Temperature-dependent oil absorption of poly(oxypropylene)amine-intercalated clays for environmental remediation

Abstract

Natural silicate clays with layered structures were intercalated with polyether-amines via an ionic exchange reaction and subsequently applied for oil absorption. Polyether-amines, including hydrophobic poly(oxypropylene)-amines (POP-amines) and the hydrophilic poly(oxyethylene)-amine (POE-amine), were used to intercalate sodium montmorillonite (MMT). The organoclays obtained from POP-monoamine of 2000 g mol⁻¹ (POP-M2000) demonstrated the ability to absorb petroleum crude oils in water at an efficacy of 15 times the weight of the organoclay. Owing to the property of lower critical aggregation temperature (LCAT) at 50 °C, the organoclays absorbed oils and self-aggregated into oil lumps and phase separated out from water cleanly above this temperature. By comparison, a homogeneous emulsion-like dispersion was formed at lower temperatures of 15 °C. This phenomenon was explained by the inherent property of lower critical solubility temperature (LCST) due to the presence of the POP-organics intercalated in the clay galleries. In addition to the POP hydrophobicity for highly efficient oil absorption, the LCAT behavior resulted in the ease of oil recovery under heating. The oil-absorbed clay aggregates were analyzed by XRD and showed the expansion and layer exfoliation of the clay structure during the incorporation of oil. The understanding of the POP-intercalated and oil-absorbed tertiary structures has led to their potential use in the areas of secondary oil recovery, shale oil extraction and remediation of oil pollution in water.

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1. Introduction

In the past, oil spills in the ocean have occurred occasionally and have caused serious impacts on the shoreline environment.^1,2 A wide range of methods and materials for oil-spill remediation, including the use of the Pickering emulsion technique,³ absorbents,⁴ superhydrophobic materials^5,6 and skimmers,⁷ have been reported. However, most of these methods involve complicated treating procedures and are often time-consuming. The methods may have not enough efficiency for application in large scale oil spills in the ocean. In addition, other problem that is often encountered is their chemical incompatibility with the environment or the issues with harmfulness to biological systems. For example, in the disaster that occurred during the oil spill in the Gulf of Mexico in 2011, a large amount of chemical dispersants were sprayed from the air and injected into the deep-sea to dissipate the oils. Concern over the long-term effects on the deep-water ecosystem in that area is still under debate.

In view of the drawbacks of conventional methods, we utilized naturally occurring silicate clays such as sodium montmorillonite (MMT), which are considered to be environmentally benign materials. Our previous study has demonstrated the feasibility of using poly(oxyalkylene)-polyamine salts for clay intercalation in order to modify the water-swellable clay with amphiphilic properties.⁸ Using the technique of organic intercalation, the layered structure of clay galleries was expanded and hosted with hydrophobic organics to introduce an affinity for oil absorption. Our approach to clay modification is different from conventional organic intercalation used for applications such as catalysis,⁹ adsorption and polymer nanocomposites.^10–12

We adopted the basics of clay chemistry, including ionic exchange reaction and organic interactions, with the high surface silicates.^13,14 Previously,¹⁵ we studied the modification of silicate clays using poly(oxypropylene)amine-salts and secondarily encapsulation with iron-oxide nanoparticles (FeNPs). The possible incorporation of FeNPs in the nanostructure of layered silicates enabled the absorbed oils to be removed by simply applying a magnetic field. More recently, we found that the key step of hydrophobic intercalation in the clay galleries actually went through a stepwise mechanism, i.e. a first step involving an ionic exchange reaction with a critical concentration for spacing expansion and a second step involving hydrophobic phase segregation in the layered structure.⁸ The findings prompted us to further study and prepare organically modified clays more suitable for oil applications.

In this study, a series of POP-intercalated silicate clays were compared for their ability towards oil absorption. Hydrophobic and hydrophilic polyether-amines with different molecular weights were used to intercalate clay and also introduced the clay gallery expansion with oil-absorption ability. Most significantly, it was found that the POP-intercalated clays exhibited an intriguing temperature dependency during oil absorption. The oil-absorbing ability is not only related to the basal spacing expansion but also to their secondary unit aggregation or phase separation from water. In other words, the unique property of lower critical solubility temperature (LCST) of POP-amines is shown in the POP/silicate organoclays, which exhibit aggregation depending on the temperature owing to the disruption of hydrogen bonding in the POP segments in water. This temperature dependency that occurred in the clay–oil–water interactions has great potential for use in oil–water separation, secondary oil recovery and shale oil extraction. More importantly, a promising use for the remediation of oil-contaminated water can be immediately exploited.

2. Experimental section

2.1 Materials

One of the naturally occurring phyllosilicates, sodium montmorillonite (Na⁺-MMT) with a generic structure comprised of a 2 : 1 tetrahedral and octahedral aluminosilicate sheet structure, was supplied by Nanocor Co. (USA). The clay minerals are aggregates of primary units in stacks. Each individual sheet has a dimension of approximately 100 × 100 × 1 nm³ and on average contains 8–10 sheets in one primary stack.¹⁶ The smectite clay with layered structure was estimated to have a cationic exchange capacity (CEC) of 1.20 mequiv. g⁻¹. In this capacity, the organic ions may exchange with counter metal ions such as Na⁺ and other alkali metal ions (Mg²⁺, Ca²⁺, or Fe²⁺). Both of the hydrophobic and hydrophilic polyether-backboned amines were purchased from Huntsman Chemical Co. (USA). The molecular weight (M_w) of the hydrophobic poly(oxypropylene)-(POP-)monoamines with 300, 1000 and 2000 g mol⁻¹ were abbreviated as POP-M300, POP-M1000 and POP-M2000, respectively. The POP-diamine of 2000 g mol⁻¹ M_w was abbreviated as POP-D2000. Another type, the hydrophilic poly(oxyethylene)-(POE-) backboned diamine with 2000 g mol⁻¹ M_w is a water-soluble amine and was abbreviated as POE-D2000. The chemical structures of the diamines and monoamines of various molecular weights are illustrated in Table 1.

Table 1 The chemical structures of polyether-amines, including POP- and POE-amines, as the precursors for clay intercalation

Name	Chemical structure	Oxyethylene/oxypropylene ratio	Mw	Solubility^a
				Water	Toluene
a + soluble; −insoluble at 10 wt% solubility tests in water or toluene.
POP-M300		0/3	300	−	+
POP-M1000		0/14	1000	−	+
POP-M2000		6/29	2000	−	+
POP-D2000		0/33	2000	−	+
POE-D2000		39/6	2000	+	−

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2.2 Preparation of organically modified clay

The family of POP- and POE-amines, including POP-M300, POP-M1000, POP-M2000, POP-D2000, and POE-D2000, with different molecular weights were used for ionic exchange intercalation into the clay layered structure. These amines were acidified by titration with one equivalent of hydrochloric acid in water to generate the corresponding NH³⁺Cl⁻ salts. The intercalation of MMT using the HCl-treated POP- and POE-backboned polyether-amines has been reported previously.^8,10 In a typical procedure, MMT (10.0 g, 1.20 mequiv. g⁻¹) was swelled in deionized water (800 mL) at 80 °C for 3 h. The amine salts were separately prepared by treating amine with an equivalent of HCl in an acidification step. An aliquot of the amine/HCl salt was added into the MMT slurry in a stoichiometric amount on the basis of the clay CEC (1.20 mequiv. g⁻¹). The slurry was vigorously mixed by mechanical stirring at 80 °C for 0.5 h. The resulting precipitates were filtered, washed thoroughly with deionized water and dried at 50 °C for 24 h. The organoclays were analyzed by X-ray powder diffraction and used as oil absorbents.

2.3 Determination of the lower critical aggregation temperature (LCAT) for the organoclays

The organoclays, prepared via an ionic exchange reaction of MMT, including POP-D2000/MMT, POE-D2000/MMT and POP-M2000/MMT, were made into a 0.1 wt% dispersion by magnetically stirring in water at 5 °C for 1 h. The dispersions were placed in a quartz cell and the transmittance was measured at a wavelength of 550 nm during the programmed ramping of temperature from 0 °C to 80 °C. Each measurement point was held for one hour to reach equilibrium. When the temperature was increased, the original fine dispersion appeared to become turbid and a white precipitate was formed. This aggregation phenomenon was found to be reversible as the temperature gradually decreased. The aggregation temperature was recorded as the lower critical aggregation temperature (LCAT). The corresponding lower critical solubility temperature (LCST) for POP-amines was measured in the same manner.

2.4 The efficiency of oil absorption

A beaker (250 mL) equipped with a magnetic stirrer bar was charged with a petroleum crude oil/water (0.50 g/20 g) slurry and then the organoclays (0.50 g) were added under the agitation at 15 °C for 30 min. The absorption efficiency was determined by incrementally adding oils into the clay slurry to observe its oil/clay aggregation and separation from the water. The process of an incremental addition of oil under continuous agitation was performed along with changing the temperature between 15 °C and 50 °C after each oil addition. The temperature fluctuation was purposely performed to observe the LCAT phenomenon. Through this process, the oil absorption occurred gradually and the clay/oil aggregated and in some cases, precipitated out in lumps from the water phase. Finally, a clear separation of oil from water was achieved. When the oil was not entirely separated from water, the addition amount of oil was defined as beyond the absorption efficiency of the organoclay. If the oil was absorbed by clay, aggregation did not occur or only the formation of an oil/water slurry occurred and then it was defined as “none” in terms of efficiency. A vivid illustration of the oil/water separation upon adding the organoclay was recorded (ESI Movie†).

2.5 Characterization

LCAT was determined by measuring the transmittance of the organoclays at a wavelength of 550 nm through a programmed ramping of temperature from 0 °C to 80 °C using a UV-vis spectrophotometer (V-570, Jasco). X-ray powder diffraction (XRD) was performed using a PANalytical X'Pert PRO diffractometer with Cu target (k = 1.5408 Å) at a generator voltage of 45 kV and current of 40 mA to record the basal spacing (d spacing) of all the clay samples. The pattern of basal spacing was calculated according to Bragg's equation (nλ = 2d sin θ). The value for n = 1 was calculated from the observed values for the higher order of n such as n = 2, 3, 4, etc. by fitting the equation. The thermal analyses were measured using a thermogravimetric analyzer (TGA) (Perkin-Elmer Pyris 1 model). The organic fraction was calculated from the weight loss by programming the temperature from 100 °C to 800 °C at the rate of 10 °C min⁻¹.

3. Results and discussion

3.1 Intercalation of MMT with polyether-amine salts

The organoclays were prepared via the intercalation of the multilayer-structured Na⁺-MMT with the polyether-amine-salts. The hydrophobic POP-series and hydrophilic POE-series of amines were treated with hydrochloric acid to become amine-salts and then added to the ionic exchange reaction with the clay. Accordingly, the family of organically modified clays, including POP-M300/, POP-M1000/, POP-M2000/, POP-D2000/and POE-D2000/MMT, were prepared from the five different polyether-amines (as structurally illustrated in Table 1). After intercalation at 1.0 equivalent during the ionic exchange reaction, the polyether-amine-salts were incorporated into the clay multi-layered galleries and led to an expanded basal spacing (as shown by the XRD d value). As a result, the d spacing of Na⁺-MMT at 12 Å was expanded to 26 Å for POP-M300/MMT, 48 Å for POP-M1000/MMT and 73 Å for POP-M2000/MMT, as recorded in Table 2. It was notable that the expansion of the clay interlayer spacing was proportionally dependent on the end-to-end length or relative molecular weight of the POP backbones in the trend of M2000 > M1000 > M300.¹⁷ Further comparison to the POP-D2000 diamine with a similar molecular weight at a d spacing of 52 Å, the POP-M2000 monoamine gave a much higher spacing of 73 Å, under the same 1.0 CEC equivalent ratio. It can be noted that the two POP-amines have a similar POP molecular weight and were incorporated into the layered silicate in a similar weight fraction, 73 wt% versus 74 wt% of organics based on TGA. The difference in d spacing could be derived from the amine-salt termini in contact with the silicate platelet surface. The POP backbone generated hydrophobic phase segregation in the clay interlayer galleries in different density for basal spacing expansion. In other words, the difference in the terminal structure between the mono- and the di-amine generated significantly different hydrophobic phase segregation. It implies that the corresponding organoclays have different hydrophobic affinities for oily organics. The hydrophobic effect was indirectly supported by the organoclays generated with the water-soluble POE-D2000 diamine of similar molecular weight but hydrophilic POE backbone, which intercalated at only 18 Å basal spacing, due to the lack of hydrophobic phase segregation in the clay galleries and as expected, an oil affinity of none.

Table 2 Performance and characterization of the various organoclays for oil absorption

Intercalation agent	Amine/clay^a (CEC)	Intercalated organoclays		LCAT^c	Oil added/organoclay (w/w)	After oil absorption		Efficiency (absorbed oil/organoclay^d, w/w)
		Organic fraction^b (wt%)	d Spacing (Å)			Organic fraction^b (wt%)	d Spacing, (Å)
a Equivalent ratio of amine-H^+ to MMT (CEC: cation exchange capacity, 120 mequiv. 100 g^−1).b Determined by TGA.c −: no LCAT was observed, +: LCAT was observed.d The efficiency was calculated using eqn (1) for the weight ratios of oil/organoclay; the endpoint was defined by the oil/clay aggregation and clear separation from water.e None: no water/oil separation (due to lacking of LCAT of the clay).
None (Na^+-MMT)	—	0	12	−	N.D.	N.D.	N.D.	None^e
POP-M300	1.0	28	26	+	1/1	35	34	Up to 10/1
POP-M1000	1.0	57	48	+	1/1	73	60	Up to 12/1
POP-M2000	0.3	43	18	−	1/1	71	78	None
	0.5	50	72	−	1/1	77	88	None
	0.7	61	71	−	1/1	81	84	None
	1.0	73	73	+	0.1/1	76	84	0.1/1
					0.5/1	79	92	0.5/1
					1/1	82	95	1/1
					3/1	90	97	3/1
					5/1	92	111	5/1
					10/1	96	116	10/1
					15/1	98	N.D.	15/1
	2.0	82	84	+	1/1	85	103	Up to 15/1
	4.0	89	100	+	1/1	83	108	Up to 15/1
POP-D2000	0.3	36	17	−	1/1	69	18	None
	1.0	74	52	+	1/1	86	52	1/1
					2/1	91	51	2/1
					3/1	91	54	3/1
					5/1	94	52	5/1
					10/1	95	N.D.	10/1
POE-D2000	1.0	63	18	−	1/1	68	18	None

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The selected POP-monoamine (POP-M2000) was further investigated for the details of clay intercalation with different ionic exchanging CEC equivalents. In our previous studies,⁸ we reported that the intercalation of POP-monoamine into clay occurred by a stepwise mechanism to incorporate the hydrophobic organics. The POP-M2000 can intercalate into the clay spacings through the ionic exchange reaction in the first step, and continuously expand by a second mechanism of hydrophobic attraction in the POP backbones. Therefore, the intercalating organics in the second step are not required to be ionic species for the ionic exchange reaction but hydrophobic compounds for expanding the layered spacing. The second hydrophobic organics can further create more hydrophobic phases and the expansion of clay galleries. The second step of hydrophobic intercalation implies the ability of the organoclays for accommodating oil absorption.

In order to understand the hydrophobic factor affecting the efficiency of oil absorption, the organoclays with various organic fractions derived from POP-M2000 intercalation at an equivalent ratio of 0.3, 0.5, 0.7, 1.0, 2.0 and 4.0 to the clay CEC were prepared. The XRD analyses showed different d values at 18, 72, 71, 73, 84 and 100 Å. There was a critical amount of POP-M2000 to clay CEC at the equivalent ratio of 0.5, and the d spacing exhibited a sudden change from 12 Å to 72 Å. This demonstrated a critical amount of POP organics for generating the hydrophobic phase in the nanostructures. The hydrophobic phase would be formed by the methyl-POP backbone (i.e., POP-M2000 monoamine) aggregation when greater than 0.5 equivalents of POP was intercalated. Afterwards, the multi-layered silicates can allow further accommodation of incoming hydrophobic organics in the second-stage intercalation, i.e. oil absorption.

3.2 Temperature-dependent aggregation of the organoclays

As shown in Table 2, some of the organoclays have temperature-dependent dispersion behaviours in water. Three polyether-amines, POP-M2000, POP-D2000 and POE-D2000, were examined for the property of lower critical solubility temperature (LCST) in the temperature range of 0–80 °C, as shown in Fig. 1. By comparison, the solubility of the hydro-phobic POP-backboned amine was temperature-dependent, while the POE-amine has the same solubility throughout the temperature range studied up to 80 °C, without showing the LCST property. The term lower critical aggregation temperature (LCAT) was derived from the POP-intercalated organoclays, which aggregated in water depending on the temperature. Above the LCAT, the hydrogen bonding was weakened or disrupted by thermal energy, which caused the intercalated POP molecular coils to shrink and phase out from water. As a result, the POP-MMT organoclays aggregated into lumpy precipitates at elevated temperatures. This phenomenon can be measured using UV-visible transmittance. As shown in Fig. 1, the transmittance dropped suddenly due to the occurrence of turbidity upon a critical temperature, which was defined as the LCAT. The LCAT of POP-D2000/MMT was in the range of 15–30 °C and LCAT of the analogous POP-M2000/MMT was around 10–50 °C. It was notable that the POE intercalated MMT maintained its dispersion state without phase aggregation during heating (Fig. 2e and f), in contrast to the POP/MMTs (Fig. 2a vs. 2b and 2c vs. 2d). The observed LCAT properties as well as the aggregation phenomena are summarized in Table 2, indicating the lack of LCAT for the organoclays with a lower amount of POP intercalation. All POP-amines, including POP-M300, POP-M1000, POP-M2000 and POP-D2000, intercalated MMT over a 1.0 CEC equivalent ratio exhibited the temperature-responsive property. While at lower amounts of POP intercalation such as 0.3, 0.5 and 0.7 CEC ratio of POP-M2000 intercalation, 0.3 of POP-D2000 and the pristine Na⁺-MMT, no LCAT property was observed. These results showed that the POP-modified MMT can be tailored for their hydrophobic interactions.

Fig. 1 UV-visible transmittance of POP-M2000, POP-D2000 and POE-D2000 solubility in water and their aggregation with organoclays with changes in temperature.

Fig. 2 Images of the aggregation of the organoclays (0.20 g in 10 g of water) with changes in temperature; POP-M2000/MMT (a) 0 °C, (b) 50 °C; POP-D2000/MMT (c) 0 °C, (d) 50 °C; and POE-D2000/MMT (e) 0 °C, (f) 50 °C.

3.3 Oil-absorbing efficiency

The POP tailored organoclays were utilized for oil absorption. As summarized in Table 2, the water-soluble POE-amine derived organoclay had no oil-absorbing ability due to the lack of hydrophobic interactions. Oil-absorbing ability was observed for the organoclays intercalated with POP-amines, including the POP-M300, -M1000, -M2000 and D2000 amines. Their maximal capacities were measured by incrementally adding oils and monitoring the oil/clay aggregation or phase separation from water until no further absorption of the additional oils was observed. The efficiency of oil absorption.

(η) was calculated based on eqn (1), where m_{oil absorbed}, m_POP, and m_MMT were the mass by weight of absorbed oil, intercalated POP in clay, and the MMT clay, respectively. The results are summarized in Table 2, for example, the efficiency or the weight ratios was shown to be 10, 12, 15 and 10 fold for the POP-amines, -M300, -M1000, -M2000, and -D2000 intercalated MMT, respectively.

η = (m_{oil absorbed})/(m_POP) + (m_MMT) (1)

The results are summarized in Table 2, for example, the efficiency or the weight ratios were shown to be 10, 12, 15, and 10 fold for the POP-amines, -M300, -M1000, -M2000, and -D2000 intercalated MMT, respectively. It can be noted that the maximal efficiency was reached by the appropriate amount of POP intercalation into MMT based on the clay CEC capacity.

In order to understand the kinetics of oil absorption, the progressive silicate interlayer spacing was monitored by X-ray diffraction analysis. The organoclays were prepared from POP-M2000 with different basal spacing depending on the amount of POP intercalation based on the clay CEC. Several different equivalent CEC, including 0.3, 0.5, 0.7, 1.0, 2.0 and 4.0 POP/clay (CEC) ratios, were involved as mentioned above. To each of these organoclays, oil in the amount of a 1 to 1 weight ratio was added. After absorption, the oil-absorbed organoclays were analysed and an increased basal spacing after absorption within the clays primary layered structure was observed. For example, the basal spacing was expanded from 18 Å to 78 Å (0.3 CEC POP intercalation), 72 Å to 88 Å (0.5 CEC), 71 Å to 84 Å (0.7 CEC), 73 Å to 95 Å (1.0 CEC), 84 Å to 103 Å (2.0 CEC), and 100 Å to 108 Å (4.0 CEC) after oil encapsulation. With further addition of more crude oil beyond a 1 to 1 weight ratio, the clay spacing continued to expand. The expansion was reasonable due to the accommodation of more oil. This result was consistent with the previous findings,¹⁷ in which a relatively linear correlation between oil absorption and spacing expansion was observed. Fig. 3 shows the XRD patterns of POPM2000/MMT at 1.0 CEC clay intercalation, indicating the expansion of clay spacing from 84 Å to featureless by XRD, upon further incremental addition of oil. Overall, the oil-absorbing capacity was accompanied with the expansion of clay basal spacing and the hydrophobic interaction eventually reaches a maximum when the clay layered structure was exfoliated into a featureless XRD measurement. At this point, the added oil was clearly phase separated from water.

Fig. 3 Basal spacing changes after adding incremental amounts of oil from 0.1 to 15 fold by weight for oil absorption in the case of POP-M2000/MMT.

In short, the high oil-absorbing efficiency was mainly attributed to the presence of POP organics that were intercalated in the MMT clay. It appears that the weight fraction of POP intercalated in the first stage of the ionic exchange reaction was responsible for the efficiency of oil absorption. Further accommodation of incoming oils was dominated by the species of POP-amines used, which maintain the layered structural container until exfoliation by overwhelming oil absorption at the point of maximum efficiency.

3.4 Oil/water separation

The POP-amines in the clay galleries play the most important role for contributing to the efficiency of oil absorption in oil/water phase separation. The LCST property of the intercalated POP-amines can contribute the behaviour of temperature-dependant aggregation and oil separation. The oil contamination in water or the emulsion state of oil/water mixing can be effectively eliminated upon adding the POP-MMT organoclay by forming easily separated oil lumps in water. Incorporated with the LCAT property, the overall cleaning process can be designed in three stages: (1) POP-amines intercalation into MMT to prepare a considerable hydrophobic expansion of the layered silicate structure, (2) oil absorption by the organoclays undergoing oil encapsulation and further expansion of the clay basal spacing, and (3) application of the LCAT effect on inducing a clear oil-water phase separation upon increasing the temperature. In general, the selected POP-M2000 organoclay at temperatures above the LCAT renders the absorbed oils aggregated into lumps to spontaneously separate from water. The breaking of the water-oil emulsion was conceptually illustrated by the encapsulation of oils in the layered silicate structure and demonstrated by the experiment showing a clear water separation, as shown in Scheme 1. The process of oil absorption in three stages, including the preparation of organoclays, oil absorption accompanying with XRD basal spacing changes and increasing temperature above the LCAT for clean phase separation, is presented. This three-stage process was examined by analysis of the changes in the organic fraction (by TGA) and primary units of the layered silicate interlayer spacing enlargement (by XRD). The TGA technique was able to recognize the absorbed oils from the POP organics dissociation process under the heating pattern (Fig. 4). Furthermore, owing to the heat-blocking nature of the layered silicates, the delayed degradation phenomenon in TGA may differentiate the oil in the clay interlayer spacing and that surrounded freely on the clay surface.¹⁸ The implementation of highly efficient oil/water separation largely relied on the hydrophobic and LCST characteristics of POP.

Scheme 1 The proposed oil-absorption process for the primary unit of POP-layered-silicates and their secondary oil phase segregation from water.

Fig. 4 TGA traces of POP-M2000/MMT at 1.0 CEC intercalation after oil absorption.

The initial oil encapsulation in the layered structure and the second stage of oil absorption in expanding the layered spacing into a randomized structure, as well as forming oil lumps upon increasing the temperature above the LCAT have completed the process for clear water separation. The overall process was demonstrated in an attached video (see ESI†).

4. Conclusion

We used hydrophobic POP-amines to modify natural silicate clays and successfully demonstrated their effectiveness in the application of petroleum oil recovery from water. After screening the behavior of five different polyether-backboned amines, including water-soluble POE- and hydrophobic POP-amines with different molecular weights, the POP-M2000 monoamine-salt intercalated clays exhibited the highest efficiency of 15 times the weight for oil absorption. Since the pristine clay and the hydrophilic POE-intercalated clays showed no affinity for oils, the hydrophobicity of the intercalated POP organics is required for oil absorption. The POP-monoamine allowed single point intercalation as a result of the spatial expansion of the clay and its hydrophobic properties. During oil incorporation, the spatial galleries were further expanded from 73 Å to >116 Å or featureless in XRD. Furthermore, the incorporated POP-amines rendered the clay with the LCST property and the influence of oil-absorption through organoclay aggregation above the critical temperature. As a result, large oil lumps were separated out from water at ∼50 °C. Hence, the selection of the hydrophobic POP-monoamine favored oil absorption and phase separation from clean water. The selected POP-intercalated clays are suitable for applications in oil-spill remediation. The method of organic intercalation can afford a family of organoclay structures, which have potential applications in secondary oil recovery, shale-oil fracking and waste water treatment.

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Footnote

† Electronic supplementary information (ESI) available: Movie: crude oil absorption by organoclay. A supporting file of video is supplied for demonstrating the process of crude oil absorption under the LCAT phenomenon. See DOI: 10.1039/c5ra18669b

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