
m‑1)
m‑2)
10%/Å2
4
5
0.31
0.26
52.8
48.5
3.1
5.0
53.2
32.9
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exhibit such inconsistencies (Figure 4) with all compounds
behaving like typical surfactants even in the case of the parent
Table 3. Parameters for the CyclohexCx Series, Derived
from Surface Tension Measurements Using the Gibbs
Equation in Terms of Activity
activity γcac 0.1/(mN
Γcac 0.1/(10‑6
Acac
CyclohexCx
(α)
m‑1)
mol m‑2)
10%/Å2
1
3
4
5
1.20
0.30
0.19
0.08
51.1
46.8
45.0
44.0
3.3
3.1
3.3
4.7
50.0
53.2
50.9
34.8
The investigation of CyclohexC1-CyclohexC4 by surface
tension by Skauge and Splitzer15 yielded slightly higher Acac
values, at around 60−65 Å2, though still comparable to the
values ascertained in this study, and with a similar converging
trend of values. The discrepancy in Acac values could be
attributed to the difference in solution preparation,15 in that the
stock solutions were prepared to pH ∼10, whereas in this study
the pH of the solution was not controlled.
The converging limiting adsorption parameters in both
PhenCx and CyclohexCx series imply that adsorption is
dictated by the ring moieties when the linear tail is short.
The higher homologues show lower limiting surface tensions
and headgroup area values, which are more typical of common
surfactants. The reduction in γcac with n follows increased
surface chain density26 longer alkyl chains pack more closely
and have lower surface energies.
Figure 4. Surface tension (γ) vs ln(α) of CyclohexCx homologues.
Quadratic equations were fitted to the pre-cac data, to determine
adsorption parameters.
hydrotrope sodium cyclohexanecarboxylate with cac ≈ 1 M.
Only two of the PhenCx homologues are presented here due to
the presence of impurities in the other compounds, despite
being subjected to the same purification method. This can be
expected, especially for the lower homologues, as their surface
activity is quite low and hence even small amounts of impurities
impart a big effect. As such, this is also a testament to the very
high purity of the compounds that do exhibit a regular Gibbs
isotherm.
For both classes of compounds, the limiting tension γcac
shows a chain length dependence, decreasing with increasing n,
consistent with increased alkyl chain density lowering surface
energy.20 The cac values as determined by tension measure-
ments are in good agreement with those determined by
electrical conductivity (Table 1). A slight decrease in “pre-cac”
gradient with decreasing chain length is seen which was also
observed for alkylbenzoates.8 The pre-cac tensions were
analyzed in terms of the Gibbs adsorption equation
Pitt et al20 observed that compounds with similar cac values
and headgroup areas but with different tail ends, exhibited
limiting surface tensions following the trend of CF3 < CF2 <
CH3 < CH2 < phenyl-. Similarly Nave et al.27 observed that
phenyl-tipped Aerosol-OT analogues exhibited higher limiting
surface tensions to that of regular CH3-tipped Aerosol-OT.
These trends hold for this study as well, when analogues of
similar cac values are compared.
In contrast to conductivity behavior (Figure 1), there does
not appear to be any clear distinction between the air−water
adsorption behavior of the compounds belonging to either
behavioral branch of the Klevens plot. The results reveal that
difficulties encountered for the lower homologues CnBenz8
series are not a result of these compounds possessing
physicochemical properties different to common surfactants
and exclusive to “hydrotropes”; as the CyclohexCx series
exhibited well-defined behavior for all homologues, with the
lower ones possessing cac values considerably higher to the
corresponding aromatic analogues. As such surface tension
investigations in this study reiterate the conclusion of8 in that
there does not appear to be a behavioral difference between
“hydrotropes” and common surfactants.
1
dγ
Γ = −
(11)
mRT ln α
where Γ is the surface excess (mol m−2), m is a constant
associated with ion dissociation (see below), and R and T
represent the gas constant and temperature. The activity
coefficients were estimated using standard procedures.21
Using a combination of surface tension and neutron
reflectivity, a prefactor m = 222−25 has been shown for ionic
surfactants (<10 mM; in the presence of sufficient M+2
sequestrant EDTA). This value of m = 2 is taken to hold
here despite the cacs being much higher than for conventional
surfactants. The surface excesses Γ were determined by fitting
quadratics to the pre-cac curves, the function and coefficients
were used to generate local tangents and hence Γ vs c profiles.
With the coefficients and the associated derivatives used to find
the local tangents. Effective headgroup areas (Acac) were
estimated using eq 12
A point of interest is that if the surface tension-determined
cac of CyclohexC1 is used in the Klevens analysis for this series,
then all log(cac) values fall on the same line (eq 11) with
similar to that obtained by electrical conductivity (eq 8).
log(cac) = −0.2875n + 0.0776
(13)
This further emphasizes the thermodynamic similarity of the
saturated cyclohexyl-ring carbons to the linear alkane carbons,
as well as the difficulty in obtaining precise cac values at such
low levels of hydrophobicity.
NMR Proton Chemical Shift Measurements. The
difference in chemical shifts for the aromatic ring protons
between the lowest concentration and those for higher
concentrations, defined as Δδ (ppm), was studied as a function
1
ΓN
Acmc
=
(12)
a
where Na is the Avogadro number.
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of concentration. The behavior of the shift change with
concentration is shown for all PhenCx homologues in Figure 5.
homologues and the sharp breaks observed for the higher
homologues.
Small-Angle Neutron Scattering (SANS). SANS is a
technique capable of directly probing the nature of amphiphile
aggregates and other soft matter, unlike conventional
techniques which offer indirect information. By the nature of
the molecules in this work, results will be complementary to the
earlier study on n-alkylbenzoates,8 which are structurally
related.
Hydrotrope solutions in D2O were prepared as multiples of
the cac determined from conductivity measurements (Table 1),
except for CyclohexC1 where the cac was that determined by
surface tension. Figure 6 shows example SANS data for PhenC3
Figure 5. Variation in Δδ/ppm of para protons of PhenCx compounds
with concentration.
The value Δδ is approximately constant in the dilute region,
with a change in behavior at values consistent with the cac as
determined by electrical conductivity. The change in Δδ
behavior comes from a change in the environment experienced
by the protons. Lower homologues appear to go through a
smooth transition in behavior with the Δδ vs log(c) being
curved in the cac region. A sharper transition is only observed
for the PhenC6 analogue, the highest homologue in the series
studied. This Δδ vs log(c) behavior was also observed for the n-
alkylbenzoate series with a definite presence of a clear break
point in behavior appearing at the C5Benz and higher
homologues.8 All PhenCx protons shifted with increasing
concentration, which according to the reasoning found in8
indicates that the phenyl rings are embedded in the aggregates,
in accord to the observations made by Martin et al.17 who
employed the more sophisticated ROSEY 1H NMR experiment
to determine the locus of the phenyl- and cyclohexyl-tipped
DTAB analogues.
The figure for the behavior of the shift change of CyclohexCx
series protons with concentration can be found in the
Supporting Information. Again all protons shifted producing a
plateau in the dilute region and this time a break in behavior for
all homologues at values consistent with the cac values
determined by electrical conductivity. A break point is observed
for the highest homologue, CyclohexC5, whereas the smaller
ones exhibit a smooth transition. Further still all protons shifted
in the opposite direction to those of phenyl-n-alkanoate
protons.
Figure 6. (a) SANS profiles of PhenC3 in D2O at multiples of the cac
as determined by electrical conductivity (b) SANS profiles of
CyclohexC5 in D2O at multiples of the cac as determined by electrical
conductivity.
The shift direction to lower ppm values has in the past been
attributed to the protons experiencing a more apolar environ-
ment.2,28 Based on the results of Martin et al.,17 where the
cyclohexyl-ring was found to be embedded in the micelle, the
parallel of the chemical shift axis to a hydrophobic scale can
only be limited to aromatic compounds. It has been previously
speculated that the change in direction of the proton chemical
shift of the terminal CH3− protons of surfactants is due to alkyl
chain bending toward the aggregate palisade.29,30
and CyclohexC5. For all compounds in this study, the
scattering profiles are consistent with charged aggregates,
features observed for anionic surfactants like SDS above the
cmc.31 The intensity I(Q) decreases with concentration to the
point where it may not be detectable at 1 × cac and below.
Such behavior supports a surfactant-like “on-off” mode of
aggregation crossing the cac (cmc) and is observed across the
homologous series for both phenyl-n-alkanoate and cyclohexyl-
n-alkanoate classes [see the Supporting Information].
1
With respect to the mode of aggregation these H NMR
studies provide an ambiguous picture as due to the existence of
a distinction between the smooth transitions of the lower
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Figure 7 shows scattering profiles of PhenCx and Cyclo-
hexCx D2O solutions at 4 × cac, showing broadly common
Table 4. Parameters Obtained by Fitting SANS Data to the
Charged Ellipsoid Aggregate Model for PhenCx and
CyclohexCx Homologous Series at 4 × cac
P(Q)
S(Q)
c
PhenCx
R1/Å
X
RS(Q)
κ/Å‑1
5
4
3
7.9
6.7
5.0
4.8
2.2
2.0
2.1
1.2
10.6
0.221
0.403
0.771
0.460
8.92
8.02
5.5
a
1
CyclohexCx
5
4
3
2
10.1
8.9
7.8
6.9
4.7
2.5
15.0
0.186
0.268
0.375
0.731
0.852
1.6
11.0
8.8
1.96
1.54
2.3
9.5
b
1
7.94
a
Parameters for C0Benz (sodium benzoate) at 4 × cac taken from ref
b
8. Parameters for CyclohexC1 (sodium cyclohexanecarboxylate) at 2
× cac. RS(Q) apparent radius in the S(Q) model.
c
In the fitting procedure the Tanford equation (eq 5) was used
as a first approximation to the length of the molecules. The
aggregate sizes formed for phenyl-n-alkanoates were smaller
than the predicted molecular length values, and for cyclohexyl-
n-alkanoates to a lesser extent. For the latter, these
discrepancies became smaller with longer molecules.
SANS is a direct report of the presence or absence of
(commonly) H-domains in D2O. Hence its capabilities extend
in distinguishing between a gradual stepwise and sharp “on−
off” mode of aggregation. The scattering profiles in this study
are in line with the previous findings;8 an “on−off” aggregation
mechanism is observed for a further two classes of organic salts.
Despite the structural differences between CnBenz, PhenCx
and CyclohexCx compounds, which are subtle, though
significant, the profiles are consistent with charged ellipsoid
structures in all cases. Previous studies by Triolo et al.6
demonstrated that a wide range of hydrotropes do associate in
forms consistent with spherical aggregates near the cac and
ellipsoid aggregates at higher concentrations. Pal et al.32 found
that sodium butylbenzenesulfonate formed ellipsoid aggregates
at approximately eight times the cac as determined by
Balasubramanian et al.7
Figure 7. (a) SANS profiles of PhenC3−PhenC5 in D2O at 4 × cac
(PhenC4 offset by +0.8, PhenC3 offset by +1.6) measured on D16 (b)
SANS profiles of CyclohexCx series in D2O at 4 × cac (CyclohexC4
offset by +0.5, CyclohexC3 offset by +1, CyclohexC1 offset by +1.5).
CyclohexC2−4 measurements were carried out on SANS2d while
CyclohexC5 was measurements were carried out on D16. Lines are
data fits using a charged ellipsoid model described in the text.
CONCLUSIONS
■
The dilute aqueous phase properties of two classes of cyclic-
tipped alkanoates have been investigated, with the results
reinforcing the notion that hydrotropes with very different
chemical structures exhibit similar solution, interfacial, and
association properties as common surfactants. These findings
support the observations made previously for the linear chain n-
alkylbenzoate series.8
behavior across the series. Increasing chain length leads to
increasing I(Q), with S(Q) also becoming more pronounced,
suggesting stronger aggregate interactions. The data were fitted
using the protocol and model described in the Experimental
section, being consistent with charged prolate ellipsoidal
aggregates at concentrations of 4 × cac (Table 4) and 2 ×
cac (Supporting Information).
The data and analyses show a persistent aggregation behavior
across the homologous series: from the short chain homologues
considered “true hydrotropes” to the longer chain homologues,
considered closer to “true surfactants”, in both saturated
cyclohexyl and aromatic phenyl classes.
Conductivity measurements with both classes of homo-
logues, report a gradual physicochemical change with
concentration, which would be consistent with a stepwise
aggregation mechanism for the lower homologues, whereas a
sharp break in behavior for higher homologues, more
reminiscent of typical surfactant behavior. Klevens analyses of
the chain length dependence of critical aggregation (as in ref 8)
show that phenyl-n-alkanoates exhibit a break in behavior at
four linear carbons, in agreement with other observations with
related compounds.7,8 Comparing compounds of equivalent
carbon number, phenyl-n-alkanoates and n-alkylbenzoates have
broadly similar cacs. Interestingly lower homologues of the
When the scattering parameters in Table 4 and those
reported in ref 8 are compared, it becomes apparent that for
equal carbon number molecules aggregates follow the size
order:
CyclohexCx > CnBenz > PhenCx
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cyclohexyl-n-alkanoate series are more hydrophilic than the
equivalent n-alkylbenzoates and phenyl-n-alkanoates. However,
this trend is inverted for higher homologues, with the
cyclohexyl-n-alkanoates being the most hydrophobic of the
three classes. The cyclohexyl-n-alkanoates appear to also
parallel regular n-alkanoates from the first added −CH2−
onward, behavior arising at much shorter chain length than for
the aromatic series. These observations reinforce the concept of
a thermodynamic balance between ring carbons and linear
carbons. Aromatic carbons appear to dominate the aggregation
behavior up to a certain value of n, after which the linear
−CH2− groups take over. Cyclohexyl, −CH2−, being saturated,
is closer in nature to the linear tail −CH2− carbons, hence the
break in behavior occurs at a much shorter chain length. Surface
tension measurements do not exhibit such a distinction in
behavior, with homologues belonging to either dlog(cac)/dn
branches of the Klevens plot, exhibiting typical Gibbs
adsorption profiles.
NMR experiments are consistent with lower homologues
gradually forming ill-defined structures, whereas higher
homologues present a more abrupt change more typical of
classical surfactants.
SANS profiles on the other hand show clear transitions from
apparently nonstructured solutions, then giving clear evidence
for the presence of aggregates above a certain concentration. As
with previous reports on related compounds,6,8,31 these
additives appear to form ellipsoid-shaped aggregates.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Additional details of hydrotrope solution properties, surface
tensions, NMR, and neutron scattering data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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The authors thank Dr. Craig Butts for useful discussions on
NMR. M.H.H. thanks Infineum UK and the EPSRC through
the University of Bristol School of Chemistry Doctoral
Training Account for the provision of a Ph.D. studentship.
We also acknowledge ILL, ISIS, and STFC for the allocation of
beam time, travel, and consumables.
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